Cyclophosphamide is a cytostatic agent from the group of oxazaphosphorines, chemically related to nitrogen mustard. Cyclophosphamide is inactive in vitro and is activated in vivo mainly by microsomal enzymes in the liver to 4-hydroxycyclophosphamide, which is in equilibrium with its tautomer aldophosphamide. These tautomers undergo partly spontaneous, partly enzymatic conversion to inactive and active metabolites (especially phosphoramide mustard and acrolein).