Mutagenicity evaluation of amino-oxazoline derivatives using in vitro and in vivo short-term tests

During routine investigation of potential drugs it was found that 1-methyl-4-(2-oxazoline-2-ylamino)-indazole (1) compound 1 was mutagenic for Salmonella typhimurium TA1535 and TA100. The genotoxicity of compound 1 was confirmed by the following in vitro test systems: V79 Chinese hamster cells (HGPRT-locus), Saccharomyces cerevisiae D7 (mitotic gene conversion, mutations, aberrations), and Escherichia coli (rec-assay). On the other hand, when compound 1 was tested in vivo (micronucleus test in mice and sister chromatid exchange in Chinese hamsters) it did not show evidence of genotoxic activity. In order to study structure/activity relationships, different analogues of compound 1 were tested in Salmonella typhimurium TA1535. The tests showed that (1) most 2-amino-oxazolines were mutagenic for the test organism; (2) the 2-amino-imidazoline and 2-amino-thiazolidine derivatives tested were not mutagenic; (3) substituents bound to the extranuclear nitrogen of the 2-aminooxazoline ring had only a weak influence on the mutagenic potential; and (4) methylation of the oxazoline ring, most conspicuously at the carbon in position 5, strongly reduced the mutagenic effect. From these observations it is concluded that the reaction of nucleophilic DNA sites with the most electrophilic site of the oxazoline ring, ie, the carbon in position 5, is responsible for the genotoxicity of amino-oxazoline compounds. Due to the genotoxicity observed in these in vitro tests this class of compounds was no longer developed, but dropped, even without performing a long-term carcinogenicity study or considering the negative in vivo findings.