Prediction of the human in vivo antiplatelet effect of S- and R-indobufen using population pharmacodynamic modeling and simulation based on in vitro platelet aggregation test

Indobufen (Ibustrin®), a reversible inhibitor of platelet aggregation, exists in two enantiomeric forms in 1:1 ratio. Here, we characterized the anti-platelet effect of S- and R-indobufen using response surface modeling using NONMEM® and predicted the therapeutic doses exerting the maximal efficacy of each enantioselective S- and R-indobufen formulation. S- and R-indobufen were added individually or together to 24 plasma samples from drug-naïve healthy subjects, generating 892 samples containing randomly selected concentrations of the drugs of 0–128 mg/L. Collagen-induced platelet aggregation in platelet-rich plasma was determined using a Chrono-log Lumi-Aggregometer. Inhibitory sigmoid I(max) model adequately described the anti-platelet effect. The S-form was more potent, whereas the R-form showed less inter-individual variation. No significant interaction was observed between the two enantiomers. The anti-platelet effect of multiple treatments with 200 mg indobufen twice daily doses was predicted in the simulation study, and the effect of S- or R-indobufen alone at various doses was predicted to define optimal dosing regimen for each enantiomer. Simulation study predicted that 200 mg twice daily administration of S-indobufen alone will produce more treatment effect than S-and R-mixture formulation. S-indobufen produced treatment effect at lower concentration than R-indobufen. However, inter-individual variation of the pharmacodynamic response was smaller in R-indobufen. The present study suggests the optimal doses of R-and S-enantioselective indobufen formulations in terms of treatment efficacy for patients with thromboembolic problems. The proposed methodology in this study can be applied to the develop novel enantio-selective drugs more efficiently.

Development of a validated liquid chromatography-tandem mass spectrometry assay for the quantification of simvastatin acid, the active metabolite of simvastatin, in human plasma

Simvastatin is a lipid-lowering drug that is metabolized to its active metabolite simvastatin acid (SA). We developed and validated a sensitive liquid chromatography-tandem mass spectrometry (LC/MS/MS) method to quantitate SA in human plasma using a liquid-liquid extraction method with methanol. The protonated analytes generated in negative ion mode were monitored by multiple reaction monitoring. Using 500-mL plasma aliquots, SA was quantified in the range of 0.1-100 ng/mL. Calibration was performed by internal standardization with lovastatin acid, and regression curves were generated using a weighting factor of 1/χ2. The linearity, precision, and accuracy of this assay for each compound were validated using quality control samples consisting of mixtures of SA (0.1, 0.5, 5, and 50 ng/mL) and plasma. The intra-batch accuracy was 95.3-107.8%, precision was -2.2% to -3.7%, and linearity (r2) was over 0.998 in the standard calibration range. The chromatographic running time was 3.0 min. This method sensitively and reliably measured SA concentrations in human plasma and was successfully used in clinical pharmacokinetic studies of simvastatin in healthy Korean adult male volunteers.