Separation of 1,4-Dihydropyridine Derivative and Its Oxidized Form

Derivatives of 1,4-dihydropyridine (DHP) still play an important role in treatment of cardiovascular diseases. Typical degradation of the DHP ring is aromatization to pyridine ring which occurs both chemically and biochemically. It is, therefore, important to have a reliable and robust analytical method for separation of DHPs from their oxidized counterparts. Separation of closely-related substances possessing similar hydrophobicity, such as DHP and its oxidized form, can be challenging on conventional alkyl-bonded sorbents. In this study, an impact of reversed-phase (RP) liquid chromatography conditions on separation of the DHP/Ox pair has been investigated. Initially, a systematic study has been performed on 33 commercial RP columns with mobile phase acetonitrile/water for separation of foridone and its corresponding oxidized form. The retention and selectivity are discussed in view of the hydrophobic-subtraction model. Best separation was found replacing conventional C₁₈ sorbents with ones containing an embedded polar group due to polar interactions. Similarly, application of cyano columns resulted in efficient separation of analytes. Organic modifier of mobile phase (methanol vs. acetonitrile) contributed significantly to separation of foridone from its oxidized counterpart. Separation of six chemically diverse DHPs from corresponding oxidized forms was studied on seven RP columns (traditional C₁₈ sorbent, alkyl sorbent with polar embedded group, two different aromatic phases, pentafluorophenylpropyl sorbent and sorbent with straight chain perfluorohexyl ligand). Both acetonitrile and methanol were applied as organic modifier. It was found that application of alkyl sorbent with an embedded polar group (column Zorbax Bonus RP) or cyano sorbent (column ACE CN) yields clear separation of chemically diverse DHPs from their oxidized forms.