Reversible Spatiotemporal Control of Induced Protein Degradation by Bistable photoPROTACs

Off-target effects are persistent issues of modern inhibition-based therapies. By merging the strategies of photopharmacology and small molecule degraders, we introduce a novel concept for persistent spatiotemporal control of induced protein degradation that potentially prevents off-target toxicity. Building on the successful principle of bifunctional all-small molecule Proteolysis Targeting Chimeras (PROTACs), we designed photoswitchable PROTACs (photoPROTACs) by including ortho-F₄-azobenzene linkers between both warhead ligands. This highly bistable yet photoswitchable structural component leads to reversible control over the topological distance between both ligands. The azo-cis-isomer is observed to be inactive because the distance defined by the linker is prohibitively short to permit complex formation between the protein binding partners. By contrast, the azo-trans-isomer is active because it can engage both protein partners to form the necessary and productive ternary complex. Importantly, due to the bistable nature of the ortho-F₄-azobenzene moiety employed, the photostationary state of the photoPROTAC is persistent, with no need for continuous irradiation. This technique offers reversible on/off switching of protein degradation that is compatible with an intracellular environment and, therefore, could be vastly useful in experimental probing of biological signaling pathways – especially those crucial for oncogenic signal transduction. Additionally, this strategy may be suitable for therapeutic implementation in a wide variety of disease phenotypes. By enabling reversible activation and deactivation of protein degradation, photoPROTACs offer advantages over conventional photocaging strategies that irreversibly release active agents.