Intestinal Absorption of Drugs. III. The Influence of Taurocholate on the Disappearance Kinetics of Hydrophilic and Lipophilic Drugs from the Small Intestine of the Rat

The influence of sodium taurocholate (TC) on the intestinal absorption of drugs was studied in vivo in a chronically isolated internal loop in the rat. The hydrophilic drugs paracetamol (PA) and theophylline (TP) and the lipophilic drugs griseofulvin (GF) and ketoconazole (KE) were used as model drugs. The drug concentrations were kept below the saturation concentration. Absorption kinetics of the drugs were evaluated on the basis of disappearance rates of the drug from luminal solutions in the intestinal loop. Concentrations of TC above the critical micelle concentration (CMC) did not affect the absorption rate of the hydrophilic drugs PA and TP; the barrier function of the intestinal wall for PA and TP was not altered in the presence of taurocholate. The addition of concentrations of TC above the CMC in the perfusion solution resulted in a reduction of the absorption rate of GF and KE. The reduction in the absorption kinetics of GF in the presence of TC correlated well with the reduction of the drug-free fraction in solution due to micellar solubilization. For KE this relation was less clear. It was not possible to determine, on the basis of the phase-separation model, to what extent the fraction of the drug incorporated in TC micelles contributes to the overall diffusion of GF and KE across the preepithelial diffusion barrier. It was concluded that TC exhibits only a minor, if not negligible, effect on the barrier function of the aqueous diffusion barrier adjacent to the intestinal wall.     

Transmucosal Passage of Liposomally-Entrapped Drugs in Rat Small Intestine

Intestinal absorption of liposomally-entrapped drugs was investigated for egg yolk phosphatidylcholine-cholesterol (2:1 by molar ratio) liposomes (EggPC liposome) and distearoylphosphatidylcholine-cholesterol (2:1) liposomes (DSPC liposome). The release of carboxyfluorescein, an aqueous phase marker, induced by the presence of everted rat intestine was 40 % and 6 % in one hour from DSPC liposomes and EggPC liposomes, respectively, and it is suggested that EggPC liposomes are more stable in the intestinal lumen. The transport of a liposomally-entrapped drug was examined with fluoresceinisothiocyanate-conjugated dextran (FITC-D) as a model drug that has a small mucosal-to-serosal clearance because of its high average molecular weight (64200). The clearance of FITC-D entrapped in DSPC liposomes was largely reduced and could be accounted for by the clearance of the extraliposomal FITC-D concentration in the preparation. On the other hand, the calculated clearance of EggPC liposome-associated FITC-D was similar to or even higher than that of free FITC-D. The serosal appearance of the EggPC liposome-associated drug was inhibited by colchicine, cytochalasin B, and iodoacetate, suggesting that the liposome was incorporated into the epithelial cells by endocytosis. However, the observation that a lipid phase marker, ¹⁴C-dipalmitoylphosphatidylcholine, failed to be transported into the serosal fluid indicates the absence of the penetration by an intact liposomal form.     

Comparative Assessment of Intestinal Transport of Hydrophilic Drugs Between Small Intestine and Large Intestine

Intestinal passive transport of several hydrophilic drugs (and probe compounds) was examined in the large intestine (colon), in comparison with that in the small intestine, in an effort to obtain basic information for developing rational colonic drug delivery strategies. The drugs tested were polyethylene glycol (PEG 900), L-glucose, D-xylose, 5-fluorouracil (5-FU) and urea. In everted intestinal sacs, the uptake of every drug was larger in the small intestine than in the large intestine, although by various extents. The uptake of urea was larger than those of D-xylose and L-glucose in both the small intestine and large intestine and associated with a larger large intestine (LI)/small intestine (SI) uptake ratio. Assuming that passive transport via the paracellular route (or aqueous pore) is predominant for them, the large intestine may have smaller paracellular (or aqueous) pores, restricting the transport of those monosaccharides compared with smaller molecules such as urea by a larger extent in the large intestine than in the small intestine. The passive transport of 5-FU was significantly larger than those of the monosaccharides in both the small intestine and large intestine and associated with a larger LI/SI uptake ratio, even though 5-FU has a molecular weight close to that of the monosaccharides. 5-FU may be transported predominantly by transcellular diffusion, because its oil-to-water partition coefficient is about 200 times larger than those of the monosaccharides. Although transport mechanisms, including transport pathways, are yet to be fully clarified, drugs with physicochemical properties similar to those of 5-FU or urea may be more feasible for colonic drug delivery than those with physicochemical properties similar to those of monosaccharides.     

Region-Dependent Role of the Mucous/Glycocalyx Layers in Insulin Permeation Across Rat Small Intestinal Membrane

The regional difference in the contribution of the mucous/glycocalyx layers in rat small intestine, as a diffusional or enzymatic barrier, to the absorption of insulin was investigated by in vitro studies. The mucous/glycocalyx layers from the duodenum, the jejunum, and the ileum in rat were successfully removed without damaging membrane integrity, by exposing them to a hyaluronidase solution in situ. In an in vitro transport experiment, the apparent permeability coefficient (P_app) of insulin for the hyaluronidase-pretreated group was significantly increased compared to the PBS-pretreated (control) group in all small intestinal regions, and the P_app of insulin in both PBS- and hyaluronidase-pretreated groups increased in the following order: duodenum < jejunum < ileum. On the other hand, irrespective of small intestinal regions, the P_app of FD-4 and of antipyrine, respectively the passive para- and transcellular permeation marker, exhibited no significant differences between PBS- and hyaluronidase-pretreated group. In addition, a significant amount of insulin was degraded in the mucous/glycocalyx layers compartment removed by hyaluronidase pretreatment, and the degradation activity in the mucous/glycocalyx layers showed regional differences in the following order: duodenum > jejunum > ileum. These findings suggest that, irrespective of small intestinal regions, the mucous/glycocalyx layers contributed to insulin permeation predominantly as an enzymatic barrier, and not as a diffusional barrier. Furthermore, the variation of the enzymatic activities in the mucous/glycocalyx layers and in the brush-border membrane would be one factor that accounts for the regional differences in the transport of insulin.     

Absorption of Organic Anions from the Rat Small Intestine

Abstract

1. Hippuric acid, sulphanilic acid, p-aminohippuric acid, and phenol red, highly ionized compounds of very low lipoid solubilities, were absorbed from the rat small intestine at rates which varied over a 25-fold range.

2. Absorption rates ranked in the same order as the chloroform-to-water partition coefficients of the compounds but not in the order of the molecular weights or degrees of ionization.

3. The results suggested that these anions are absorbed mainly by simple diffusion through lipoid regions of the intestinal boundary.     

Differences in Transport Rate of Oxytocin and Vasopressin Analogues Across Proximal and Distal Isolated Segments of the Small Intestine of the Rat

The transmural intestinal passage of some oxytocin and vasopressin analogues (oxytocin, OT; [Mpa¹, D-Arg⁸]vasopressin, dDAVP; [Mpa¹, Tyr (OMe)², carba⁶]oxytocin, carbetocin; [Mpa¹, D-Tyr (OEt)², Thr⁴, Orn⁸]vasotocin, antocin II; [Mpa¹, D-Tyr (OEt)², Thr⁴, desPro⁷Orn⁸Gly⁹NH₂]tocinoic acid-NH(CH₂)₃NH₂, desPOG-antocin II-NH(CH₂)₃NH₂) was studied using isolated proximal and distal segments in the rat. All peptides (measured as peptide-like immunoreactivity) displayed a higher transport rate across distal intestinal segments as determined by radioimmunoassay (RIA). The smallest peptide, des POG-antocin II-NH(CH₂)₃NH₂, was transported at the fastest rate. No correlation of lipophilicity with transport rate was observed. Determination of the amount of peptide remaining in the mucosal media at the end of the incubation period by HPLC did not reveal any visible degradation products. However, the large difference in transport rate between [³H]OT and immuno-reactive OT indicates mucosal metabolism of this peptide. [³H]d-DAVP was distributed in a larger mucosal volume than the extracellular space marker [³H]inulin, indicating tissue uptake, but was too low (<100% of buffer concentration) to make an active transport mechanism likely. The differences in peptide transport rates between proximal and distal intestinal segments are most likely due to a higher distal paracellular permeability despite a decreased absorptive surface area at this region.     

Transport Functions of Riboflavin Carriers in the Rat Small Intestine and Colon: Site Difference and Effects of Tricyclic-Type Drugs

The present study was aimed at kinetically characterizing the newly found carrier-mediated riboflavin transport system in the rat colon, comparing it with that in the small intestine, and also probing the potential roles of these transport systems in intestinal drug absorption. Riboflavin transport, evaluated by measuring the initial uptake into everted intestinal tissue sacs, was saturable with a Michaelis constant (K_m     

Absorption of Organic Arsenical Compounds from the Rat Small Intestine

Abstract

1. The pentavalent organic arsenical compounds carbarsone, tryparsamide, and cacodylic acid were absorbed from the rat small intestine at rates directly proportional to concentration over a 100-fold range.

2. Absorption half-times (min) were: carbarsone, 87; tryparsamide, 184; and cacodylic acid, 201.

3. Absorption rates (first-order rate constants) ranked in the same order as the CHCl₃-to-water partition coefficients of the compounds as measured at pH 5·3.

4. The results suggested that these organo-metallic compounds are absorbed mainly by a process of simple diffusion.     

P-Glycoprotein Increases from Proximal to Distal Regions of Human Small Intestine

Purpose. The contribution of the efflux transporter P-glycoprotein (P-gp) as a barrier to drug absorption may depend on its level of expression at the site of absorption. Accordingly, the distribution of P-gp was examined along the entire length of the human small intestine. Methods. Homogenates prepared from mucosal scrapings from every other 30-cm segment of four unrelated human donor small intestines were analyzed for P-gp and the control protein villin by Western blot. Results. In each donor intestine, relative P-gp expression (P-gp/villin integrated optical density ratio) progressively increased from proximal to distal regions. Among individuals, relative P-gp levels varied 2.1-fold in the duodenal/proximal jejunal region, 1.5- to 2.0-fold in the middle/distal jejunal region, and 1.2- to 1.9-fold in the ileal region. Within-donor variation was somewhat greater, from 1.5- to 3.0-fold. Conclusions. These results provide further evidence that the site of absorption can represent another source for the interindividual variation in the oral bioavailability of drugs.     

Route-Dependent Metabolism of Morphine in the Vascularly Perfused Rat Small Intestine Preparation

Purpose. 1. To compare the disposition of tracer morphine ([³H]M)following systemic and intraduodenal administration in therecirculating, rat small intestine preparation in absence or presence of verapamil(V), an inhibitor of P-glycoprotein. 2. To develop a physiological modelto explain the observations. Methods. A bolus dose of [³H]M was added to the reservoir or injectedinto the duodenum of the rat small intestine preparation. V (200 Min reservoir) was either absent (control studies) or present. Intestinalmicrosomal, incubation studies were performed to evaluate the effectof V on morphine glucuronidation. Results. After systemic administration, [³H]M was not metabolizedbut was exsorbed into lumen. By contrast, both [³H]M and the3-glucuronide metabolite, [³H]M3G, appeared in reservoir and lumenafter intraduodenal administration. A physiologically-based model thatencompassed absorption, metabolism and secretion was able to describethe route-dependent glucuronidation of M. The presence of V resultedin diminished levels of M3G in perfusate and lumen and mirrored theobservation of decreased glucuronidation in microsomal incubations.Verapamil appeared to be an inhibitor of glucuronidation and notsecretion of M. Conclusions. M was secreted and absorbed by the rat small intestine.Route-dependent glucuronidation of M was explained by physiologicalmodeling when M was poorly partitioned in intestinal tissue, with alow influx clearance from blood and a even poorer efflux clearancefrom tissue. The poor efflux rendered a much greater metabolism ofM that was initially absorbed from the lumen. V increased the extentof M absorption through inhibition of M glucuronidation.This work was supported by the Medical Research Council of Canada (MA9104 and MOP-36457) and was presented in part at the ASPET meeting in San Diego, 1997 and the AAPS meeting in Boston, 1997.     

Cholinergic Modulation of Electrogenic Ion Transport in Different Regions of the Rat Small Intestine

Acetylcholine acting via muscarinic receptors located in the intestinal mucosa controls ion and fluid transport. This study examined the pathway(s) by which cholinergic receptors mediate secretion in rat isolated duodenum, jejunum and ileum using the short-circuit current (Isc) as an index of electrogenic Cl^? secretion. Carbachol and bethanechol induced electrogenic Cl^? transport which was insensitive to the neural blocker tetrodotoxin, indicating their direct action on the enterocytes. Functional characterization of electrogenic secretion activated via muscarinic receptors on jejunal and ileal enterocytes was achieved by use of selective muscarinic antagonists in the presence of tetrodotoxin. In both regions the rank order of potency of these compounds (atropine > 4-diphenylacetoxy-N-piperidine methiodide (4-DAMP) > hexahydro-sila-difenidol (HHSiD) > pirenzepine > methoctramine) indicated the M₃ receptor subtype. Secretion activated by the muscarinic agonist 4-[[(3-chlorophenyl)amino]carbonyl]-N,N,N-trimethyl-2-butyn-1-ammonium chloride (McN-A-343) was sensitive to tetrodotoxin and pirenzepine but not to the ganglionic blocker, hexamethonium, indicating the M₁ receptor subtype on post ganglionic neurons. Regional differences for bethanechol-activated secretion showed an increasing gradient in secretory capacity (Isc max) in a proximal-to-distal direction along the small intestine. Responses to McN-A-343 also showed regional differences but these were unlike those of bethanechol. These results show that cholinomimetic-induced electrogenic Cl^? secretion in rat isolated small intestine appears to be mediated by two dissimilar populations of muscarinic receptor: M₃ muscarinic receptors positioned on enterocytes and M₁ muscarinic receptors sited on submucosal neurons.     

The Predominant Contribution of Oligopeptide Transporter PepT1 to Intestinal Absorption of β-Lactam Antibiotics in the Rat Small Intestine

Although recent evidence suggests that certain β-lactam antibiotics are absorbed via a specific transport mechanism, its nature is unclear. To confirm whether peptide transport in the rat can be largely ascribed to the intestinal oligopeptide transporter PepT1, the transporter has been functionally characterized and its significance in the intestinal absorption of β-lactam antibiotics was evaluated. For evaluation of transport activity complementary RNA (cRNA) of rat PepT1 was synthesized in-vitro and expressed in Xenopus laevis oocytes. cRNA induced uptake of several β-lactam antibiotics and the dipeptide [¹⁴C]glycylsarcosine; this was specifically inhibited by various dipeptides and tripeptides but not by their constituent amino acids or by terra- or pentapeptides. The transport activity of PepT1 for β-lactam antibiotics correlated well with their in-vivo intestinal transport and absorption. Furthermore, mutual inhibitory effects on uptake were observed between glycylsarcosine and β-lactam antibiotics. Hybrid depletion of the functional expression of rat PepT1 in oocytes injected with rat intestinal epithelial total mRNA was studied using an antisense oligonucleotide corresponding to the 5′-coding region of PepT1. In oocytes injected with rat mRNA pre-hybridized with the antisense oligonucleotide against rat PepT1, the uptake of [¹⁴C]glycylsarcosine was almost completely abolished, whereas its uptake was not influenced by a sense oligonucleotide for the same region of PepT1. Similarly, the uptake of β-lactam antibiotics was also reduced by the antisense oligonucleotide against rat PepT1. These results demonstrate that the intestinal proton-coupled oligopeptide transporter PepT1 plays a predominant role in the carrier-mediated intestinal absorption of β-lactam antibiotics and native oligopeptides in the rat.     

Effect of Ganglionic Blocking Compounds on In-vivo Fluid Secretion in the Rat Small Intestine

It is well-known that enteric, secreto-motor nerves mediate cholera toxin-induced fluid secretion in the rat small intestine. This notion is, in part, derived from experiments on anaesthetized animals in which the response to cholera toxin was antagonized by the ganglionic nicotinic receptor antagonist, hexamethonium. In the current study, such anti-secretory action of ganglionic blocking compounds was analysed in an experiment designed to minimize any possible negative effect of general anaesthesia on intestinal secretion. Rats were anaesthetized with ether for 5–10 min, during which time a jejunal loop (10–12 cm) was constructed. The loop was challenged with one of the secretagogues, cholera toxin, prostaglandin E₁ (PGE₁) or okadaic acid. Saline (control) or either of the ganglionic blockers, hexamethonium and chlorisondamine, was administered intravenously. The rats were killed 5 h (cholera toxin) or 1–5 h (PGE₁ and okadaic acid) after challenge, and the amount of fluid accumulated in the loops was determined. Cholera toxin-induced secretion was unchanged by hexamethonium but reduced by approximately 80% by chlorisondamine. The difference in effect between the two blockers might relate to the duration of ganglionic blockade. Chlorisondamine blocked secretion induced by either PGE₁ or okadaic acid by approximately 60%. It is suggested that the anti-secretory effect of ganglionic blocking compounds might be a result of blockade of secreto-motor nerves but other mechanisms, for example interference with haemodynamic factors, cannot be ruled out.     

Drug Absorption from the Small Intestine of the Triparanol–Treated Rat in Situ

Abstract The effect of treatment with triparanol (25 mg/kg by gavage every 24 hours for three weeks) on the absorption of phenobarbitone, sulphafurazole, isoniazid, mecamylamine and quinidine from the rat small intestine was studied in situ by measuring their disappearance from the intestinal lumen. The appearance of sulphafurazole and mecamylamine in the intestinal lumen was also studied after their intravenous administration, and the partitioning of mecamylamine between the buffer solution and the intestinal tissue was measured in vitro. Treatment with triparanol retarded the absorption of sulphafurazole, whereas the absorption of mecamylamine was accelerated. The amount of sulphafurazole and mecamylamine in the intestinal lumen after their intravenous administration was relatively slight. The in vitro partitioning of mecamylamine into the intestinal tissue was higher in triparanol–treated than in control intestines. Triparanol did not change the absorption of phenobarbitone, isoniazid or quinidine. Phenobarbitone in the whole blood at the end of the experiment was increased after triparanol, but the levels of other drugs were unchanged. Triparanol did not modify drug concentrations in the intestinal wall at the end of the experiment. The relatively slight changes in drug absorption induced by triparanol are probably due to changes in the morphology and composition of the intestinal wall.     

五酯胶囊对环孢素在大鼠小肠吸收的影响

目的：研究五酯胶囊（WuZhi—capsule，WZ）对环孢素（cyclosporinA，CsA）在大鼠小肠吸收的影响。方法：三组大鼠分别以CsA试液、含WZ100mg&#183;L^-1的CsA试液、含WZ200mg&#183;L^-1的CsA试液进行在体肠段吸收实验，HPLC法测定CsA浓度。结果：与单用CsA相比，回肠段合用WZ组CsA吸收率明显降低，空肠段合用WZ组CsA吸收无明显变化。结论：WZ对CsA小肠吸收有抑制作用。     

外翻肠囊法评价及比较5种药物大鼠肠透膜能力

目的:考察外翻肠囊法在评价及比较多种药物肠透膜能力方面的可行性与实用性。方法:按外翻肠囊法制备大鼠肠吸收离体模型,测定西酞普兰、布洛芬、酮洛芬、氯霉素、盐酸地尔硫5种药物的累积吸收率,并与该5种药物经脂质体毛细管电泳(LCE)法所测得的脂水分配系数(logP)排序结果比较。结果:5种药物在大鼠小肠壁均有不同程度的吸收,累积吸收率由高至低依次为西酞普兰>盐酸地尔硫>布洛芬>酮洛芬>氯霉素,与LCE法测定结果一致。结论:外翻肠囊法可评价及比较多种药物的肠透膜能力。     

In Vivo Absorption of Theophylline and Salicylic Acid from Rat Small Intestine

Abstract: In vivo absorption of theophylline and salicylic acid from the rat small intestine was studied by a closed segment technique. The drugs were administered at 1 and 4 mM in dissolved form, both in the presence and absence of phlorizin (0.01 mM). Drug concentrations were measured by high-pressure liquid chromatography. Phlorizin inhibited the absorption of theophylline (pK_a = 8.6) at 4 mM but not at 1 mM. In contrast, the absorption of salicylic acid (pK_a = 3) both at I and 4 mM was unaffected by phlorizin. This suggests that an active transport system, sensitive to phlorizin, is involved in the rat intestinal absorption of theophylline, but not in that of salicylic acid. Apart from differences in drug structure, differences in the degree of ionization may influence the access of acidic drugs to this transport system.     

Effect of Colchicine on Drug Absorption from the Rat Small Intestine in Situ and in Vitro

Abstract The effect of colchicine (1 mg/kg intraperitoneally on two successive days) on the absorption of isoniazid, quinidine and sulphafurazole (sulfisoxazole) from the rat small intestine was studied in situ and in vitro. Colchicine produced two different types of histological damage in the small intestine, one with degenerative and the other with regenerative changes predominating. The small intestinal surface area was variably reduced. The colchicine-treated rats were lethargic and hypothermic as compared to controls. Colchicine retarded the disappearance of fluid and all three drugs from the small intestinal lumen in situ 2 days after the first colchicine injection. In vitro the total amounts of fluid and drugs passed through the intestinal wall were not significantly changed by colchicine, although there was a slight tendency towards an increased absorption of quinidine. Hence, colchicine as an antimitotic drug decreases drug absorption from the rat small intestine in situ, apparently due to the decreased surface area of the small intestine, the decreased water flux through the intestinal wall, the retarded intestinal motility and hypothermia of the rats. In vitro the changes are small, which makes the in vitro tests less suitable for studying the effect of colchicine on absorption.     

Effect of Methotrexate on Drug Absorption from the Rat Small Intestine in Situ and in Vitro

Abstract The effect of methotrexate (20 mg/kg intramuscularly) on the absorption of phenobarbitone, sulphafurazole, mecamylamine, quinidine and isoniazid from the rat small intestine was studied in situ and in vitro. The disappearance of all drugs studied from the intestinal fluid in situ was retarded on the third day after methotrexate administration. The fluid transfer and the amount of drugs passed through the intestinal wall in vitro were also decreased. The absorption of phenobarbitone was reversible within six days, whereas the absorption of quinidine was still retarded on the sixth day after methotrexate administration. Methotrexate did not modify the amount of quinidine excreted into the intestinal lumen after intravenous administration. The levels of other drugs except isoniazid in the blood at the end of the experiment showed changes corresponding to their disappearance from the intestinal lumen. In situ the drug levels in the intestinal wall were much lower than in vitro. Their levels in the intestinal wall reflected drug absorption in vitro but not in situ. The methotrexate-induced reversible decrease in absorption seems to be attributable at least partly to diminished water flux through the intestinal wall, although other mechanisms may also exist.