Drug Interaction Report

GENERALLY AVOID: Coadministration with strong CYP450 3A4 inhibitors may increase plasma concentrations of thiotepa, and decrease concentrations of its active metabolite triethylenephosphoramide (TEPA). Thiotepa is a prodrug that is primarily converted to TEPA by the isoenzyme. A pharmacokinetic study evaluating six breast cancer patients receiving high-dose chemotherapy (cyclophosphamide 1,500 mg/m2/day, thiotepa 120 mg/m2/day, and carboplatin AUC 5 mg min/mL/day) with the moderate CYP450 3A4 inhibitor aprepitant (125 mg one day before chemotherapy, then 80 mg daily during and for three days after) showed a 15% increase in total thiotepa exposure, a 33% decrease in TEPA formation, and 20% reduction in TEPA exposure. Although clinical evidence is limited, strong CYP450 3A4 inhibitors may more significantly increase thiotepa exposure and reduce TEPA exposure, potentially resulting in more thiotepa-related adverse effects and reduced efficacy.

MANAGEMENT: The use of thiotepa in combination with strong CYP450 3A4 inhibitors should generally be avoided. If concomitant use of a strong CYP450 3A4 inhibitor is required, it is recommended that patients be carefully monitored for reduced efficacy and thiotepa-related toxicities such as myelosuppression, cutaneous toxicity, and neurotoxicity.

GENERALLY AVOID: Administration with a high-fat meal may decrease the oral bioavailability of amprenavir. The mechanism is unknown. In healthy volunteers, consumption of a standardized high-fat meal decreased the peak plasma concentration (Cmax) and area under the concentration-time curve (AUC) of amprenavir (1200 mg single oral dose) by 36% and 21%, respectively, compared to administration in the fasted state. The time to reach Cmax (Tmax) was increased 44% following a high-fat meal.

Grapefruit juice does not appear to significantly affect the pharmacokinetics of amprenavir. In 12 healthy volunteers, administration with grapefruit juice (200 mL) decreased the mean peak plasma concentration (Cmax) of amprenavir (1200 mg single oral dose) by 22% compared to water. The median time to reach Cmax (Tmax) was prolonged from 0.75 to 1.13 hours. These pharmacokinetic changes are not thought to be clinically significant, since antiretroviral response is more closely associated with systemic exposure (AUC) and trough plasma concentration (Cmin), which were not affected in the study.

MANAGEMENT: Amprenavir may be taken with or without food, but should not be taken with a high-fat meal.