Doxorubicin is a useful
antineoplastic drug with multiple mechanisms of cytotoxicity. One such mechanism involves the reductive bioactivation of the
quinone ring to a semiquinone radical, which can exert direct toxic effects and/or undergo redox cycling. We hypothesized that human
NADPH-cytochrome p450 reductase (CYPRED) catalyzes
doxorubicin reduction and that overexpression of this
enzyme sensitizes human
breast cancer cell lines to the aerobic cytotoxicity of
doxorubicin.
cDNA-expressed human CYPRED catalyzed
doxorubicin reduction, measured as the rate of
doxorubicin-stimulated
NADPH consumption. Using a bank of 17 human liver microsomal samples, the rate of
doxorubicin reduction correlated with CYPRED catalytic activity and CYPRED
protein immunoreactivity.
Diphenyliodonium chloride, a mechanism-based inactivator of CYPRED, inhibited CYPRED activity and
doxorubicin reduction in human liver microsomes with similar concentration dependence. Stably transfected clones of MDA231 human
breast cancer cells overexpressing human CYPRED immunoreactive
protein and catalytic activity showed enhanced sensitivity to the aerobic cytotoxicity of
tirapazamine, a bioreductive drug known to be activated by CYPRED; however, no sensitization to the cytotoxic effects of
doxorubicin was observed. Although human CYPRED is an important catalyst of
doxorubicin reduction, overexpression of this
enzyme does not confer enhanced sensitivity of human
breast cancer cells to the aerobic cytotoxicity of
doxorubicin.