A Series of Analogues to the AT2R Prototype Antagonist C38 Allow Fine Tuning of the Previously Reported Antagonist Binding Mode

Abstract

We here report on our continued studies of ligands binding to the promising drug target angiotensin II type 2 receptor (AT₂R). Two series of compounds were synthesized and investigated. The first series explored the effects of adding small substituents to the phenyl ring of the known selective nonpeptide AT₂R antagonist C38, generating small but significant shifts in AT₂R affinity. One compound in the first series was equipotent to C38 and showed similar kinetic solubility, and stability in both human and mouse liver microsomes. The second series was comprised of new bicyclic derivatives, amongst which one ligand exhibited a five-fold improved affinity to AT₂R as compared to C38. The majority of the compounds in the second series, including the most potent ligand, were inferior to C38 with regard to stability in both human and mouse microsomes. In contrast to our previously reported findings, ligands with shorter carbamate alkyl chains only demonstrated slightly improved stability in microsomes. Based on data presented herein, a more adequate, tentative model of the binding modes of ligand analogues to the prototype AT₂R antagonist C38 is proposed, as deduced from docking redefined by molecular dynamic simulations.

Keywords: AT2 receptor; angiotensin II; medicinal chemistry; molecular docking; prototype antagonist.

Figure 1

The first selective nonpeptide AT₂R agonist C21, the structurally related AT₂R antagonist C38, and the AT₂R antagonist EMA401. [a] AT₂R from pig uterus membrane assay. [b] HEK‐293 cells expressing human AT₂R. [c] See Ref. [19].

Scheme 1

Synthesis of AT₂R ligands 40–51. a) ZnCl₂, isobutylmagnesium chloride, Pd(t‐Bu₃P)₂, toluene, THF; b) 1. n‐BuLi (4.5 eq.), triisopropyl borate, THF; 2. methyliminodiacetic acid, DMSO, toluene; c) imidazole, DCM (to 16–17) ; d) 1. TEA, MsCl, DCM; 2. imidazole, DMF (to 18–19, 22–23, 25–27); e) 1. thionyl chloride, DCM; 2. imidazole DMF (to 20–21, 24); f) PdCl₂(dppf), K₂CO₃, DME, H₂O (to 28–33, 35–39); g) Pd(PPh₃)₄, K₂CO₃, EtOH, H₂O, THF (to 34); h) 1. TFA; 2. butyl chloroformate, Na₂CO₃, DCM, H₂O.

Scheme 2

Synthesis of compounds 57–64. a) Acetic anhydride, K₂CO₃, MeCN (to 57–61); b) cyclopropanecarbonyl chloride, DIPEA, DCM (to 62); c) methyl chloroformate, DIPEA, DCM (to 63); d) MsCl, DIPEA, DCM (to 64).

Scheme 3

Synthesis for compounds 68–70. e) 1. Thionyl chloride, DMF, toluene; 2. methylamine, H₂O (to 66); f) 1. thionyl chloride, DMF, toluene; 2. ethylamine, H₂O (to 67); g) paraformaldehyde, Eaton's reagent; h) NaH, MeI, DME.

Scheme 4

Synthesis of AT₂R ligands 83–96. i) PdCl₂(dppf), K₂CO₃, DME, H₂O; j) 1. TFA; 2. butyl chloroformate, TEA, DCM (to 83–87); k) 1. TFA; 2. ethyl chloroformate, DMAP, DIPEA, DCM (to 88–96).

Figure 2

Compound 41 (blue) biding to the AT₂ receptor (gray), overlaid with the co‐crystalized ligand L‐161,638 (PDB 5UNG, gray sticks).

Figure 3

The docked ligands (40–45) in the most common pose on the modeled conformation of the AT₂R. The ligands are color coded based on the binding affinities, blue – high, green – moderate, and orange – low binding affinity. The N‐terminal, EL3 and parts of TM6‐TM7 of the AT₂R are not shown for better clarity.