Effect of temperature and light on the stability of latanoprost and its clinical relevance

Purpose: To examine the effect of controlled heat and ultraviolet exposures on the stability of latanoprost (Xalatan, Pharmacia & Upjohn, Kalamazoo, M I) using highperformance liquid chromatography to derive practical recommen dations for patients regarding its use and storage. Methods: Using serial dilution of a latanoprost stock solution, varying con centrations were prepared to obtain a standard curve. The accuracy and prec ision of the high-performance liquid chromatography assay conditions were v alidated using between-day and within-day studies. The original latanoprost containers were stored at 4, 25, 50, and 70 degreesC, and the concentratio n of latanoprost remaining was measured by high-performance liquid chromato graphy at different times for up to 1 month. In addition, the original lata noprost containers were exposed to known amounts of ultraviolet A and B rad iation for 4 hours, and the concentration of latanoprost was measured at I- hour intervals using high-performance liquid chromatography. Results: The increased temperature studies showed that latanoprost remained stable at 4 and 25 degreesC for the 30-day study duration. Analysis of con centration versus time curves for 50 and 70 degreesC yielded a t(90) (time for 10% degradation) of 8.25 and 1.32 days, respectively. Ultraviolet B rad iation caused a rapid degradation of latanoprost, whereas ultraviolet A rad iation was less effective in causing the degradation of latanoprost. Conclusions: Latanoprost exhibits thermal and solar instability and should ideally be stored below room temperature and in the dark. The importance of these storage conditions should be conveyed clearly to the patient.