Combined Peptide and Small-Molecule Approach toward Nonacidic THIQ Inhibitors of the KEAP1/NRF2 Interaction

Abstract

The NRF2-ARE pathway is an intrinsic mechanism of defense against oxidative stress. Inhibition of the interaction between NRF2 and its main negative regulator KEAP1 is an attractive strategy toward neuroprotective agents. We report here the identification of nonacidic tetrahydroisoquinolines (THIQs) that inhibit the KEAP1/NRF2 protein-protein interaction. Peptide SAR at one residue is utilized as a tool to probe structural changes within a specific pocket of the KEAP1 binding site. We used structural information from peptide screening at the P2 pocket, noncovalent small-molecules inhibitors, and the outcome from an explorative SAR at position 5 of THIQs to identify a series of neutral THIQ analogs that bind to KEAP1 in the low micromolar range. These analogs establish new H-bond interactions at the P3 and P2 pockets allowing the replacement of the carboxylic acid functionality by a neutral primary carboxamide. X-ray crystallographic studies reveal the novel binding mode of these molecules to KEAP1.

Figure 1

Structures of noncovalent inhibitors of KEAP1/NRF2 PPI.

Figure 2

Crystal structure of 6 (green) bound to KEAP1 (KEAP1 [325–609]: 6; PDB code 6SP1) overlaid with (A) docked peptide 13 (blue) and docked peptide 17 (m-phenylcarboxamide) (blue carbon atoms).

Figure 3

(A) Crystal structure of 23 (pink carbon atoms) bound to KEAP1 (KEAP1[324–609]:23; PDB code 6SP4). (B) Overlaying of X-ray structures of compound 23 (pink) and compound 6 (green) bound to KEAP1.

Scheme 1. Synthesis of Compounds 6–11 and 19–23

Reagents and conditions: (a) BBr₃, DCM, rt, 18 h; (b) RCO₂H, HATU, DIPEA, DMF, rt or 60 °C, 18 h; (c) H₂ (1 atm), Pd/C, MeOH or EtOAc, rt; (d) ROH, PS–PPh₃, THF, 0 °C to rt; (e) NH₃, TBTU, DMF, rt, 1 h; (f) Tf₂O, TEA, DCM, −15 °C, 30 min; (g) alkyne, CuI, (Ph₃P)₂Cl₂Pd, TBAI, TEA, DMF, 100 °C, 1 h; (h) TFA/DCM (1:10), 0 °C to rt.