Toremifeneinteractions

Drugs that decrease renal calcium excretion, e.g., thiazide diuretics, may increase the risk of hypercalcemia in patients receiving FARESTON. Agents that prolong QT should be avoided. Coadministration with a strong CYP3A4 inducer may result in a relevant decrease in FARESTON exposure and should be avoided. Coadministration with a strong CYP3A4 inhibitor can result in a relevant increase in FARESTON exposure and should be avoided. CYP2C9 substrates with a narrow therapeutic index such as warfarin or phenytoin with FARESTON should be used with caution and require careful monitoring.

7.1 Drugs that

Decrease Renal Calcium Excretion Drugs that decrease renal calcium excretion, e.g., thiazide diuretics, may increase the risk of hypercalcemia in patients receiving FARESTON.

7.2 Agents that

Prolong QT The administration of FARESTON with agents that have demonstrated QT prolongation as one of their pharmacodynamic effects should be avoided. Should treatment with any of these agents be required, it is recommended that therapy with FARESTON be interrupted. If interruption of treatment with FARESTON is not possible, patients who require treatment with a drug that prolongs QT should be closely monitored for prolongation of the QT interval. Agents generally accepted to prolong QT interval include Class 1A (e.g., quinidine, procainamide, disopyramide) and Class III (e.g., amiodarone, sotalol, ibutilide, dofetilide) antiarrhythmics; certain antipsychotics (e.g., thioridazine, haloperidol); certain antidepressants (e.g., venlafaxine, amitriptyline); certain antibiotics (e.g., erythromycin, clarithromycin, levofloxacin, ofloxacin); and certain anti-emetics (e.g., ondansetron, granisetron). In patients at increased risk, electrocardiograms (ECGs) should be obtained and patients monitored as clinically indicated.

7.3 Effect of

Strong CYP3A4 Inducers on Toremifene Strong CYP3A4 enzyme inducers, such as dexamethasone, phenytoin, carbamazepine, rifampin, rifabutin, phenobarbital, St. John's Wort, lower the steady-state concentration of toremifene in serum.

7.4 Effect of

Strong CYP3A4 Inhibitors on Toremifene In a study of 18 healthy subjects, 80 mg toremifene once daily coadministered with 200 mg of ketoconazole twice daily increased the toremifene Cmax and AUC by 1.4- and 2.9-fold, respectively. N-demethyltoremifene Cmax and AUC were reduced by 56% and 20%, respectively. The administration of FARESTON with agents that are strong CYP3A4 inhibitors (e.g., ketoconazole, itraconazole, clarithromycin, atazanavir, indinavir, nefazodone, nelfinavir, ritonavir, saquinavir, telithromycin, and voriconazole) increase the steady-state concentration in serum and should be avoided. Grapefruit juice may also increase plasma concentrations of toremifene and should be avoided. Should treatment with any of these agents be required, it is recommended that therapy with FARESTON be interrupted. If interruption of treatment with FARESTON is not possible, patients who require treatment with a drug that strongly inhibits CYP3A4 should be closely monitored for prolongation of the QT interval.

7.5 Effect of

Toremifene on CYP3A4 Substrates In a study of 20 healthy subjects, 2 mg midazolam once daily (days 6 and 18) coadministered with toremifene as a 480 mg loading dose followed by 80 mg once daily for 16 days. Following coadministration on days 6 and 18 relevant increases in midazolam and α-hydroxymidazolam Cmax and AUC were not observed. Following coadministration on day 18 midazolam and α-hydroxymidazolam Cmax and AUC were reduced by less than 20%. Clinically relevant exposure changes in sensitive substrates due to inhibition or induction of CYP3A4 by toremifene appear unlikely.

7.6 Effect of

Toremifene on CYP2C9 Substrates In a study of 20 healthy subjects, 500 mg tolbutamide once daily (days 7 and 19) coadministered with toremifene as a 480 mg loading dose followed by 80 mg once daily for 16 days. Following coadministration on days 7 and 19 plasma tolbutamide Cmax and AUC were increased by less than 30%. A reduction of similar magnitude was observed for hydroxytolbutamide and carboxytolbutamide Cmax and AUC. Toremifene is a weak inhibitor of CYP2C9. Concomitant use of CYP2C9 substrates with a narrow therapeutic index such as warfarin or phenytoin with FARESTON should be done with caution and requires careful monitoring (e.g., substrate concentrations (if possible), appropriate laboratory markers, and signs and symptoms of increased exposure).