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Solubility is a measurement of the amount of a substance that can stay in a solvent without precipitation. Solubility may be expressed in units of concentration, molality, mole fraction, mole ratio and other units. For the purposes of biopharmaceutics, the substance of interest is usually the API and the solvent will vary depending upon the physiological region of interest; for example the stomach, small intestine or pulmonary fluid.

A drug's structure determines its solubility as the chemical structure will also determine lipophilicity, hydrogen bonding, molecular volume, crystal energy and ionisability. Thus during lead compound optimisation, there is scope to balance potency with solubility [1].

The solubility of a substance can change when pressure, temperature and/or the composition of the solvent changes thus it is important to accurately detail solubility data to provide sufficient information.