Eyes are considered complex organs with various compartments (cornea, iris, retina, sclera and choroid). Ocular drugs could be administered systemically or topically. Systemic route for administering ocular drugs suffers low bioavailability at the site of action, besides the undesirable side effects. On the other hand, effective topical ocular drug delivery is challenging owing to the physiological protective nature which acts as a barrier [1]. Administering these drugs by conventional dosage forms via the topical route doesn't achieve the targeted bioavailability, owing to the turnover of tears, insufficient residence period as well as the drainage through the nasolacrimal duct [2].

Various previous trials were conducted to enhance the ocular bioavailability of different poorly soluble drugs via the topical route, through their incorporation in nanoparticles as niosomes [3], liposomes [4], bilosomes [5], elastosomes [2,6], olaminosomes [7,8], cubosomes [9] and leciplexes [10,11] or enhancing their solubilities by microemulsions or nanosuspensions. It's important to take into consideration that chronic ocular diseases require frequent drug instillation which decreases the compliance of patients. Consequently, formulating sustained release systems was explored frequently through the fabrication of mucoadhesive ocular systems incorporating chitosan [8,12] or hyaluronic acid [2,13].

Glaucoma is an overwhelming disease that can lead to blindness. Glaucoma means elevated intraocular pressure, that happens due to the over production of aqueous humor or the blockage of its drainage [8]. The main consequences of high intraocular pressure are optic nerve degeneration and permanent vision loss [6]. Treatment of glaucoma could be classified into either drugs that affect the secretion of aqueous humor (including inhibitors of carbonic anhydrase, both adrenergic antagonists or agonists) or others that impact its drainage (including donors of nitric oxide, prostaglandin, inhibitors of rho kinase). Moreover, novel treatment drugs as activators of tyrosine kinase, natriuretic peptide as well as melatonin analogs, and modulators of adenosine receptors couldn't be neglected [14].

Felodipine (FELO), being a dihydropyridine calcium channel blocker, is commonly used to treat high blood pressure and angina pectoris [15]. FELO, being a mineralocorticoid antagonist, was reported to decrease the intraocular pressure (IOP) after ocular application [16]. This is well in line with the results of Swain et al. who proved successful felodipine docking with mineralocorticoid receptor with a score of −5.2 kcal/mol [17]. It is worth mentioning that nimodipine revealed successful IOP reduction [18,19]. Also, verapamil was reported to manage glaucoma after topical application in human eyes [20].

FELO is classified as a BCS class II drug, showing low solubility and high permeability. Moreover, FELO suffers extensive hepatic metabolism, and therefore diminished oral bioavailability (around 15 %) [15]. Many approaches were explored for improving the oral bioavailability of FELO. Yet, investigating the pharmacodynamic effect of FELO in managing intraocular pressure through the ocular route was explored only once by Swain et al., in 2023 [17]. FELO loaded film using dimethyl sulphoxide as a plasticizer revealed improved ocular permeation in 7 h and enhanced IOP reduction up to 8 h post ocular application [17].

Herein, hyaluobilosomes (hyaluronic acid enriched bilosomes) were examined as a dual approach amalgamating the merits of hyaluronic acid as well as the traditional bilosomes. Owing to their lipophilic nature, bilosomes are expected to achieve outstanding encapsulation of hydrophobic drugs and controlled drug release [17]. Bilosomes could also enhance the bioavailability of topically applied ocular drugs by their protection from metabolic degradation [5,6]. Cholesterol is mainly responsible for constituting the bilosomal matrix [21], span 60®, being a non-ionic surfactant, could decrease the particle size of the developed vesicles and edge activators (EA) enhance the vesicles’ elasticity and hence permeation [8,22]. Besides, hyaluronic acid is a mucoadhesive polymer that prolongs the ocular residence time and enhances the permeability through the cornea [2,6]. Previous studies explored the ocular delivery of the anti-glaucomic agomelatine via the development of bilosomes [5] and elastosomes enriched with hyaluronic acid [6], as well as the ocular delivery of the anti-fungal voriconazole by the fabrication of hyaluronic acid enriched ultra-deformable elastosomes [2].

In view of the aforementioned, FELO-loaded bilosomes were successfully developed adopting a 23 factorial design. Two cholesterol: span 60® ratios (1:2 or 2:1), two EA types (NDC: sodium deoxycholate or NTC: sodium taurocholate) and two EA amounts (5 or 15 mg) were investigated. The statistical significance of the in-vitro characterization was tested to choose the optimum system. Following, the pharmacodynamic studies of the optimum system were conducted in normotensive rabbits; in comparison to FELO dispersion. The protocol of participating animals in research studies was firstly submitted, then revised and finally approved (PI-3849) by the Research Ethics Committee for experimental studies, Faculty of Pharmacy, Cairo University, Egypt. In vivo studies were conducted in agreement with EU Directive 2010/63/EU for animal experiments.