Levodopa: a review of its pharmacological properties and therapeutic use with particular reference to Parkinsonism

Levodopa therapy for Parkinson disease: A look backward and forward

Although levodopa is widely recognized as the most effective therapy for Parkinson disease (PD), its introduction 5 decades ago was preceded by several years of uncertainty and equivocal clinical results. The translation of basic neuroscience research by Arvid Carlsson and Oleh Hornykiewicz provided a logical pathway for treating PD with levodopa. Yet the pioneering clinicians who transformed PD therapeutics with this drug--among them Walther Birkmayer, Isamu Sano, Patrick McGeer, George Cotzias, Melvin Yahr, and others--faced many challenges in determining whether the concept and the method for replenishing deficient striatal dopamine was correct. This article reviews highlights in the early development of levodopa therapy. In addition, it provides an overview of emerging drug delivery strategies that show promise for improving levodopa's pharmacologic limitations.

Sympathetic modulation by levodopa reduces vascular risk factors in Parkinson disease

BACKGROUND

Sympathetic nervous system hyperactivity promotes vascular disorders by its catabolic effects and by increasing arterial blood pressure. Levodopa-derived dopamine modulates sympathetic overactivity and is known to reduce blood pressure, but its effects on glucose and lipid metabolism have not been studied in large series of patients.

METHODS

We retrospectively examined 483 consecutive parkinsonian patients, admitted to a single institute between 1970 and 1987, before statins were available. We compared risk factors for vascular disease in the 305 who were on levodopa with the 178 who had never received the drug.

RESULTS

On admission levodopa-treated patients had significantly lower plasma levels of triglycerides, total cholesterol and lipids, and lower frequency of diabetes and hypertension than untreated patients. Mean body mass index, resting blood pressure, fasting plasma glucose, and smoking did not differ between the groups. A year after enrollment 160 patients were re-hospitalized; of these 63 had started levodopa during first hospitalization. In these new levodopa users total cholesterol, triglycerides and lipids had reduced to levels comparable with those of longer-term levodopa users.

CONCLUSION

Levodopa use in parkinsonian patients is associated with reduced vascular risk factors. In causal terms this finding might be attributed to the inhibitory action of levodopa-derived dopamine on the sympathetic nervous system.

A randomized crossover trial of levodopa in congestive heart failure

BACKGROUND

The short- and long-term effects of levodopa (L-dopa), an oral dopaminergic prodrug, were assessed in patients with severe left ventricular dysfunction.

METHODS AND RESULTS

Initially, 26 patients were included in the study group. After clinical, radiographic, and radionuclide examination, each patient underwent right heart catheterization (Swan-Ganz thermodilution catheter). Plasma noradrenaline levels were measured. In two patients, a favorable hemodynamic response to L-dopa was not observed, another two required permanent pacemaker implantation. These four patients were excluded from the study. Two patients required permanent pacemaker implantation. The remaining 22 patients with favorable hemodynamic response to L-dopa (increase in cardiac index, stroke volume index, reduction in total systemic resistance) were randomized in a nonblinded fashion to the conventional (11 patients) or conventional plus L-dopa (11 patients) treatment groups. During the study period, two patients, one from each group, died. They were excluded from the analysis. The final analyzed study group consisted of 20 men, aged 33-69, in New York Heart Association functional class IV (9 patients) and III (11 patients). The cause of congestive heart failure was primary dilated cardiomyopathy in 11 patients and ischemic heart disease in 9 patients. After 3 months' treatment, all patients were crossed over. Clinical, radiographic, radionuclide, and hemodynamic evaluation was repeated at the end of the 3-month treatment period. After 3 months of therapy with L-dopa in each group (covariance analysis), there was improvement in clinical, radiographic (relative heart volume, -128 mL/m2), radionuclide (left ventricular ejection fraction, +4.6; right ventricular ejection fraction, +4.8%), hemodynamic (mean pulmonary wedge pressure, -8 mmHg; total systemic resistance, -1.8 Wu; total pulmonary resistance, -3.5 Wu), and neurohumoral (noradrenaline, -218 pg/mL) measures.

CONCLUSIONS

The addition of L-dopa to conventional therapy has beneficial short- and long-term effects in patients with severe left ventricular dysfunction.

Refractory Heart Failure: A Therapeutic Approach

Optimal “triple therapy” for patients with chronic congestive heart failure (CHF) includes diuretics, digoxin, and either angiotensin-converting enzyme inhibitors or hydralazine plus nitrates. Refractory CHF is defined as symptoms of CHF at rest or repeated exacerbations of CHF despite “optimal” triple-drug therapy. Most patients with refractory CHF require hemodynamic monitoring and treatment in the intensive care unit. If easily reversible causes of refractory CHF cannot be identified, then more aggressive medical and surgical interventions are necessary. The primary goal of intervention is to improve hemodynamics to palliate CHF symptoms and signs (i.e., dyspnea, fatigue, edema). Secondary goals include improved vital organ and tissue perfusion, discharge from the intensive care unit, and, in appropriate patients, bridge to cardiac transplantation. Medical interventions include inotropic resuscitation (e.g., adrenergic agents, phosphodiesterase inhibitors, allied nonglycoside inodilators), load resuscitation (e.g., afterload and preload reduction with nitroprusside or nitroglycerin; preload reduction with diuretics and diuretic facilitators, such as dopaminergic agents or ultrafiltration), and electrical resuscitation (e.g., prevention of sudden death, correction of new or rapid atrial fibrillation, or dual chamber pacing in the setting of relative prolongation of the PR interval and diastolic mitral/tricuspid regurgitation). Surgical interventions are temporizing (e.g., intra-aortic balloon pump and other mechanical assist devices) or definitive (e.g., coronary artery revascularization, valvular surgery, and cardiac transplantation). Although these interventions may improve immediate survival in the short term, only coronary artery revascularization and cardiac transplantation have been shown to improve long-term survival.

The short-term effect of nicotine chewing gum in patients with Parkinson's disease

Because of the inverse association of cigarette smoking with the risk of Parkinson's disease, we performed a short-term, double-blind, randomized controlled trial of nicotine polacrilex resin gum in patients with this disease. Forty-eight subjects were randomly assigned to chew either nicotine gum or placebo gum three times at 2-h intervals, with evaluation of symptoms before and after the trial. The nicotine gum was reasonably well tolerated, but there were no substantial differences in Parkinson's disease symptoms between the two treatment groups.

Systemic and coronary hemodynamic and neurohumoral effects of levodopa in chronic congestive heart failure

Systemic and neurohumoral effects of oral levodopa were evaluated in 17 patients with severe chronic heart failure. The maximum mean dopamine level achieved after 1.5 g of oral levodopa was 19.6 +/- 16.4 ng/ml. At peak dopamine level, cardiac index increased by 14% from baseline (1.95 +/- 0.55 to 2.27 +/- 0.45 liters/min/m2, p less than 0.05), stroke volume index increased by 14% (22.4 +/- 6.0 to 25.9 +/- 5.8 ml/min/m2, p less than 0.01). There was a trend toward reduced systemic vascular resistance of 13% (1,773 +/- 769 to 1,535 +/- 432 dynes.s.cm-5, p = 0.08). There was no significant change from baseline in heart rate, mean arterial pressure, right atrial pressure, pulmonary capillary wedge pressure, pulmonary vascular resistance and rate-pressure product. In addition, as the arterial dopamine level increased there was a concomitant decrease in plasma norepinephrine level that was sustained for the period of observation. In a subgroup of 8 patients, there was no change in coronary sinus blood flow, myocardial oxygen consumption, myocardial oxygen extraction, lactate extraction and transmyocardial release of catecholamines after levodopa. These findings suggest that oral levodopa, 1.5 g, can improve left ventricular function without adversely affecting myocardial energetics and catecholamine balance.

Evaluation of long-term oral levodopa therapy in chronic congestive heart failure

To evaluate the long-term effects of orally administered levodopa, 11 patients with chronic congestive heart failure (NYHA III-IV) were studied during maintenance therapy (30 +/- 1 days) and after withdrawal from levodopa. The daily levodopa dose was 4 g in six patients; because of side effects the levodopa dose was reduced to 2-3 g in the remaining patients. After withdrawal of levodopa, mean pulmonary capillary wedge pressure and mean right atrial pressure increased significantly (from 19 +/- 2 to 24 +/- 3 and from 7 +/- 2 to 9 +/- 2 mmHg, respectively). Effective renal plasma flow was 329 +/- 57 during levodopa therapy and decreased significantly to 252 +/- 27 ml/min after withdrawal of levodopa. The number of ventricular premature contractions and couplets increased during levodopa therapy and decreased again significantly after withdrawal of levodopa. No significant differences between on and off levodopa were observed in resting heart rate, arterial blood pressure, cardiac index, stroke work index, systemic vascular resistance, sodium and water excretion, or creatinine clearance. Seven patients improved on levodopa therapy by one NYHA class; four of these seven patients deteriorated again by one NYHA class after withdrawal of levodopa. Regarding both clinical and hemodynamic changes after withdrawal of levodopa, three patients were classified as responders to long-term levodopa therapy. All three responders received 4 g levodopa per day. Average dopamine plasma level was 5.3 +/- 0.8 ng/ml in the responder group and 2.0 +/- 0.5 ng/ml in the nonresponder group. Long-term administration of oral levodopa is associated with beneficial clinical and hemodynamic response in only a minority of patients with chronic congestive heart failure.

Psychopharmacological investigations of a lead-induced long-term cognitive deficit in monkeys

This study investigated pharmacological manipulations of the cholinergic (ACh) and dopaminergic (DA) transmitter systems in monkeys with a long-term lead-induced cognitive deficit on delayed spatial alternation (DSA). Both ACh and DA have been found to be affected by developmental lead exposure and to be involved with performance on spatial learning and memory tasks. The lead-induced deficit in performance accuracy on DSA persisted throughout the 2 years of this experiment, which ended more than 8 years after the end of the postnatal lead exposure. Acute administration of agonists and antagonists of the ACh and DA systems did not elicit differential effects from the lead-exposed and control groups in terms of DSA per cent correct performance. The ACh antagonist, scopolamine, caused a dose-related decline in performance in both groups. Significant amelioration of the lead-induced DSA deficit was achieved by chronic treatment with the DA agonist, L-dopa. After withdrawal from L-dopa, the lead-related deficit reappeared. Improvement in performance of the lead-treated group was also seen after chronic amphetamine administration, but this effect was not significant. These data implicate DA mechanisms in the long-lasting cognitive effects of developmental lead exposure. The alleviation of the deficit with chronic administration of a DA precursor points to a possible line of treatment for the cognitive effects of developmental lead exposure.

The inotropic effects of dopamine and its precursor levodopa on isolated human ventricular myocardium

The direct positive inotropic effects of dopamine and its precursor, levodopa, were measured using isolated, contracting human papillary muscle strips taken from patients during mitral valve replacement. Levodopa did not produce any positive inotropic effect at concentrations up to 3 X 10(-3) M. The positive inotropic effects of dopamine were observed at concentrations above 1 X 10(-5) M with the maximal effect at 3 X 10(-3) M - concentrations higher than those observed in therapy. This inotropic effect was reduced by the beta 1-selective antagonist, 1-practolol (1 X 10(-6) M); the beta 2-selective antagonist, ICI 118,551 HCl (1 X 10(-6) M); the dopamine antagonist, haloperidol (3 X 10(-6) M); the neuronal uptake inhibitor, cocaine (3 X 10(-5) M), but not by the alpha 1-antagonist, prazosin (1 X 10(-7) M). This indicates that dopamine exerts its positive inotropic effects on human heart muscle mainly through release of noradrenaline, together with possible interactions at beta- and dopamine-receptors. The maximal inotropic effect of dopamine was about 50% that of calcium (15 mM, 6.2 +/- 0.7 mN) or ouabain (1 X 10(-7) M, 5.0 +/- 0.8 mN) when measured in the same muscle strips, possibly due to the reduced cardiac noradrenaline content together with the reduced beta-receptor number in congestive heart failure. This concentration of ouabain (1 X 10(-7) M) gave almost maximal inotropy without marked toxicity; when dopamine was then added, only toxicity developed without any further increases in force of contraction. Any haemodynamic benefits of dopamine therapy in optimally digitalis-treated patients are probably due to other cardiovascular effects such as vasodilatation.

Does unilateral dopamine deficit contribute to depression?

A system of emotional control of behavior is believed to be lateralized to the right hemisphere. Given that dopaminergic pathways are involved in affective behavior, depression, which is recognized as an integral part of Parkinson's disease, may be associated with a dopamine imbalance. The present study examined this hypothesis in patients with unilateral symptomatology indicating either left hemisphere parkinsonism (LHP) or right hemisphere parkinsonism (RHP). Sixteen patients were tested on a battery of neuropsychological tests and several scales for evaluating mood. The two groups did not differ significantly on either cognitive or emotional measures. However, RHP patients rated themselves higher on the Present Scale of Cantril, and showed some neglect of the left visual field, as compared to LHP patients.

Beneficial hemodynamic effects of oral levodopa in heart failure. Relation to the generation of dopamine

Among the positive inotropic drugs available to improve myocardial contractility in congestive heart failure, only digitalis glycosides are suitable for oral administration. In this study, we administered oral levodopa (1.5 to 2.0 g), which is decarboxylated to form dopamine, to 10 patients with severe congestive heart failure. Peak hemodynamic responses occurred one hour after the ingestion of levodopa, with the mean (+/- S.E.M.) cardiac index increasing from 1.8 +/- 0.1 to 2.4 +/- 0.2 liters per minute per square meter of body-surface area (P less than 0.01) and systemic vascular resistance declining from 1905 +/- 112 to 1513 +/- 121 dyn X sec X cm-5 (P less than 0.01). These effects persisted for four to six hours. Left ventricular filling pressure, heart rate, and mean arterial pressure were unchanged. Plasma concentrations of dopamine rose to a peak level of 34 +/- 5 ng per milliliter one hour after drug ingestion and decreased toward base line over the ensuing five hours. A significant correlation was observed between plasma dopamine levels and changes in cardiac index (r = 0.8; P less than 0.02). Five patients enrolled in a trial to evaluate the effectiveness of long-term therapy with levodopa had similar hemodynamic responses to the drug after 6.8 +/- 1.7 months of treatment. Thus, oral administration of levodopa to patients with severe heart failure produced a sustained improvement in cardiac function. The hemodynamic responses observed can be attributed to the activation of beta 1-adrenergic, dopamine, and dopamine receptors by dopamine derived from levodopa.

Sexual behavior of the male schizophrenic: the impact of illness and medications

The literature concerning the impact of (a) the schizophrenic illness and (b) the neuroleptic drugs (which are the most commonly employed medications for this disorder) on male sexual behavior is critically reviewed in the light of what is currently known about the interaction of both the schizophrenic illness and the neuroleptic drugs with hormones and neurotransmitters known to play a role in male sexual behavior. The effect of the schizophrenic illness on male sexual behavior is unclear, but there are some indications that chronic, severe schizophrenia may exert detrimental effects on many aspects of male sexual behavior. As for neuroleptic drugs, a wealth of evidence suggests that they have many detrimental effects on male sexual behavior. Nevertheless, since the introduction of these drugs, the reproductive rates of male schizophrenics have increased. The multiplicity of factors involved in the sexual behavior of the schizophrenic patient is emphasized. It is concluded that the sexual behavior of the male schizophrenic provides an important forum for studying the interaction between psychological, sociological and biochemical-pharmacological factors which determine sexual behavior.

[Effects of precursors and metabolites of catecholamines on motricity of isolated rat duodenum. Particular reference to dopamine]

Studying the effects, on the isolated rat duodenum motricity, of ten compounds precursors or metabolites of catecholamines, the following results were obtained: The direct metabolites of epinephrine and norepinephrine (metanephrine, normetanephrine), are either ineffective at concentrations below 5 X 10(-6) M, or weakly inhibitory at higher concentrations. Such inhibitory effects are prevented by alpha- and beta-blockers. 3-methoxy, 4-hydroxyphenylglycol and vanylmandelic acid have no significant effect. The catecholamine precursor, dopamine, the related compounds DOPA, 3 methoxytyramine, and to a lesser extent, 3-O methyl DOPA and homovanillic acid, have excito-motor effects at concentrations ranging mainly from 10(-7) M to 10(-5) M. At higher concentrations, the same compounds frequently exhibit inhibitory effects. The excito-motor effects might be due to a serotoninergic mechanism, since they are suppressed by the serotoninergic blocking agents methysergide and cyproheptadine. Furthermore, in the case of DOPA, we were able to establish a relationship between the excito-motor effects and duodenal serotonin stores. As for the inhibitory effects, they may be prevented by using alpha and beta blocking agents. Dihydroxyphenylacetic acid has no effect on the isolated rat duodenum motricity. The fact that dopamine and related compounds may have excitomotor effects at some concentrations, correlated with some physiopathological observations in man and animal allows some considerations about the eventual role of dopamine on intestinal motricity.

Clinical pharmacokinetics of levodopa in parkinson's disease

Although levodopa has provided a major advance in the treatment of parkinsonism, its maximum benefits have not yet been realised, in part because of its complicated pharmacokinetics. This review summarises that available pharmacokinetic data involving levodopa, especially as it relates to therapeutic response of parkinsonian patients. A large number of factors, including protein intake, gastric emptying time, pyridoxine ingestion, and dopa decarboxylase activity, affect plasma levels of levodopa attained following oral administration of this drug. Other variables influence the rate of brain uptake of levodopa from the blood. Even so, plasma levodopa concentration correlates significantly with dosage size in a large parkinsonian population and also coincides with therapeutic response in many, but not all, patients. Therefore, in certain instances, valuable information may be derived by correlating clinical response with plasma levodopa concentration. Cerebrospinal fluid levels of homovanillic acid, a major metabolite of dopamine, may have some value in predicting clinical response to levodopa. This relationship, however, has not been firmly established. Concentration of homovanillic acid or levodopa in body fluids may also be closely related to certain adverse side-effects, including abnormal involuntary movements, gastric discomfort and psychiatric disturbances. Evidence indicates that a clearer understanding of levodopa pharmacokinetics may improve the clinical management of parkinsonism.

Effect of chronic L-DOPA administration of catecholamine metabolism in the rat

1. L-DOPA was administered to rats by twice daily intraperitoneal injection for a period of eleven days. The daily dose was gradually increased from 250 mg/kg to 1 g/kg. The animals were killed 12 h after the last injection and tissue catechol-O-methyl transferase, DOPA-decarboxylase and monoamine oxidase activity determined.2. Catechol-O-methyl transferase activity was reduced in the liver (46%) and red blood cells (38%) but was approximately doubled in the heart.3. Liver DOPA-decarboxylase activity was reduced by approximately 25%.4. L-DOPA administration did not significantly affect the activity of any of the enzymes studied in brain tissue.