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 <b>3e</b> was selected as the best photoswitchable ACKR3 agonist in the series. Compound <b>3e</b> can be effectively switched from its thermodynamically stable <i>trans</i> state to the less active <i>cis</i>-isomer with a photostationary state of 96%. The thermodynamically less stable <i>cis-</i> <b>3e</b> only slowly switches back to the <i>trans</i> state (<i>t</i> _1/2,37°C ∼ 15 days), and <i>trans</i>-<b>3e</b> binds and activates ACKR3 at 10-fold lower concentrations compared to its <i>cis</i>-isomer. Compound <b>3e</b> 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 <i>trans</i>-<b>3e</b> is proposed that is perfectly in line with the observed SAR and the reduction in interaction with ACKR3 upon photoswitching. ACKR3 agonist <b>3e</b> (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.