A series of analogues of the previously described compound N-[2-(2-methyl-5-nitroimidazol-1H-yl)ethyl]-4-(2-nitroimidazol- 1H-yl)butanamide (4), a novel hypoxic cell
cytotoxin and radiosensitizer, have been prepared and evaluated for
hypoxia-selective cytotoxicity and hypoxic cell radiosensitization in vitro. The new derivatives were designed to overcome the low aqueous solubility of 4 and its slow kinetics of killing under
hypoxia. The nitroheterocycle unit had a significant effect on solubility, with 3-nitrotriazoles being about 6-fold more soluble than the corresponding 2-nitroimidazoles. Analogues with a range of neutral linker chains (polyhydroxy, alkanesulfonamide, and bisamide) showed only slightly improved solubility and were unable to be fully evaluated. However, a series of analogues with cationic
amine linkers had adequate aqueous solubility (up to 280 mM). The
amine analogues could not be prepared by direct reduction of precursor
amides such as 4 and were most conveniently synthesized by aza-Wittig condensation of the appropriate
azide and
aldehyde components. The
amine-linked compounds were more cytotoxic than 4, with the symmetrical bis(2-nitroimidazole) derivatives (13 and 14) up to 9-fold more potent. They showed hypoxic selectivities comparable to that of 4 (ca. 200-fold) but had much more rapid kinetics of killing under
hypoxia, resulting in high hypoxic selectivity at early times in culture. The nature of the mechanism of cytotoxicity of these compounds remains unclear but appears not to be
DNA cross-linking, with the compounds showing a lack of
hypersensitivity toward repair-deficient UV4 cells. The enhanced solubility and
hypoxia-selective cytotoxicity (at early times) of 13 compared with 4 represent significant potential advantages.