DNA-Targeted 1,2,4-Benzotriazine 1,4-Dioxides:  Potent Analogues of the Hypoxia-Selective Cytotoxin Tirapazamine

Tirapazamine (TPZ, 1,2,4-benzotriazin-3-amine 1,4-dioxide) is a bioreductive hypoxia-selective cytotoxin, currently in phase II/III clinical trials in combination with radiotherapy and with cisplatin-based chemotherapy. We have prepared a series of 1,2,4-benzotriazine 1,4-dioxide (BTO) analogues of TPZ where a DNA-targeting chromophore is attached at the 3-position via a flexible linker. DNA binding affinity was modified through variation of the chromophore or the pK a of the linker chain. The association constants (K DNA) for calf thymus DNA ranged from 1 × 102 to 5.6 × 105 M-1 (ionic strength of 0.01 M). DNA binding affinity was dependent on the presence of a positive charge, either in the linker chain or in the chromophore, and (for a series of 4-acridine carboxamide chromophore analogues) correlated strongly with linker chain pK a. The efficacy of these BTOs in killing aerobic and hypoxic mouse SCCVII tumor cells in vitro was determined by clonogenic survival. Cytotoxicity was measured as the concentration required to reduce plating efficiency to 10% of controls (C 10), and the hypoxic cytotoxicity ratio (HCR) for each BTO was calculated as C 10(aerobic)/C 10(hypoxic). BTOs bearing a positive charge showed increased hypoxic cytotoxicity (1.5−56-fold) compared to TPZ and mostly modest HCRs (8−51), but some excellent (>167 and 400) values. There was a strong correlation between K DNA and hypoxic cytotoxicity but no correlation between K DNA and HCR. Cytotoxicity in HT-29 human colon carcinoma cells, determined using IC50 assays, showed similar relationships with a correlation between K DNA and hypoxic cytotoxicity but no correlation between K DNA and HCR. In this cell line, a higher proportion of compounds (7 of 11) had HCRs greater than or equal to that of TPZ. The data confirm that DNA targeting is a useful concept for increasing potency while maintaining hypoxic selectivity and provide a direction for the further development of DNA-targeted analogues of TPZ.