Studies on Mitozolomide (CCRG 81010), a new antineoplastic agent

Abstract

The rational for the synthesis of the novel 1,2 ,3 ,5-imidazotetrazinone ring system and in particular for that of mitozolomide [8-carbamoyl-3-(2-chloroethyl)imidazo [5,1-d] [1,2,3,S]tetrazin-4-(3H)-oneJ is discussed. The antitumour activity of the triazenes, the chemistry of the 1,2,3-benzotriazinones and the imidazo f 1,2 ,4]-triazinones which contributed to the hypothesis that mitozolomide might possess antitumour activity are reviewed. Various mechanisms of action are proposed. Chiefly via the production of the metabonates 2-chloroethyl isocyanate, Diazo-IC (5-diazo imidazole-4-carboxamide) and MCTIC (5-[3-(2-chloroethvl) triazen-1-yl] imidazole-4-carboxamide). No evidence of isocyanate generation by mitozolomide could be found using the inhibition of enzymes sensitive to the effect s of carbamoylation. It was concluded that 2-chloroethyl isocyanate and Diazo-IC production did not occur in intact cells. In precursor incorporation experiments mitozolomide and MCTIC were shown to have similar effects. The generation of MCTIC was further supported by the finding that [14c]-imidazo mitozolomide decomposed to produce [ 14c]-AIC, (4-aminoimidazole-5-carboxamide) which is an accepted metabonate of MCTIC. BCNU (l,3-bis(2-chloroethyl)-l-nitrosourea) a known antitumour agent produces a chloroethydiazo moiety, which is fundamental to its mode of action. If mitozolomide does qenerate MCTTC which is also a chlorodiazo species it is possible that both mitozolomide and BCNU possess a common mechanism of action. Flow cytometry of Lewis lung carcinoma cells treated in vivo and in vitro with mitozolomide revealed a marked G2/M block. Such a result would be expected of an alkylation agent. Again the effects of MCTIC were similar to those of mitozolomide. The production of 06 -chloroethylguanine adducts in DNA by mitozolomide was found to be an important feature of mitozolomide toxic ity using methyl excision repair proficient and deficient cell lines. The stability of mitozolomide and its analogues was investigated,-but no obvious correlation with antitumour activity existed. In conclusion, mitozolomide generates MCTIC and hence alkylates DNA in a manner analoqous to that of the nitrosoureas. A number of DNA lesions are presumably produced, of which the 06 -chloroethylquanine adduct may be particularly important.