Stepwise intravenous infusion of apomorphine to determine the therapeutic window in patients with Parkinson's disease

A new experimental strategy was applied to determine the concentration-effect relation and the therapeutic window of apomorphine in individual patients with Parkinson's disease. Apomorphine was administered by a stepwise intravenous infusion. The infusion rate was increased by 10 micrograms/kg/h every 20 minutes, up to 100 micrograms/kg/h or less when adverse effects occurred. Thereafter, the infusion rate was decreased in a stepwise fashion until zero. Plasma apomorphine concentrations were measured every 20 minutes. Clinical efficacy (tapping score and tremor), dyskinesia, and adverse effects were monitored at the same time. The mean clearance of apomorphine was 4.5 L/min (2.2 to 6.6 L/min). Of the 10 patients, 8 responded to apomorphine. The effects were quantal rather than continuous. Within each patient, the concentrations at onset and offset of effect generally were similar. Significant interpatient variability was observed with respect to minimal concentration for each of the effects. Clinical efficacy occurred at a mean minimal effective concentration (MEC) of 4.7 ng/mL (range 1.4 to 10.7 ng/mL). Dyskinesia was observed at a mean concentration of 8.5 ng/mL (range 2.7 to 20 ng/mL). This value was not significantly different from the MEC. The mean minimal toxic concentration was 16.7 ng/mL (8.5 to 24.5 ng/mL) and was significantly different from the mean MEC. In conclusion, the stepwise increase and decrease of the intravenous infusion rate is a suitable tool for the establishment of the concentration-effect relation of apomorphine in individual patients. The finding of a narrow therapeutic window, in which the onset concentrations vary from patient to patient, underlines the need for accurate and individualized dosing.