Decreased absorption as a possible cause for the lower bioavailability of a sustained-release propranolol

Pharmacokinetic and pharmacodynamic integration for optimal dosage of cefquinome against Streptococcus equi subsp. equi in foals

Cefquinome is administered in horses for the treatment of respiratory infection caused by Streptococcus equi subsp. zooepidemicus, and septicemia caused by Escherichia coli. However, there have been no attempts to use cefquinome against Streptococcus equi subsp. equi (S. equi), the causative agent of strangles. Hence the objective of this study was to calculate an optimal dosage of cefquinome against S. equi based on pharmacokinetics and pharmacodynamics integration. Cefquinome (1.0 mg/kg) was administered by intravenous and intramuscular routes to six healthy thoroughbred foals. Serum cefquinome concentrations were determined by high-performance liquid chromatography. The in vitro and ex vivo antibacterial activity were determined from minimum inhibitory concentrations (MIC) and bacterial killing curves. The optimal dosage was calculated from the integration of pharmacokinetic parameters and area under the curve (AUC_24h/MIC) values. Total body clearance and volume of distribution of cefquinome after intravenous administration were 0.06 L/h/kg and 0.09 L/kg, respectively. Following intramuscular administration, a maximum concentration of 0.73 μg/mL at 1.52 h (T_max) and a systemic bioavailability of 37.45% were observed. The MIC of cefquinome against S. equi was 0.016 μg/mL. The ex vivo AUC_24h/MIC values representing bacteriostatic, and bactericidal activity were 113.11, and 143.14 h, respectively. Whereas the %T > MIC for bactericidal activity was 153.34%. In conclusion, based on AUC_24h/MIC values and pharmacokinetic parameters, cefquinome when administered by intramuscularly at a dosage of 0.53 mg/kg every 24 h, would be effective against infection caused by S. equi in foals. Further studies may be necessary to confirm its therapeutic efficacy in a clinical environment.

Rapid induction of P-glycoprotein expression by high permeability compounds in colonic cells in vitro: a possible source of transporter mediated drug interactions?

P-glycoprotein (PGP) substrates with high membrane permeability, such as propranolol and verapamil, are considered to be essentially "transparent" to PGP since the transporter does not significantly limit their absorption or elimination. However, the question of whether such compounds can modulate PGP expression in epithelial cells following short-term exposure, with potential consequences for drug interactions, has not been addressed. LS180 colonic epithelial cells were exposed to propranolol or verapamil at concentrations (50-300 microM) consistent with those likely to be present in the gut lumen during oral dosing. Both compounds stimulated four to six-fold increases in MDR1 mRNA and PGP protein expression measured by quantitative real-time PCR and immunoblotting, respectively. These changes were accompanied by an induction in transporter activity measured by rhodamine 123 efflux. In contrast, metoprolol, a compound with similar permeability but no affinity for PGP had no effect on PGP expression. The induction of PGP by propranolol and verapamil was rapid with significant increases occurring within 3h with maximal stimulation after 6h exposure. Rifampicin, shown to cause clinical drug interactions via a PXR-mediated increase in PGP expression, exhibited a very similar time-course and extent of induction. In conclusion, verapamil and propranolol, whose trans-epithelial permeability are unaffected by PGP, appear to be effective inducers of PGP expression in gut epithelial cells in vitro. While the in vivo significance of these observations is unknown, this questions whether high permeability, "PGP-transparent" compounds, currently favoured in drug selection strategies, should be evaluated in terms of their potential for transporter-mediated drug interactions.

PKQuest: a general physiologically based pharmacokinetic model. Introduction and application to propranolol

BACKGROUND

A "physiologically based pharmacokinetic" (PBPK) approach uses a realistic model of the animal to describe the pharmacokinetics. Previous PBPKs have been designed for specific solutes, required specification of a large number of parameters and have not been designed for general use.

METHODS

This new PBPK program (PKQuest) includes a "Standardhuman" and "Standardrat" data set so that the user input is minimized. It has a simple user interface, graphical output and many new features: 1) An option that uses the measured plasma concentrations to solve for the time course of the gastrointestinal, intramuscular, intraperotineal or skin absorption and systemic availability of a drug - for a general non-linear system. 2) Capillary permeability limitation defined in terms of the permeability-surface area products. 4) Saturable plasma and tissue protein binding. 5) A lung model that includes perfusion-ventilation mismatch. 6) A general optimization routine using either a global (simulated annealing) or local (Powell) minimization applicable to all model parameters.

RESULTS

PKQuest was applied to measurements of human propranolol pharmacokinetics and intestinal absorption. A meal has two effects: 1) increases portal blood flow by 50%; and 2) decreases liver metabolism by 20%. There is a significant delay in the oval propranolol absorption in fasting subjects that is absent in fed subjects. The oral absorption of the long acting form of propranolol continues for a period of more than 24 hours.

CONCLUSIONS

PKQuest provides a new general purpose, easy to use, freely distributed www.pkquest.com and physiologically rigorous PBPK software routine.

Determination of unbound warfarin enantiomers in human plasma and 7-hydroxywarfarin in human urine by chiral stationary-phase liquid chromatography with ultraviolet or fluorescence and on-line circular dichroism detection

Enantiomers of warfarin and 7-hydroxywarfarin in human plasma and urine, respectively, were determined by high-performance liquid chromatography using a cellulose-derivative column with UV or fluorescent detection, and their absolute configuration was determined simultaneously by a circular dichroism spectropolarimeter connected in series. Enantiomers of warfarin and its major metabolites [i.e., (R)-6-hydroxywarfarin, (S)-7-hydroxywarfarin and (RS)-warfarin alcohol] were well resolved. The method was precise and sensitive: within- and between-day coefficients of variation were <9.6% for warfarin enantiomers in plasma and <7.1% for 7-hydroxywarfarin enantiomers in urine, respectively, and the lower detection limits were 20 ng/ml for (R)-warfarin, 40 ng/ml for (S)-warfarin, 2.5 ng/ml for (R)-7-hydroxywarfarin and 4.5 ng/ml for (S)-7-hydroxywarfarin in 0.5 ml of both plasma and urine. The ultrafiltration technique was used for determining unbound concentrations of warfarin enantiomers in plasma using [14C]warfarin enantiomers resolved by the present HPLC system. Clinical applicability of the method was evaluated by determining unbound concentrations of warfarin enantiomers in five consecutive plasma samples obtained from a patient exhibiting an unstable anticoagulant response to warfarin (4 mg/day, p.o.). Results indicated that the present method would be useful in clarifying factors responsible for a large intra- and inter-patient variability in warfarin effects with regard to unbound plasma enantiomer pharmacokinetics.

Bioavailability and in-vitro/in-vivo correlation for propranolol hydrochloride extended-release bead products prepared using aqueous polymeric dispersions

The influence of formulation and extrinsic factors has been investigated for the in-vitro release of propranolol hydrochloride from controlled-release beads prepared using aqueous polymeric dispersions, Aquacoat and Surelease. A single-dose three-way crossover bioavailability study of two extended-release experimental formulations (80 mg), Inderal LA (80 mg) and an Inderal immediate-release dosage form (2 x 40 mg) was also conducted and a comparative analysis of pharmacokinetic parameters and the in-vitro release profiles was performed to assess in-vitro/in-vivo correlation. Analysis showed that the in-vitro release data appeared to follow zero-order release kinetics. Intensity of agitation and dissolution method were found to have no significant effect on drug release from beads prepared using either of the coating dispersions studied or Inderal LA. Release of drug from beads coated with Aquacoat was faster in basic media than in acidic media; Surelease-coated beads, however, showed release characteristics that were less sensitive to changes in the pH of the dissolution fluid, and Inderal LA beads showed slower release profiles in acidic medium than in other dissolution media studied. Pharmacokinetic analysis of the data revealed sustained-release absorption characteristics without any evidence of dose-dumping from any of the extended-release dosage forms studied. Regression analysis of the fraction of drug absorbed against the percentage of the drug released in-vitro, at the corresponding times, yielded good in-vitro/in-vivo correlation (level A) for all the extended-release formulations studied. The results showed that there was no dose-dumping from any of extended-release formulations and that the relative bioavailabilities of the experimental formulations were superior to that of the marketed formulation.