Revealing Local Structure of Angiotensin Receptor-Neprilysin Inhibitor (S086) Drug Cocrystal by Linear and Nonlinear Infrared Spectroscopies

Structurally knowing the active sites of a drug is important for understanding its therapeutic functions. S086 is a novel angiotensin receptor-neprilysin inhibitor that consists of the molecular moieties of EXP3174 (the active metabolite of the angiotensin receptor blocker losartan) and sacubitril (a neprilysin inhibitor prodrug) in a 1:1 molar ratio. There are two forms of cocrystals of S086, namely, ξ-crystal and α-crystal, which were formed both via intermolecular coordination bonding to calcium ions, with the aid of internal water. The binding state of multiple carboxyl anions (COO^-) to Ca²⁺ of EXP3174 and sacubitril was examined in this study using infrared (IR) absorption spectroscopy, in which the asymmetric stretching (<i>as</i>) and symmetric stretching (<i>ss</i>) modes of the COO^- groups were used as IR probes. Ultrafast two-dimensional (2D) IR spectroscopy was utilized for spectrally assigning the origin of multiple COO^- groups by the presence or absence of interchromophore vibrational coupling. Key structural variation between the two crystal forms was found: in the unit cell of ξ-crystal, the ratio of "bridging" and "bidentate" types of COO^- binding to Ca²⁺ for four EXP3174 molecules is 2:2, while the ratio is predicted to be 3:1 in the case of α-crystal. However, in both crystals, four sacubitril molecules are believed to similarly form a "trident" type of COO^- binding to Ca²⁺. Our study demonstrates that linear and nonlinear IR spectroscopies can be used to characterize local crystal structures of drugs and reveal subtle difference between similar crystal structures.