Formulation Screening and Characterization of PLGA-Based Injectable In Situ Gel Loaded with Progesterone

Objective: Conventional progesterone (P4) formulations suffer from low bioavailability, severe local irritation, and poor patient adherence due to P4’s poor aqueous solubility. This work aimed to develop and screen a biodegradable PLGA/NMP (Poly(lactic-co-glycolic acid)/N-methyl-2-pyrrolidone) injectable in situ gel for sustained P4 delivery to overcome these clinical limitations. Significance: Commercial oral, vaginal, and oil-based intramuscular P4 preparations cannot maintain stable long-term drug exposure, and they cause injection-site pain/inflammation. The screened in situ depot system reduces administration frequency and local tissue irritation, supporting convenient luteal phase support and pregnancy maintenance. Methods: Nine formulations with variable P4 loading (10–50% w/w) and PLGA concentration (15–55% w/w) were fabricated. Formulations were screened via three core endpoints: injectability (injection force and discharge rate), in vitro sustained release in 10% Hydroxypropyl-β-cyclodextrin (HP-β-CD)-Phosphate-buffered saline (PBS) sink medium, and 7-day subcutaneous histocompatibility in rats. high-performance liquid chromatography (HPLC) was validated for progesterone quantification; Hematoxylin and eosin (H&E) staining assessed local inflammatory responses. Results: Formulations with progesterone ≤ 30% w/w and PLGA ≤ 35% w/w exhibited acceptable injectability (injection force < 50 N; discharge rate > 79%). Higher PLGA concentrations suppressed initial burst release (16.74% at 8 h for 35% PLGA vs. 29.8% for 20% PLGA). All formulations formed stable ellipsoidal subcutaneous depots and completed progesterone release within 4 days. Histopathology revealed only mild local inflammation (histological score = 1) without severe necrosis, superior to highly irritating oil injections in formulation control groups. Conclusions: The screened PLGA-based progesterone in situ gel resolves critical drawbacks of traditional progesterone dosage forms. This low-irritation, sustained-release injectable platform provides a scalable industrial formulation candidate for long-acting hormone therapy.