Development of lipophilic anticancer agents for the treatment of brain tumors by the esterification of water-soluble chlorambucil.

The lipophilic derivatives of the anticancer alkylating agent chlorambucil, chlorambucil-methyl, -isopropyl and -tertiary butyl esters, were synthesized and administered i.v. to anesthetized rats. Plasma and brain concentrations of these agents and of their active metabolites, chlorambucil and phenylacetic mustard, then were determined by high-performance liquid chromatography between 5 and 60 min. Whereas large amounts of chlorambucil-tertiary butyl ester entered and were maintained in brain, lower amounts of chlorambucil-isopropyl ester and no chlorambucil-methyl ester were found in brain. The comparative brain/plasma concentration-time integral ratios of the total active agents generated from chlorambucil-tertiary butyl, -isopropyl and -methyl esters were 0.85, 0.12 and 0.06, respectively, compared to a ratio of 0.02 for chlorambucil. In vitro alkylating activity of each ester was compared to that of equimolar chlorambucil, by reaction with 4-(p-nitrobenzyl)pyridine. Each ester possessed high intrinsic alkylating activity, equal to 38.4, 57.0 and 69.9% of chlorambucil activity, for the -tertiary butyl, -isopropyl and -methyl esters, respectively. Therefore each is an active antineoplastic agent irrespective of whether or not chlorambucil is regenerated. The rates of ester hydrolysis of these derivatives to chlorambucil were measured in fresh rat blood and in liver and brain homogenates at 37 degrees C. Chlorambucil-methyl and -isopropyl esters were hydrolysed quickly within 30 s in blood and liver, whereas chlorambucil-tertiary butyl ester was more stable with half-lives of approximately 7 h and 2 h, respectively. All proved to be relatively stable in brain homogenate. Steric hindrance around the ester linkage of chlorambucil-tertiary butyl ester reduces its affinity to and rate of hydrolysis by plasma and liver esterases, and allows it to accumulate within the brain. Chlorambucil-tertiary butyl ester maintains high levels in brain despite rapidly declining plasma concentrations and, due to these favorable pharmacokinetics and to its intrinsic anticancer activity, it possess promising characteristics for the treatment of malignant brain tumors.

Brain and plasma pharmacokinetics and anticancer activities of cyclophosphamide and phosphoramide mustard in the rat.

By a sensitive and quantitative fluorometric assay, brain and plasma time-dependent concentration profiles were generated for phosphoramide mustard (PM) and active alkylating metabolites derived from cyclophosphamide (CPA) administration to rats. Whereas PM rapidly disappeared from plasma, with a monophasic half-life of 15.1 min, equimolar administration of CPA generated active metabolites in plasma that disappeared monoexponentially, with a composite half-life of 63 min. As a consequence, the time-dependent concentration integral of active alkylating metabolites derived from CPA administration, calculated between 5 min and infinity, was 3-fold that of PM. Pharmacokinetic parameters were calculated for each compound. The brain/plasma concentration-integral ratios of PM and active alkylating metabolites derived from CPA were 0.18 and 0.20, respectively. The cerebrovascular permeability-surface area product of PM was 7.5 x 10(-5) s-1, which is similar to that of other water-soluble anticancer agents that are restricted from entering the brain. The activities of a range of daily doses of PM and CPA were assessed against subcutaneous and intracerebral implants of Walker 256 carcinosarcoma tumor in rats. Inhibition of subcutaneous tumor growth by 50% was caused by CPA and PM doses of 6.6 and 12.0 mg/kg (daily for 5 consecutive days, starting 36 h after tumor implantation), respectively. However, administration of daily doses of up to 40 mg/kg did not significantly increase the survival of animals with intracerebral tumor implants. These studies indicate that active metabolites of CPA are restricted from entering the brain and that only subtherapeutic concentrations are achieved in brain tissue after systemic administration of CPA or PM.