Does tirapazamine (SR-4233) have any cytotoxic or sensitizing effect on three human tumour cell lines at clinically relevant partial oxygen pressure?

Solid human tumours contain areas with low oxygen tension (pO2). For bioreductive drugs it is important to define the cytotoxic effect according to drug concentration and to clinically relevant pO2. In this study, the pO2 dependence of the survival of three human cell lines (HRT 18, Na11 +, and MEWO), exposed to tirapazamine (SR-4233) alone or combined with ionizing radiation, was studied in vitro. Gas changes were made to obtain five different oxygen concentrations: air (20.9% O2), 10, 2, 0.2 and 0.02% O2 (hypoxia). Tirapazamine below a concentration of 100 microM was not cytotoxic in air or at 10% O2. At 100 microM tirapazamine was toxic in 2% O2, and at 50 microM in 0.2% O2. For pO2 < 0.2% O2, there was a marked increase in cell killing when 10 microM tirapazamine was combined with 2 Gy, compared with either 10 microM or 2 Gy given alone (p < 0.03). The cytotoxic effect of tirapazamine on human tumour cells in vitro is highly dependent on clinically relevant pO2's. The activation of tirapazamine at a low concentration and at a pO2 found mainly in tumours could yield a very beneficial therapeutic ratio.