4-oxoCyclophosphamide is an inactive metabolite of the alkylating agent cyclophosphamide .1,2It is formed from cyclophosphamide through a 4-hydroxycyclophosphamide intermediate by aldehyde dehydrogenases.2 1.Struck, R.F., Kirk, M.C., Mellett, L.B., et al.Urinary metabolites of the antitumor agent cyclophosphamideMol. Pharmacol.7(5)519-529(1971) 2.de Jonge, M.E., Huitema, A.D.R., Rodenhuis, S., et al.Clinical pharmacokinetics of cyclophosphamideClin. Pharmacokinet.44(11)1135-1164(2005)