The evaluation of short- and long-term stability studies for brimonidine in aqueous humor by DPV/BDDE method—possible application for direct assay in native samples

Abstract

A novel voltammetric method was developed for brimonidine (BRIM) determination in deproteinized aqueous humor, simplifying preparation of biological samples for analysis for stability studies. The differential pulse voltammetric (DPV) method using boron doped diamond electrode (BDDE), based on characteristic oxidation peaks, was proposed and successfully applied. The linearity range was within 5.0 × 10−6 to 5.0 × 10−5 M of brimonidine, and limit of detection and limit of quantitation were 1.94 × 10−6 M and 6.46 × 10−6 M, respectively. Intra-day and inter-day precision and accuracy were evaluated and all results were in accordance with validation ICH guidelines. The best short-term stability study results were obtained for a concentration level of 3.0 × 10−5 M expressed by deviation of + 1.86% between initial and post storage concentrations. A long-term stability study was performed for two concentrations of 3.0 × 10−5 M and 5.0 × 10−5 M and resulted in deviations of + 1.63% and + 3.56%, respectively. A freeze and thaw stability study indicated that samples might be frozen only once. The enhancement of DPV/BDDE method sensitivity gained by modification, for the analysis of immeasurable BRIM quantities in native, untreated aqueous humor, was reached for quantities of 6 or 12 nmol/0.1 mL aqueous humor with acceptable accuracy (up to + 7.5%). The nature of the process—the irreversible one electron oxidation voltammetric peak of BRIM—limited the sensitivity. Only electrochemical pre-treatment of the BDD electrode before each measurement significantly speeded up the whole procedure. The advantages of the proposed method are simplicity, short-time performance, and good specificity/selectivity, as well as satisfactory accuracy, and no chemical modification of BDDE was necessary.