Effect of enzyme digestion on the permeability barrier in keratinizing and non-keratinizing epithelia

Full-Thickness Intraoral Mucosa Barrier Models for In Vitro Drug-Permeation Studies Using Microneedles

The use of permeation enhancers such as microneedles (MNs) to increase drug penetration across intraoral mucosa has increased in recent years. Permeation studies, commonly performed using vertical diffusion cells, are a well-established way to preview formulations and enhance their performance during the development stage. However, to our knowledge, the existing intraoral mucosa barrier models do not permit permeation using MN-pretreated mucosa due to their insufficient thickness. Therefore, the objective of this study was to develop a barrier model using thick palate tissues to perform in vitro permeation studies, with physical enhancement of the permeability of intraoral mucosa by pretreatment with MNs. The adapted Franz-type cells used in the permeation experiments were validated (cell dimensions and volume, sealing effectiveness, stirring and dissolution efficiency, temperature control, and establishment of uniaxial flux). Commercially available MNs were used in the palatal mucosa. Optical images of the mucosa were acquired to analyze the microperforations created. In vitro permeation studies were conducted with the MN-pretreated mucosa. This work presents a new in vitro method for the evaluation of MNs as permeation enhancers, with the aim of improving the absorption of drug formulations topically applied within the oral cavity.

Factors and strategies for improving buccal absorption of peptides

Peptides and polypeptides have important pharmacological properties but only a limited number (e.g. insulin, oxytocin, vasopressin) have been exploited as therapeutics because of problems related to their delivery. The buccal mucosa offers an alternative route to conventional, parenteral administration. Peptides are generally not well absorbed through mucosae because of their molecular size, hydrophilicity and the low permeability of the membrane. Peptide transport across buccal mucosa occurs via passive diffusion and is often accompanied by varying degrees of metabolism. This review describes various approaches to improve the buccal absorption of peptides including the use of penetration enhancers to increase membrane permeability and/or the addition of enzyme inhibitors to increase their stability. Other strategies including molecular modification with bioreversible chemical groups or specific formulations such as bioadhesive delivery systems are also discussed.

Transport of thyrotropin releasing hormone in rabbit buccal mucosa in vitro

The transport of thyrotropin releasing hormone (TRH) in rabbit buccal mucosa in vitro has been investigated with respect to (a) rate and type of metabolism of TRH on mucosal and serosal sides of buccal mucosa, (b) mechanism of TRH transport including charge effect on its permeability, and (c) pathway and rate-limiting regions of TRH movement. In addition, the integrity of excised buccal mucosa has been evaluated for purposes of in vitro solute diffusion experiments using tissue ATP level data, transmission electron microscopy, and TRH transport kinetic data. The results indicate that excised rabbit buccal mucosa can be used for TRH diffusion studies for approximately 6 hr. In addition, TRH apparently traverses buccal mucosa by simple diffusion with a steady-state permeability of about 10(-7) cm/sec, and this permeability is independent of pH. Moreover, the primary pathway appears to be via the intercellular space in the rate-limiting barrier, i.e., the upper 50 microns of the epithelium. Finally, TRH is degraded predominantly by deamidase activity, which is followed by, to a lesser degree, carboxypeptidase metabolism.

Drug delivery via the mucous membranes of the oral cavity

The delivery of drugs via the mucous membranes lining the oral cavity (i.e., sublingual and buccal), with consideration of both systemic delivery and local therapy, is reviewed in this paper. The structure and composition of the mucosae at different sites in the oral cavity, factors affecting mucosal permeability, penetration enhancement, selection of appropriate experimental systems for studying mucosal permeability, and formulation factors relevant to the design of systems for oral mucosal delivery are discussed. Sublingual delivery gives rapid absorption and good bioavailability for some small permeants, although this site is not well suited to sustained-delivery systems. The buccal mucosa, by comparison, is considerably less permeable, but is probably better suited to the development of sustained-delivery systems. For these reasons, the buccal mucosa may have potential for delivering some of the growing number of peptide drugs, particularly those of low molecular weight, high potency, and/or long biological half-life. Development of safe and effective penetration enhancers will further expand the utility of this route. Local delivery is a relatively poorly studied area; in general, it is governed by many of the same considerations that apply to systemic delivery.

The lipid composition of porcine epidermis and oral epithelium

The permeability barrier of mammalian skin has been analysed in terms of its lipid composition whereas that of the oral mucosa is uncharacterized. As there are differences in permeability between different regions of the oral mucosa, and between the oral mucosa and skin, there may be corresponding differences in the nature of the barrier layer. Lipids were identified by thin-layer chromatography after solvent extraction of separated epithelium; their localization was determined in histological sections using standard histochemical stains. Keratinized oral epithelium and epidermis showed a similar pattern of lipid distribution, the majority of neutral lipids being ceramides, localized around the cells of the stratum corneum. The non-keratinized epithelium contained few neutral lipids but polar lipids, particularly cholesterol sulphate and glucosylceramides, were clearly evident. Histochemical staining suggested that these were localized in intercellular regions throughout the epithelium. The differences in the types and distribution of lipids accord well with known permeability differences, which show that keratinized oral epithelium and epidermis have a similar impermeability to water and that non-keratinized regions have greater permeability.

Differences in human skin between the epidermal growth factor receptor distribution detected by EGF binding and monoclonal antibody recognition

Two methods have been used to examine epidermal growth factor (EGF) receptor distribution in human scalp and foreskin. The first employed [125I]EGF viable explants and autoradiography to determine the EGF binding pattern while the second used a monoclonal antibody to the human EGF receptor to map the distribution on frozen skin sections of an extracellular epitope on the EGF receptor. The [125I]EGF binding experiments showed accessible, unoccupied EGF receptors to be present on the epidermal basal cells (with reduced binding to spinous cells), the basal cells of the hair shaft and sebaceous gland, the eccrine sweat glands, capillary system, and the hair follicle outer root sheath, generally similar in pattern to that previously reported for full-thickness rat skin and human epidermis. The same areas also bound EGF-R1 but in addition the monoclonal antibody recognized a cone of melanin containing presumptive cortex cells, excluding the medulla, lying around and above the upper dermal papilla of anagen hair follicles, epithelial cells around the lower dermal papilla region, and in some tissue samples the cell margins of the viable differentiating layers of the epidermis. In a control study, to clarify whether EGF-R1 could recognize molecules unrelated to the EGF receptor, the EGF binding and EGF-R1 recognition profiles were compared on cultures of SVK14 cells, a SV40 transformed human keratinocyte cell line. EGF binding and EGF-R1 monoclonal antibody distribution on these cells was found to be similar, indicating that, at least for SVK14 cells, EGF-R1 binding provides a reliable marker for EGF binding. Explanations for the discrepancies between these two methods for determining EGF receptor distribution in human skin are discussed, including the possibility that latent EGF receptors, unable to bind [125I]EGF, may be present in some differentiating epithelial compartments.