Automated enzyme inhibition assay method for the determination of atorvastatin-derived HMG-CoA reductase inhibitors in human plasma using radioactivity detection

Introduction: A Tecan-based Enzyme inhibition assay has been developed for the determination of atorvastatin-derived 'active' and 'total' (active inhibitors plus atorvastatin lactone and other potential inhibitors following base hydrolysis) 3-hydroxy-3-methylglutaryl-Coenzyme A (HMG-CoA) reductase inhibitor concentrations in human plasma. Atorvastatin is an inhibitor of HMG-CoA reductase, which is a key rate-limiting Enzyme in the Cholesterol biosynthesis. Previously, atorvastatin-derived HMG-CoA reductase inhibitors were measured via Enzyme inhibition assays by manual operation.

Methods: In this work, an Enzyme assay procedure based on 8-tip Tecan robotics and set-up in a 96-well plate format with customized hardware is presented. Following protein precipitation of the plasma sample, an aliquot of the resulting supernatant is mixed with HMG-CoA reductase and (14)C-labeled HMG-CoA prior to incubation. The product, (14)C-mevalonic acid, is lactonized, separated from unreacted (14)C-substrate, and counted in a liquid scintillation counter. Plasma HMG-CoA reductase inhibitor concentrations are measured against atorvastatin as the standard. Tecan Genesis 150 and 200 robotic workstations were used for the protein precipitation, Enzyme incubation, and product separation.

Results: The standard calibration range for the assay was 0.4-20 ng eq/mL. Intra-day precision (%CV) data for the calibration standard and quality control (QC) samples (n=5 replicates) were both <or=8%, with an accuracy between 88 and 113% of nominal values. Initial inter-day precision of the QC samples was <or=6%, with an accuracy range of 94-111% of nominal values.

Discussion: The assay procedure provides high throughput analysis of clinical samples to support pharmacokinetic studies.