Glutathione-responsive PROTAC for targeted degradation of ERα in breast cancer cells

ER alpha (estrogen receptor-alpha)-targeting PROTACs (PROteolysis TArgeting Chimeras) have emerged as a novel and promising modality for breast cancer therapeutics. However, ER alpha PROTACs-induced degradation in normal tissues raises concerns about potential off-tissue toxicity. Tumor microenvironment-responsive strategy provides potential for specific control of the PROTAC's on-target degradation activity. The glutathione (GSH) level has been reported to be significantly increased in tumor cells. Here, we designed a GSH-responsive ER alpha PROTAC, which is generated by conjugating an o-nitrobenzenesulfonyl group to the hydroxyl group of VHLbased ER alpha PROTAC through a nucleophilic substitution reaction. The o-nitrobenzenesulfonyl group as a protecting group blocks the bioactivity of ER alpha PROTAC (ER P1), and that can be specifically recognized and removed by highly abundant GSH in cancer cells. Consequently, the GSH-responsive ER alpha PROTAC (GSH-ER-P1) exhibits significantly enhanced degradation of ER alpha in cancer cells compared to that in normal cells, leading to a remarkable inhibition of breast cancer cell proliferation and less toxic effects on normal cells. This study provides a potentially valuable strategy for breast cancer treatment using tumor microenvironmentresponsive PROTACs.