Absorption-partition relationships for true homologous series of xenobiotics as a possible approach to study mechanisms of surfactants in absorption. II. Aromatic amines in rat small intestine

Closed-Loop Doluisio (Colon, Small Intestine) and Single-Pass Intestinal Perfusion (Colon, Jejunum) in Rat-Biophysical Model and Predictions Based on Caco-2

PURPOSE

The effective rat intestinal permeability (P _eff ) was deconvolved using a biophysical model based on parameterized paracellular, aqueous boundary layer, transcellular permeabilities, and the villus-fold surface area expansion factor.

METHODS

Four types of rat intestinal perfusion data were considered: single-pass intestinal perfusion (SPIP) in the jejunum (n = 40), and colon (n = 15), closed-loop (Doluisio type) in the small intestine (n = 78), and colon (n = 74). Moreover, in vitro Caco-2 permeability values were used to predict rat in vivo values in the rat data studied.

RESULTS

Comparable number of molecules permeate via paracellular water channels as by the lipoidal transcellular route in the SPIP method, although in the closed-loop method, the paracellular route appears dominant in the colon. The aqueous boundary layer thickness in the small intestine is comparable to that found in unstirred in vitro monolayer assays; it is thinner in the colon. The mucosal surface area in anaesthetized rats is 0.96-1.4 times the smooth cylinder calculated value in the colon, and it is 3.1-3.6 times in the small intestine. The paracellular permeability of the intestine appeared to be greater in rat than human, with the colon showing more leakiness (higher P _para ) than the small intestine.

CONCLUSION

Based on log intrinsic permeability values, the correlations between the in vitro and in vivo models ranged from r² 0.82 to 0.92. The SPIP-Doluisio method comparison indicated identical log permeability selectivity trend with negligible bias.

Valence topological charge-transfer indices for dipole moments: percutaneous enhancers

Valence topological charge-transfer (CT) indices are applied to the calculation of dipole moments. The algebraic and vector semisum CT indices are defined. The combination of CT indices allows the estimation of the dipole moments. The model is generalized for molecules with heteroatoms. The ability of the indices for the description of the molecular charge distribution is established by comparing them with the dipole moments of homologous series of percutaneous enhancers (phenyl alcohols and 4-alkylanilines). Linear and quadratic correlation models are obtained. CT indices improve the multivariable quadratic regression equations for the dipole moment. The variance decreases 97% (4-alkylanilines). No superposition of the corresponding G(k)-J(k)and G(k)(V)-J(k)(V) pairs is observed in the fits, which diminishes the risk of co-linearity. The inclusion of the heteroatom in the pi-electron system is beneficial for the description of the dipole moment, owing to either the role of the additional p orbitals provided by the heteroatom or the role of steric factors in the pi-electron conjugation. Inclusion of a conjugated double bond in the alkyl chain lends to more rigid structures with dipole moment variations lower than 1%.

Physiochemical and physiological mechanisms for the effects of food on drug absorption: the role of lipids and pH

Drugs are absorbed after oral administration as a consequence of a complex array of interactions between the drug, its formulation, and the gastrointestinal (GI) tract. The presence of food within the GI tract impacts significantly on transit profiles, pH, and its solubilization capacity. Consequently, food would be expected to affect the absorption of co-administered drugs when their physicochemical properties are sensitive to these changes. The physicochemical basis by which ingested food/lipids induce changes in the GI tract and influence drug absorption are reviewed. The process of lipid digestion is briefly reviewed and considered in the context of the absorption of poorly water-soluble drugs. The effect of food on GI pH is reviewed in terms of location (stomach, upper and lower small intestine) and the temporal relationship between pH and drug absorption. Case studies are presented in which postprandial changes in bioavailability are rationalized in terms of the sensitivity of the physicochemical properties of the administered drug to the altered GI environment.

Partition and distribution coefficients of solutes and drugs in brush border membrane vesicles

Partition and distribution coefficients (log P, log D) into rat small intestinal brush border membrane (BBM) were measured for a variety of ionizable and non-ionizable drugs and solutes using a novel technique. The log P values were compared with those determined with model solvents, octanol and propylene glycol dipelargonate (PGDP). Non-ionizable solutes with log P values up to 3.0 showed that octanol was a better model for partition into the BBM than PGDP. With one exception, BBM partition coefficients of greater than 3 were not observed, even for solutes with log P values in model solvents that were greater than 5. Liposomes prepared from BBM lipids, or synthetic lipid mixtures of similar composition to BBM, demonstrated similar trends in partition coefficients to the intact BBM. Two cationic drugs, Atenolol and Xamoterol were investigated for partition into BBM lipid liposomes. An apparent enhancement of log D with respect to octanol was attributed to a "surfactant-like" orientation in the membrane and an interaction of the ionized drug with anionic phospholipid head groups. The anionic drug Proxicromil shows the expected decrease in log D with increasing pH, at low NaCl concentrations. Changes in electrophoretic mobility of liposomes after incorporation of Proxicromil into them were consistent with the negative charge of the ionized drug being at the membrane surface. It was concluded that Proxicromil also associates with membranes in a "surfactant-like" orientation and that increased extraction with increasing NaCl concentrations is a result of ionic strength effects. Partition of solutes into BBM vesicles is more complex than into organic solvents and probably represents an important step in overall intestinal permeation of solutes.

Pharmacokinetics and bioavailability of diclofenac in the rat

Diclofenac sodium is a widely used drug with interesting absorption and disposition features when administered to laboratory animals. The present study was undertaken to assess the pharmacokinetics of the drug after iv and gastrointestinal dosing to rats. Renal excretion of unchanged drug was negligible, but biliary excretion of the drug (unchanged and conjugated) was detected in bile duct-cannulated rats; it accounted for 27.2 and 31.2% of the total dose following iv and intraduodenal administration, respectively. Most of the drug excreted in the bile was conjugated diclofenac; unchanged drug accounted for only 4.7 and 5.4% of total diclofenac excreted in the bile after iv and intraduodenal dosing, respectively. In normal animals, intestinal absorption of the drug excreted in the bile resulted in higher drug concentrations in plasma than those obtained in bile duct-cannulated rats, but only after 60 min of dosing. When administered directly into the duodenum, diclofenac absorption was extremely fast and the maximum plasma diclofenac concentration was reached within 2 min. After oral dosing, an early peak was also observed, but it was lower than that obtained after intraduodenal dosing: 71% diclofenac bioavailability was found in bile duct-cannulated rats intraduodenally dosed, whereas in normal animals dosed by mouth a bioavailability of 79% was obtained. In normal animals intraduodenally dosed, an apparent bioavailability of 106% was observed. All of these features, particularly the influence of enterohepatic circulation on drug bioavailability, are discussed.

In vitro studies of intestinal drug absorption. Determination of partition and distribution coefficients with brush border membrane vesicles

Brush border membrane vesicles (BBMV) were isolated from rat small intestine and characterized in terms of relative enrichment of specific organelle marker enzymes (20-fold enrichment; 20% yield), contamination by other subcellular organelles (less than 1%) and functional integrity (Na(+)-dependent glucose uptake). Using these vesicles, techniques were developed for the determination of partition and distribution coefficients for the model solutes, nitrobenzene, toluene and benzoic acid. No gender, age or regional variation along the small intestine in partition coefficient (log P) values was detected. There was no temperature (10-40 degrees) or pH (4.5-8.0) dependence in partition coefficients of nitrobenzene and toluene. Fair agreement was obtained for log P and log D values for these two solutes determined with BBMV and those reported with octanol and propylene glycol dipelargonate. Selective removal of proteins, both ecto-brush border and micro-villus core proteins, did not alter the partition coefficients of the three model solutes. In contrast, depletion of the BBMV of non-esterified fatty acids significantly decreased the partition coefficients. Liposomes prepared from BBMV lipid extracts were also used for partition coefficient determinations and gave similar values to intact BBMV; addition of increasing amounts of cholesterol to the lipid extract caused small increases in the partition coefficients of the model solutes in the liposomes. It was concluded that the partition coefficients of the BBMV were related to the lipid and not to the protein composition of the vesicles. The method offers a rapid and reliable means of measuring the partition coefficient of non-protein bound drugs and nutrients in isolated intestinal BBMV and should assist in the subsequent modelling and prediction of intestinal absorption in vivo.