Chemical approaches to improve the oral bioavailability of peptidergic molecules

S-adenosyl-l-methionine: transcellular transport and uptake by Caco-2 cells and hepatocytes

S-adenosyl-l-methionine (SAMe) is an endogenous molecule that is known to be protective against hepatotoxic injury. Although oral SAMe appears to be absorbed across the intestinal mucosa, its systemic bioavailability is low. The reason for this is unknown. Using the Caco-2 cell culture model for enterocyte absorption, we determined the mode by which SAMe is transported across this cell monolayer. We also determined the extent it is taken up by both Caco-2 cells and hepatocytes. In Caco-2 cells transport was observed in both apical to basolateral and basolateral to apical directions. The apparent permeability coefficients (Papp) appeared to be concentration independent and were similar in both directions (0.7 times 10−6 and 0.6 times 10−6 cms−1, respectively), i.e. identical to that of the paracellular transport marker mannitol (0.9 times 10−6 and 0.7 times 10−6 cms−1). This mode of transport was supported by a four-fold increase in the Papp for SAMe transport in Ca++-free buffer. Cellular uptake of SAMe was examined in both Caco-2 cells and cultured rat hepatocytes. Uptake by hepatocytes was not saturable in a concentration range of 0.001–100 μm. Accumulation by both cell types was very low, with a cell:medium ratio at equilibrium of only 0.2–0.5. This low cell accumulation supports the finding of paracellular transport as the only mode of cell membrane transport. Increased hepatocellular protection for SAMe may be accomplished by converting SAMe to a more lipid-soluble prodrug.

Past, present, and future technologies for oral delivery of therapeutic proteins

Biological drugs are usually complex proteins and cannot be orally delivered due to problems related to degradation in the acidic and protease-rich environment of the gastrointestinal (GI) tract. The high molecular weight of these drugs often results in poor absorption into the periphery when administered orally. The most common route of administration for these therapeutic proteins is injection. Most of these proteins have short serum half-lives and need to be administered frequently or in high doses to be effective. So, difficulties in the administration of protein-based drugs provides the motivation for developing drug delivery systems (DDSs) capable of maintaining therapeutic drug levels without side effects as well as traversing the deleterious mucosal environment. Employing a polymer as an entrapment matrix is a common feature among the different types of systems currently being pursued for protein delivery. Protein release from these matrices can occur through various mechanisms, such as diffusion through or erosion of the polymer matrix, and sometimes a combination of both. Encapsulation of proteins in liposomes has also been a widely investigated technology for protein delivery. All of these systems have merit and our worthy of pursuit.

Specific permeability modulation of intestinal paracellular pathway by chitosan-poly(isobutylcyanoacrylate) core-shell nanoparticles

This work is focused on the evaluation of the in vitro permeation modulation of chitosan and thiolated chitosan (chitosan-TBA) coated poly(isobutylcyanoacrylate) (PIBCA) nanoparticles as drug carriers for mucosal administration. Core-corona nanoparticles were obtained by radical emulsion polymerisation of isobutylcyanoacrylate (IBCA) with chitosan of different molecular weights and different proportions of chitosan/chitosan-TBA. In this work, the effect of these nanoparticles on the paracellular permeability of intestinal epithelium was investigated using the Ussing chamber technique, by adding nanoparticle suspensions in the mucosal side of rat intestinal mucosa. Results showed that permeation of the tracer [14C]mannitol and the reduction of transepithelial electrical resistance (TEER) in presence of nanoparticles were more pronounced in those formulations prepared with intermediate amounts of thiolated polymer. This effect was explained thanks to the high diffusion capacity of those nanoparticles through the mucus layer that allowed them to reach the tight junctions in higher extent. It was concluded that, although a first contact between nanoparticles and mucus was a mandatory condition for the development of a permeation enhancement effect, the optimal effect depended on the chitosan/chitosan-TBA balance and the conformational structure of the particles shell.

Synthesis and antinociceptive activity of orally active opioid peptides: improvement of oral bioavailability by esterification

To improve the oral bioavailability of a dermorphin tetrapeptide analog, N(alpha)-1-iminoethyl-Tyr-D-MetO-Phe-MebetaAla-OH (III), which has a potent analgesic activity after oral administration, various derivatives were synthesized to increase lipophilicity by esterification of the C-terminal carboxyl group and/or acylation of the phenolic hydroxyl group on Tyr1. Antinociceptive activity was evaluated after subcutaneous or oral administration using the mouse tail pressure test. As a result, increased antinociceptive activity after oral administration as well as an improved ED50(p.o.)/ED50(s.c.) ratio, which is an indicator of oral bioavailability, were found for some compounds. With regard to the improvement of bioavailability, derivatives with acylation of the phenolic hydroxyl group on Tyr1 showed better results than derivatives with esterification of the C-terminal carboxyl group. In particular, an ED50(p.o.)/ED50(s.c.) ratio equivalent to that of morphine was found for an acetylated derivative, N(alpha)-1-iminoethyl-Tyr(COMe)-D-MetO-Phe-MebetaAla-OH (7a), as well as for a methoxycarbonylated derivative, N(alpha)-1-iminoethyl-Tyr(CO2Me)-D-MetO-Phe-MebetaAla-OH (7l).

Pyrazolinone-piperidine dipeptide growth hormone secretagogues (GHSs). Discovery of capromorelin

Novel pyrazolinone-piperidine dipeptide derivatives were synthesized and evaluated as growth hormone secretagogues (GHSs). Two analogues, capromorelin (5, CP-424391-18, hGHS-R1a K(i)=7 nM, rat pituicyte EC(50)=3 nM) and the des-methyl analogue 5c (hGHS-R1a K(i)=17 nM, rat pituicyte EC(50)=3 nM), increased plasma GH levels in an anesthesized rat model, with ED(50) values less than 0.05 mg/kg iv. Capromorelin showed enhanced intestinal absorption in rodent models and exhibited superior pharmacokinetic properties, including high bioavailabilities in two animal species [F(rat)=65%, F(dog)=44%]. This short-duration GHS was orally active in canine models and was selected as a development candidate for the treatment of musculoskeletal frailty in elderly adults.

Protein drug oral delivery: the recent progress

Rapid development in molecular biology and recent advancement in recombinant technology increase identification and commercialization of potential protein drugs. Traditional forms of administrations for the peptide and protein drugs often rely on their parenteral injection, since the bioavailability of these therapeutic agents is poor when administered nonparenterally. Tremendous efforts by numerous investigators in the world have been put to improve protein formulations and as a result, a few successful formulations have been developed including sustained-release human growth hormone. For a promising protein delivery technology, efficacy and safety are the first requirement to meet. However, these systems still require periodic injection and increase the incidence of patient compliance. The development of an oral dosage form that improves the absorption of peptide and especially protein drugs is the most desirable formulation but one of the greatest challenges in the pharmaceutical field. The major barriers to developing oral formulations for peptides and proteins are metabolic enzymes and impermeable mucosal tissues in the intestine. Furthermore, chemical and conformational instability of protein drugs is not a small issue in protein pharmaceuticals. Conventional pharmaceutical approaches to address these barriers, which have been successful with traditional organic drug molecules, have not been effective for peptide and protein formulations. It is likely that effective oral formulations for peptides and proteins will remain highly compound specific. A number of innovative oral drug delivery approaches have been recently developed, including the drug entrapment within small vesicles or their passage through the intestinal paracellular pathway. This review provides a summary of the novel approaches currently in progress in the protein oral delivery followed by factors affecting protein oral absorption.

New renin inhibitors with pseudodipeptidic units in P(1)-P(1') and P(2')-P(3') positions

A series of four new potential renin inhibitors has been synthesized. The structure of the compounds was designed in such a way as to produce agents resistant to enzymatic degradation, metabolically stable, possibly potent and with improved oral absorption. All positions of the 8-13 fragment of the human angiotensinogen were occupied by unnatural units (two unnatural amino acids in positions P(3) and P(2) and two pseudodipeptides in positions P(1)-P(1') and P(2')-P(3')). Both N- and C-terminal functions of the inhibitors were blocked with tert-Boc and ethyl ester groups. Their hydrophobicity evaluated as a log P value, calculated by a computer method, was 6.57 and 6.08 respectively. All peptides were obtained by the carbodiimide method in solution and purified by chromatography on the SiO(2) column. Their resistance to enzymatic degradation was assayed by determination of stability against chymotrypsin activity. The potency was measured in vitro by a spectrofluorimetric method (assay of Leu-Val-Tyr-Ser released from the N-acetyltetradecapeptide substrate by renin in the presence of the inhibitor). All inhibitors were stable to chymotrypsin. Their IC(50) (M/l) values were: 9.6 x 10(-4) (12), 1.6 x 10(-5) (17), 1.0 x 10(-5) (22) and 1.0 x 10(-5) (23) respectively.

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.

Targeted prodrug design to optimize drug delivery

Classical prodrug design often represents a nonspecific chemical approach to mask undesirable drug properties such as limited bioavailability, lack of site specificity, and chemical instability. On the other hand, targeted prodrug design represents a new strategy for directed and efficient drug delivery. Particularly, targeting the prodrugs to a specific enzyme or a specific membrane transporter, or both, has potential as a selective drug delivery system in cancer chemotherapy or as an efficient oral drug delivery system. Site-selective targeting with prodrugs can be further enhanced by the simultaneous use of gene delivery to express the requisite enzymes or transporters. This review highlights evolving strategies in targeted prodrug design, including antibody-directed enzyme prodrug therapy, gene-directed enzyme prodrug therapy, and peptide transporter-associated prodrug therapy.

Controlled release opportunities for oral peptide delivery in aquaculture

Over the last decade, fish supplies for human consumption have reached over 100 million tons. Due to overfishing, future increases in demand can only be met from the aquaculture industry. This will require increased research in areas such as the control and manipulation of fish reproduction. There is increasing interest in the oral delivery of peptides that control gamete reproduction. However, compared to mammalian species, little is known about the barriers to peptide delivery and methods to improve such delivery. The three major barriers to peptide delivery are the enzymatic barriers sourced from the host luminal and membrane bound peptidases, the immunological cells present within both the enterocytes and underlying connective tissue and the physical barrier of the epithelial cells. Furthermore, the anatomy and physiology of the gastrointestinal tract of these species are markedly different when compared to higher vertebrates and therefore must be considered when designing appropriate delivery systems. Research to date has focused on the oral delivery and subsequent pharmacodynamic responses to the peptides associated with growth and reproduction. However, minimal work has been undertaken to overcome the identified barriers and therefore any future investigations need to attend to these obstacles before the oral delivery of bioactive peptides can become a commercial reality.

Strategies toward predicting peptide cellular permeability from computed molecular descriptors

The therapeutic efficacy of an orally administered drug is dictated not only by its pharmacological properties such as potency and selectivity, but also its pharmacokinetic properties such as its access to the site of activity. Thorough evaluation of the physicochemical and biological barriers to drug delivery is essential to the selection and successful development of drug candidates. We have demonstrated previously that cellular permeability, as a primary component of drug delivery, is principally dependent upon the desolvation potential of the polar functionalities in the molecule and, secondarily, upon the solute lipophilicity [Conradi, R.A., Hilgers, A.R., Ho, N.F.H., Burton, P.S. (1992). The influence of peptide structure on transport across Caco-2 cells. II. Peptide bond modification which results in improved permeability. Pharm. Res. 9, 473-479]. Increasingly sophisticated computational methods are becoming available for describing molecular structural features proposed to correlate with such molecular physicochemical determinants of permeability. Herein we examine the relationships of various computationally derived molecular geometric descriptors for a set of peptides and peptidomimetics, in the context of experimentally measured hydrogen-bond potentials and lipophilicities, with their cellular permeabilities. These descriptors include molecular volume, polar and non-polar surface areas and projected molecular cross-sectional areas. Particular attention is paid to the roles of solvation treatments and other computational factors in descriptor generation, deconvolution of cellular transport mechanisms and statistical analyses of the resulting data for the development of valid, structure-based and mechanistically meaningful models of cellular permeability. No significant correlation of cellular permeability with computed descriptors was found. This was primarily because of our inability to identify surrogates for hydrogen-bond desolvation potential for the solutes from among these descriptors.

Molecular dynamics study of disulfide bond influence on properties of an RGD peptide

Three 1 ns length molecular dynamics simulations of an RGD peptide (Ac-Pen-Arg-Gly-Asp-Cys-NH2, with Pen denoting penicillamine) have been performed in aqueous solution, one for the disulfide bridged, and two for the unbridged form. The trajectories were analyzed to identify conformations explored by the two forms and to calculate several properties: NMR vicinal coupling constants, order parameters, dipole moments and diffusion coefficients, in an effort to describe the physical role of the disulfide bond. The cyclic peptide was able to explore several distinct backbone conformations centered around a turn-extended-turn structure. However, its flexibility was limited and it appeared to be 'locked in' into a a family of structures characterized by a high dipole moment and a well-defined conformation of the pharmacophore, which has been previously identified as biologically active. Excellent agreement between the simulated and observed NMR vicinal coupling constants indicates that realistic structures were sampled in the cyclic peptide simulation. The linear form of the peptide was much more flexible than the cyclic one. In the two independent 1 ns simulations of the linear form the explored conformations could be roughly grouped into two classes, of cyclic-like and extended type. Within each simulation the peptide switched between the two classes of structures several times. Exact matches between conformations in the two linear peptide simulations were not found; several conformational regions with backbone rms deviations below 1A were identified, suggesting that representative structures of the linear form have also been identified. In the linear peptide simulations the RGD pharmacophore is able to adopt a wide range of conformations, including the one preferred by the cyclic form. The lower biological activity of the linear peptide compared to the cyclic one may be correlated with the lower population of this structure in the absence of the disulfide bond.

Improvement of intestinal absorption of peptide drugs by glycosylation: transport of tetrapeptide by the sodium ion-dependent D-glucose transporter

A tetrapeptide (Gly-Gly-Tyr-Arg, GGYR), which is not transported by di- or tripeptide transporters, was glycosylated with p-(succinylamido)phenyl alpha- or beta-D-glucopyranoside (alpha,beta-SAPG) to investigate whether these glycosylated molecules are transported by the Na+-dependent D-glucose transporter. Their uptake into brush border membrane vesicles (BBMVs) and transport through the intestinal membrane were examined using the rapid filtration technique and the everted sac method. It was observed that glycosylation at the alpha-amino position of GGYR increased resistance to aminopeptidase activity and inhibited its degradation. When alpha- and beta-SAPG-GGYR were incubated with BBMVs, overshoot uptake was observed about 2 min after the start of incubation in the presence of an inward Na+ gradient. This uptake remained unaffected by the addition of GGYR while it was significantly inhibited when Na+ was replaced with K+ or alpha- and beta-SAPG-GGYR were incubated with BBMVs at 4 degrees C. Uptake was also markedly inhibited either with 1 mM phloridzin or 10 mM D-glucose. These findings suggested that the Na+-dependent glucose transporter (SGLT-1) played an important role in the uptake of both alpha- and beta-SAPG-GGYR into BBMVs. A comparison of alpha- with beta-SAPG-GGYR revealed that the amount of beta-SAPG-GGYR taken up was greater than that of alpha-SAPG-GGYR. From the everted sac method data, it was shown that the elimination clearance from the mucosal side, CLel, and permeation clearance to the serosal side, CLp, were 15.82+/-6.83 and 0.83+/-0.06 microL/min/cm for alpha-SAPG-GGYR and 44.52+/-3.61 and 3.50+/-0.81 microL/min/cm for beta-SAPG-GGYR, respectively, and that alpha-SAPG-GGYR was more resistant to enzymatic degradation than beta-SAPG-GGYR. Permeation of both alpha- and beta-SAPG-GGYR was inhibited in the presence of D-glucose and in the absence of a Na+ gradient, suggesting that both alpha- and beta-SAPG-GGYR were transported by the Na+-dependent D-glucose transporter. The permeation clearance transported by the Na+-dependent D-glucose transporter, (CLp)Na+, of beta-SAPG-GGYR was about 5 times greater than that for alpha-SAPG-GGYR. This result may be ascribable to the fact that the beta-form of glucose has higher affinity to SGLT-1 than the alpha-form. The results of the present study encourage further investigations on improvements in intestinal absorption of peptide drugs by glycosylation.

Human jejunal effective permeability and its correlation with preclinical drug absorption models

This review focuses on intestinal permeability measurements in humans and various aspects of in-vivo transport mechanisms. In addition, comparisons of human data with preclinical models and the blood-brain barrier is discussed. The regional human jejunal perfusion technique has been validated by several crucial points. One of the most important findings is that there is a good correlation between the measured human effective permeability values and the extent of absorption of drugs in humans determined by pharmacokinetic studies. We have also shown that it is possible to determine the effective permeability (Peff) for carrier-mediated transported compounds, and to classify them according to the proposed Biopharmaceutical Classification System (BCS). Furthermore, it is possible to predict human in-vivo permeability using preclinical permeability models, such as in-situ perfusion of rat jejunum, the Caco-2 model and excized intestinal segments in the Ussing chamber. The permeability of passively transported compounds can be predicted with a particularly high degree of accuracy. However, special care must be taken for drugs with a carrier-mediated transport mechanism, and a scaling factor has to be used. It is also suggested that it is possible to roughly estimate the permeability of the blood-brain barrier using measurements of intestinal permeability, even if the quantitative role of efflux of P-glycoprotein(s) in-vivo still remains to be clarified. Finally, the data obtained in-vivo in humans emphasize the need for more clinical studies investigating the effect of physiological in-vivo factors and molecular mechanisms influencing the transport of drugs across the intestinal and as well as other membrane barriers. It is also important to study the effect of anti-transport mechanisms, such as efflux by P-glycoprotein(s), and gut wall metabolism, for example CYP 3A4, on the bioavailability.