Structure-activity relationships of 1,2,4-benzotriazine 1,4-dioxides as hypoxia-selective analogues of tirapazamine

Tirapazamine (TPZ, 1,2,4-benzotriazin-3-amine 1,4-dioxide) is a bioreductive hypoxic cytotoxin currently in Phase II/III clinical trials in combination with radiotherapy and with cisplatin-based chemotherapy. As part of a program to develop TPZ analogues with improved solubility/potency and therapeutic indices, we synthesized 34 1,2,4-benzotriazin-3-amine 1,4-dioxides (BTO) to examine structure-activity relationships (SAR) for ring substitution. The electronic, hydrophobic, and steric parameters of substituents at the 5-, 6-, 7-, and 8-positions were systematically varied, and the aqueous solubility and one-electron reduction potentials [E(1)] of the analogues were determined. For each compound, we determined cell killing of mouse SCCVII tumor cells in vitro under aerobic and hypoxic conditions by clonogenic survival and determined their relative hypoxic toxicity (RHT; relative to TPZ) and hypoxic cytotoxicity ratio (HCR). A subset of compounds was independently evaluated using a 96-well SRB proliferation assay, the data from which correlated well with that derived by the clonogenic endpoint. Most substituents, except 5- and 8-dimethylamino and 8-diethylamino, gave analogues less soluble than TPZ. EM values ranged from -240 mV through -670 mV (with TPZ having a value of -456 mV) and correlated well with the electronic parameter sigma for substituents at the 5-, 6-, 7-, and 8-positions. Aerobic cytotoxic potency showed a strong positive correlation with E(1) (i.e., electron-withdrawing substituents increased aerobic toxicity). Hypoxic cytotoxicity also generally increased with increasing E(1), with a maximum (RHT up to 3.9-fold) seen in halo- and trifluoromethyl-substituted BTO derivatives having E(1) between ca. -370 to -400 mV. Analogues with high HCRs (>50) all had E(1)s in the range -450 to -510 mV (weakly electron-donating substituents) with the exception of the 8-CF3 analogue, which had an HCR of 112 against SCCVII despite a high E(1) of -372 mV). The results suggest that ring-A substituents in BTO analogues can be used to predictably vary one-electron reduction potentials and also provide a much better definition than previously of the optimum range of these reduction potentials for a desirable biological activity profile (high HCR, RHT, and solubility).