Inhibition of carboxyethylphosphoramide mustard formation from 4-hydroxycyclophosphamide by carmustine

It has been reported that the toxicity of carmustine (BCNU)/cyclophosphamide (CY)/etoposide regimen (when BCNU is split into 4 doses) is less than that of BCNU/CY/cisplatin regimen (when the same amount of BCNU is administered as a single dose). We hypothesized that this might in part be due to the inhibition of aldehyde dehydrogenase 1 (ALDH1) by BCNU or its degradation product, 2-chloroethyl isocyanate, which is likely to be more pronounced at the higher BCNU dose. The effects of BCNU and 2-chloroethyl isocyanate on the formation of carboxyethylphosphoramide mustard (CEPM) from 4-hydroxycyclophosphamide (HCY) was evaluated in human liver cytosol incubations. We found that CEPM formation from HCY was inhibited strongly by BCNU and weakly by 2-chloroethyl isocyanate. The mechanism of inhibition of ALDH1 activity by BCNU was elucidated using indole-3-acetaldehyde (IAL) as the probe substrate in ALDH1 prepared from human erythrocytes. BCNU was a competitive inhibitor of ALDH1 activity with a K(i) of 1.95 microM. The inhibition was independent of preincubation time and reversible by dialysis. The calculated %inhibition of ALDH1 activity by acrolein and BCNU in patients receiving BCNU in 4 split doses with CY was 81%, and it increased to 92% in single dose BCNU regimen. Thus, the calculation indicates that residual operating ALDH1 activity is halved in the presence of single-dose BCNU compared to split-dose BCNU. The inhibition of ALDH1 may contribute to the observed lower incidence of toxicity when BCNU was split into 4 doses compared with single dose and coadministered with CY although dose-dependent effects of BCNU on glutathione and glutathione reductase are also likely to contribute.