Pharmacokinetics of chlorambucil-tertiary butyl ester, a lipophilic chlorambucil derivative that achieves and maintains high concentrations in brain.

Equimolar doses of chlorambucil (10 mg/kg) and the lipophilic chlorambucil derivative, chlorambucil-tertiary butyl ester (13 mg/kg), were given i.v. to rats. Plasma and brain concentrations of chlorambucil and its active metabolites, 3,4-dehydrochlorambucil and phenylacetic mustard, as well as of chlorambucil-tertiary butyl ester were then determined by HPLC between 2 and 240 min after drug administration. Chlorambucil demonstrated a monophasic disappearance from plasma following its administration, with a half-life of 28 min. Significant amounts of phenylacetic mustard were detected after 15 min, and this agent maintained high levels of active compounds in plasma throughout the study. Only low concentrations of chlorambucil and phenylacetic mustard were detected in brain between 2 and 120 min. Following equimolar chlorambucil-tertiary butyl ester administration, it rapidly disappeared from plasma, with a half-life of approximately 2 min, and maintained low plateau concentrations between 15 and 120 min after treatment. It was not detected thereafter, although significant amounts of chlorambucil and phenylacetic mustard were detected throughout the study. Significant amounts of chlorambucil-tertiary butyl ester entered and remained within the brain, achieving a peak concentration at 15 min and disappearing thereafter with a half-life of 37 min. Low levels of chlorambucil and phenylacetic mustard were also detected. Calculated from the areas under the concentration vs time curves of total active compounds derived from chlorambucil and chlorambucil-tertiary butyl ester in brain and plasma, the brain:plasma concentration integral ratios were 0.018 and 0.68, respectively. Following equimolar doses of chlorambucil and chlorambucil-tertiary butyl ester, a 7-fold greater concentration integral was achieved by chlorambucil-tertiary butyl ester in brain at a 5-fold lower plasma concentration integral. Chlorambucil-tertiary butyl ester may be of value in the treatment of brain-sequestered tumors.