Characterization of MGI 114 (HMAF) histiospecific toxicity in human tumor cell lines

Purpose: The acylfulvenes are a class of antitumor agents derived from the fungal toxin illudin S. One acylfulvene derivative, MGI 114 (HMAF), demonst rates marked efficacy in xenograft carcinoma models when compared to the pa rent acylfulvene or related illudin compounds. The maximum tolerated dose ( MTD) of the two analogs in animals, however, is similar. To help elucidate the basis of the increased therapeutic efficacy of MGI 114, we determined t he in vitro cytotoxicity, cellular accumulation and DNA incorporation of th is drug and compared the results with those from the parent acylfulvene ana log. Method's: The cytotoxicity of acylfulvene analogs was tested in vitro against a variety of tumor cell lines. Radiolabeled MGI 114 was used for ce llular accumulation and DNA incorporation studies. Results: MGI 114 retaine d relative histiospecific toxicity towards myeloid leukemia and various car cinoma cell lines previously noted with the parent acylfulvene compound. Ma rkedly fewer intracellular molecules of MGI 114 were required to kill human tumor cells in vitro as compared to the parent acylfulvene, indicating tha t MGI 114 was markedly more toxic on a cellular level. At equitoxic concent rations, however, the incorporation of MGI 114 into genomic tumor cell DNA was equivalent to that of acylfulvene. Analysis of cellular accumulation of MGI 114 into tumor cells revealed a lower Vmax for tumor cells, and a mark edly lower Vd for diffusion accumulation as compared to acylfulvene. Conclu sions: The addition of a single methylhydroxyl group to acylfulvene to prod uce MGI 114 results in a marked increase in cytotoxicity in vitro towards t umor cells as demonstrated by the reduction in IC50 values. There was a cor responding decrease in the number of intracellular molecules of MGI 114 req uired to kill tumor cells, but no quantitative alteration in covalent bindi ng of the drugs to DNA at equitoxic concentrations. This indicates that cel lular metabolism plays a role in the in vitro cytotoxicity of MGI 114. The equivalent incorporation into genomic DNA at equitoxic doses suggests that DNA damage produced by acylfulvene and MGI 114 is equivalent in regard to c ellular toxicity and ability to repair DNA. This increased cellular toxicit y, together with the decrease in diffusion rate, may explain the increased therapeutic efficacy of MGI 114 as compared to the parent acylfulvene analo g.