Role of the 4-hydroxy intermediate in the in vitro embryotoxicity of cyclophosphamide and dechlorocyclophosphamide.

Cyclophosphamide must be metabolically activated to produce malformations in cultured rat embryos. A 4-hydroxylated intermediate, 4-hydroxycyclophosphamide is initially formed during this activation. While 4-hydroxycyclophosphamide (and/or its open-ring tautomer, aldophosphamide) is believed to act as a transport form in mediating the antineoplastic activity of cyclophosphamide, its role in the teratogenicity of this drug is not known. In this study the effects of two "preactivated" cyclophosphamide analogs on cultured Day 10 rat embryos were determined. The first analog, 4-hydroperoxycyclophosphamide, is converted to 4-hydroxycyclophosphamide in aqueous solutions, releasing both acrolein and phosphoramide mustard, while the second, 4-hydroperoxydechlorocyclophosphamide, releases, in a similar manner, acrolein and the inactive metabolite, phosphoric acid diamide. Both cyclophosphamide analogs were teratogenic, embryolethal, and growth retarding in vitro, but the effective concentrations and the types of malformations produced were different. 4-Hydroperoxycyclophosphamide produced embryo deaths and malformations and decreases in embryonic growth and protein content at concentrations in the range of 5 to 25 microM. In contrast, 4-hydroperoxydechlorocyclophosphamide did not produce embryo deaths at concentrations below 100 microM and produced embryo malformations and growth retardation only at 125 microM. The concentration-response curve and the spectrum of malformations produced by 4-hydroperoxycyclophosphamide resembled those previously reported for phosphoramide mustard, while the concentration-response curve and types of malformations produced by 4-hydroperoxydechlorocyclophosphamide more closely resembled those observed with acrolein. Thus, the 4-hydroxy intermediates are similar as teratogens to the most potent of the metabolites which they produce; the 4-hydroxy compounds may serve as a transport form of cyclophosphamide but do not appear themselves to have a major role in teratogenicity.