Permeation of unfolded basic fibroblast growth factor (bFGF) across rabbit buccal mucosa--does unfolding of bFGF enhance transport?

Enhancing the buccal mucosal delivery of peptide and protein therapeutics

With continuing advances in biotechnology and genetic engineering, there has been a dramatic increase in the availability of new biomacromolecules, such as peptides and proteins that have the potential to ameliorate the symptoms of many poorly-treated diseases. Although most of these macromolecular therapeutics exhibit high potency, their large molecular mass, susceptibility to enzymatic degradation, immunogenicity and tendency to undergo aggregation, adsorption, and denaturation have limited their ability to be administered via the traditional oral route. As a result, alternative noninvasive routes have been investigated for the systemic delivery of these macromolecules, one of which is the buccal mucosa. The buccal mucosa offers a number of advantages over the oral route, making it attractive for the delivery of peptides and proteins. However, the buccal mucosa still exhibits some permeability-limiting properties, and therefore various methods have been explored to enhance the delivery of macromolecules via this route, including the use of chemical penetration enhancers, physical methods, particulate systems and mucoadhesive formulations. The incorporation of anti-aggregating agents in buccal formulations also appears to show promise in other mucosal delivery systems, but has not yet been considered for buccal mucosal drug delivery. This review provides an update on recent approaches that have shown promise in enhancing the buccal mucosal transport of macromolecules, with a major focus on proteins and peptides.

Comparative permeability of various chemical markers through human vaginal and buccal mucosa as well as porcine buccal and mouth floor mucosa

A number of drugs undergo extensive first-pass metabolism after oral administration, necessitating large doses for effective therapeutic responses in the body. Buccal administration of drugs is becoming more popular because the drugs diffuse into the systemic circulation directly, circumventing the first-pass metabolism. Lower concentrations thus need to be administered and side effects may be minimized. In this study, one of the classic models for human buccal permeability, i.e. the porcine buccal mucosal model, is compared with the more recent human vaginal model and both these are in turn further compared to porcine mouth floor mucosa. To determine the permeability of the different markers (arecoline, 17beta-estradiol, water and vasopressin), a continuous flow-through perfusion system was used (20 degrees C, 24h). Mean steady state flux values were compared statistically using a t-test at a significance level of 5%. Porcine buccal mucosa showed a consistently lower permeability towards all the markers than the other mucosae tested. Porcine mouth floor mucosa was found to be more permeable than porcine buccal mucosa. From these studies we concluded that human vaginal and porcine mouth floor mucosae were superior models for human buccal mucosa than porcine buccal mucosa, using in vitro permeability studies with various chemical markers.

Shape imposed by secondary structure of a polypeptide affects its free diffusion through liquid-filled pores

The purpose of the present study was to investigate the effect of secondary structure of three model polypeptides on their apparent permeability (P(app)) across a synthetic, microporous membrane. Poly-L-lysine (PL), poly-L-glutamate (PGlu), and poly-L-lysine-L-phenylalanine (1:1) (PLP) were selected because a solution environment in which their predominant secondary structure is random coil (RC), alpha-helix, and beta-sheet, respectively, is easily achieved. The conformation of each polypeptide was verified by circular dichroism (CD). Diffusion studies were conducted under sink conditions at 25 degrees C across a microporous polyester membrane using a donor concentration of 0.02 mM for each model polypeptide. NMR was utilized to obtain a second estimation of the diffusion coefficient for each polypeptide. The equivalent hydrodynamic radii (R(e)) of the three model polypeptides were calculated using the values of the diffusion coefficient obtained by both NMR and the classic in vitro diffusion studies. The viscosity of each polypeptide solution was also determined to investigate the effect of viscosity on the aqueous diffusion coefficient. Statistical analysis demonstrated a significant (P < 0.05) difference in both P(app) and the aqueous diffusion coefficient (D(aq)), as well as the calculated R(e) values, between all three model polypeptides and there was no significant (P > 0.05) difference in the viscosity of the polypeptide solutions. Values of D(aq) and R(e) calculated from the diffusion studies were in relatively close agreement to those obtained using NMR. The logarithm of P(app) was highly correlated (r = -0.961) with the values of R(e) calculated from NMR (R(e (NMR))) rather than the mw of the polypeptides (r = 0.681). Values of the Perrin or shape factor which deviate substantially from unity are suggestive of a non-spherical or ellipsoid shape and were 1.22 +/- 0.20, 1.55 +/- 0.11, and 2.38 +/- 0.20 for PGlu, PL, and PLP, respectively. In conclusion, the observed difference in the membrane transport/diffusion of the three model polypeptides is suggested to be due to the shape associated with the secondary structure of each macromolecule, rather than the polypeptide's mw or the viscosity of the dilute polypeptide solution.

Peptidase activity on the surface of the porcine buccal mucosa

Peptide drugs in buccal bioadhesive delivery systems are exposed to the surface of the buccal mucosa at high concentrations over long periods of time. The peptidase activity on the surface of the buccal mucosa has not been evaluated as a barrier to peptide buccal delivery. The in vitro stability of various synthetic substrates on the surface of intact porcine buccal mucosa was determined. No carboxypeptidase or dipeptidyl peptidase IV activity was detected on the buccal mucosa, while aminopeptidase N activity was detected using Leu-p-nitroanilide. No endopeptidase activity was observed towards the peptide substrates. Insulin and insulin B-chain were intact at the 2 h time point at 37 degrees C, while the percent of parent Leu-enkephalin remaining was 18+/-9 (mean+/-S.D., n=9). In the presence of aminopeptidase inhibitors, amastatin, sodium deoxycholate and EDTA, the degradation of Leu-enkephalin was dramatically reduced. This work suggests that the buccal route maybe advantageous for the delivery of peptides that are susceptible to such activities. The inclusion of aminopeptidase inhibitors in buccal bioadhesive delivery systems could improve buccal bioavailability of Leu-enkephalin. We suggest that compared with the existing in vitro metabolism methods, the analysis of peptide or protein metabolism on intact buccal mucosa could better predict the degradation of the drug as it crosses the tissue.

Transcellular and lipophilic complex-enhanced intestinal absorption of human growth hormone

PURPOSE

To evaluate the transcellular mechanism of novel enhancers absorption enhancement of human growth hormone (hGH), by examining the involvement of a P-glycoprotein-like efflux system, changes in membrane fluidity, and membrane damage.

METHODS

Caco-2 cell monolayers were grown on Snapwell filter supports and placed in a side-by-side diffusion apparatus. Transport in both the apical to basolateral (AP to BL) and basolateral to apical (BL to AP) direction was measured at different temperatures and in the presence of potential inhibitors. Fluorescence anisotropy measurement was used to measure membrane fluidity. The fluorescence anisotropy of DPH- and TMA-DPH-labeled cell suspensions was measured at room temperature. LDH (a measure of cytosolic lactate dehydrogenase) leakage assay was used to evaluate cytotoxicity.

RESULTS

The bi-directional transepithelial fluxes of hGH in the presence of these novel enhancers across Caco-2 cells showed marked asymmetry. Average permeability coefficient values obtained in the apical to basolateral (AP to BL) direction were lower than those of the reverse (BL to AP) direction. On the other hand, the fluxes for hGH alone were symmetric. When P-gp-like efflux inhibitors were included in the transport medium, the permeability coefficient value of BL to AP direction was significantly decreased while the transport was increased in the reverse direction in the presence of novel enhancers. In addition, lowering the temperature to 25 degrees C completely eliminated the asymmetry of hGH transport in the presence of novel enhancers. It was also shown by fluorescence anisotropy that these novel enhancers alone only slightly increased membrane fluidity. On the other hand, upon addition of hGH to the novel enhancers, the cell membrane showed a dramatic change as compared to treatment with novel enhancers alone. The results from the LDH assay showed that the novel enhancers and/or hGH did not cause cell damage, at least up to 1 hour, and the damage seen at the 2 hour point is also much lower than other known enhancers.

CONCLUSIONS

This study shows that human growth hormone alone cannot be transported across Caco-2 cells, except in small quantities, by passive diffusion, but in the presence of novel enhancers, human growth hormone permeation is substantial. In addition, the asymmetry of transport of the complexed hGH appears to be due to a P-gp-like efflux system. Assuming that the present substrate specificity of the P-gp-like efflux system shows the same preference for hydrophobic molecules as p-gp, the present work also indirectly shows that human growth hormone has become more lipophilic in the presence of these novel enhancers. Furthermore, membrane fluidity data also supports the premise that these novel enhancers interact and stabilize hGH, to make them more hydrophobic and easier to be transported through cell membranes.

Heparan sulfate mediates bFGF transport through basement membrane by diffusion with rapid reversible binding

Basic fibroblast growth factor (bFGF) is a pluripotent cytokine with a wide range of target cells. Heparan sulfate binds bFGF, and this interaction has been demonstrated to protect bFGF against physical denaturation and protease degradation. The high concentrations of heparan sulfate in basement membranes have implicated these matrices as storage sites for bFGF in vivo. However, the mechanisms by which basement membranes modulate bFGF storage and release is unknown. To gain insight into these mechanisms, we have developed experimental and mathematical models of extracellular growth factor transport through basement membrane. Intact Descemet's membranes isolated from bovine corneas were mounted within customized diffusion cells and growth factor transport was measured under a variety of conditions that decoupled the diffusion process from the heparan sulfate binding phenomenon. Transport experiments were conducted with bFGF and interleukin 1beta. In addition, bFGF-heparan sulfate binding was disrupted in diffusion studies with high ionic strength buffer and buffers containing protamine sulfate. Transport of bFGF was enhanced dramatically when heparan sulfate binding was inhibited. This process was modeled as a problem of diffusion with fast reversible binding. Experimental parameters were incorporated into a mathematical model and independent simulations were run that showed that the experimental data were accurately predicted by the mathematical model. Thus, this study indicated that basement membranes function as dynamic regulators of growth factor transport, allowing for rapid response to changing environmental conditions. The fundamental principles controlling bFGF transport through basement membrane that have been identified here might have applications in understanding how growth factor distribution is regulated throughout an organism during development and in the adult state.

Evaluation of a mucoadhesive buccal patch for delivery of peptides: in vitro screening of bioadhesion

We have assessed the bioadhesive properties of several different mucoadhesive buccal patches. The patches consisted of custom coformulations of silicone polymers and Carbopol 974P. The contact angle of water was measured for each of the test formulations, using an ophthalmic shadow scope. The corresponding work of adhesion between the water and the patches (W1), and between the patches and freshly-excised rabbit buccal mucosa (W2) was then calculated, using a modification of Dupre's equation. The bioadhesive strength between the patches and excised rabbit buccal mucosa was also assessed. The results of the contact-angle measurements indicated that the contact angle decreased with an increase in the amount of Carbopol in the formulation. Additionally, the calculated values of both W1 and W2 increased with an increase in the amount of Carbopol in the buccal-patch formulations. A correlation (r not equal to 0.9808) was found between the measured contact angle and the calculated values for W2. The direct measurement of the force required to separate a buccal patch from excised rabbit buccal mucosa with the INSTRON demonstrated that the adhesive strength increased with an increase in the amount of Carbopol. This preliminary study has shown that the measurement of contact angles alone may provide a useful technique for estimating the work of adhesion, and may serve as a convenient and rapid screening procedure to identify potential mucoadhesive buccal-patch formulations.