A Randomly Coiled, High-Molecular-Weight Polypeptide Exhibits Increased Paracellular Diffusion in Vitro and in Situ Relative to the Highly Ordered ?-Helix Conformer

Methyl-β-cyclodextrin quaternary ammonium chitosan conjugate: nanoparticles vs macromolecular soluble complex

Purpose

The present study aimed to compare a novel cyclodextrin–polymer–drug complex in solution with a dispersed supramolecular nanosize system, made of the same complex, for ability to carry dexamethasone (DEX) across excised rat intestine.

Results

Methyl-β-cyclodextrin-quaternary ammonium chitosan conjugate (QA-Ch-MCD) was obtained by covalent grafting through a 10-atom spacer. The conjugate was characterized by ¹H-NMR, resulting in 24.4% w/w of MCD content. Phase solubility profile analysis of the QA-Ch-MCD/DEX complex yielded an association constant of 14037 M⁻¹, vs 4428 M⁻¹ for the plain MCD/DEX complex. Nanoparticle (NP) dispersions resulted from ionotropic gelation of the QA-Ch-MCD/DEX complex by sodium tripolyphosphate, leading to 9.9%±1.4% drug loading efficiency. The mean diameter and zeta potential for NP were 299±32 nm (polydispersity index [PI] 0.049) and 11.5±1.1 mV, respectively. Those for QA-Ch-MCD/DEX were 2.7±0.4 nm (PI 0.048) and 6.7±0.6 mV. QA-Ch-MCD/DEX solutions and corresponding NP dispersions were compared in vitro for water-assisted transport through mucus, DEX permeation through excised rat intestine, and ex vivo mucoadhesivity. The complex showed higher mucoadhesion and lower transport rate through mucus; also, it provided faster drug permeation across excised rat intestine.

Conclusion

Carrier adhesion to mucus surface has played a most important role in favoring transepithelial permeation. Then, within the concerns of the present study, the use of NP seems not to provide any determinant advantage over using the simpler macromolecular complex.

Systematic studies on the paracellular permeation of model permeants and oligonucleotides in the rat small intestine with chenodeoxycholate as enhancer

The objective of this study was to mechanistically and quantitatively analyze chenodeoxycholate-enhanced paracellular transport of polar permeants and oligonucleotides in the rat jejunum and ileum. Micellar chenodeoxycholate solutions were used to perturbate the tight junctions. Supporting studies included assessment of the aqueous boundary layer (ABL) with ABL-controlled permeants, measurements of the permeability coefficients and fluxes of the bile acid in dilute and micellar concentrations, and determinations of pore sizes with paracellular probes (urea, mannitol, and raffinose). The paracellular permeability coefficients, P(para), of two model oligonucleotides (ON3 and ON6; 12- and 24-mers with 11 and 23 negative charges, respectively) were determined. The enhanced permeabilities paralleled the increased fluxes of micellar bile salt solutions into mesenteric blood and the opening of the tight junctions as compared to controls. As the pore radius increased from 0.7 nm to a maximum of 2.4 nm in the jejunum and ileum, the absorption of ON3 was enhanced up to sixfold in the jejunum and about 14-fold in the ileum with P(para) values between 0.5 x 10(-6) and 6 x 10(-6) cm/s, whereas ON6 was enhanced up to twofold in the jejunum and fivefold in the ileum with permeabilities between 0.3 x 10(-6) and 2 x 10(-6) cm/s.

Effect of structural and conformation modifications, including backbone cyclization, of hydrophilic hexapeptides on their intestinal permeability and enzymatic stability

A library of 18 hexapeptide analogs was synthesized, including sub-libraries of N- or C-methylation of the parent hexapeptide Phe-Gly-Gly-Gly-Gly-Phe, as well as backbone cyclized analogs of each linear analog with various ring sizes. N- or C-methylation as well as cyclization (but not backbone cyclization) have been suggested to improve intestinal permeability and metabolic stability of peptides in general. Here we aimed to assess their applicability to hydrophilic peptides. The intestinal permeability (Papp) of the 18-peptide library was in the range of 0.2-6.8 x 10-6 cm/sec. Based on several tests, we concluded that the absorption mechanism of all tested analogs is paracellular, regardless of the structural or conformational modifications. In all cases, backbone cyclization increased Papp (5-fold) in comparison to the linear analogs due to the smaller 3D size and also dramatically decreased peptide proteolysis by brush border enzymes. N- or C-methylation did not enhance the permeability of the linear analogs in this series.

Diffusion coefficient and the secondary structure of poly-L-glutamic acid in aqueous solution

The diffusion coefficients (D) of poly-L-glutamic acid (PLG) at various pHs are investigated by the laser-induced transient-grating method with a new photoreactive probe molecule. The pH dependence of D is compared with that of the helical content of PLG measured by circular dichroism. It is found that the pH dependences of both quantities are very similar. Since the frictions of the translational diffusion of charged and protonated carboxyl groups are found to be similar each other, it is concluded that the conformation of the main polymer chain is the main factor in determining the diffusion process; in other words, the alpha-helix conformation makes the molecular diffusion faster. This result indicates that the conformational change of a protein can be detected by monitoring the diffusion coefficient.

Interplay of Secondary Structure and Charge on the Diffusion of a Polypeptide through Negatively Charged Aqueous Pores

PURPOSE

This study was conducted to investigate the interplay of secondary structure and charge of a polypeptide on its permeability through negatively charged pores of synthetic porous membranes and Caco-2 cell monolayers.

MATERIALS AND METHODS

Poly(D-glutamic acid) [Poly(D-Glu)] was used as a model polypeptide. Transport studies were conducted at 37 degrees C through both track-etched polycarbonate membranes (using side-by-side diffusion cells) and Caco-2 cell monolayers. Apparent permeability coefficients and diffusion coefficients were calculated.

RESULTS

When diffusion was unhindered, poly(D-Glu) appeared to be transported at the same rate regardless of whether it existed in the random coil or the alpha-helix secondary structure. When moderately hindered diffusion was evaluated, poly(D-Glu) with partial alpha-helix secondary structure, exhibited significantly greater transport than when the polypeptide predominantly existed as the highly negatively charged random coil. This trend was reversed when the diffusion was severely hindered by the tight junctions of the Caco-2 cell monolayers.

CONCLUSIONS

Neither charge, nor secondary structure, played a significant role in the unhindered diffusion of poly(D-Glu). When the molecules were moderately hindered, polypeptide/membrane charge interactions significantly influenced the rate of aqueous diffusion. As the overall molecular dimensions of the polypeptide approached the pore size, the inherent molecular flexibility of the random coil secondary structure overcame the effect of charge repulsion.

Therapeutic utility of a novel tight junction modulating peptide for enhancing intranasal drug delivery

Previously, a novel tight junction modulating (TJM) peptide was described affording a transient, reversible lowering of transepithelial electrical resistance (TER) in an in vitro model of nasal epithelial tissue. In the current report, this peptide has been further evaluated for utility as an excipient in transepithelial drug formulations. Chemical stability was optimal at neutral to acidic pH when stored at or below room temperature, conditions relevant to therapeutic formulations. The TJM peptide was tested in the in vitro tissue model for potential to enhance permeation of a low-molecular-weight (LMW) drug, namely the acetylcholinesterase inhibitor galantamine, as well as three peptides, salmon calcitonin, parathyroid hormone 1-34 (PTH(1-34)), and peptide YY 3-36 (PYY(3-36)). In all cases, the TJM peptide afforded a dramatic improvement in drug permeation across epithelial tissue. In addition, a formulation containing PYY(3-36) and TJM peptide was dosed intranasally in rabbits, resulting in a dramatic increase in bioavailability. The TJM peptide was as or more effective in enhancing PYY(3-36) permeation in vivo at a 1000-fold lower molar concentration compared to using LMW enhancers. Based on these in vitro and in vivo data, the novel TJM peptide represents a promising advancement in intranasal formulation development.

Current strategies used to enhance the paracellular transport of therapeutic polypeptides across the intestinal epithelium

The intent of this paper is to update the reader on various strategies which have been utilized to increase the paracellular permeability of protein and polypeptide drugs across the intestinal epithelium. Structural features of protein and polypeptide drugs, together with the natural anatomical and physiological features of the gastrointestinal (GI) tract, have made oral delivery of this class of compounds extremely challenging. Interest in the paracellular route for the transport of therapeutic proteins and polypeptides following oral administration has recently intensified and continues to be explored. The assumption that molecules with a large molecular weight are not able to diffuse through the tight junctions of the intestinal membrane has been challenged by current research, along with an increased understanding of tight junction physiology.