Correlation between structures of organic cations and their binding behaviours to brush border membrane isolated from rat small intestine

Influence of enhancers on the absorption and on the pharmacokinetics of cefodizime using in-vitro and in-vivo models

In the development of novel antibiotics, more and more compounds have been found that cannot be absorbed orally and, therefore, must be administered intravenously or intramuscularly. Because of the obvious drawbacks of drug delivery by injection, the development of alternatives with enhanced oral bioavailability has received much attention in pharmaceutical research. Cefodizime, a novel third-generation cephalosporin with significant advantages in the parenteral treatment of common infections, was used as a model drug. Cefodizime behaves as a highly hydrophilic compound, as shown from its extremely low partition coefficient. The effect of cationic absorption enhancers (hexadecyldimethyl-benzylammonium chloride, N-hexadecylpyridinium bromide, dodecyltrimethylammonium bromide and hexadecyltrimethylammonium bromide) on the lipophilicity of cefodizime was investigated by means of the n-octanol/water system. Results showed that the counter-ions had a positive influence on the solubility of cefodizime. These results on partitioning coefficients in the n-octanol/buffer system were confirmed using an in-vitro transport model with artificial and biological membranes (Caco-2-cells). Furthermore, the physiological compatibility of the absorption enhancers was investigated using the active d-glucose transport. The pharmacokinetic profile of cefodizime was evaluated in rabbits after intraduodenal administration with and without an absorption enhancer.

Permeation characteristics of a hydrophilic basic compound across a bio-mimetic artificial membrane

In the present study, the permeation characteristics of a hydrophilic basic compound (HBC) in a bio-mimetic parallel artificial membrane permeability assay (bio-mimetic PAMPA) were investigated in detail. The bio-mimetic PAMPA membrane was constructed on a hydrophobic filter by impregnating a lipid solution consisting of phosphatidylcholine (0.8%, w/w), phosphatidylethanolamine (0.8%, w/w), phosphatidylserine (0.2%, w/w), phosphatidylinositol (0.2%, w/w), cholesterol (1.0%, w/w), and 1,7-octadiene (97.0%, w/w). The pH-permeability curve (pH 3-10), the effect of lipid composition, concentration dependency (0.02-2.00 mM), and inhibition by other cationic compounds, were investigated for several HBCs. Ketoprofen and methylchlorpromazine were also employed as an acidic and a quaternary ammonium compound, respectively. At pH 3-6, the permeability of timolol, a HBC, was higher than expected from the pH-partition hypothesis, especially in the PI-containing membrane, whereas the pH-permeability curve of ketoprofen followed the pH-partition hypothesis. Permeation of HBC was saturable and inhibited by basic and quaternary ammonium compounds. Similar results were also found for methylchlorpromazine. The permeation characteristics of HBC observed in the present study are not usually expected in a passive permeation process across an artificial membrane. The participation of facilitated permeation of cationic species was suggested, in addition to a simple passive diffusion of un-dissociated species. Ion pair transport was suggested as a possible permeation mechanism of cationic species. However, further investigation is necessary to clarify the reason for the permeation characteristics of HBC.

Enhancement of intestinal transport of thyrotropin-releasing hormone via a carrier-mediated transport system by chemical modification with lauric acid

The transport characteristics of thyrotropin-releasing hormone (TRH) and its chemically modified derivative with lauric acid (Lau-TRH) across the rat small or large intestine were estimated by means of an in vitro everted sac experiment. Both compounds were especially absorbed from the upper small intestine. The penetration of TRH across the upper small intestine was significantly increased by conjugation with lauric acid. Lau-TRH administered to the mucosal side appeared as a native TRH form in the serosal side. On the other hand, a temperature dependency and a directional difference in the transfer rates of these compounds were observed in the everted and non-everted sacs of the upper small intestine. Moreover, the penetration of TRH and Lau-TRH across the upper small intestine was inhibited by 0.25 mM 2,4-dinitrophenol and 10 mM glycylglycine. In addition, Lau-TRH was very stable in the cytosolic fraction of the small intestinal mucosa, while it was gradually converted to the native TRH in the brush-border membrane (BBM) fraction. The binding amounts of TRH to the BBM were remarkably enhanced by the lauric acid conjugation; however, its binding was nonspecific. Therefore, it was suggested that Lau-TRH rapidly bound to the BBM in the small intestine, where Lau-TRH is converted to TRH, and this released TRH is efficiently transported by an oligopeptide transporter which exists in the upper small intestine.

Carrier-mediated transport system for choline and its related quaternary ammonium compounds on rat intestinal brush-border membrane

The characteristics of the intestinal transport system for choline were investigated using isolated brush-border membrane vesicles from rat small intestine. In spite of the diminutive lipid solubility, the uptake of choline by membrane vesicles reflected smooth permeation into intravesicular space rather than the binding to the membrane surface. Physiological conditions, present in the intact intestine, such as an inward-directed Na+ or H+ gradient and inside negative membrane potentials, didn't directly involve in choline transport across the brush-border membrane. Moreover, an outward-directed H+ gradient had no significant effect on the time course of choline transport. However, in the absence of a driving-force, the initial uptake of choline exhibited a saturable manner. A kinetic analysis of the initial uptake rate gave an apparent Km of 159 microM. Furthermore, unlabeled choline caused both cis-inhibition and trans-stimulation for labeled choline transport, suggesting the existence of a carrier-mediated transport system for choline, classified as so-called 'facilitated diffusion'. Since tetramethylammonium, acetylcholine, and N1-methylnicotinamide caused both cis-inhibition and trans-stimulation, they appear to be accepted as the substrate of choline carrier. On the other hand, quaternary ammonium compounds (QACs) such as those which possessed hydrophobic parts in their molecules exhibited only cis-inhibition. They also inhibited Na(+)-dependent D-glucose transport, indicating that they influenced various carrier-mediated transport systems non-specifically due to interaction with the membrane. These findings strongly suggest that the choline transport system on the brush-border membrane of rat intestine recognizes only small molecular QACs as its substrate.

Gastrointestinal absorption of quaternary ammonium compounds correlated to their binding to small intestinal brush border membrane in rat

The relationship between absorption of quaternary ammonium compounds (QACs) from rat intestine and their in-vitro binding to isolated brush-border membrane has been examined, using a series of n-alkyltrimethylammoniums. The binding of these QACs gradually increased with each extension of unbranched hydrocarbon chain from octyltrimethylammonium to tetradecyltrimethylammonium. However, hexyltrimethylammonium and heptyltrimethylammonium failed to bind to the membrane. On the other hand, the disappearance of these QACs from rat jejunal loop also increased with the length of hydrocarbon chain over the range of 8.9 to 71.3%. A good correlation was found between binding to the brush-border membrane and disappearance from jejunal loop. From these results, it was suggested that the size of the hydrophobic part of a QAC molecule was a principal determinant of both absorption and membrane binding, and that the absorption of QACs with an appropriate sized hydrophobic part, was closely associated with the degree of binding to the membrane.