Comprehensive Analytical Studies on the Solubility and Dissolution Rate Enhancement of Tadalafil with Type IV Lipid Formulations

<b>Background</b>: This work aimed to enhance the solubility of Tadalafil (TDL), a BCS Class II drug, by preparing Type IV lipid-based formulations. <b>Methods</b>: Type IV formulations were prepared using surfactants and/or hydrophilic co-surfactants, resulting in oil-free systems. <b>Results</b>: Based on the solubility test, Transcutol^® HP exhibited the highest solubility for TDL (48.33 ± 0.004 mg/mL) and was selected as the co-surfactant. Among surfactants, Kolliphor^® PS80 (42.74 ± 2.29 mg/mL), Kolliphor^® EL (41.87 ± 2.50 mg/mL), Kollisolv^® PEG 400 (40.70 ± 0.30 mg/mL), and Kolliphor^® HS15 (31.40 ± 3.63 mg/mL) demonstrated high solubilization capacity. These were used to prepare formulations without the addition of an oil phase. The developed formulations resulted in a system with a nano-droplet size (<50 nm) and PDI values < 0.3, which was clear, transparent, and resistant to pH dilutions. The optimum Type IV lipid formulations were further characterized and demonstrated good thermodynamic stability under temperature and pH changes. The optimized formulation was adsorbed onto different carriers and transformed into solid TDL-loaded formulations. The in vitro dissolution rate of the drug from the solidified lipid formulations was studied in various dissolution media. It was observed that the solid formulations prepared with Neusilin US2^® (2:1) exhibited a significantly higher dissolution of over 95% within 5 min compared to the marketed product. The in vitro lipolysis studies demonstrated that F2 formulation maintained TDL in a supersaturated state throughout digestion, with limited enzymatic degradation of the excipients. Cytotoxicity evaluation using the MTT assay in Caco-2 cells confirmed the biocompatibility of both drug-free and TDL-loaded formulations, with IC₅₀ values of 19.55 µg/mL and 17.55 µg/mL, respectively. <b>Conclusions</b>: The overall results suggested that the developed solid Type IV lipid formulations can improve the dissolution rate of TDL, which would potentially lead to an improvement in its oral bioavailability and, consequently, a reduction in the treatment dose as a safe delivery system.