Getting physical in drug discovery: a contemporary perspective on solubility and hydrophobicity

Suboptimal physical properties have been identified as a particular shortcoming of compounds in contemporary drug discovery, contributing to high attrition levels. An analysis of the relationship between hydrophobicity (calculated and measured) and ∼100 k measured kinetic solubility values has been undertaken. In line with the General Solubility Equation, estimates of hydrophobicity, particularly ACD c log DpH7.4, give a useful indication of the likely solubility classification of particular molecules. Taking ACD c log DpH7.4 values together with the number of aromatic rings in a given molecule provides enhanced prediction. The ‘Solubility Forecast Index’ (SFI = c log DpH7.4 + #Ar) is proposed as a simple, yet effective, guide to predicting solubility. Moreover, analysis of measured distribution/partition coefficient values highlighted statistically significant shortcomings in the applicability of octanol/water as a model system for hydrophobicity determination with poorly soluble compounds.