Drug Interactions between amoxicillin / clarithromycin / lansoprazole and tamsulosin

GENERALLY AVOID: Coadministration with potent inhibitors of CYP450 3A4 may significantly increase the plasma concentrations of tamsulosin, which is primarily metabolized by the hepatic microsomal isoenzymes CYP450 3A4 and 2D6. Severe hypotension and priapism may occur. In 24 healthy volunteers, administration of a single 0.4 mg dose of tamsulosin with the potent CYP450 3A4 inhibitor ketoconazole (400 mg once daily for 5 days) resulted in a 2.2-fold increase in tamsulosin peak plasma concentration (Cmax) and 2.8-fold increase in systemic exposure (AUC). The magnitude of interaction may be increased further in individuals who have genetic polymorphisms of CYP450 2D6 resulting in reduced or absent enzyme activity, or so-called CYP450 2D6 poor metabolizers (approximately 7% of Caucasians and less than 2% of Asians and individuals of African descent). When a single 0.4 mg dose of tamsulosin was given to 24 healthy volunteers with the potent CYP450 2D6 inhibitor paroxetine (20 mg once daily for 9 days), tamsulosin Cmax and AUC increased by a factor of 1.3 and 1.6, respectively. A similar increase in exposure is expected in CYP450 2D6 poor metabolizers as compared to extensive metabolizers, hence a potentially greater impact of CYP450 3A4 inhibition.

MANAGEMENT: Since CYP450 2D6 poor metabolizers cannot be readily identified, concomitant use of tamsulosin with potent CYP450 3A4 inhibitors should generally be avoided. Some authorities recommend avoiding concomitant use of tamsulosin during and for 2 weeks after treatment with itraconazole. If tamsulosin administration is discontinued for several days or more at either the 0.4 or 0.8 mg dose, therapy should be reinitiated with the 0.4 mg once-daily dose and titrated gradually as needed.

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.

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.