Peripheral pharmacokinetics of apomorphine in humans

Apomorphine, a potent dopamine agonist, has been used in acute and chronic studies of parkinsonism and other neurological disorders. To define its peripheral pharmacokinetics, we administered apomorphine by subcutaneous injection, by subcutaneous infusion, and by intravenous infusion to 15 patients with parkinsonism and measured plasma apomorphine levels by high-performance liquid chromatography with electrochemical detection. The peak drug levels and area under the curve were closely correlated with the dose administered; time to peak was brief and was independent of dose. The variation in absorption was high between subjects but low within individual subjects. In 11 of 15 subjects, the disappearance of drug could be described by a two-compartment model, with a distribution half-life of 5 minutes and an elimination half-life of 33 minutes. The drug absorption, volume of distribution, plasma clearance, and half-lives were similar for subcutaneous injection, subcutaneous infusion, and intravenous infusion. We conclude that apomorphine is rapidly and completely absorbed from subcutaneous tissue, correlating with the rapid onset of clinical effects, and that the brief duration of clinical action of the drug is explained by its rapid clearance.