Design, synthesis, and characterization of new 5-fluorocytosine salts

5-Fluorocytosine (FC), an antifungal drug and a cytosine derivative, has a complex solid-state landscape that challenges its development into a drug product. A total of eight new FC salts, both cytosinium and hemicytosinium, with four strong acids were prepared by controlling acid concentration in the crystallization medium. The pharmaceutically acceptable saccharin salt of FC exhibits superior phase stability and, hence, has the potential to address the instability problem of FC associated with hydration.