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The chemical alkylating activities of four prototype phosphoraziridine antineoplastic agents were compared with their biological effects on V-79 Chinese hamster lung fibroblasts. It was found that the chemical reactivity patterns correlate well with all of the biological parameters examined in this study, i.e. cytotoxicity, DNA synthesis, and production of alkali labile strand breaks. Specifically, the 2,2-dimethylaziridine derivatives (AB-132 and AB-163) showed higher initial activities reaching a plateau after a short reaction time in all of the systems used in this study while the unsubstituted aziridine derivatives (AB-100 and D-63) reacted more slowly but continued to exert their action in a linear fashion to produce greater overall effects. These findings are consistent with the conclusion that the difference between the time-dependent biological activities of these drugs closely follows the different chemical mechanisms of their alkylating reactions (SN1 vs SN2). The more rapid action and subsequent hydrolytic inactivation of the 2,2-dimethylphosphoraziridines as effective alkylators could be the basis of their lower hemopoietic toxicity compared to conventional alkylating agents including their own C-unsubstituted aziridine analogs. The much more rapid action of the 2,2-dimethylphosphoraziridines on DNA inside the cell may have some bearing on their radiation potentiating activity, but this aspect and the cholinesterase inhibitory activity of these agents (which may depend on phosphorylation) were not investigated in the present study.

DNA polymerase alpha from calf thymus was relatively insensitive to the action of partially thiolated polycytidylic acid (MPC) which had been shown previously to be a potent inhibitor of the corresponding enzyme from regenerating rat liver, competitive with the activated DNA template. In contrast, partially thiolated polyuridylic acid (MPU) strongly inhibited the calf thymus enzyme as well, but showed non-competitive kinetics with respect to the activated DNA template. The much more potent inhibitory activity of MPU compared to MPC was attributed to the less rigid conformation of the former. Methyl substitution on the 5-mercapto groups of MPU substantially decreased but did not abolish its inhibitory activity. MPU was also a potent inhibitor of the herpes virus (HSV-1) induced DNA polymerase which, too, showed little sensitivity toward MPC; in this case, the inhibition by MPU was uncompetitive with respect to the DNA template. In preliminary experiments, MPU showed significant (61%) inhibition of the replication of HSV-1, while MPC was inactive. The results demonstrate that the inhibitory activity of partially thiolated synthetic polynucleotides toward certain DNA polymerases is dependent on the base composition.