Enhancing the buccal permeability potential of model ARV drugs : permeability and histo-morphological evaluations.

Abstract

Buccal delivery of antiretroviral (ARV) drugs may overcome disadvantages such as low bioavailability due to extensive first pass effects and gastrointestinal degradation associated with the oral route. However, the small mucosal surface area and low membrane permeability are challenges to buccal drug permeation. Identification of new permeation enhancers as well as new permeation enhancing strategies have been shown to overcome these limitations, thereby delivering adequate amounts of drug through the buccal mucosa. Polymeric excipients with previously reported mucoadhesive and controlled release properties could also possess additional buccal permeation enhancing effects and may therefore serve as multifunctional excipients in a buccal drug delivery system. Therefore the aims of this study were: 1) to identify and compare the buccal permeability potential of tenofovir (TNF) and didanosine (ddI). 2) to identify the buccal permeation effects of potential multifunctional excipients ie. carboxymethylcellulose (CMC), sodium alginate (SA), polyacrylic acid (PAA) and polyethylene glycol (PEG) for TNF and ddI, and 3) to identify the buccal permeation potential of saquinavir (SQV) and assess the effect of high-energy ball milling on its permeability. All in vitro permeation studies across porcine buccal mucosa were performed using vertical Franz diffusion cells with TNF, ddI and SQV being quantified using UV spectrophotometry at 262 nm, 250 nm and 239 nm respectively. The histomorphological evaluations were undertaken by light microscopy (LM) and Transmission Electron Microscopy (TEM). Ball milling of SQV samples for 1, 3, 15 and 30 hours was performed in a high-energy planetary mill. The integrity of the buccal mucosa was assessed by transepithelial electrical resistance (TEER) measurements using a Millicell ERS meter connected to a pair of chopstick electrodes. Both TNF and ddI were able to permeate the buccal mucosa in a concentration-dependent manner. A higher permeability was observed for ddI (Flux = 181.62 ± 23.62 μg/cm2h) as compared to TNF (Flux = 102.10 ± 19.80 μg/cm2h). The permeation of these drugs in the absence of enhancers was attributed to passive diffusion via the paracellular route with transcellular route being an additional possibility for ddI. The addition of PAA, SA, CMC and PEG increased the permeability of TNF whilst only PEG was able to increase the permeability of ddI. The effect of these polymeric excipients appeared to be dependent on their ionic charges as well as that of the respective drugs. Permeability enhancement ratios for ddI and TNF were 1.63 and 1.74 respectively with PEG (0.5 %w/v) and CMC (0.5 %w/v) respectively. A maximum enhancement ratio of 2.13 for TNF was achieved with 4 %w/v PEG. Furthermore PEG was identified as the optimal permeation enhancer for TNF and ddI. Histological investigations revealed no significant loss in cellular integrity for mucosa treated with either TNF or ddI alone or when coupled with PEG as an enhancer. The differences in histomorphological changes in response to TNF and ddI alone could support the greater permeation observed with ddI. The histological findings proved useful in assessing the effects of drug and enhancer on mucosal integrity and provided insight into permeation pathways across the mucosa. Selective polymeric excipients therefore provide an effective means to increase the penetration of ddI and TNF. Their previously reported mucoadhesive and controlled release properties coupled with their permeation enhancing effects shown in this study highlight their potential use as multifunctional excipients for the design of buccal drug delivery systems. SQV, a candidate for buccal drug delivery, is limited by its poor solubility. Therefore, Aim 3 identified the effects of high energy ball milling on the buccal permeability of SQV and compared it’s enhancing effect to the conventional use of common chemical enhancers together with unmilled SQV i.e. ethylenediaminetetraacetic acid (EDTA), sodium lauryl sulphate (SLS), PEG and beta cyclodextrin (ß-cyclodextrin). Unmilled SQV was able to permeate through the buccal mucosa with a flux of 3.99 ± 0.11μg/cm2h. Ball milling of SQV at all the time periods led to an increase in its permeability with optimal enhancement obtained at 15 hours with an enhancement ratio of 2.61. The enhanced permeability of the milled SQV samples was attributed to a contribution of various factors such as solubility, particle size, surface area, crystallinity, morphology and the formation of solid dispersions. The chemical permeation enhancers were also able to increase the permeability of unmilled SQV across the buccal mucosa, with SLS achieving the greatest enhancement ratio of 1.75. However, ball milling of SQV without any chemical permeation enhancers displayed to a greater enhancement ratio (2.61) as compared to the best permeation enhancer SLS at 0.5 %w/v (1.75). Histological investigations revealed no significant loss in cellular integrity for mucosa treated with either unmilled or milled SQV samples. The presence of larger intercellular spaces in the treated tissue suggests that SQV also uses the paracellular route of transport in combination with the transcellular route across the mucosa. High energy ball milling of SQV is therefore an effective approach for increasing buccal permeability when formulating SQV for a buccal delivery system, as compared to incorporating common chemical enhancers studied at 0.5% w/v for this purpose. The findings in this study will therefore contribute to formulation optimization strategies for the development of novel buccal delivery systems for ARV drugs, thereby optimising treatment of patients with HIV and AIDS.