Toremifene: pharmacologic and pharmacokinetic basis of reversing multidrug resistance.

Triphenylethylene compounds, such as tamoxifen, have shown chemosensitizing activity independent of estrogen receptor status in doxorubicin-resistant cells. We examined the chemosensitizing activity of a new triphenylethylene, toremifene, and its major metabolites in a doxorubicin-resistant human breast cell line, MCF-7/DOX. In addition, we examined the chemosensitizing activity of unbound plasma toremifene and its metabolites isolated from patients treated with toremifene doses of 20 to 400 mg/d. MCF-7/DOX cells were exposed to ultrafiltrate plasma specimens in the absence and presence of doxorubicin. These latter studies were single-blinded. Toremifene and its major metabolites were capable of sensitizing multidrug-resistant cells to doxorubicin. The degree of chemosensitizing activity in vitro correlated with the plasma concentrations of toremifene and its metabolites (P less than .05). Plasma samples isolated from patients receiving high-dose toremifene (400 mg/d) had the greatest chemosensitizing activity. We present evidence that toremifene and its metabolites can sensitize resistant MCF-7/DOX cells to doxorubicin, that this effect is concentration-dependent, and that sensitizing activity can be detected at clinically achieved concentrations.