Drug Interactions between amoxicillin / clarithromycin / omeprazole and modafinil

MONITOR: Coadministration with modafinil (the racemate) or armodafinil (the R-enantiomer) may increase the plasma concentrations of drugs that are primarily metabolized by the CYP450 2C19 isoenzyme. The mechanism is decreased clearance due to inhibition of CYP450 2C19 activity by modafinil and armodafinil. In pharmacokinetic studies, coadministration of a single 400 mg dose of armodafinil with omeprazole 40 mg increased systemic exposure to omeprazole by approximately 40%. A case report implicated modafinil in the development of clozapine toxicity characterized by dizziness, ataxia, and tachycardia. Clozapine serum levels increased from 761 ng/mL to 1400 ng/mL several weeks after initiation of modafinil therapy.

MANAGEMENT: Caution is advised if modafinil or armodafinil must be used concurrently with medications that undergo metabolism by CYP450 2C19, particularly those with a narrow therapeutic range. Dosage adjustments as well as clinical and laboratory monitoring may be appropriate for some drugs whenever modafinil or armodafinil is added to or withdrawn from therapy.

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.

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.

Coadministration with inhibitors of CYP450 3A4 may increase the plasma concentrations of modafinil (the racemate) and armodafinil (the R-enantiomer), which are both partially metabolized by the isoenzyme. Conversely, the plasma levels of some of these inhibitors may decrease, since many of them are also substrates of CYP450 3A4, and modafinil and armodafinil have been found to be modest inducers of CYP450 3A4. The clinical significance of this potential interaction is unknown. Clinical monitoring for altered effects of modafinil and armodafinil as well as the CYP450 3A4 inhibitor may be appropriate following addition or withdrawal of one or the other drug. Dose adjustments may be required if an interaction is suspected.