STUDIES OF THE STABILITY OF A SUBSTITUTED PHOSPHOROTHIOATE USING AN IMPROVED HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY ASSAY COUPLED WITH ELECTROCHEMICAL DETECTION

A sensitive, reproducible and rapid HPLC assay was developed for prefo rmulation and stability studies of a potential radioprotector, S-2-(3- methylaminopropylamino) ethylphosphorothioate, WR-3689. This method is based on the elution of WR3689 by high performance ion-pair reverse p hase liquid chromatography using an electrochemical detector with an o xidation potential set at + 0.19 v. The detector response was found to be linear over the concentration range of 0.02-20 mu g/mL with an abs olute limit of quantitation of 170 pg. The coefficient of variation ra nged from 0.75-6.52% for the interday reproducibility. This HPLC assay was employed to determine the stability of WR-3689 at various pH (2, 4, 7, 10 and 12) and temperatures (22 degrees C, 35 degrees C, 43 degr ees C and 100 degrees C). Results of the stability studies indicated t hat WR-3689 is more stable in basic than in acidic aqueous solutions. For example, the half-life of WR-3689 at 45 degrees C was 13.8 days at pH 12 as compared to 12.7 h at pH 4. With regard to the effect of tem perature, the half-life of this compound increased from 19.2 h at 43 d egrees C to 41.8 days at 22 degrees C. In conclusion, the excellent re producibility and stability indicating capacity of this assay makes it a useful method for formulation studies. The high sensitivity and spe cificity of this assay make it useful for pharmacokinetic studies.