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We examined the effects of supplemental standard carbidopa/levodopa (Std-L) on early morning akinesia in patients with Parkinson's disease (PD) who were being treated with sustained-release carbidopa/levodopa (L-CR). We compared plasma dopa levels and clinical response in 15 PD patients after a dose of Std-L and L-CR (2 hours later) and after a dose of L-CR and placebo in a double-blind, placebo-controlled, crossover study. Plasma dopa levels, total motor score, walking time, and finger tapping time were assessed every 15 minutes for the first 2 hours and then every 30 minutes for the next 3 hours. The time of onset in clinical benefit was significantly earlier with Std-L (47 minutes, range 15 to 75 minutes) as the first dose as compared with L-CR (58 minutes, range 30 to 105 minutes). Similarly, there was a significant difference in the peak plasma dopa levels (Cmax) and the time to reach peak plasma dopa levels (Tmax) with administration of Std-L (Tmax 36 minutes; Cmax 1,501 micrograms/ml) as compared with L-CR (Tmax 111 minutes, Cmax 1,260 micrograms/ml). There was no significant difference in dyskinesias between the two treatment arms. An initial morning dose of Std-L alleviates the problem of delayed-onset clinical response that commonly occurs with L-CR.

We gave increasing daily doses of clozapine to six patients with advanced Parkinson's disease (PD) and levodopa-induced dyskinesias. Clozapine reduced the daily dyskinesia time five-fold, increased "on" time eight-fold, and doubled the serum [DOPA] producing half-maximal dyskinesia. Parkinsonism scores after overnight DOPA withdrawal improved with increasing daily clozapine intake, and there was no clozapine dose-related shift in levodopa dose response for relief of parkinsonism. Patients experienced sedation, sialorrhea, and orthostatic hypotension. Clozapine appears to be an effective agent for suppression of levodopa-induced dyskinesias in PD.

We characterized the clinical dose-response curves for relief of parkinsonism and production of dyskinesias as a function of plasma levodopa and 3-O-methyldopa levels in six patients with advanced Parkinson's disease (PD) and fluctuating responses to oral levodopa/carbidopa. Dose response to ramped intravenous levodopa infusion was measured after overnight drug withdrawal on two occasions: first after chronic, intermittent oral levodopa/carbidopa, and second after 3 to 5 days of continuous intravenous levodopa. Continuous intravenous levodopa shifted the dyskinesia dose-response curve to the right, reduced maximum dyskinesia activity, but did not significantly alter dose response for relief of parkinsonism. Improvement in dyskinesia was apparent by the second day of continuous levodopa, during which ratios of plasma dopa/3-O-methyldopa remained constant. Our results support the hypothesis that relief of parkinsonism and production of dyskinesia by levodopa occur by separate mechanisms.

Seven parkinsonian patients who had severe on-off effects during chronic treatment with levodopa were studied. Marked swings in plasma dopa levels as well as in motor function followed each oral dose of levodopa. A constant intravenous infusion of levodopa resulted in stable plasma dopa concentrations and virtual disappearance of motor fluctuations. Notwithstanding steady plasma dopa levels, an abrupt decline in the antiparkinsonian response to levodopa attended postural tilting or cold pressore stimulation. Although these maneuvers were accompanied by a significant rise in plasma norepinephrine, the intravenous infusion of this catecholamine failed to influence the effect of levodopa on parkinsonian signs. The results suggest that central noradrenergic mechanisms as well as alterations in circulating dopa may contribute to the on-off response to levodopa.

Serial determinations of plasma dihydroxyphenylalanine (dopa) in 16 Parkinson's disease patients receiving levodopa showed a negative correlation between plasma dopa levels and disability scores among patients who exhibited daily fluctuations of signs and symptoms. This suggests that the amount of levodopa delivered to the brain from the periphery is of major importance in the production of the "on-off" phenomenon. A close relationship between plasma dopa levels and abnormal involuntary movements was present in six patients. In three a striking dissociation between control of Parkinson's disease and abnormal involuntary movements was present, suggesting that in some patients these two effects are mediated through different underlying mechanisms. Administration of levodopa in such a way as to prevent both high and low levels of dopa in plasma minimizes disability in Parkinson's disease and may lessen abnormal involuntary movements in patients with the "on-off" effect.

Administration of a low-protein diet to parkinsonian patients with "on-off" syndromes consistently increased the total daily time of "on" states when compared with a high-protein diet. The clinical effect of the low-protein diet may be due to a marked decrease in the plasma concentration of large neutral amino acids that compete with L-dopa for carrier-mediated transport into the brain.

The pathogenesis of "random" fluctuations in parkinsonian mobility, which are not clearly related to the dosing schedule of levodopa, has not been determined. We rated parkinsonian mobility and assayed plasma dopa in one patient with clinically random fluctuations during two modes of administration of levodopa/carbidopa: (1) standard oral route and (2) direct duodenal delivery via nasoduodenal tube. During oral therapy, mobility varied unpredictably in relation to levodopa dosing, suggesting a clinically random pattern. During duodenal delivery, however, a predictable and dramatic pattern of recurrent end-of-dose deterioration was observed; each intraduodenal levodopa dose resulted in 60 to 90 minutes of benefit. Plasma dopa levels correlated closely with mobility ratings for both modes of administration. Our findings indicate that erratic gastric emptying of levodopa is responsible for apparently "random" oscillations in mobility in at least one patient with Parkinson's disease and probably in others.

In four patients with Parkinson disease, we compared carbidopa combined with levodopa (Sinemet) and benserazide combined with levodopa (Madopar). All of these patients had responded to treatment, first with levodopa and then with Sinemet; after 6 years two continued to show a good response, while two developed marked "on-off" phenomena. Clinically, Sinemet and Madopar were similar; however, DOPA levels were higher, but with a shorter half-life, on Madopar. The higher DOPA levels may have been offset by the shorter half-life, resulting in no clinical change. DOPA levels were lower and half-life was shorter in patients with on-off phenomena. These differences may be responsible in part for the on-off phenomena.

In 19 patients with Parkinson disease, we studied the relationship of the therapeutic effect of levodopa, or dyskinesia, to the plasma content of DOPA and growth hormone (GH). Those with stable responses to levodopa, individually or as a group, showed stable and lower plasma DOPA levels than those with unstable symptomatic responses. These results show that stable and oscillating clinical responses in Parkinson disease parallel plasma DOPA levels, suggesting that there are different mechanisms in peripheral levodopa metabolism and that extracerebral mechanisms are important in regulating the availability of levodopa to the brain. Plasma GH did not differ in the two groups, suggesting that the secretion of GH is independent of the effects of levodopa in parkinsonism.