A novel S-nitrosocaptopril monohydrate for pulmonary arterial hypertension: H2O and –SNO intermolecular stabilization chemistry

S-nitrosocaptopril (CapNO) possesses dual capacities of both Captopril and an NO donor with enhanced efficacy and reduced side effects. CapNO crystals are difficult to make due to its unstable S-NO bond. Here, we report a novel stable S-nitrosocaptopril monohydrate (CapNO·H2O) that is stabilized by intermolecular five-membered structure, where one H of H2O forms a hydrogen bond with O- of the stable resonance zwitterion Cap-S+=N-O-, and the O in H2O forms the dipole-dipole interaction with S+ through two unpaired electrons. With the chelation and common ion effect, we synthesized and characterized CapNO·H2O that is stable at 4 °C for 180 days and thereafter without significant degradation. Compared to Captopril, CapNO showed direct vasorelaxation and beneficial effect on PAH rats, and could be self-assembled in rat stomach when Captopril and NaNO2 were given separately. This novel CapNO·H2O with low entropy paves an avenue for its clinical trials and commercialization.