Aldehyde dehydrogenase-mediated cellular relative insensitivity to the oxazaphosphorines

As judged by findings in preclinical models, determinants of cellular sensitivity to cyclophosphamide and other oxazaphosphorines include two cytosolic aldehyde dehydrogenases, viz., ALDH1A1 and ALDH3A1. Each catalyzes the detoxification of the oxazaphosphorines; thus, cellular sensitivity to these agents decreases as cellular levels of ALDH1A1 and/or ALDH3A1 increase. Of particular clinical relevance may be that stable sublines, relatively insensitive to the oxazaphosphorines due to elevated ALDH1A1 or ALDH3A1 levels, emerged when cultured human tumor cells were exposed only once to a high concentration of one of these agents for 30 to 60 minutes. Whether differences in cellular levels of either enzyme accounts for the clinically-encountered uneven therapeutic effectiveness of the oxazaphosphorines remains to be determined. However, it has already been established that measurable levels of these enzymes are found in some, but not all, tumor types, and that in those tumor types where measurable levels are present, e.g., infiltrating ductal carcinomas of the breast, they vary widely from patient to patient. Potentially useful clinical strategies that might be pursued if it turns out that ALDH1A1 and/or ALDH3A1 are, indeed, clinically operative determinants of cellular sensitivity to the oxazaphosphorines include 1) individualizing cancer chemotherapeutic regimens based, at least in part, on the levels of these enzymes in the malignancy of interest, and 2) sensitizing tumor cells that express relatively large amounts of ALDH1A1 and/or ALDH3A1 to the oxazaphosphorines by preventing the synthesis of these enzymes, e.g., with antisense RNA, or by introducing an agent that directly inhibits the catalytic action of the operative enzyme. Further, the fact that ALDH1A1 and ALDH3A1 are determinants of cellular sensitivity to the oxazaphosphorines provides the rationale for the investigation of two additional strategies with clinical potential, viz., decreasing the sensitivity of vulnerable and essential normal cells, e.g., pluripotent hematopoietic cells, to the oxazaphosphorines by selectively transferring into them the genetic information that encodes 1) ALDH1A1 or ALDH3A1, or 2) a signaling factor, the presence of which would directly or indirectly, stably upregulate the expression of these enzymes.