Significant Inhibition of Protein Binding of Phenytoin

Abstract

Aim: This research was aimed to assess the potential for inhibition of protein binding of phenytoin by tizoxanide in human plasma. Phenytoin was used because studies have proven to be a highly plasma protein-bound drug with a narrow therapeutic index and non-linear pharmacokinetics. Tizoxanide is another highly protein-bound active metabolite of an anti-infective prodrug nitazoxanide. Both drugs are expected to be administered together due to their therapeutic indications. Study Design: The study was divided into two phases. Phase 1 produced a reference line of phenytoin protein binding values. Phase 2 was conducted to reveal the effect of the interactant tizoxanide on the protein binding of phenytoin. The results obtained from phase 1 were compared with those of phase 2. Methodology: Protein binding of phenytoin was studied using a centrifugal ultrafiltration method. Protein binding of three phenytoin concentrations was studied: 25, 50, and 100 µg/mL. The concentrations of phenytoin were analyzed by validated HPLC method. Each experiment was performed in triplicate. Results: Co-administration of tizoxanide significantly inhibited protein binding of phenytoin for all concentrations tested. Tizoxanide increased unbound fraction (fu) of phenytoin by 4.4, 3.7, and 2.8-fold for concentration of 25, 50, and 100 µg/ml, respectively. Phenytoin was displaced from protein binding sites what resulted in amplified unbound plasma levels of the antiepileptic drug. Conclusion: Tizoxanide significantly inhibited protein binding of phenytoin in human plasma. The interaction could potentially result in altered elimination and increased toxicity of phenytoin leading to neuronal side effects and hypersensitivity reactions. Caution also should be taken when administering nitazoxanide concurrently with other highly plasma protein-bound drugs, especially drugs with narrow therapeutic indices, as competition for binding sites may occur.