Abraham Model Solute Descriptors for Favipiravir: Case of Tautomeric Equilibrium and Intramolecular Hydrogen-Bond Formation

Experiment-based Abraham model solute descriptors are calculated based on recently published solubility data for favipiravir dissolved in 12 chemically diverse organic mono-solvents. The calculated descriptor values indicate that favipiravir engages in intramolecular hydrogen formation that renders the hydroxyl functional group on the pyrazine ring system unable to form intermolecular hydrogen bonds with the surrounding solvent molecules. Our study further shows that the existing group contribution and machine learning methods provide rather poor estimates of the experiment-based solute descriptors of favipiravir. Poor estimation likely occurs, in part, because the methods fail to account for the intramolecular hydrogen-bonds that are believed to be formed. In the current study, it was found that the solute descriptors estimated using three different methods provided rather poor estimates of the observed solubility behavior of favipiravir, with the overall average standard errors between the experimental and predicted molar solubilities exceeding 0.40 log units.