Activation mechanisms of mafosfamide and the role of thiols in cyclophosphamide metabolism.

cis-Mafosfamide (cis-5) (ASTA Z7557), a stable analogue of cis-4-hydroxycyclophosphamide (cis-2), undergoes rapid decomposition in aqueous phosphate buffer or plasma at pH 7.4 and 37 degrees C. The reaction kinetics of cis-5 are complex, and trans-mafosfamide (trans-5) and cis-2 are produced and subsequently disappear over the course of the reaction. The rates of decomposition of cis-5 as well as cis-2 were much faster in plasma than in buffer. The cis-trans isomerization of cis-5 occurred by a specific-base-catalyzed process via iminocyclophosphamide (8) as a transient intermediate. In contrast, formation of cis- and trans-mafosfamide (5) from cis-2 and MESNA (sodium 2-mercaptoethanesulfonate) proceeded by an acid-catalyzed process via the hemithioacetal intermediate (6). The significance of these findings with respect to cyclophosphamide metabolism is discussed.