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

Abstract

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 (as) and symmetric stretching (ss) 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.

Scheme 1. Chemical Composition of S086

From Left to Right: Sacubitril, EXP3174, Calcium Ion, and Water, with Molar Ratio = 1:1:1.5:2.

Figure 1

FTIR spectrum of S086 in the ξ-crystal form in CH₂Cl₂ (a) and the α-crystal form in acetone (b) in the spectral range of 1500–1650 cm^–1, with peak area normalized, and spectra fitted using Voigt functions. In particular, two asymmetric stretching modes of carboxylic groups in EXP3174 are estimated with an area ratio of 1:1 in (a) and 1:3 in (b). The strong and sharp peak is due to the asymmetric stretching mode of carboxylic acid, and the high-frequency peak is assigned to the amide-I mode in sacubitril.

Figure 2

Illustration of a COO^– group and its binding with a calcium ion in different forms. The frequency difference between asymmetric and symmetric COO^– stretching vibration modes (Δυ_a-s = υ_as – υ_ss) is in ascending order from left to right.

Figure 3

Calculated frequency separation between the asymmetric and symmetric COO^– stretching frequencies for the deuterated acetic acid (CD₃COO^–) in dichloromethane (CH₂Cl₂, black) and dimethyl sulfoxide (DMSO, red) solutions with calcium ions to form the “unidentate,” “bridging,” “bidentate,” and “trident” structures, in comparison to that of the “ionic” form.

Figure 4

Asymmetric and symmetric COO^– vibrational frequency difference analysis. Δυ_a-s (ionic) (black dashed lines) were obtained from EXP3174: sacubitril = 1:1 (without Ca²⁺) in CH₂Cl₂ (a and c) or in acetone (b and d), and Δυ_{a-s (sample)} (colored solid lines) were obtained from S086 in the ξ-crystal form in CH₂Cl₂ (a and c) and the α-crystal form in acetone (b and d). Pink lines: derived from the low-frequency as-COO^– modes of EXP3174; red solid lines: derived from the high-frequency as-COO^– modes of EXP3174; blue solid lines: derived from the as-COO^– modes of sacubitril. Arrows show the values of [Δυ_{a-s (sample)} – Δυ_a-s (ionic)]. Data were summarized in Tables S5 and S6 of SI. See the text for details.

Figure 5

Illustration of local structures in the ξ-crystal form (a and c) and the α-crystal form (b and d) of S086 deduced from the vibrational frequency analysis of the carboxyl groups of EXP3174 (upper row) and sacubitril (lower row). Six Ca²⁺ ions are numbered in (b). Internal water molecules are ignored for simplification in (a) and are unknown in (b).