In vitro intestinal transport of vasopressin and its analogues

Mesoporous silica nanoparticles: An emerging approach in overcoming the challenges with oral delivery of proteins and peptides

Proteins and peptides (PPs), as therapeutics are widely explored in the past few decades, by virtue of their inherent advantages like high specificity and biocompatibility with minimal side effects. However, owing to their macromolecular size, poor membrane permeability, and high enzymatic susceptibility, the effective delivery of PPs is often challenging. Moreover, their subjection to varying environmental conditions, when administered orally, results in PPs denaturation and structural conformation, thereby lowering their bioavailability. Hence, for effective delivery with enhanced bioavailability, protection of PPs using nanoparticle-based delivery system has gained a growing interest. Mesoporous silica nanoparticles (MSNs), with their tailored morphology and pore size, high surface area, easy surface modification, versatile loading capacity, excellent thermal stability, and good biocompatibility, are eligible candidates for the effective delivery of macromolecules to the target site. This review highlights the different barriers hindering the oral absorption of PPs and the various strategies available to overcome them. In addition, the potential benefits of MSNs, along with their diversifying role in controlling the loading of PPs and their release under the influence of specific stimuli, are also discussed in length. Further, the tuning of MSNs for enhanced gene transfection efficacy is also highlighted. Since extensive research is ongoing in this area, this review is concluded with an emphasis on the potential risks of MSNs that need to be addressed prior to their clinical translation.

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.

Impact of non-proteinogenic amino acids in the discovery and development of peptide therapeutics

With the development of modern chemistry and biology, non-proteinogenic amino acids (NPAAs) have become a powerful tool for developing peptide-based drug candidates. Drug-like properties of peptidic medicines, due to the smaller size and simpler structure compared to large proteins, can be changed fundamentally by introducing NPAAs in its sequence. While peptides composed of natural amino acids can be used as drug candidates, the majority have shown to be less stable in biological conditions. The impact of NPAA incorporation can be extremely beneficial in improving the stability, potency, permeability, and bioavailability of peptide-based therapies. Conversely, undesired effects such as toxicity or immunogenicity should also be considered. The impact of NPAAs in the development of peptide-based therapeutics is reviewed in this article. Further, numerous examples of peptides containing NPAAs are presented to highlight the ongoing development in peptide-based therapeutics.

Recent insights in the use of nanocarriers for the oral delivery of bioactive proteins and peptides

Bioactive proteins and peptides have been used with either prophylactic or therapeutic purposes, presenting inherent advantages as high specificity and biocompatibility. Nanocarriers play an important role in the stabilization of proteins and peptides, offering enhanced buccal permeation and protection while crossing the gastrointestinal tract. Moreover, preparation of nanoparticles as oral delivery systems for proteins/peptides may include tailored formulation along with functionalization aiming bioavailability enhancement of carried proteins or peptides. Oral delivery systems, namely buccal delivery systems, represent an interesting alternative route to parenteric delivery systems to carry proteins and peptides, resulting in higher comfort of administration and, therefore, compliance to treatment. This paper outlines an extensive overview of the existing publications on proteins/peptides oral nanocarriers delivery systems, with special focus on buccal route. Manufacturing aspects of most commonly used nanoparticles for oral delivery (e.g. polymeric nanoparticles using synthetic or natural polymers and lipid nanoparticles) advantages and limitations and potential applications of nanoparticles as proteins/peptides delivery systems will also be thoroughly addressed.

Intestinal transepithelial permeability of oxytocin into the blood is dependent on the receptor for advanced glycation end products in mice

Plasma oxytocin (OT) originates from secretion from the pituitary gland into the circulation and from absorption of OT in mother's milk into the blood via intestinal permeability. However, the molecular mechanism underlying the absorption of OT remains unclear. Here, we report that plasma OT concentrations increased within 10 min after oral delivery in postnatal day 1-7 mice. However, in Receptors for Advanced Glycation End Products (RAGE) knockout mice after postnatal day 3, an identical OT increase was not observed. In adult mice, plasma OT was also increased in a RAGE-dependent manner after oral delivery or direct administration into the intestinal tract. Mass spectrometry evaluated that OT was absorbed intact. RAGE was abundant in the intestinal epithelial cells in both suckling pups and adults. These data highlight that OT is transmitted via a receptor-mediated process with RAGE and suggest that oral OT supplementation may be advantageous in OT drug development.

Approaches for enhancing oral bioavailability of peptides and proteins

Oral delivery of peptide and protein drugs faces immense challenge partially due to the gastrointestinal (GI) environment. In spite of considerable efforts by industrial and academic laboratories, no major breakthrough in the effective oral delivery of polypeptides and proteins has been accomplished. Upon oral administration, gastrointestinal epithelium acts as a physical and biochemical barrier for absorption of proteins resulting in low bioavailability (typically less than 1-2%). An ideal oral drug delivery system should be capable of (a) maintaining the integrity of protein molecules until it reaches the site of absorption, (b) releasing the drug at the target absorption site, where the delivery system appends to that site by virtue of specific interaction, and (c) retaining inside the gastrointestinal tract irrespective of its transitory constraints. Various technologies have been explored to overcome the problems associated with the oral delivery of macromolecules such as insulin, gonadotropin-releasing hormones, calcitonin, human growth factor, vaccines, enkephalins, and interferons, all of which met with limited success. This review article intends to summarize the physiological barriers to oral delivery of peptides and proteins and novel pharmaceutical approaches to circumvent these barriers and enhance oral bioavailability of these macromolecules.

Intestinal Absorption of Fibrinolytic and Proteolytic Lumbrokinase Extracted from Earthworm, Eisenia andrei

To investigate the intestinal absorption of a fibrinolytic and proteolytic lumbrokinase extracted from Eisenia andrei, we used rat everted gut sacs and an in situ closed-loop recirculation method. We extracted lumbrokinase from Eisenia andrei, and then raised polyclonal antibody against lumbrokinase. Fibrinolytic activity and proteolytic activity in the serosal side of rat everted gut sacs incubated with lumbrokinase showed dose- and time-dependent patterns. Immunological results obtained by western blotting serosal side solution using rat everted gut sacs method showed that lumbrokinase proteins between 33.6 and 54.7 kDa are absorbed mostly by the intestinal epithelium. Furthermore, MALDI-TOF mass spectrometric analysis of plasma fractions obtained by in situ recirculation method confirmed that lumbrokinase F1 is absorbed into blood. These results support the notion that lumbrokinase can be absorbed from mucosal lumen into blood by oral administration.

FDA perspective on peptide formulation and stability issues

Traditionally, peptide drugs are prepared as sterile solutions and administered to patients by daily injection. However, this form of drug delivery causes pain and inconvenience to patients and thus has been poorly accepted. In addition to improving patient compliance, many novel delivery systems have been developed to address the need for prolonged, localized (targeted), or pulsatile drug action. Examples include, but are not limited to oral, nasal, or long-acting controlled release injectable dosage forms; a number of them have been approved by FDA recently. The unique characteristics and the relevant regulatory issues with respect to each type of delivery system are presented.

Current concepts in intestinal peptide absorption

Today there is considerable interest in oral peptide delivery. However, oral administration of peptides is limited by a low bioavailability and a high variability in plasma levels. A review is given of the literature describing the major barriers in peptide absorption, the basic mechanisms of intestinal peptide transport, the experimental models and the pharmaceutical approaches currently used in the investigation of peptide and protein absorption processes.

Subcellular distribution of proteolytic activities degrading bioactive peptides and analogues in the rat small intestinal and colonic enterocytes

The objective of this study was to compare, in rat small intestinal and colonic enterocytes, subcellular distributions of activities degrading the large peptides, neurotensin, acetylneurotensin (8-13), GRF(1-29)NH2 (human growth hormone releasing factor fragment), (desNH2Tyr1,D-Ala2,Ala15)-GRF(1-29)NH2, insulin, and insulin B-chain. Proteolytic activities degrading individual peptides in the 10,000-g pellet, rich in intracellular organelles, 27,000-g pellet, rich in brush-border membrane, 100,000-g pellet, and 100,000-g supernatant, rich in cytosol, were determined and compared for both the small intestine and colon. In colonic fractions, the cytosol had highest activity (g protein)-1 degrading three out of four peptides tested, while in small intestinal fractions, the 27,000-g pellet had the highest activity (g protein)-1, degrading four out of five peptides tested. In both small intestine and colon, the cytosol had a higher percentage of total proteolytic activity degrading each of the above polypeptides and the highest insulin-degrading activity (g protein)-1. The results suggest that at pH 7.5, proteolytic activities (g protein)-1 in the fraction of subcellular organelles are much lower than those in cytosol and that cytosolic proteolytic activities degrading polypeptides and analogues are significant.

Use of everted intestinal rings for in vitro examination of oral absorption potential

The ability to predict in vivo oral absorption potential based on ex vivo screening in an everted intestinal ring model was examined. In vitro drug accumulation in cross sectional rings of everted rat jejunum was determined with 12 compounds whose in vivo absorptions (as distinct from bioavailabilities) are well characterized. The compounds examined ranged from well- to poorly-absorbed and included compounds absorbed by active and passive mechanisms. The effects of drug concentration, pH, cosolvents, and tissue origin site on drug accumulation were determined. Light microscopic observation indicated that the mucosal tissue remained intact up to 3 h after the intestine was excised. Accumulations of two nonabsorbable markers were also determined as measures of tissue integrity. A strong correlation (slope = 23 pmol/mg of tissue weight per percent oral absorption, r2 = 0.9430 by linear regression analysis) of in vitro uptake into everted rings from a 10 mM drug solution versus the known in vivo bioavailability for each compound was observed. These results indicated that under appropriate conditions, in vitro uptake of drug by the everted intestinal ring model closely paralleled known in vivo bioavailability and was relatively independent of pH, cosolvent, and tissue origin.

Susceptibility of ebiratide to proteolysis in rat intestinal fluid and homogenates and its protection by various protease inhibitors

In order to estimate the intestinal stability of ebiratide [H-Met(O2)-Glu-His-Phe-D-Lys-Phe-NH(CH2)8-NH2], a novel adrenocorticotropic hormone (ACTH) analogue, after oral administration, the hydrolytic properties of ebiratide were determined in the rat small intestinal fluid and mucosal homogenates. Ebiratide was extremely stable in the rat small intestinal fluid, while it was degraded in various intestinal mucosal homogenates. Regional differences were observed in its proteolytic properties; e.g., the hydrolytic rates of ebiratide in jejunal and ileal mucosal homogenates were 2-3 times faster than that in duodenal and colonic homogenates. Degradation of ebiratide was markedly inhibited by aminoprotease inhibitors such as sodium glycocholate, puromycin, bestatin and bacitracin. These results suggest that co-administration of certain protease inhibitors are useful to improve the stability of ebiratide in the intestine.

Absolute bioavailability of an aqueous solution of 1-deamino-8-D-arginine vasopressin from different regions of the gastrointestinal tract in man

The absolute bioavailability of an aqueous solution of 1-deamino-8-D-arginine vasopressin (dDAVP) from different regions of the gastrointestinal (GI) tract (stomach, duodenum, jejunum, ileum, colon, rectum) has been studied in 6 healthy, male volunteers aged 24 to 35 years, followed for 12 h after each drug administration. For i.v. administration the subjects received 4 micrograms dDAVP. For intestinal administration 400 micrograms dDAVP was directly applied to six distinct sites in the GI tract via two or four channel tubes with or without a distal occlusive balloon. Biological effects were assessed and plasma and urinary levels of dDAVP were measured using a specific, sensitive RIA. Urine osmolality remained elevated and diuresis decreased for 12 h following dDAVP administration irrespective of the site of application. After i.v. administration, the half-life of elimination of dDAVP was 60.0 min, plasma clearance 1.7 ml.min-1.kg-1, amount excreted in urine 2.0 micrograms and renal clearance was 0.8 ml.min-1.kg-1. The mean bioavailability (f) after gastric application was 0.19% (range 0.02-0.35%). f was 0.24% after duodenal application (range 0.04-0.62%), 0.19% after jejunal (range 0.01-0.41%), 0.03% after distal ileal (range 0.01-0.08%), 0.04% after proximal colonic (range 0.01-0.12%) and 0.04% after rectal (0.01-0.10%) application. The bioavailability was significantly higher in the three upper GI regions in comparison to the three lower regions. The bioavailability of dDAVP after gastric, duodenal and jejunal application was similar to that after swallowing a tablet in a previous study. Absorption from the ileum was lower than expected and no preferential site of absorption was found.

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 immunoreactive 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 (less than 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.

Intestinal absorption of the octapeptide SMS 201-995 visualized by fluorescence derivatization

The absorption of an intact oligopeptide was investigated in rat and dog small intestine using the metabolically stable somatostatin analogue SMS 201-995. The synthetic octapeptide was coupled to 4-nitrobenzo-2-oxa-1,3-diazol to have a fluorescent label for the direct visualization. The 4-nitrobenzo-2-oxa-1,3-diazol-labeled peptide was active in displacing the corresponding hormone 125I-Tyr3-SMS 201-995 (Sandostatin; Sandoz Pharmaceuticals, Basel, Switzerland) from its high-affinity binding site in rat cortex membranes with an IC50 = 4.6 x 10(-10) mol/L. The release of growth hormone from cultured anterior pituitary cells was inhibited by the fluorescent somatostatin analogue with the same potency as by somatostatin 14 (IC50 = 6 x 10(-10) mol/L). Incubation with mucosal scrapings followed by high-performance thin-layer chromatography analysis showed that the peptide was stable against proteolysis. 4-Nitrobenzo-2-oxa-1,3-diazol SMS 201-995 was well absorbed from enterocytes of rat small intestine. The absorption was highest into jejunal cells and it could be inhibited by an excess of unlabeled peptide. A significantly lower absorption was detected in crypts compared with villus tips. No fluorescence could be seen in intestinal mucin and goblet cells. After oral administration, the 4-nitrobenzo-2-oxa-1,3-diazol-labeled peptide rapidly appeared in the blood of rats and dogs, reaching a bioavailability of 4.3% and maintaining pharmacological activity. This suggests that enterocytes are able to absorb intact oligopeptides being stabilized against proteolytic degradation through a transcellular mechanism.

Biliary excretion of the vasopressin analogue DDAVP after intraduodenal, intrajugular and intraportal administration in the conscious pig

The biliary excretion of the vasopressin analogue 1-deamino-8-D-arginine vasopressin (dDAVP) was determined in the pig after three administration routes, intrajugular venous, intraportal venous and intraduodenal. In all cases the biliary excretion was less than 1% of the administered dose. The plasma/bile concentration ratio was less than 1:1. A significant first-pass effect was found when the liver was exposed to a high intraportal dose of dDAVP. Possible uptake and degradation/biotransformation was evaluated by incubating [3H]dDAVP with liver tissue slices showing that [3H]dDAVP was rapidly removed from the incubation medium. The following conclusions can be drawn from these experiments: 1) The intestinal mucosa constitutes the major barrier to intestinal absorption of dDAVP. 2) dDAVP is excreted in bile in small amounts. 3) Indirect evidence suggests that the dDAVP molecule is degraded/biotransformed in the liver at its C-terminus.

Biologically active peptides in the gastrointestinal lumen

The release of a variety of biologically active peptides into the gastrointestinal lumen via gastric, duodenal and intestinal secretions, as well as in the saliva, pancreatic juice and bile, has been explored. The key features of luminal secretion of peptides such as secretion at high concentrations, neurohormonal regulation, luminal orientation of stimulated secretion, stability of peptides in the gastrointestinal lumen, altered secretion under pathophysiological conditions, and biological activity of luminally administered peptides are discussed. This review develops a detailed picture of the current understanding of luminal secretion of peptides and their possible biological functions under normal and pathophysiological conditions.

Degradation of [mercaptopropionic acid1, D-arginine8]-vasopressin (dDAVP) in pancreatic juice and intestinal mucosa homogenate

The degradation of the vasopressin analogue dDAVP was studied by reversed phase high-performance liquid chromatography (HPLC) after incubations in pancreatic juice and intestinal mucosa homogenates. dDAVP remained stable in pancreatic juice for a period of 60 min. while the parent hormone arginine vasopressin (AVP) was completely degraded within 5 min. In intestinal mucosa homogenates dDAVP was degraded with half-lives of 9 min. (fast phase) and 161 min. (slow phase), about four times slower than AVP. By amino-acid analysis it was confirmed that the metabolite [Mpa1, Des-D-Arg8-Gly9 NH2]-vasopressin was gradually produced. No other breakdown products were observed. These findings may be of value for the further development of more stable peptide analogues which may be effective upon oral administration.

Decreased passage of the nonapeptide dDAVP over the intestinal epithelium during development in the young rat

The nonapeptide 1-deamino-cysteine-8-D-arginine vasopressin (dDAVP) was gavage-fed together with cow's milk whey protein to young, developing rats. The transepithelial passage of dDAVP in the gastrointestinal (GI) tract was assessed by a specific RIA as immunoreactive levels in blood serum extracts and as urinary excretion of dDAVP 0.5-8 h after feeding. In 14-day-old rats the passage of dDAVP was higher than in 30-day-old rats, since the 14-day-old rats had significantly higher serum levels (5-10 times) 0.5-2 h after feeding and a urinary excretion approaching 0.15% of the administered amount after 8 h. In the 30-day-old rats urinary excretion increased up to 0.05% after 2 h and then levelled off. It was also clear that 30-day-old rats had a slower transfer to and faster elimination from serum than 14-day-old rats. dDAVP appeared to be passed over the GI tract mucosa independently of intestinal proteolysis since feeding it to 30-day-old rats together with the proteinase inhibitors, soya-bean trypsin inhibitor and pepstatin did not influence the serum or urinary levels. Thus, dDAVP was taken up from the GI tract into the blood circulation and excreted in the urine of young rats. The decrease in the passage of dDAVP found around weaning appears to be related to developmental processes affecting the permeability of the intestinal epithelium rather than intestinal proteolysis.