Development, validation and standardization of oromucosal ex-vivo permeation studies for implementation in quality-controlled environments

Oral Mucosa Models to Evaluate Drug Permeability

Due to its numerous advantages, such as excellent drug accessibility, rapid absorption, and bypass of first-pass metabolism, the route of drug administration that involves crossing the oral mucosa is highly favored. As a result, there is significant interest in investigating the permeability of drugs through this region. The purpose of this review is to describe the various ex vivo and in vitro models used to study the permeability of conveyed and non-conveyed drugs through the oral mucosa, with a focus on the most effective models. Currently, there is a growing need for standardized models of this mucosa that can be used for developing new drug delivery systems. Oral Mucosa Equivalents (OMEs) may provide a promising future perspective as they are capable of overcoming limitations present in many existing models.

A method for evaluating drug penetration and absorption through isolated buccal mucosa with highly accuracy and reproducibility

The purpose of the project is to establish a standardized operation method of the in vitro permeability model to maximize mucosal integrity and viability. The model drug lidocaine permeability, 20 kDa fluorescein isothiocyanate-dextran, H&E staining, and mucosal viability were used as evaluation indicators. Firstly, the buccal mucosae of rats, rabbits, dogs, porcine, and humans were analyzed by H&E staining and morphometric analysis to compare the differences. Then, we studied a series of operation methods of isolated mucosa. The buccal mucosae were found to retain their integrity in Kreb's bicarbonate ringer solution at 4 °C for 36 h. Under the long-term storage method with program cooling, freezing at -80 °C, thawing at 37 °C, and using cryoprotectants of 20% glycerol and 20% trehalose, mucosal integrity and biological viability can be maintained for 21 days. The heat separation method was used to prepare a permeability model with a mucosal thickness of 500 μm, which was considered to be the optimal operation. In summary, this study provided an experimental basis for the selection and operation of in vitro penetration models, standardized the research process of isolated mucosa, and improved the accuracy of permeability studies.

Formulation Development of Sublingual Cyclobenzaprine Tablets Empowered by Standardized and Physiologically Relevant Ex Vivo Permeation Studies

Suitable ex vivo models are required as predictive tools of oromucosal permeability between in vitro characterizations and in vivo studies in order to support the development of novel intraoral formulations. To counter a lack of clinical relevance and observed method heterogenicity, a standardized, controlled and physiologically relevant ex vivo permeation model was established. This model combined the Kerski diffusion cell, process automation, novel assays for tissue integrity and viability, and sensitive LC-MS/MS analysis. The study aimed to assess the effectiveness of the permeation model in the sublingual formulation development of cyclobenzaprine, a promising agent for the treatment of psychological disorders. A 4.68-fold enhancement was achieved through permeation model-led focused formulation development. Here, findings from the preformulation with regard to pH and microenvironment-modulating excipients proved supportive. Moreover, monitoring of drug metabolism during transmucosal permeation was incorporated into the model. In addition, it was feasible to assess the impact of dosage form alterations under stress conditions, with the detection of a 33.85% lower permeation due to salt disproportionation. Integrating the coherent processes of disintegration, dissolution, permeation, and metabolization within a physiological study design, the model enabled successful formulation development for cyclobenzaprine sublingual tablets and targeted development of patient-oriented drugs for the oral cavity.

Development and evaluation of a composite dosage form containing desmopressin acetate for buccal administration

Desmopressin acetate (DDAVP) is an oligopeptide indicated for the treatment of primary nocturnal enuresis, for example. The poor oral bioavailability of DDAVP accelerated a shift to alternative routes of administration like nasal and oromucosal, whereby nasal administration results in high fluctuations increasing the risk of undesirable side effects. Aim of the study was to use a new composite dosage form (solid matrix attached to a bilayer mucoadhesive film) to make DDAVP available via oromucosal route, reducing the risk of undesirable side effects through precise dosing. DDAVP was incorporated into a solid matrix in the form of a minitablet, and both direct tableting (AV > 30) and granulation followed by tableting (AV = 17.86) were compared. Minitablets with content uniformity could only be obtained by granulation and loss supplementation (AV = 11.27) with immediate drug release (>80% after 7–8 min) and rapid disintegration (<49 s). Permeation studies were performed with a clinically relevant dose (200 μg) in a time interval of up to one hour, resulting in apparent permeation coefficients of 4.90 × 10⁻⁶ cm/s (minitablet) and 2.04 × 10⁻⁶ cm/s (composite). Comparable fluctuations showed no inferiority of composite and minitablet regarding dosing accuracy. Thus, a step towards controlled and dose-accurate transmucosal delivery of systemically active DDAVP could be achieved.

Resistivity Technique for the Evaluation of the Integrity of Buccal and Esophageal Epithelium Mucosa for In Vitro Permeation Studies: Swine Buccal and Esophageal Mucosa Barrier Models

Permeation assays are important for the development of topical formulations applied on buccal mucosa. Swine buccal and esophageal epithelia are usually used as barriers for these assays, while frozen epithelia have been used to optimize the experimental setup. However, there is no consensus on these methods. In transdermal studies, barrier integrity has been evaluated by measuring electrical resistance (ER) across the skin, which has been demonstrated to be a simple, fast, safe, and cost-effective method. Therefore, the aims here were to investigate whether ER might also be an effective method to evaluate buccal and esophageal epithelium mucosa integrity for in vitro permeation studies, and to establish a cut-off ER value for each epithelium mucosa model. We further investigated whether buccal epithelium could be substituted by esophageal epithelium in transbuccal permeation studies, and whether their permeability and integrity were affected by freezing at -20 °C for 3 weeks. Fresh and frozen swine buccal and esophageal epithelia were mounted in Franz diffusion cells and were then submitted to ER measurement. Permeation assays were performed using lidocaine hydrochloride as a hydrophilic drug model. ER was shown to be a reliable method for evaluating esophageal and buccal epithelia. The esophageal epithelium presented higher permeability compared to the buccal epithelium. For both epithelia, freezing and storage led to decreased electrical resistivity and increased permeability. We conclude that ER may be safely used to confirm tissue integrity when it is equal to or above 3 kΩ for fresh esophageal mucosa, but not for buccal epithelium mucosa. However, the use of esophageal epithelium in in vitro transmucosal studies could overestimate the absorption of hydrophilic drugs. In addition, fresh samples are recommended for these experiments, especially when hydrophilic drugs are involved.