Value of early pharmacodynamic and pharmacokinetic investigations with anticancer drugs: data from phase I tolerance studies on a new vinca alkaloid derivative

1. A novel anticancer vinca alkaloid derivative (I) has been given as an i.v. bolus to cancer patients, using four different dosage regimens with dose levels ranging from 0.04 to 0.84 mg/m2 (equivalent to between 0.12 and 1.35 mg per dose), and the pharmacokinetics determined up to 72 h after dosing. In addition, secondary effects of leukopenia and neutropenia, were related to drug exposure using a sigmoid Emax model. 2. Plasma levels of I declined in a triphasic manner with a terminal half-life of approximately 50 h; most drug elimination (55%) being associated with the terminal phase. 3. Clearance of I was relatively low (245 +/- 160 ml/min) and remained constant with increasing doses. Initial distribution volume was low (approximately 71) but once distribution was complete, it was comparatively high (327 +/- 2121). 4. Both leukopenia and neutropenia were fitted successfully to a sigmoid Emax model showing that these effects were related to the total exposure to the drug. The Hill constant was less than 1, indicating a relatively shallow exposure/response curve and a predictable, graded increase in response with increasing I exposure, rather than a sudden quantal response. 5. Pharmacokinetically, I shows some similarities to other vinca alkaloids in its plasma level decline profile, although there are some notable differences which can be exploited clinically. In addition, the ability to model both leukopenia and neutropenia to the exposure to I, provides a valuable tool in the design of the most appropriate dosage regimen for the drug, as well as for dose adjustment taking into account inter-individual variations.