Characterisation of steroids

Abstract

The extent of action of steroids is affected by their characteristics. To date, there are few studies experimentally characterising difluprednate (DFBA) and 6α,9α- difluoroprednisolone (DFP) in literature. The aim of the study was to characterise DFBA and DFP by determining their respective pKa and logP values using experimental methods. A literature review was conducted using HyDi and Reaxys to identify pKa and logP determination methods for steroids. The purity of steroids was evaluated using a melting point apparatus. The solubility of steroids was assessed in acetonitrile (ACN) or methanol (MeOH) and water (H20) at different ratios and determined using UV spectroscopy at 242nm to aid solvent selection for Potentiometric Titration (PT). PT was conducted to determine the pKa of the steroids using H20 with ACN or MeOH. A shake-flask method was conducted to determine the logP. A High-Performance Liquid Chromatography coupled to UV-Visible spectroscopy (HPLC/UV-Vis) method was conducted for the simultaneous determination of pKa and logP. The melting points of DFBA and DFP were 188-189C and 210-220C respectively. The highest solubility of DFBA was found to be 0.72mg/ml in 10% MeOH and that DFP of 1.92mg/ml in 10% ACN. PT for DFP using H20:ACN indicated a pKa of less than 9. The shake-flask method could not be used to accurately determine the logP of the steroids due to emulsion formation. The logP and pKa of DFP were found to be 1.460, and 1.159 and 10.818 respectively, and those of DFBA were 3.322, and 1.451 and 7.240 respectively, using an isocratic HPLC method. The gradient HPLC method was unsuccessful in determining the pKa and logP of the selected steroids. The melting points determined were found to be in accordance with literature values. DFBA was found to be more soluble in MeOH and DFP in ACN. Due to the weak acidic nature of the steroids, pKa determination using PT was unsuccessful. Emulsion formation led to challenges during experimentation as it affected concentration determination. The determined logP of DFP differed from literature values while that of DFBA was similar. The pKa values of DFBA determined experimentally differ to literature values.