P-31 NMR INVESTIGATIONS OF PHOSPHORAMIDE MUSTARD - EVALUATION OF PH CONTROL OVER THE RATE OF INTRA-MOLECULAR CYCLIZATION TO AN AZIRIDINIUM ION AND THE HYDROLYSIS OF THIS REACTIVE ALKYLATOR

The metabolic transformations shown in Scheme I for the anticancer drug cyclophosphamide (1)1 are generally viewed as an initial “activation step” (1 → 2/3) which is followed by partitioning of the primary metabolites among competing toxification (2/3 → 4/5) and reversible deactivation2 (2/3 → 6) pathways. Causal relationships between these events and the oncostatic specificity of 1 have not as yet been clearly established; however, the toxicity of phosphoramide mustard (5)3 is reasonably associated with DNA cross-linking.4, 5 Friedman6 originally rationalized the high alkylating reactivity of 5 in terms of an electronically enhanced rate of intramolecular cyclization of conjugate base 5b to form aziridinium ion 7 (Scheme I), the trapping of which has been recently reported by Colvin et al.7 The influence of pH on the alkylating activity of 5 has been recognized as an important reactivity feature;6-9 however, details in this connection have been heretofore unavailable. This lack of fundamental information concerning cyclophosphamide's ultimate alkylating metabolites prompted our investigation of 5 by high-resolution NMR, and we now report preliminary31P NMR measurements which accurately define the pH-rate profile for 5 → 7 and also reveal a striking pH dependency for subsequent hydrolytic reactions of 7. These findings bear upon numerous recent studies involving 52, 5, 7, 10-14 and provide the basis for a new perspective from which the oncostatic specificity of 1 may be considered. © 1979, American Chemical Society. All rights reserved.