CONVERSION OF 4-HYDROPEROXYCYCLOPHOSPHAMIDE AND 4-HYDROXYCYCLOPHOSPHAMIDE TO PHOSPHORAMIDE MUSTARD AND ACROLEIN MEDIATED BY BIFUNCTIONAL CATALYSTS

  • 1 January 1982
    • journal article
    • research article
    • Vol. 42  (3) , 830-837
Abstract
The rates at which 4-hydroperoxycyclophosphamide and 4-hydroxycyclophosphamide are converted to phosphoramide mustard and acrolein were determined as a function of buffer composition, buffer concentration and pH. Conversion of 4-hydroperoxycyclophosphamide to 4-hydroxycyclophosphamide in 0.5 M Tris buffer, pH 7.4, 37.degree., was 1st-order (k = 0.016 min-1), but subsequent conversion of 4-hydroxycyclophosphamide to phosphoramide mustard and acrolein under these conditions was negligible. Phosphoramide mustard and acrolein were readily generated from 4-hydroxycyclophosphamide when either of these agents was placed in phosphate buffer. Conversion of 4-hydroxycyclophosphamide to phosphoramide mustard and acrolein was 1st-order with respect to 4-hydroxycyclophosphamide (k = 0.126 min-1 in 0.5 M phosphate buffer, pH 8, 37.degree.) as well as 1st-order with respect to phosphate serving as a catalyst. The rate-determining step in the reaction was pH-dependent only insofar as the H+ concentration governed the relative amounts of monobasic and dibasic phosphate present. Pseudo-1st-order rate constants were 0.045 M-1 min-1 for monobasic phosphate and 0.256 M-1 min-1 for dibasic phosphate. The role of phosphate in this reaction was as that of a bifunctional catalyst. The reaction was not subject to specific or general, acid or base, catalysis. Other bifunctional catalysts such as G-6-P and bicarbonate also catalyzed the reaction, although less efficiently. Aldophosphamide apparently exists only transiently; its presence could not be established by 31PNMR spectroscopy. In the reaction sequence 4-hydroxycyclophosphamide .fwdarw. aldophosphoramide .fwdarw. phosphoramide mustard + acrolein, the conversion of 4-hydroxycyclophosphamide to aldophosphamide is rate-limiting and is subject to bifunctional catalysis; this reaction can proceed efficiently only in the presence of a bifunctional catalyst. Assuming that the oncotoxic specificity of cyclophosphamide resides with 4-hydroxycyclophosphamide and that its cytotoxic effect at therapeutic doses is largely mediated by phosphoramide mustard released within cells, these observations offer the possibility that the intracellular concentration of bifunctional catalysts, whether in the form of inorganic phosphates, organic phosphates, enzymes or other species, serve as important determinants with regard to the oncotoxic potential and specificity of cyclophosphamide. Similarly, the concentration of bifunctional catalysts in the urine as well as the pH of the urine may be important with regard to the potential of cyclophosphamide [an important antitumor prodrug] to induce, via acrolein, hemorrhagic cystitis.