Drug Interactions between dabrafenib and Prevpac

MONITOR: Coadministration with clarithromycin may increase the plasma concentrations of lansoprazole. The proposed mechanism is clarithromycin inhibition of intestinal (first-pass) and hepatic metabolism of lansoprazole via CYP450 3A4. Although lansoprazole is primarily metabolized by CYP450 2C19 in the liver, 3A4-mediated metabolism is the predominant pathway in individuals who are 2C19-deficient (approximately 3% to 5% of the Caucasian and 17% to 20% of the Asian population). Additionally, inhibition of P-glycoprotein intestinal efflux transporter by clarithromycin may also contribute to the interaction, resulting in increased bioavailability of lansoprazole. In 18 healthy volunteers--six each of homozygous extensive metabolizers (EMs), heterozygous EMs, and poor metabolizers (PMs) of CYP450 2C19--clarithromycin (400 mg orally twice a day for 6 days) increased the peak plasma concentration (Cmax) of a single 60 mg oral dose of lansoprazole by 1.47, 1.71- and 1.52-fold, respectively, and area under the concentration-time curve (AUC) by 1.55-, 1.74- and 1.80-fold, respectively, in each of these groups compared to placebo. The AUC ratio of lansoprazole to lansoprazole sulphone, which is considered an index of CYP450 3A4 activity, was significantly increased by clarithromycin in all three groups. However, elimination half-life of lansoprazole was prolonged by 1.54-fold only in PMs. Mild diarrhea was reported in two subjects and mild abdominal disturbance in six subjects during clarithromycin coadministration. These side effects continued until day 6 and ameliorated the day after discontinuation of clarithromycin, whereas no adverse events were reported during placebo administration or after lansoprazole plus placebo. In another study, clarithromycin induced dose-dependent increases in the plasma concentration of lansoprazole in a group of 20 patients receiving treatment for H. pylori eradication. Mean 3-hour plasma lansoprazole concentration was 385 ng/mL for the control subjects who received lansoprazole 30 mg and amoxicillin 750 mg twice a day for 7 days; 696 ng/mL for patients coadministered clarithromycin 200 mg twice a day; and 947 ng/mL for patients coadministered clarithromycin 400 mg twice a day.

MANAGEMENT: Although lansoprazole is generally well tolerated, caution may be advised during coadministration with clarithromycin, particularly if higher dosages of one or both drugs are used. Dosage adjustment may be necessary in patients who experience excessive adverse effects of lansoprazole.

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GENERALLY AVOID: Coadministration with potent inhibitors of CYP450 3A4 and/or 2C8 may increase the plasma concentrations of dabrafenib and its active metabolites. In vitro studies have shown that dabrafenib is a substrate of CYP450 3A4 and 2C8, while hydroxy-dabrafenib and desmethyl-dabrafenib are substrates of CYP450 3A4. In a pharmacokinetic study, administration of dabrafenib 75 mg twice daily in combination with the potent CYP450 3A4 inhibitor ketoconazole 400 mg once daily for 4 days increased dabrafenib systemic exposure (AUC) by 71%, hydroxy-dabrafenib AUC by 82%, and desmethyl-dabrafenib AUC by 68%. When dabrafenib was given similarly with the potent CYP450 2C8 inhibitor gemfibrozil 600 mg twice daily for 4 days, dabrafenib AUC increased by 47%, but AUC of the metabolites did not change.

MONITOR: Coadministration with dabrafenib may decrease the plasma concentrations of drugs that are substrates of CYP450 3A4, including many of the known inhibitors of the isoenzyme such as conivaptan, delavirdine, nefazodone, telithromycin, and most azole antifungal agents, macrolide antibiotics, and protease inhibitors. Dabrafenib has been found in vitro to be a dose-dependent inducer of CYP450 3A4. Onset of induction is likely to occur after 3 days of repeat dosing with dabrafenib; however, transient inhibition of CYP450 3A4 may be observed during the first few days of treatment. In 12 study subjects, administration of the CYP450 3A4 probe substrate midazolam following repeat doses of dabrafenib 150 mg twice daily for 15 days reduced midazolam peak plasma concentration (Cmax) by 61% and systemic exposure (AUC) by 74%.

MANAGEMENT: The use of dabrafenib with potent CYP450 2C8 inhibitors such as gemfibrozil or potent CYP450 3A4 inhibitors such as ceritinib, clarithromycin, cobicistat, conivaptan, delavirdine, erythromycin, idelalisib, nefazodone, telithromycin, and most protease inhibitors and azole antifungal agents should generally be avoided if possible. Some authorities recommend avoiding concomitant use of dabrafenib during and for 2 weeks after treatment with itraconazole. Otherwise, patients should be closely monitored for development of adverse effects such as febrile reactions (high fever or fever accompanied by rigors, hypotension, dehydration, or renal failure), hyperglycemia, uveitis, and cutaneous malignancies (e.g., squamous cell carcinoma, keratoacanthoma, melanoma). During coadministration of dabrafenib with a CYP450 3A4 inhibitor, the potential for diminished therapeutic effects of the inhibitor should also be considered.

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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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