Selective D-1 dopamine receptor agonist treatment of parkinson's disease

J Neural Transm (1987) 68:41-50 dournal of N e u m I T r ' a n s m l s s i , on �9 by Springer-Verlag 1987

Selective D-1 Dopamine Receptor Agonist Treatment of Parkinson's Disease

A. Braun, G. Fabbrini, M.M. Mouradian, C. Serrati, P. Barone, and T .N. Chase

Experimental Therapeutics Branch, National Institute of Neurological and Communicative Disorders and Stroke, Bethesda, Maryland, U.S.A.

With I Figure

Received May 9, 1986

Summary

Preclinical evidence suggests that the D-1 dopamine receptor contribu- tes to the generation of behaviors used as models for human extrapyramidal disorders. To evaluate the potential of D-1 receptor stimulation in neuro- logic disease, SKF 38393, a selective D-1 dopamine receptor agonist, was administered to seven patients with idiopathic Parkinson's disease in a double-blind, placebo controlled study. SKF 38393 was found to be rapidly absorbed when administered orally, and to occur in micromolar concentra- tions in spinal fluid. No change in scores of parkinsonian severity were noted when SKF38393 was administered alone, or when the drug was combined with intravenous levodopa. The results support the view that the pathophysiology of Parkinson's disease may relate exclusively to the D-2 subclass of dopamine receptors.

Key words:Parkinson's disease, D-1 dopamine receptor agonist; SKF 38393, intravenous levodopa.

Introduction

Extrapyramidal dysfunction in Parkinson's disease is largely attributable to degeneration of the nigrostriatal dopamine system. Symptoms can generally be reversed by the administration of the catecholaminergic precursor, levodopa, or by direct acting

42 A. Braun et aL

dopamine agonists. Two broad subclasses of dopamine receptors are now recognized: D-1 receptors mediate the effects of dopamine on adenylate cyclase, while D-2 receptors are either negatively linked to cyclase or are unassociated with the enzyme (Kebabian and Calne, 1979; Stool and Kebabian, 1981).

Although the specific roles of these receptors in the pathophysio- logy and pharmacotherapy of Parkinson's disease remain uncharacte- rized, the D-2 receptor has generally been considered the principal mediator of dopamine's effects on animal behaviors used as models of human extrapyramidal dysfunction (Creese et al., 1983). On the other hand, recent preclinical evidence implicates the D-1 receptor in the generation of motor behaviors that serve as models for Parkin- son's disease and certain of the side effects that accompany its treat- ment (Arnt and Hyttel, 1984; Christensen et al., 1984; Gershanik et al., 1983; Mailman et al., 1984; Pugh el al., 1986; Braun et al., 1986). To date, no parallel clinical investigations have made use of any of the newly developed, highly selective D-1 agonists or antagonists.

A selective D-1 agonist, SKF 38393 (Setler et al., 1978; Molloy and Waddington, 1984, 1985; Kaiser, 1983), is now available for clinical study. We have administered this agent to patients with Parkinson's disease in order to evaluate the effects of independent D-1 receptor stimulation and of concurrent D-1 and D-2 stimulation on parkinsonian signs and to investigate the contribution of Dq recep- tor mediated events to the dyskinesias which may complicate dop- aminomimetic treatment of Parkinson's disease.

Methods

Seven patients with idiopathic Parkinson's disease (5 men, 2 women; ages 56 to 73 years) consented to participate in this study after full disclosure of its purposes, risks and potential benefits. Symptoms had been present from 4 to 27 years; severity in the untreated state ranged between stage II and IV on the Hoehn and Yahr scale (Hoehn and Yahr, 1967). All study subjects had been chronically treated with levodopa-carbidopa (Sinemet); dopa- minergic agents other than levodopa-carbidopa were withdrawn at least 2 weeks prior to study; one patient remained on a stable dose ofamantadine and 2 others continued to receive a fixed amount of an anticholinergic preparation.

SKF 38393 (Research Biochemicals, Inc., Wayland, MA) was administer- ed orally in a double blind, placebo-controlled, cross-over design. In an initial dose finding phase of this study, SKF 38393 was added to the patients' existing levodopa-carbidopa regimen. The dose of SKF 38393 was gradually increased over about 1 week to a daily maximum of 14.5 4-1.1 mg/kg (range

Selective D-1 Dopamine Receptor Agonist Treatment 43

8.2 to 19.2 mg/kg). During the evaluation phase of this study, orally ad- ministered levodopa-carbidopa was discontinued and each patient received either levodopa or a placebo solution (5% dextrose in water) by continuous intravenous infusion in combination with oral carbidopa (50 mg every 3 hours while awake). Two levodopa infusion rates were employed: one ad- justed to provide a mildly suboptimal antiparkinsonian response (19 4- 3.9 mg/hour; range 10 to 30 rag/hour), the other to provide a slightly supraoptimal response (66 4-7.3 mg/hour; range 44 to 80 rag/hour). The effect of SKF 38393 (12 4- 0.9 mg/kg per day on the day of testing; range 7.8 to 16 mg/kg) or of matching placebo capsules administered three times ,daily was evaluated after the levodopa dosage had been stabilized for at least 12 hours. For both SKF 38393 and levodopa, drug and placebo order were randomized.

The effects of each drug combination on motor function were evaluated 30, 60 and 90 minutes following the third daily dose of SKF38393 (7.2 +.54 mg/kg; range 4.1 to 8.1 mg/kg) or placebo. Overall parldnsonian and dyskinesia severity were each rated on a scale of 0 (absent) to 4 (very severe). Since motor function did not differ significantly at any of these times, mean values for all 3 ratings are reported.

Cognitive function was assessed in 5 patients at the supraoptimal rate of levodopa infusion 60 minutes following administration of the second daily dose of SKF 38393 (4.2_ .48 mg/kg; range 2.7-5.4 mg/kg) or placebo. The Mini Mental State exam (Folstein et al., 1975) was modified to include tests of visuospatial memory (hidden objects test) and selective attention (digit repetition and random letter tests) (Strub and Black, 1977).

The effects of SKF 38393 alone on simple upper limb reaction time and movement time were evaluated in 4 patients 60 minutes following the second daily dose of drug (4.2-4-.48 mg/kg; range 2.7-5.1mg/kg) or placebo. A computerized system designed to assess motor function was utilized which is described in detail elsewhere (Gopinathan et al., 1981).

SKF 38393 levels were monitored in blood and spinal fluid. Serial venous samples were obtained from each patient during the 90 minute period of neurologic ratings. In a separate experiment, three patients under- went lumbar puncture 3 hours after SKF 38393 administration (14 + 1.6 rag/ kg; range 10.5, to 15.4 mg/kg). Drug levels in these individuals were assayed both in spinal fluid and in simultaneously obtained plasma samples.

In an attempt to determine the blood-brain partition for SKF 38393 as well as to estimate pharmacologically effective cerebral drug levels, rats were injected intraperitoneally with a dose of SKF 38393 (8 mg/kg) which re- producibly influenced motor behavior. Rats, sacrificed at the time of maximal response, were perfused prior to brain removal. SKF 38393 levels were measured in both brain homogenates and mixed venous blood.

SKF 38393 was assayed by HPLC with electrochemical detection utiliz- ing a Bioanalytical Systems electrochemical detector at an oxidation poten- tial of.65 nanoamps per volt. Spinal fluid samples were lyophilized and the residue dissolved in 500 ml of.1 M dibasic sodium phosphate. This solution was extracted twice into 1 ml of ethyl acetate. Ethylacetate was extracted into

44 A. Braun et al.

.3 ml of mobile phase. Blood and brain homogenate samples were precipitat- ed with equal volumes of.4 N perchloric acid and placed on ice for 15 minu- tes, then centrifuged for 15 minutes at 16,000 x g; supernatant was collected and diluted 1 : 1 with distilled water for analysis. A Watman C 18 column and a mobile phase consisting of a sodium acetate/citric acid buffer, pH 4, with 20% volume: volume of methanol were used. Eluted peaks quantified as drug had retention times identical with those of freshly prepared samples of SKI: 38393. No endogenous substances eluted at these times when samples of drug-free CSF were analyzed.

Data are reported as the means +_ S.E.M. and were analyzed statistically by two-way Analysis of Variance with repeated measures.

Results

Plasma SKF 38393 levels rose significantly above baseline values 60 minutes following drug ingestion (Fig. 1). Ninety minutes after SKF 38393 administration, concentrations in the general circulation were increased 5 times above residual levels. In another p a t i e n t group, given a single oral dose of SKF 38393 averaging 14 4- 1.6 mg/ kg, plasma drug levels 180 minutes later ranged from 20 to 110 ng/ml

150

100 r 03 03 oo 03

~ 5o _..1 13_

I 0

I I I 30 60 90

MINUTES

Fig. 1. Effect of orally administered SKF 38393 on plasma drug levels. Mean -4- S.E.M. values are from 3 parkinsonian patients after receiving SKF 38393 (7.2 • .54 mg/kg) at

time 0

Selective D-1 Dopamine Receptor Agonist Treatment

Table 1. SKF38393 levels in plasma, spinal fluid and brain

45

Spinal Plasma Fluid Brain Partition

n (ng/ml) (ng/ml) (ng/g) Coefficient

Rat 3 20 + 7.3 - 1.6 + .37 13 : 1 Man 3 53 4-29 2.1 --+ .26 - 26 : 1

Mean _+ S.E.M. values are reported for rats 60 minutes after the intraperitoneal injection of SKF 38393 8 mg/kg, and for man 180 minutes after the oral ingestion of 14 -+ 1.6 mg/kg.

Table 2. Effect of SKF 38393 alone on parkinsonian severity

Patient Number Placebo Drug

1 1.5 1.7 2 0.9 0.7 3 1.3 0.7 4 3.0 1.7 5 3.2 4.0 6 4.0 4.0 Mean + S.E.M. 2.3 -- 0.5 2.1-4- 0.6

Values are the mean of 3 ratings during a period of 90 minutes following the oral administration of placebo or SKI: 38393 at a dose of 7.2-t-.54 mg/kg. Parkinsonian severity was rated on a scale of 0 (absent) to 4 (very severe).

(Table 1). Simultaneously collected spinal fluid contained SKF 38393 at concentrations ranging between 1.6 and 2.5 ng/ml, suggesting a mean blood-brain partition coefficient of 26 : 1. Rats sacrificed at the time of their peak motoric response to SKF 38393 (8 mg/kg) had plasma drug levels less than half those found in man (Table 1). Rodent brain concentrations and blood-brain partition coefficients did not differ significantly from the spinal fluid concentrations and blood-spinal fluid coefficients found in the patient group (Table 1).

SKI: 38393 given alone produced no consistent change in par- kinsonian severity (Table2), induced no abnormal involuntary movements, nor did it produce any significant changes in simple reaction or movement times. When combined with levodopa given intravenously at subtherapeutic dose rates, SKI: 38393 also failed to alter parkinsonian scores (Table 3) and did not produce dyskinesias. In patients receiving levodopa at therapeutically supraoptimal rates, SKI: 38393 again had no significant effect on parkinsonian symptoms, but did reduce oro-facial and/or appendicular dyskinesias in 5 of 6 patients evidencing these levodopa-induced movements

46 A. Braun et al.

(Table 4). This change for the group as a whole did not attain statistical significance. Scores on the modified Mini Mental State exam did not differ significantly between SKI: 38393 and placebo treatments although there was a trend towards improvement in tests of selective attention during drug treatment: errors on the random letters test decreased from 2 + 1 to 0; digit spans without error in- creased from 4 +__ .4 to 5 _ .3 (forward) and 3 + .4 to 4 +_ .5 (reverse).

Adverse effects of SKF 38393 were limited to hypotension. Four patients manifested transient reductions of diastolic pressure ranging from 15 to 30 mm of mercury at doses above 4.5 mg/kg. There were no orthostatic changes in blood pressure, and patients remained

Table 3. Effect of SKF 38393 on the response to a suboptimal dose of levodopa

Patient Number Placebo Drug

1 1.3 1.3 2 1.0 0.7 3 1.4 0.8 4 0 0.8 5 3.9 4.0 6 4.0 3.7 7 -- -- Mean + S.E.M. 1.9 • .67 1.9 +-- .63

Values are the means of 3 ratings of parkinsonian severity on a scale of 0 (absent) to 4 (very severe), during a 90 minute period following the oral administration of placebo or SKF 38393 (7.2 -}- .54 mg/kg) to patients receiving intravenous levodopa ( 1 9 - 3.9 mg/hour). Dysldnesia ratings were 0 in all patients.

Table 4. Effect of SKF38393 on dyskinesias induced by a supraoptimal dose of levopoda

Patient Number Placebo Drug

1 .25 .21 2 1.2 0 3 1.6 .25 4 3.0 2.0 5 2.9 2.7 6 0.5 1.0 Mean _+ S.E.M. 1.6 + .48 1.0 _+ .45

Values are the means of 3 ratings of dyskinesia severity on a scale of 0 (absent) to 4 (very severe) during a 90 minute period following the oral administration of placebo or SKF 38393 (7.2-{-.54 mg/kg) to patients receiving intravenous levodopa (66 -+- 7.3 rag/hour).

Selective D-1 Dopamine Receptor Agonist Treatment 47

asymptomatic throughout. A single patient experienced transient nausea without emesis which did not appear to be dose related. Hematological parameters and blood chemistries remained within normal limits.

Discussion

The present results suggest that the acute administration of a selective D-1 agonist either alone, or in combination with levodopa, may not influence motor function in parkinsonian patients. The doses used in this study were comparable with those which modify motor behavior in rats (Molloy and Waddington, 1984, 1985; Braun et al., 1986; Barone et al., 1986). Plasma SKF 38393 levels in the patients averaged more than twice those found in responding rodents. The possiblitity that SKF38393 may have relatively limited access to the human central nervous system appears unlikely, since the plasma to CSF partition in man and the plasma to brain partition in rats did not differ significantly. Furthermore, spinal fluid drug levels in patients were comparable with brain levels found in rats manifesting drug-induced alterations in motor function. Serial SKI? 38393 measurements in patient's plasma suggested that adequate circulating levels of the drug were maintained throughout the 90 minute period of clinical observation.

The apparent lack of response to SKF 38393 was unexpected in light of clinical and preclinical observations suggesting that the D-1 receptor system may play a role in the pathogenesis and treatment of Parkinson's disease. The unilaterally 6-hydroxydopamine lesioned rat has (Ungerstedt and Arbuthnott, 1970) been widely used to screen drugs for potential antiparkinsonian activity. SKF 38393 potently stimulates contralateral rotation in this preparation (Arnt and Hyttel, 1984; Gershanik et al., 1983). On the other hand, SKF 38393 failed to affect motor function in marmosets rendered parldnsonian by MPTP pretreatment (Nomoto el aL, 1985). Since these latter observations are more consistent with our clinical findings, the MPTP lesioned primate may be a more accurate predictor ofdopaminomimetic drug efficacy in parkinsonian patients.

The failure of a D-1 agonist to alter parkinsonian severity when administered alone, however, is not inconsistent with preclinical and clinical data suggesting that concurrent stimulation of both D-1 and D-2 receptors may be necessary to achieve a maximal effect on motor function. Recent studies in the normosensitive rat have suggested an obligatory interaction between D-1 and D-2 receptors in the elicita- tion of certain motor behaviors. For example, the full expression of

48 A. Braun et al.

stereotypic behavior, a standard behavioral index of striatal dopami- nergic activity, requires concurrent activation of D-1 and D-2 recep- tors (Braun et al., 1986). Indeed, in catecholamine depleted animals, D-2 receptor stimulation produces virtually no motor response without concurrent activation of D-1 receptors (Barone et al., 1986; Braun et al., 1986). Clinical experience also suggests that combined D-1 and D-2 receptor stimulation may have a synergistic effect in parkinsonian patients. With dopamine agonists such as bromocrip- tine or lisuride, which preponderantly stimulate D-2 receptors (Pieri et al., 1978; Markstein and Herding, 1978), the addition oflevodopa, which has nonselective D-1 and D-2 effects, often appears necessary to achieve a maximal antiparkinsonian response (Lees and Stern, 1981; Obeso et al., 1986). Nevertheless, we were unable to document any effect on parkinsonian signs-even when the D-1 agonist was administered concurrently with levodopa.

The fact that levodopa stimulates both D-t and D-2 receptors might, in theory, obscure any effect of a concurrently administered D-1 receptor agonist. A subtherapeutic rate of levodopa infusion was thus chosen to allow SKF38393 stimulation of D-1 receptors beyond the level produced by levodopa alone. Nevertheless, no change in neurologic status was observed in spite of the administra- tion of relatively high doses of SKF 38393.

The foregoing results suggest that the pathophysiolo~ and pharmacotherapy of Parkinson's disease may relate exclusively to the D-2 subclass of dopamine receptors. On the other hand, an altered sensitivity of the D-1 receptor system in this disease may preclude any clinically significant effects of selective D-1 receptor stimulation. Although measurements of D-1 receptor changes in parkinsonian subjects have recently been carried out (Raisman el aL, 1985; Pimoule et al., 1985) this issue remains unclear.

Acknowledgements

We wish to thank Anne M. Kask for her expert technical collaboration.

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Authors' address: Dr. T.N. Chase, NINCDS/NIH, Building 10, Room 5 C 103, Bethesda, MD 20892, U.S.A.