Stability of Levodopa/Carbidopa Rectal Suspensions

Objective: The combination of levodopa and carbidopa (L/C) is used as an effective therapy for treating Parkinson's disease; however when oropharyngeal dysphagia develops or when insertion of an nasogastric tube is not possible, then rectal administration could be used. To reduce compounding workload and establish a beyond use date, this study was conducted to determine the stability of 2 L/C rectal suspension formulations when stored at either 22°C or 5°C. Methods: Two formulations of L/C rectal suspension were compounded and then packaged in amber polypropylene bottles. Three bottles of each formulation were stored at either 22°C or 5°C and analyzed at 11 time periods. Physical parameters such as caking, ease of resuspending, and pH were also determined at each time period. A validated stability-indicating high-performance liquid chromatography (HPLC) method was used to analyze both active ingredients. Results: All solutions were easy to resuspend, there were no signs of caking, and there was no significant change in pH over the 35 days of storage at either temperature. The glycerin-based formulation (formulation 1) was less stable at 22°C (10 days) than formulation 2 (24 days). Both rectal suspensions were stable for 35 days when stored at 5°C. Conclusions: The physical compatibility and chemical stability of 2 formulations of L/C rectal suspension packaged in amber polypropylene bottles was determined to be either 10 or 24 days when stored at 22°C or 35 days at 5°C.