Mucoadhesive tacrolimus formulations for buccal application: a formulation and drug delivery feasibility study for oral lichen planus

Abstract

Tacrolimus is a potent calcineurin inhibitor suitable for the local treatment of oral mucosal immune-mediated diseases; however, no buccal formulation is currently available. The main objective of this work was to develop a mucoadhesive, semi-solid formulation containing tacrolimus that has adequate mucoadhesion salivation, and effective mucosal penetration with low systemic availability. A series of tacrolimus containing semisolid formulations based on chitosan, carbomer and hypromellose (HPMC) as mucoadhesive polymers were developed. Their rheologic behavior, pH value, macroscopic and microscopic properties as well as mucoadhesion and mucosal penetration were investigated.

Lactate buffer was successfully used to stabilize chitosan against hydrolysis. It was shown that diols of different chain length, like 1,2-propanediol 1,2-pentanediol and 1,2-hexanediol improved mucosal penetration but reduced mucoadhesion due to gel matrix weakening. After 60 min in the mucoadhesion cell, mucosal tacrolimus amounts versus salivary wash-off for the diols were 0.78 ± 0.09 µg/cm² versus 0.47 ± 0.20% for 20% 1,2-propanediol, 1.99 ± 0.23 µg/cm² versus 14.26 ± 3.01% for 40% 1,2-propanediol, 3.56 ± 0.29 µg/cm² versus 25.04 ± 1.68% for 20% 1,2-pentanediol, and 3.66 ± 0.25 µg/cm² versus 28.13 ± 4.00% for 20% 1,2-hexanediol. A series of tacrolimus formulations with different degrees of mucoadhesion, salivary retention and penetration characteristics was successfully developed.

Introduction

Oral immunogenic diseases such as systemic lupus erythematosus, Sjögren’s syndrome, Behçet syndrome, pemphigus vulgaris or oral lichen planus often require long-term immunosuppressant treatment in the oral cavity (Lourenço et al. Citation2007; Margaix-Muñoz et al. Citation2009; Cizenski et al. Citation2017; Greco et al. Citation2018; Husein-ElAhmed et al. Citation2019). While systemic corticosteroids or immunosuppressants are effective, systemic therapy is associated with adverse effects like osteoporosis, hypertension, nephrotoxicity, hepatotoxicity, fluid buildup in body, diabetes and increased infection ratio (Ruiz and Kirk Citation2015). Topically used immunomodulators—specifically tacrolimus—offer an attractive alternative due to high local efficacy. If used topically, tacrolimus has shown therapeutic potency comparable to high-potency corticosteroids but without systemic toxicities, making it a strong candidate for local application in the oral cavity and thereby circumventing systemic adverse effects (Saccucci et al. Citation2018; Utz et al. Citation2022).

Despite this therapeutic potential, currently no marketed buccal formulation containing tacrolimus exists. For skin use, Protopic® ointment containing tacrolimus has been used effectively against immunogenic diseases already for two decades, but it shows neither mucoadhesive capabilities nor appropriate organoleptic properties for the buccal area (Zuo et al. Citation2020). Due to the lack of suitable formulations, Protopic® is still used in the buccal area. To this end, a mucoadhesive paste may be prepared by compounding. In this paste, HPMC is used as mucoadhesive agent, which is dispersed in the hydrophobic Protopic® ointment. This improves the use properties of Protopic® but is still far from a suitable formulation for buccal use. Developing such a formulation requires overcoming challenges inherent to buccal delivery: salivary flow (Carpenter Citation2013), constant mechanical stress (Cook et al. Citation2017), and the need for strong mucoadhesion to maintain contact with mucosa. Therefore, the aim of this work was to formulate a mucoadhesive formulation for the buccal area, containing tacrolimus. Most preferably, a formulation suited for clinical use allowing pharmacy compounding should be developed to enable direct translation to the clinics. To be therapeutically effective, sufficiently high local tissue concentrations within the buccal epithelium and lamina propria need to be achieved while maintaining the formulation’s mucoadhesive properties and minimizing systemic absorption. Also, tacrolimus presents formulation challenges due to its low aqueous solubility, and inherently poor permeability across oral mucosa.

The buccal mucosa lines the inner cheeks and represents a non-keratinized, highly vascularized region of the oral cavity (Laine and Smoker Citation1995; Okur et al. Citation2021). Its epithelial layer is approximately 250–700 µm thick, consisting of stratified squamous cells supported by connective tissue, the lamina propria (Di Stasio et al. Citation2019; Edmans et al. Citation2020). Unlike the keratinized gingiva or palate in the mouth and the epidermis in the skin, the buccal mucosa lacks a dense lipid-rich barrier, making it more permeable and suitable for local or systemic drug delivery. Nevertheless, it still acts as a prominent barrier. Continuous saliva flow from dedicated salivary cells keeps the mucosal surface hydrated but also contributes to formulation dilution and removal, making mucoadhesion an important requirement for effective topical treatment.

Mucoadhesive semisolid formulations should provide prolonged residence time at the application site (= high substantivity) but also provide sufficient localized drug penetration. Among the polymers commonly used, chitosan offers particular advantages because of its cationic character, intrinsic mucoadhesion, and capacity to modulate epithelial barrier properties (Smith et al. Citation2004; Kulkarni et al. Citation2023). HPMC is also widely used and for example contained in the Protopic® paste that is used as reference. Carbomer has also been shown to provide strong mucoadhesion, and was thus included as an alternative. Since the buccal mucosa acts as a barrier to xenobiotics, it hinders or slows down the permeation and penetration of substances. To bypass this physiological barrier, chemical penetration enhancers, like 1,2-diols (e.g. propylene glycol, 1,2-pentanediol and 1,2-hexanediol) can be used. They are safe, non-irritating and their effect is reversible so that the barrier integrity is quickly restored (Senel and Hincal Citation2001; Nicolazzo et al. Citation2005).

To develop a new mucoadhesive drug delivery system the evaluation of its mucoadhesive property is an important step. Mucoadhesive agents can be characterized by evaluating their adhesion properties by in vitro and in vivo tests. In vitro methods measure adhesion between the formulation and a mucosal tissue, or a mucosa-mimetic material (e.g. mucin hydrogel) (Mansuri et al. Citation2016). At present, there is no standardized method available for studying mucoadhesion (Cook and Khutoryanskiy Citation2015; Komati et al. Citation2019). To investigate mucosa penetration, Franz cells are typically used, although no external saliva flow could be implemented to date (Diaz-Del Consuelo et al. Citation2005; Malhotra et al. Citation2025). Additionally, a custom mucoadhesion cell was developed. This was used in this study. It provides a temperature-controllable aluminum casket with a fixed mucosa, while simulating salivary flow and enabling abrasion testing. Here, mucoadhesion is defined as the capability of the tested formulation to withstand removal by salivary flow. Another advantage of the system is the possibility to measure mucoadhesion and mucosal penetration simultaneously.

The aim of this study was to develop mucoadhesive semi-solid tacrolimus formulation with high local delivery and minimal systemic exposure and favorable microstructural characteristics. The influence of penetration enhancers on tacrolimus delivery was investigated. Furthermore, penetration results obtained via Franz diffusion cells were compared to results obtained via a custom-build mucoadhesive cell developed by Hoffmann et al. (Hoffmann and Daniels Citation2018) and improved by Söpper et al. (Citation2021), to investigate, whether both cells could be used hand-in-hand to explore the formulations penetrative properties on mucosa alone and mucosa with constant salivary flow. Together, these findings will contribute to a clearer understanding of how mucoadhesive formulations with tacrolimus can be optimized to achieve both mucoadhesive stability and effective mucosal drug delivery and enabled the development of a promising formulation candidate for clinical use. As control formulation, a paste-based mucoadhesive formulation with tacrolimus was included, which was proposed in literature (DAC/NRF Compendium Citation2018). The paste contained Protopic 0.1% ointment, which is largely based on white petrolatum (Avoxa Citation2016), and hypromellose as a mucoadhesive polymer.

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Materials

Micronized tacrolimus monohydrate was of Ph. Eur. grade and purchased from Fagron GmbH (Glinde, Germany). Metolose® 60SH 4000 and 90SH 4000 of Ph. Eur. grade was donated by Shin-Etsu Chemical Co. (Tokyo, Japan). Protopic® 0.1% ointment Leo Pharma A/S (Ballerup, Denmark) was obtained from a local pharmacy. Chitosan (95% deacetylated) was of pharmaceutical grade and purchased from Harke Pharma GmbH (Mülheim-Ruhr, Germany). α–Amylase from Bacillus subtilis (380 U/mg) and mucin from porcine stomach type III were purchased from Sigma-Aldrich Chemie GmbH (Steinheim, Germany). NEG 50 was purchased from Thermo Fisher Scientific Inc. (Waltham, MA, USA). Aqueous solutions of glycerol 85% (m/m), lactic acid 90% (m/m) and sodium lactate 50% (m/m) were obtained as ready to use and were of Ph. Eur. grade. Carbopol® 980 NF, dipotassium hydrogen phosphate trihydrate, potassium chloride, potassium sorbate, propylene glycol, sodium chloride, sorbitol and trometamol were of Ph. Eur. grade. Acetonitrile and methanol were of HPLC (high performance liquid chromatography) grade. Hydrochloric acid 37%, n-hexane and phosphoric acid 85% were of laboratory grade. Ibuprofen was donated by BASF SE (Ludwigshafen am Rhein, Germany) and micronized prior to use. Ultra-pure water was used for all solutions and formulations (Elga Maxima, High Wycombe, UK). Porcine esophagi were supplied by a local butcher (Moessingen, Germany). The Department of Pharmaceutical Technology is registered for the use of animal products at the District Office of Tübingen (registration number: DE 08 4161052 21, approval date: 22 December 2015).

Hoffreiter, A., Graf, E., & Lunter, D. J. (2026). Mucoadhesive tacrolimus formulations for buccal application: a formulation and drug delivery feasibility study for oral lichen planus. Pharmaceutical Development and Technology, 1–24. https://doi.org/10.1080/10837450.2026.2738760