BIOREDUCTIVE METABOLISM OF THE NOVEL FLUORINATED 2-NITROIMIDAZOLE HYPOXIA PROBE DROXY-3,3,3-TRIFLUOROPROPYL)-2-(2-NITROIMIDAZOLYL) ACETAMIDE (SR-4554)

The aim of this work was to study the metabolic characteristics of the novel fluorinated 2-nitroimidazole hypoxia probe droxy-3,3,3-trifluor opropyl)-2-(2-nitroimidazolyl) acetamide (SR-4554). HPLC and F-19 NMR methods were employed to evaluate the rate of reductive metabolism of SR-4554 and the nature of the resulting metabolites, respectively. SR- 4554 was enzymatically reduced by mouse liver microsomes (1.1 +/- 0.1 nmol of SR-4554 reduced/min/mg protein), purified rat and human NADPH: cytochrome P450 reductase (17.8 +/- 0.4 and 5.0 +/- 0.5 nmol of SR-45 54 reduced/min/mg protein, respectively), and SCCVII tumour homogenate s (2.3 +/- 0.3 nmol of SR-4554 reduced/min/g tumour) under nitrogen. N ADPH:cytochrome P450 reductase was a major microsomal enzyme involved in the bioreduction of SR-4554 by liver microsomes. In a panel of muri ne and human tumour xenografts, cytochrome P450 reductase activities w ere found to be low and only varied by 3-fold between different tumour types, suggesting that enzyme activities within the rumours are unlik ely to influence markedly in vivo reductive metabolism. Reduction of S R-4554 by mouse liver microsomes showed a characteristic oxygen depend ence with a half-maximal inhibition of 0.48 +/- 0.06%. Thus, the reduc tive metabolism of SR-4554 can be employed to detect the low oxygen te nsions that occur within both murine and human rumours. Soluble, low m olecular weight reductive metabolites of SR-4554 were identified by F- 19 NMR. These metabolite peaks appeared (up to 0.12 ppm) downfield of the parent drug peak. In conclusion, SR-4554 undergoes an oxygen-depen dent metabolism that involves NADPH:cytochrome P450 reductase. F-19 NM R is capable of identifying reduced metabolites that are undetectable by HPLC. (C) 1997 Elsevier Science Inc.