The low potential for drug interactions with zanamivir

Objective: The objective of this study was to assess the potential of zanamivir, a specific inhibitor of influenza A and B virus neuraminidase, to interact with other coadministered therapies in the clinical setting. Design: Potential interactions with zanamivir were examined in a series of in vitro and in vivo model systems. Interventions: The expression of microsomal cytochrome P450 (CYP) isoenzymes was examined after daily treatment of rats with intravenous zanamivir. The ability of zanamivir to inhibit the metabolism of CYP probe substrates was studied in human liver microsomes. The binding of zanamivir to human and animal red blood cell fractions and plasma proteins was measured. Finally, the effect of commonly coadministered drugs on the ability of zanamivir to inhibit viral replication in vitro was tested. Results: Zanamivir had no effect on the expression of microsomal CYP isoenzymes after daily intravenous treatment of rats with zanamivir 1, 9 or 90 mg/kg for 5 weeks. Zanamivir at concentrations up to 500 mu mol/L (150 mg/L) had no effect on the metabolism of the CYP probe substrates bufuralol, chlorzoxazone, coumarin, ethoxyresorufin, mephenytoin, midazolam, phenacetin and tolbutamide by human liver microsomes. The binding of zanamivir 0.05 to 10 mg/L to human, dog and rat red blood cells and plasma proteins was low. The in vitro potency of zanamivir against influenza virus in Madin Darby canine kidney cells was not adversely affected by aspirin (acetylsalicylic acid) 1.2 mmol/L, paracetamol (acetaminophen) 6.6 mmol/L, ibuprofen 243 mu mol/L, phenylephrine 6 mmol/L, oxymetazoline 350 mu mol/L, promethazine 35 mu mol/L and co-amoxiclav (amoxicillin-clavulanic acid) 1.66 mmol/L. Conclusions: These data suggest the following: (i) there is no theoretical basis for expecting metabolic interactions between zanamivir and other coadministered compounds; (ii) zanamivir is unlikely to interact with coadministered compounds that are protein bound; and (iii) commonly coadministered drugs will not interfere with the antiviral activity of zanamivir in vivo. Although none of these in vitro, or in vivo studies were exhaustive, and although none were performed in humans, all the data are consistent with zanamivir having a very low potential for interactions with coadministered drugs in the clinical setting.