Role of aldehyde dehydrogenase in the metabolism-dependent biological activity of cyclophosphamide.

It has been proposed that cytosolic aldehyde dehydrogenase, measured with acetaldehyde as the substrate, is involved in the in vitro and in vivo detoxification of activated cyclophosphamide (CP) metabolites. Higher levels of this enzyme in some normal tissues, such as liver and kidney, are believed to protect these tissues against the toxic effects of CP, whereas relatively lower levels of this enzyme in tumor tissues are believed to be responsible for the selective toxicity of CP against tumor cells. In the present studies, we investigated in detail the role of liver aldehyde dehydrogenase, measured in vitro with acetaldehyde as the substrate, in the metabolism-dependent biological ef fects of CP. Eight inbred strains of mice and two F, hybrids, with and without pretreatment with phénobarbital, were used to study hepatic microsome-mediated metabolism of [cWoroefriy7-3H]CP ([3H]CP), and [4-14C]CP ([14C]CP), and cytosolic (100,000 x g supernatant) aldehyde dehydrogenase activity. Total metabo lism (chloroform-extractable, water-soluble, and macromolecular-binding metabolites) of [3H]CP and [14C]CP was measured; analysis of the data revealed a good correlation (r = 0.8) for [3H]CP versus [14C]CP metabolism and a poor correlation (r = 0.26) for CP metabolism versus aldehyde dehydrogenase ac tivity. Wide variability in the induction by phénobarbital of aldehyde dehydrogenase activity was observed; the inducibility ratio ranged from 1 to 10. The induction of the metabolism of [3H]CP or [14C]CP in various strains ranged from 1.4 to 2.6. C3H/StHa mice were inducible 10-fold for aldehyde dehy drogenase and about 2.5-fold for CP metabolism, whereas DBA/2Ha-D mice were not inducible for aldehyde dehydrogen ase but were about 1.5-fold inducible for CP metabolism. Using these two strains as genetic models, the role of hepatic alde hyde dehydrogenase in in vivo macromolecular binding of [3H]CP, in in vivo metabolism of [3H]CP, and in chemotherapeutic activity of CP against compatible tumors in these two strains was evaluated. The interpretation of our extensive studies on these and other aspects of CP metabolism leads to the following conclusions: (a) although dehydrogenase-oxidases are in volved in the detoxification of CP, aldehyde dehydrogenase, measured with acetaldehyde as the substrate, does not appear to play a significant role in CP metabolism or in the biological effects dependent upon CP metabolism; (b) phosphoramide mustard is probably the major transport and alkylating form of CP, although transport of 4-hydroxycyclophosphamide to extrahepatic tissues followed by some intracellular decomposition to phosphoramide mustard is not entirely excluded; (c) 4-(pnitrobenzylpyridine assay is a fairly good method for measuring total metabolism of CP.