Physicochemical and pharmacokinetic parameters of seven lipophilic chlorambucil esters designed for brain penetration

This report describes the physicochemical and pharmacokinetic parameters of seven chlorambucil esters, which were compared with those of chlorambucil. These esters were designed as chlorambucil prodrugs to increase the brain penetration and concentration vs time profile of chlorambucil within the CNS for potential treatment of brain tumors. They include four aliphatic esters from one to eight carbon chains in length (chlorambucil-methyl, -propyl, -hexyl, and -octyl esters) and three aromatic esters, including the phenylmethyl, phenylethyl and prednisolone ester of chlorambucil, prednimustine. The esters were lipophilic and possessed log octanol:water partition coefficients (log P values) that ranged from 4.05 to greater than 8.0. All retained alkylating activity, which was reduced compared with that of chlorambucil. In addition, all were metabolized in vivo in the rat to yield chlorambucil alone. Measurement of the in vitro rate of ester hydrolysis of the compounds to yield chlorambucil in rat plasma demonstrated that short-chain aliphatic and aromatic chlorambucil esters were rapidly broken down to their parent compound. The plasma half-lives of the compounds increased with the increasing length and complexity of their ester chain. This may have been related to an increase in the binding of the long-chain esters to plasma proteins, protecting the ester from nonspecific plasma esterases, and to a reduced affinity of plasma esterases to these esters. Pharmacokinetic analysis of chlorambucil-hexyl, -octyl, and -prednisolone esters by HPLC demonstrated that following their intravenous administration in the rat (in doses equivalent to equimolar chlorambucil, 10 mg/kg), they yielded only low concentrations of active compounds in plasma and brain. The brain:plasma ratio of these was low and similar to that of chlorambucil, and no ester demonstrated anticancer activity superior to that obtained after the administration of equimolar chlorambucil (5 mg/kg i.v., days 1-5) against brain-sequestered Walker 256 carcinosarcoma in the rat.