The pK a is an important physical property to take into account in drug development. The pK a value is used to indicate the strength of an acid. A lower pK a value indicates a stronger acid, indicating the acid more fully dissociates in water. Molecules with multiple ionizable centers have multiple pK a values, and knowledge of the pK a of each of the ionizable moieties allows for the percentage of ionised/neutral species to be calculated at a given pH (if activity coefficients are known/assumed). pK a plays a particularly important role in drug development because the ionization state of molecules at physiological pH can have important ramifications in terms of drug-target interactions (e.g., ionic interactions) and/or by influencing other key determinants of drug absorption, distribution, metabolism and excretion (ADME) , such as lipophilicity, solubility, membrane permeability and plasma protein binding .