Gan Wang

PURPOSE: Estrogen receptor (ER) alpha signaling is a known driver of ER-positive (ER+)/human epidermal growth factor receptor 2 negative (HER2-) breast cancer. Combining endocrine therapy (ET) such as fulvestrant with CDK4/6, mTOR, or PI3K inhibitors has become a central strategy in the treatment of ER+ advanced breast cancer. However, suboptimal ER inhibition and resistance resulting from the ESR1 mutation dictates that new therapies are needed. EXPERIMENTAL DESIGN: A medicinal chemistry campaign identified vepdegestrant (ARV-471), a selective, orally bioavailable, and potent small molecule PROteolysis-TArgeting Chimera (PROTAC) degrader of ER. We used biochemical and intracellular target engagement assays to demonstrate the mechanism of action of vepdegestrant, and ESR1 wild-type (WT) and mutant ER+ preclinical breast cancer models to demonstrate ER degradation-mediated tumor growth inhibition (TGI). RESULTS: Vepdegestrant induced ≥90% degradation of wild-type and mutant ER, inhibited ER-dependent breast cancer cell line proliferation in vitro, and achieved substantial TGI (87%-123%) in MCF7 orthotopic xenograft models, better than those of the ET agent fulvestrant (31%-80% TGI). In the hormone independent (HI) mutant ER Y537S patient-derived xenograft (PDX) breast cancer model ST941/HI, vepdegestrant achieved tumor regression and was similarly efficacious in the ST941/HI/PBR palbociclib-resistant model (102% TGI). Vepdegestrant-induced robust tumor regressions in combination with each of the CDK4/6 inhibitors palbociclib, abemaciclib, and ribociclib; the mTOR inhibitor everolimus; and the PI3K inhibitors alpelisib and inavolisib. CONCLUSIONS: Vepdegestrant achieved greater ER degradation in vivo compared with fulvestrant, which correlated with improved TGI, suggesting vepdegestrant could be a more effective backbone ET for patients with ER+/HER2- breast cancer.

Heterobifunctional proteolysis targeting chimeras (PROTACs) are proven to degrade disease-causing proteins, and many PROTACs have already entered into clinical trials. The majority of these PROTACs recruit cereblon (CRBN) or von Hippel-Lindau (VHL) substrate receptors of cullin RING E3 ubiquitin ligases, but there remains a need for alternative E3 ligase ligands. In this study, we enable DDB1 as an E3 ligase adapter protein for PROTAC drug discovery, describe a DNA-encoded library (DEL) ligand discovery campaign, and report the identification of a novel DDB1 ligand. Structure-guided modifications allowed DDB1 ligands to be developed from the initial DEL hit with nanomolar potency. Biochemical assays, cellular target engagement, and X-ray crystallography analysis demonstrated binding of the ligand to a unique pocket within DDB1. This chemical series furthers our understanding of ligand binding pockets within DDB1 and expands the repertoire of small molecules that may be suitable for the incorporation into PROTACs.

<div>AbstractPurpose:<p>Estrogen receptor (ER) alpha signaling is a known driver of ER-positive (ER<sup>+</sup>)/human epidermal growth factor receptor 2 negative (HER2<sup>−</sup>) breast cancer. Combining endocrine therapy (ET) such as fulvestrant with CDK4/6, mTOR, or PI3K inhibitors has become a central strategy in the treatment of ER<sup>+</sup> advanced breast cancer. However, suboptimal ER inhibition and resistance resulting from the <i>ESR1</i> mutation dictates that new therapies are needed.</p>Experimental Design:<p>A medicinal chemistry campaign identified vepdegestrant (ARV-471), a selective, orally bioavailable, and potent small molecule PROteolysis-TArgeting Chimera (PROTAC) degrader of ER. We used biochemical and intracellular target engagement assays to demonstrate the mechanism of action of vepdegestrant, and <i>ESR1</i> wild-type (WT) and mutant ER<sup>+</sup> preclinical breast cancer models to demonstrate ER degradation-mediated tumor growth inhibition (TGI).</p>Results:<p>Vepdegestrant induced ≥90% degradation of wild-type and mutant ER, inhibited ER-dependent breast cancer cell line proliferation <i>in vitro,</i> and achieved substantial TGI (87%–123%) in MCF7 orthotopic xenograft models, better than those of the ET agent fulvestrant (31%–80% TGI). In the hormone independent (HI) mutant ER Y537S patient-derived xenograft (PDX) breast cancer model ST941/HI, vepdegestrant achieved tumor regression and was similarly efficacious in the ST941/HI/PBR palbociclib-resistant model (102% TGI). Vepdegestrant-induced robust tumor regressions in combination with each of the CDK4/6 inhibitors palbociclib, abemaciclib, and ribociclib; the mTOR inhibitor everolimus; and the PI3K inhibitors alpelisib and inavolisib.</p>Conclusions:<p>Vepdegestrant achieved greater ER degradation <i>in vivo</i> compared with fulvestrant, which correlated with improved TGI, suggesting vepdegestrant could be a more effective backbone ET for patients with ER<sup>+</sup>/HER2<sup>−</sup> breast cancer.</p></div>