Enhancement of intestinal transport of thyrotropin-releasing hormone via a carrier-mediated transport system by chemical modification with lauric acid

Recent Progress in the Oral Delivery of Therapeutic Peptides and Proteins: Overview of Pharmaceutical Strategies to Overcome Absorption Hurdles

Purpose

Proteins and peptides have secured a place as excellent therapeutic moieties on account of their high selectivity and efficacy. However due to oral absorption limitations, current formulations are mostly delivered parenterally. Oral delivery of peptides and proteins (PPs) can be considered the need of the hour due to the immense benefits of this route. This review aims to critically examine and summarize the innovations and mechanisms involved in oral delivery of peptide and protein drugs.

Methods

Comprehensive literature search was undertaken, spanning the early development to the current state of the art, using online search tools (PubMed, Google Scholar, ScienceDirect and Scopus).

Results

Research in oral delivery of proteins and peptides has a rich history and the development of biologics has encouraged additional research effort in recent decades. Enzyme hydrolysis and inadequate permeation into intestinal mucosa are the major causes that result in limited oral absorption of biologics. Pharmaceutical and technological strategies including use of absorption enhancers, enzyme inhibition, chemical modification (PEGylation, pro-drug approach, peptidomimetics, glycosylation), particulate delivery (polymeric nanoparticles, liposomes, micelles, microspheres), site-specific delivery in the gastrointestinal tract (GIT), membrane transporters, novel approaches (self-nanoemulsifying drug delivery systems, Eligen technology, Peptelligence, self-assembling bubble carrier approach, luminal unfolding microneedle injector, microneedles) and lymphatic targeting, are discussed. Limitations of these strategies and possible innovations for improving oral bioavailability of protein and peptide drugs are discussed.

Conclusion

This review underlines the application of oral route for peptide and protein delivery, which can direct the formulation scientist for better exploitation of this route.

Naringenin alters the pharmacokinetics of ranolazine in part through the inhibition of cytochrome P450 (3A4) and P-glycoprotein

Background

This study set out to look at how naringenin affected the pharmacokinetics of ranolazine in rats. The pharmacokinetic investigation of ranolazine in rats following oral administration of ranolazine with or without coadministration of naringenin was successfully conducted using the established technique. Animals were administered the same medications for 7 days as part of a multiple dosage study (MDS), and the amount of ranolazine in plasma was calculated on 18 days. The intestinal transit of ranolazine in the presence and absence of naringenin and verapamil was examined in an in vitro experiment using the intestinal sacs of rats and chickens (P-glycoprotein inhibitor).

Results

Naringenin raised the maximal level (Cmax) of ranolazine from 231 ± 10.16 to 303.67 ± 9.46 and 325.67 ± 21.81 ng/mL in SDS and MDS, respectively. Moreover, naringenin elevated the area under the curve (AUC) of ranolazine from 1293.54 ± 37.18 to 1505.38 ± 100.30 and 1575.42 ± 76.98 ng/mL/h in SDS and MDS. In the presence of naringenin, there was an increase in the transfer of ranolazine from the mucosal side to the serosal side. Naringenin inhibits the enzymes Cytochrome P450 (3A4) or (CYP3A4) and P-glycoprotein (P-gp). The findings showed that naringenin might have a considerable impact on ranolazine pharmacokinetics, including extending its t1/2 and raising its AUC.

Conclusions

The findings of the study showed that naringenin inhibits the enzymes CYP3A4 and P-gp. Therefore, naringenin might have a considerable impact on ranolazine pharmacokinetics, including extending its t1/2 and raising its AUC.

Graphical abstract

Instability Challenges and Stabilization Strategies of Pharmaceutical Proteins

Maintaining the structure of protein and peptide drugs has become one of the most important goals of scientists in recent decades. Cold and thermal denaturation conditions, lyophilization and freeze drying, different pH conditions, concentrations, ionic strength, environmental agitation, the interaction between the surface of liquid and air as well as liquid and solid, and even the architectural structure of storage containers are among the factors that affect the stability of these therapeutic biomacromolecules. The use of genetic engineering, side-directed mutagenesis, fusion strategies, solvent engineering, the addition of various preservatives, surfactants, and additives are some of the solutions to overcome these problems. This article will discuss the types of stress that lead to instabilities of different proteins used in pharmaceutics including regulatory proteins, antibodies, and antibody-drug conjugates, and then all the methods for fighting these stresses will be reviewed. New and existing analytical methods that are used to detect the instabilities, mainly changes in their primary and higher order structures, are briefly summarized.

An Update on Pharmaceutical Strategies for Oral Delivery of Therapeutic Peptides and Proteins in Adults and Pediatrics

While each route of therapeutic drug delivery has its own advantages and limitations, oral delivery is often favored because it offers convenient painless administration, sustained delivery, prolonged shelf life, and often lower manufacturing cost. Its limitations include mucus and epithelial cell barriers in the gastrointestinal (GI) tract that can block access of larger molecules including Therapeutic protein or peptide-based drugs (TPPs), resulting in reduced bioavailability. This review describes these barriers and discusses different strategies used to modify TPPs to enhance their oral bioavailability and/or to increase their absorption. Some seek to stabilize the TTPs to prevent their degradation by proteolytic enzymes in the GI tract by administering them together with protease inhibitors, while others modify TPPs with mucoadhesive polymers like polyethylene glycol (PEG) to allow them to interact with the mucus layer, thereby delaying their clearance. The further barrier provided by the epithelial cell membrane can be overcome by the addition of a cell-penetrating peptide (CPP) and the use of a carrier molecule such as a liposome, microsphere, or nanosphere to transport the TPP-CPP chimera. Enteric coatings have also been used to help TPPs reach the small intestine. Key efficacious TPP formulations that have been approved for clinical use will be discussed.

Peptide Lipidation - A Synthetic Strategy to Afford Peptide Based Therapeutics

Peptide and protein aberrant lipidation patterns are often involved in many diseases including cancer and neurological disorders. Peptide lipidation is also a promising strategy to improve pharmacokinetic and pharmacodynamic profiles of peptide-based drugs. Self-adjuvanting peptide-based vaccines commonly utilise the powerful TLR2 agonist Pam_nCys lipid to stimulate adjuvant activity. The chemical synthesis of lipidated peptides can be challenging hence efficient, flexible and straightforward synthetic routes to access homogeneous lipid-tagged peptides are in high demand. A new technique coined Cysteine Lipidation on a Peptide or Amino acid (CLipPA) uses a 'thiol-ene' reaction between a cysteine and a vinyl ester and offers great promise due to its simplicity, functional group compatibility and selectivity. Herein a brief review of various synthetic strategies to access lipidated peptides, focusing on synthetic methods to incorporate a Pam_nCys motif into peptides, is provided.

Absorption-enhancing effects of gemini surfactant on the intestinal absorption of poorly absorbed hydrophilic drugs including peptide and protein drugs in rats

In general, the intestinal absorption of small hydrophilic molecules and macromolecules like peptides, after oral administration is very poor. Absorption enhancers are considered to be one of the most promising agents to enhance the intestinal absorption of drugs. In this research, we focused on a gemini surfactant, a new type of absorption enhancer. The intestinal absorption of drugs, with or without sodium dilauramidoglutamide lysine (SLG-30), a gemini surfactant, was examined by an in situ closed-loop method in rats. The intestinal absorption of 5(6)-carboxyfluorescein (CF) and fluorescein isothiocyanate-dextrans (FDs) was significantly enhanced in the presence of SLG-30, such effect being reversible. Furthermore, the calcium levels in the plasma significantly decreased when calcitonin was co-administered with SLG-30, suggestive of the increased intestinal absorption of calcitonin. In addition, no significant increase in the of lactate dehydrogenase (LDH) activity or in protein release from the intestinal epithelium was observed in the presence of SLG-30, suggestive of the safety of this compound. These findings indicate that SLG-30 is an effective absorption-enhancer for improving the intestinal absorption of poorly absorbed drugs, without causing serious damage to the intestinal epithelium.

A two-step strategy to enhance activity of low potency peptides

Novel strategies are needed to expedite the generation and optimization of peptide probes targeting G protein-coupled receptors (GPCRs). We have previously shown that membrane tethered ligands (MTLs), recombinant proteins comprised of a membrane anchor, an extracellular linker, and a peptide ligand can be used to identify targeted receptor modulators. Although MTLs provide a useful tool to identify and/or modify functionally active peptides, a major limitation of this strategy is the reliance on recombinant protein expression. We now report the generation and pharmacological characterization of prototype peptide-linker-lipid conjugates, synthetic membrane anchored ligands (SMALs), which are designed as mimics of corresponding MTLs. In this study, we systematically compare the activity of selected peptides as MTLs versus SMALs. As prototypes, we focused on the precursor proteins of mature Substance P (SubP) and Cholecystokinin 4 (CCK4), specifically non-amidated SubP (SubP-COOH) and glycine extended CCK4 (CCK4-Gly-COOH). As low affinity soluble peptides these ligands each presented a challenging test case for assessment of MTL/SMAL technology. For each ligand, MTLs and corresponding SMALs showed agonist activity and comparable subtype selectivity. In addition, our results illustrate that membrane anchoring increases ligand potency. Furthermore, both MTL and SMAL induced signaling can be blocked by specific non-peptide antagonists suggesting that the anchored constructs may be orthosteric agonists. In conclusion, MTLs offer a streamlined approach for identifying low activity peptides which can be readily converted to higher potency SMALs. The ability to recapitulate MTL activity with SMALs extends the utility of anchored peptides as probes of GPCR function.

Drugs from slugs. Part II--conopeptide bioengineering

The biological transformation of toxins as research probes, or as pharmaceutical drug leads, is an onerous and drawn out process. Issues regarding changes to pharmacological specificity, desired potency, and bioavailability are compounded naturally by their inherent toxicity. These often scuttle their progress as they move up the narrowing drug development pipeline. Yet one class of peptide toxins, from the genus Conus, has in many ways spearheaded the expansion of new peptide bioengineering techniques to aid peptide toxin pharmaceutical development. What has now emerged is the sequential bioengineering of new research probes and drug leads that owe their lineage to these highly potent and isoform specific peptides. Here we discuss the progressive bioengineering steps that many conopeptides have transitioned through, and specifically illustrate some of the biochemical approaches that have been established to maximize their biological research potential and pharmaceutical worth.

Polyamidoamine dendrimers as novel potential absorption enhancers for improving the small intestinal absorption of poorly absorbable drugs in rats

Effects of polyamidoamine (PAMAM) dendrimers on the intestinal absorption of poorly absorbable drugs were examined by an in situ closed loop method in rats. 5(6)-Carboxyfluorescein (CF), fluorescein isothiocyanate-dextrans (FDs) with various molecular weights, calcitonin and insulin were used as model drugs of poorly absorbable drugs. The absorption of CF, FD4 and calcitonin from the rat small intestine was significantly enhanced in the presence of PAMAM dendrimers. The absorption-enhancing effects of PAMAM dendrimers for improving the small intestinal absorption of CF were concentration and generation dependent and a maximal absorption-enhancing effect was observed in the presence of 0.5% (w/v) G2 PAMAM dendrimer. However, G2 PAMAM dendrimer had almost no absorption-enhancing effect on the small intestinal absorption of macromolecular drugs including FD10 and insulin. Overall, the absorption-enhancing effects of G2 PAMAM dendrimer in the small intestine decreased as the molecular weights of drug increased. However, G2 PAMAM dendrimer did not enhance the intestinal absorption of these drugs with different molecular weights in the large intestine. Furthermore, we evaluated the intestinal membrane damage with or without G2 PAMAM dendrimer. G2 PAMAM dendrimer (0.5% (w/v)) significantly increased the activities of lactate dehydrogenase (LDH) and the amounts of protein released from the intestinal membranes, but the activities and amounts of these toxic markers were less than those in the presence of 3% Triton X-100 used as a positive control. Moreover, G2 PAMAM dendrimer at concentrations of 0.05% (w/v) and 0.1% (w/v) did not increase the activities and amounts of these toxic markers. These findings suggested that PAMAM dendrimers at lower concentrations might be potential and safe absorption enhancers for improving absorption of poorly absorbable drugs from the small intestine.

Lipid Core Peptide System for Gene, Drug, and Vaccine Delivery

A vast number of biologically active compounds await efficient delivery to become therapeutic agents. Lipidation has been demonstrated to be a convenient and useful approach to improve the stability and transport across biological membranes of potential drug molecules. The lipid core peptide (LCP) system has emerged as a promising lipidation tool because of its versatile features. This review discusses the progress in the development of the LCP system to improve cell permeability of nucleotides, physicochemical properties of potential drugs, and vaccine immunogenicity. Emphasis was put on the application of the LCP system to deliver antigens for the prevention of group A streptococcus infection, novel techniques of conjugation of target molecules to the LCP, and new alterations of the LCP system itself.

Characterization of the influence of nitric oxide donors on intestinal absorption of macromolecules

To characterize the influence of nitric oxide (NO) donors on the intestinal absorption of macromolecules, the relationship between the release rate of NO from NO donors and their absorption-enhancing effects and the effects of several scavengers and generators on the absorption-enhancing effects of NO donor were investigated. The t1/2 values of the NO release rate from 3-(2-hydroxy-1-methylethyl-2-nitrosohydrazino)-1-propanamine (NOC5), 3-(2-hydroxy-1-methylethyl-2-nitrosohydrazino)-N-methyl-1-propanamine (NOC7) and N-ethyl-2-(1-ethyl-hydroxy-2-nitrosohydrazino)-ethanamine (NOC12) are 25, 5 and 100min, respectively. The absorption-enhancing effects of NO donors on the absorption of fluorescein isothiocyanate dextrans with an average molecular weight of 4400 (FD-4) are NOC5 > NOC7 > NOC12 in the colon. The lowest enhancing effect of NOC12 may be due to the slow rate of NO release. The enhancing effect of NOC7 rapidly disappeared compared with the effect of NOC5. The results raise the possibility that the difference between NOC5 and NOC7 on enhancing effect is related to the t1/2 of the NO release. The NOC7-induced enhancing effect was prevented by the co-administration of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazole-1-oxyl-3-oxide sodium salt (C-PTIO), an NO scavenger; tiron, an O2(-) scavenger; mannitol, an OH* scavenger, and deferoxamine, peroxynitrate scavenger. Pyrogallol, an O2(-) generator, potentiated the NOC7-induced enhancing effect. These results support a role for peroxynitrate, and possibly OH*, in the NO donor-induced intestinal enhancing effect.

Evaluation of Dodecylmaltoside as a Permeability Enhancer for Insulin Using Human Carcinoma Cells

Dodecylmaltoside (DDM), an alkylglycoside showing tissue-permeability-enhancing properties, has been successful in improving nasal and ocular transport of poorly absorbed drugs. It was hypothesized that optimization of DDM concentration would improve the transport of insulin across epithelial monolayers without causing cell damage. Samples of markers and insulin were collected over a 6-h period and transepithelial electrical resistance was measured at concurrent time points to ascertain the effect of DDM on tight junctions. Samples were analyzed for lucifer yellow and insulin using reversed-phase high-performance liquid chromatography and for (3)[H]-mannitol by scintillation counting. A significant increase in the transport of markers and insulin was recorded in DDM-treated cells compared with controls. DDM enhanced the transport of markers and insulin in a concentration-dependent manner. Decreased transepithelial electrical resistance values confirmed that enhanced transport is caused by loosening of tight junctions. Cell recovery was >95% in 8 h indicating the potential of DDM as a penetration enhancer for clinical administration of insulin and other poorly absorbed drugs without causing cell damage.

Functional characterization of brain peptide transporter in rat cerebral cortex: identification of the high-affinity type H+/peptide transporter PEPT2

In this report, we studied the functional characteristics of a brain peptide transporter using synaptosomes prepared from rat cerebral cortex. Crude synaptosomes (P(2) fraction) were prepared from cerebral cortices in male Wistar rats. Uptake of [14C]glycylsarcosine (Gly-Sar), a substrate for H(+)/oligopeptide transporters PEPT1 and PEPT2, and [3H]histidine, a substrate for peptide/histidine transporters PHT1 and PHT2, was measured at 37 degrees C by a rapid filtration technique. The uptake of [14C]Gly-Sar into synaptosomes was stimulated by an inwardly directed H(+)-gradient. The uptake system exhibited a Michaelis-Menten constant (K(t)) of 110+/-20 microM for Gly-Sar. This value is comparable to the K(t) value for Gly-Sar uptake via the high-affinity H(+)/peptide transporter PEPT2. The H(+)-dependent uptake of [14C]Gly-Sar into synaptosomes was inhibited by di- and tripeptides and beta-lactam antibiotics, but was unaffected by amino acids glycine and histidine. In particular, kyotorphin (Tyr-Arg) completely inhibited Gly-Sar uptake with the K(i) value of 29+/-14 microM. These uptake properties of the brain peptide transporter (i.e., the K(t) value for Gly-Sar uptake and the K(i) value of kyotorphin for Gly-Sar uptake) are very similar to those of PEPT2. RT-PCR and Western blotting analyses revealed that PEPT2 is actually expressed in the cerebral cortex in rat. These results indicate that a H(+)-coupled high affinity peptide transport system is functionally expressed in the cerebral cortex and that this transport system is identical to PEPT2.

Enhanced transdermal delivery of phenylalanyl-glycine by chemical modification with various fatty acids

We synthesized three novel lipophilic derivatives of phenylalanyl-glycine (Phe-Gly), C4-Phe-Gly, C6-Phe-Gly and C8-Phe-Gly by chemical modification with butyric acid (C4), caproic acid (C6) and octanoic acid (C8). The effect of the acylation on the stability, permeability and accumulation of Phe-Gly in the skin was investigated by in vitro studies. The stability of Phe-Gly in skin homogenates was low, but was significantly improved by the acylation. In the transport studies, a Franz-type diffusion cell was used for the permeability experiments with Phe-Gly and its acyl derivatives. The permeability of acyl-Phe-Gly derivatives across the intact skin was higher than that of native Phe-Gly. Of all the acyl-Phe-Gly derivatives, C6-Phe-Gly was the most permeable compounds across the intact skin. On the other hand, the permeability of acyl-Phe-Gly derivatives across stripped skin was less than that of native Phe-Gly in the initial time period of transport studies, but their permeability was higher than that of native Phe-Gly at the end of the transport studies. When the skin was pretreated with ethanol, which could inactivate the peptidases responsible for the degradation of Phe-Gly, the permeability of native Phe-Gly was higher than that of acyl derivatives. These findings indicated the involvement of peptidases on the permeability of Phe-Gly across the skin. The relationship between the lipophilic indexes of Phe-Gly derivatives and the permeability coefficients indicated that there is an optimal carbon number of fatty acid for improving the transdermal permeability of Phe-Gly by the acylation. A good correlation was found between the accumulation of these acyl-Phe-Gly derivatives in the intact skin and their lipophilicity. These results suggest that the stability and permeability of Phe-Gly were improved by chemical modification with fatty acids and this enhanced permeability of Phe-Gly by the acylation may be attributed to the protection of Phe-Gly from the enzymatic degradation in the skin and the increase in the partition of Phe-Gly to the stratum corneum.

Enhanced Permeability of Phenylalanyl-glycine (Phe-Gly) Across the Intestinal Membranes by Chemical Modification with Various Fatty Acids

We synthesized four novel lipophilic derivatives of phenylalanyl-glycine (Phe-Gly), C4-Phe-Gly, Phe-Gly-C4, C6-Phe-Gly and C8-Phe-Gly by chemical modification with butyric acid (C4), caproic acid (C6) and octanoic acid (C8). The effect of the acylation on the stability, permeability and accumulation of Phe-Gly in the intestine was investigated by in vitro studies. The stability of Phe-Gly in homogenates of duodenal and colonic membranes was low, but was significantly improved by the acylation except for Phe-Gly-C4. In the transport studies, a modified Ussing chamber was used for the intestinal permeability experiments with Phe-Gly and its acyl derivatives. The permeability of native Phe-Gly and Phe-Gly-C4 across the intestinal membrane was not observed during the transport studies. However, the permeability of Phe-Gly was much improved by chemical modification with various fatty acids to its N-terminal portion. The permeability of acyl-Phe-Gly derivatives across the intestinal membrane decreased with increasing the chain length of fatty acids. In addition, the intestinal tissue accumulation of acyl-Phe-Gly derivatives at the end of the transport studies was much higher than that of native Phe-Gly. The intestinal tissue accumulation of acyl-Phe-Gly in the duodenum increased as the chain length of fatty acids increased. Furthermore, intestinal permeability of C4-Phe-Gly was slightly inhibited in the presence of 5 mM ceftibuten and was significantly reduced under low temperature condition. We observed a directional difference in the transport of C4-Phe-Gly (the mucosal to serosal transport of C4-Phe-Gly was higher than its serosal to mucosal transport) suggesting that C4-Phe-Gly might be transported by a carrier-mediated process as well as other dipeptides. These findings indicate that acylation might be useful approach to enhance the transport of Phe-Gly, a model dipeptide, transported by a carrier-mediated process.

[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.

Factors and strategies for improving buccal absorption of peptides

Peptides and polypeptides have important pharmacological properties but only a limited number (e.g. insulin, oxytocin, vasopressin) have been exploited as therapeutics because of problems related to their delivery. The buccal mucosa offers an alternative route to conventional, parenteral administration. Peptides are generally not well absorbed through mucosae because of their molecular size, hydrophilicity and the low permeability of the membrane. Peptide transport across buccal mucosa occurs via passive diffusion and is often accompanied by varying degrees of metabolism. This review describes various approaches to improve the buccal absorption of peptides including the use of penetration enhancers to increase membrane permeability and/or the addition of enzyme inhibitors to increase their stability. Other strategies including molecular modification with bioreversible chemical groups or specific formulations such as bioadhesive delivery systems are also discussed.

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.

Drug transport and targeting. Intestinal transport

A wide variety of transporters are found in the intestine, and are involved in the membrane transport of daily nutrients as well as drugs. These intestinal transporters are located in the brush border membrane as well as basolateral membrane. Each transporter exhibits its own substrate specificity, and some have broader specificities than others. In addition, the distribution and characteristics of the intestinal transporters exhibit regional differences along the intestine, implying diverse physiologic functions and in some cases pathologic responses. Indeed several genetic disorders have been shown to result from deficient intestinal transporters. The development of prodrugs that target to intestinal transporters has been successful in improving oral absorption. For example, the intestinal peptide transporter is utilized in order to increase the bioavailability of several classes of peptidomimetic drugs, especially ACE inhibitors and beta-lactam antibiotics. The bioavailability of poorly absorbed drugs can be improved by utilization of the transporters responsible for the intestinal absorption of various solutes and/or by inhibiting the transporter involved in the efflux system. Recent advances in gene cloning and molecular biology techniques make it possible to study the characteristics and distribution of transporters at the molecular level. Based on molecular characterizations of membrane transporters and accumulated biochemical data on their specificities and kinetics, structural modification and targeting of a specific transporter is a promising strategy for the design of drugs that improve bioavailability and tissue distribution.

Enhanced permeability of insulin across the rat intestinal membrane by various absorption enhancers: their intestinal mucosal toxicity and absorption-enhancing mechanism of n-lauryl-beta-D-maltopyranoside

We have examined the in-vitro permeability characteristics of insulin in the presence of various absorption enhancers across rat intestinal membranes and have assessed the intestinal toxicity of the enhancers using an in-vitro Ussing chamber method. The absorption enhancing mechanism of n-lauryl-beta-D-maltopyranoside was studied also. The permeability of insulin across the intestinal membranes was low in the absence of absorption enhancers. However, the permeability was improved in the presence of enhancers such as sodium glycocholate and sodium deoxycholate in the jejunum, and sodium glycocholate, sodium deoxycholate, n-lauryl-beta-D-maltopyranoside, sodium caprate and ethylenediaminetetraacetic acid (EDTA) in the colon. Overall, the absorption enhancing effects were greater on the colonic membrane than on the jejunal membrane. The intestinal membrane toxicity of these enhancers was characterized using the release of cytosolic lactate dehydrogenase from the colonic membrane. A marked increase in the release of lactate dehydrogenase was observed in the presence of sodium deoxycholate and EDTA. The release of lactate dehydrogenase in the presence of these absorption enhancers was similar to that seen with sodium dodecyl sulphate (SDS), used as a positive control, indicating high toxicity of these enhancers to the intestinal membrane. In contrast, sodium glycocholate and sodium caprate caused minor releases of lactate dehydrogenase, similar to control levels, suggesting low toxicity. In addition, the amount of lactate dehydrogenase in the presence of n-lauryl-beta-D-maltopyranoside was much less than that seen with sodium deoxycholate, EDTA and SDS. Therefore, sodium glycocholate, sodium caprate and n-lauryl-beta-D-maltopyranoside are useful absorption enhancers due to their high absorption enhancing effects and low intestinal toxicity. To investigate the absorption enhancing mechanisms of n-lauryl-beta-D-maltopyranoside, the transepithelial electrical resistance (TEER), voltage clamp experiments and the circular dichroism spectra were studied. n-Lauryl-beta-D-maltopyranoside decreased the TEER values in a dose-dependent manner, suggesting that the enhancer may open the tight junctions of the epithelium, thereby increasing the permeability of insulin via a paracellular pathway. This speculation was supported by the findings that 20 mM n-lauryl-beta-D-maltopyranoside produced a greater increase in the paracellular flux rate than in the transcellular flux rate by the voltage clamp studies. Evaluating the circular dichroism spectra we found that insulin oligomers were not dissociated to monomers by the addition of n-lauryl-beta-D-maltopyranoside, but dissociation did occur with the addition of sodium glycocholate. Thus, the dissociation of insulin was not a major factor in the absorption enhancing effect of n-lauryl-beta-D-maltopyranoside. These findings provide basic information to select the optimal enhancer for the intestinal delivery of peptide and protein drugs including insulin.

Intestinal peptide transport systems and oral drug availability

The intestinal peptide transport system has broad substrate specificities. In addition to its physiological function of absorbing di- and tripeptides resulting from the digestion of dietary proteins, this transport system also absorbs some orally administered peptidomimetic drugs, including beta-lactam antibiotics, angiotensin converting enzyme inhibitors, renin inhibitors, bestatin, thrombin inhibitors, and thyrotropin-releasing hormone and its analogues. There have been several studies on the mechanism and substrate structure-affinity relationship for this transport system. Rapid progress has been made recently in studies on the molecular basis of the intestinal peptide transport system. A protein apparently involved in peptide transport has been isolated from rabbit small intestines, and genes for human intestinal peptide transporters have been cloned, sequenced and functionally expressed. This review summarizes these studies and addresses the pharmaceutical potential of the intestinal peptide transport system.

Transport kinetics of leucine enkephalin across Caco-2 monolayers: quantitative analysis for contribution of enzymatic and transport barrier

In this study, we determined the activities of four aminopeptidases such as aminopeptidase B (APB), M (APM), N (APN) and dipeptidylpeptidase IV (DPP IV) in Caco-2 cells and compared with those in the rat intestinal mucosae. The activities of APB, APM and APN appeared to be highest in rat small intestinal mucosa, while DPP IV activity was much higher in Caco-2 cells than that in the rat intestinal mucosa. Next the inhibitory effects of various protease inhibitors were examined in Caco-2 homogenate. Three tested inhibitors, bacitracin, amastatin and puromycin, effectively inhibited the activities of APM, APN and DPP IV except for APB. Further, we quantitatively evaluated the permeation and degradation properties of leucine enkephalin (Leu-Enk) in the presence or absence of inhibitors in Caco-2 monolayer system. Leu-Enk had a high degradation clearance (CLd) and a low permeation clearance (CLp) in Caco-2 monolayers. This finding indicates that the very rapid degradation of Leu-Enk on the apical side of Caco-2 monolayers was due to aminopeptidases. However, these protease inhibitors besides sodium glycocholate were able to reduce the CLd values markedly, thereby increasing the permeation amount of Leu-Enk across Caco-2 monolayers. In particular, amastatin significantly decreased the CLd value and increased the CLp value. This enhanced CLp value was further increased by the coadministration with an absorption enhancer, EDTA or laurylmaltoside. These findings are relevant to the oral administration of peptide drugs and to developing an efficient oral delivery system.

Effects of different absorption enhancers on the permeation of ebiratide, an ACTH analogue, across intestinal membranes

The permeation of ebiratide (H-Met(O2)-Glu-His-Phe-D-Lys-Phe-NH(CH2)8NH2), a novel ACTH analogue, across the intestinal mucosae has been examined by use of isolated intestinal membranes from rats in a modified Ussing chamber. Regional differences were observed in the permeation of ebiratide across intestinal membranes; the order of membrane permeability was jejunum > ileum > duodenum > colon. Overall, the permeation of ebiratide was relatively poor. The effects of various absorption enhancers were examined to increase the intestinal permeability to ebiratide. Sodium glycocholate and sodium caprate had no significant enhancing effect on the permeability of the jejunal membrane, but significantly enhanced the permeation of ebiratide through the colonic membrane. On the other hand, N-dodecyl-beta-D-maltopyramoside (LM) significantly enhanced the permeation of ebiratide through both jejunal and colonic membranes. In general, the absorption-enhancing effects of these agents were more predominant in the colon than in the jejunum. Membrane damage by the absorption enhancers was evaluated by measuring the amount of protein released from the intestinal membrane. It was found that all the absorption enhancers slightly increased the amount of protein released, but that the amounts of protein released in the presence of these enhancers were much less than in the presence of ethylenediaminetetraacetic acid (EDTA), used as a positive control. These findings suggest that the absorption enhancers, especially LM might be useful adjuvants for improving the intestinal absorption of peptide and protein drugs, including ebiratide.