Ionic liquids (ILs) have evolved from salts studied primarily for their physical properties (low melting salts which could be used as solvents) to tunable materials based upon the physical, chemical, and even biological properties that can be introduced through either ion. Interesting evolution work in this growing field can lead to possible future directions, such as the use of ILs as Active Pharmaceutical Ingredients (APIs).

Ionic liquids (ILs) are defined as salts composed solely of ions, with a melting point below 100° C. Although compounds fitting this definition have been known for over a century, it has been only relatively recently, first with the utilization of ILs as electrochemical solvents and later with the suggestion that ILs could be ‘‘green solvents,’’ that a phenomenal growth in industrial and academic interest has occurred.

Biologically active cations such as quaternary ammonium compounds and anions such as acesulfamate and saccharinate have previously been used to form ILs. These were paired with traditional IL-forming counter ions to control physical properties, especially melting point, but with a focus on preparing ILs, not on preparing biologically active salts. Specific ILs with biologically active ions have been prepared and shown to retain their biological activity; specifically, IL salts of anti-bacterial quaternary ammonium cations were shown to be active against various types of bacteria and in some cases, an increased anti-bacterial effect was observed.

Since ILs are composed of a minimum of two ions (a cation and an anion), both may impart biological activity to the resulting salt. This dual functionality inherent in ILs is rarely exploited. Given that polymorphism and solvate formation cannot be predicted; that the exact crystalline state affects chemical (e.g., dissolution rate, solubility), biological (e.g., bioavailability, pharmacokinetics), mechanical, and physical properties, as well as, manufacturing processes; and that polymorphs and solvates may inconveniently interconvert—what are needed are chemical compositions that a) are effective for their intended purpose; b) have controlled and tunable chemical, biological, and physical properties, c) are in a form that is not subject to polymorphism, and d) for which controlled tunable dissolution and solubility are possible, which are basic traits of ionic liquids.

A modular IL strategy could potentially revolutionize the pharmaceutical and medical industries and provide a platform for improved activity, new treatment options, or even personalized medication; or it could lead to such complexity that these materials would never be accepted as pharmaceuticals. The possibility to overcome problems such as polymorphism, solubility, and bioavailability, that have stopped the use of many proposed pharmaceuticals, might give abandoned APIs a second life; or speed the adoption of new candidate drugs. While the future of ILs as potential APIs cannot be certainly predicted, the pharmaceutical industry should join the growing number of other industries that are considering the use of ILs as materials for many different process options, and not just as solvents.