Drug Interaction Report

GENERALLY AVOID: Zalcitabine can cause peripheral neuropathy in up to one-third of patients with advanced HIV disease, and concurrent use of other agents that are also associated with this adverse effect can potentiate the risk and/or severity of nerve damage. Zalcitabine-related peripheral neuropathy is a sensorimotor neuropathy characterized initially by numbness and burning dysesthesia involving the distal extremities. These symptoms may be followed by sharp shooting pains or severe continuous burning pain if the drug is not withdrawn, and progress to severe pain requiring narcotic analgesics. The neuropathy is potentially irreversible. However, with prompt discontinuation of zalcitabine, it is usually slowly reversible, although symptoms may initially progress following discontinuation.

MANAGEMENT: Use of zalcitabine with other drugs that have the potential to cause peripheral neuropathy should be avoided whenever possible. Otherwise, careful monitoring is recommended for symptoms of neuropathy such as burning, tingling, pain, numbness, or weakness in the extremities, particularly in patients with a low CD4 cell count or diabetes. Since the development of peripheral neuropathy appears to be dose-related, the recommended dosage of zalcitabine should not be exceeded. Patients should be advised to promptly discontinue zalcitabine therapy and contact their physician if neuropathy develops. Therapy may be reinstituted following resolution of neuropathy symptoms, but dosage should be reduced to one-half the initially recommended dosage. Zalcitabine should be permanently discontinued in patients who develop severe peripheral neuropathy during treatment.

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GENERALLY AVOID: Grapefruit juice may significantly increase the plasma concentrations of orally administered amiodarone. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits. In 11 nonsmoking, healthy volunteers, grapefruit juice (300 mL with drug administration, then 3 hours and 9 hours later) increased the mean peak plasma concentration (Cmax) and area under the concentration-time curve (AUC) of amiodarone (17 mg/kg single dose) by 84% and 50%, respectively, compared to water. Formation of the pharmacologically active metabolite, N-desethylamiodarone (N-DEA), was completely inhibited. Clinically, this interaction can lead to altered efficacy of amiodarone, since antiarrhythmic properties of amiodarone and N-DEA appear to differ. In the study, mean increases in PR and QTc intervals of 17.9% and 11.3%, respectively, were observed 6 hours postdose with water, while increases of 10.2% and 3.3%, respectively, were observed after administration with grapefruit juice.

ADJUST DOSING INTERVAL: Food increases the rate and extent of absorption of amiodarone. The mechanism appears to involve the effect of food-induced physiologic changes on drug release from its formulation. In 30 healthy volunteers, administration of a single 600 mg dose of amiodarone following a high-fat meal resulted in a Cmax and AUC that were 3.8 and 2.4 times the respective values under fasting conditions. The time to reach peak plasma concentration (Tmax) was decreased by 37%, indicating an increased rate of absorption. Mean Cmax and AUC for the active metabolite, N-DEA, also increased by 32% and 55%, respectively, but there was no change in the Tmax.

MANAGEMENT: Patients treated with oral amiodarone should avoid consumption of grapefruits and grapefruit juice. In addition, oral amiodarone should be administered consistently with regard to meals.

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Zalcitabine bioavailability may be decreased by 14% if taken with meals. The mechanism and clinical significance are unknown.

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