Ligand-embedded photoswitching PROTAC for spatiotemporal tubulin degradation

This study developed a novel light-switchable proteolysis-targeting chimera (PROTAC) by integrating azobenzene-modified combretastatin A4 (Azo-CA4) as a photocontrollable tubulin ligand. In contrast to conventional light-regulated PROTACs that modulate linker conformation, our strategy embeds the photoswitch directly within the target protein ligand (Azo-CA4). Under 365 nm UV light, Azo-CA4 isomerizes to its cis-configuration, enabling high-affinity tubulin binding and subsequent ubiquitin-proteasome-dependent degradation. The lead compound AC2 exhibited pronounced light-dependent antitumor activity against triple-negative breast cancer (MDA-MB-231 cells), with a 15-fold enhancement in potency (IC₅₀ = 4.05 ± 0.13 μM under UV vs. 63.64 μM in the dark). Furthermore, AC2 exhibited minimal toxicity in normal breast epithelial cells (MCF-10A) under both light and dark conditions (IC₅₀ > 100 μM), highlighting its favorable selectivity. Mechanistic analyses established reversible β-tubulin degradation, ubiquitin-proteasome system (UPS) dependency (inhibited by MG132), and robust ternary complex formation (binding energy: −5.96 kcal/mol). ADMET profiling indicated moderate membrane permeability (Log P_o/w = 3.19) but this permeability limited oral bioavailability, attributable to its high-molecular-weight (645 Da) and poor solubility. This ligand-embedded approach enhances spatiotemporal precision while mitigating off-target toxicity, establishing a novel therapeutic paradigm for targeted cancer therapy.