Effect of the transport medium composition on in vitro drug permeation across excised pig intestinal tissue

Abstract

A crucial step in the process of drug research and development is the investigation of permeation characteristics of new chemical entities across biological membranes to give an indication of their pharmacokinetic properties. Permeation studies are necessary to ensure that drug candidates have acceptable permeability properties before proceeding to more expensive clinical trials. Various techniques are currently employed by researchers to determine the pharmacokinetic properties of pharmaceutical compounds. In vitro techniques are commonly used as screening tools to quickly evaluate the ADME-Tox properties of pharmacologically active compounds. Ex vivo methods, a subdivision of in vitro techniques, include the evaluation of drug transport across excised segments of intestinal tissue. Excised pig intestinal tissue models are commonly used in ex vivo pharmacokinetic studies. Due to physiological, biochemical and anatomical similarities to humans, the pig model is considered to be sufficiently accurate to be used to predict the pharmacologically active compound’s absorption and efflux rates in humans. Due to the difficulty associated with obtaining aspirated intestinal fluid, simple aqueous buffers are often employed in permeation and solubility studies. These simple buffer systems are, however, not necessarily adequate to predict intestinal solubility or transport. This is due to the effect of pH, stomach content and to a certain extent the ionisation of the drug. Transport medium components used in in vitro permeation tests may significantly alter the effect of P-gp on the transport of compounds across the intestinal epithelium. This may be attributed to the fact that the transport media may alter the “tightness” of the intercellular juctions as indicated by the trans-epithelial electrical resistance (TEER) values. The media may either inhibit or stimulate P-gp proteins to such an extent that the rate of P-gp mediated efflux is modulated. Due to these factors, the results obtained from the bi-directional transport studies may differ significantly from that which is encountered in a live animal. To preserve the predictive value of screening tests, it is important to know what influence the transport medium composition may have on the activity of the efflux transporter proteins, tight junctions and solubility of the model drugs The aim of this study was to compare different types of transport media (which included Phosphate buffer, Krebs Ringer Bicarbonate buffer and simulated intestinal fluids in the fed and fasted states) on the bi-directional transport of four model compounds from the different classes of the Biopharmaceutical Classification System (BCS) in the Sweetana-Grass diffusion chamber apparatus across excised pig intestinal tissue.