Characterization of gastrointestinal absorption of digoxin involving influx and efflux transporter in rats: application of mdr1a knockout (-/-) rats into absorption study of multiple transporter substrate

The Rat Continuous Intestine Model Predicts the Impact of Particle Size and Transporters on the Oral Absorption of Glyburide

Oral drug absorption is known to be impacted by the physicochemical properties of drugs, properties of oral formulations, and physiological characteristics of the intestine. The goal of the present study was to develop a mathematical model to predict the impact of particle size, feeding time, and intestinal transporter activity on oral absorption. A previously published rat continuous intestine absorption model was extended for solid drug absorption. The impact of active pharmaceutical ingredient particle size was evaluated with glyburide (GLY) as a model drug. Two particle size suspensions of glyburide were prepared with average particle sizes of 42.7 and 4.1 μm. Each suspension was dosed as a single oral gavage to male Sprague Dawley rats, and concentration-time (C-t) profiles of glyburide were measured with liquid chromatography coupled with tandem mass spectrometry. A continuous rat intestine absorption model was extended to include drug dissolution and was used to predict the absorption kinetics of GLY depending on particle size. Additional literature datasets of rat GLY formulations with particle sizes ranging from 0.25 to 4.0 μm were used for model predictions. The model predicted reasonably well the absorption profiles of GLY based on varying particle size and varying feeding time. The model predicted inhibition of intestinal uptake or efflux transporters depending on the datasets. The three datasets used formulations with different excipients, which may impact the transporter activity. Model simulations indicated that the model provides a facile framework to predict the impact of transporter inhibition on drug C-t profiles. Model simulations can also be conducted to evaluate the impact of an altered intestinal lumen environment. In conclusion, the rat continuous intestine absorption model may provide a useful tool to predict the impact of varying drug formulations on rat oral absorption profiles.

Digoxin absorption decreased independently of P-gp activity in rats with irinotecan-induced gastrointestinal damage

Background

Irinotecan (CPT-11) is clinically known to cause severe diarrhea and gastrointestinal damage. Recently, we have reported that CPT-11-induced gastrointestinal damage is associated with the upregulation of intestinal P-glycoprotein (P-gp) expression and decreased absorption of its substrate, dabigatran etexilate (DABE), using a rat model. However, the P-gp activity or its contribution to the decreased absorption remains unclear. The aim of this study was to quantitatively evaluate how P-gp activity changes in rats with CPT-11-induced gastrointestinal damage, as assessed by the absorption of digoxin (DGX), a typical P-gp substrate.

Methods

Male Sprague-Dawley rats were intravenously administered CPT-11 at a dose of 60 mg/kg/day for 4 days to induce gastrointestinal damage. Then, the rats were administered DGX orally (40 μg/kg), after some of them were orally administered clarithromycin (CAM; 10 mg/kg), a P-gp inhibitor. DGX (30 μg/kg) was administered intravenously to determine the bioavailability (BA). The rats’ DGX plasma concentration profiles were determined using LC-MS/MS.

Results

CPT-11 treatment decreased the maximum concentration (C_max) and area under the plasma concentration-time curve (AUC_po) of DGX, which does not contradict to the DABE study. Although in the CPT-11-treated group the BA of DGX was significantly decreased to 40% of the control value, CAM did not affect the BA of DGX in the CPT-11-treated group.

Conclusions

Increased P-gp expression in rats with CPT-11-induced gastrointestinal damage is not necessarily associated with increased P-gp activity or contribution to the drug absorption in vivo. The decreased DGX absorption observed in this study might be attributable to other factors, such as a reduction in the absorptive surface area of the gastrointestinal tract.

Effect of excipients on oral absorption process according to the different gastrointestinal segments

INTRODUCTION

Excipients are necessary to develop oral dosage forms of any Active Pharmaceutical Ingredient (API). Traditionally, excipients have been considered inactive and inert substances, but, over the years, numerous studies have contradicted this belief. This review focuses on the effect of excipients on the physiological variables affecting oral absorption along the different segments of the gastrointestinal tract. The effect of excipients on the segmental absorption variables are illustrated with examples to help understand the complexity of predicting their in vivo effects.

AREAS COVERED

The effects of excipients on disintegration, solubility and dissolution, transit time, and absorption are analyzed in the context of the different gastrointestinal segments and the physiological factors affecting release and membrane permeation. The experimental techniques used to study excipient effects and their human predictive ability are reviewed.

EXPERT OPINION

The observed effects of excipient in oral absorption process have been characterized in the past, mainly in vitro (i.e. in dissolution studies, in vitro cell culture methods or in situ animal studies). Unfortunately, a clear link with their effects in vivo, i.e. their impact on Cmax or AUC, which need a mechanistic approach is still missing. The information compiled in this review leads to the conclusion that the effect of excipients in API oral absorption and bioavailability is undeniable and shows the need of implementing standardized and reproducible preclinical tools coupled with mechanistic and predictive physiological-based models to improve the current empirical retrospective approach.

Determination of intestinal permeability using in situ perfusion model in rats: Challenges and advantages to BCS classification applied to digoxin

The purpose of this work was to describe the closed loop in situ perfusion method in rats and to compare the difficulties and advantages with other methods proposed by regulatory agencies for BCS classification and finally to illustrate its application to evaluate the permeability of digoxin at relevant clinical concentrations. Digoxin was evaluated at two concentration levels: 1.0 μg/ml (with and without sodium azide 65.0 μg/ml) and 6.0 μg/ml. These concentrations correspond to the ratio of the highest dose strength (0.25 mg) and the highest single dose administered (1.5 mg) and the 250 ml of water. In situ closed loop perfusion studies in rats were performed in the whole small intestine and also in duodenum, jejunum and ileum segments to evaluate the relevance of P-gp secretion in the overall permeability. A kinetic modelling approach involving passive permeation and efflux transport mechanism allowed the estimation of the passive diffusional component and the Michaelis-menten parameters. The estimated K_m value demonstrated that at clinical luminal concentrations the efflux process is not saturated and then it could be inhibited by other drugs, excipients or food components leading to the already reported clinical drug-drug and drug-food interations. The present data confirms from a mechanistic point of view these interactions.

When barriers ignore the "rule-of-five"

Why are a few drugs with properties beyond the rule of 5 (bRo5) absorbed across the intestinal mucosa while most other bRo5 compounds are not? Are such exceptional bRo5 compounds exclusively taken up by carrier-mediated transport or are they able to permeate the lipid bilayer (passive lipoidal diffusion)? Our experimental data with liposomes indicate that tetracycline, which violates one rule of the Ro5, and rifampicin, violating three of the rules, significantly permeate a phospholipid bilayer with kinetics similar to labetalol and metoprolol, respectively. Published data from experimental work and molecular dynamics simulations suggest that the formation of intramolecular H-bonds and the possibility to adopt an elongated shape besides the presence of a significant fraction of net neutral species facilitate lipid bilayer permeation. As an alternative to lipid bilayer permeation, carrier proteins can be targeted to improve absorption, with the potential drawbacks of drug-drug interactions and non-linear pharmacokinetics.