9-Amino-20(S)-camptothecin (9-AC) is a water-insoluble topoisomerase I inhibitor with evident schedule-dependent antitumor activity in preclinical studies. The pharmacokinetic behavior of 9-AC given as a bolus i.v. infusion (1.0 mg/m2 over 5 min) was recently characterized in 12 patients in a bioavailability study. Remarkable rebound concentrations of 9-AC total drug (i.e. lactone plus carboxylate forms) were observed at about 2-3 h after dosing. In vitro experiments indicated that this phenomenon was associated with a substantial uptake of 9-AC lactone by erythrocytes immediately after dosing and its subsequent release followed by accumulation of 9-AC carboxylate in the plasma compartment mediated by a pH-dependent hydrolysis of the lactone form, which is unable to diffuse across cell membranes. The preferential binding of 9-AC carboxylate to human serum albumin shifts the equilibrium between the lactone and carboxylate forms of 9-AC to the pharmacological inactive carboxylate form.