INTRINSIC CELLULAR-RESISTANCE TO OXAZAPHOSPHORINES EXHIBITED BY A HUMAN COLON-CARCINOMA CELL-LINE EXPRESSING RELATIVELY LARGE AMOUNTS OF A CLASS-3 ALDEHYDE DEHYDROGENASE

A cultured human colon carcinoma cell line, viz. colon C, exhibiting i ntrinsic cellular resistance to mafosfamide mediated by relatively ele vated levels of a cytosolic class-3 aldehyde dehydrogenase was identif ied. Colon C cells were found to be much less sensitive/more resistant (about 10-fold as judged by LC(90) values) to mafosfamide than were t wo other cultured human colon carcinoma cell lines, viz. RCA and HCT 1 16b, and, as compared to the barely detectable aldehyde dehydrogenase activity (NADP-dependent enzyme-catalyzed oxidation of benzaldehyde to benzoic acid) in RCA and HCT 116b cells, that in colon C cells was ab out 200-fold greater. The three cell lines were equisensitive to phosp horamide mustard. Aldehyde dehydrogenase activity was confined to the cytosol in colon C cells (as well as in the other two cell lines) and, on the basis of its physical, immunological and catalytic characteris tics, the operative enzyme was judged to be a Type-1 ALDH-3 identical to the Type-1 ALDH-3 expressed in methylcholanthrene-treate d human br east adenocarcinoma MCF-7/0 cells and very nearly identical to the Typ e-1 ALDH-3 expressed in human normal stomach mucosa. Class-1 and class -2 aldehyde dehydrogenases were not found in these cells. The relative insensitivity to mafosfamide on the part of colon C cells was not obs erved when exposure to mafosfamide was in the presence of benzaldehyde or 4-(diethylamino)benzaldehyde, each a relatively good substrate for ALDH-3, whereas it was retained when exposure to mafosfamide was in t he presence of acetaldehyde, a relatively poor substrate for this enzy me. Sensitivity to mafosfamide on the part of HCT 116b and RCA cells, and to phosphoramide mustard on the part of all three cell lines, was unaffected when drug exposure was in the presence of any of the three aldehydes. Together with earlier reports from our laboratory, these ob servations demonstrate that intrinsic, as well as stable and transient acquired, resistance to oxazaphosphorines, such as mafosfamide and cy clophosphamide, can be mediated by relatively increased levels of cyto solic class-3 aldehyde dehydrogenases.