Influence of stereoselective pharmacokinetics in the development and predictability of an IVIVC for the enantiomers of metoprolol tartrate

PURPOSE

To investigate the ability of an IVIVC developed with a racemate drug as well as each enantiomer in predicting the in vivo enantiomer drug performance.

METHODS

Dissolution of metoprolol extended release tablets with different release characteristics (e.g., fast (F), moderate (M), and slow (S)) was performed using USP Apparatus I, pH 1.2, 50 rpm. Metoprolol racemate tablets (S, M, and F, 100 mg) and 50 mg oral solution were administered to healthy volunteers, blood samples were collected over 24 (solution) and 48 (tablet) hours and assayed. IVIVC models developed were: (1) Racemate-fraction of drug dissolved (FRD) vs Racemate-fraction of drug absorbed (FRA), (2) R-FRD vs R-FRA, and (3) S-FRD vs S-FRA for combinations of formulations (S/M/F, S/M, S/F, and M/F). Enantiomer Cmax and AUC prediction errors (PEs) were estimated for model evaluation after convolution of in vivo release rates.

RESULTS

The R-IVIVC and S-IVIVC accurately predicted the R- and S-metoprolol pharmacokinetic profiles, respectively. The averaged prediction errors (PE) for the enantiomer Cmax and AUC were less than 10% for S/M/F, M/F, and S/F IVIVC models. Racemate-IVIVC (M/F) was able to predict S-enantiomer with an average %PE of 2.52 for S-Cmax and 4.3 for S-AUC. However, the racemate-IVIVC was unable to predict the R-enantiomer pharmacokinetic profile.

CONCLUSIONS

Metoprolol racemate data cannot be used to accurately predict R-enantiomer drug concentrations. However, the racemate data was predictive of the active stereoisomer.

The influence of the menstrual cycle on triazolam and indocyanine green pharmacokinetics

The aim of this study was to investigate the effects of the menstrual cycle phase on the pharmacokinetics of two high-clearance agents, triazolam and indocyanine green (ICG). Eleven nonsmoking, healthy, eumenorrheic women were enrolled in this study. Triazolam (0.25 mg) was administered orally, and indocyanine green was administered as an i.v. bolus (0.5 mg/kg) during the follicular, ovulatory, and luteal phases of a single menstrual cycle. Blood samples were collected over 10 hours for triazolam and over 30 minutes for ICG. Triazolam and indocyanine green concentrations were quantitated by electron capture gas chromatography and spectrophotometry, respectively. Noncompartmental analysis was used to determine relevant pharmacokinetics parameters, which were statistically assessed using two-way ANOVA (p < 0.05). No statistical differences for triazolam were observed. Vd/F was lower in the luteal phase (107 L) as compared to the follicular (138 L) and ovulatory (133 L) phases. Clearance of triazolam was comparable in the follicular (583 ml/min), ovulatory (565 ml/min), and luteal (538 ml/min) phases. ICG also revealed no significant differences across the phases. These results suggest that the phases of the menstrual cycle do not influence triazolam or ICG pharmacokinetics.