Simultaneous fitting of R- and S-ibuprofen plasma concentrations after oral administration of the racemate

Aims To assess the pharmacokinetic equivalence of two different formulation s of ibuprofen lysinate with special focus on the expected effects. Methods Sixteen healthy volunteers received cross-over ibuprofen lysinate a s either one tablet of 400 mg ('test') or two tablets of 200 mg ('reference '). Ibuprofen plasma concentrations were followed up for 10 h. Bioequivalen ce was assessed by standard noncompartmental methods. Ibuprofen plasma conc entrations were fitted with a model that took bioinversion of R- to S-ibupr ofen into account. Results Peak plasma concentrations of R- and S-ibuprofen were 18.1 and 20 m ug ml(-1) (test), and 18.2 and 20 mug ml(-1) (reference). Areas under the p lasma concentration vs time curves were 39.7 and 67.5 mug ml(-1) h (test), and 41.1 and 68.2 mug ml(-1) h (reference). Clearance of R-ibuprofen was 5. 2 (test) and 5 l h(-1) (reference). A specific plasma concentration was rea ched with the test formulation about 5 min later than with the reference. P arameters from compartmental modelling were (given for R-and then for S-ibu profen): body clearance: 4.9 and 4.64 l h(-1), central volume of distributi on: 2.8 and 4.1 l, intercompartment clearance: 5.1 and 5.45 l h(-1), periph eral volume of distribution: 4.1 and 5.2 l. The absorption rate constant wa s 1.52 h(-1), and the test but not the reference formulation had a lag time of 0.1 h. Simulations showed similarity between formulations of the expect ed effects except for a calculated delay of 6 min with the test formulation . Conclusions Ibuprofen formulations were bioequivalent. The pharmacokinetic model may serve as a basis for future pharmacokinetic/pharmacodynamic calcu lations after administration of racemic ibuprofen.