Mechanism and reactivity of chlorambucil and chlorambucil–spermidine conjugate

Abstract

The mechanism and kinetics of hydrolysis of chlorambucil and chlorambucil–spermidine conjugate in aqueous buffered solutions have been compared. In the absence of added chloride ion the reactions are shown to be first-order in the nitrogen mustard and independent of the nucleophile concentration. In the presence of high concentrations of sodium chloride the reaction is reversible and is subject to a significant common-ion effect. The rates of hydrolysis of both compounds are independent of pH in the range 8 to 3.5, and both rates begin to drop rapidly below pH 3.5 which corresponds to the pK_as of the aryl amine groups. The relative rates of alkylation of a range of nucleophiles by chlorambucil have been deduced from the isokinetic points, and have shown that the phosphate dianion, imidazole base and particularly thiolates are all capable of competing with water for the aziridinium ion at comparatively low concentrations. The rates of reaction of chlorambucil and the chlorambucil-spermidine conjugate have been shown to be sensitive to the medium, and, in particular, there is a large micellar inhibition of the hydrolysis of chlorambucil (60-fold reduction in rate) in the presence of hexadecyltrimethylammonium chloride that is not seen for the conjugate. These data are all accounted for in terms of a rate limiting formation of the aziridinium ion intermediate in each case. No evidence for any other mechanistic pathways was found.