Pharmacokinetics of pirmenol enantiomers and pharmacodynamics of pirmenol racemate in patients with premature ventricular contractions

The pharmacokinetics and pharmacodynamics of pirmenol were investigated in 12 patients with premature ventricular contractions (PVCs) after oral administration of racemic pirmenol, 100 mg and 200 mg every 12 hours. Holter monitoring was performed and serial blood samples were collected after the seventh doses. Plasma concentrations of pirmenol enantiomer were determined using a stereospecific liquid chromatographic assay. Clearance of total (-)-pirmenol was 20% higher than that of total (+)-pirmenol, and the difference in unbound clearance was 45% between enantiomers. Total pirmenol showed a smaller difference because of stereoselective protein binding, with 25% (100-mg dose) or 27% (200-mg dose) higher fraction unbound for (+)-pirmenol than for (-)-pirmenol. Distribution volume was similar for both enantiomers. Dose-dependent clearance was observed for unbound pirmenol enantiomers, as both enantiomers showed 20% lower unbound clearance at the higher dose. Antiarrhythmic effect (% reduction in PVCs from baseline) was correlated with plasma concentrations of pirmenol using a sigmoid maximum drug effect model, and patients showed a large variability in their antiarrhythmic response to plasma concentrations of pirmenol. The median value for minimum effective plasma concentration of racemic pirmenol was 1.5 micrograms/mL.