Treatment of Parkinson's disease with levodopa. (Levodopa: A pharmacologic miracle four decades later)
The designation of levodopa as a miracle drug is no exaggeration. It was first used to treat Parkinson’s disease (PD) in the early 1960s, and by the time it was approved by the U.S. FDA in 1967, it was hailed as one of the most important advances in the pharmacotherapy of neurologic diseases of the first half of the twentieth century.
Its development was based on a series of major advances in the understanding of the neurochemical mechanisms underlying the disease. Now, almost 40 years later and despite significant advances in the pharmacotherapy of PD, levodopa remains the “gold standard” of treatment. The gold, however, has been tarnished by a variety of intrinsic problems and complications of long-term use, such as motor fluctuations (the “on-off” phenomenon) and dyskinesia, which can be no less disabling than the parkinsonian symptoms it suppresses. Moreover, the early and unrealistic belief that levodopa could actually cure or at least slow the process of neurodegeneration by the simple replacement of a depleted neurotransmitter was quickly undercut by the harsh truth of practical experience and scientific discovery. In fact, one of the most active controversies swirling around levodopa today centers on whether it might increase the pace of neuronal degeneration by promoting oxidative neurotoxicity.
In this article we review the history of levodopa’s development and the major milestones that punctuate its maturation as the mainstay of treatment for PD. We also cover the impact of levodopa on the mortality and morbidity of PD, the proposed mechanism of levodopa induced complications (fluctuations and dyskinesia), and the data regarding levodopa toxicity.
In his classic 1817 monograph An Essay on the Shaking Palsy, James Parkinson (1) described a series of 6 patients afflicted by the highly visible malady that now bears his name. The precision of much of the description is remarkable considering that Parkinson examined only 3 of the patients directly, whereas the others were observed by him as a vigilant spectator on the streets of London. His language has forever captured the cardinal manifestations of the disease, although the accuracy of some of his observations (i.e., the italicized segments, italics added) has been refuted by modern experience: “Involuntary tremulous motion, with lessened muscular power, in parts not in action and even when supported: with the propensity to bend the trunk forward and to pass from a walking to a running pace: the senses and the intellect being unimpaired.” Parkinson himself did not comment on how to treat this new condition. Instead, he concluded his essay with an appeal “to those who humanely employ anatomical examination in detecting the courses and nature of diseases” to study the brain and find the cause.
Later in the nineteenth century, Jean-Martin Charcot (2) described a “pill rolling” tremor and masked face as particular features of PD. As a corrective revision, he commented on the absence of weakness and the occasional impairment of intellect. By the beginning of the twentieth century, the clinical picture of the disease was well defined but the pathologic substrate remained unknown. In 1913, Lewy (3) first described the characteristic eosinophilic intracytoplasmic inclusion bodies in various regions of the brainstem but mistakenly reported that the substantia nigra (SN) was not affected. It was only a few years later, in 1919, that Tretiakoff (4) discovered that neuronal degeneration of the SN was a consistent pathologic signature in the brains of patients with clinical parkinsonism.
The anticholinergic belladona was the first pharmacologic agent used to treat PD. Ordenstein (5) observed, in 1867, that stiffness and tremor improved in addition to the drooling that belladonna was intended to treat. Antiparkinsonian drug development was slow to evolve, but by the early 1950s, synthetic anticholinergic drugs, such as benztropine mesylate and trihexyphenidyl, had been introduced. Although benefit was modest and inconsistent, anticholinergics remained the cornerstone of therapy for nearly 100 years. How anticholinergics ameliorate symptoms in PD is not clear. It is believed that they block muscarinic receptors in the striatum and thereby restore balance to the biochemical polarity that normally characterizes the relationship between dopamine and acetylcholine (6).
Because limited pharmacologic therapy in the early twentieth century was largely ineffective (except in dampening tremor and modestly reducing rigidity), neurosurgical ablation of a variety of sites in the brain and spinal cord became a popular alternative. The clinical finding that parkinsonian tremor was abolished by a stroke in the contralateral brain—an observation first made by James Parkinson in his essay—led to the conclusion that well-placed surgical lesions in particular motor centers might be useful in suppressing symptoms (7). Early surgical trials showed that lesions in the cortex or descending pyramidal tracts could truly arrest tremor, but often at the expense of paralysis.
In 1952, Cooper (8) serendipitously discovered that accidental ligation of the anterior choroidal artery abolished parkinsonian tremor and rigidity (without causing paralysis) by producing a lesion in the globus pallidus. A variety of stereotactic surgical approaches were subsequently used to target the globus pallidus (9) and later the thalamus (10), specifically for tremor suppression but at considerable risk to the patient of major adverse effects and little chance that the underlying progressive disability could be favorably modified. The advent and widespread use of levodopa rapidly eclipsed the application of surgical treatment in the management of PD, until the gradual appearance of motor complications associated with long-term levodopa therapy brought about the revival of a more sophisticated version of targeted lesioning. The evolution of both thinking and technology eventually led to electrical stimulation of key anatomic sites in the basal ganglia, today’s standard of care in the surgical management of the symptoms of PD in selected patients.
Dopamine and the Rational Treatment of Parkinson’s Disease
The development of levodopa as effective pharmacotherapy for PD was a logical outcome of advances in understanding the pathophysiology of parkinsonism. In the early 1950s, Brodie et al. (11) discovered that reserpine depleted serotonin in the brains of rats by altering storage in synaptic vesicles, following which Carlsson et al. (12) demonstrated that reserpine had the same effect on dopamine. Brodie (11) further showed that the motor slowing, or bradykinesia, induced by administration of reserpine to rabbits could be reversed by administration of levodopa, an inert precursor of dopamine. Subsequent investigations by Carlsson et al. (13) and by Bertler and Rosengren (14) showed that dopamine was highly concentrated in the caudate and putamen of the basal ganglia (the striatum), compared with other biogenic amines such as noradrenaline, which accumulated in the brainstem.
By the end of the 1950s, these findings had led to the hypothesis that dopamine deficiency was the biochemical link to the pathophysiology of PD. Hornykiewiecz (15), whose seminal work and landmark publication in 1960 proved the hypothesis correct, subsequently wrote, “On the basis of these findings (in animals), it was to my mind a very simple and very logical step to go from animal to human brain to see whether it was possible to discover any abnormalities of dopamine metabolism in certain neurological disorders involving the basal ganglia.” Ehringer and Hornykiewicz (16) studied the brains of patients dying of PD and consistently demonstrated a 90% reduction in the concentration of dopamine in the striatum and substantia nigra. They also showed that the content of striatal homovanillic acid (HVA), a stable by-product of dopamine metabolism, directly correlated with the degree of dopamine deficiency and of cell loss in the SN (17). They further observed that parkinsonism associated with chronic manganese poisoning was also characterized by degeneration of nigral neurons and a decrease in dopamine and HVA in the striatum and SN (18).
However, authors of a recent review (Perl D, Olanow CW. The neuropathology of manganese induced parkinsonism. J Neuropath Exp Neurol 2007; 66:675–682) of the subject of manganism have disputed Ehringer and Hornykiewicz’s conclusion in this single case of presumed manganese induced parkinsonism, since the patient developed symptoms of parkinsonism ten years after exposure to manganese had ceased and had Lewy bodies in the SN, thereby strongly suggesting that the patient, despite the remote history of exposure to manganese, actually had PD as the basis for the neurologic illness and not manganism. Moreover, it is now generally accepted, as described in the recent review, that the pathology of manganism is confined to the medial globus pallidus. The SN in manganism is unaffected.
By the mid-1960s, sophisticated histofluorescence techniques had been developed and new knowledge quickly accumulated. Anden et al. (19), using this new methodology, demonstrated that dopamine was concentrated in neurons of the SN pars compacta (pc) and that axonal terminals projected cephalad to the striatum. Poirier and Sourkes (20), by showing that a unilateral nigral lesion in the monkey could cause ipsilateral depletion of striatal dopamine, mapped the previously unknown nigrostriatal pathway and thus explained the relationship between neuronal loss in the SNpc and dopamine depletion downstream in the striatum.
These pivotal discoveries provided the foundation for the logical and imaginative next step: treatment of PD by replacing the depleted neurotransmitter dopamine. The simplicity of the concept had to be a siren signal that implementation would not be easy.
One of the early findings of research into the pharmacologic properties of dopamine was that it did not penetrate the blood–brain barrier (BBB). Its immediate amino acid precursor, dihydroxyphenylalanine (dopa), could, however, cross the barrier and enter the brain, where it was enzymatically converted to dopamine. Proof of access to the central nervous system lay in levodopa’s ability to reverse the behavioral effects of reserpine in experimental animals and elevate brain dopamine levels when administered systemically.
In 1961, two independent research groups launched clinical trials of dopa in patients with advanced PD. Birkmayer and Hornykiewicz (21) administered dopa intravenously to parkinsonian patients in doses up to 150 mg. They observed “complete abolition or substantial reduction” of parkinsonian akinesia. Barbeau et al. (22) reported similar results following oral doses up to 300 mg of dopa daily (22). These early therapeutic experiments were soon repeated by many other investigators, with conflicting and frequently unimpressive results. Birkmayer and Hornykiewicz (23) later reported a positive response to dopa in only half of the patients and saw no recognizable effect if the patient’s previous anticholinergic medication had been withdrawn.
In 1964, McGeer and Zeldowitz (24) treated 10 patients with an oral dose of dopa ranging from 1 to 3 g daily. Only 2 of 10 patients improved at the highest dose, and the investigators concluded that dopa was not useful. In their experiment, dopa was combined with pyridoxine because of the erroneous belief that pyridoxine, a cofactor for dopa decarboxylase in the dopa-to-dopamine conversion reaction, would enhance the therapeutic impact. The fact that pyridoxine actually diminishes the effect of dopa by potentiating peripheral decarboxylation was not appreciated until 5 years later, when it was described by Duvoisin and coworkers (25).
Challenged by the inconsistent and even disappointing early experience with dopa therapy, Cotzias achieved what others could not by bringing dedication and an unswerving vision to the goal of proving that dopa really works. After years of deliberative trial and error, he and his colleagues reported in 1967 (26) that high doses (4 to 16 g) of oral racemic (D, L) dopa brought about “either complete, sustained disappearance or marked amelioration of parkinsonism” in 8 of 16 patients. There was a clear dose–response relationship, with only the patients on the highest dose responding. However, 25% of the patients developed granulocytopenia attributable to the drug and withdrew from the trial. In the same study the patients were exposed to melanocyte-stimulating hormone and phenylalanine, a dopa precursor, both of which aggravated the parkinsonian symptoms. The authors concluded that D, L dopa was effective in certain cases of parkinsonism but that the significant risk of granulocytopenia nullified its potential as a useful antiparkinsonian drug.
Two years later, Cotzias et al. (27) published the results of a study of levodopa (L-dopa) in 28 patients with PD, of whom 20 experienced marked and sustained improvement for up to 20 years. The daily dose of levodopa ranged from 4.5 to 8 g. None of the patients experienced the granulocytopenia observed with the use of D, L dopa. Nausea and vomiting—adverse drug effects common to other studies of dopa in PD—were overcome by the use of low-doses at the initiation of treatment followed by slow dose escalation. In the same study, the authors observed involuntary choreiform movements in 14 of 28 patients, ranging from mild to severe and correlating in severity with theduration of disease.
Within the next few years similar studies using large oral doses of levodopa were reported by Yahr et al. (28), McDowell et al. (29), Markham (30), Godwin-Austen (31), and others, confirming the dramatic findings reported by Cotzias. A major breakthrough in the treatment of PD was duly recognized—Cotzias received the Lasker award in 1970—but just as important was the idea that replacement pharmacotherapy could be successful and might be applicable to other neurodegenerative disorders involving specific biochemical defects.
The next 5 years were marked by a number of studies that supported pronounced and sustained response of all parkinsonian symptoms to treatment with levodopa. Markham (30) demonstrated that the overall response to medication achieved at 1 year was sustained at 2 1/2 years. Similar results were published by Yahr (32) and Cotzias et al. (33) as well as Godwin-Austen (31). At the same time, clinicians observed that despite levodopa’s broad efficacy, tremor tended to respond less predictably than rigidity or bradykinesia (34).
The complete or near complete reversal of the physical signs and symptoms of a chronic progressive neurodegenerative disorder had not been previously seen or reported. The spectacle of patients being able to get out of bed and wheelchair to resume long lost daily and athletic activities was truly incredible to professional and lay witnesses alike. Furthermore, the evidence that levodopa was having a long-term impact on the natural history of PD began to accumulate. When progression of parkinsonian disability was compared in 182 patients receiving levodopa treatment with a cohort of patients from the prelevodopa era, Hoehn (35,36) found that patients on levodopa remained stable in each Hoehn and Yahr stage of the disease 3 to 5 years longer than was the case in the prelevodopa era. Also, the number of patients classified as disabled or dead at each stage was reduced by 30% to 50%. Yet there was no detectable difference in the severity of the parkinsonian symptoms between treated and untreated patients evaluated in the setting of withdrawal from levodopa, suggesting that the benefit derived from using levodopa was purely symptomatic, quickly reversible, and not in any way disease-modifying.
Levodopa’s success was shadowed from the beginning by the emergence of a unique set of drug-related complications not seen by previous observers of the phenomenology of PD. Cotzias (26) was among the first to report that abnormal involuntary movements (called dyskinesia) became increasingly common and problematic with chronic use of levodopa. As early as 1968, the issue of levodopa-induced dyskinesia was the subject of a symposium (37). Later, Godwin-Austen (38), summarizing 4 years of experience using levodopa, noted the intractable nature of the involuntary movements in many cases and the continued progression of the parkinsonian disability in most, despite early improvement in motor function in the majority. The high frequency of organic mental symptoms, including confusion and visual hallucinations, was also disturbing. These and other drug-related obstacles to effective treatment have not only frustrated treating physicians but also fueled the collective efforts of researchers to refine levodopa’s cruder aspects and search for viable alternative therapies.
The biochemistry and metabolism of levodopa
Levodopa’s complicated metabolism, a short (90 minutes) half-life, a diminishing capacity for a dying pool of nigrostriatal neurons to store and convert levodopa to dopamine, and other molecular and pharmacodynamic reasons are responsible for the motor complications of long-term therapy. Levodopa is a large neutral amino acid (LNAA), which is absorbed in the proximal small intestine via an energy-dependent, carrier-mediated mechanism (38). The carrier is shared with other LNAAs and has saturation kinetics (39), which explains why dietary protein blocks the transport of individual oral doses of levodopa to the brain in some but not all patients.
Cotzias reported this negative interaction in the early trials of levodopa in advanced PD. Peripherally, levodopa is rapidly metabolized to dopamine by the enzyme aromatic amino acid decarboxylase (AADC) and to 3-O methyl dopa by the enzyme catechol-O-methyl transferase (COMT). Its elimination half-life is approximately 90 minutes (40). The small amount of levodopa that eventually reaches the brain after a single oral dose— estimated at 1%—depends on the speed of gastric emptying, presence of competition for transport of the alternative amino acids, and, most of all, the degree of peripheral metabolism (41, 42). Coadministration of a peripheral AADC inhibitor (carbidopa or benserazide) doubles the bioavailability of levodopa without changing its elimination half-life (43) and allows more unchanged BBB permissible levodopa to reach the brain (44). Increased bioavailability means at least an 80% reduction in the amount of levodopa required to achieve the same clinical effect as when levodopa is taken without an AADC inhibitor, as well as a decrease in peripheral dose-related side effects such as nausea, vomiting, and hypotension.
After crossing the BBB, levodopa is taken up by the surviving striatal neurons and converted by intraneuronal AADC to dopamine (DA), which is, in turn, released presynaptically (45). According to this simplified model of levodopa’s central metabolism, the response to a single dose of levodopa should decline in proportion to the progressive loss of nigral cells and the loss of their capacity to convert exogenous levodopa to DA. However, experience has shown that some PD patients, after many years of illness and near-total depletion of nigral neurons, still respond well to levodopa. There is some evidence to indicate that decarboxylation can occur in nondopaminergic striatal interneurons and in glia (46). Under physiologic conditions, dopamine is released at the synapse mainly through tonic activity in dopamine neurons (47). Released dopamine interacts in a complex fashion with dopamine receptors.
Types D1 and D2 are the best characterized and are mainly represented in the motor striatum (48). Nigrostriatal denervation in PD is associated with upregulation of postsynaptic D2 receptors. It has been postulated that the combination of receptor upregulation, nonphysiologic pulsatile stimulation of receptors by exogenous DA, and abnormal signal transduction resulting from altered gene expression is responsible for the development of motor complications, specifically, “on-off” or wearing-off fluctuations and dyskinesia (49, 50). Dopamine is metabolized via the reuptake system or by the enzymes monoamine oxidase (MAO-B) and COMT (41). Oxidation via MAO-B converts dopamine to the stable, inactive byproduct homovanillic acid (HVA) (51). COMT methylates dopamine to produce 3-methoxytyramine (3-OMD), which is oxidized to HVA (44).
Complications of levodopa therapy
The immediate adverse effects of levodopa, particularly nausea and vomiting, were shown to be caused by the peripheral decarboxylation of levodopa to DA and were managed in the majority of cases by combining levodopa with carbidopa, a peripheral dopa decarboxylase inhibitor (DDI). In resistant cases, alternative peripheral dopamine antagonists (e.g., domperidone), which block dopamine receptors in the brainstem vomiting center, have been useful. However, the more insidious and ultimately disabling adverse drug effects generally emerge later, after several years of chronic use. Specifically, the evolution of motor fluctuations, drug-induced dyskinesia, and psychosis in the setting of advanced disease represents the most important challenge to the physician treating PD in the post levodopa era.
Cotzias and coworkers (27) described dyskinesia in 14 out of 28 patients treated with levodopa for up to 2 years. Since 1969, multiple volumes have been dedicated to the discussion of etiology and management of levodopa-induced complications, largely because the great majority of patients with PD must eventually learn to cope with the inevitable consequences of using an increasingly indispensable drug (52–56). For example, over 50% of the 352 patients enrolled in the DATATOP study of early PD developed fluctuations or wearing off. One-third of the sample reported dyskinesia within 20.5 (#8.8) months of starting levodopa therapy (54). The only study that provided a more optimistic profile of drug-induced complications was the CR FIRST clinical trial, which randomly compared the effects of initiating treatment with standard-formulation carbidopa/levodopa versus the controlled-release formulation (57). In that study, only 22% of the patients developed fluctuations or dyskinesia during a 5-year prospective follow-up.
The lower incidence of complications in CR FIRST can partly be explained by methodologic differences (fluctuations were recorded only if more than 20% of the day was spent in the “off” state and 10% of the day was spent with dyskinesia). Moreover, patients enrolled in CR FIRST, compared with other studies, required surprisingly low doses of standard and CR carbidopa/levodopa for a satisfactory response (158).
The most recent data on the incidence of dyskinesia in levodopa-treated patients with early PD come from the ELLDOPA study (Earlier versus Later Levodopa Therapy in Parkinson’s Disease) (58). This study was designed to address the perennial debate of the impact of early initiation of levodopa therapy on the rate of progression of PD, which revolves around the possibility that levodopa might accelerate the progression of neurodegeneration because of its known ability to enhance oxidant stress in some in vitro experimental preparations (see below).
In the ELLDOPA study, patients with recently diagnosed very early clinical parkinsonism and no prior exposure to long-term dopaminergic therapy were randomly assigned to four groups: placebo or carbidopa/levodopa at the daily levodopa dose of 150, 300, or 600 mg. Study duration was 40 weeks, followed by a 2-week washout period. The primary outcome measure was the rate of progression of PD based on the total score of the Unified Parkinson’s Disease Rating Scale (UPDRS) performed by a blinded rater at the end of the washout period. Levodopa therapy reduced the worsening of PD symptoms in a dose–response pattern, with the greatest benefit seen in the group treated with the highest dose of levodopa, 600 mg daily. However, after less than a year’s exposure, 16.5% of subjects treated with the 600-mg dose developed dyskinesia, and 30% developed wearing off by the end of the 40-week study period. The incidence of dyskinesia and wearing off in the groups receiving 150 and 300 mg was much lower and comparable to that in the placebo group.
The ELLDOPA study confirmed the high risk of levodopa-induced motor complications even early in the course of treatment, which correlated with the higher dose exposure. However, the study was not designed or powered either to quantify the degree of motor complications or to investigate the impact of motor complications on the patient’s quality of life and the overall efficacy of therapy. The more interesting (and unexpected) finding of ELLDOPA was the inverse correlation between the dose of levodopa used during the study and the UPDRS score after the 2-week washout, thereby suggesting a potential neuroprotective (rather than adverse) effect of chronic levodopa therapy.
The pathogenesis of levodopa-associated motor fluctuations and dyskinesia is not fully understood, although two major pathophysiologic mechanisms are thought to be involved (50, 59, 60). First, progressive nigrostriatal degeneration reduces the capacity of the brain to store dopamine, and dopamine receptors increase in number as a result of denervation hypersensitivity. Second, intermittent or pulsatile stimulation (versus the more physiologic continuous stimulation) of dopamine receptors by exogenous levodopa appears to alter or heighten receptor sensitivity with untoward behavioral consequences, resulting in the emergence of motor complications. Third, the pathologic loss of dopamine causes significant disruption of intracellular signaling pathways linked to the healthy transcription of regulatory genes in the basal ganglia.
As a result, altered gene expression can induce changes in the homeostatic biochemistry of neurotransmitters and neuropeptides, leading to unmodulated motor responses to drugs, especially levodopa, used to replace depleted dopamine. The precise nature of this common complication of chronic levodopa therapy is not known, especially because postmortem and positron emission tomographic (PET) studies in PD indicate that chronic levodopa therapy causes downregulation or a reduction in the number of dopamine receptors, the physiologic effect of which should be a lowered behavioral sensitivity (56). Experience in patients with PD and animals with MPTP-induced parkinsonism has shown that monotherapy with dopamine agonists in those previously untreated with levodopa is much less likely to induce dyskinesia, whereas dopamine agonists used to supplement levodopa where motor fluctuations are already present can easily induce dyskinesia (61, 62).
The duration of PD and the duration and amount of levodopa may influence the occurrence and timing of motor fluctuations (63). Early in the course of PD, patients experience a smooth response to levodopa treatment, with dosing only 2 to 3 times a day. Considering the short half-life of levodopa (90 minutes), this long-duration response (LDR) might be explained by presynaptic dopamine storage in unaffected axons and tonic release of the transmitter (41). As nigral degeneration progresses and nigrostriatal axons die back, storage capacity for dopamine becomes increasingly reduced and the LDR is transformed into a short-duration response, which conforms more or less with the pharmacokinetic short half-life of levodopa (64–66).
This “storage hypothesis’’ is supported by a reduction in the striatal uptake of 18F fluorodopa (an indicator of dopamine storage), on PET in fluctuating but not in stable PD patients (67). However, some patients develop fluctuations and dyskinesia soon after starting levodopa, irrespective of severity or duration of disease or of the dose of levodopa, as in the high-dose subgroup of ELLDOPA. Therefore loss of storage capacity may be necessary but is not sufficient to explain the complexities of motor fluctuations (56). The remarkably wide range of clinical variability among people who develop PD is a vivid reminder of how incompletely we understand the underlying basis for many aspects of PD, especially the complications of levodopa therapy.
The evidence demonstrating a postsynaptic pharmacodynamic mechanism to explain motor fluctuations and dyskinesia has also been accumulating. Several studies have shown that the motor response to intravenous apomorphine, a direct dopamine agonist that does not depend on presynaptic storage and whose action is identical to that of dopamine, diminishes in proportion to the duration of illness and levodopa therapy (68, 69, 70). Similarly, in patients with asymmetric motor symptomatology, the duration of response to apomorphine is shorter on the more affected side (71). These experiments suggest that apomorphine’s loss of efficacy is caused by the downregulation of postsynaptic receptors.
A number of studies have shown that chronic infusions of direct dopamine agonists, such as lisuride or apomorphine, ameliorate or significantly decrease on-off fluctuations and dyskinesias (72, 73). Similar results have been obtained with a continuous (nonpulsatile) infusion of levodopa (74, 75). These data suggest that motor fluctuations associated with chronic oral levodopa treatment are partly the result of nonphysiologic discontinuous delivery, in contrast to the tightly balanced mix of tonic and phasic release of dopamine in normal subjects (76).
The hypothesis of the relationship between pulsatile stimulation of postsynaptic dopamine receptors and induction of dyskinesia has been tested in two large randomized, double blind (RDB), placebo controlled clinical trials of dopamine agonists (DAs) versus levodopa for monotherapy of early PD. The primary endpoint of the studies was time of onset of motor complications. The design of the studies was based on the rationale that, compared to levodopa, DAs have a longer half-life and do not require presynaptic storage. Thus chronic use of DA should be associated with a lower risk of motor complications. Indeed, presently there is a solid body of clinical trials data with essentially all available DA demonstrating that treatment with DA produces fewer motor fluctuations and dyskinesia than levodopa (77–79). The two pivotal studies were conducted with two most commonly used DAs: pramipexole and ropinirole.
The pramipexole-versus-levodopa study for patients with early PD (CALM PD) demonstrated a 24% incidence of dyskinesia in the pramipexole-treated group versus 54% in the levodopa-treated group over 4 years. (81). The ropinirole-versus-levodopa study had similar results in regard to dyskinesia: 20% in the ropinirole group versus 45% in the levodopa group over 5 years (82). The design of both studies allowed open-label levodopa supplementation in case the efficacy of the initial treatment agent was not sufficient. Patients who received DA and levodopa combination therapy still experienced fewer motor complications, which could have been a result of a lower dose of levodopa used in that subgroup compared to the group treated with levodopa monotherapy. The results of these studies could be interpreted as supporting the possibility that a longer half-life of dopaminergic agents leads to less dyskinesia.
It has been postulated that pulsatile stimulation of dopaminergic receptors can elicit long-term potentiation of excitotoxic glutamate-mediated responses because of disinhibition of the subthalamic nucleus, the major glutamatergic nucleus in the basal ganglia. The role of glutamate in causing motor fluctuations and dyskinesia is further supported by the observation that blockers of the NMDA (glutamate) receptor can ameliorate levodopainduced dyskinesia in PD patients (61). For example, amantadine, a putative NMDA receptor blocker (80), has been demonstrated to reduce drug-induced dyskinesia when used at high dosages (83).
A continuing controversy : Is levodopa toxic?
The Oxidant Stress Hypothesis Despite its shortcomings, levodopa has stood the test of time and remains the most effective drug for treating the symptoms of PD. However, the pervasive, often disabling motor and mental complications associated with chronic levodopa usage have sustained an unresolved debate over the possibility that the drug itself is toxic and can accelerate neurodegeneration (84,85). It has been postulated that levodopa’s toxicity is based on the formation of oxygen free radicals and other reactive oxygen species (86), which can be destructive to the lipid substructure of cell membranes, among other areas, and may lead to cell death.
Oxidative reactions are ubiquitous in the human body and are an intrinsic part of the oxidative phosphorylation chain reactions resulting in the production of ATP. Hydrogen peroxide and free radical by-products can react with and damage not only cell membranes but also DNA and proteins (87). Normally, their production is balanced by endogenous antioxidants, which effectively quench the toxic potential of these products. The most important of these intrinsic antioxidants are vitamins A, E, and C and the enzymes superoxide dismutase (SOD), catalase, and glutathione peroxidase (88). If the capacity of natural antioxidants is exceeded, cell death may result. Cells with high metabolic demand, such as the pigmented dopaminergic neurons in the SN, are particularly vulnerable to oxidative stress. Several characteristics peculiar to these neurons create a high risk for oxidative damage, including the presence of neuromelanin (89), iron (90), and MAO, all of which promote autooxidation. MAO catalyzes the oxidative deamination of dopamine in the SN and forms hydrogen peroxide (H2O2) in the process (91, 92). H2O2 itself is an oxidizing agent, but it also can react with ferrous iron (Fe2#), to form the highly toxic hydroxyl radical (OH–), a prime mediator of oxidative damage (Figure 38-2C). Iron is important in catalyzing oxidation reactions because of its ability to exist in 2 valence forms; thus it can donate a free electron, which promotes the formation of free radicals, including OH– (88). H2O2 and oxygen radicals can also be generated nonenzymatically by the autooxidation of dopamine to form quinones (Q) and semiquinones (SQ) (93).
Neuromelanin, which is generated from dopamine autooxidation (93, 94), is associated with the generation of oxyradicals and hydrogen peroxide (95). Levodopa is a potential source of toxic free radicals as a result of its decarboxylation to dopamine and oxidation to neuromelanin. Under normal circumstances, the buffering capacity of the brain’s antioxidants is sufficient to detoxify H2O2 and other free radicals. For example, glutathione is one of the most powerful naturally occurring antioxidants in the nervous system. Most tissue glutathione exists in the reduced form (GSH). Oxidation of glutathione is catalyzed by glutathione peroxidase, and this is the pathway by which H2O2 is cleared from the brain.
The average ratio of reduced glutathione to oxidized gluthathione (GSH:GSSG) in most normal tissues is more than 50:1. Although GSH is a potent antioxidant, GSSG can be potentially toxic. Levels of GSH have been shown to be selectively lowered in the SN of patients with early PD, compared with normal controls and patients with other neurodegenerative disorders (96–98). In addition, it is normal in other brain regions in PD. Even clinically normal subjects with incidental Lewy bodies in the SN (considered to be a marker of preclinical PD) on postmortem examination had decreased levels of GSH, similar to those in patients with advanced PD (99), but they showed no increase in GSSG, as might be predicted if it were a mere consequence of increased hydrogen peroxide load. The latter observation suggests that GSH depletion is not a pure consequence of oxidant stress. Rather, it could be a primary biochemical defect leading to the programmed death of dopaminergic neurons. The use of exogenous levodopa may add further stress to an already overtaxed and inadequate supply of GSH. According to the oxidant stress hypothesis, this combination of a high rate of oxyradical formation (from dopamine) and insufficient levels of antioxidation (due to depleted GSH) is the metabolic mechanism responsible for accelerated cell death in PD.
A number of other possible mechanisms for levodopa-induced cell death have been reported and discussed; they include mitochondrial respiratory chain dysfunction (100, 101), apoptosis (102–107), and excitotoxicity (108).
Is Levodopa Neurotoxic in Vitro?
Levodopa is toxic to dopaminergic neurons in tissue culture (109–114). In these experiments, cells were exposed to levodopa concentrations ranging from 100 to 250 μM for 1 to 5 days. Postulated mechanisms by which levodopa enhanced cell death in these studies include the production of reactive oxygen radicals, which, in turn, cause cell death by either apoptosis or necrosis. Walkinshaw and Waters (104) showed that levodopa, not dopamine, was toxic and that toxicity was inhibited by antioxidants. In general, the concentration of levodopa used in tissue culture experiments has exceeded, by a large margin, the doses of levodopa used by patients with PD (5 to 50 μM) (115).
In a study of dopaminergic stimulation in a preparation of human lymphocytes, Blandini et al. (114) found mixed but mostly adverse effects of dopamine on antiapoptotic protein, Bcl-2, proapoptotic enzyme caspase-3, and antioxidant/antiapoptotic enzyme Cu/Zn superoxide dismutase.
Most in vitro experiments demonstrating levodopa toxicity have been conducted in neuronal cultures with few if any astrocytes—a major deficiency, since astrocytes have been shown to prevent autooxidation (116) and in vivo may help protect against the potential oxidative toxicity of dopamine. H2O2 resulting from the enzymatic metabolism of dopamine can be efficiently detoxified by abundant supplies of catalase and glutathione peroxidase located in glial cells (108). One astrocyte has the capacity to protect 20 neurons against the toxicity induced by the application of 100 μM of H2O2 (117). Dopaminergic neurons survive longer in glial-conditioned media, and this environment also protects them from the toxic effects of levodopa concentrations as high as 200 μM (118). Recent studies have demonstrated that the toxicity of levodopa in vitro is directly proportional to its concentration (119). In fact the addition of low concentrations of levodopa (50 μM) to cultures of fetal midbrain neurons increased survival and promoted neurite extension of dopaminergic neurons (120). Exposure of cultures containing mesencephalic neurons and glia to levodopa actually increased the cell concentration of GSH (119) and enhanced neuronal protection.
There is another rationale for a possible neuroprotective, rather then neurotoxic, effect of levodopa therapy on dopaminergic neurons in PD. The hallmark of pathology in PD is presence of intracytoplasmic Lewy bodies (LB), which form as a result of the misfolding and abnormal fibrillization (aggregation) of the protein alpha synuclein. Dopamine combines with #-synuclein through oxidation to form dopamine-#-syn adducts, which can block the development of toxic amyloid fibrillization but instead form protofibrils that also can be neurotoxic (121). Therefore any intervention designed to protect vulnerable dopaminergic neurons and prevent neurodegeneration must block the formation of the entire fibrillization process.
A more recent in vitro experiment documented that an intermediate by-product of dopamine oxidation, dopaminochrome, inhibits #-synuclein fibrillization by combining with the 125 to 129 amino acid residue of the #-synuclein molecule (122). Fully oxidized/polymerized dopamine is an ineffective inhibitor of #-synuclein fibrillization. The result is a conformational change in #-synuclein that leads to the formation of nontoxic oligomeric, soluble spheres which are unable to mature and are potentially toxic amyloid fibrils inside nigral neurons. In contrast to others, these investigators were unable to detect dopamine-#-synuclein adducts in their experimental model. Furthermore, in one transgenic animal model of PD, the #-synuclein A53T mutant mouse, insoluble synuclein aggregates were present in many parts of the brain but not the SN, where dopamine is primarily manufactured (122). Therefore, based on data from Norris and Giasson et al. (123), the presence of dopamine protects nigrostriatal neurons, and it is only when the cascade of factors responsible for the neuronal degeneration peculiar to PD begins to deplete dopaminergic cells that oxidant stress can gain an accelerating foothold to propel the degenerative process.
In summary, high concentrations of levodopa have been shown to be cytotoxic to pure dopaminergic neuron cultures and human lymphocytes; however, levodopa can also protect neurons in culture, especially when mixed with astrocytes in tissue cultures that approximate in vivo conditions more closely (124).
Is Levodopa Toxic in Normal Animals?
Exposure of normal animals to high concentrations of levodopa failed to demonstrate a neurotoxic effect (126–129). No reduction in the number of dopaminergic cells was observed in the substantia nigra of rats and mice fed with high doses of levodopa for 18 months (128). Administration of levodopa to normal rats did not increase the levels of striatal oxidized gluthathione (GSSG) despite a marked increase in dopamine turnover (130). GSSG, a by-product of the clearance of H2O2, could be considered a marker of increased oxidative load. The absence of an elevation in its concentration argues against the presence of levodopa-induced oxidative stress in normal animals.
Primates given high doses of levodopa for 3 months showed no evidence of nigral degeneration or decrease in the density of striatal dopamine terminals (131). However, intrastriatal administration of high doses of levodopa to rats did produce degeneration of presynaptic dopaminergic terminals (132). Mytilineou et al, (133) explored the role of oxidative stress as an enhancer to the putative levodopa toxicity in dopaminergic cell culture and in neonatal rats: while glutathione inhibition enhanced levodopa-induced cell loss in tissue culture, there was no evidence of dopaminergic cell loss either with levodopa therapy or with glutathione inhibition in healthy neonatal rats. High concentrations of levodopa are potentially toxic to normal dopaminergic neurons in some species and not in others.
No study has shown that systemic administration of levodopa in human-equivalent doses causes degeneration of dopaminergic cells in normal animals, although Pearce et al. (125) showed that dyskinesia can occur in normal monkeys given large doses of levodopa, thereby suggesting that exposure to supramaximal amounts of levodopa for any length of time can overwhelm the normal buffering capacity of striatal neurons for rapid reuptake and recycling of dopamine released at synaptic terminals.
Is Levodopa Toxic in Animals with a Lesioned Nigrostriatal System?
A number of studies have evaluated the effect of exposure to levodopa on the 6-hydroxy dopamine (6-OHDA) animal model of PD. Blunt et al. (134) lesioned the nigrostriatal pathway in rats and investigated the effect of chronic levodopa exposure on dopaminergic cell survival. Animals were assigned to a control group (lesion but no levodopa) and a treated group (lesion plus levodopa/ carbidopa feedings for 27 weeks). The experiment demonstrated that the animals fed with levodopa/carbidopa had greater loss of dopaminergic cells on the lesioned side than control animals, especially in the ventral segmental area. The number of dopaminergic cells on the nonlesioned contralateral side was not affected in either group. The authors concluded that a damaged dopaminergic system is susceptible to further damage from levodopa-induced oxidative stress, which translates into an increased risk of treatment-related accelerated neurodegeneration.
The study by Murer and colleagues (136) used a similar design but expanded it considerably. Animals exposed to levodopa or placebo for 26 weeks had either a sham or actual unilateral 6-hydroxydopamine lesion that caused moderate or severe damage. When the various groups were compared, there was no significant difference in the number of surviving dopaminergic neurons between rats treated with levodopa versus rats treated with placebo. In contrast, surviving cells in the SN of the moderately lesioned rats treated with levodopa had a higher concentration of dopaminergic neurons compared with placebo-treated animals. The authors concluded that chronic levodopa exposure is not toxic to dopaminergic neurons of either healthy or 6-OHDA–lesioned rats.
On the contrary, the authors postulated that levodopa can actually promote dopamine function and recovery in the rats with moderate degrees of 6-OHDA–induced damage. The difference between their results and those obtained by Blunt et al. was attributed to the variable degree of the 6-OHDA–induced lesion, which was more extensive in the Blunt study. The number of surviving dopaminergic cells before levodopa exposure was the key determinant of a neurotrophic levodopa effect. A number of more recent studies support that conclusion: Ferrario et al. (137) demonstrated no evidence of enhanced striatal cells degeneration with exposure to levodopa compared to a vehicle in 6-OHDA–lesioned rats. Reveron et al. (138) came to the same conclusion in the dopamine-depleted mouse model.
Is Levodopa Toxic in Humans?
There are no convincing data from human studies to assess a beneficial or detrimental effect of levodopa on the rate of PD progression or on nigral cell death in PD. The recently completed ELLDOPA study (cited earlier) was a pivotal attempt to address the issue of levodopa toxicity in PD patients and the impact of the drug on the rate of PD progression (58). The primary endpoint of the study was the rate of progression of PD over a 9½-month (40 weeks) period of exposure to a low (150 mg daily), medium (300 mg daily), and high (600 mg) daily dose of levodopa versus placebo in patients with early PD. The clinical marker of PD progression was the rate of change of the total UPDRS score over the duration of the study measured after 2 weeks of levodopa washout.
Based on the short half-life of levodopa and previous clinical observations, the assumption was made that 2 weeks off medication was a sufficiently long time to wash out the symptomatic effect of the drug. The study enrolled 317 patients, and 86% completed the final clinical evaluation. The clinical outcome measures demonstrated that levodopa reduced the accumulation of PD disability in a dose–response fashion as measured by the UPDRS scale over the 40 weeks of observation. The effect was most robust in the high-dose levodopa (600 mg) group. After 2 weeks of washout, the scores in the levodopa-treated arms worsened but did not return to pretreatment baseline values. As discussed before, the high-dose levodopa arm also experienced the highest rate of adverse events, specifically motor complications, dyskinesias, and wearing off. The conclusion of the clinical arm of the study was that “levodopa either slows the progression of PD or has a prolonged effect on the symptoms of PD.”
The ELLDOPA study did not provide conclusive results on the impact of levodopa on the rate of progression of PD, since the benefit attributed to the highest-dose subgroup could have reflected greater storage capacity of the brain in early PD for exogenous levodopa, beyond the ability of a 2-week washout to measure. However, neither did the study demonstrated any clinical evidence of a detrimental effect of levodopa. There are a number of limitations of this study that make its interpretation difficult: (a) The short duration of the study provides limited insight into the long-term effect of the drug on the rate of progression of PD, and (b) The use of a washout in this study may be responsible for an unwarranted conclusion that levodopa is protective, since the persistence of a possible symptomatic effect of levodopa that exceeded the 2-week washout period could account for the improved status of the high-dose subgroup as a result of storage.
There is no good information on how long exogenous levodopa can be “stored” in the brain, but it is not unreasonable to consider that it is still robust in patients with newly diagnosed PD. The unreliability of drug washouts must be considered in the design of future studies. The ELLDOPA-2 study is now planned to address these issues. In the interim, clinicians should be reassured that there is still no evidence that levodopa is toxic in humans with PD.
The Role of Dopamine Imaging in Evaluating the Impact of Levodopa on the Rate of PD Progression.
One of the major constraints in defining the impact of levodopa on the natural history of PD has been lack of a reliable biomarker that can provide an accurate quantifiable estimate of progression of the disease and on which the symptomatic effect of treatment has no impact. Recent developments in the technology of in vivo neurotransmitter imaging and the assumption that these imaging modalities provide an accurate biomarker of the degree of preservation of the dopaminergic system have made imaging an attractive modality. Several ligands that utilize single photon emission computed tomography (SPECT) or positron emission tomography (PET) focused on the dopamine system have been developed (139).
The 2 most commonly used ligands in PD measure the integrity of presynaptic dopamine function: 18F fluorodopa PET labels dopa decarboxylase and -CIT SPECT labels the dopamine transporter (140). Both ligands have been demonstrated to reliably separate subjects with normal dopaminergic function from those with a parkinsonian disorder(140). The degree of decline of ligand uptake correlates with the severity of PD symptoms and the degree of dopaminergic cell loss as demonstrated in postmortem tissue (141, 142). These properties of the ligands make them an attractive research tool in measuring the rate of progression of PD and studying the impact of pharmacological agents as disease-modifying interventions.
The ELLDOPA study was one of a series of clinical trials performed over the last decade that attempted to use dopamine imaging as a surrogate marker to complement the clinical outcome measures (58). In addition to clinical evaluations, a subset of 142 study subjects underwent -CIT SPECT imaging. The scans were performed before initiation of treatment and at week 40, before levodopa taper. The imaging substudy demonstrated that the patients treated with levodopa had a higher percent decrease in -CIT uptake than the placebo-treated cohort. That analysis was performed after the exclusion of 19 subjects (14.5%) with normal -CIT uptake. The conclusion of the imaging part of the ELLDOPA study was that “the neuroimaging data suggest that levodopa either accelerates the degree of dopaminergic cell loss or that its pharmacological effects modify the dopamine transporter.” The major unanswered question is the pharmacological effect of the treatment agent (levodopa) on the level of transporter binding and how it affects the results of the scans.
Clinical trials of dopamine agonists versus levodopa in early PD also utilized dopamine imaging scans (143–145). REAL PET evaluated the impact of ropinirole versus levodopa on the results of 18F fluorodopa PET scan obtained 4 weeks after the initiation of the treatment and again after 24 months of therapy (145). The study demonstrated a 34% relative reduction in the decline of tracer uptake in the ropinirole group compared to the levodopa group at 24 months. Clinical outcome measures demonstrated the superiority of levodopa treatment in controlling PD symptoms as measured by the motor UPDRS scale on medication. The pramipexole versus levodopa (CALM PD) study used -CIT SPECT obtained at baseline and again at 24 and 46 months of treatment.
At 24 months, there was no difference in imaging outcome between the treatment groups; but at 46 months there was a significant one-third reduction in the rate of tracer uptake decline in the pramipexole group (143, 144). The clinical outcome measure, motor UPDRS score off medication, did not differ between the treatment-assignment groups in the imaging substudy; but, as discussed previously, levodopa was superior to pramipexole in the whole CALM PD cohort. Both studies demonstrated a beneficial effect of dopamine agonists or detrimental effect of levodopa therapy on the imaging outcome measures, and these effects were consistent across the studies despite the use of different imaging ligands and different dopamine agonists. However, the interpretation of these studies is challenging due to potential direct pharmacological effect of the treatment agent on the level of ligand binding, which can influence the results of the scans (139, 146).
There is a concern of potential dopamine transporter upregulation by dopamine agonists but not by levodopa (146, 147). Levodopa and dopamine agonists can also have differential effects on metabolism of 18F fluorodopa (146). Last, there is a discrepancy between the clinical outcomes that favor the effect of levodopa and the imaging outcomes that favor the effect of DA on the rate of disease progression. Provided that the intervention has a true neuroprotective effect, it should be expected to cause both outcome measures to point in the same direction.
In conclusion, the use of dopamine imaging as the surrogate marker of the rate of PD progression is presently premature. Additional data are necessary to clarify the impact of the treatment agents on the results of the imaging studies in the short and long term (139). Such studies are under way at present. In the interim, dopamine scans will continue to be used as exploratory tools in clinical trials, but the efficacy of the treatment interventions will still be based on clinical outcome measures. Imaging studies have not helped to clarify the issue of the protective versus detrimental effect of levodopa on the rate of progression of PD.
Another approach to assessment of the potential neuroprotective versus neurotoxic effect of long-term levodopa therapy is to evaluate the treatment effect on normal individuals and conduct large-scale epidemiological studies. Individuals treated with chronic levodopa as a result of a mistaken diagnosis of PD do not develop parkinsonism or changes in striatal metabolism on PET scans (148, 149), and nigral degeneration is not present at autopsy in these cases (148, 150). However, it can be argued that the normal brain is more resistant to oxidative stress than the parkinsonian brain. Autopsy data from patients with PD have not demonstrated any difference in the number of surviving nigral cells between levodopa-treated and levodopa-untreated patients (151), notwithstanding the difficulty of making quantitative comparisons in a SN severely depleted of neurons by end-stage PD. Moreover, active axonal outgrowth has been demonstrated in a fetal mesencephalic transplant performed on a levodopa-treated patient with PD who subsequently died of unrelated causes (152). Such active fetal tissue proliferation despite continuous levodopa treatment argues against drug toxicity in vivo.
Enhanced survival of PD patients in the postlevodopa era is used as another argument against levodopa-induced neurotoxicity. The landmark study of the natural history of PD in the prelevodopa era by Hoehn and Yahr (in about 1967) (36) reported a mortality rate 2.9 times greater than that in the age-matched population. Studies performed soon after the introduction of levodopa revealed a favorable but variable effect on mortality rates. Yahr (153) reexamined 597 of the patients who were treated with levodopa between 1967 and 1973 and showed that the mortality had decreased from 2.9 to 1.46 times the expected rate. Hoehn (154) reported a similar mortality ratio of 1.5 in 182 patients who were followed since the advent of levodopa therapy and concluded that life expectancy in the treated patients was close to that of the general population.
Diamond and colleagues (155) demonstrated a positive correlation between early initiation of levodopa and reduced mortality. Other studies showed less optimistic results, and the mortality ratios ranged from 1.85 to 2.5 (156–160). One of the explanations for the discrepancy in mortality ratios between the early and later studies is that reduced mortality is a somewhat transitory benefit experienced during the first years of levodopa therapy but is partly reversed by the progressive nature of PD even in the face of optimal and sustained levodopa therapy (161). After a mean of 6 years of follow-up, Lees and Stern observed that the mortality ratio was 1.46 in a cohort of 178 patients treated with levodopa between 1969 and 1977, but it increased to 2.59 after a mean of 12 years of follow-up. Louis and colleagues (162) reported that the combination of PD and dementia was associated with the highest mortality, although the actual cause of death could not be separated into motor and mental components.
In conclusion, the question of levodopa’s potential neurotoxicity remains unanswered (84, 163). In fact, levodopa may even be neuroprotective (85). However, since in vitro and in vivo models of human PD are imperfect, the practical impact of this debate devolves to a critical interpretation of the total body of clinical data.
Refinement of levodopa therapy
Controlled-Release Levodopa/Carbidopa: Sinemet CR
The declining capacity of nigrostriatal neurons to store and physiologically release dopamine in progressive PD leads to a growing dependence on a rapidly metabolized, exogenous source of levodopa for effective motor control. This combination of forces may be responsible for the emergence of motor fluctuations and dyskinesia, whereby rapid cycling of exogenous dopamine at the synapse exposes the dopamine receptors to a less physiologic and potentially harmful pulsatile stimulation by the released neurotransmitter.
One of the proposed ways to reverse the negative impact of pulsatile activation of the dopamine receptor and the associated motor fluctuations is to provide a steady levodopa plasma concentration and, consequently, a more continuous stimulation of these receptors (187). Multiple attempts to develop a controlled release (CR) preparation of levodopa/ carbidopa were made during the 1970s, but the clinical effects were inconsistent. After numerous clinical trials, controlled-release (CR) carbidopa/levodopa (Sinemet) reached optimal development with the CR4 formulation. This was a slowly erodible matrix that released its contents in the most favorable temporal relationship with gastric emptying and offered the best kinetic advantage over immediate release (IR) Sinemet to offset CR’s lower bioavailability (70% of IR) (188).
The superiority of CR over IR was validated in phase III trials, and CR was approved by the FDA for marketing in the United States in the summer of 1991. Sinemet CR (in the United States) and Madopar HBS (in the United Kingdom and Canada), the CR preparations used today, produce a constant elevation of plasma levodopa levels for 3 to 4 hours longer than the IR preparation. Peak plasma levodopa levels are decreased and the half-life is prolonged (189). Several open-label and double-blind studies of CR have demonstrated a significant reduction in “off” time, improvement in clinical disability, and decreased frequency of dosing when compared with IR (190). However, two 5-year, randomized, double-blind trials comparing IR and CR in early untreated patients (134 in one study and 618 in the other) showed no major differences in the development of motor fluctuations or in performance on the UPDRS (57,191,192). The only statistically significant finding favoring CR (the Koller or CR FIRST study) was improvement in performance of ADLs and emotional reaction–social isolation scores on the Nottingham health profile (NHP), a measure of quality of life. CR was well tolerated. In one open-label study, 24 patients with advanced PD were converted from IR to CR and followed for 6 months (193).
There was no significant difference in frequency of dosing, degree of fluctuations, and dyskinesias between the two groups, but a majority of patients preferred CR over IR and the NHP scores showed improvement in degree of social isolation, emotional reaction, and quality of sleep comparable to the favorable effect demonstrated by CR FIRST. In patients with early stages of PD and mild disability, the evidence from the CR FIRST clinical trial does not confer enough of an advantage to offset the higher expense of CR. Moreover, CR can cause increased and at times uncontrollable dyskinesias late in the day, as the concentration of levodopa in the blood increases.
Refinement of levodopa therapy: Enzyme inhibition
Efforts to improve and refine levodopa’s antiparkinsonian potential began in parallel with the earliest clinical trials. Enthusiasm for levodopa’s phenomenal clinical success as an oral drug for PD was tempered by 2 major pharmacologic shortcomings: First, the short (90 minute) half-life caused the short duration response (SDR) to predominate over the long duration response (LDR) in many patients. Second, the high doses of levodopa that Cotzias found necessary for the best suppression of parkinsonian symptoms and signs caused intolerable nausea because of the stimulating effect of dopamine (decarboxylated peripherally from levodopa) on the vomiting center in the floor of the fourth ventricle, which lies outside the BBB. Creative chemists at Hoffman–La Roche, the manufacturer of levodopa, saw 3 ways to manipulate levodopa’s metabolic pathway to achieve clinical advantage: block peripheral conversion to dopamine (inhibit dopa decarboxylase) so that nausea could be prevented and a smaller amount of precursor would be required to enter the brain; prolong levodopa’s duration of action by blocking the enzymes that degrade it (inhibit MAO and COMT); and retard intestinal absorption and thereby sustain bioavailability (controlled-release preparation).
Inhibitors of Dopa Decarboxylase
The earliest attempt to combine levodopa with a decarboxylase inhibitor (DDI) occurred when Roche sponsored Birkmayer’s successful trial of levodopa and the DDI benserazide in the 1960s (164, 165). Cotzias et al. (27, 166) reported a similar effect with the L-isomer of an alternative DDI, alpha-methyldopahydrazine. In 1973, Rinne, Sonninen and Siirtola (167) demonstrated that levodopa and a DDI in a 4:1 ratio allowed a five-fold dose reduction of the dose of levodopa with comparable clinical benefit and significant amelioration of drug-induced nausea and vomiting. These unequivocally positive results convinced the Federal Drug Administration in the United States to approve a combination of levodopa and the DDI carbidopa for commercial distribution under the name of Sinemet (from the Latin: without nausea). Its counterpart in Europe, a combination of levodopa and benserazide, was released as Madopar. Optimists predicted that the reduced levodopa burden would greatly diminish the incidence of motor fluctuations, a central complication of long-term levodopa exposure. However, experience quickly showed that only the peripheral dopaminergic side effects (nausea and vomiting) were impacted.
Inhibitors of Enzymes that Degrade Dopamine: Monoamine Oxidase-B and Catechol-O-Methyl Transferase
The successful inhibition of MAO-B and COMT by safe and effective pharmaceuticals represents another advance in the refinement of levodopa as the premier agent for treating PD (see Chapters 33, 35). Birkmayer and Horniekiewicz (168), in the early 1960s, were the first to attempt to enhance levodopa’s duration of action by combining it with a nonselective MAO inhibitor. The combination therapy potentiated the effect of levodopa but caused severe hypertension, tachycardia, and toxic delirium.
The conclusion that a MAO inhibitor could not be used in conjunction with levodopa did not take into account—because it was not known at the time—that the adverse drug effects were manifestations of sympathetic overactivity caused by the catalytic action of MAO-A on norepinephrine (NE) and serotonin (5-HT), and not dopamine, which is metabolized by MAO-B. The toxic side effects observed by the investigators were precipitated by an increase in the concentration of tyramine (producing the “cheese’’ effect) because of the blockade of tyramine metabolism by MAO-A. Deprenyl, a selective MAO-B inhibitor, first introduced by Knoll et al. (169) as a “psychic energizer,’’ is devoid of tyramine side effects in doses used in humans. It was subsequently shown to benefit patients with PD when used jointly with levodopa by modulating motor fluctuations while at the same time allowing for a reduction in the effective dose of levodopa (170). However, deprenyl is metabolized to methamphetamine and amphetamine, which may cause cognitive side effects.
In the presence of a DDI, COMT becomes the major enzyme responsible for the peripheral metabolism of levodopa (171). The principal by-product, 3-O-methyldopa (3-OMD), weakly competes with levodopa for uptake into the brain (172) but does not compromise its central therapeutic action. COMT inhibitors (COMTIs) can significantly decrease the peripheral metabolism of levodopa and boost central bioavailability of the drug. The initial attempts to develop COMTIs, in the early 1970s, were halted because of the toxic effects of the compounds and lack of efficacy (173). In the 1990s, 2 new COMTIs—tolcapone and entacapone—were developed to address the problem of motor fluctuations. A number of level I clinical trials (randomized, doubleblind, placebo controlled) showed that COMTIs reduced “off” time by an average of 90 to 120 minutes per day while permitting a variable reduction in levodopa dosage by as much as 25% (174).
Tolcapone is a more potent inhibitor of COMT, acting peripherally and centrally with a longer half-life, whereas entacapone inhibits only peripheral COMT and must be given more frequently with each dose of carbidopa/levodopa (175, 176). Both drugs improve the availability of levodopa as measured by the area under the plasma concentration–time curve without increasing the maximum concentration (Cmax) of plasma levodopa after single dose (177, 178, 186). However, with multiple doses, there is gradual escalation of the peak plasma concentration, which translates into increased potential to induce peak-dose dyskinesia. Indeed, dyskinesia was one of the most prominent adverse effects (along with nausea, diarrhea and insomnia) in the early trials of tolcapone. Tolcapone was associated with an asymptomatic and reversible elevation of hepatocellular enzymes in a small percentage of users in the trials, but the occurrence of 3 deaths from hepatic failure in patients using tolcapone forced its removal from the marketplace in Canada and Europe and restriction of its use by the FDA in the United States (179). Current FDA regulation requires biweekly liver function tests for the first year of administration, followed by monthly testing.
The virtual demise of tolcapone in the aftermath of the hepatic deaths has allowed entacapone to become the dominant COMTI worldwide. A recent review of 8 level I clinical trials of carbidop/levodopa/entacapone compared with carbidopa/levodopa/placebo (1560 patients followed for 2 to 12 months) (180) concluded that entacapone reduced “off” time by 1 to 2 hours per day and levodopa dosage by 10% to 15%. Best motor function, ADLs, and quality of life were only modestly improved if at all.
The possibility that the combination of carbidopa/ levodopa and entacapone might be more effective than carbidopa/levodopa alone in nonfluctuating Parkinson’s patients has been evaluated in 2 recent studies. Olanow et al. (181) conducted a 26-week, multicenter, randomized, double-blind, placebo-controlled, parallel trial of 750 patients (373 entacapone, 377 placebo) with mild to moderate (Hoehn and Yahr stage II), typical PD and a stable response to carbidopa/levodopa. Changes in the UPDRS (Part II-ADL, Part III-motor and total) and a quality-of-life (QOL) scale were the primary efficacy measures. There was no significant difference between the groups, although some items on the QOL scale trended positively. The results were confounded by a 25% dropout rate and an increased carbidopa/levodopa requirement in the placebo group.
In the second study of similar design, Brooks et al. (182) evaluated 172 fluctuators and 128 nonfluctuators over 6 months, with a 2:1 treatment:control ratio. The primary efficacy measure was change in part II of the UPDRS. As predicted, the fluctuators had significantly increased “on” time (average 1.3 hours per day) and reduced levodopa dosage. The nonfluctuators showed a modest (1-point) but significant improvement in UPDRS ADL score.
These 2 studies of nonfluctuating patients treated with carbidopa/levodopa and entacapone reached different conclusions, although differences in study design, the second study’s small sample size, and the minimal absolute improvement in the primary efficacy measure could account for the variance. It was inferred from the use of entacapone combined with carbidopa/levodopa (marketed as Stalevo in the United States and Europe) in early, nonfluctuating patients that the combination produces more sustained blood levels of levodopa and less pulsatile, more physiologic delivery of the drug to the brain’s dopamine receptors (135, 183) than carbidopa/ levodopa without entacapone. Hence it was felt that early use of the combination might prevent or at least postpone the time when motor fluctuations occurred in the chronic levodopa users. This hypothesis is supported by evidence in animal models of parkinsonism that early use of entacapone with carbidopa/levodopa therapy can prevent motor fluctuations when compared with carbidopa/ levodopa alone (184, 185). An RDB, placebo-controlled clinical trial comparing Stalevo with regular Sinemet as initial pharmacotherapy is currently under way to test the hypothesis in humans.
Adverse events associated with levodopa’s use as an antiparkinsonian drug prompted investigators to search for alternative treatment strategies. Dopamine agonists (DAs) (see Chapter 34) were appealing because they represented a fundamentally different and potentially advantageous approach to treatment. First, unlike levodopa, DAs act directly on postsynaptic dopamine receptors and are not dependent on a supply of presynaptic enzymes to convert levodopa to dopamine (194). Second, DAs have a longer half-life than levodopa, thereby providing the benefits of sustained instead of pulsatile stimulation of postsynaptic dopamine receptors (195). Third, DAs have a greater affinity for the D2 subgroup of dopamine receptors, which are not as likely to be involved with the generation of dyskinesia as the D1 subgroup (196). Fourth, DAs have the potential to decrease endogenous dopamine turnover through negative feedback to nigral neurons and can indirectly reduce the formation of dopamine-generated oxygen free radicals, which in turn have the capacity to accelerate neuronal cell death (194, 197).
Clinical trials in the early 1970s demonstrated the efficacy of the DA bromocriptine in PD, but experience quickly and clearly showed that DAs were effective mainly as modulators of and not as substitutes for levodopa except in the earliest phase of illness, when disability is mild (78, 79). The first generation of DAs (bromocriptine, pergolide) were derivatives of ergot alkaloid. In the few studies of these agents as monotherapy, a majority of patients required the addition of levodopa to the agonist within a year (195, 198). Such a short duration of benefit coupled with an average 30% rate of discontinuation of therapy in clinical trials because of immediate side effects (nausea, vomiting, hypotension) and lack of efficacy halted further studies in patients with newly diagnosed PD. Instead, these drugs were reserved as adjunctive therapy in advancing disease (195). Pergolide was reevaluated as monotherapy for early PD, and a 3-month double-blind, placebo-controlled trial demonstrated its efficacy and safety (199).
Pramipexole and ropinirole were developed in the early 1990s and approved by the FDA in 1997. Unlike the earlier DAs, these agents were evaluated systematically as monotherapy in early PD (195, 198). Long-term experience with ropinirole and pramipexole comes from the 2 pivotal RDB placebo-controlled studies discussed above: the Requip 056 study (82) and the CALM PD study (pramipexole) (81). Both studies were designed to compare DA and levodopa with respect to the potential to delay motor fluctuations and dyskinesia over a long-term treatment period: 5 years for ropinirole and 4 years for pramipexole. The Requip 056 study enrolled 268 patients with early PD.
Patients were randomly assigned to receive ropinirole or levodopa, both of which were titrated to symptomatic effect. Physicians were allowed to supplement study patients with openlabel levodopa in both groups if PD symptoms were not adequately controlled by the primary assigned drug. The ropinirole study demonstrated that 59.8% and 34% of patients, respectively, who completed the study remained on ropinirole monotherapy without the need for supplemental levodopa at 3 and 5 years of follow- up (82). The primary outcome measure was time to onset of drug-induced dyskinesia. There was a significant difference in the incidence of dyskinesia in favor of ropinirole regardless of levodopa supplementation: 20% in the ropinirole group and 45% in the levodopa group. Only 5% of study patients treated with ropinirole alone reported dyskinesia at the 5-year follow-up, although only 34% of the patients who completed the study were in this group (82).
The efficacy of treatment based on the UPDRS motor scores favored levodopa. However, the statistically significant difference was small and did not translate into a change in activities of daily living (ADL) scores. The study had a nearly 50% dropout rate in both treatment groups. Withdrawal due to the side effects was also comparable in both groups. The study concluded that ropinirole monotherapy with as-needed levodopa supplementation provides adequate control of symptoms in early PD and reduces the risk of developing dyskinesia.
The role of pramipexole versus levodopa as initial treatment for PD was investigated in the CALM-PD study (81), which was designed like the ropinirole protocol, but the endpoint was incidence of all motor complications rather than dyskinesia only (81). The study again demonstrated the superiority of DA over levodopa in the incidence of motor complications: 52% in the pramipexole group versus 74% in the levodopa group. The incidence of dyskinesia was comparable to that in the ropinirole study: 24% in the pramipexole group, independent of levodopa supplementation, compared with 54% in the levodopa group. Levodopa was also superior to DA in the level of efficacy of treatment of PD symptoms based on the UPDRS motor score, but the quality-of-life scores did not differ between the groups. Pramipexole, like ropinirole, was effective as monotherapy early on. Approximately 70% of patients remaining in the study responded satisfactorily to monotherapy at the 2-year follow-up; by 4 years, however, 72% of the patients in pramipexole group required levodopa supplementation (81). Trials of open-label use of pramipexole report similar results, with about 50% of patients maintaining monotherapy for up to 36 months (200).
The results of these two studied unequivocally support the ability of DAs to delay the onset of motor complications for several years. However, there still remains a debate on the long-term significance of that effect (146). While DAs are associated with a lower risk of developing motor complications, they are less effective in controlling PD motor disability, associated with higher incidence of drug-related side effects (specifically somnolence, confusion, and leg edema), and substantially more expensive and complicated to use. Although the apparent advantages of using DAs as monotherapy in early PD have not been firmly proven, the appeal of postponing the use of levodopa is strong among neurologists (201).
Even if there is no long-term benefit in a degenerative disease like PD, which runs a protracted course, the ability of DA monotherapy to reduce the risk of early motor fluctuations deserves serious consideration in the process of deciding which treatment is appropriate for an individual patient. The same reasoning applies to the earlier combination of levodopa and DA in patients who require increased motor benefit. The algorithm for the management of Parkinson’s disease presented by Olanow and Watts (202) suggests a choice between DAs and levodopa as initial monotherapy: DAs in the “younger,” fit patients with a milder burden of disease and levodopa in older patients with a higher burden of disease, especially if cognitive dysfunction is an issue,. There are compelling empirical arguments for initiating the treatment of PD with either a DA or with levodopa.
The American Academy of Neurology’s (AAN) Quality Standards Subcommittee and the International Movement Disorder Society (MDS) have each published reviews of the various pharmacologic therapies for PD to guide the treating physician through the proliferating and often conflicting literature on the subject. Each used standard biomedical databases (e.g., MEDLINE, Cochrane Library) to select publications that met high inclusion standards with emphasis on well-designed randomized controlled clinical trials (level I studies). Each also reached evidence-based conclusions on efficacy and safety. The MDS review (174) addresses each drug or class of drugs individually and compares efficacies where data permit. The AAN review (203) concludes with general recommendations for treating PD patients with the entire spectrum of drugs. In this report, the Quality Standards Subcommittee recommended the following: “For PD patients requiring initiation of symptomatic therapy, either levodopa or a dopamine agonist can be used. Levodopa provides superior motor benefit but is associated with a higher risk of dyskinesia” (203). Moreover, none of the antiparkinson drugs reviewed were shown convincingly to have neuroprotective properties.
Continuous Levodopa Administration
The rationale for continuous drug delivery in PD is to simulate with exogenous levodopa the “normal’’ steady state of dopamine release at the striatal synapse. A number of such systems providing continuous delivery of levodopa to the brain have been evaluated. Patients receiving brief continuous enteral or intravenous infusions of levodopa have experienced excellent control or a significant reduction of fluctuations and dyskinesia (204). Shoulson and Chase (205) first used a constant intravenous infusion of levodopa in 5 patients with wearing off and demonstrated the ability to abolish motor fluctuations once a stable plasma levodopa concentration was achieved. Subsequently, other authors confirmed that continuous intravenous infusion of levodopa could effectively ameliorate complex fluctuations (206). However, long-term continuous intravenous delivery of the drug is not practical because of the corrosive effect of chronically infused levodopa on veins and soft tissue.
Continuous duodenal or jejunal infusion of levodopa has been employed as a way of neutralizing the contribution of erratic gastric emptying and uneven intestinal absorption to the problem of motor fluctuations in patients with advanced PD. Several open-label studies have demonstrated significant improvement in quality of life during “on’’ time (207,208). Kurth et al. (209) performed a small double-blind, placebo-controlled study of duodenal infusion of levodopa/carbidopa in 10 patients with advanced PD; 7 of the 10 improved, and 5 continued using duodenal infusion for 20 months after completion of the study. New gel preparation of carbidopa/ levodopa for intraduodenal infusion was recently developed in Sweden (210–214), and the results of a 6-week randomized crossover study were reported (214). In a cohort of 24 patients with motor fluctuations, daytime inraduodenal infusion provided a 34% reduction of UPDRS motor scores compared to conventional PD medications. The infusion resulted in a 20% increase in “on” time without a corresponding increase in dyskinesia. PD quality-of-life scales also improved in the infusion group. At the end of the study, 16 patients elected to remain on the infusion. In that short-term study, the gel preparation was delivered via nasoduodenal tube. The same group reported their experience with long-term intraduodenal infusion (210).
Between 1991 and 1998, a total of 28 patients with advanced PD were treated with intraduodenal carbidopa/levodopa infusion (210). They required insertion of an intraabdominal pump to deliver the drug and infusion was limited to the daytime hours due to concern of development of tolerance if it were continuous and around the clock. Patients were allowed to supplement the infusion with oral PD medications. Twenty-two patients remained on the infusion therapy long term, although the rate of pump-related complications was relatively high: 6 patients had tube infection, and gastroscopy-guided catheter adjustment was performed 35 times over the duration of follow-up. In summary, continuous duodenal drug delivery has been revived with advancing technology. However, each of the studies cited included small numbers of patients, and all authors agreed that the demands of maintaining the duodenal delivery system limit its use to a select subgroup of highly motivated patients with advanced disease who fail other treatment options.
A liquid suspension of carbidopa/levodopa, stabilized with ascorbic acid and taken orally, has been used as an alternative to cumbersome intraduodenal infusion. The mixture is stable for up to 72 hours at room temperature without specific handling (216). The results of this simple technique for facilitating oral–intestinal absorption have been modestly positive. In one study reported by Pappert et al. (216), some patients overcame unpredictable “off’’ symptoms by an earlier boost of the peak plasma levodopa level but otherwise realized no advantage over standard carbidopa/levodopa tablets. The frequency of dosing actually increased.
Parcopa, an orally dissolvable preparation of carbidopa/levodopa, was approved in 2005 (215). It offers the convenience of avoiding the need to swallow a pill, can be taken without water, but has no additional benefit over standard carbidopa/levodopa. It can be a useful alternative for postoperative and dysphagic patients and a convenient option if carbidopa/levodopa cannot be taken with a liquid beverage.
Levodopa methyl ester, a dispersable oral agent now in development as a liquid or tablet, contains the same active ingredients as standard carbidopa/levodopa but is more rapidly absorbed and has a shorter time to peak plasma concentration. It has no effect on the duration of on time but has the potential for quickly reversing severe early morning akinesia. Levodopa methyl ester has the same short half-life as standard carbidopa/levodopa and therefore is no less likely to be associated with the development of motor fluctuations (217). Alternative routes of delivery of levodopa methyl ester—including intranasal, subcutaneous, and intravenous—have also been investigated (218).
Another way to optimize levodopa therapy is to develop a levodopa compound that will circumvent the obstacles of gastrointestinal absorption and systemic metabolism encountered by the standard formulation. A number of new compounds are now being investigated. One such product, levodopa ethyl ester, is a levodopa prodrug, which, as a result of hydrolysis by esterases in the GI tract, has a faster onset of action and higher maximum concentration compared with standard carbidopa/levodopa. It can also be used parenterally as rescue therapy for severe “off’’ symptoms, especially in postoperative patients unable to take anything by mouth (219). An RDB study comparing levodopa ethyl ester and carbidopa/levodopa as a remedy for the reduction of morning “off” time and the latency between swallowing a pill and its onset of action showed no difference between the two drugs (220).
All of these pharmacologic strategies represent incremental progress, but they still fall short of disease modification, the holy grail of pharmacotherapy. Research on fetal cell transplantation has slowly and deliberately evolved for more than 2 decades (221), and early results were promising. However, the results of two recent RDB shamcontrolled trials showed no difference between implanted patients and controls (222, 223) despite promising results of the open-label protocols (224, 225). Levodopa or dopamine-secreting cell lines, encapsulated in slow-release polymer systems and implanted subcutaneously or directly into the striatum, are being investigated in animals and pilot human trials (226). Another approach is delivery of the growth factors to stimulate intrinsic nigrostriatal production of dopamine. A pilot open-label protocol with glial-derived neurotrophic factor (GDNF) demonstrated substantial benefit in 5 patients after 2 years of therapy (227). A subsequent double-blind randomized study was terminated prematurely at 6 months due to lack of efficacy and safety concerns. Finally, studies of gene therapy aimed at encoding enzymes responsible for dopamine biosynthesis, employing a variety of vectors, are on the near horizon. The most recent studies have focused on genes that offer neuroprotective or even neurorestorative function like production of growth factors (228, 229).
Forty years of experience have taught us much about the strengths, shortcomings, and travails of using levodopa to treat PD. As we move further into the new millennium and reflect on the remarkable story of this true pharmaceutical miracle, we conclude this chapter with a condensed list of lessons learned.
Levodopa remains the most effective drug for treating the symptoms of PD, notwithstanding the problems related to chronic use. Maximal levodopa efficacy is achieved by the concomitant use of inhibitors of the peripheral catabolic enzymes dopa decarboxylase, monoamine oxidase, and COMT.
- There is no evidence that levodopa is toxic to human beings, notwithstanding the experimental findings supporting toxicity in vitro. To the contrary, it may be neuroprotective!
- Motor complications associated with chronic oral levodopa therapy occur in most patients from the interaction between progressive nigrostriatal degeneration, unique pharmacodynamic properties of dopamine receptors, and altered molecular plasticity regulated by genes of the basal ganglia.
- The question of whether treatment with levodopa should be initiated early or later in the course of illness remains unanswered. The pendulum swings back and forth. That debate may continue into the twenty-second century.
- The fashion of treating “young’’ parkinsonians with dopamine agonists as a levodopa-sparing strategy has a sound theoretical and empirical basis. It is not yet clear that this strategy makes a difference over the long course of PD.
- Combination pharmacotherapy is a major advance in the management of PD. The evolution of this practice is the result of major achievements in drug development and the emergence of the randomized clinical trial as the most rigorous measure of drug efficacy.
- The Achilles’ heel of levodopa therapy is the drug’s short half-life and erratic intestinal absorption, both of which produce a nonphysiologic pulsatile delivery of levodopa to the brain. A simple and effective system of continuous parenteral delivery has been pursued but not realized. A cure for PD is nowhere on the horizon but is still the ultimate dream of all researchers, no matter how they focus their investigative attention. Development of a completely new generation of antiparkinson drugs based on discoveries of mutated genes and their by-products may offer the best hope of major breakthroughs that will bring the dream closer to reality. Then and only then will levodopa be truly obsolete.
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