The primary metabolite of cyclophosphamide (CP, 1), i.e. 4-hydroxy-CP 2, has high pharmacological activity, but it is a very unstable compound. Chemical approaches to the stabilization involved in the substitution of the hydroxy group at the C 4-position, especially by a sulfoalkylthio-moiety. Within this new class of compounds ASTA Z 7557 (2-(bis-(2-chloroethyl]-amino-cis-4-[2-sulfoethyl)-thio)-tetrah ydr o-2H-1,3, 2-oxaza-phosphorine-r-2-oxide cyclohexylamine salt, i.e. cis-4-sulfoethylthio-CP, cis-13) was chosen for further evaluation. Cis-13 was synthesized by condensation of compound 2 and 2-mercapto-ethanesulfonic acid cyclohexylamine salt 14 in aqueous acetone yielding the cis-isomer with high stereoselectivity. It is a white crystalline powder, m.p. 126-134 degrees C, stable at room temperature, with a solubility of 16% in water. The stereochemistry was confirmed by NMR-data and X-ray diffraction. In 0.07 M phosphate buffer at pH 7 and 37 degrees C cis-13 isomerizes to the epimer trans-13, equilibrating at a cis-trans-ratio of 59 to 41 within less than 5 minutes. Simultaneously a rapid initial hydrolysis occurs to 2 and 14 followed by a time period with lower degradation due to the decomposition of 2. The rate of release of 2 increases with decreasing concentration and especially by addition of an oxidant. It could be retarded by addition of the corresponding thiol mesna, sodium 2-mercapto-ethanesulfonate 15, or of another thiol.(ABSTRACT TRUNCATED AT 250 WORDS)