Abstracts Biological Sciences

Metabolism of cyclophosphamide: implications for hematopoietic stem cell transplantation

by Song Ren

The overall goal of the project initiated by the work described in this dissertation is to develop a strategy by which dosing of cyclophosphamide can be modified in bone marrow transplantation patients to diminish toxicity while not compromising the success of the procedure. In order to accomplish this, it is necessary to understand the relationship between the exposure to the active metabolites formed from cyclophosphamide and the outcome of bone marrow transplantation and to understand the factors that govern exposure to the important metabolites.This dissertation reports on 53 patients undergoing bone marrow transplantation in whom the relationship between cyclophosphamide metabolism and pharmacokinetics and the outcome was sought. We found that patients who had greater exposure to two of the presumed detoxification metabolites of the active metabolite 4-hydroxycyclophosphamide also had a higher incidence of severe liver toxicity. These two metabolites could be viewed as surrogate markers for the exposure to 4-hydroxycyclophosphmide. 4-Hydroxycyclophosphamide is the metabolite that delivers phosphoramide mustard into cells, where it decomposes to form phosphoramide mustard, which cross-links DNA resulting in cell death. The factors governing exposure to the key metabolites of cyclophosphamide were sought through studies in subcellular preparations of human liver and in patients undergoing bone marrow transplantation. In these studies it was found that approximately 70% of a 60 mg/kg dose of cyclophosphamide is oxidized by cytochrome P450 2C and 3A to 4-hydroxycyclophosphamide. It was discovered that cyclophosphamide induces its own conversion to 4-hydroxycyclophsophamide, However, it was also found that acrolein, a product formed in parallel with phosphoramide mustard from 4-hydroxycyclophosphamide, inhibits the detoxification of 4-hydroxycyclophosphamide by inhibiting aldehyde dehydrogenase-1.At the outset of these studies, it was hypothesized that a polymorphism in aldehyde dehydrogenase-1 may be the most important single determinant of exposure to the cytotoxic metabolites of cyclophosphamide. The discovery that this enzyme is significantly inhibited by acrolein diminishes its importance in determining the exposure to cytotoxic metabolites. Thus, aldehyde dehydrogenase-1 genotype or phenotype is unlikely to aid appropriate dose selection. The work described herein suggests that the most effective intervention strategy is likely to be based on the actual patient-specific pharmacokinetic observations.