Pharmaceutical salts and cocrystals involving amino acids: A brief structural overview of the state-of-art

Salification of new drug substances in order to improve physico-chemical or solid-state properties (e.g. dissolution rate or solubility, appropriate workup process, storage for further industrial and marketing development) is a well-accepted procedure. Amino acids, like aspartic acid, lysine or arginine take a great part in this process and are implicated in several different formulations of therapeutic agent families, including antibiotics (amoxicillin from beta lactam class or cephalexin from cephalosporin class), NSAIDs (ketoprofen, ibuprofen and naproxen from profen family, acetylsalicylic acid) or antiarrhythmic agents (e.g. ajmaline). Even if more than a half of known pharmaceutical molecules possess a salifiable moiety, what can be done for new potential drug entity that cannot be improved by transformation into a salt? In this context, after a brief review of pharmaceutical salts on the market and the implication of amino acids in these formulations, we focus on the advantage of using amino acids even when the target compound is not salifiable by exploiting their zwitterionic potentialities for cocrystal edification. We summarize here a series of new examples coming from literature to support the advantages of broadening the application of amino acids in formulation for new drug substances improvement research for non-salifiable molecules.