Evaluation of Permeation Enhancers for Vaginal Delivery of Buserelin Acetate Using a Validated Chromatographic Method and Ex Vivo Porcine Model

Background/Objectives: This study aimed to enhance the vaginal permeation of buserelin acetate (BA), a synthetic gonadotropin-releasing hormone (GnRH) analogue, by evaluating various permeation enhancers (PEs) using a validated reversed-phase high-performance liquid chromatography (RP-HPLC) method and an ex vivo porcine vaginal model. Methods: A robust RP-HPLC method was developed and validated according to ICH Q2 (R2) guidelines to enable accurate quantification of BA in permeation samples. The analytical method demonstrated high specificity, linearity (R2 = 0.9999), accuracy (98–102%), precision (%RSD < 2%), robustness, and stability. Using this method, ex vivo permeation studies were conducted with six different PEs: 2-hydroxypropyl-β-cyclodextrin, sodium dodecyl sulfate, poloxamer 188, Span 80, Tween 80, and chitosan. Results: Among all tested PEs, chitosan demonstrated the best enhancement of BA permeation. It achieved the highest flux (J) (0.64 ± 0.03 × 10−2 µg/cm2·h) and apparent permeability coefficient (Papp) (16.20 ± 0.84 × 10−5 cm/h), both of which were statistically significantly higher (p < 0.05) than those of all other enhancer groups. Kinetic modelling indicated a non-Fickian, biphasic permeation mechanism best described by the Makoid–Banakar model. Conclusions: These findings highlight chitosan’s potential as an effective intravaginal delivery vehicle for peptide therapeutics and establish the validated HPLC method as a reliable platform for future formulation development and translational studies in mucosal drug delivery.