Photo‐click Proteolysis‐Targeting Chimeras Enable Intracellular Generation of PROTACs for Precise Dual Protein Degradation

ABSTRACT Proteolysis‐targeting chimeras (PROTACs) are an emerging therapeutic modality via targeted protein degradation, but plagued by concerns about systemic toxicity and a poor pharmacokinetic profile. To tackle the issues, we have devised a photo‐click proteolysis targeting chimera (PCPTAC) that enables spatiotemporally controllable intracellular synthesis of PROTACs by photo‐triggered bioorthogonal ligation. A photocaged dibenzosilacycloheptyne (photo‐DBSH) and the complementary azide were deployed to tag the oncoprotein ligands (i.e., (+)‐JQ1 for BRD4 and Olaparib for PARP1) and the E3 ligase ligand (i.e., Pomalidomide for CRBN), respectively, for a proof‐of‐concept study and potential treatment for triple‐negative breast cancer (TNBC). Upon light irradiation, photo‐DBSH‐JQ1/‐Olap was rapidly uncaged to give the reactive cycloalkyne‐JQ1/‐Olap, which immediately underwent a strain‐promoted azide–alkyne cycloaddition with azide‐Pomalidomide, in situ generating dual PROTACs for simultaneous degradation of BRD4 and PARP1 in TNBC MDA‐MB‐231 cells, with 25‐ and 2.4‐fold more potent antiproliferative activity than the un‐irradiated inhibitors and the corresponding PROTAC combination, respectively (IC 50 = 0.032 µM vs. 0.846 µM and 0.075 µM). Further in zebrafish models, PCPTAC promoted BRD4 degradation leading to thinner yolk sac extension and achieved 94% tumor inhibition in HeLa xenografts. This split‐and‐photoclick strategy paves a new avenue for developing safer and more efficacious PROTACs with synergistic antitumor effects.