Mutagenicity of the antitumor antibiotic CC-1065 and its analogues in mammalian (V79) cells and bacteria.

CC-1065 is a very potent antitumor antibiotic which binds in the minor groove of DNA with alkylation at N-3 of adenine. Since CC-1065 caused delayed deaths in mice at therapeutic doses, analogues were prepared whose antitumor and biochemical activities have been reported. In this study, the mutagenicity for V79 cells (6-thioguanine resistance) and Salmonella (histidine auxotrophy or azaguanine resistance) of selected analogues was compared to DNA-binding activity and the structure-activity relationship was determined. CC-1065, U-62,736, U-66,866, U-66,694, U-67,786, and U-68,415 all have an A segment with an intact cyclopropyl group and different B segments. The cyclopropyl group is absent from U-66,226 and U-63,360. Elimination of the cyclopropyl ring diminished the cytotoxic and mutagenic potency of the compounds such that U-63,360 was nearly three orders of magnitude less potent than CC-1065 in V79 cells. For the compounds with an intact cyclopropyl group, the order of cytotoxic and mutagenic potency (molar basis) in V79 cells generally correlated with binding to calf thymus DNA, and increased with the length of the B segment. Thus, the order of cytotoxicity was CC-1065 greater than U-68,415 greater than U-66,694 greater than U-66,866 greater than U-62,736. U-67,786 fell outside this pattern since it was more cytotoxic and mutagenic than U-66,694, although it was of a similar size and had similar DNA-binding activity. These results show that an electrophilic carbon afforded by an intact cyclopropyl group of this type is necessary but not sufficient to account for the high cytotoxic and mutagenic potency of CC-1065 and U-68,415. The size and characteristics of the B segment also affect the potency. At an equitoxic (10 or 50% lethal dose) dose, an inverse relationship exists between cytotoxic and mutagenic potency such that at the 50% lethal dose, the least cytotoxic compound (U-62,736) was more mutagenic than the most cytotoxic compound (CC-1065). We speculate that the more cytotoxic analogues are less mutagenic (at an equitoxic dose) because they may have greater structure-directed binding to less mutable DNA sites in the minor groove.