Improvement of intestinal absorption of leucine enkephalin by sugar coupling and peptidase inhibitors

Immense Insulin Intestinal Uptake and Lymphatic Transport Using Bile Acid Conjugated Partially Uncapped Liposome

We provide immense insulin absorption from the gastrointestinal tract, combining apical sodium-dependent bile acid transporter-mediated intestinal uptake and the lymphatic transport pathway. This strategy has proven to employ chondroitin sulfate- g-taurocholic acid coated, insulin-loaded partially uncapped liposome (IPUL-CST) for type 1 diabetes mellitus (T1DM) treatment. The loading efficiency of insulin in IPUL-CST increased significantly from 33% to 75% via the partially uncapped liposome preparation method. Moreover, the IPUL-CST revealed an improved insulin protection efficacy in GIT simulated pH and digestive enzyme conditions. The high dose of IPUL-CST in the small intestine was detected 4 h post-oral administration using ex vivo optical imaging and fluorescence intensity. The IPUL-CST exhibited significantly enhanced intestinal absorption (oral bioavailability, 34%; T_max, 9 h) and reduced blood glucose levels for 16 h in T1DM. The results demonstrated that the new investigated IPUL-CST is a promising carrier for oral insulin delivery.

A review on the strategies for oral delivery of proteins and peptides and their clinical perspectives

In the modern world, a number of therapeutic proteins such as vaccines, antigens, and hormones are being developed utilizing different sophisticated biotechnological techniques like recombinant DNA technology and protein purification. However, the major glitches in the optimal utilization of therapeutic proteins and peptides by the oral route are their extensive hepatic first-pass metabolism, degradation in the gastrointestinal tract (presence of enzymes and pH-dependent factors), large molecular size and poor permeation. These problems can be overcome by adopting techniques such as chemical transformation of protein structures, enzyme inhibitors, mucoadhesive polymers and permeation enhancers. Being invasive, parenteral route is inconvenient for the administration of protein and peptides, several research endeavors have been undertaken to formulate a better delivery system for proteins and peptides with major emphasis on non-invasive routes such as oral, transdermal, vaginal, rectal, pulmonary and intrauterine. This review article emphasizes on the recent advancements made in the delivery of protein and peptides by a non-invasive (peroral) route into the body.

Lipophilic derivatives of leu-enkephalinamide: in vitro permeability, stability and in vivo nasal delivery

Leu-enkephalin is an endogenous pain modulating opioid pentapeptide. Its development as a potential pharmaceutic has been hampered by poor membrane permeability and susceptibility to enzymatic degradation. The addition of an unnatural amino acid containing a lipidic side chain at the N-terminus and the modification of the C-terminus to a carboxyamide was performed to enhance the nasal delivery of the peptide. Two lipidic derivatives with varying side chain lengths (C(8)-Enk-NH(2) (1), C(12)-Enk-NH(2) (2)) and their acetylated analogues were successfully synthesised. Caco-2 cell monolayer permeability and Caco-2 cell homogenate stability assays were performed. C(8)-Enk-NH(2) (1) and its acetylated analogue Ac-C8-Enk-NH(2) (3) exhibited apparent permeabilities (mean±SD) of 2.51±0.75×10(-6)cm/s and 1.06±0.62×10(-6), respectively. C12-Enk-NH(2) (2) exhibited an apparent permeability of 2.43±1.26×10(-6) cm/s while Ac-C12-Enk-NH(2) (4) was not permeable through the Caco-2 monolayers due to its poor solubility. All analogues exhibited improved Caco-2 homogenate stability compared to Leu-Enk-NH(2) with t(½) values of: C8-Enk-NH(2) (1): 31.7 min, C(12)-Enk-NH(2) (2): 14.7 min, Ac-C8-Enk-NH(2) (3): 83 min, Ac-C(12)-Enk-NH(2) (4): 27 min. However, plasma stability assays revealed that the diastereoisomers of C8-Enk-NH(2) (1) did not degrade at the same rate, with the l isomer (t(1/2)=8.9 min) degrading into Leu-enkephalinamide and then des-Tyr-Leu-Enk-NH(2), whereas the d isomer was stable (t(1/2)=120 min). In vivo nasal administration of C(8)-Enk-NH(2) to male rats resulted in concentrations of 5.9±1.84×10(-2) μM in the olfactory bulbs, 1.35±1.01×10(-2) μM in the brain and 6.53±1.87×10(-3) μM in the blood 10 min after administration.

Does the well-stirred model assess the intestinal first-pass effect well?

The pre-systemic intestinal extraction ratio (Eg) has been estimated by an equation based on the well-stirred model, which does not have a term of membrane transport. In this report, we have identified the application limitations of the well-stirred model equation to assess the pre-systemic intestinal extraction ratio. The Eg of metoprolol (CYP2D6 substrate) was assessed by three methods. Intrinsic clearances for metoprolol metabolism in hepatic and gastrointestinal microsomes were from a published report. Method 1 (model-independent method): the Eg of 0.228 was obtained according to the equation, F = Ff × (1 — Eg) × Fh, where F, Ff and Fh were the bioavailability, the fraction entering the intestinal tissue and the hepatic availability, respectively. Method 2: the Eg of 0.0071 was calculated according to the well-stirred model equation, and was much lower than the value of 0.228. Method 3: the Eg of 0.213 was obtained by the transport-metabolism-flow (TMF) model equation, and was much closer to the value of 0.228 obtained by the model-independent method than the Eg of 0.0071 calculated by the well-stirred model equation. Therefore, we propose that the factor of membrane transport process be incorporated into the pharmacokinetic model for the assessment of the pre-systemic intestinal extraction ratio.

Dietary polyphenols (-)-epicatechin and chrysin inhibit intestinal glucuronidation metabolism to increase drug absorption

The effect of dietary polyphenols on the intestinal glucuronidation and absorption of a model phenolic drug, alpha-naphthol (alpha-NA), was studied in isolated rat small intestine. (-)-Epicatechin significantly inhibited the glucuronidation of alpha-NA. Chrysin, (-)-epigallocatechin galleate (EGCG), and quercetin decreased the rate of glucuronidation, although not significantly. Baicalin did not affect the glucuronidation. The rate of absorption of alpha-NA in the presence of these polyphenols also varied. The absorption clearance (CLabs) and the metabolic clearance (CLmet) were inversely correlated, and this relationship was well explained in the metabolic inhibition model with kinetic parameters (knowledge-based prediction) which characterizes the relationship between the CLabs and CLmet of alpha-NA (Biochim Biophys Acta 1998, 1425, 398.). These results indicate that the concomitant intake of some polyphenols can increase the absorption of a phenolic drug, and the effect is predictable. (-)-Epicatechin and chrysin are effective for the inhibition of glucuronidation and promotion of intestinal drug absorption.

Determination of transport in the Caco-2 cell assay of compounds varying in lipophilicity using LC-MS: enhanced transport of Leu-enkephalin analogues

PURPOSE

To synthesize a number of analogues of Leu-enkephalin with different lipophilicities and to develop an LC-MS method for determining the Caco-2 cell permeability values of these compounds.

METHODS

A number of sugar and sugar plus lipoamino acid analogues of Leu-enkephalin were synthesized by solid-phase and solution methods. An LC-MS method was developed for analyzing the Caco-2 cell assay samples and validated against the traditional method using radiolabelled compounds.

RESULTS

A sensitive and specific LC-MS assay was developed. Standard curves were linear in the range of 0.025-5 microM. Apparent permeability values determined by LC-MS and liquid scintillation counter were identical, for both a hydrophilic drug, cephalexin and a lipophilic Leu-enkaphalin analogue. Caco-2 permeability values for the analogues of Leu-enkephalin were determined and it was found that attachment of sugar or sugar and lipoamino acid to the Leu-enkephalin peptide resulted in an increase in the apparent permeability values compared to the native peptide, which was not transported across the Caco-2 cell monolayers.

CONCLUSIONS

A rapid, generic LC-MS method for analyzing a range of compounds was developed. Attachment of a sugar or sugar and lipoamino acid to Leu-enkephalin improves the apparent permeability across Caco-2 cell monolayers.

Electrical charge on protein regulates its absorption from the rat small intestine

The effect of the electrical charge on the intestinal absorption of a protein was studied in normal adult rats. Chicken egg lysozyme (Lyz), a basic protein with a molecular weight of 14,300, was selected and several techniques for chemical modification were applied. Then the intestinal absorption of Lyz derivatives was evaluated by measuring the radioactivity in plasma and tissues, after the administration of an (111)In-labeled derivative to an in situ closed loop of the jejunum. After the administration of (111)In-Lyz, the level of radioactivity in plasma was comparable with the lytic activity of Lyz, supporting the fact that the radioactivity represents intact Lyz. (111)In-cationized Lyz showed a 2-3 times higher level of radioactivity in plasma, whereas the radioactivity of (111)In-anionized Lyz was much lower. The absorption rate of (111)In-Lyz derivatives calculated by a deconvolution method was correlated for the strength of their positive net charge. A similar relationship was observed using superoxide dismutase. These findings indicate that the intestinal absorption of a protein is, at least partially, determined by its electrical charge.

Kinetic Impact of Presystemic Intestinal Metabolism on Drug Absorption: Experiment and Data Analysis for the Prediction of In Vivo Absorption from In Vitro Data

Orally administered drugs suffer from attack by metabolic enzymes not only in the liver, but also in the gastrointestine during the absorption process across the intestinal tissue. Although kinetic study on hepatic metabolism has been done well, the intestinal metabolism has not been well focused on compared with hepatic metabolism. In order to emphasize the role of intestinal metabolism in drug absorption and bioavailability, I have reviewed the experimental methods for intestinal absorption and metabolism, and the data analysis. Since Klippert et al. reported the prediction of intestinal first-pass effect of phenacetin in the rat from enzyme kinetic data in 1982, several reports have showed a good prediction, but others have not. Although intestinal absorption is an integrated process of transport (transporters) and metabolism (metabolic enzymes), most of the researchers missed the pathway of intestinal drug absorption and applied the kinetic model effective on only systemic metabolism to presystemic intestinal metabolism for their analysis of intestinal metabolism of orally administered drugs. A kinetic model, which incorporated factors of membrane transport, metabolic activity and protein binding, was structured to compare the equations in the reported models. In conclusion, we need more studies including kinetic modeling and experiments to understand the impact of intestinal metabolism on drug absorption. That knowledge must lead to the construction of ADME in silico (e-ADME).

[Improvement of transmucosal absorption of biologically active peptide drugs]

Peptide and protein drugs are becoming a very important class of therapeutic agents. However, the oral bioavailability of peptide and protein drugs is generally poor because they are extensively degraded by proteases in the gastrointestinal tract or impermeable through the intestinal mucosa. For the systemic delivery of the peptide and protein drugs, parenteral administration is currently required to achieve their therapeutic activities. However, this administration is poorly accepted by patients and may cause allergic reactions and serious side effects. Therefore, various approaches have been examined to overcome the delivery problems of these peptides when they are administered into the gastrointestinal tract and other mucosal sites. These approaches include (1) to use additives such as absorption enhancers and protease inhibitors, (2) to develop an administration method for peptides that can serve as an alternative to oral and injection administration, (3) to modify the molecular structure of peptide and protein drugs to produce prodrugs and analogues, and (4) to use the dosage forms to these peptide drugs. In this study, we demonstrated that the transmucosal absorption of various peptides including insulin, calcitonin, tetragastrin and thyrotropin releasing hormone (TRH) could be improved by the use of these approaches. Therefore, these approaches may give us basic information to improve the transmucosal absorption of peptide and protein drugs.

Development of novel lipophilic derivatives of DADLE (leucine enkephalin analogue): intestinal permeability characateristics of DADLE derivatives in rats

PURPOSE

The objective of this study is to examine the intestinal permeability of novel lipophilic derivatives of DADLE (Tyr-D-Ala-Gly-Phe-D-Leu), an enkephalin analogue, using isolated rat intestinal membranes.

METHODS

The novel lipophilic derivatives of DADLE were synthesized by chemical modification with various fatty acids at the C terminus. The pharmacological activities of these DADLE derivatives were assessed by a hot plate test. The intestinal permeability of these derivatives was estimated by the in vitro Ussing chamber method.

RESULTS

We obtained four different DADLE derivatives including acetyl-DADLE (DADLE-C2), butyryl-DADLE (DADLE-C4), caproyl-DADLE (DADLE-C6), and caprylyl-DADLE (DADLE-C8). All the derivatives of DADLE had at least 75% of the activity of native DADLE, suggesting that chemical modification of DADLE at the C terminus did not markedly affect its pharmacological activity. These DADLE derivatives were more stable than native DADLE in jejunal and colonic homogenates. A "bell-shaped" profile was observed between the apparent permeability coefficients (Papp) of DADLE derivatives and lipophilicity. In particular, DADLE-C4 had the greatest permeability characteristics across the intestinal membrane of the acyl derivatives studied in this experiment. The permeability of DADLE-C4 across the jejunal membrane was further improved in the presence of puromycin, amastatin, and sodium glycocholate (NaGC), all at a concentration of 0.5 mM.

CONCLUSIONS

We suggest that the combination of chemical modification with butyric acid and the application of a protease inhibitor are effective for improving the absorption of DADLE across the intestinal membrane.

Investigations of the in-vitro metabolism of three opioid tetrapeptides by pancreatic and intestinal enzymes

The metabolism of three opioid tetrapeptides, Tyr-D-Arg-Phe-Nva-NH2, Tyr-D-Arg-Phe-Phe-NH2 and Tyr-D-Ala-Phe-Phe-NH2, was investigated in the presence of pure pancreatic enzymes (trypsin, chymotrypsin, elastase, carboxypeptidase A and carboxypeptidase B), as well as in the presence of pure carboxylesterase and aminopeptidase N. The cleavage patterns of the pure pancreatic enzymes were then compared with those found in rat and human jejunal fluid. Metabolism was also studied in homogenates from different intestinal regions (duodenum, jejunum, ileum and colon) and in enterocyte cytosol from rats. The effect of various protease inhibitors was investigated in the jejunal homogenate. The parent peptides were assayed by high-performance liquid chromatography and metabolites were identified by means of liquid chromatography-mass spectrometry. Of the pure enzymes, the quickest hydrolysis of the peptides was observed for the pancreatic enzymes chymotrypsin, trypsin and carboxypeptidase A. In most cases they formed the corresponding deamidated tetrapeptides (chymotrypsin and trypsin) or tripeptides with a missing C-terminal amino acid (carboxypeptidase A). Regional differences in intestinal metabolism rates were found for all three peptides (P < 0.001), with the highest rates observed in jejunal and/or colonic homogenates. The deamidated tetrapeptides were formed both in rat intestinal homogenates and in enterocyte cytosol. Metabolism in the jejunal homogenate was markedly inhibited by some serine and combined serine and cysteine protease inhibitors. In conclusion, the C-terminal amide of these tetrapeptides did not fully stabilise them against intestinal deamidase and carboxypeptidase activities. The significant hydrolysis of the peptides by pure chymotrypsin, trypsin and carboxypeptidase A showed that lumenal pancreatic proteases might be a clear metabolic obstacle in oral delivery even for small peptides such as these tetrapeptides.

Intestinal transport and metabolism of glucose-conjugated kyotorphin and cyclic kyotorphin: metabolic degradation is crucial to intestinal absorption of peptide drugs

Intestinal transport and metabolism of modified kyotorphin (KTP) were studied in rats. Modified KTPs studied were C-terminally modified KTP with p-aminophenyl-beta-D-glucoside (KTP-pAPbeta glc), N-terminally modified KTP-pAPbeta glc with t-butyloxycarbonyl group (Boc-KTP-pAPbeta glc) and the N- and C-terminally modified KTP by cyclization (cyclic KTP). KTP-pAPbeta glc was metabolized at a similar rate to that of KTP, and did not appear on the serosal side. Although Boc-KTP-pAPbeta glc was also metabolized, it was more stable than KTP and appeared on the serosal side. Cyclic KTP was also quite stable and appeared on the serosal side. The modified KTPs were evaluated kinetically for absorption consisting of membrane transport and metabolism. Absorption clearance (CL(abs)) of cyclic KTP, Boc-KTP-pAPbeta glc and Boc-KTP was higher than that of KTP (0.247 microl/min/cm) (Mizuma et al., Biochim. Biophys. Acta 1335 (1997) 111-119), which is the theoretical maximum by complete inhibition of peptidase activity, indicating that derivatization of KTP increases the membrane permeability. Furthermore, the data clearly showed that the greater the metabolic clearance (CL(met)) of KTP and the KTP derivatives, the lower the absorption clearance (CL(abs)). These results and further simulation study led to the conclusion that metabolic degradation in the intestinal tissues is more critical than membrane permeability (transport) for oral delivery of peptide drugs. Based on the stability of cyclic KTP in serum, this appears to be a good candidate analgesic peptide drug.

Intestinal transport of beta-thioglycosides by Na+/glucose cotransporter

Intestinal metabolism and transport of p-nitrophenyl beta-D-thioglucoside (p-NPbetaSglc) and p-nitrophenyl beta-D-thiogalactoside (p-NPbetaSgal) by the Na+/glucose cotransporter were studied in excised small intestine of the rat. p-NPbetaSglc and p-NPbetaSgal were stable enough on the mucosal side to be transported to the serosal side. Transport of p-NPbetaSglc was inhibited in the presence of phloridzin (a Na+/glucose cotransporter inhibitor), glucose, or 3-O-methylglucose (3-OMG). p-NPbetaSglc transport was dependent on Na+ concentration in a sigmoidal manner. The activation energy for transport was 19.4 kcal mol(-1). The distribution of transport activity of p-NPbetaSglc in each region of the small intestine correlated well with that of 3-OMG. These results indicate that p-NPbetaSglc is transported by the Na+/glucose cotransporter in small intestine. The order of turnover rate for glycoside transport by Na+/glucose cotransporter was 3-OMG > p-nitrophenyl beta-O-glucoside > p-NPbetaSglc > p-NPbetaSgal, indicating that the presence of a galactose moiety and a sulphur between the monosaccharide moiety and aglycone decreases the turnover rate of the Na+/glucose cotransporter in the transport of glycosides. In an inhibition study using stable p-NPbetaSglc as a Na+/glucose cotransporter-transportable marker glucoside, it was also shown that the Na+/glucose cotransporter recognized several types of glycosides. In conclusion, displacement of the oxygen at carbon C-1 of glucose by sulphur in thioglycosides decreases the turnover rate of the Na+/glucose cotransporter, but thioglycosides are stable in the small intestine and are transported by the Na+/glucose cotransporter.

Inhibitory effect of phloridzin and phloretin on glucuronidation of p-nitrophenol, acetaminophen and 1-naphthol: kinetic demonstration of the influence of glucuronidation metabolism on intestinal absorption in rats

Intestinal glucuronidation and absorption of p-nitrophenol (p-NP), acetaminophen (APAP) and 1-naphthol (alpha-NA) in the presence of phloridzin (inhibitor of Na+/glucose cotransporter) and phloretin (aglycone of phloridzin) were studied. Glucuronides of p-NP, APAP and alpha-NA appeared on both the serosal and mucosal sides. The amounts of glucuronides on the serosal side were decreased in the presence of phloridzin and phloretin. p-NP, APAP and alpha-NA appeared on the serosal side as well, and the amounts of p-NP, APAP and alpha-NA on the serosal side were increased by the presence of phloridzin and phloretin. Furthermore, the intestinal glucuronidation and absorption of alpha-NA at various concentrations were studied in the presence and absence of phloretin. Metabolic clearance was decreased in the presence of phloretin, and the absorption clearance was increased. The higher concentrations of alpha-NA caused higher absorption clearance. The lower the metabolic clearance, the higher the absorption clearance. The relationship between glucuronidation metabolism and absorption in intestine was kinetically analyzed by the metabolic inhibition model. Complete inhibition of glucuronidation improved the intestinal absorption of alpha-NA, and the absorption clearance increased to 7.17 microliter/min/cm. The formation of phloretin and an unknown metabolite from phloridzin were observed. An unknown metabolite from phloretin was observed, and was suppressed by the presence of alpha-NA. This suggests that phloridzin was hydrolyzed to phloretin, which was metabolized to glucuronide, and thereby inhibited glucuronidation of p-NP, APAP and alpha-NA.

The Metabolic Inhibition Model Which Predicts the Intestinal Absorbability and Metabolizability of Drug: Theory and Experiment

The intestinal absorption of analgesic peptides (leucine enkephalin and kyotorphin) and modified peptides in rat were studied. Although these peptides were not absorbed, the absorbability (absorption clearance) of these peptides were increased in the presence of peptidase inhibitors. In order to kinetically analyze these phenomena, we proposed the metabolic inhibition model, which incorporated the metabolic clearance (metabolizability) with the absorption clearance. Metabolic activity was determined with intestinal homogenates. The higher the metabolic clearance was, the lower was the absorption clearance. The relationships between the absorption clearance and the metabolic clearance of the experimental data as well as of the theoretical values were hyperbolic. This model predicted the maximum absorption clearances of cellobiose-coupled leucine enkephalin (0.654 &mgr;l/min/cm) and kyotorphin (0.247 &mgr;l/min/cm). Details of the experimental methods are described.

Intestinal absorption of stable cyclic dipeptides by the oligopeptide transporter in rat

Intestinal absorption of four cyclic dipeptides was studied in the everted small intestine of the rat. Cyclic seryltyrosine (cyclo(Ser-Tyr)) was stable enough to be transported whereas linear seryltyrosine was not. The absorption clearance of cyclo(Ser-Tyr) was concentration-dependent, and for cyclo(Ser-Tyr) at 125 microM decreased in the presence of glycylsarcosine (10 mM) or cephalexin (10 mM), which were reported to be absorbed by oligopeptide transporter. The absorption clearance was also reduced at 4 degrees C and in the presence of 1 mM dinitrophenol. Kinetic analysis of cyclo(Ser-Tyr) absorption showed that Km and Vmax were 19.8 microM and 0.295 nmol min(-1) cm(-1), respectively. It was also suggested that cyclic aspartylphenylalanine and cyclic histidylphenylalanine were absorbed by oligopeptide transporters, but cyclic histidylproline was not. The absorption clearance of cyclo(Ser-Tyr) in the control was much higher than the value of the correlation line representing a plot of passive transport (which was obtained from the absorption clearance of cyclic peptides in the presence of glycylsarcosine (10 mM)) against hydrophobicity (oil-water partition coefficient). These results indicate that cyclo(Ser-Tyr) is absorbed by the oligopeptide transporter.

Intestinal absorption of stable cyclic glycylphenylalanine: comparison with the linear form

The absorption, especially the stability and transportability, of the cyclic peptide cyclic glycylphenylalanine (cyclo(Gly-Phe)) and the linear peptides glycylphenylalanine, glycyl-D-phenylalanine and phenylalanylglycine have been studied in rat small intestine. Linear peptides were degraded on the mucosal side and only glycyl-D-phenylalanine appeared on the serosal side. However, cyclo(Gly-Phe) was stable on the mucosal side and appeared on the serosal side. Furthermore, the absorption clearance of cyclo(Gly-Phe) was higher than that of glycyl-D-phenylalanine. In the presence of the peptidase inhibitor bestatin, the degradation of linear peptides was reduced and linear peptides appeared on the serosal side, but only phenylalanylglycine, which is transported by the oligopeptide transporter, was absorbed faster than cyclo(Gly-Phe). The absorption clearance of cyclo(Gly-Phe) was reduced as its concentration was increased from 125 microM to 500 microM. Furthermore, the absorption clearance of cyclo(Gly-Phe) at 125 microM was reduced at 4 degrees C or in the presence of glycylsarcosine and cephalexin, which are transported by the oligopeptide transporter. These results indicated that cyclo(Gly-Phe) was stable enough to be absorbed and was transported in part by the oligopeptide transporter rather than completely by passive diffusion.

Intestinal transport and metabolism of analgesic dipeptide, kyotorphin: rate-limiting factor in intestinal absorption of peptide as drug

Intestinal transport and metabolism of kyotorphin (KTP) were studied in rat everted small intestine. KTP on the mucosal side was metabolized completely within 60 min, and any amounts of KTP were not detected on the serosal side. On the other hand, [D-Arg2]-KTP (D-KTP) was stable on the mucosal side to appear on the serosal side. However, N-t-butoxycarbonyl-KTP (Boc-KTP), which was metabolized on the mucosal side faster than KTP, appeared on the serosal side. In intestinal homogenate, KTP was metabolized, and the metabolic clearance (CL(met)) was decreased by peptidase inhibitors, bestatin, o-phenanthrolin and tryptophan hydroxamate. In the presence of these peptidase inhibitors, the absorption clearance (CL(abs)) of KTP was increased. The less the CL(met) of KTP was, the more the CL(abs) of KTP was. Meanwhile, Boc-KTP in intestinal homogenate was stable even in the absence of peptidase inhibitors. The CL(abs) of Boc-KTP was constant irrespective of the stability on the mucosal side. Kinetic analysis by the metabolic inhibition model indicated that the stabilization of KTP in the intestinal tissue could increase the CL(abs) up to 0.247 microl/min per cm, which was as much as the CL(abs) of stable D-KTP. These results led to the conclusion that rate-limiting process in intestinal absorption of KTP is metabolic degradation in intestinal tissue during the absorption.