MECHANISM AND REACTIVITY OF CHLORAMBUCIL AND CHLORAMBUCIL-SPERMIDINE CONJUGATE

The mechanism and kinetics of hydrolysis of chlorambucil and chlorambu cil-spermidine conjugate in aqueous buffered solutions have been compa red. In the absence of added chloride ion the reactions are shown to b e first-order in the nitrogen mustard and independent of the nucleophi le concentration. In the presence of high concentrations of sodium chl oride the reaction is reversible and is subject to a significant commo n-ion effect. The rates of hydrolysis of both compounds are independen t of pH in the range 8 to 3.5, and both rates begin to drop rapidly be low pH 3.5 which corresponds to the pK(a)s of the aryl amine groups. T he relative rates of alkylation of a range of nucleophiles by chloramb ucil 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 compara tively low concentrations. The rates of reaction of chlorambucil and t he chlorambucil-spermidine conjugate have been shown to be sensitive t o 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 t he conjugate. These data are all accounted for in terms of a rate limi ting formation of the aziridinium ion intermediate in each case. No ev idence for any other mechanistic pathways was found.