Determining compound solubility is an essential tool for early stages of the drug discovery process, as well as for lead optimization. Low solubility can lead to unpredictable and unreliable results during in vitro testing, thereby increasing the development costs. Solubility issues at the later stages of the drug discovery may lead to poor bioavailability, underestimated toxicity and other obstacles, lowering the chances of a given drug candidate for success. Solubility can be measured as either a kinetic or thermodynamic value.
Typically, for early-stage drug discovery the kinetic solubility method is used, as it is fast and well suited for HTS format. In this case, solid compounds are first dissolved in DMSO and then linear serial dilutions of each compound are added to aqueous buffer and observed for precipitate formation when the compound is not completely soluble. Precipitate appearance can be evaluated by light scattering (laser nephelometry method).  For better precision, the solution can be subjected to high-speed centrifugation or filtration using special solubility filter plates and then the compound concentration is measured in the saturated solution directly by UV or LC/MS using separately built calibration curves (shake-flask method).
Thermodynamic solubility is important for lead optimization and drug formulation stages. It is usually determined for pure compounds: crystalline powders, amorfous substances and liquids. In this modification of solubility assay long-term (12-24 hours) incubation is used. Measurements are usually performed by the shake-flask method with LC/MS detection.