Salt disproportionation: A material science perspective
While screening the counter-ions for salt selection for an active pharmaceutical substance, there is often an uncertainty about disproportionation of the salt and hence physical stability of the final product formulation to provide adequate shelf life. Several examples of disproportionation reactions are reviewed to explain the concepts of pHmax, microenvironmental pH, and buffering capacity of excipients and APIs to gain mechanistic understanding of disproportionation reaction. Miscellaneous factors responsible for disproportionation are examined. In addition to the dissolution failure due to the formation of less soluble unionized form, various implications of the disproportionation are evaluated with specific examples. During lead optimization and early stages of development, when only a limited amount of material is available, use of predictive tools like mathematical models and model free kinetics to rank order the various counter-ions are discussed in detail. Finally, analytical methods and mitigation strategies are discussed to prevent the disproportionation by detecting it during early stages of drug development.