Chemical mechanism of the radiation potentiating effects of 2,2-dimethylaziridine-type antitumor agents

Comparative chemical and biological studies of four prototype phosphoraziridine antineoplastic agents

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.

A pilot study with ethyl bis (2,2-dimethyl-1-aziridinyl) phosphinate (AB-163) and radiation therapy

Ethyl bis (2,2 dimethyl-1-aziridinyl) phosphinate (AB-163), a TEPA analogue, was used with radiation therapy in treating 18 patients with advanced malignancies. There were 12 patients with esophageal carcinoma; 3 with adenocarcinomas of the gastrointestinal tract; one, squamous carcinoma of the cervix; and one, adenocarcinoma of the ovary. One hundred mg/M2 AB-163 was given by rapid i.v. drip one half-hour before conventional radiation therapy. The majority of patients received 10 combined treatments. Three of those with squamous cell carcinomas (two in the esophagus and one in the cervix) remained disease-free for more than 2 years. One with liver metastasis and unresectable carcinoma of the stomach survived for 9 months. The drug causes side effects mainly involving the central nervous system and gastrointestinal tract. Drug-related myelosuppression has not been observed. The mode of action is speculated to be a result of active intermediate hydrolysis products which appear capable of phosphorylating X ray induced DNA strand damage. However, much additional investigation is required, both in vitro and clinically, before its efficacy and safety can be demonstrated.