The labelling of EMT-6 tumours in BALB/C mice with 14C-misonidazole.

14C-misonidazole was injected IP into BALB/C mice bearing EMT-6 tumours. Tumour and normal tissue levels of radioactivity were determined by liquid scintillation procedures for times up to 72 h after drug administration. At times longer than 8 h, levels of 14C in tumour were 2-6 times higher than 14C levels measured in other normal tissues, with the exception of liver. The levels of 14C from misonidazole retained in liver tissue were comparable to those retained in the tumours. The clearance of 14C-misonidazole from the tumours and all normal tissues could be characterised by two distinct phases: a rapid phase (0.7 h half-life) followed by a much slower phase (approximately 50 h half-life). The distribution of retained 14C from labelled misonidazole within tumour-bearing mice was also measured by whole animal autoradiographic techniques. This procedure confirmed the biodistribution of 14C-misonidazole determined by liquid scintillation procedures. To enhance binding to hypoxic cells in vivo, 14C-misonidazole was administered in multiple doses over a three-hour period, simulating a prolonged exposure to the drug. The 14C from labelled misonidazole retained in tumour tissue was 4-15 times greater than that retained in normal tissues, liver tissue being again an exception. Myocardial ischaemia was induced by isoproterenol treatment and a two-fold increase of 14C from labelled misonidazole was found in the heart tissue of mice treated by the drug compared to controls. The increased binding of 14C-misonidazole to tumour cells after prolonged exposure to the drug and the increased binding of 14C-misonidazole to ischaemic heart tissue suggests that hypoxia is a factor in sensitiser adduct formation in vivo.