Drug Interactions between Omeclamox-Pak and venlafaxine

MONITOR: Coadministration with inhibitors of CYP450 3A4 such as azole antifungal agents and ketolide/macrolide antibiotics may increase the plasma concentrations of venlafaxine and its metabolite, O-desmethylvenlafaxine (ODV). The possibility of prolonged and/or increased pharmacologic effects of venlafaxine should be considered. Although venlafaxine is primarily metabolized by CYP450 2D6, there is some evidence that CYP450 3A4 may play a minor role, thus any alteration in its activity levels could conceivably affect the metabolism of venlafaxine. The interaction may be of greater concern in patients who are CYP450 2D6-deficient, or so-called poor metabolizers of CYP450 2D6 (approximately 7% of Caucasians and less than 2% of Asians and individuals of African descent). In healthy volunteers, administration of single-dose venlafaxine with the potent CYP450 3A4 inhibitor ketoconazole (100 mg twice daily) increased venlafaxine systemic exposure (AUC) by 21% in extensive metabolizers and 70% (range -2% to 206%) in poor metabolizers of CYP450 2D6; increased AUC of ODV by 23% in extensive metabolizers and 33% (range -38% to 105%) in poor metabolizers; and increased combined AUC of venlafaxine and ODV by 23% in extensive metabolizers and 53% in poor metabolizers (range 4% to 134%). Venlafaxine and ODV peak plasma concentration (Cmax) also increased by 26% and 14%, respectively, in extensive metabolizers and 48% and 29%, respectively, in poor metabolizers.

MONITOR: Venlafaxine may cause dose-related prolongation of the QT interval. Theoretically, coadministration with other agents that can prolong the QT interval such as azole antifungal agents and ketolide/macrolide antibiotics may result in additive effects and increased risk of ventricular arrhythmias including torsade de pointes and sudden death. In general, the risk of an individual agent or a combination of these agents causing ventricular arrhythmia in association with QT prolongation is largely unpredictable but may be increased by certain underlying risk factors such as congenital long QT syndrome, cardiac disease, and electrolyte disturbances (e.g., hypokalemia, hypomagnesemia). In addition, the extent of drug-induced QT prolongation is dependent on the particular drug(s) involved and dosage(s) of the drug(s).

MANAGEMENT: Caution is recommended when venlafaxine is used with CYP450 3A4 inhibitors that can prolong the QT interval such as azole antifungal agents and ketolide/macrolide antibiotics. Patients should be advised to seek prompt medical attention if they experience symptoms that could indicate the occurrence of torsade de pointes such as dizziness, lightheadedness, fainting, palpitation, irregular heart rhythm, shortness of breath, or syncope.

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Although some in vitro data indicate synergism between macrolide antibiotics and penicillins, other in vitro data indicate antagonism. When these drugs are given together, neither has predictable therapeutic efficacy. Data are available for erythromycin, although theoretically this interaction could occur with any macrolide. Except for monitoring of the effectiveness of antibiotic therapy, no special precautions appear to be necessary.

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Clarithromycin may increase and prolong the omeprazole plasma concentration. The mechanism may be related to clarithromycin inhibition of hepatic cytochrome P450 enzymes responsible for omeprazole metabolism. Coadministration of omeprazole may result in an increase in clarithromycin and 14-(R)-hydroxyclarithromycin plasma concentrations. These increases may be due to the effect of omeprazole on gastric pH.

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