Role and pharmacologic significance of cytochrome P‐450 2D6 in oxidative metabolism of toremifene and tamoxifen

We investigated the in vitro metabolism and estrogenic and antiestrogenic activity of toremifene (TOR), tamoxifen (TAM) and their metabolites to better understand the potential effects of cytochrome P-450 2D6 (CYP2D6) status on the activity of these drugs in women with breast cancer. The plasma concentrations of TOR and its N-desmethyl (NDM) and 4-hydroxy (4-OH) metabolites during steady-state dosing with TOR were also determined. Unlike TOR, TAM and its NDM metabolite were extensively oxidized to 4-OH TAM and 4-OH-NDM TAM by CYP2D6, and the rate of metabolism was affected by CYP2D6 status. 4-OH-NDM TOR concentrations were not measurable at steady state in plasma of subjects taking 80 mg of TOR. Molecular modeling provided insight into the lack of 4-hydroxylation of TOR by CYP2D6. The 4-OH and 4-OH-NDM metabolites of TOR and TAM bound to estrogen receptor (ER) subtypes with fourfold to 30-fold greater affinity were 35- to 187-fold more efficient at antagonizing ER transactivation and had antiestrogenic potency that was up to 360-fold greater than their parent drugs. Our findings suggest that variations in CYP2D6 metabolic capacity may cause significant differences in plasma concentrations of active TAM metabolites (i.e., 4-OH TAM and 4-OH-NDM TAM) and contribute to variable pharmacologic activity. Unlike TAM, the clinical benefits in subjects taking TOR to treat metastatic breast cancer would not likely be subject to allelic variation in CYP2D6 status or affected by coadministration of CYP2D6-inhibiting medications.