Shift of the high-performance liquid chromatographic retention times of metabolites in relation to the original drug on an RP8 column with acidic mobile phase

The effect of the structural change in the metabolization of drugs on the HPLC retention time with an RP8 column with an acetonitrile-phosphate buffer (pH 2.3) as the mobile phase was investigated at model compound pairs of 29 functionalization reactions. A more or less typical region for T-M=log(k'(M)/k'(D)) was found for each of these reactions (with k'(M) and k'(D) being the capacity factors of the metabolite and the drug, respectively), which can be explained by an increase or a decrease of the hydrophilic properties caused by the structural change. This effect is superimposed by an essential influence of the unchanged part of the molecule and in some cases by special intramolecular interactions like the hydrogen bond. Despite the more complicated structure of real drugs the results obtained at the model compound pairs were confirmed for most of the 55 metabolite/drug pairs. The practical use of the T-M values as a support to distinguish between different metabolites in the HPLC-DAD analysis of intoxications is demonstrated with cases of poisoning with diphenhydramine, propafenone and methaqualone.