Stability Assessment of Compounded Niaprazine Oral Solutions to Support an Evidence-Based Beyond-Use Date

Background/Objectives: Niaprazine is widely used for the management of sleep disorders in pediatric and geriatric patients; however, no commercially available oral liquid formulation is currently available in Italy, making extemporaneous compounding necessary. In routine practice, the beyond-use date (BUD) is often limited to 30 days, potentially affecting therapeutic continuity. This study aimed to evaluate the chemical, physical, and microbiological stability of a compounded niaprazine syrup to support evidence-based BUD and to assess the suitability of selected ready-to-use compounding vehicles for preparing alternative niaprazine oral liquid formulations. Methods: A niaprazine syrup (3 mg·mL−1) was prepared in a sucrose-based vehicle acidified with tartaric acid and preserved with potassium sorbate. Chemical stability of niaprazine was evaluated by high-performance liquid chromatography coupled with diode array detector (HPLC-DAD), whose specificity was confirmed by forced degradation studies. Stability was monitored for up to 9 months at 4–8, 25, and 40 °C and confirmed after 12 months by HPLC-DAD and mass spectrometry (MS). Physical stability of the formulation was monitored by pH and visual inspection up to 12 months. Microbiological quality was assessed for 2 months at 4–8 and 25 °C according to the European Pharmacopoeia. In parallel, four selected ready-to-use compounding vehicles were evaluated for their suitability to prepare stable niaprazine oral liquid formulations. Results: Niaprazine concentrations in the syrup remained within pharmacopeial acceptance limits (±10%) at all temperatures, although a decrease was observed under accelerated conditions (40 °C). pH remained stable (≤0.5-unit variation) and the formulation stayed clear and homogeneous throughout the study, with only minor visual changes after prolonged storage at 40 °C. Statistically significant differences (p < 0.05) were observed in both the HPLC-DAD and HPLC-MS datasets. Microbiological testing confirmed compliance up to 2 months at 4–8 and 25 °C (TAMC ≤ 103 CFU/mL; TYMC ≤ 102 CFU/mL; Escherichia coli absent). Comparable chemical and physical stability was observed for three of the four formulations prepared with the ready-to-use vehicles over at least two months. Conclusions: The compounded niaprazine syrup demonstrated chemical, physical, and microbiological stability under refrigerated and room-temperature storage, supporting evidence-based beyond-use dating of up to two months under the tested conditions. Ready-to-use vehicles may represent a practical complementary approach, offering standardized alternatives for the preparation of niaprazine oral liquid formulations.