Development and validation of an enantioselective HPLC-UV method using Chiralpak AD-H to quantify (+)- and (-)-torcetrapib enantiomers in hamster plasma--application to a pharmacokinetic study

Abstract

A chiral selective, accurate and reproducible high-performance liquid chromatographic (HPLC) method was developed and validated for direct separation of individual enantiomers of torcetrapib (TTB) [(+)-TTB and (-)-TTB]. TTB enantiomers and IS were extracted from a small aliquot of plasma (100 microL) by simple liquid-liquid extraction using acetonitrile as extraction solvent. The enantiomers were resolved on Chiralpak AD-H (250 mm x 4.6 mm, 5 microm) with the mobile phase consisting of n-hexane:isopropyl alcohol (IPA) in the ratio of 95:5 (v/v). The eluate was monitored using an UV detector set at 254 nm. Baseline separation of the TTB enantiomers and the internal standard (IS, DRL-17859), free from endogenous interferences was achieved. The resolution factor between the enantiomers was optimized and found to be not less than five. During method development, the IPA content in the mobile phase was optimized for separation of peaks of interest. Additionally, both flow rate and column temperature were optimized for an improved baseline separation of the enantiomers. Ratio of peak area of each enantiomer to IS was used for quantification of plasma samples. Nominal retention times of (+)-TTB, (-)-TTB and IS were 9.4, 13.8 and 17.5 min, respectively. The standard curves for TTB enantiomers were linear (r(2)>0.999) in the concentration range 0.1-10 microg/mL for each enantiomer. Absolute recovery, when compared to neat standards, was 88.7-90.0% for TTB enantiomers and 100% for IS from the hamster plasma. The lower limit of quantification (LLOQ) for each enantiomer of TTB was 0.1 microg/mL. The inter-day precisions were in the range of 4.57-6.32 and 5.66-11.0% for (+)-TTB and (-)-TTB, respectively. The intra-day precisions were in the range of 1.60-7.36 and 2.76-13.6% for (+)-TTB and (-)-TTB, respectively. Accuracy in the measurement of quality control (QC) samples was in the range of 95.6-109% and 92.7-108% for (+)-TTB and (-)-TTB, respectively. Both enantiomers were stable in a series of stability studies, viz. bench-top (up to 12h), auto-sampler (up to 24h) and freeze/thaw cycles (n=3). Stability of TTB enantiomers was established in hamster plasma for 15 days at -80 degrees C. The application of the assay to a pharmacokinetic study of (-)-TTB in hamsters is described.