Nucleophilic selectivity ratios of model and clinical alkylating agents by 4-(4�-nitrobenzyl) pyridine competition

SPEARS, C. P. Nucleophiic selectivity ratios of model and clinical alkylating agents by 4-(4’-nitrobenzyl)pyridine competition. Mol. Pharmacol. 19:496-504 (1981). An aqueous acetone 4-(4’-nitrobenzyl)pyridine (NBP) assay for alkylating activity was used at neutral pH to reproduce the Swain-Scott-Ogston reactivity sequence of nucleo-phile n constants, based on bimolecular competition between NBP and added nucleophiles for alkylation by nitrogen mustards, methanesulfonate esters, methyl iodide, epichloroh-ydrin, or 1,3-bis(2-chloroethyl)- 1-nitrosourea (BCNU). Nucleophile concentration-depen-dent decreases in the 540-nm NBP product(s) permitted determination of the second-order rate constant ratio, k /k0bp, for alkylation of the competing nucleophile/NBP. The slope, s , of several log(k /knbp) values versus n constants closely paralleled literature Swain-Scott s constants and could be derived from the value for thiosulfate competition, 1og(k590/kb). Alkylating activity, defined by the maximal 540-nm absorbance found for reactions up to 1 hr, was clearly related to an alkylating agent’s nucleophiic sensitivity [s, s, or log(k.0/kb) value]. Values oflog(kS,O./kflbP) corresponding to s constants 0.78-1.39 for clinical alkylating agents increased in the order 1-chloroethyl-3-(4-methylcycloh-exyl)-1-nitrosourea streptozotocin < 1,4-bis(methanesulfonoxy)butane BCNU < 1, 1’,l “-phosphinothioylidynetrisaziridine methyl-bis(2-chloroethyl)amine < 4-[bis(2-chloroethyl)amino]-L-phenylalanine. NBP competition methods for determination of nucleophiic selectivity ratios should be highly useful for rapid identification of protective nucleophile-clinical alkylating agent combinations, and for classification of alkylating agent biological mechanism of action according to recent interpretations of alkylating agent kinetic behavior