The effect of tirapazamine (SR-4233) alone or combined with chemotherapeutic agents on xenografted human tumours.

Recent data have shown that the in vitro and in vivo cytotoxicity of bioreductive drugs could be significantly increased when combined with chemotherapy drugs such as cisplatinum, depending on the timing of administration. The aim of this study was to define the toxicity (animal lethality) and the activity (growth delay assay, excision assay) of a bioreductive drug, tirapazamine, alone and combined with chemotherapy agents (5-FU, VP16, bleo, DTIC and c-DDP) on nude mice bearing xenografted human tumours: a rectal carcinoma (HRT18) and a melanoma (Na11+). Animal lethality was markedly increased when tirapazamine at the lethal dose 10% was combined with the other drugs. For the HRT18 tumour the combination of tirapazamine and bleomycin significantly increased the delay of regrowth compared with bleomycin alone (P = 0.04) and was more cytotoxic than tirapazamine alone (P = 0.04). For the Na11+ tumours the combination of tirapazamine with VP16 significantly increased tumour doubling time compared with the controls (P = 0.001) or VP16 alone. The combination of tirapazamine and VP16 was more cytotoxic than VP16 alone (P = 0.0001). When compared with c-DDP or tirapazamine alone, there was a significant decrease in plating efficiency when tirapazamine and c-DDP were given at the same time (P = 0.04), but not when tirapazamine was given 3 h before c-DDP. In conclusion, tirapazamine was shown to be cytotoxic against clonogenic human tumour cells. Its efficacy in vivo may depend on its combination with already active chemotherapy drugs on the tumour model used. The timing of administration may be less important than previously thought.