Determination of the partitioning, stability, and metabolite formation of isosorbide dinitrate in human and rat blood using an improved gas-liquid chromatographic assay

A simple and highly sensitive gas-liquid chromatographic method using electron-capture detection has been developed for the simultaneous determination of isosorbide dinitrate (ISDN) and its mononitrate metabolites in rat and human plasma. This method has a limit of quantitation of about 5 ng/ml for the mononitrates and of 1 ng/ml for ISDN using 0.1 ml of plasma, and is thus useful for pharmacokinetic studies of these compounds in small animals, and in humans when the available volume of blood is limited. Using this method, we found the apparent in vitro partitioning ratio of ISDN between erythrocyte and plasma in rat and human blood at 37°C to be 0.22 and 0.13, respectively. In spite of this poor affinity for red blood cells, ISDN degradation in whole blood was mediated primarily via this blood fraction. Loss of ISDN in blood appeared to proceed exclusively through its mononitrate metabolites, resulting in a 6:1 product ratio of the 5-mononitrate to its 2-isomer. These data suggest that although blood degradation of ISDN and erythrocyte partitioning occur per se, these phenomena do not contribute significantly to the very rapid in vivo clearance of ISDN observed in man and in the rat.