Buccal mucosa permeability

Keywords Buccal mucosa Permeability barrier Passive diffusion Excised tissue Nonkeratinized epithelium Diffusion chamber... 

Buccal administration offers certain unique advantages for drugs which cannot be easily or efficiently administered by the oral or intravenous route. However, transbuccal drug delivery has received relatively little attention and few well-controlled studies of buccal mucosa permeability have been conducted. 

A recent study, however, has shown that aminopeptidase activity is present on the surface of porcine buccal mucosa, and that various aminopeptidase inhibitors, including amastatin and sodium deoxycholate, reduce the mucosal surface degradation of the aminopeptidase substrate, leucine-enkephalin [149], Since the peptidases are present on the surface of the buccal mucosa, they may act as a significant barrier to the permeability of compounds which are substrates for the enzyme. In addition to proteolytic enzymes, there exist some esterases, oxidases, and reductases originating from buccal epithelial cells, as well as phosphatases and carbohydrases present in saliva [154], all of which may potentially be involved in the metabolism of topically applied compounds. 

Methods Employed to Assess the Permeability of the Buccal Mucosa... 

One of the most common in vivo methods used to assess the permeability of the buccal mucosa is the buccal absorption test of Beckett and Triggs [13]. In this test, a known volume of a drug solution is introduced into the oral cavity of a subject, who swirls it around for a specified period of time and then expels it. The subject then rinses his or her mouth with an aliquot of distilled water or buffer solution, and the expelled drug solution and rinse are combined and analyzed for drug content. The difference between the initial and final drug concentration in the solution is assumed to be the amount of drug taken up into the oral mucosa. 

Table 4.1 Epithelial thickness and permeability coefficient (P) for tritiated water through the buccal mucosa of different species together with epithelial thickness.

While Franz-type diffusion cells are commonly used to assess in vitro penetration of compounds across the skin, they have also been used for the assessment of compound permeability across the buccal mucosa [19, 71, 104], In this system, buccal mucosa is sandwiched between two chambers, and compound solution is added to the donor chamber with compound-free buffer in the receptor chamber. The receptor chamber is then periodically sampled to assess the amount of compound that has permeated the tissue over time. 

The appropriate calculations for determining the flux and permeability coefficient across the buccal mucosa using this approach are detailed in the appendix containing the detailed method used in our laboratory for assessing buccal permeation. 

Because of the possible effects of active and carrier-mediated processes and metabolic biotransformation, the issue of tissue viability is important for in vitro buccal mucosal experiments. The barrier nature of the buccal mucosa resides in the upper layers of the epithelium, where unlike in the stratum corneum, the cells contain a variety of functional organelles [119, 122, 125, 150], and so tissue viability may be an important component of the barrier function of the tissue. Various methods have been employed to assess the viability of excised buccal mucosa, including measurement of biochemical markers, microscopic methods, and linearity of transport data [42], While biochemical methods, including measurement of adenosine 5 -triphosphate (ATP) levels and utilization of glucose, provide information on the metabolic activity of the tissue, this does not necessarily relate to the barrier function of the tissue. In excised rabbit buccal mucosa, levels of ATP were measured and found to decline by 40% in 6 h, and this correlated well with transmission electron microscopic evaluation of the tissue (intact superficial cells) [32], In addition, the permeability of a model peptide was unaltered up to 6 h postmortem, but at 8 h, a significant change in permeability was observed [32], These investigators therefore claimed that excised rabbit buccal mucosa could be used for diffusion studies for 6 h. 

Recently it has been claimed that the tissue can be considered viable if the drug permeability does not change over the course of the experiment, and thus the actual permeability experiments themselves may provide insight into the viability of the tissue [109, 157], This method was employed in permeation experiments using porcine buccal mucosa, where the permeability of compounds was assessed in two consecutive permeability experiments to ensure the nature of the barrier was not compromised [111, 112]. While this demonstrates that the barrier nature of the tissue was unaltered between the permeation experiments, the tissue may have already undergone tissue death in the time between the excision and the commencement of the initial permeation experiment, and thus the permeability rate obtained in vitro may not be representative of the in vivo situation. Therefore, more studies assessing the dependence of the barrier nature of the buccal mucosa on tissue viability are... 

While there are limitations associated with the use of an in vitro permeability model for assessing the transport of compounds across the buccal mucosa, it can still be useful in assessing and comparing the permeability of compounds under different conditions, such as pH, temperature, and osmolarity, which provide valuable information on the mechanisms involved in drug transport. Additionally, the preliminary effects of potential chemical penetration enhancers or formulation excipients may be assessed, and these may provide a substantial rationale for subsequently assessing the effect of these agents in man. 

The use of buccal cell cultures for assessing the permeability of the buccal mucosa has attracted recent attention (see Chap. 7 for a more extensive summary). In order to culture buccal epithelial cells, the cells must be harvested from an appropriate source and cultured under specific conditions using an appropriate growth medium, temperature, and humidity [46], Cell cultures have been successfully grown from hamster cheek pouch. These cultured cells, however, did not differentiate to form a complete keratinized surface as seen in the normal hamster cheek pouch, and they consequently displayed a greater permeability to compounds when compared with keratinized hamster cheek pouch mucosa [134], Therefore, the cultured hamster cheek cells more closely mimicked the human buccal mucosa due to their lack of keratinization, and so this may be an appropriate model for predicting permeability through the human buccal mucosa. 

TR146 cell culture. With the development of tissue culture techniques, it is anticipated that various cell culture models may be developed with similar morphological and barrier properties to normal intact buccal mucosa. Such models may be very useful in assessing the buccal permeability and metabolism of many compounds. 

The buccal mucosa does serve as an alternative route for administering compounds systematically however, to ensure particular compounds are candidates for delivery across this biological tissue, preclinical screening is essential. While in vivo human permeability studies are ideal, due to their costs and associated issues, it is necessary to perform such screening in vitro. Assessment of compound permeability across porcine buccal mucosa has been widely used and can provide the preclinical biopharmaceutical scientist with much information relating to permeability, routes of transport, and effects of various chemical penetration enhancers. 

Jasti BR, Zhou S, Mehta RC, Li X (2000) Permeability of antisense oligonucleotides through porcine buccal mucosa. Int J Pharm 208 35-39... 

Nicolazzo JA, Reed BL, Finnin BC (2003) The effect of various in vitro conditions on the permeability characteristics of the buccal mucosa. J Pharm Sci 92 2399-2410... 

Nielsen HM, Rassing MR (2000b) TR146 cells grown on filters as a model of human buccal epithelium IV. Permeability of water, mannitol, testosterone and P-adrenoceptor antagonists. Comparison to human, monkey and porcine buccal mucosa. Int J Pharm 194 155-167... 

Nielsen HM, Rassing MR (2002) Nicotine permeability across the buccal TR146 cell culture model and porcine buccal mucosa in vitro Effect of pH and concentration. Eur J Pharm Sci 16 151-157... 

Senel S, Hoogstraate AJ, Spies F, Verhoef JC, Bos-van Geest A, Junginger HE, Bodde HE (1994) Enhancement of in vitro permeability of porcine buccal mucosa by bile salts Kinetic and histological studies. J Control Release 32 45-56... 

Keywords Buccal Mucosa In vitro Absorption Permeability Culture... 

As noted earlier, the choice of species for experimentation is critical due to anatomical differences and it may also reflect species, as well as individual, differences in the expression/activity of transporter and metabolic proteins [43, 44], Since the fraction absorbed across buccal mucosa in vivo is not established for many compounds in different species including humans, the potential existence of a correlation between in vitro permeability coefficients in freshly isolated pig, dog, monkey, and human buccal mucosa was investigated (Figure 7.3). The correlation coefficient obtained for porcine and canine tissue was poor (0.65 and 0.67, respectively, at the 95% confidence level). Results for relatively high permeability compounds in porcine tissue resemble those previously reported where permeability coefficients were by an order of magnitude... 

Van Eyk AD, and Van der Biijl P (2004) Comparative permeability of various chemical markers through human vaginal and buccal mucosa as well as porcine buccal and mouth floor mucosa. Arch. Oral Biol. 49 387-392. 

Agarwal V, Habib W, and Moe D (2005) Effervescent mediated transmucosal enhancement of fentanyl permeability Comparison across ex vivo porcine buccal mucosa and in vitro cultured human buccal epithelium test. American Association Pharm. Sci. Abs. T2126. 

---