SEPARATION OF GEOMETRICAL SYN AND ANTI ISOMERS OF OBIDOXIME BY ION-PAIR HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

Obidoxime chloride (Toxogonin) is one of the fastest reactivating oximes to restore acetylcholinesterase activity after intoxication with organophosphates. Reversed-phase HPLC analysis with ion-pairing reagents revealed the existence of three geometrical isomers, both in solid obidoxime dichloride monohydrate and in solution (Toxogonin ampoules). The syn-syn isomer prevails, the asymmetric syn-anti isomer amounts to 5% (solid) and 3% (solution) and the symmetric anti-anti isomer less than 0.1%. The structure of the syn-anti isomer isolated by ion-exchange chromatography was confined by H-1 NMR spectroscopy. The acidity of the oxime protons decreased from the syn-syn isomer (mean pK(a) 8.0) to the syn-anti (mean pK(a) 8.3) and the anti-anti form (mean pk(a) 8.6). In addition, the absorption maximum of the oximate was shifted from 356 (syn-syn) to 360 (syn-anti) and 367 nm (anti-anti). Analysis of outdated Toxogonin ampoules that had been stored at about 20 degrees C for up to 19 years indicated a shelf-life (10% decomposition) of ca. 20 years, which is less than predicted earlier from advanced decomposition studies. Cyanide formation was negligible, and the ratio of syn-syn- to syn-anti-obidoxime still remained at 3%.