Saccharin salts of active pharmaceutical ingredients, their crystal structures, and increased water solubilities

Abstract

Salts of active pharmaceutical ingredients (APIs) have been traditionally used in drug formulations because of improved properties with respect to solubility, stability, or bioavailability. Saccharin has been used as an acid for salt formation with APIs in a few cases hitherto. In this paper, we have explored the generality of this property and have isolated saccharinates of quinine, haloperidol, mirtazapine, pseudoephedrine, lamivudine, risperidone, sertraline, venlafaxine, zolpidem, and amlodipine. These salts have been characterized with single-crystal X-ray methods. The structures contain many hydrogen bonds of the O-H···N^(-), N⁽⁺⁾-H···N^(-), N⁽⁺⁾-H···O, N-H···O, O-H···O and N-H···N type, with auxiliary C-H···N^(-) and C-H···O interactions. These saccharinates are mostly very soluble in water when compared to the free base. Additionally, aqueous solutions of these API saccharinates are of moderate pH. Both these properties may be advantageous in the pharmaceutical industry. In general, most saccharinates would appear to have high water solubility, and this follows from the molecular structure of the anion, which is donor-poor and acceptor-rich in terms of hydrogen-bonding functionalities. If an API of insufficient basicity is treated with saccharin, it may form a hydrogen-bonded cocrystal wherein proton transfer from saccharin to the API does not take place. This phenomenon was found in the cocrystal saccharin-piroxicam.