Evaluation of the Pharmaceutical Equivalence of Lidocaine and Prilocaine Creams

Background: Lidocaine and prilocaine cream is a compounded topical anesthetic formulation comprising lidocaine and prilocaine. Upon application, the active ingredients are locally released and permeate into the subcutaneous tissue, exerting anesthetic effects by blocking ion channels involved in nerve impulse transmission. Variations in drug permeation may influence the onset time, anesthetic efficacy, and duration of action. Methods: This study investigates the in vitro properties of the innovator formulation Emla® and five generic formulations through in vitro bioequivalence studies and Q3 Characterization tests. Results: With the exception of TS, the generic formulations exhibited notable differences compared to the innovator product. Specifically, TB and TL demonstrated significantly higher in vitro release than Emla®, yet exhibited lower in vitro permeation rates. In contrast, TH and TT showed release rates comparable to Emla®, while their permeation rates were similarly reduced. Conclusions: These findings indicate that in vitro release rate does not directly predict in vitro permeation. Permeation behavior is significantly influenced by emulsion globule size, rheological characteristics (viscosity and elasticity), and pH, collectively underscoring the multifactorial nature of this phenomenon.