Development and Chemical Stability Assessment of a Methyl-β-Cyclodextrin-Containing Dextromethorphan/ Quinidine Oral Solution for Extemporaneous Compounding.

Pseudobulbar affect is a distressing condition frequently observed in patients with amyotrophic lateral sclerosis (ALS), for which a fixed-dose combination of dextromethorphan and quinidine is approved in the United States. In Europe, no commercial product is available, and compounded capsules or suspensions are used; however, capsules may be unsuitable for patients with dysphagia, and suspensions require shaking before administration. The aim of this study was to develop simple fully water-soluble formulation of dextromethorphan hydrobromide and quinidine sulfate using methyl-ß-cyclodextrin as a solubilizing agent and to evaluate its stability under different storage conditions, enabling pharmacies to compound the preparation without complex infrastructure. Preliminary qualitative solubility experiments showed that the dextromethorphan hydrobromide/ quinidine sulfate powder mixture did not dissolve in water without methyl-ß-cyclodextrin. Addition of methyl-ß-cyclodextrin improved dissolution; however, the initial preparation targeting 20 mg/mL dextromethorphan hydrobromide and 10 mg/mL quinidine sulfate in a final volume of 100 mL remained incompletely dissolved. Increasing the final volume to 250 mL yielded a clear formulation containing nominal concentrations of 8.4 mg/mL dextromethorphan hydrobromide and 4.2 mg/mL quinidine sulfate. Three independently prepared batches were stored in amber PET bottles providing light protection and fitted with child-resistant closure at 2-8°C, 15-25°C, and 40°C and analyzed by HPLC for four weeks. HPLC assay values remained within 10% of the corresponding baseline values at each storage condition, and no visible precipitation was observed. The results support the physical clarity and chemical consistency of the tested methyl-ß-cyclodextrin- containing formulation over four weeks under the evaluated storage conditions. Microbiological stability, preservative effectiveness, formal method validation in the author's laboratory, and the extent of cyclodextrin inclusion complexation were not assessed. This formulation may serve as a basis for further pharmaceutical, microbiological, and clinical evaluation of an extemporaneously compounded oral liquid.