Animal models for investigating intestinal drug absorption: various antibiotics

pH and P.E.G.-400 effects on flucloxacillin absorption

1. Intestinal clearances of flucloxacillin significantly decrease with the increase of pH, concomitantly with a decrease of the percent absorbed at 60 min. 2. The apparent rate constants for unionized (Ku) and ionized species are 0.173 min-1 and 0.017 min-1 respectively. 3. The addition of P.E.G.-400 at different concentrations significantly enhanced intestinal clearances of flucloxacillin parallel to an increase of net water flux from the lumen of the small intestine.

Kinetics and mechanism of in vitro uptake of amino-beta-lactam antibiotics by rat small intestine and relation to the intact-peptide transport system

By utilizing the everted jejunum of rats, the initial uptake rates of several antibiotics were measured over a wide range of concentrations. The uptakes followed mixed-type kinetics involving saturable and non-saturable processes in parallel. The pertinent kinetic parameters for the uptake of each antibiotic were determined. The effect of cephalexin on the uptake of cyclacillin obeyed competitive inhibition kinetics, and the inhibition constant Ki was found to be equal to the Michaelis constant Kt for the uptake of cephalexin itself. In a similar way, the uptake of cephalexin was inhibited by cyclacillin. Uptakes of both cyclacillin and cephalexin were reduced significantly by several metabolic inhibitors. From the effect of temperature on the uptakes of cyclacillin and cephalexin, activation energies of 24.8 and 23.1 kcal/mole were obtained respectively. These results indicate the involvement of an active transport mechanism for cyclacillin and cephalexin. It was found that several dipeptides markedly inhibited the uptakes of cyclacillin and cefadroxil. Furthermore, the uptake of glycylglycine, a typical dipeptide, was inhibited by cyclacillin, cefadroxil, cephalexin, and cephradine. The kinetics of mutual inhibition of the uptakes of cyclacillin and glycylglycine were consistent with competitive-type inhibition. This is the first report which establishes, from a kinetic point of view, the involvement of a common transport system in the in vitro uptakes of the dipeptides and the antibiotics.

Intestinal absorption mechanism of amphoteric beta-lactam antibiotics I: Comparative absorption and evidence for saturable transport of amino-beta-lactam antibiotics by in situ rat small intestine

The disappearance of various beta-lactam antibiotics from in situ rat small intestinal loops was studied at pH 7.4. For monobasic penicillins, despite the wide variety of apparent partition coefficients in isobutyl alcohol-water, the disappearance from the jejunal loops was almost 30% (+/- 5% SD). On the other hand, the disappearance of amphoteric derivatives of penicillins and cephalosporins having very low lipid solubility varied widely between 12 and 80%. The peak blood levels after intraduodenal administration to the rats correlated well with the extent of disappearance of amphoteric penicillins from the intestinal loops. Absorption studies utilizing in situ intestinal loops were performed at variable dose ranges to yield a clear dose-dependent disappearance. It is suggested that certain carrier-mediated transport systems underlie the absorption mechanism of amphoteric beta-lactam antibiotics.

In vivo absorption of phenytoin from rat small intestine and its inhibition by phlorizin

In vivo absorption of phenytoin from the small intestine was studied by an in vivo closed segment technique. Phenytoin in concentrations of 1000, 2000, and 4000 mumol/l was administered in dissolved form. Polythylene glycol 4000 was used as a non-absorbable marker. The concentrations of phenytoin in the intestinal lumen, in the mucosa, and in cardiac blood were measured both by spectrophotometry and by gas chromatography. Phenytoin was absorbed very rapidly, and the proportion absorbed increased with increasing dose. Thus, during the first 10 min. about 85 per cent of the largest dose but only 25 per cent of the smallest dose had been absorbed. The phenytoin concentration in mucosa and serum increased in an analogous way; maximum values were observed within the first ten minutes. The concentrations in mucosa and serum were dose dependent during the first ten minutes. 0.01 mmol/l and 1 mmol/l phlorizin significantly reduced the transfer of phenytoin (4000 and 2000 mumol/l) from the gut lumen to the mucosa. No inhibition was observed when the initial phenytoin dose was 1000 mumol/l. The results suggest that an active transport mechanism, sensitive to phlorizin, is involved in the intestinal absorption of phenytoin in the rat.

GI absorption of beta-lactam antibiotics. III: Kinetic evidence for in situ absorption of ionized species of monobasic penicillins and cefazolin from the rat small intestine and structure-absorption rate relationships

Absorption rates of monobasic beta-lactam antibiotics were measured as a function of lumen solution pH between 4 and 9 by utilizing the rat intestinal recirculating method in situ. Between pH 6.5 and 9, the absorption rate constants of ionized antibiotics were almost identical; but, at pH 4, the unionized species were highly absorbed, depending on their lipophilicity through the GI membrane lipoidal barrier. The structure-absorption rate relationship was established with the unstirred layer model.

GI absorption of beta-lactam antibiotics II: deviation from pH--partition hypothesis in penicillin absorption through in situ and in vitro lipoidal barriers

The absorption of propicillin from the rat stomach and small intestine in situ was examined as a function of recirculating solution pH. The in vitro interphase transport from an aqueous buffer of various pH values to the octanol phase was also studied for several penicillins by the use of a two-phase rolling cell. The rate--pH profiles obtained from both in situ and in vitro experiments deviated significantly from the dissociation curves. The degrees of the shifts were approximately 2 pH units for the in situ intestinal absorption of propicillin and in vitro transport of propicillin and cloxacillin, approximately 1.5 pH units for the in vitro transport of penicillin V, and 0.8 pH unit for the in situ stomach absorption of propicillin. These discrepancies from the classical pH--partition hypothesis can be interpreted by the permeation through the lipoidal barrier of the undissociated species of penicillins transported through the aqueous diffusion layer adjacent to the lipoidal surface. All in situ and in vitro experiments tend to support this theory.