In recent decades, a growing concern has emerged with approximately 60 % of newly discovered compounds exhibiting poor solubility in aqueous media and low oral bioavailability (Bergström et al., 2016, Stegemann et al., 2023). Attempts to address the problem by optimizing the aqueous solubility have been used with variable success. However, the effectiveness of solubility-enhancing approaches is additionally hindered by the fact that many poorly water-soluble molecules can be likened to ‘brick dust’. These molecules exhibit limited solubility in both aqueous and organic media due to the strong intermolecular forces within their crystal units (Wassvik et al., 2008). Thus, bio-enabling strategies are often required for the efficient delivery of these drug molecules. Particularly, utilizing LbF as a bio-enabling strategy provides the benefit of introducing the poorly soluble drug to the gastrointestinal tract (GIT) in a pre-solubilized state, effectively bypassing the rate-limiting dissolution step(Holm et al., 2023, Porter et al., 2007). The digestion of exogenous lipid components creates a solubilizing environment for the drug in the GIT and maintains the drug in a solubilized state. Lipid formulation may trigger endogenous solubilizers like bile-lipid aggregates to effectively shuttle the drug to the site of absorption, and facilitate lymphatic absorption of certain drugs (Suys et al., 2021, Williams et al., 2013). To date, lipid formulations have been utilized for oral delivery of number of FDA-approved compounds, particularly those requiring low target doses, in the form of lipid suspensions and lipid solutions. (Savla et al., 2017). However, the complete utilization of LbFs is constrained by the high target dose and insufficient lipid solubility of brick dust molecules, presenting the need for advancements in LbFs to effectively incorporate these molecules in lipid solutions.

The synthesis of lipophilic salts or complexes for brick dust molecules represents a promising strategy for enhancing their lipid solubility and subsequently incorporating them into lipid solutions to achieve the target dose. Molecular properties of compounds like polar surface area and melting point are pivotal determinants influencing the solubility in lipids (Bergström et al., 2016). Therefore, pairing a drug compound with a counterion is a strategic approach to enhance solubility in lipid-based solutions by altering the aforementioned molecular properties (Koehl et al., 2022, Lai et al., 2022). This enhancement can be attributed to two distinct mechanisms: firstly, the reduction of drug crystallinity by diminishing intermolecular crystalline forces; and secondly, the lipophilicity of the counterion, which promotes the absorption of the drug into the lipid phase (Ditzinger et al., 2019, Ford et al., 2020). The structural features of the counterion (such as molecular weight, steric hindrance, branching, and chiral centres) are pivotal factors influencing the disruption of crystal lattice energy and, subsequently lipid solubility (Tokuda et al., 2005). In the literature, various types of counterions including alkyl sulfate, carboxylic acid-based, phospholipid-based, and cationic surfactant-based counterions have been employed for the fabrication of lipophilic salts. Particularly, we thoroughly explored different counterions within alkyl sulfate and carboxylic acid-based lipophilic categories (Bharate, 2021; Ristroph D. and Prud’homme, R., 2019). Alkyl sulfate-based counterions form stable lipophilic salts (i.e. rely on complete proton transfer) with APIs due to a substantial difference in pKa values between the API and the counterion. This disparity enhances the probability of stable salt formation while concurrently diminishing the likelihood of dissociation in solution (Ford et al., 2020). Due to the amphiphilic nature, aliphatic sulfate anions have been extensively investigated for the preparation of ionic salts. This exploration is based on the expectation that the hydrogen bond acceptor sites on the sulfate moiety will complement the hydrogen bond donor site on positively charged APIs. Additionally, the long aliphatic chain is anticipated to enhance the potential for favourable van der Waals interactions between the lipophilic salt and non-polar lipids, thereby good miscibility with a range of LbF (Kaczmarek et al., 2019, Williams et al., 2014). Apart from alkyl sulfate counterions, carboxylic acid-based counterions, including linolenic acid, oleic acid, stearic acid, cholic acid, or their sodium salts are extensively studied by medicinal chemists for imparting a lipophilic character and improving the membrane permeability through ion pairing with hydrophilic small molecules and peptides (Ditzinger et al., 2019, Md Moshikur et al., 2020). However, the selection of optimal counterion is dependent upon various factors including the chemistry of both the drug and counterion (including the molecular size of the counterion, carbon chain length, and pKa difference), drug loading capacity, targeted release profile, and safety considerations (Gamboa et al., 2020, Ristroph and K., K. Prud’homme, R., , 2019).

Continued efforts are directed towards refining the hydrophilic-lipophilic balance (HLB) of pharmaceutical compounds through salt formation or complexation, yet the applicability of these approaches is limited within the development framework of LbFs. Recently, a few studies demonstrated that the use of these approaches can increase the liposolubility of pharmaceutical compounds such as ceritinib, cabozantinib, ranitidine, amlodipine, and metformin (Saeed et al., 2021, Williams et al., 2018a, Williams et al., 2018b). The lipophilic salt formation demonstrated a modulation of apparent solubility under digestive conditions compared to both the unformulated crystalline drug and LbF comprising a crystalline drug. This suggests that incorporating the lipophilic counterion may play a crucial role in enhancing drug absorption. Additionally, Williams et al., 2014 also studied that lipophilic salts based LbF showed effective interaction with bile lipid aggregates in the GI milieu resulting in the formation of highly dispersed species with increased access to the absorptive surface thereby enhancing the absorption profile of Danazol. Despite these advancements, there exists a significant lack of comprehensive research into the impact of the incorporation of lipophilic salt or complex into LbF on critical aspects such as physical stability, aqueous solubilization, and absorption flux for difficult-to-deliver molecules. In the current investigation, Nilotinib (Nil), a BCS class IV compound, is chosen as the model drug due to its brick-dust characteristics and dissolution-limited absorption (Koehl et al., 2019). Being a weakly basic molecule, its solubility is highly dependent on pH, and it is practically insoluble at elevated pH levels (Chougule et al., 2023). Recently, Koehl et al., 2019 and Zakkula et al., 2020 demonstrated that Nil solubility is limited (below 10 mg/g) in lipid solutions due to strong intermolecular forces within the crystal lattice and inadequate interaction with the lipids. According to the FDA product label, the recommended dosage regimen for the commercial Nilotinib product (Tasigna®) is within the range of 300 to 400 mg, to be taken orally twice daily (US Food and Drug Administration, Tasigna® Product Label) (USFDA, 2024). Hence, the inherent characteristics of Nil and its high dosage regimen make it an ideal candidate for a combined approach of salt formation and their incorporation into LbF to address the aforementioned challenges.

In consideration of these factors, our research aims to synthesize different lipophilic salts/complexes with Nil to disrupt drug crystallinity and compare their solubilization behaviour in aqueous and lipid components. The synthesized lipophilic salts/ complex were incorporated into prototype LbF and evaluated the potential advantage gain in terms of drug loading compared to crystalline Nil. The prototype LbF was a type IIIA-based LbF comprising Capmul® MCM EP (50 % w/v), Kolliphor® RH40 (30 % w/v), and PEG 200 (20 % w/v). Further, the effect of different lipophilic counterions on the dispersion behaviour and aqueous solubilization in the digestive conditions was assessed in vitro. Additionally, we assessed the absorptive flux through the biomimetic membrane and the pharmacokinetic behaviour of prepared formulations. Subsequently, we compared these results with those of unformulated crystalline Nil and conventional LbF. Conventional LbF was prepared by dissolving crystalline Nil in a prototype liquid LbF, where the drug load was equivalent to the equilibrium solubility of Nil in the lipid solution.