The discovery of new cocrystals of 5-fluorocytosine using amine-carboxylate supramolecular synthon

Supramolecular synthon strategy is an often-employed principle to guide the discovery of novel pharmaceutical cocrystals. The amine-carboxylate (N-F center dot center dot center dot-OOC) intermolecular interaction has been confirmed as a member of the supramolecular toolbox, and a cocrystal of 5-fluorocytosine (FC) with L-praline (FC-L-PRO) containing this hetemsynthon was previously harvested. Herein, two new cocrystals of FC with sarcosine (SAR) or dimethylglycine (DMG) were successfully screened out. Similar to FC-L-PRO, the N-H center dot center dot center dot-OOC synthon presents as the most important hetem-molecular interaction between FC and SAR/DMG in their crystal structures. And the FC molecular ribbons via homo-molecular N-H center dot center dot center dot N and N-H center dot center dot center dot O hydrogen bonds were observed in all of the cocrystal structures, which also exist in the structure of FC. Theoretical calculations show that the formations of FC-L-PRO and FC-DMG are thermodynamically favorable. While for FC-SAR-MeOH, the calculated result is not applicable. The coformer competitive experiments reveal that FC-L-PRO and FC-DMG have similar relative stability and FC-SAR-MeOH is the relative unstable form. These cocrystals exhibit weakened physical stability under high humidity condition (75% RH), then should be stored in controlled humidity conditions (<= 50% RH). Additionally, FC-L-PRO demonstrates reduced intrinsic dissolution rate (IDR) in pH 1.2 medium (simulated gastric fluid), which may provide positive contribution to alleviate its toxic side effects.