Mechanism Of Intestinal Absorption Research Articles

5'-Amino acid esters of antiviral nucleosides, acyclovir, and AZT are absorbed by the intestinal PEPT1 peptide transporter.

General use of nucleoside analogues in the treatment of viral infections and cancer is often limited by poor oral absorption. Valacyclovir, a water soluble amino acid ester prodrug of acyclovir has been reported to increase the oral bioavailability of acyclovir but its absorption mechanism is unknown. This study characterized the intestinal absorption mechanism of 5' -amino acid ester prodrugs of the antiviral drugs and examined the potential of amino acid esters as an effective strategy for improving oral drug absorption. Acyclovir (ACV) and Zidovudine (AZT) were selected as the different sugar-modified nucleoside antiviral agents and synthesized to L-valyl esters of ACV and AZT (L-Val-ACV and L-Val-AZT), D-valyl ester of ACV (D-Val-ACV) and glycly ester of ACV (Gly-ACV). The intestinal absorption mechanism of these 5' -amino acid ester prodrugs was characterized in three different experimental systems; in situ rat perfusion model, CHO/hPEPT1 cells and Caco-2 cells. Testing 5' -amino acid ester prodrugs of acyclovir and AZT, we found that the prodrugs increased the intestinal permeability of the parent nucleoside analogue 3- to 10-fold. The dose- dependent permeation enhancement was selective for L-amino acid esters. Competitive inhibition studies in rats and in CHO cells transfected with the human peptide transporter, hPEPT1, demonstrated that membrane transport of the prodrugs was mediated predominantly by the PEPT1 H+/dipeptide cotransporter even though these prodrugs did not possess a peptide bond. Finally, transport studies in Caco-2 cells confirmed that the 5' - amino acid ester prodrugs enhanced the transcellular transport of the parent drug. This study demonstrates that L-amino acid-nucleoside chimeras can serve as prodrugs to enhance intestinal absorption via the PEPT1 transporter, providing a novel strategy for improving oral therapy of nucleoside drugs.

Intestinal absorption of aluminum: effect of sodium and calcium.

Aluminum (Al) is recognized as a toxin in patients with deficient renal function. Moreover, Al may play a role in some neurodegenerative diseases. It is hypothesized that more than one intestinal absorption mechanism exists for Al, related to various intraluminal chemical species, and that Al shares intestinal transport routes for essential inorganic substances due to similarities in their chemical speciation characteristics. The purpose of the present investigation was to study the effects of ionic Na (0-120 mmol/l) and Ca (0-10 mmol/l), alone and in combination, on the intestinal absorption of ionic Al (20 and 30 mmol/l). A previously standardized method of perfusion of rat small intestine was used, combined with serial blood sampling. Mucosal uptake and systemic appearance of Al in the blood were monitored during 1 h perfusion, together with Na and Ca; the metals were given as the chloride. The intestinal absorption of Na and Ca was taking place according to previously reported mechanisms. A one-sided negative interaction was shown to exist between Ca and Al resp. Al and Na during both mucosal uptake and systemic appearance; Ca interacts negatively with Na during systemic appearance, but enhances mucosal uptake of Na. It may be speculated that Al mimicks Ca in its Na-dependent intestinal passage.

Influence of Dietary Fat on the Intestinal Absorption of Lipophilic Compounds in Goldfish (Carassius auratus)

Dietary uptake of a mixture of pp′DDT and four chlorobenzenes from diets with different lipid contents was measured in goldfish (Carassius auratus) in order to investigate the mechanism of intestinal absorption of organic compounds. The results of the experiments suggest that intestinal absorption is basically controlled by chemical diffusion rather than lipid coassimilation. The extent of dietary uptake as indicated by biomagnification factor was strongly correlated with the chemical logKow, indicating that uptake of the chemicals from the gastrointestinal fluid is similar to the uptake from other aqueous environments and lipid content of the food in the range used in these experiments (2.9–10.9%) could not influence the uptake of lipophilic chemicals.

Contrasting Nutrient Effects on the Plasma Levels of an Amino Acid-Like Antiepileptic Agent from Jejunal Administration in Dogs

The absorption of gabapentin was investigated by monitoring drug plasma levels as a function of time following midjejunal administration in mongrel dogs. From previous work, dose-dependent absorption had been postulated to be a consequence of carrier-mediated transport and a paracellular pathway had been postulated to contribute to the passive absorption component in mammalian small intestine. The potential for amino acid inhibition of the carrier-mediated absorption component was investigated by drug coinfusion with leucine and phenylalanine. The potential for monosaccharide-enhanced increases in drug absorption was studied by drug coinfusion with D-glucose and 3-O-methylglucose. While lower drug plasma levels were observed with amino acid coinfusion versus controls in each of the dogs studied, mean area under the plasma level time curves (AUC) were not statistically significantly different (p ≤ 0.07). Monosaccharide coinfusion significantly increased gabapentin AUC over control studies (p≤0.014) and over coinfusion with L-system amino acids (p≤0.0025). Implications for the mechanisms of intestinal absorption of this amino acid-like antiepileptic drug in this canine model are discussed.

Small-intestinal absorption of cadmium and the significance of mucosal metallothionein.

1 Although food intake is among the most important routes of Cd exposure, not many details are known about the intestinal absorption mechanisms of Cd. In this respect Cd is representative of most other nonessential, merely toxic metals. 2 Based on a concept of two distinguishable steps, intestinal absorption of Cd is characterized by high accumulation within the intestinal mucosa and a low rate of diffusive transfer into the organism. 3 After uptake into the mammalian organism, Cd is sequestered into hepatic metallothionein (MT). It is assumed that hepatic Cd-MT then gradually redistributes Cd to the kidney, which is the main target organ for chronic Cd toxicity. 4 When feeding low levels of dietary CdCl2, however, Cd accumulates preferentially in the kidney and to a lesser degree in the liver, a distribution pattern also found after intravenous and peroral administration of the Cd-MT complex itself. As dietary Cd induces intestinal MT, intestinal Cd-MT complexes could be at least partly responsible for the renal accumulation of dietary Cd. 5 For this mechanism, however, serosal release of mucosal Cd-MT is required. In fact, in vitro findings in rats reveal a concentration-dependent release of intestinal MT to the serosal side of the small intestine. These results indicate that endogenous intestinal MT may deliver Cd-MT to other inner organs, thus contributing to the preferential renal accumulation of ingested Cd.

INTESTINAL ABSORPTION KINETICS OF AMIODARONE IN RAT SMALL INTESTINE

Amiodarone is a widely used antiarrhythmic agent with highly variable therapeutic effects. These seem to be related, at least in part, to the pharmacokinetics of the drug and particularly to some features of its gastrointestinal absorption process. The drug exhibits physico-chemical properties highly suitable for diffusion across lipophilic absorbing membranes, but its low aqueous solubility can act as the rate limiting step for absorption, making the process erratic and variable. In order to gain an insight into the intestinal absorption mechanism of the drug and detect possible non-linearities, a series of experiments using a classical rat gut in situ preparation were carried out with three amiodarone hydrochloride solutions (10, 75, and 200 μg mL−1). A synthetic non-ionic surfactant, polysorbate 80, at supramicellar concentration (2 mM) was used as the drug solubilizer. Amiodarone was assayed in biological samples by HPLC using a rapid, sensitive technique that was validated. The amiodarone first-order absorption rate constants obtained in these conditions were similar. No significant differences between ka values were found. Amiodarone absorption was clearly identified as a passive diffusion process. © 1997 John Wiley & Sons, Ltd.

Intestinal absorption kinetics of amiodarone in rat small intestine.

Amiodarone is a widely used antiarrhythmic agent with highly variable therapeutic effects. These seem to be related, at least in part, to the pharmacokinetics of the drug and particularly to some features of its gastrointestinal absorption process. The drug exhibits physico-chemical properties highly suitable for diffusion across lipophilic absorbing membranes, but its low aqueous solubility can act as the rate limiting step for absorption, making the process erratic and variable. In order to gain an insight into the intestinal absorption mechanism of the drug and detect possible non-linearities, a series of experiments using a classical rat gut in situ preparation were carried out with three amiodarone hydrochloride solutions (10, 75, and 200 micrograms mL-1). A synthetic non-ionic surfactant, polysorbate 80, at supramicellar concentration (2 mM) was used as the drug solubilizer. Amiodarone was assayed in biological samples by HPLC using a rapid, sensitive technique that was validated. The amiodarone first-order absorption rate constants obtained in these conditions were similar. No significant differences between ka values were found. Amiodarone absorption was clearly identified as a passive diffusion process.

Intestinal absorption of the quaternary trospium chloride: permeability-lowering factors and bioavailabilities for oral dosage forms

The intestinal absorption mechanism, permeability and bioavailability of the parasympatholytic trospium chloride has been investigated in vitro and in vivo in rats, in order to gain a better mechanistic explanation for the underlying cause leading to low bioavailability of quaternary compounds following peroral dosing. Permeability determinations were done in Ussing-type chambers with rat jejunum and human Caco-2 cells. In vivo bioavailability and mass balance studies were done in rats. Absorption was studied from trospium chloride solutions in saline and from w/o microemulsions and cyclodextrin complex formulations. The absorption mechanism of trospium chloride across the intestinal epithelium is rather complex and involves P-glycoprotein-mediated secretion and saturable binding to intestinal mucus. Trospium permeability across the intestinal epithelium increases nonlinearly with rising drug concentrations at the apical membrane. Neither the microemulsions nor the cyclodextrin formulations increased the permeability of trospium in vitro, leading to lower or equal bioavailability of these formulations in vivo as compared with the aqueous solution control.

Intestinal absorption of sodium dodecyl sulfate in the rodent: evidence for paracellular absorption.

Experiments were performed to define the mechanism of intestinal absorption of dodecyl sulfate (DS), an amphipathic organic anion whose chemical structure resembles that of dodecanoate, a C12 fatty acid anion. With jejunal segments perfused in single-pass fashion in the anesthetized rat, steady-state absorption of DS was concentration dependent, with the apparent permeability constant (P(app)) ranging from 4 to 22 x 10(-5) cm/s. When DS concentration was held constant and net water absorption was induced by decreasing perfusate osmolality, DS absorption increased in direct proportion to water absorption, suggesting absorption by solvent drag via the paracellular route. However, DS absorption continued even when water secretion was induced by a hypertonic perfusate. Consequently, for all experiments, DS absorption could be empirically described as the sum of two terms: 1) absorption in the absence of water absorption (P(app) = 5.6 x 10(-5) cm/s) and 2) absorption induced by water movement [(delta P(app)/delta water absorption) = 0.2 x 10(-5) cm x s(-1) x microl segment(-1) x min(-1)]. In a polarized epithelial monolayer of renal epithelial cells (Madin-Darby canine kidney cells), DS was absorbed predominantly by a paracellular pathway, as the absorption rate increased threefold when paracellular junction pore size was increased by the addition of cytochalasin D. The calculated apparent radius was 2.9 A, indicating that the cross section of the molecule, not its length, determined the rate of absorption. It is concluded that absorption of DS in the intact animal occurs slowly and mostly via the paracellular route, because the fixed negative charge on the molecule retards rapid passive entry into the enterocyte, as occurs with protonated fatty acids. That absorption of DS persisted despite net water secretion suggests a low level of transcellular absorption across the jejunal enterocyte also occurs.

Inhibitory Mechanism of Intestinal Ethanol Absorption Induced by High Acetaldehyde Concentrations: Effect of Intestinal Blood Flow and Substance Specificity

This study describes the effects of high blood acetaldehyde concentrations on the intestinal absorption of ethanol and 2-butanone using an in situ single-pass perfusion technique on the rat intestine and the colored microsphere method to measure intestinal blood flow. We found that high blood acetaldehyde concentrations inhibit intestinal ethanol absorption in an inverse proportion to peak acetaldehyde concentrations, decrease intestinal blood flow, and inhibit intestinal absorption of 2-butanone. The decrease of the intestinal blood flow, induced by high blood acetaldehyde concentrations, is a major mechanism inhibiting intestinal ethanol absorption, but other mechanisms are also thought to inhibit absorption. Therefore, inhibition by high acetaldehyde concentrations is not the only factor affecting ethanol absorption.

Iron nutritional status in female karatekas, handball and basketball players, and runners

The iron nutritional status was studied in 84 sportswomen (19 karatekas, 20 handball players, 20 basketball players, and 25 middle and long distance runners) and in 82 nonathletic females of similar characteristics (control group). After a 7-day nutritional survey by means of the food weighing method, it was found that iron intake was significantly higher in the handball players ( p < 0.05), basketball players ( p < 0.01), and runners ( p < 0.01) with regard to the control group; the basketball players were the only ones to cover the recommended minimum intake (15 mg/day). The heme iron intake was significantly greater in the handball and basketball players ( p < 0.01), who, together with the runners, reached the value of 1.5 mg/day, which is considered to be optimal. In relation to the control group, the karatekas and handball and basketball players had lower levels of serum ferritin, although their iron intake was greater, whereas the runners had higher values that were very similar to those of the control group, due to the iron supplementation they had received. Despite finding a marked prevalence of inadequate iron intake, both in the sportswomen and in the control group, the manifest cases of anemia are relatively scarce. The organic iron stores do not seem to depend exclusively on the iron intake but also on intimate mechanisms of intestinal absorption and diverse causes of iron loss.

Intestinal absorption of azetirelin, a new thyrotropin-releasing hormone (TRH) analogue. II. In situ and in vitro absorption characteristics of azetirelin from the rat intestine.

Intestinal absorption characteristics of azetirelin, a new thyrotropin-releasing hormone (TRH) analogue, were studied in rats by means of in situ closed loop and in vitro everted sac experiments. Plasma concentrations of azetirelin obtained in the in situ closed loop experiments were not significantly different among the intestinal segments. Area under the plasma concentration-time curve (AUC) of azetirelin following administration into the duodenal loop increased in proportion to the dose. The serosal to mucosal concentration ratio of the analogue in the everted sac experiment was constant over the mucosal drug concentration range of 0.01-10 mM. There was no directional difference in the transfer rate of azetirelin across the everted and non-everted sacs of the duodenum. Furthermore, its transport across the duodenum was not influenced by low incubation temperature (25 degrees C), addition of dipeptide (Gly-Gly), or pretreatment of the mucosal surface with 2,4-dinitrophenol, while that of TRH was inhibited under these conditions. These results suggest that the intestinal absorption mechanism of azetirelin is different from that of TRH, and that azetirelin is predominantly transported via a passive diffusion.

Effects of inhibition of serotonin synthesis on 5-hydroxyindoleacetic acid excretion, in healthy subjects.

The urinary excretion of 5-hydroxyindoleacetic acid (5-HIAA), the main metabolite of serotonin, reflects the content and turnover of gastrointestinal (GI) serotonin. Employing longitudinal measurements of 5-HIAA, the authors investigated in healthy subjects (n = 43) how manipulations of serotonin synthesis affect GI serotonin. Under conditions of serotonin-free diets, the intersubject and intrasubject variability (coefficient of variation) for 5-HIAA excretion averaged 33% and 14%, respectively. Dietary tryptophan restrictions to 50% of minimal daily requirements (which is equivalent to a 10-fold reduction in baseline tryptophan intake) decreased by half the urinary excretion of 5-HIAA, irrespective of the caloric content of diet. Restoration to the regular tryptophan intake produced a rapid normalization of the 5-HIAA excretion. Neutral amino acids are known to compete with the intestinal transport absorption mechanisms of tryptophan. Administration of neutral amino acids (1.8 g, by mouth, three times a day, before each meal) or of carbidopa (50 mg, by mouth, three times a day for 3 days) to a normal tryptophan diet failed to alter significantly the 5-HIAA excretion. Further, neutral amino acids failed to enhance the reduction in 5-HIAA produced by the low-tryptophan diet. The failure of these treatments to reduce 5-HIAA excretion could be due to large capacity transport and decarboxylation systems for tryptophan. Other possibilities are discussed. In summary, dietary tryptophan is essential for the maintenance of GI serotonin. Reductions or increases in dietary tryptophan are the easiest and most effective method to alter GI serotonin.(ABSTRACT TRUNCATED AT 250 WORDS)

STUDIES ON THE MECHANISMS FOR INTESTINAL ABSORPTION AND RENAL EXCRETION OF QUINOLONE ANTIBACTERIAL AGENTS

Transport mechanisms of intestinal absorption and renal excretion for new-quinolone antibactrial agents were investigated. The absorption of enoxacin (ENX) from rat jejunal loop exhibited a pH-dependent profile. The greater absorption at acidic pH of medium was observed, and ciprofloxacin (CPFX) reduced the ENX plasma concentration after simultaeously oral administration to rat. In the uptake experiments by the intestinal brush-border membrane vesicles (BBMV), initial rate and time-course of ENX and sparfioxacin (SPFX) were significantly dependent on the pH of medium (pH5.5 > pH7.5), and CPFX inhibited compleately the pH dependent uptake of ENX. The ion diffusion potential affected the uptake of these drugs by the intestinal BBMV. On the other hand, ENX uptake by the renal BBMV was contributed by not only the ionic diffusion potential but also the cation-H+ antiport system of organic cation secretion.

Use of CACO-2 Cells as an in vitro Intestinal Absorption and Metabolism Model

The Caco-2 cell line, a human colorectal carcinoma cell line, is an established in vitro model for the study of drug transport in the human intestine. We have routinely utilized this in vitro model to 1) elucidate intestinal absorption mechanisms of small drug molecules and peptide-like therapeutic agents (e.g. paracellular/transcellular passive diffusion and carrier-mediated active transport), 2) screen and select orally active therapeutic agents, 3) identify optimum luminal pH's for drug absorptions, 4) address dissolution rate-related absorption problems, 5) assess mucosal toxicity of therapeutic agents, and 6) evaluate prodrug approaches for enhanced drug absorptions. We have also utilized this in vitro model to assess the metabolic stability of therapeutic agents in the intestinal epithelium. Demonstrated in this report are primarily the techniques for the elucidation of absorption mechanisms. Examples of the characterization of paracellular/ transcellular passive diffusion pathways and carrier-mediated active transport will be given. Application of the Caco-2 model to the process of drug development will also be discussed.

Effects of surfactants on amiodarone intestinal absorption. I. Sodium laurylsulfate.

Amiodarone is a widely used antiarrhythmic agent with high variability in therapeutic effects, which appears to be related, at least in part, to its pharmacokinetics, and in particular, gastrointestinal absorption. The drug exhibits physico-chemical properties highly suitable for diffusion across lipophilic absorbing membranes but its low aqueous solubility can act as the rate limiting step for absorption, making it erratic and variable. In studying the intestinal absorption mechanism of amiodarone, a series of experiments using a rat gut in situ preparation was performed in the presence of a synthetic anionic surfactant, as a drug solubilizer, i.e., sodium laurylsulfate, at variable supramicellar concentrations (from 2.6 to 104 mM). Absorption rate constants of amiodarone decreased as surfactant concentration increased, the absorption being unusually fast at lower surfactant concentrations. Equations were developed to evaluate the relationship between absorption rate constant and surfactant concentration in the intestinal luminal fluid.

Identification of the gastrointestinal absorption site for cadmium chloride in vivo.

Most models for the study of the mechanism of intestinal absorption of Cadmium (Cd) have been using intestinal tissue in vitro or in situ. The in vivo experiments reported in this article were performed in an attempt to localize the site for gastrointestinal absorption of cadmium chloride during natural physiological conditions by oral exposure of mice to 109Cd-labelled CdCl2. Independent of exposure via drinking water or oral administration of a single dose, Cd was primarily deposited in the most proximal duodenum. Thus the present study as well as others indicate that absorption takes place in the proximal part of the intestine. Absorbed Cd is initially transported to the liver and deposited before being redistributed and accumulated in the kidneys. In this experimental model, dietary tetraethylthiuram disulfide exposure was shown to change the intestinal labelling profile and increase the whole-body retention as well as the intestinal deposition of Cd.

Characterization of the Oral Absorption of Some β-Lactams Effect of the α-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, ʃmax= 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, ʃmax = 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 ʃmax 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 β-lactam antibiotics correlated well with their absorption numbers obtained from P*w values in rats. These results further demonstrate that an α-amino group is not necessary for transport by the intestinal peptide transporter.

Intestinal absorption and biomagnification of organochlorines

AbstractDietary uptake rates of several organochlorines from diets with different lipid contents were measured in goldfish (Carassius auratus) to investigate the mechanism of intestinal absorption and biomagnification of organic chemicals. The results suggest that intestinal absorption is predominantly controlled by chemical diffusion rather than lipid cotransport. Data for chemical uptake in human infants are presented to illustrate that biomagnification is caused by the digestion of food in the gastrointestinal tract. The findings are discussed in the context of two conflicting theories for the mechanism of biomagnification, and a mechanistic model is presented for the dietary uptake and biomagnification of organic chemicals in fish and mammals.

Mechanism of intestinal absorption of ranitidine and ondansetron: transport across Caco-2 cell monolayers.

We have investigated the transport of ranitidine and ondansetron across the Caco-2 cell monolayers. The apparent permeability co-efficients (Papp) were unchanged throughout the concentration range studied, indicating a passive diffusion pathway across intestinal mucosa. No metabolism was observed for ranitidine and ondansetron during the incubation with Caco-2 cell monolayers. Papp values for ranitidine and ondansetron (bioavailability of 50 and approximately 100% in humans, respectively) were 1.03 +/- 0.17 x 10(-7) and 1.83 +/- 0.055 x 10(-5) cm/sec, respectively. The Papp value for ranitidine was increased by 15- to 20-fold in a calcium-free medium or in the transport medium containing EDTA, whereas no significant change occurred with ondansetron, indicating that paracellular passive diffusion is not rate determining for ondansetron. Uptake of ondansetron by Caco-2 cell monolayers was 20- and 5-fold higher than that of ranitidine when the uptake study was carried out under sink conditions and at steady state. These results suggest that ranitidine and ondansetron are transported across Caco-2 cell monolayers predominantly via paracellular and transcellular pathways, respectively.