In vitro and in vivo stability of oseltamivir within a bioequivalence trial

A simple, precise, and rapid method to simultaneously determine the levels of oseltamivir (OS) and oseltamivir carboxylate (OSC) in human plasma was developed. Additionally, the stability of both substances in plasma was investigated under different conditions. The method involved protein precipitation (0.01 % HCl in acetonitrile), and then the supernatant was injected into the high-performance liquid chromatography (HPLC)-MS/MS. The chromatographic separation was achieved on a YMC-Triart C18 (100 × 2.0 mm, 5 μm) column using acetonitrile/water (30:70, v/v) containing 0.1 % formic acid as the mobile phase. Sample volume was 5 μl. The linearity of the method was established in the concentration range of 0.5-100 ng/mL for OS and 1.0-1000 ng/mL for OSC. The intra-day precision and accuracy for oseltamivir were 1.5-8.9 and 94.4-101.0 %, respectively. For oseltamivir carboxylate, the intra-day precision and accuracy were 3.2-12.7 and 92.8-108.8 %, respectively, whereas the inter-day precision and accuracy were 5.5-11.5 and 94.6-104.0 % for oseltamivir and 4.7-11.5 and 99.9-103.9 % for oseltamivir carboxylate, respectively. The application of this method was demonstrated by a bioequivalence study in 28 healthy humans with 75 mg oseltamivir phosphate capsules (Tamiflu®). Sodium fluoride (2.4 mg/mL) with potassium oxalate (3 mg/mL) was used as anticoagulant within sampling of trial. The assay reproducibility was established by reanalysis of 80 incurred samples.