Pharmacokinetics and metabolism of chlorambucil in man: a preliminary report

A sensitive and specific gas-chromatographic-mass spectrometric (GC-MS) assay for chlorambucil and its major metabolite in biological fluids, phenylacetic acid mustard, has been developed. It has been applied to the study of oral chlorambucil pharmaco-kinetics in 6 patients. The assay has a sensitivity limit of 50 ng/ml and a precision of 94.3±1.3% at a plasma chlorambucil concentration of 200 ng/ml. In vitro chlorambucil recovery is temperature dependent with recovery rate constants at 37°C of k = 0.43 hr −1 for water and k = 0.12 hr −1 for plasma. After an oral bolus dose of 0.6–1.2 mg/kg in 4 patients the mean terminal phase half-life of chlorambucil was 91.7±19.3 min, its mean peak plasma concentration, 1.1±0.6 μg/ml (adjusted to a dose of 0.6 mg/kg) and its mean plasma concentration-time product, 143±102 μg min/ml (adjusted to a dose of 0.6 mg/kg). The urinary excretion of chlorambucil over the first 24 h was 0.54±0.16% and its mean renal clearance was 0.029±0.014 ml/min/kg. The hepatic extraction of chlorambucil in these 4 patients averaged 0.24±0.13. The mean phenylacetic acid mustard plasma terminal phase half-life was approximately 1.6 times greater than that of its parent compound. The mean 24 h metabolite urinary excretion was 0.29±0.16%. This preliminary pharmacokinetic data suggests that oral chlorambucil undergoes rapid gastrointestinal absorption and plasma clearance, and that it is almost completely metabolized, having extremely low urinary excretion.