Enzymatic detoxification of phosphoramide mustard by soluble fractions from rat organ tissues

Enzymatic degradation of phosphoramide mustard (PM), the ultimate cytotoxic metabolite of cyclophosphamide (CP), by the cleavage of the phosphorous-nitrogen (P-N) bond was investigated in vitro using 65,000g-soluble fractions from rat organ tissues. In the presence of physiologic bicarbonate in vivo, the P-N bond cleavage of PM was previously found to form 3-(2-chloroethyl)-1,3-oxazolidin-2-one (CNM), which is devoid of antitumor activity. This product was thus quantitated in the present studies using GC/MS and a deuterium-labeled analog (CNM-d8) as the internal standard. Under the experimental conditions, CNM was found to be enzymatically produced from PM in soluble fractions of rat liver, kidney, spleen, and intestine. Mean KM and Vmax values in soluble fractions of rat liver at pH 6.7 and 37 degrees C were determined to be 1.65 +/- 0.536 mM (N = 7) and 6.38 +/- 1.37 microM/min (N = 6), respectively. The enzymic activity of the rat liver soluble fraction was significantly reduced following boiling at 100 degrees C for 5 min. No CNM production was detected from PM incubated in plasma. The P-N bond cleavage for CP and for two other metabolites, 4-ketocyclophosphamide and alcophosphamide, was also investigated using soluble fractions from rat liver similar to that for PM. None of these compounds has been found to form CNM, however, indicating enzyme specificity for the P-N bond in PM. This enzyme probably resembles the previously described phosphamidases.(ABSTRACT TRUNCATED AT 250 WORDS)