Transport of leuprolide across rat intestine, rabbit intestine and Caco-2 cell monolayer

Nanocarriers transport across the gastrointestinal barriers: The contribution to oral bioavailability via blood circulation and lymphatic pathway

Oral administration is the preferred route of drug delivery in clinical practice due to its noninvasiveness, safety, convenience, and high patient compliance. The gastrointestinal tract (GIT) plays a crucial role in facilitating the targeted delivery of oral drugs. However, the GIT presents multiple barriers that impede drug absorption, including the gastric barrier in the stomach and the mucus and epithelial barriers in the intestine. In recent decades, nanotechnology has emerged as a promising approach for overcoming these challenges by utilizing nanocarrier-based drug delivery systems such as liposomes, micelles, polymeric nanoparticles, solid lipid nanoparticles, and inorganic nanoparticles. Encapsulating drugs within nanocarriers not only protects them from degradation but also enhances their transport and absorption across the GIT, ultimately improving oral bioavailability. The aim of this review is to elucidate the mechanisms underlying nanocarrier-mediated transportation across the GIT into systemic circulation via both the blood circulation and lymphatic pathway.

Extrahepatic in vitro metabolism of peptides; comparison of human kidney and intestinal S9 fraction, human plasma and proximal tubule cells, using cyclosporine A, leuprorelin, and cetrorelix as model compounds

Peptide therapeutics showcase number of advantages compared to the traditional small molecule drugs, e,g. they usually have higher affinity to target and lower toxicity profiles. Endogenous peptides are mostly cleared from the body through renal clearance or proteolytic hydrolysis. As a part of drug discovery, metabolite identification is an important part in their development to identify metabolic hot spots and to further improve their stability. As the catabolism of the peptides and peptide-like drugs is often considered to be extrahepatic, the use of in vitro systems derived from these organs might be beneficial. In this study, multiple extrahepatic metabolic systems were evaluated for the applicability for peptide metabolism studies. Three peptide drugs (leuprorelin, cetrorelix, cyclosporin) were incubated in kidney and intestinal S9 fraction ( ± NADPH), fresh plasma (anticoagulants EDTA and heparin separately), and plated proximal tubule cells. Additionally, leuprorelin was also incubated with human kidney microsomes and cytosol to further investigate the NADPH-dependent metabolism detected in kidney S9 fraction. Both substrate disappearance and metabolite formation were monitored, using UPLC/HR-MS analysis of the collected samples.Overall, the largest number of metabolites was formed in the incubation with kidney S9 fraction, followed by intestinal S9, while incubations with proximal tubule cells produced lower number of metabolites All investigated peptides were stable in plasma and only a few metabolites were detected, likely because the studied peptide drugs have been optimized to be stable in plasma. Leuprorelin showed NADPH-dependent metabolite formation in kidney S9 fraction, while the metabolism of cetrorelix was more NADPH independent. As expected, formation of cytochrome P450 (CYP) catalyzed metabolism of cyclosporine was not observed with the employed extrahepatic systems. The NADPH-dependent metabolism of leuprorelin was detected also in the incubation with kidney cytosol, but not with kidney microsomes, and was thus not caused by CYPs or FMOs, but with cytosolic NADPH-dependent drug metabolizing enzymes. These enzymes could, in principle, activate the amide bond via reductive or oxidative metabolism outside the amide bond. The identity of the involved drug metabolizing enzymes in this process is still unknown.

Influence of permeability enhancers on the paracellular permeability of metformin hydrochloride and furosemide across Caco-2 cells

Permeability enhancers can affect absorption of paracellularly transported drugs. This study aims to evaluate effects of permeability enhancers (chitosan, methyl-β -cyclodextrin, sodium caprate, sodium lauryl sulfate, etc.) on the permeability of paracellularly absorbed furosemide and metformin hydrochloride. Methyl thiazole tetrazolium bromide test was carried out to determine the drug concentrations in permeability study. Trans-epithelial electrical resistance (TEER) values determined to assess the integrity of tight junctions. Permeability enhancers were applied at different concentrations alone, in dual/triple combinations. Permeability was determined using human colorectal adenocarcinoma (Caco-2) cells (TEER > 400 Ω·cm²). Permeability enhancers have no significant effect (<2-fold; p > 0.05) on the permeability of furosemide (1.80 × 10^-5 ± 4.55 × 10^-7 cm/s); however, metformin permeability (1.36 × 10^-5 ± 1.25 × 10^-6 cm/s) increased significantly (p < 0.05) with 0.3% and 0.5% (w/v) chitosan (2.0- and 2.7-fold, respectively), 1% methyl-β -cyclodextrin (w/v) (3.5-fold), 10 and 20 µmol/L sodium caprate (2.2- and 2.8-fold, respectively), and 0.012% sodium lauryl sulfate (w/v) (1.9-fold). Furosemide permeability increased significantly (p < 0.05) with chitosan-sodium lauryl sulfate combination (1.7-fold), and all triple combinations (1.4- to 1.9-fold). Chitosan containing dual/triple combinations resulted in significant increase (p < 0.05) in metformin permeability (1.7 to 2.8-fold). All results indicated that absorption of furosemide and metformin can be improved by the combination of permeability enhancers. Therefore, it can be evaluated for the formulation of development strategies containing furosemide and metformin by the pharmaceutical industry.

S-protected thiolated chitosan: synthesis and in vitro characterization

Purpose of the present study was the generation and evaluation of novel thiolated chitosans, so-named S-protected thiolated chitosans as mucosal drug delivery systems. Stability of all conjugates concerning swelling and disintegration behavior as well as drug release was examined. Mucoadhesive properties were evaluated in vitro on intestinal mucosa. Different thiolated chitosans were generated displaying increasing amounts of attached free thiol groups on the polymer, whereby more than 50% of these thiol groups were linked with 6-mercaptonicotinamide. Based on the implementation of this hydrophobic residue, the swelling behavior was 2-fold decreased, whereas stability was essentially improved. Their mucoadhesive properties were 2- and 14-fold increased compared to corresponding thiolated and unmodified chitosans, respectively. Release studies out of matrix tablets comprising the novel conjugates revealed a controlled release of a model peptide. Accordingly, S-protected thiomers represent a promising type of mucoadhesive polymers for the development of various mucosal drug delivery systems.

Everted gut sac model as a tool in pharmaceutical research: limitations and applications

OBJECTIVES

This review discusses the limitations and applications of the everted gut sac model in studying drug absorption, metabolism, and interaction.

KEY FINDINGS

The mechanism of drug absorption, interaction and the effect of factors such as age, sex, species, chronic therapy, and disease state on drug absorption have been summarized. The experimental conditions and their effects on the outcomes of trials have been discussed also.

SUMMARY

The everted sac model is an efficient tool for studying in-vitro drug absorption mechanisms, intestinal metabolism of drugs, role of transporter in drug absorption, and for investigating the role of intestinal enzymes during drug transport through the intestine.

Thiolated chitosan: development and in vivo evaluation of an oral delivery system for leuprolide

The aim of the present study was to develop an oral delivery system for the peptide drug leuprolide. Gel formulations based on unmodified chitosan/reduced glutathione (GSH) and chitosan-thioglycolic acid (chitosan-TGA)/GSH were prepared, and their effect on the absorption of leuprolide was evaluated in vitro and in vivo in male Sprague Dawley rats. Transport studies were performed with freshly excised rat intestinal mucosa mounted in Ussing-type chambers. Due to the addition of gel formulations comprising 0.5% (m/v) unmodified chitosan/0.5% (m/v) GSH and 0.5% (m/v) chitosan-TGA/0.5% (m/v) GSH, the transport of leuprolide across excised mucosa was improved up to 2.06-fold and 3.79-fold, respectively, in comparison with leuprolide applied in buffer (P(app)=2.87 ± 0.77 × 10⁻⁶ cm/s). In vivo, the addition of oral gel formulation comprising 8 mg of unmodified chitosan, 1mg of GSH and 1mg of leuprolide increased the area under the plasma concentration-time curve (AUC₀₋₈) of leuprolide 1.39-fold in comparison with leuprolide having been administered just in saline. Moreover, the administration of oral gel formulation comprising 8 mg of chitosan-TGA, 1mg of GSH and 1mg of leuprolide resulted in a further enhanced leuprolide plasma concentration, and the area under the plasma concentration-time curve (AUC₀₋₈) of leuprolide was increased 3.72-fold in comparison with the control. With the oral gel formulation comprising 8 mg of chitosan-TGA, a relative bioavailability (versus s.c. injection) of 4.5% was achieved in contrast to the control displaying a relative bioavailability of 1.2%. Thus, according to the achieved results, it is suggested that chitosan-TGA in combination with GSH is a valuable tool for improving the oral bioavailability of the peptide drug leuprolide.

Concentration and surface of absorption: concepts and applications to gastrointestinal patches delivery

Gastrointestinal patches represent a novel multiparticulate drug delivery system able to increase the intestinal absorption of drugs with poor bioavailability. The number of patches to administer is a critical issue since it is related to the surface and drug concentration at the absorption site. The objective of this article is to evaluate the effect of the number of administered patches on the final absorption of leuprolide, a peptide chosen as model drug, assuming complete adhesion of all the devices to the intestinal membrane. The same dose of leuprolide was encapsulated into 2, 4 and 6 patches; the resulting intestinal absorption profiles were measured with the Ussing chamber ex vivo experimental setup and compared between them. The results showed that varying the number of patches, the final absorption does not present statistically significant changes, indicating that changes in concentration are balanced by change in absorption surface. These experimental findings can also be explained considering the equation that links the drug flow to surface and concentration at the absorption site, showing that the drug flow is related only to the geometry of each individual patch.

Mucoadhesive polymeric hydrogels for nasal delivery of acyclovir

The study evaluated different mucoadhesive polymeric hydrogels for nasal delivery of acyclovir. Gels containing poly-N-vinyl-2-pyrrolidone (PVP) were prepared with crosslinking achieved by irradiation with a radiation dose of 15 kGy being as efficient as 20 kGy. Gels containing chitosan and carbopol were also evaluated. The mucoadhesive properties of gels were measured by a modification of a classical tensile experiment, employing a tensile tester and using freshly excised sheep nasal mucosa. Considering the mucoadhesive force, chitosan gel and gel prepared with 3% PVP in presence of polyethylene glycol (PEG) 600 were the most efficient. The in vitro drug release depended on the gel composition. Higher release rates were obtained from PVP gels compared to chitosan or carbopol gels. The release rate of drug from PVP gels was increased further in presence of PEG or glycerol. Histopathological investigations proved that the PVP was a safe hydrogel to be used for mucosal delivery. The PEG in gel formulations caused less damages to the nasal mucosal compared to formulation containing glycerol.

The potential of cystine-knot microproteins as novel pharmacophoric scaffolds in oral peptide drug delivery

Within this study, the potential of three clinically relevant microproteins (SE-AG-AZ, SE-EM and SE-EP) with cystine-knot architecture as pharmacophoric scaffolds for oral peptide delivery was investigated. Cystine-knot microproteins (CKM) were analysed regarding their stability towards the most important gastrointestinal secreted and membrane bound proteases in physiological concentrations. In addition, their permeation behaviour through freshly excised rat intestinal mucosa as well as important parameters such as aggregation behaviour, stability in rat plasma and isoelectric point were evaluated and compared to the properties of the model peptide drugs bacitracin and insulin. Aggregation studies indicate that under physiological conditions between 25 and 70% of the CKMs occur as monomers, whereas the rest forms di- and trimers. Pepsin and elastase cause no or only minor degradation to CKMs, whereas trypsin and chymotrypsin degrade CKMs extensively. Removing the theoretical chymotrypsin cleavage site from a CKM, however, led to stabilization towards this protease. Two of the three evaluated CKMs are stable against membrane bound proteases. P(app) values were determined to be 5.96 +/- 0.98 x 10(-6) and 6.63 +/- 0.47 x 10(-6) cm/s. In conclusion, this study indicates that CKM are promising novel pharmacophoric scaffolds for oral peptide delivery.

Permeation characteristics of oligoarginine through intestinal epithelium and its usefulness for intestinal peptide drug delivery

Cell-penetrating peptides such as HIV-1 Tat and oligoarginine are attractive tools for the intracellular delivery of therapeutic macromolecules. Although we have found that oligoarginine enhances the intestinal absorption of therapeutic peptides, especially insulin, the mechanism underlying the ability of oligoarginine to increase intestinal drug absorption is unclear. In addition, there is no information about the permeation characteristics of these functional peptides through the biological intestinal membrane. Therefore, in this study the permeation characteristics of oligoarginine itself across the intestinal membrane were first determined to obtain the information about absorption enhancement mechanisms. Incubation at low temperature and coincubation with heparin reduced the tissue distribution and permeation of fluorescein-labeled oligoarginine (FL-d-R6) through the rat ileal membrane. These results suggest that the attachment of FL-d-R6 to cell-surface proteoglycans and energy-dependent endocytosis are involved in its permeation through the ileal epithelial membrane. Based on the characteristics of oligoarginine, we attempted to facilitate the intestinal permeation of the peptide drug, leuprolide, using the function of oligoarginine. However, leuprolide permeation was not achieved when leuprolide was applied with oligoarginine to mucosal side of rat ileal sheets or when a leuprolide-oligoarginine conjugate was administered. These findings emphasize that any strategy using oligoarginine to improve intestinal drug permeation requires an intermolecular interaction, such as an electrostatic interaction, and a covalent linkage between the macromolecular drug and oligoarginine may hamper the ability of oligoarginine to enhance intestinal epithelial permeation of therapeutic peptides and proteins.

Correlation of in vitro and in vivo models for the oral absorption of peptide drugs

The aim of this study was to evaluate two in vitro models, Caco-2 monolayer and rat intestinal mucosa, regarding their linear correlation with in vivo bioavailability data of therapeutic peptide drugs after oral administration in rat and human. Furthermore the impact of molecular mass (Mm) of the according peptides on their permeability was evaluated. Transport experiments with commercially available water soluble peptide drugs were conducted using Caco-2 cell monolayer grown on transwell filter membranes and with freshly excised rat intestinal mucosa mounted in Using type chambers. Apparent permeability coefficients (P (app)) were calculated and compared with in vivo data derived from the literature. It was shown that, besides a few exceptions, the Mm of peptides linearly correlates with permeability across rat intestinal mucosa (R (2) = 0.86; y = -196.22x + 1354.24), with rat oral bioavailability (R (2) = 0.64; y = -401.90x + 1268.86) as well as with human oral bioavailability (R (2) = 0.91; y = -359.43x + 1103.83). Furthermore it was shown that P (app) values of investigated hydrophilic peptides across Caco-2 monolayer displayed lower permeability than across rat intestinal mucosa. A correlation between P (app) values across rat intestinal mucosa and in vivo oral bioavailability in human (R (2) = 0.98; y = 2.11x + 0.34) attests the rat in vitro model to be a very useful prediction model for human oral bioavailability of hydrophilic peptide drugs. Presented correlations encourage the use of the rat in vitro model for the prediction of human oral bioavailabilities of hydrophilic peptide drugs.