Zeffix Drug Interactions

The likelihood of metabolic interactions is low due to limited metabolism and plasma protein binding and almost complete renal elimination of unchanged drug.
Lamivudine is predominantly eliminated by active organic cationic secretion. The possibility of interactions with other drugs administered concurrently should be considered, particularly when their main route of elimination is active renal secretion via the organic cationic transport system e.g. trimethoprim. Other drugs (e.g. ranitidine, cimetidine) are eliminated only in part by this mechanism and were shown not to interact with ZEFFIX.
Drugs shown to be predominately excreted either via the active organic anionic pathway, or by glomerular filtration are unlikely to yield clinically significant interactions with ZEFFIX.
Interactions relevant to lamivudine: Trimethoprim/sulphamethoxazole: Administration of trimethoprim/sulphamethoxazole 160 mg/800 mg increased ZEFFIX exposure by about 40%. ZEFFIX had no effect on the pharmacokinetics of trimethoprim or sulphamethoxazole. However, unless the patient has renal impairment, no dosage adjustment of ZEFFIX is necessary.
Zidovudine: A modest increase in C_max (28%) was observed for zidovudine when administered with ZEFFIX, however overall exposure (AUC) was not significantly altered. Zidovudine had no effect on the pharmacokinetics of ZEFFIX (see Pharmacology: Pharmacokinetics under Actions).
Emtricitabine: ZEFFIX may inhibit the intracellular phosphorylation of emtricitabine when the two medicinal products are used concurrently. ZEFFIX is not recommended for use in combination with emtricitabine.
Sorbitol: Coadministration of sorbitol solution (3.2 g, 10.2 g, 13.4 g) with a single 300 mg dose (Adult HIV daily dose) of lamivudine oral solution resulted in dose-dependent decreases of 14%, 32%, and 36% in lamivudine exposure (AUC_∞) and 28%, 52%, and 55% in the C_max of lamivudine in adults. When possible, avoid use of lamivudine with sorbitol-containing medicines or consider more frequent monitoring of HBV viral load when chronic coadministration cannot be avoided.
Alpha-interferon: ZEFFIX has no pharmacokinetic interaction with alpha-interferon when the two drugs are concurrently administered. There were no observed clinically significant adverse interactions in patients taking ZEFFIX concurrently with commonly used immunosuppressant drugs (e.g. cyclosporin A). However, formal interaction studies have not been performed.