Transport mechanisms of beta-lactam antibiotics across everted rat gut

Oral absorption of ampicillin: role of paracellular route vs. PepT1 transporter

The beta-lactam antibiotic ampicillin has a relatively poor oral bioavailability in animals and man (30-40%), and its widespread agricultural use in livestock may be contributing to the emergence of antibiotic resistance in the environment. The aim of this study was to define the absorption mechanism by which ampicillin crosses the small intestinal epithelium. The improved rat everted gut sac system was used, with an emphasis on the role of the PepT1 transporter. The absorption kinetics, effects of pH and the use of competitive substrates failed to provide any substantive evidence that the transporter played a major role in ampicillin absorption. Ethylenediaminetetraacetic acid enhanced the absorption, and tissue levels remained low, suggesting that paracellular transport was predominant. pH and competition studies with glycylsarcosine, the widely used PepT1 substrate, also failed to show any transporter activity. Despite evidence from studies with Caco-2 cells that beta-lactam antibiotics are transported by the PepT1 transporter in rat small intestine, the results rather suggest that paracellular diffusion is the major mechanism of absorption, at least for beta-lactam antibiotics with poor bioavailability, such as ampicillin. We suggest that the use of Caco-2 cells underestimates the role of the paracellular route in the absorption of hydrophilic drugs in vivo, and may exaggerate the role of influx transporters.

Characterization of the oral absorption of some beta-lactams: effect of the alpha-amino side chain group

The intestinal absorption mechanisms of cefixime, 7-aminocephalosporanic acid (7-ACA) and 6-aminopenicillanic acid (6-APA) were determined from the results of single-pass perfusion experiments in rats by modified boundary layer analysis. The estimated absorption parameters (SEM) were as follows: for cefixime, J*max = 0.016 (0.008) mM, Km = 0.031 (0.015) mM, P*m = 0.184 (0.037), P*c = 0.523 (0.051); for 7-ACA, J*max = 6.39 (1.57) mM, Km = 19.33 (5.64) mM, P*c = 0.33 (0.03) mM; and for 6-APA, P*m = 0.41 (0.11), where J*max is the maximal flux of peptide transport system, Km is the intrinsic Michaelis constant, P*m is the dimensionless membrane permeability, and P*c is the dimensionless carrier permeability. Cefixime was absorbed by a carrier-mediated mechanism because its wall permeability (P*w) was concentration dependent and significantly inhibited by cephradine. A concentration-dependent permeability of 7-ACA was observed, but an inhibition study failed to show significant inhibition by cephradine. The absorptions of 6-APA and penicillin V were not inhibited by cephradine or cefixime. The fractions of dose absorbed of several beta-lactam antibiotics correlated well with their absorption numbers obtained from P*w values in rats. These results further demonstrate that an alpha-amino group is not necessary for transport by the intestinal peptide transporter.

Characterization of the oral absorption of several aminopenicillins: determination of intrinsic membrane absorption parameters in the rat intestine in situ

The absorption mechanism of several penicillins was characterized using in situ single-pass intestinal perfusion in the rat. The intrinsic membrane parameters were determined using a modified boundary layer model (fitted value +/- S.E.): Jmax* = 11.78 +/- 1.88 mM, Km = 15.80 +/- 2.92 mM, Pm* = 0, Pc* = 0.75 +/- 0.04 for ampicillin; Jmax* = 0.044 +/- 0.018 mM, Km = 0.058 +/- 0.026 mM, Pm* = 0.558 +/- 0.051, Pc* = 0.757 +/- 0.088 for amoxicillin; and Jmax* = 16.30 +/- 3.40 mM, Km = 14.00 +/- 3.30 mM, Pm* = 0, Pc* = 1.14 +/- 0.05 for cyclacillin. All of the aminopenicillins studied demonstrated saturable absorption kinetics as indicated by their concentration-dependent wall permeabilities. Inhibition studies were performed to confirm the existence of a nonpassive absorption mechanism. The intrinsic wall permeability (Pw*) of 0.01 mM ampicillin was significantly lowered by 1 mM amoxicillin and the Pw* of 0.01 mM amoxicillin was reduced by 2 mM cephradine consistent with competitive inhibition.

Contribution of passive transport mechanisms to the intestinal absorption of beta-lactam antibiotics

The transport characteristics of aminopenicillins (ampicillin and amoxicillin), aminocephalosporins (cephalexin, cephradine and cefadroxil) and cefazolin have been compared with those of an actively transported substance (D-glucose) and a passively transported substance (L-glucose). Although the initial uptake of the aminocephalosporins was stimulated in the presence of an inward H+ gradient, there was no overshoot in the uptake of any of the drugs tested, even in the presence of an H+ gradient. Also, the time course and the degree of uptake of these drugs were similar to those of L-glucose, especially in the absence of an H+ gradient. These results suggest that the beta-lactam antibiotics tested, like L-glucose, pass through the rat intestinal brush border membrane mainly by passive diffusion. However, the differences in absorption between these drugs, like the differences in their disappearance from a proximal loop of rat intestine, cannot be explained by a simple permeation process alone.

Effect of chlorpromazine on the permeability of beta-lactam antibiotics across rat intestinal brush border membrane vesicles

The effect of chlorpromazine on the membrane permeability of beta-lactam antibiotics (benzylpenicillin, ampicillin, cephradine and cephalexin) and actively transported substances (glycylglycine and D-glucose) has been studied using rat intestinal brush border membrane vesicles. Except for cephalexin, the initial uptakes at 25 degrees C of these antibiotics were significantly enhanced in the presence of chlorpromazine. In contrast, the transport of glycylglycine and D-glucose was significantly inhibited. These results suggest that the two groups, drugs and actively transported substances, have a different permeation process. The effect of chlorpromazine concentration on membrane lipid fluidity, as assessed by the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-anilino-8-naphthalene sulphonate (ANS), was also examined. The fluorescence polarization of ANS decreased with increasing concentration of chlorpromazine, while that of DPH increased suggesting an increase of membrane surface fluidity might affect the permeation of beta-lactam antibiotics and actively transported substances in a different manner.

Characterization of the oral absorption of beta-lactam antibiotics. I. Cephalosporins: determination of intrinsic membrane absorption parameters in the rat intestine in situ

The oral absorption of five cephalosporin antibiotics, cefaclor, cefadroxil, cefatrizine, cephalexin, and cephradine, has been studied using a single-pass intestinal perfusion technique in rats. Intrinsic membrane absorption parameters, "unbiased" by the presence of an aqueous permeability (diffusion or stagnant layer), have been calculated utilizing a boundary layer mathematical model. The resultant intrinsic membrane absorption parameters are consistent with a significant carrier-mediated, Michaelis-Menten-type kinetic mechanism and a small passive component in the jejunum. Cefaclor colon permeability is low and does not exhibit concentration dependent behavior. The measured carrier parameters (+/- SD) for the jejunal perfusions are as follows: cefaclor, J*max = 21.3 (+/- 4.0), Km = 16.1 (+/- 3.6), P*m = 0, and P*c = 1.32 (+/- 0.07); cefadroxil, J*max = 8.4 (+/- 0.8), Km = 5.9 (+/- 0.8), P*m = 0, and P*c = 1.43 (+/- 0.10); cephalexin, J*max = 9.1 (+/- 1.2), Km = 7.2 (+/- 1.2), P*m = 0, and P*c = 1.30 (+/- 0.10); cefatrizine, J*max = 0.73 (+/- 0.19), Km = 0.58 (+/- 0.17), P*m = 0.17 (+/- 0.03), and P*c = 1.25 (+/- 0.10); and cephradine, J*max = 1.57 (+/- 0.84), Km = 1.48 (+/- 0.75), P*m = 0.25 (+/- 0.07), and P*c = 1.06 (+/- 0.08). The colon absorption parameter for cefaclor is P*m = 0.36 (+/- 0.06, where J*max (mM) is the maximal flux, Km (mM) is the Michaelis constant, P*m is the passive membrane permeability, and P*c is the carrier permeability.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct photoaffinity labelling of binding proteins for beta-lactam antibiotics in rabbit intestinal brush border membranes with [3H]benzylpenicillin

Brush border membrane vesicles from rabbit small intestine were used to study the intestinal uptake system for beta-lactam antibiotics. Benzylpenicillin inhibited the H+-dependent uptake of alpha-aminocephalosporins in a concentration-dependent manner suggesting a common transport system for alpha-aminocephalosporins and benzylpenicillin. Benzylpenicillin is therefore a suitable probe to characterize this transport system. Irradiation of [3H]benzylpenicillin using light sources having their maximum of radiation at 300 or 254 nm resulted in a covalent incorporation of radioactivity into penicillin binding proteins as was shown with serum albumin. Hence [3H]benzylpenicillin can be used for direct photoaffinity labeling of penicillin binding proteins in different cells and tissues. In brush border membrane vesicles from rabbit small intestine predominantly a membrane polypeptide with an apparent molecular weight of 127,000 was labeled by [3H]benzylpenicillin. Competition labeling experiments demonstrated that beta-lactam antibiotics--penicillins and cephalosporins--specifically interact with this protein, whereas amino acids, sugars or bile acids had no effect on the labeling pattern. Compounds which decreased the labeling of the 127,000 molecular weight membrane polypeptide also inhibited the H+-dependent uptake of the alpha-aminocephalosporin cephalexin into intestinal brush border membrane vesicles. These results suggest that a polypeptide of molecular weight 127,000 in the brush border membrane from rabbit small intestine is a constituent of a common transport system responsible for the uptake of orally effective beta-lactam antibiotics and dipeptides. beta-Lactam antibiotics which are not absorbed from the small intestine also bind from the luminal site to this transport system, but are not transported across the brush border membrane.

Binding of amino beta-lactam antibiotics to soluble protein from rat intestinal mucosa--I. Purification of drug-binding protein

Several amino beta-lactam antibiotics, including ampicillin, amoxicillin, cyclacillin, cephalexin, cephradine and cefadroxil, were found to bind in vitro to specific components in 105,000 g supernatant of homogenate obtained from rat intestinal mucosa. The major binding component (fraction b) was purified by chromatography on DEAE-cellulofine and by gel filtration on Sephadex G-50. The molecular weight of fraction b was determined by SDS polyacrylamide gel electrophoresis (15,000 Da). The binding behaviour of these amino beta-lactam antibiotics to fraction b were estimated by equilibrium dialysis. There were significant high affinities of all tested amino beta-lactam antibiotics which were well absorbed from intestine, but there was not a good correlation between binding and absorption of these drugs. It was also found that poorly absorbed cephalosporins which lack aminobenzyl group in their structure, cefazolin and cephaloridine, did not bind to fraction b.

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.

Phase equilibria of nafcillin sodium-water

The phase diagram for the binary system nafcillin sodium-water, was determined using differential scanning calorimetry (DSC) and polarized light microscopy. In the temperature range of -20-30 degrees, three crystalline forms and an amphiphilic liquid crystalline phase were detected. The stable crystalline form of nafcillin sodium (form alpha) and water exhibit a eutectic mixture containing 28% drug (w/w) at -1 degrees. In more dilute solutions a lower temperature eutectic (-5 degrees) occurs. The composition and form of nafcillin sodium in this eutectic are not known. The form -5 degrees eutectic was found to be metastable above 28% concentration and converted to form alpha. Two other crystalline forms were observed at 9 degrees (form beta) and 22 degrees (form gamma) at concentrations above 40%. The crystalline forms could not be further characterized due to their transient nature and existence in highly concentrated mixtures. A lamellar mesophase is present near ambient temperature in mixtures containing more than 55% nafcillin sodium. The phase equilibria were highly susceptible to supercooling. Temperature cycling methods were devised which gave reproducible DSC data and allowed construction of the phase diagram.

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.

The use of liposomes as a model for drug absorption: beta-lactam antibiotics

Liposomes were prepared from egg phosphatidylcholine-cholesterol-diacetylphosphate (80:20:5) and total lipid extracts of rat intestinal mucosa, and the permeabilty of the liposomal membrane to eight beta-lactam antibiotics was studied by using a dynamic dialysis method. Although all the antibiotics used here are ionized and poorly lipid-soluble at pH 6.5, some of them are orally active and efficiently absorbed from the small intestine. The release rate constants from the aqueous dispersion of drug-entrapped liposomes were approximately in the order of their absorbability. Intestinal lipid liposomes were more permeable to the antibiotics than egg lecithin liposomes and the release rate constants for the drugs from intestinal lipid liposomes were strongly correlative with their absorption rate constants, except for cephalothin and ampicilin, the deviations of which could be explained by their surface activity. It is suggested that lipid components of the intestinal mucosa and the bilayer structure may play an important role in the absorption process of the antibiotics. The validity of liposomes as a model for the intestinal absorption of drugs is also discussed.

Pharmacokinetics of ampicillin and its prodrugs bacampicillin and pivampicillin in man

Five healthy fasting male subjects were each given single doses of intravenous ampicillin (471 mg), oral ampicillin tablets (495 mg), oral bacampicillin hydrochloride tablets (562 mg ampicillin equivalent), and oral pivampicillin hydrochloride capsules (491) mg ampicillin equivalent) in a crossover experiment. The resulting concentrations of ampicillin were determined in plasma and urine. The pharmacokinetic analysis was made according to a two-compartment open model. The total distribution volume of unbound ampicillin during the disposition phase was 0.247 +/- 0.045 (SD) liter/kg, which is only slightly more than the extracellular fluid, suggesting that tissue binding and intracellular distribution of ampicillin are limited. The bioavailability of the esters bacampicillin (86 +/- 11%) and pivampicillin (92 +/- 18%) was significantly greater than that of ampicillin (62 +/- 17%); however, the difference between the esters was not statistically significant. The adsorption for all drugs given orally proceeded at a constant rate, suggesting zero-order release rates from the products. The adsorption rate was highest for bacampicillin (0.89 +/- 0.39 of dose absorbed per minute), followed by pivampicillin (0.64 +/- 0.19) and ampicillin (0.58 +/- 0.16). Bacampicillin also had the shortest lag time for the start of absorption (7.0 +/- 0.9 min) under the present conditions. Thus, in comparison with ampicillin, the esters have a higher bioavailability, which, in fact, is close to the theoretically highest possible value by clearance concepts. The higher bioavailability in connection with higher absorption rates may be clinically important in ampicillin treatment by the oral route.

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.

Effect of ionization on absorption of cephalosporins

To explore the relative absorbabilities of different ionic forms of cephalosporins, the absorption rates of four compounds were measured in the pH 5-9 region using an in situ rat gut technique. Cephalexin, cephradine, and cephaloglycin have some oral activity, while 3-[(acetyloxy)methyl]-8-oxo-7-[[(4-oxo-1(4H-pyridinyl)acetyl]-amino]-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid (I) has insignificant oral activity. The pH-species profiles calculated from their ionization constants showed that cephalexin, cephradine, and cephaloglycin have a large proportion of uncharged molecules plus zwitterions in the pH range of the small intestine, while I exists as the anion throughout this range. When the species profiles are compared with the pH-absorption rate profiles for cephalexin, cephradine, and I, the results are consistent with a model in which the zwitterionic and/or uncharged forms of the molecules are well absorbed, whereas the anions show little or no absorption. Although it has a pH profile for zwitterions plus uncharged molecules similar to cephalexin, cephaloglycin shows poor absorption, suggesting that the ratio of uncharged molecules to zwitterions may be important in absorption.

Absorption of amino penicillins from everted rat intestine

1. Using an in vitro everted gut sac method based on that of Wilson & Wiseman (1954), a number of amino penicillins were tested in order to identify the involvement of any specialized transport mechanisms in their absorption across rat intestine. 2. Only one of the amino penicillins, cyclacillin (1-amino-cyclohexyl penicillin) was shown to be actively transported. The other penicillins appeared to diffuse passively across the intestine. 3. Cyclacillin was found to concentrate against a gradient at 37 degrees C but not at 19 degrees C. 4. Transport of cyclacillin across the mucosal membrane was saturated at mucosal concentrations greater than 1000 microgram/ml. 5. The rate of the forward flux of cyclacillin was many times that of its back flux. 6. No relationship between the active transport of cyclacillin and that of amino acids could be demonstrated.