In the presence of ketoconazole, the clearance of docetaxel in cancer patients was shown to decrease by 50%. When docetaxel and ketoconazole are administered together, dosage reduction in docetaxel may be necessary in order to minimize the incidence of toxicities associated with docetaxel.
Concomitant administration of ketoconazole and protease inhibitors metabolized by CYP3A4, such as indinavir and saquinavir, may increase plasma concentrations of these protease inhibitors. Dosage reduction of indinavir is recommended when administering ketoconazole concomitantly. No dosage adjustments are recommended when saquinavir and ketoconazole are coadministered for a short period of time.
Ketoconazole tablets may alter the metabolism of methylprednisolone, resulting in elevated plasma concentrations of methylprednisolone. Dose adjustments may be required if methylprednisolone is given concomitantly with ketoconazole tablets.
Oral imidazole compounds such as ketoconazole may enhance the anticoagulant effect of coumarin-like drugs, thus the anticoagulant effect should be carefully titrated and monitored.
Because severe hypoglycemia has been reported in patients concomitantly receiving oral miconazole (an imidazole) and oral hypoglycemic agents, such a potential interaction involving the latter agents when used concomitantly with ketoconazole tablets (an imidazole) cannot be ruled out.
Ketoconazole was shown to inhibit the CYP-mediated metabolism of rifabutin in vitro. Co-administration with ketoconazole tablets may result in elevated plasma concentrations of rifabutin.
Ketoconazole had been shown to increase sildenafil plasma concentrations. When used concomitantly with ketoconazole tablets, a 50% reduction in sildenafil starting dose should be considered.
Multiple-dose ketoconazole had been shown to increase sirolimus Cmax and AUC by 4.3-fold and 10.9-fold, respectively. The concomitant use of ketoconazole tablets and sirolimus is not recommended.
Ketoconazole had been shown to decrease the oral clearance of tacrolimus thereby leading to a 2-fold increase in tacrolimus oral bioavailability. Adjustment in tacrolimus dosage may be required if tacrolimus is given concomitantly with ketoconazole tablets.
Ketoconazole increased the AUC of telithromycin by 1.5 to 2-fold. Use caution when administering telithromycin concurrently with ketoconazole tablets since this may result in an increased risk for telithromycin associated adverse events.
In the presence of ketoconazole, the apparent oral clearance of tolterodine decreased resulting in at least a two-fold increase in tolterodine. For patients receiving ketoconazole, a 50% reduction in the initial tolterodine dosage is recommended.
In vitro data suggest that trimetrexate is extensively metabolized by CYP3A4. In vitro animal models have demonstrated that ketoconazole potently inhibits the metabolism of trimetrexate. Patients treated concomitantly with trimetrexate and ketoconazole tablets should be carefully monitored for trimetrexate-associated toxicities.
Findings of in vitro metabolic studies indicate that verapamil is metabolized by enzymes including CYP3A4. Ketoconazole may increase verapamil serum concentrations. Caution should be taken when co-administering verapamil with ketoconazole tablets.
Ketoconazole may inhibit the metabolism of vinca alkaloids metabolized by CYP3A4. Close monitoring for toxicities associated with vincristine, vinblastine, or vinorelbine is recommended when co-administered with ketoconazole tablets.
Studies have shown that absorption of ketoconazole is impaired when gastric acid production is decreased. Reduced plasma concentrations of ketoconazole were reported when ketoconazole tablets were administered with antacids, antimuscarinics, histamine H2 -blockers, proton pump inhibitors (omeprazole, lansoprazole) and sucralfate. (See PRECAUTIONS, Drug Interactions (General) section.)
2.2 Drugs that were shown or are expected to significantly reduce the systemic exposure to ketoconazole
Co-administration of ketoconazole with potent CYP3A4 enzyme inducers is not recommended.
Concomitant administration of ketoconazole tablets with carbamazepine may alter the metabolism of one or both of the drugs. Close monitoring for both plasma concentrations of carbamazepine and reduced ketoconazole efficacy is recommended.
Ketoconazole AUC and Cmax decreased by a median of 63% and 40%, respectively, in HIV-infected patients who were given nevirapine 200 mg once daily for two weeks along with ketoconazole 400 mg daily. Concomitant administration of ketoconazole tablets and nevirapine is not recommended.
Concomitant administration of ketoconazole with phenytoin may alter the metabolism of one or both of the drugs. Close monitoring for both plasma concentrations of phenytoin and reduced efficacy of ketoconazole tablets is recommended.
Concomitant administration of rifampin and rifabutin with ketoconazole tablets reduces the blood concentrations of the latter. INH (Isoniazid) was also reported to affect ketoconazole concentrations adversely. These antitubercular drugs should not be given concomitantly with ketoconazole tablets.
Concomitant administration of ritonavir with ketoconazole tablets increases was shown to increase the oral bioavailability of ketoconazole. Therefore, when ritonavir is to be given concomitantly, higher doses (>200 mg/day) of ketoconazole tablets should not be used.
Rare cases of a disulfiram-like reaction to alcohol have been reported. These experiences have been characterized by flushing, rash, peripheral edema, nausea, and headache. Symptoms resolved within a few hours.
After the co-administration of 200 mg oral ketoconazole twice daily and one 20 mg dose of loratadine to 11 subjects, the AUC and Cmax of loratadine averaged 302% (±142 S.D.) and 251% (± 68 S.D.), respectively, of those obtained after cotreatment with placebo. The AUC and Cmax of descarboethoxyloratadine, an active metabolite, averaged 155% (± 27 S.D.) and 141% (± 35 S.D.), respectively. However, no related changes were noted in the QTc on ECG taken at 2, 6, and 24 hours after the coadministration. Also, there were no clinically significant differences in adverse events when loratadine was administered with or without ketoconazole.
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