Engineering ERα degraders with pleiotropic ubiquitin ligase ligands maximizes therapeutic efficacy by co-opting distinct effector ligases

Proximity-inducing compounds that modulate target protein homeostasis are an emerging therapeutic strategy [1]. While the inherent complexity of these bifunctional compounds poses challenges for rational design and bioavailability, their composition also provides opportunities to co-opt specific cellular proteins to maximize therapeutic impact. Here, we systematically evaluate the cellular efficacy, biophysical mechanisms, and therapeutic benefits of a series of bifunctional degrader compounds, that are all engineered with the Estrogen Receptor-alpha (ERα)-inhibitor endoxifen linked to different bioactive ubiquitin ligase ligands. Bifunctional ERα degraders that incorporate CRL4-CRBN-binding ligands promoted the most potent ERα degradation, whereas those incorporating either CRL2-VHL- or IAP-binding ligands maximized the depth of ERα degradation. Notably, ERα degraders containing pan-IAP antagonist ligands significantly decreased the proliferation of ERα-dependent cells relative to clinical-stage ERα-degraders, including the SERDs fulvestrant and GDC-9545 and the bifunctional degrader ARV-471. Mechanistic studies revealed that pan-IAP antagonist-based ERα degraders uniquely promote TNFα-dependent cell death, unlike the clinical-stage comparators. Remarkably, the pan-IAP antagonist-ERα-degraders co-opt distinct effector ligases to achieve dual therapeutic effects: they harness XIAP within tumor cells to promote ERα degradation, and activate cIAP1/2 within tumor and immune cells to induce TNFα that drives tumor cell death. Our studies demonstrate a broader concept that co-opting the discrete functions of a selected set of cellular effectors, while simultaneously modulating therapeutic target protein homeostasis, are dual strategies that can be leveraged to maximize the efficacy of induced proximity therapeutics.