Xin Chen

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.

A commercial process for vepdegestrant (1), the most advanced PROTAC protein degrader in human clinical trials, has been developed to support clinical and commercial needs. The process features an efficient convergent synthetic strategy through the final reductive amination of two advanced chiral intermediates, as well as several highly efficient telescoped processes and robust crystallization for purity control. The final commercial process of vepdegestrant (1) consists of seven proposed regulatory GMP steps with five isolations in an overall yield of 29%.

<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>

<p>Extended Methods</p>