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The search for an optimized solid form was conventionally restricted to the free drug or associated salts; from which the form with the best physico-chemical properties was chosen. However, today the options in the search for the optimised solid form can be increased by co-crystals which, in simplistic terms, consist of two or more components that are solid at room temperature. This characterization is well-suited for describing new solid forms for pharmaceutical applications, however, co-crystals are different from salts which have a proton transferred from the acidic to the basic functionality of the crystallization partner, as the pKa difference between the partners is sufficiently large, no such transfer takes place in co-crystals. Co-crystals exhibit different properties than free drugs or salts and consequently have different physico-chemical properties such as solubility, dissolution rate, melting point, chemical stability and hygroscopicity, which can result in a form with much better properties, and therefore represent a class of compounds that should be investigated in the search for the optimal solid form.
