Effects of tirapazamine on experimental colorectal liver metastases after radiofrequency ablation.

Radiofrequency ablation (RFA) is a common procedure for the management of colorectal liver metastases. RFA-generated lesions are surrounded by a rim of hypoxia that is associated with aggressive outgrowth of intrahepatic micrometastases. Hypoxia-activated prodrugs such as tirapazamine are designed selectively to induce apoptosis in tumour cells under hypoxic conditions. Therefore, it was hypothesized that tirapazamine may have therapeutic value in limiting hypoxia-associated tumour outgrowth following RFA. Murine C26 and MC38 colorectal cancer cells were grown under hypoxia and normal oxygenation in vitro, and treated with different concentrations of tirapazamine. Apoptosis and cell cycle distribution were assessed by western blot and fluorescence-activated cell sorting analysis. Proliferative capacity was tested by means of colony-formation assays. Mice harbouring microscopic colorectal liver metastases were treated with RFA, followed by a single injection of tirapazamine (60 mg/kg) or saline. Tumour load was assessed morphometrically 7 days later. Tirapazamine induced apoptosis of colorectal tumour cells under hypoxia in vitro. Under normal oxygenation, tirapazamine caused a G2 cell cycle arrest from which cells recovered partly. This reduced, but did not abolish, colony-forming capacity. A single dose of tirapazamine largely prevented accelerated outgrowth of hypoxic micrometastases following RFA. Tirapazamine administration was associated with minimal toxicity. Tirapazamine induced apoptosis in colorectal cancer cells in a hypoxia-dependent manner and potently suppressed hypoxia-associated outgrowth of liver metastases with limited toxicity. This warrants further study to assess the potential value of tirapazamine, or other hypoxia-activated prodrugs, as adjuvant therapeutics following RFA treatment of colorectal liver metastases.