Design, Synthesis and Pharmacological Characterization of the First Photoswitchable Small-Molecule Agonist for the Atypical Chemokine Receptor 3

Photopharmacology offers the promise of optical modulation of cellular signaling in a spatially and temporally controlled fashion with light-sensitive molecules. This study presents the first small-molecule photoswitchable agonist for an atypical G protein-coupled receptor (GPCR), the atypical chemokine receptor 3 (ACKR3). Inspired by a known benzylpiperidine-based ACKR3 agonist scaffold, 12 photoswitchable azobenzene-containing analogs were synthesized and characterized for their interaction with ACKR3. After analysis of concise Structure-Photochemistry and Structure-Affinity Relationships (SAR), compound 3e was selected as the best photoswitchable ACKR3 agonist in the series. Compound 3e can be effectively switched from its thermodynamically stable trans state to the less active cis-isomer with a photostationary state of 96%. The thermodynamically less stable cis-3e only slowly switches back to the trans state (t_1/2,37°C ∼ 15 days), and trans-3e binds and activates ACKR3 at 10-fold lower concentrations compared to its cis-isomer. Compound 3e demonstrates selectivity for ACKR3 within in a wide panel of chemokine receptors. Using the recently published ACKR3 cryo-EM structures in computational studies, a binding mode for trans-3e is proposed that is perfectly in line with the observed SAR and the reduction in interaction with ACKR3 upon photoswitching. ACKR3 agonist 3e (VUF25471) is the first photoswitchable ligand for an atypical GPCR and will be a useful tool to investigate the role of ACKR3 in biological settings.