Oxazaphosphorine-specific resistance in human MCF-7 breast carcinoma cell lines expressing transfected rat class 3 aldehyde dehydrogenase.

Overexpression of either class 1 or class 3 aldehyde dehydrogenase (ALDH) has been found in cell lines selected for resistance to the oxazaphosphorine (OAP) alkylating anticancer agent cyclophosphamide (CPA). Direct oxidation of the CPA metabolic intermediate aldophosphamide (ALDO) is catalyzed efficiently in vitro by the class 1 ALDH isozyme, but the involvement of the class 3 isozyme in OAP resistance is problematic since in vitro studies do not show efficient oxidation of ALDO. Cell lines were established that express stably transfected rat class 3 ALDH to model the potential role of this isozyme in OAP resistance. Clonogenic survival assay data indicated that even modest expression of rat class 3 ALDH was associated with resistance (2-4-fold) to the CPA analog mafosfamide and that the fold resistance was directly proportional to the class 3 ALDH activity expressed in clonal transfectants. Pretreatment of the highest activity cell line (3A1-31A) with 75 microM diethylaminobenzaldehyde, an ALDH substrate and inhibitor of benzaldehyde oxidation, effectively reversed the 3.8-fold resistance in this line; drug sensitivity was unaffected by diethylaminobenzaldehyde in the control transfected cell line. The resistance conferred by ALDH to mafosfamide is OAP-specific since the 3A1-31A line is also resistant to 4-hydroperoxycyclophosphamide (2.9-fold) and 4-hydroperoxyifosfamide (3.2-fold) but not to the non-oxazaphosphorine drugs phosphoramide mustard and melphalan, which cannot be detoxified by aldehyde dehydrogenase enzymes.