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Background: HR+/HER2– mBC, the most common subset of breast cancer, is treated with sequential endocrine therapy + targeted agents followed by sequential single-agent chemotherapy (CT), with increasingly shorter benefit duration with each subsequent treatment. High Trop-2 expression is observed in breast cancer regardless of subtype. SG is a Trop-2-directed antibody-drug conjugate approved for pre-treated metastatic triple-negative breast cancer. In the phase 3 TROPiCS-02 study, SG showed both significant progression-free survival (PFS) benefit (HR, 0.66; P10- 0. Of Trop-2-evaluable pts, 192 (42%) and 270 (58%) had H-scores 10- Table Citation Format: Hope Rugo, Aditya Bardia, Frederik Marmé, Javier Cortés, Peter Schmid, Delphine Loirat, Olivier Trédan, Eva Ciruelos, Florence Dalenc, Patricia Gómez Pardo, Komal Jhaveri, Monica Motwani, Oh Kyu Yoon, Hao Wang, Wendy Verret, Sara Tolaney. Sacituzumab Govitecan (SG) vs Treatment of Physician’s Choice (TPC): Efficacy by Trop-2 Expression in the TROPiCS-02 Study of Patients (Pts) With HR+/HER2– Metastatic Breast Cancer (mBC) [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr GS1-11.

Introduction: Phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) is mutated in ~40% of patients (pts) with hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2−) advanced breast cancer (ABC). PIK3CA mutations are associated with resistance to endocrine therapy (ET) and worse overall survival. Alpelisib (ALP), an α-selective PI3K inhibitor and degrader, is indicated in combination with fulvestrant (FUL) for pts with PIK3CA-mutated (mut) HR+, HER2− ABC following progression on/after ET-based treatments. In the Phase 2, open-label, 3-cohort, noncomparative BYLieve study, clinical benefit of ALP in combination with ET was observed in the post-cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) setting in pts with PIK3CA-mut, HR+, HER2− ABC. Here we report the results of a biomarker analysis using paired baseline (Cycle 1 Day 1) and end-of-treatment (EOT) circulating tumor DNA (ctDNA) samples from pts in BYLieve Cohorts A and B. Methods: In the BYLieve study, pts with PIK3CA-mut, HR+, HER2− ABC had CDK4/6i + aromatase inhibitor (Cohort A; N=127) or CDK4/6i + FUL (Cohort B; N=126) as treatment immediately prior to receiving ALP + FUL and ALP + letrozole, respectively. In this biomarker analysis, gene alterations were detected in ctDNA at baseline and EOT using next-generation sequencing (PanCancer V2 panel). Pts included in this interim analysis had confirmed PIK3CA mutations and matched baseline/EOT samples with enough sequencing coverage and ctDNA fraction to detect mutations at both time points. ctDNA fractions, tumor mutation burden (TMB) distributions, genomic landscapes, gain/loss of PIK3CA and estrogen receptor 1 (ESR1), chromosome 8/11 amplification profiles, and alterations in PI3K pathway and potential CDK4/6i resistance markers were assessed across time points. Sample sizes were small; results should thus be interpreted with caution. Results: Forty-three pts were included in the Cohort A biomarker population and 40 pts were included in Cohort B. ctDNA fraction was numerically higher at EOT compared with baseline in both cohorts; further analyses will be presented. In Cohort A, no significant differences were observed in TMB at EOT compared with baseline (P=0.21). In Cohort B, TMB was higher at EOT compared with baseline (P=0.053). Chromosome 8/11 amplifications were consistent between baseline and EOT for both cohorts. Small variations were observed in ESR1/PIK3CA mutations between baseline and EOT on both cohorts (Table). The status of potential CDK4/6i resistance markers was relatively unchanged at EOT (Table). Loss-of-function mutations in PTEN, a known PI3K inhibitor resistance marker, increased from 9% at baseline to 14% at EOT in Cohort A and from 12% at baseline to 22% at EOT in Cohort B. Conclusions: Between baseline and EOT, only small variations in gene alterations in PIK3CA-mutated HR+, HER2– ABC were observed in the post-CDK4/6i setting. As the disease progressed, increases in loss-of-function mutations in PTEN at EOT in both Cohorts A and B suggested loss of PTEN in PI3K pathway may drive resistance to ALP. Early intervention with ALP, when the tumor is particularly driven by PIK3CA oncogenic mutations and before it develops more genomic complexity, may potentially provide better clinical outcomes. Table. Gene Alteration Gain/Loss at Baseline/EOT Across Cohorts A and B Citation Format: Dejan Juric, Nicholas Turner, Sherene Loi, Fabrice Andre, Stephen K. Chia, Komal Jhaveri, Patrick Neven, Rebecca Dent, Eva Ciruelos, Mukta Joshi, Estelle Roux, Heather Patino, Murat Akdere, Hope Rugo. Baseline and End-of-Treatment Biomarkers in Patients With PIK3CA-Mutated, Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative Advanced Breast Cancer From BYLieve Study Cohorts A and B [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-09-12.

PURPOSE To update recommendations on appropriate use of breast cancer biomarker assay results to guide adjuvant endocrine and chemotherapy decisions in early-stage breast cancer. METHODS An updated literature search identified randomized clinical trials and prospective-retrospective studies published from January 2016 to October 2021. Outcomes of interest included overall survival and disease-free or recurrence-free survival. Expert Panel members used informal consensus to develop evidence-based recommendations. RESULTS The search identified 24 studies informing the evidence base. RECOMMENDATIONS Clinicians may use Onco type DX, MammaPrint, Breast Cancer Index (BCI), and EndoPredict to guide adjuvant endocrine and chemotherapy in patients who are postmenopausal or age > 50 years with early-stage estrogen receptor (ER)–positive, human epidermal growth factor receptor 2 (HER2)–negative (ER+ and HER2–) breast cancer that is node-negative or with 1-3 positive nodes. Prosigna and BCI may be used in postmenopausal patients with node-negative ER+ and HER2– breast cancer. In premenopausal patients, clinicians may use Onco type in patients with node-negative ER+ and HER2– breast cancer. Current data suggest that premenopausal patients with 1-3 positive nodes benefit from chemotherapy regardless of genomic assay result. There are no data on use of genomic tests to guide adjuvant chemotherapy in patients with ≥ 4 positive nodes. Ki67 combined with other parameters or immunohistochemistry 4 score may be used in postmenopausal patients without access to genomic tests to guide adjuvant therapy decisions. BCI may be offered to patients with 0-3 positive nodes who received 5 years of endocrine therapy without evidence of recurrence to guide decisions about extended endocrine therapy. None of the assays are recommended for treatment guidance in individuals with HER2-positive or triple-negative breast cancer. Treatment decisions should also consider disease stage, comorbidities, and patient preferences. Additional information is available at www.asco.org/breast-cancer-guidelines .

Overall sequencing CONSORT diagram. Flow diagram of study patients and analyzed biospecimens processed through and selected for central sequencing using MSK-IMPACT and Guardant360. cfDNA, cell-free DNA; FFPE, formalin-fixed paraffin-embedded; MSK-IMPACT, Memorial Sloan Kettering-Integrated Mutation Profiling of Actionable Cancer Targets; QC, quality control.

HER2 mutations define a subset of metastatic breast cancers with a unique mechanism of oncogenic addiction to HER2 signaling. We explored activity of the irreversible pan-HER kinase inhibitor neratinib, alone or with fulvestrant, in 81 patients with HER2-mutant metastatic breast cancer. Overall response rate was similar with or without estrogen receptor (ER) blockade. By comparison, progression-free survival and duration of response appeared longer in ER+ patients receiving combination therapy, although the study was not designed for direct comparison. Preexistent concurrent activating HER2 or HER3 alterations were associated with poor treatment outcome. Similarly, acquisition of multiple HER2-activating events, as well as gatekeeper alterations, were observed at disease progression in a high proportion of patients deriving clinical benefit from neratinib. Collectively, these data define HER2 mutations as a therapeutic target in breast cancer and suggest that coexistence of additional HER signaling alterations may promote both de novo and acquired resistance to neratinib.Significance:HER2 mutations define a targetable breast cancer subset, although sensitivity to irreversible HER kinase inhibition appears to be modified by the presence of concurrent activating genomic events in the pathway. These findings have implications for potential future combinatorial approaches and broader therapeutic development for this genomically defined subset of breast cancer.This article is highlighted in the In This Issue feature, p. 161

Supplementary data

Supplementary Methods

The phosphoinositide 3–kinase (PI3K) pathway regulates proliferation, survival, and metabolism and is frequently activated across human cancers. A comprehensive elucidation of how this signaling pathway controls transcriptional and cotranscriptional processes could provide new insights into the key functions of PI3K signaling in cancer. Here, we undertook a transcriptomic approach to investigate genome-wide gene expression and transcription factor activity changes, as well as splicing and isoform usage dynamics, downstream of PI3K. These analyses uncovered widespread alternatively spliced isoforms linked to proliferation, metabolism, and splicing in PIK3CA-mutant cells, which were reversed by inhibition of PI3Kα. Analysis of paired tumor biopsies from patients with PIK3CA-mutated breast cancer undergoing treatment with PI3Kα inhibitors identified widespread splicing alterations that affect specific isoforms in common with the preclinical models, and these alterations, namely PTK2/FRNK and AFMID isoforms, were validated as functional drivers of cancer cell growth or migration. Mechanistically, isoform-specific splicing factors mediated PI3K-dependent RNA splicing. Treatment with splicing inhibitors rendered breast cancer cells more sensitive to the PI3Kα inhibitor alpelisib, resulting in greater growth inhibition than alpelisib alone. This study provides the first comprehensive analysis of widespread splicing alterations driven by oncogenic PI3K in breast cancer. The atlas of PI3K-mediated splicing programs establishes a key role for the PI3K pathway in regulating splicing, opening new avenues for exploiting PI3K signaling as a therapeutic vulnerability in breast cancer.Significance:Transcriptomic analysis reveals a key role for the PI3K pathway in regulating RNA splicing, uncovering new mechanisms by which PI3K regulates proliferation and metabolism in breast cancer.See related commentary by Claridge and Hopkins, p. 2216

Supplemental Figure 2. Tumor PU-H71 retention at 24 hours post-injection (quantified by standard uptake value) is not explained by plasma radiotracer exposure (quantified by plasma AUC from 10 min to 4 h post-injection, see Methods), as indicated by Spearman Test R-value.

Supplementary Figures: Supplementary Figure 1. Summary of duration on AZD9496 therapy, previous therapies, and circulating biomarkers status at baseline for each patient ranked by ascending dose; Supplementary Figure 2. Tumor pharmacodynamics. Supplementary Tables: Supplementary Table 1. Study Plan showing time-points of collection of biosamples for exploratory research; Supplementary Table 2. ddPCR primers and probes;Supplementary Table 3. Antibodies and conditions for IHC analysis Antibodies for IHC analysis; Supplementary Table 4. Patients with {greater than or equal to}1 CTC/7.5 mL WB within 120 days of fulvestrant wash-out period prior C1D1.

Supplemental Figure 1. Tumor retention of PU-H71 tracer varied considerably between patients. Tumor retention expressed as tumor:bloodpool SUV ratio (y-axis) measured on PET at 4 and 24 hours post tracer-injection.

Tumor PU-H71 retention at 24 hours post-injection (quantified by standard uptake value) is not explained by plasma radiotracer exposure (quantified by plasma AUC from 10 min to 4 h post-injection, see Methods), as indicated by Spearman Test R-value.

Supplemental Tables 1-4. Table S1. Sequences of sgRNAs and oligonucleotides; Table S2. GSEA pathway enrichment analysis of enriched and dropout genes in MCF7 and T47D cells; Table S3. The top 2.5% (3745) enriched sgRNAs of whole-genome CRISPR/Cas9 sgRNA knockout screen in MCF7 cells; Table S4. The top 2.5% (3745) enriched sgRNAs of whole-genome CRISPR/Cas9 sgRNA knockout screen in T47D cells.

Purpose:Common resistance mechanisms to endocrine therapy (ET) in estrogen receptor (ER)–positive metastatic breast cancers include, among others, ER loss and acquired activating mutations in the ligand-binding domain of the ER gene (ESR1LBDm). ESR1 mutational mediated resistance may be overcome by selective ER degraders (SERD). During the first-in-human study of oral SERD AZD9496, early changes in circulating tumor cells (CTCs) and circulating tumor DNA (ctDNA) were explored as potential noninvasive tools, alongside paired tumor biopsies, to assess pharmacodynamics and early efficacy.Experimental Design:CTC were enumerated/phenotyped for ER and Ki67 using CellSearch in serial blood draws. ctDNA was assessed for the most common ESR1LBDm by droplet digital PCR (BioRad).Results:Before starting AZD9496, 11 of 43 (25%) patients had ≥5 CTC/7.5 mL whole blood (WB), none of whom underwent reduction to +, two of whom had CTC-ER+ reduction. CTC-Ki67 status did not change appreciably. Patients with ≥5 CTC/7.5 mL WB before treatment had worse progression-free survival (PFS) than patients with P = 0.0003). Fourteen of 45 (31%) patients had ESR1LBDm+ ctDNA at baseline, five of whom had ≥2 unique mutations. Baseline ESR1LBDm status was not prognostic. Patients with persistently elevated CTC and/or ESR1LBDm+ ctDNA at C1D15 had worse PFS than patients who did not (P = 0.0007).Conclusions:Elevated CTC at baseline was a strong prognostic factor in this cohort. Early on-treatment changes were observed in CTC-ER+ and ESR1LBDm+ ctDNA, but not in overall CTC number. Integrating multiple biomarkers in prospective trials may improve outcome prediction and ET resistance mechanisms' identification over a single biomarker.

Supplementary figures and methods

PK parameters for M5 following a single dose (Day 1) and multiple doses (Day 15)

Purpose:Activating mutations in PIK3CA promote resistance to HER2-targeted therapy in breast cancer; however, inhibition of PI3K alone leads to escape via feedback upregulation of HER3. Combined inhibition of HER2, HER3, and PI3K overcomes this mechanism preclinically.Patients and Methods:This phase I study investigated the MTD of alpelisib given in combination with trastuzumab and LJM716 (a HER3-targeted antibody) in patients with PIK3CA-mutant HER2-positive (HER2+) metastatic breast cancer (MBC) using the continual reassessment method. Secondary analyses included efficacy and exploratory correlative studies.Results:Ten patients were treated initially with daily alpelisib (arm A). Grade ≥3 adverse events seen in ≥2 patients included diarrhea (n = 6), hypokalemia (n = 3), abnormal liver enzymes (n = 3), hyperglycemia (n = 2), mucositis (n = 2), and elevated lipase (n = 2). The MTD of alpelisib in arm A was 250 mg daily. This prompted the opening of arm B in which 11 patients received intermittently dosed alpelisib. Grade ≥3 adverse events seen in ≥2 patients included diarrhea (n = 5), hypokalemia (n = 3), and hypomagnesemia (n = 2). The MTD of alpelisib in arm B was 350 mg given 4 days on, 3 days off. Among 17 patients assessed, 1 had a partial response, 14 had stable disease, and 2 had disease progression at best response. Five patients had stable disease for >30 weeks. mRNA profiling of pre- and on-treatment tissue demonstrated PIK3CA target engagement by alpelisib via induction of downstream signaling and feedback pathways.Conclusions:Combination treatment with alpelisib, trastuzumab, and LJM716 was limited by gastrointestinal toxicity. Further efforts are warranted to target the PI3K pathway in HER2+ MBC.

PET detects subcentimeter PU-H71 retention by breast cancer metastases. 65-year-old female with stage IV breast cancer, off-treatment. 124I-PU-H71 PET (right), CT (left), and fusion PET/CT (middle) images show corresponding axial planes of chest, obtained 24 hours post-injection. Tracer has cleared from blood (asterisk, aorta) with sustained PU-H71 retention by biopsy-proven subcentimeter pulmonary metastasis (arrow) and left internal mammary and mediastinal nodal metastases (arrowheads).

PDF file, 37K, Human PBMCs Sample Preparation, Western Blot assay for Hsp90 Client Proteins, Luminex Assay for Hsp90 Client Proteins.

GAZD9496 metabolite, M3 (Panel A) and M5 (Panel C), geometric mean plasma concentration following a single dose of AZD9496 on Day 1 (semi-log scale). AZD9496 metabolite, M3 (Panel B) and M5 (Panel D), geometric mean plasma concentration following multiple doses of AZD9496 on Day 15 (semi-log scale). On Day 15 at all doses except 20 mg, the 24 h time point is the 12 h trough concentration following the evening dose of AZD9496 and therefore not shown. aPooled data from dose escalation and expansion groups. BID=twice daily; QD=once daily; EXP=expansion cohort.

Supplementary Tables

Purpose: AZD9496 is an oral nonsteroidal, small-molecule inhibitor of estrogen receptor alpha (ERα) and a potent and selective antagonist and degrader of ERα. This first-in-human phase I study determined the safety and tolerability of ascending doses of oral AZD9496 in women with estrogen receptor (ER)+/HER2− advanced breast cancer, characterized its pharmacokinetic (PK) profile, and made preliminary assessment of antitumor activity.Patients and Methods: Forty-five patients received AZD9496 [20 mg once daily (QD) to 600 mg twice daily (BID)] in a dose-escalation, dose-expansion “rolling 6” design. Safety, tolerability, and PK activity in each cohort were reviewed before escalating to the next dose. PK was determined by mass spectrometry. Adverse events (AEs) were graded according to the Common Terminology Criteria for Adverse Events (CTCAE) v4.0. Objective tumor response was evaluated by Response Evaluation Criteria in Solid Tumors (RECIST) v1.1.Results: Most common causally related AEs were diarrhea (35.6%), fatigue (31.1%), and nausea (22.2%), and seven patients had grade ≥3 AEs. Three patients experienced a dose-limiting toxicity: one each at 150 mg BID (abnormal hepatic function), 400 mg BID (diarrhea and elevated liver function tests), and 600 mg BID (diarrhea), and all were reversible. The maximum tolerated dose was not reached. Partial response was confirmed in one patient, who also had decreased tumor marker Ca15.3. Four patients had stable disease at 12 months' follow-up.Conclusions: AZD9496 is well tolerated with an acceptable safety profile, showing evidence of prolonged disease stabilization in heavily pretreated patients with ER+/HER2− advanced breast cancer. Clin Cancer Res; 24(15); 3510–8. ©2018 AACR.See related commentary by Jordan, p. 3480

Purpose:124I-PU-H71 is an investigational first-in-class radiologic agent specific for imaging tumor epichaperome formations. The intracellular epichaperome forms under cellular stress and is a clinically validated oncotherapeutic target. We conducted a first-in-human study of microdose 124I-PU-H71 for PET to study in vivo biodistribution, pharmacokinetics, metabolism, and safety; and the feasibility of epichaperome-targeted tumor imaging.Experimental Design:Adult patients with cancer (n = 30) received 124I-PU-H71 tracer (201±12 MBq, Results:124I-PU-H71 PET detected tumors of different cancer types (breast, lymphoma, neuroblastoma, genitourinary, gynecologic, sarcoma, and pancreas). 124I-PU-H71 was retained by tumors for several days while it cleared rapidly from bones, healthy soft tissues, and blood. Radiation dosimetry is favorable and patients suffered no adverse effects.Conclusions:Our first-in-human results demonstrate the safety and feasibility of noninvasive in vivo detection of tumor epichaperomes using 124I-PU-H71 PET, supporting clinical development of PU-H71 and other epichaperome-targeted therapeutics.

Supplementary Figures

A total of 45 patients were recruited and four to six patients were allocated to cohorts to receive escalating doses of AZD9496. In the BID cohorts, patients received QD dosing on Day 1 and BID dosing thereafter. Six patients entered the expansion cohort, receiving AZD9496 250 mg BID. BID=twice daily, DLT=dose-limiting toxicity; n=number of patients included; LFT=liver function tests; QD=once daily.

PET detects PU-H71 retention by lymphoma. 62-year-old female with non-Hodgkin lymphoma. 124I-PU-H71 fusion PET/CT (left), CT (middle), and F-18 FDG fusion PET/CT (right) images show corresponding axial planes of chest. 124I-PU-H71 PET/CT obtained 24 hours post-injection. Mediastinal lymphomatous adenopathy (arrowhead) demonstrates PU-H71 avidity and FDG avidity.

Purpose: We conducted a phase I dose-escalation study to define the maximum tolerated dose (MTD), pharmacokinetics (PK), and pharmacodynamics of alvespimycin (17-DMAG), a heat shock protein 90 (Hsp90) inhibitor, given in combination with trastuzumab.Experimental Design: Patients were treated with trastuzumab followed by intravenous alvespimycin on a weekly schedule. Hsp90 client proteins were measured at baseline and serially in peripheral blood lymphocytes (PBL) during cycle 1. Patients with advanced solid tumors progressing on standard therapy were eligible.Results: Twenty-eight patients (25, breast; 3, ovarian) were enrolled onto three dose cohorts: 60 (n = 9), 80 (n = 13), and 100 mg/m2 (n = 6). Dose-limiting toxicities (DLT) were: grade III left ventricular systolic dysfunction presenting as congestive heart failure in 1 patient (100 mg/m2), and reversible grade III keratitis in two patients (80 mg/m2). Drug-related grade III toxicity included one episode each of fatigue, diarrhea, myalgia, and back pain. Common mild to moderate toxicities included diarrhea, fatigue, myalgia, arthralgia, nausea, blurry vision, headache, back pain, and dry eyes. There was one partial response and seven cases of stable disease (range, 4–10 months), all in HER2+ MBC. In addition, an ovarian cancer patient had complete resolution of ascites and pleural effusion that lasted 24.8 months. There was no change in PK upon weekly dosing. Hsp70 effect continued to increase across four weeks and was most pronounced at 80 and 100 mg/m2.Conclusion: The combination of alvespimycin and trastuzumab is safe and tolerable at MTD. Antitumor activity was seen in patients with refractory HER2+ MBC and ovarian cancer. The recommended dose of alvespimycin for further study in this combination is 80 mg/m2 weekly. Clin Cancer Res; 18(18); 5090–8. ©2012 AACR.

The phosphoinositide 3–kinase (PI3K) pathway regulates proliferation, survival, and metabolism and is frequently activated across human cancers. A comprehensive elucidation of how this signaling pathway controls transcriptional and cotranscriptional processes could provide new insights into the key functions of PI3K signaling in cancer. Here, we undertook a transcriptomic approach to investigate genome-wide gene expression and transcription factor activity changes, as well as splicing and isoform usage dynamics, downstream of PI3K. These analyses uncovered widespread alternatively spliced isoforms linked to proliferation, metabolism, and splicing in PIK3CA-mutant cells, which were reversed by inhibition of PI3Kα. Analysis of paired tumor biopsies from patients with PIK3CA-mutated breast cancer undergoing treatment with PI3Kα inhibitors identified widespread splicing alterations that affect specific isoforms in common with the preclinical models, and these alterations, namely PTK2/FRNK and AFMID isoforms, were validated as functional drivers of cancer cell growth or migration. Mechanistically, isoform-specific splicing factors mediated PI3K-dependent RNA splicing. Treatment with splicing inhibitors rendered breast cancer cells more sensitive to the PI3Kα inhibitor alpelisib, resulting in greater growth inhibition than alpelisib alone. This study provides the first comprehensive analysis of widespread splicing alterations driven by oncogenic PI3K in breast cancer. The atlas of PI3K-mediated splicing programs establishes a key role for the PI3K pathway in regulating splicing, opening new avenues for exploiting PI3K signaling as a therapeutic vulnerability in breast cancer. Significance: Transcriptomic analysis reveals a key role for the PI3K pathway in regulating RNA splicing, uncovering new mechanisms by which PI3K regulates proliferation and metabolism in breast cancer. See related commentary by Claridge and Hopkins, p. 2216

Background: HER2 mutations are oncogenic drivers in a subset of metastatic breast cancers (MBC). Neratinib (N) is an oral, irreversible pan-HER tyrosine kinase inhibitor with preclinical and clinical activity against HER2 mutations. Genomic analyses from paired biopsies following N ± fulvestrant (F) suggest that resistance to N may occur via amplification of the mutant allele or by acquisition of secondary HER2 mutations. Addition of trastuzumab (T) to N+F showed encouraging clinical activity with durable responses in the SUMMIT trial in hormone receptor-positive (HR+), HER2-mutant MBC, including patients (pts) who had previously received cyclin-dependent kinase 4 & 6 inhibitors (CDK4/6i) [Jhaveri et al. SABCS 2020]. On the basis of these findings, and in order to better understand the contribution of N to the activity of the N+F+T combination, SUMMIT has recently been expanded to include a randomized Simon 2-stage comparison of N+F+T vs. F+T vs. F in pts with HR+, HER2-mutated, HER2-negative MBC who were exposed to CDK4/6i. Enrollment for stage 1 is now complete (N+F+T, n=7; F+T, n=7; F, n=7), and results will be forthcoming once the data are mature. Here we report updated findings from the breast cancer cohorts of the SUMMIT trial for which data are currently available. Methods: The phase 2 SUMMIT trial (NCT01953926) enrolled pts with HR+, HER2-negative MBC whose tumors harbored activating HER2 mutation(s) identified by genomic sequencing. Prior to starting the randomized portion of the trial, these patients were enrolled in a non-randomized cohort and received N+F+T (oral N 240 mg/d, i.m. F 500 mg d1&15 of cycle 1 then q4w, i.v. T 8 mg/kg initially then 6 mg/kg q3w). Following initiation of the randomized portion of the trial, these pts received N+F+T, F+T or F (1:1:1 ratio; dose schedules as above). Pts with HER2-mutant triple-negative breast cancer (TNBC) were enrolled in a non-randomized cohort and received N+T (dose schedules as above). Loperamide prophylaxis was mandatory during the first 2 treatment cycles. There was no restriction on the number of prior lines of systemic therapy for MBC. Efficacy endpoints: investigator-assessed objective response rate and clinical benefit rate (RECIST v1.1 or other defined criteria); duration of response; best overall response. Results: Prior to enrolling the randomized cohort, 24 pts with HR+, HER2-mutated MBC who had previously received CDK4/6i were enrolled in the non-randomized cohort and received N+F+T, and 17 pts with HER2-mutant TNBC were enrolled and received N+T, as of 18-Jun-2021. Data for randomized pts are not yet mature. HER2 allelic variants across both cohorts (pts may have >1 mutation): kinase domain hotspots (n=26); exon-20 insertion (n=9); extracellular domain hotspot (n=4); exon-19 deletion (n=1); transmembrane domain missense (n=1); kinase domain non-hotspot (n=2). Efficacy findings are reported in the Table. Diarrhea was the most commonly reported adverse event: N+F+T (non-randomized cohort), 96%; N+T (TNBC cohort), 94%. No grade 4 diarrhea was reported. Conclusions: N+F+T is a promising combination for pts with HR+, HER2-mutated MBC with prior exposure to CDK4/6 inhibitors. N+T also showed encouraging activity in HER2-mutated TNBC. The first results from the randomized comparison of N+F+T vs. F+T vs. F in pts with HR+, HER2-mutated MBC (Simon stage 1 analysis) will be presented at the meeting. Table: Efficacy findingsHR+, HER2-mutated, HER2-non-amplified MBCHER2-mutant TNBCN+F+T (n=24)N+T (n=17)Confirmed objective response,a n (%)11 (46)5 (29)CR0 (0)1 (6)PR11 (46)4 (24)ORR, % (95% CI)46 (26–67)29 (10–56)Best overall response, n (%)13 (54)7 (41)CR0 (0)1 (6)PR13 (54)6 (35)Best overall response rate, % (95% CI)54 (33–74)41 (18–67)Medianb DOR, months (95% CI)14.4 (6.4–NR)NRClinical benefit, n (%)14 (58)6 (35)CR or PR11 (46)5 (29)SD ≥24 weeks3 (13)1 (6)CBR,b % (95% CI)58 (37–78)35 (14–62)aORR defined as either a CR or PR confirmed no less than 4 weeks after the response criteria are met; bCBR defined as confirmed CR or PR or SD for ≥24 weeks. Note: Tumor response is based on investigator tumor assessments per RECIST v1.1 for HR+, HER2-mutated cohort, and RECIST v1.1 or modified PERCIST for HER2-mutated TNBC cohort. CBR, clinical benefit rate; CI, confidence interval; CR, complete response; DOR, duration of response; F, fulvestrant; HR+, hormone receptor-positive; MBC, metastatic breast cancer; N, neratinib; NR, not reached; ORR, objective response rate; PERCIST, Positron Emission Tomography Response Criteria in Solid Tumors; PR, partial response; RECIST, Response Evaluation Criteria in Solid Tumors; SD, stable disease; T, trastuzumab, TNBC, triple-negative breast cancer. Citation Format: Komal Jhaveri, Haeseong Park, James Waisman, Jonathan W Goldman, Angel Guerrero-Zotano, Valentina Boni, Barbara Haley, Ingrid A Mayer, Adam Brufsky, Eddy S Yang, José A García-Sáenz, François-Clement Bidard, John Crown, Bo Zhang, Aimee Frazier, Irmina Diala, Lisa D Eli, Brian Barnett, Hans Wildiers. Neratinib + fulvestrant + trastuzumab for hormone receptor-positive, HER2-mutant metastatic breast cancer and neratinib + trastuzumab for triple-negative disease: Latest updates from the SUMMIT trial [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr GS4-10.

Background: Oncogenic mutations involving PIK3CA gene (PI3Kα) are observed in over 40% of patients with hormone receptor-positive (HR+) advanced breast cancer (ABC). Clinical efficacy of PI3K inhibition was demonstrated in a recent trial, but data on the natural history of disease by PIK3CA status are limited in ABC. We assessed patient and disease characteristics, treatment patterns, and clinical outcomes among patients with ABC treated at the Memorial Sloan Kettering Cancer Center in the United States, prior to the clinical availability of alpelisib (a PI3Kα inhibitor). Methods: A retrospective medical record review of adult patients diagnosed with HR+ and human epidermal growth factor receptor 2-negative (HER2-) ABC between January 2014 and December 2018 was conducted. Data collection is ongoing and updated results will be presented. Cohorts were determined by eligible patients’ PIK3CA gene status (i.e., mutant or wild-type [WT]), and were frequency matched to have an equal distribution of the year of ABC diagnosis between cohorts. Study measures were descriptively summarized, and the Kaplan-Meier method was used to estimate progression-free survival (PFS) and left truncation-adjusted overall survival (OS), in months. To assess the association of PIK3CA mutation status on OS, a multivariate Cox proportional hazard model adjusted for age, race, stage at diagnosis, menopausal status, visceral disease, bone only disease, and Charlson Comorbidity Index score was computed. Additional adjustment on treatment modalities is planned. Results: Data for 249 PIK3CA-mutant and 143 PIK3CA-WT patients was analyzed. For the mutant cohort, the mean (SD) age was 59.0 (11.2) years, 80.7% were White, 30.9% were diagnosed with de novo ABC, and 54.6% were postmenopausal at initial BC diagnosis. Visceral- and bone only-disease were observed among 41.4% and 21.7% of patients, respectively. Among patients with a single PIK3CA mutation (n=220, 88.4%), the most common mutations were H1047R (37.3%), E545K (20.5%), and E542K (12.7%). Multiple PIK3CA mutations were observed in 29 (11.6%) patients. For the WT cohort, the mean (SD) age was 56.3 (12.1) years, 77.6% were White, 39.9% were diagnosed with de novo ABC, and 45.5% were postmenopausal at initial BC diagnosis. Visceral- and bone-only disease were observed among 51.8% and 16.8% of patients, respectively. Most common 1st-line treatments for ABC were cyclin-dependent kinase 4 and 6 inhibitor (CDK4/6i) + aromatase inhibitor (AI) (mutant: 38.6%; WT: 40.6%), AI only (mutant: 21.3%; WT:22.4%), CDK4/6i + fulvestrant (mutant: 15.7%; WT: 7.7%), and chemotherapy-based regimens (mutant: 9.6%; WT: 11.2%). Chemotherapy-based regimens were the most common 2nd- (mutant: 25.6%; WT: 21.3%) and 3rd-line (mutant: 41.4%; WT: 33.9%) therapies for both cohorts. Results for PFS and OS are presented in Table 1, and the survival risk was similar for both cohorts (adjusted HR: 1.18, 95% CI: 0.88-1.60). Conclusion: In this preliminary analysis of HR+/HER2- ABC, CDK4/6i-based regimens were the most common 1st-line therapy, and PFS to 1st-3rd line therapies and OS were not statistically significantly different. The recent availability of PI3K inhibitor therapy may improve clinical outcomes among patients with PIK3CA-mutant tumors. Table 1.Progression-free Survival and Overall Survival Among Patients with HR+/HER2- ABC.PIK3CA MutantPIK3CA Wild-typeProgression-free Survival (months)1st line – all patients (n)249143Median (95% CI)12.4 (10.4-15.2)13.5 (10.3-16.0)1st line – CDK4/6i + AI (n)9658Median (95% CI)21.0 (17.0-27.9)19.1 (9.4-28.6)2nd line – all patients (n)211127Median (95% CI)7.3 (6.3-10.4)6.8 (5.4-8.8)2nd line – chemotherapy-based regimens (n)5427Median (95% CI)5.1 (3.7-6.3)5.2 (3.3-6.3)3rd line – all patients (n)186115Median (95% CI)4.5 (3.7-5.4)5.7 (4.7-8.9)3rd line – chemotherapy-based regimens (n)7739Median (95% CI)4.3 (3.0-5.1)4.7 (3.0-7.7)Overall Survival (months)Median (95% CI)47.4 (42.8-52.4)55.5 (44.9-61.9) Citation Format: Pedram Razavi, Rohan C Parikh, Barbara Acevedo, Abigail Hitchens, Mehnaj Ahmed, Anton Safonov, Emanuela Ferraro, Komal Jhaveri, Michelle Sidel, Stacey Simmons, Mark Robson, Sarat Chandarlapaty, Sanjeev Balu. Real-world natural history data among patients with PIK3CA-mutant and PIK3CA-wild-type advanced breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-18-18.

Introduction: Despite initial efficacy, most patients (pts) with HR+/HER2– advanced breast cancer (BC) eventually progress on endocrine therapy (ET) plus a CDK4/6 inhibitor (CDK4/6i). In preclinical models, resistance to ET + CDK4/6i can be overcome by concomitant inhibition of CDK2. Therefore, simultaneous inhibition of CDK2/4/6 may provide a new therapeutic strategy to prolong clinical benefit. This first-in-human dose escalation/expansion study (NCT03519178) is evaluating the safety, pharmacokinetics (PK), pharmacodynamics (PD) and preliminary antitumor activity of the first-in-class selective CDK2/4/6 inhibitor PF-06873600 (PF-3600) as monotherapy and combined with ET. The results of Part 1 monotherapy dose escalation and initial combination with ET are presented.Methods: This is a phase 1/2a, open-label, non-randomized, study of PF-3600 as monotherapy and combined with letrozole or fulvestrant (F). Pts with locally advanced/metastatic HR+/HER2– BC whose tumor progressed on previous ET + CDK4/6i and with ≤2 prior lines of chemotherapy (Cx), triple-negative breast cancer (TNBC) with ≤2 prior lines of Cx, and platinum-resistant ovarian cancer (OVCA) with ≤3 prior lines of treatment (Tx) were eligible for Part 1 dose escalation. Pts received escalating doses of PF-3600 (1–50 mg) BID orally. Dose escalation followed the modified toxicity probability interval method. Tx continued until disease progression, toxicity requiring discontinuation or pt withdrawal. Primary objectives for Part 1 were to evaluate safety including dose-limiting toxicities (DLTs) in the 1st 28-day cycle, adverse events (AEs), and laboratory abnormalities; and to select the recommended dose for expansion (RDE) in combination with ET. Results: As of 24 June 2021, 67 pts (HR+/HER2– BC, n=59; TNBC, n=2; OVCA, n=6) received PF-3600 (n=58) or PF-3600 + F (n=9). In the PF-3600 group, 22 (38%) and 36 (62%) pts had ECOG performance status (PS) 0 and 1, respectively. In the PF-3600 + F group, 6 (67%) and 3 (33%) pts had ECOG PS 0 and 1, respectively. The median (range) number of prior Tx in the metastatic setting was 4.0 (1, 8) for PF-3600 and 4.0 (3, 6) for PF-3600 + F. The most frequently reported treatment-related AEs (all grade/grade ≥3) for PF-3600 alone and PF-3600 + F, respectively, were nausea (50%/0% and 67%/0%), anemia (38%/14% and 44%/11%) and neutropenia (29%/16% and 22%/11%). Six (13%) of 48 DLT-evaluable pts treated with monotherapy had DLTs; all occurred at the highest doses (35 and 50 mg BID). At the RDE of 25 mg BID (PF-3600 alone, n=17; PF-3600 + F, n=9), there were no DLTs. PF-3600 exposure increased with dose, was not significantly impacted by F and at the RDE remained above the preclinical efficacious concentrations in most of the tested models for >70% of the dosing interval. The disease control rate was 48% (28/58, PF-3600) and 67% (6/9, PF-3600 + F) including 3 pts who remained stable for >13 mos with one ongoing for >28 mos. Three heavily pre-treated pts with HR+/HER2– BC (including prior ET + CDK4/6i and Cx) treated with PF-3600 alone had partial responses (2 confirmed), including 1 pt who remained on Tx for >13 mos. Modulation of cell cycle PD biomarkers (pRb and Ki-67) in paired tumor biopsies was observed. Data are not final and subject to change. Updated data will be presented.Conclusions: PF-3600 can be combined with ET with a manageable AE profile and demonstrates promising preliminary antitumor activity in heavily pretreated pts, including those with HR+/HER2– BC progressing on ET + CDK4/6i and Cx. Dose expansion in combination with ET in pts with HR+/HER2– BC with and without prior CDK4/6i Tx is ongoing. Citation Format: Timothy A. Yap, Cynthia Basu, Jonathan W. Goldman, Michael Gordon, Erika Hamilton, Adrianne Kelly, Feng Liu, Allison R. Moreau, Heather Neumann, Kyriakos Papadopoulos, Hope S. Rugo, Geoffrey I. Shapiro, Shaveta Vinayak, Li Zhou, Komal Jhaveri. A first-in-human phase 1/2a dose escalation/expansion study of the first-in-class CDK2/4/6 inhibitor PF-06873600 alone or with endocrine therapy in patients with breast or ovarian cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-16-06.

Background: The estrogen receptor (ER) is the key therapeutic target for ER-positive (ER+), human epidermal growth factor receptor 2-negative (HER2-) breast cancer. Novel degraders of ER may overcome resistance to available endocrine therapy (ET) while providing consistent oral bioavailability and convenience of administration. LY3484356 is a novel, orally bioavailable selective estrogen receptor degrader (SERD) with pure antagonistic properties resulting in sustained inhibition of ER-dependent gene transcription and -cell growth. In the phase 1a/b EMBER trial, LY3484356 monotherapy demonstrated a favorable safety profile with pharmacokinetic (PK) exposures that exceeded fulvestrant. LY3484356 also showed single agent efficacy in patients with ER+, HER2- metastatic breast cancer (MBC), including in patients with baseline ESR1 mutations and fulvestrant- and/or CDK4/6 inhibitor- refractory disease1. Trial Design: EMBER-3 is a randomized, open-label, global phase 3 study comparing LY3484356 versus investigator’s choice of ET (fulvestrant or exemestane), in patients with ER+, HER2- locally advanced or MBC. Approximately 500 patients will be randomized 1:1 to receive LY3484356 (400 mg PO QD continuously in 28-day cycles) or investigator’s choice of ET (dosed per label). Males and pre-menopausal women will receive concomitant treatment with a GnRH agonist. Eligibility criteria: Eligible patients are adult males and females (pre- or post-menopausal) with ER+, HER2- locally advanced or MBC who have received prior treatment with an aromatase inhibitor, alone or in combination with a CDK4/6 inhibitor. No other prior therapy for advanced disease is permitted. Patients must have evaluable disease (measurable or non-measurable bone only). Study endpoints: The primary endpoint is investigator-assessed progression-free survival (PFS) per RECIST v1.1. Secondary endpoints include BIRC-assessed PFS, overall survival, objective response rate, duration of response, clinical benefit rate, safety and tolerability, PK and patient reported outcomes. Recruitment for the EMBER-3 study begins in Q3 2021. Reference: 1Jhaveri et al. J Clin Oncol 2021 39:15_suppl, 1050 Citation Format: Komal Jhaveri, Nadia Harbeck, Philippe Aftimos, Sung-Bae Kim, Xavier Pivot, Cristina Saura, Giuseppe Curigliano, Monica Casalnuovo, Xuejing Aimee Wang, Suzanne R.L. Young, Lillian Smyth, Joyce O'Shaughnessy. EMBER-3: A randomized phase 3 study of LY3484356, a novel, oral selective estrogen receptor degrader vs investigator’s choice of endocrine therapy of either fulvestrant or exemestane, in patients with estrogen receptor-positive, human epidermal growth factor receptor 2-negative, locally advanced or metastatic breast cancer previously treated with endocrine-based therapy [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr OT2-11-01.

Background: Extended release onapristone (onapristone ER) is a progesterone receptor (PR) antagonist that inhibits hormone-mediated PR activation and stabilizes PR association with corepressors, resulting in an antineoplastic effect when applied alone or in combination with antiestrogen therapy to breast cancer cells in vitro. Recent preclinical studies further suggest that onapristone adds to inhibition of cell proliferation when combined with CDK4/6 inhibitors and fulvestrant. Elevations in ctDNA can precede overt disease progression by a matter of months in metastatic breast cancer and may represent an opportunity for proactive therapeutic intervention. Trial Design: This is an investigator initiated open-label, single institution phase 1b study of onapristone ER added as escalation therapy in patients with ER+, PR+, HER2 negative MBC, who have detectable ctDNA after six months of treatment with letrozole and palbociclib in the first line. The study is supported by Context Therapeutics and will involve two stages. Stage 1 is a dose escalation/de-escalation phase of 18 patients maximum, in which the safety and recommended phase 2 dose (RP2D) will be established for onapristone ER when used in combination with letrozole and palbociclib. In stage 2, the dose expansion phase, the RP2D of onapristone will be combined with letrozole and palbociclib in 10 patients to further explore the tolerability of the regimen. ctDNA will be collected serially while patients are on this triplet therapy. Eligibility Criteria: This study will enroll patients with radiologically measurable or evaluable metastatic or unresectable ER+/PR+/HER2-negative MBC in whom a tumor-derived somatic mutation can be detected in ctDNA at a variant allele fraction of 0.5% or greater after 6 months (+/- 4 weeks) of treatment with first line letrozole and palbociclib without progression, using our in house CLIA certified MSK-ACCESS assay. Adequate organ function and functional status for enrollment are stipulated in the protocol. Specific Aims: The primary objective of this study is to define the safety, tolerability, and recommended phase 2 dose of onapristone ER used in combination with letrozole and palbociclib. Secondary objectives include to investigate ctDNA response rate of the triplet therapy regimen, to gather early data regarding the 6-month clinical benefit rate , overall response rate , and progression free survival of this triplet escalation therapy regimen in high risk ctDNA+ patients, and to evaluate the pharmacokinetics of Onapristone ER when used in combination with letrozole and Palbociclib. Exploratory objective include to describe ctDNA dynamics during antiprogestin therapy escalation in ER+ MBC, as well as the molecular features present in responders vs. non-responders using ctDNA and pre-treatment tissue. Target Accrual: The total planned cohort for the phase I dose escalation is a maximum of 18 patients across 3 dose levels, and the total planned cohort for the dose expansion is 10 patients, with an anticipated maximum total of 28 patients. We will allot for 5 additional patients to account for inevaluability during the dose escalation and expansion portions of the trial. The trial will be open to enrollment at MSKCC in July 2021. Citation Format: Joshua Z Drago, Elaine M Walsh, Mithat Gonen, Michael F Berger, Mark E Robson, Sarat Chandarlapaty, Pedram Razavi, Komal Jhaveri. Circulating tumor DNA-guided adaptive therapy escalation in ER+ MBC: A phase 1b study with letrozole, palbociclib and onapristone ER [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr OT2-04-01.

Purpose GDC-0810 (ARN-810) is a novel, non-steroidal, orally bioavailable, selective estrogen receptor degrader (SERD) that potentially inhibits ligand-dependent and ligand-independent estrogen receptor (ER)-mediated signaling. Methods A phase Ia/Ib/IIa dose escalation, combination treatment with palbociclib or a luteinizing hormone-releasing hormone, and expansion study determined the safety, pharmacokinetics, and recommended phase 2 dose (RP2D) of GDC-0810 in postmenopausal women with ER + (HER2 −) locally advanced or metastatic breast cancer (MBC). Baseline plasma ctDNA samples were analyzed to determine the ESR1 mutation status. Results Patients (N = 152) received GDC-0810 100–800 mg once daily (QD) or 300–400 mg twice daily, in dose escalation, expansion, as single agent or combination treatment. Common adverse events regardless of attribution to study drug were diarrhea, nausea, fatigue, vomiting, and constipation. There was one dose-limiting toxicity during dose escalation. The maximum tolerated dose was not reached. GDC-0810 600 mg QD taken with food was the RP2D. Pharmacokinetics were predictable. FES reduction (> 90%) highlighting pharmacodynamic engagement of ER was observed. Outcomes for the overall population and for patients with tumors harboring ESR1 mutations included partial responses (4% overall; 4% ESR1), stable disease (39% overall; 42% ESR1), non-complete response/non-progressive disease (13% overall; 12% ESR1), progressive disease (40% overall; 38% ESR1), and missing/unevaluable (5% overall; 5% ESR1). Clinical benefit (responses or SD, lasting ≥ 24 weeks) was observed in patients in dose escalation (n = 16, 39%) and expansion (n = 24, 22%). Conclusion GDC-0810 was safe and tolerable with preliminary anti-tumor activity in heavily pretreated patients with ER + advanced/MBC, with/without ESR1 mutations, highlighting the potential for oral SERDs. Clinical Trial and registration date April 4, 2013. NCT01823835 .

Background: Adjuvant ET has been the standard of care for patients with estrogen receptor-positive (ER+), human epidermal growth factor receptor 2-negative (HER2-) EBC. A significant proportion of patients with increased risk still experience disease relapse despite available ET and more optimum ET is needed to prevent patients developing incurable metastatic cancer. Distant recurrence risk ranges from 20% to 40% after 5 years of adjuvant ET, depending on clinicopathological (clin-path) features at diagnosis. Consequently, there is a need to further optimize adjuvant treatment, particularly in those patients who are at increased risk of recurrence. Imlunestrant is an orally bioavailable selective estrogen receptor degrader (SERD) with pure antagonistic properties and the potential to overcome ET resistance. In early phase trials, imlunestrant monotherapy showed favorable safety with pharmacokinetic (PK) exposures exceeding fulvestrant and preliminary efficacy in ER+, HER2- advanced breast cancer patients (EMBER, Jhaveri 2022) along with robust biological/pharmacodynamic activity and tolerability in EBC (EMBER-2, Neven). Trial Design: EMBER-4 is a randomized, open-label, global phase 3 study comparing imlunestrant versus physicians’ choice of ET, in patients who are at an increased risk of recurrence based on clin-path features and who have received 2 to 5 years of standard adjuvant ET. Approximately 6,000 patients will be randomized 1:1 to receive imlunestrant (400 mg daily) for 5 years or physicians’ choice of adjuvant ET (tamoxifen or an aromatase inhibitor, AI, dosed per label). Study treatment duration is 5 years. Males and pre-/peri-menopausal women will receive concomitant treatment with a GnRH agonist if receiving imlunestrant or an AI. Stratification factors include time from initial adjuvant ET, use of prior adjuvant cyclin dependent kinase 4/6 inhibitors, nodal status, menopausal status, and geographic region. Eligibility criteria: Eligible patients are adult males and females (pre-, peri- or postmenopausal) with ER+, HER2- EBC who have completed definitive locoregional therapy and have received 2 to 5 years of prior adjuvant ET without disease recurrence, but who are at increased risk of recurrence based on clin-path features at diagnosis. Prior (neo) adjuvant chemotherapy and/or targeted therapy with a CDK4/6- or PARP- inhibitor is permitted. Study endpoints: The primary endpoint is invasive disease-free survival (IDFS), excluding second non-breast primary invasive cancers. Key secondary endpoints include distant relapse-free survival, overall survival, IDFS including second non-breast primary invasive cancers, safety, PK and patient reported outcomes. Recruitment for EMBER-4 begins globally in Q4 2022. Citation Format: Komal Jhaveri, Joyce O’Shaughnessy, Fabrice Andre, Matthew P. Goetz, Nadia Harbeck, Miguel Martín, Francois-Clement Bidard, Zachary M. Thomas, Suzanne Young, Roohi Ismail-Khan, Lillian M. Smyth, Michael Gnant. EMBER-4: A phase 3 adjuvant trial of imlunestrant vs standard endocrine therapy (ET) in patients with ER+, HER2- early breast cancer (EBC) with an increased risk of recurrence who have previously received 2 to 5 years of adjuvant ET [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT1-01-02.

Background: Imlunestrant is a novel, orally bioavailable selective estrogen receptor degrader (SERD) with pure antagonistic properties that result in sustained inhibition of estrogen receptor (ER)-dependent gene transcription and cell growth. Preclinically, imlunestrant has favorable efficacy and pharmacokinetic (PK) properties, including antitumor activity in ESR1-mutant models, along with enhanced efficacy when combined with abemaciclib. In dose escalation (Phase 1a) and dose expansion (Phase 1b) in the EMBER study, imlunestrant monotherapy was well tolerated with favorable safety, PK and encouraging antitumor activity in heavily pre-treated ER+, HER2- advanced breast cancer (aBC) patients (Jhaveri, ASCO 2022); imlunestrant recommended phase 2 dose (RP2D) was determined as 400mg QD. Here, we present the phase 1b dose expansion of imlunestrant with abemaciclib ± aromatase inhibitor (AI) in EMBER (NCT04188548). Methods: Phase 1b enrolled patients with ER-positive (ER+), HER2-negative (HER2-) aBC [shown prior endocrine therapy (ET) sensitivity or untreated de novo aBC; ≤1 prior therapies for aBC but must not have received a prior CDK4/6 inhibitor]. Patients were randomized, based on menopausal status and presence of visceral metastases, to receive imlunestrant + abemaciclib OR imlunestrant + abemaciclib + AI. Men and premenopausal women received a concomitant GnRH agonist. Serial plasma samples were obtained for PK and ctDNA analysis. Key endpoints included safety and tolerability, PK, objective response rate (ORR) per RECIST v1.1 (ORR: complete response [CR] or partial response [PR]) in patients with measurable disease), and clinical benefit rate (CBR: CR or PR, or stable disease ≥24 weeks) in patients enrolled ≥24 weeks prior to data cut. Results: As of 26 May 2022, 85 patients have received imlunestrant [n=80 at 400 mg (RP2D); n=5 at 800 mg] in combination with abemaciclib (150mg twice daily) ± AI. Forty-eight (56%) patients had visceral disease and 9% had at least 1 ESR1 mutation detected in ctDNA at baseline. Patients were predominantly (75%) ET pre-treated, 51% with an AI; and 8% and 5%, respectively, had received prior chemotherapy or fulvestrant, for aBC. The most common treatment-emergent adverse events were diarrhea (87%), nausea (58%), fatigue (45%), neutropenia (39%) and abdominal pain (34%). The majority of treatment-related AEs (TRAEs) were Grade 1 or 2, with Grade ≥3 TRAEs occurring in 36% of patients. Most common TRAEs at RP2D (400mg) were diarrhea (81%), nausea (45%), fatigue (33%) and neutropenia (35%). No patient discontinued treatment due to an AE. Dose reductions were required of both imlunestrant and abemaciclib in 6 (7%) patients and of either imlunestrant in 3 (4%) or abemaciclib in 22 (26%) patients. Preliminary efficacy is presented in Table 1. Conclusion: Imlunestrant in combination with abemaciclib ± AI showed acceptable safety and tolerability, comparable to the MONARCH 2 trial of fulvestrant + abemaciclib, along with evidence of clinical activity in ER+, HER2- aBC patients. These data suggest no additive toxicity of imlunestrant when administered in combination with abemaciclib, along with comparable clinical benefit to that observed in MONARCH 2. Further data will be presented at the meeting. The phase 3, EMBER-3 study is ongoing; evaluating imlunestrant, investigator’s choice ET, and imlunestrant + abemaciclib in ET pre-treated ER+, HER2- aBC patients (NCT04975308). Table 1. Preliminary efficacy in combination therapies in EMBER Citation Format: Komal Jhaveri, Hwei-Chung Wang, Cynthia Ma, Elgene Lim, Jessica J. Tao, Luis Manso, Jean-Yves Pierga, Ritesh Parajuli, Yolanda Jerez Gilarranz, Yen-Shen Lu, Muralidhar Beeram, Tim Larson, Ajay Dhakal, Roohi Ismail-Khan, Claudia Karacsonyi, Shanshan Cao, Cynthia Osborne, Shawn T. Estrem, Bastien Nguyen, Yujia Li, Eunice Yuen. PD13-12 Imlunestrant, an oral selective estrogen receptor degrader, in combination with abemaciclib with or without an aromatase inhibitor, in estrogen receptor-positive advanced breast cancer: Results from the phase 1a/b EMBER study [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD13-12.

BACKGROUND Endocrine therapy (ET) modulates estrogen synthesis and/or estrogen receptor (ER) activity and is the mainstay of ER+ breast cancer (BC) treatment. ET + a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) is the standard of care in patients (pts) with ER+, HER2– metastatic BC (mBC) in the first-line setting. Fulvestrant ± a CDK4/6i, and everolimus + exemestane, are the current regimens approved for use in the second-line setting. However, therapeutic resistance to some ETs, such as aromatase inhibitors, can arise from ESR1 mutations driving estrogen-independent transcription and proliferation. Current post-CDK4/6i treatment options are suboptimal. New therapy options are therefore needed to reduce this risk and to improve outcomes, tolerability, quality of life, and adherence to treatment. Giredestrant is a highly potent, nonsteroidal oral selective ER antagonist and degrader (SERD) that achieves robust ER occupancy and is active regardless of ESR1 mutation status. While phase I SERD combination data in the post CDK4/6 setting is encouraging, there is no randomized combination data. Combining giredestrant and everolimus may potentially improve outcomes after CDK4/6is and in pts with ESR1-mutated tumors; evERA BC is investigating this combination to address the unmet need in the post-CDK4/6i setting. TRIAL DESIGN This phase III, global, randomized, open-label, multicenter study will evaluate the efficacy and safety of giredestrant + everolimus vs exemestane + everolimus in pts with ER+/HER2– locally advanced (LA)/mBC who had previous treatment with a CDK4/6i and ET in the LABC/mBC or adjuvant setting. Pts will be randomized to either giredestrant (30 mg) + everolimus (10 mg) by mouth (PO) every day (QD) on Days 1–28 of each 28-day cycle, or exemestane (25 mg) + everolimus (10 mg) PO QD on Days 1–28 of each 28-day cycle. Pts will receive treatment until disease progression or unacceptable toxicity. Pts will use a dexamethasone mouth rinse four times QD for 8 weeks, started concurrently with study treatment. ELIGIBILITY Female/male pts ≥18 years with ER+/HER2– LA/mBC, an Eastern Cooperative Oncology Group Performance Status of 0–1, measurable disease defined per RECIST v.1.1 (or evaluable bone metastases with at least one predominantly lytic bone lesion confirmed by computed tomography or magnetic resonance imaging), disease progression ≥6 months after initiating ET + CDK4/6i in the LABC/mBC setting (and ≥4 months on most recent ET, if ET + a CDK4/6i was not the most recent therapy received), or relapsed either while taking or within 12 months of exposure to combination adjuvant ET (≥12 months) and a CDK4/6i (≥6 months). Availability of a baseline blood sample to determine ESR1 mutation status by circulating tumor DNA assay for testing at a central laboratory. Men and pre-/perimenopausal women will receive a luteinizing hormone-releasing hormone agonist on Day 1 of each 28-day cycle. AIMS Primary endpoint: investigator-assessed progression-free survival (PFS; per RECIST v1.1). Secondary endpoints: investigator-assessed PFS in pts with detectable ESR1-mutated tumors in circulating tumor DNA at baseline; overall survival; objective response rate; duration of response; clinical benefit rate; patient-reported outcomes; safety; pharmacokinetics. STATISTICAL METHODS The primary endpoint analysis will use a stratified log-rank test at an overall 0.05 significance level (two-sided). An independent data monitoring committee will be in place for safety. ACCRUAL Target enrollment is 224 pts globally, and this study is currently recruiting. CONTACT INFORMATION For more information or to refer a patient, email global.rochegenentechtrials@roche.com or call 1-888-662-6728 (USA only). Clinicaltrials.gov number NCT05306340. Citation Format: Erica L. Mayer, Sara Tolaney, Adam M. Brufsky, William Gradishar, Komal Jhaveri, Miguel Martín, Luca Moscetti, Gregory Vidal, Patricia Cortazar, Merilin Feldman, Bann-mo Day, Hope Rugo. evERA Breast Cancer: A phase III study of giredestrant (GDC-9545) + everolimus vs exemestane + everolimus in patients with estrogen receptor+, HER2– locally advanced or metastatic breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT2-01-07.

Background: In the SUMMIT trial, original cohorts of patients with HR+, HER2-negative (locally assessed), HER2-mutant MBC received N alone or N+F, with promising clinical response rates but abbreviated duration. Clinical progression coincided with emergence of additional HER2 mutations and/or amplification of the mutant allele [Smyth et al. Cancer Discov 2020;10:198–213]. Addition of T to the combination was postulated to prolong response; the combination of N+F+T in heavily pretreated patients with HR+, HER2-mutant MBC who had received CDK4/6 inhibitors (n=51) yielded a confirmed overall response rate (ORR) of 35.3%, median duration of response (DOR) of 14.3 months, clinical benefit rate (CBR) of 47.1%, and median progression-free survival (PFS) of 8.2 months [Jhaveri et al. J Clin Oncol 2022;40:1028]. Seven of these 51 patients were part of a randomized (1:1:1) cohort who received N+F+T, F+T, or F alone. Patients randomized to F+T or F could crossover to N+F+T upon progression. No patients responded to F or F+T; however, one of four patients who crossed over to N+F+T upon progression on F+T responded to the triplet, as did two of six who crossed over upon progression on F. We undertook ctDNA sequencing in patients who responded to N+F+T upfront and after crossover. Methods: Patients with HR+, HER2-negative MBC with activating HER2 mutation(s) and prior CDK4/6i received N+F+T (oral N 240 mg/d with loperamide prophylaxis, im F 500 mg d1, d15, and d29 of cycle 1 then q4w, iv T 8 mg/kg initially then 6 mg/kg q3w) or F+T or F alone. Efficacy endpoints included investigator-assessed ORR and CBR (RECIST v1.1), DOR, and best overall response. ctDNA was collected at baseline, every third cycle during treatment, and at the end of treatment and analyzed by next-generation sequencing. Samples were analyzed using the TEMPUS xF assay. Results: Sequencing results from ctDNA analysis are pending at the time of this abstract submission. Baseline HER2 mutations and co-alterations will be reported and compared with those found in tissue samples. Genomic spectrum and variant allele frequencies in samples taken at baseline, on treatment, and at the end of treatment from patients who experienced complete or partial response to N+F+T and then progressed (n=25) will be sequenced and mechanisms of acquired resistance will be posited. ctDNA genomic profiles of serial time points from patients randomized to F or F+T before and after crossover to N+F+T (n=10) will also be evaluated. Conclusions: Similarities and differences between the mechanisms of acquired resistance to N+F+T, and those previously reported to be associated with progression on N or N+F, will be discussed. Citation Format: Cynthia Ma, James Waisman, Adam M. Brufsky, Eddy S. Yang, Hans Wildiers, John P. Crown, Sarina A. Piha-Paul, Jennifer M. Suga, José Ángel García-Sáenz, Valentina Gambardella, Angel Guerrero, Salomon Stemmer, Ron Bose, Tonya Novara-Demgen, Daniel DiPrimeo, Lisa D. Eli, Komal Jhaveri. Genomic analysis of circulating tumor DNA (ctDNA) from patients with HR+, HER2-mutant metastatic breast cancer (MBC) enrolled in SUMMIT: mechanisms of acquired resistance to neratinib + fulvestrant + trastuzumab (N+F+T) [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD17-01.

ctDNA-guided Adaptive Therapy Escalation in ER+ MBC: A Phase 1b Study with Letrozole, Palbociclib and Onapristone ER Background: Extended release onapristone (onapristone ER) is a progesterone receptor (PR) antagonist that inhibits hormone-mediated PR activation and stabilizes PR association with corepressors, resulting in an antineoplastic effect when applied alone or in combination with antiestrogen therapy to breast cancer cells in vitro. Recent preclinical studies further suggest that onapristone adds to inhibition of cell proliferation when combined with CDK4/6 inhibitors and fulvestrant. Elevations in ctDNA can precede overt disease progression by a matter of months in metastatic breast cancer and may represent an opportunity for proactive therapeutic intervention. Trial Design: This is an investigator initiated open-label, single institution phase 1b study of onapristone ER added as escalation therapy in patients with ER+, PR+, HER2 negative MBC, who have detectable ctDNA after six months of treatment with letrozole and palbociclib in the first line. The study is supported by Context Therapeutics and involves two stages. Stage 1 is a dose escalation/de-escalation phase of 18 patients maximum, in which the safety and recommended phase 2 dose (RP2D) will be established for onapristone ER when used in combination with letrozole and palbociclib. In stage 2, the dose expansion phase, the RP2D of onapristone will be combined with letrozole and palbociclib in 10 patients to further explore the tolerability of the regimen. ctDNA will be collected serially while patients are on this triplet therapy. Eligibility Criteria: This study is enrolling patients with radiologically measurable or evaluable metastatic or unresectable ER+/PR+/HER2-negative MBC in whom a tumor-derived somatic mutation can be detected in ctDNA at a variant allele fraction of 0.5% or greater after 6 months (+/- 4 weeks) of treatment with first line letrozole and palbociclib without progression, using our in house CLIA certified MSK-ACCESS assay. Adequate organ function and functional status for enrollment are stipulated in the protocol. Specific Aims: The primary objective of this study is to define the safety, tolerability, and recommended phase 2 dose of onapristone ER used in combination with letrozole and palbociclib. Secondary objectives include to investigate ctDNA response rate of the triplet therapy regimen, to gather early data regarding the 6-month clinical benefit rate, overall response rate, and progression free survival of this triplet escalation therapy regimen in high risk ctDNA+ patients, and to evaluate the pharmacokinetics of Onapristone ER when used in combination with letrozole and Palbociclib. Exploratory objective include to describe ctDNA dynamics during antiprogestin therapy escalation in ER+ MBC, as well as the molecular features present in responders vs. non-responders using ctDNA and pre-treatment tissue. Target Accrual: The total planned cohort for the phase I dose escalation is a maximum of 18 patients across 3 dose levels, and the total planned cohort for the dose expansion is 10 patients, with an anticipated maximum total of 28 patients. We will allot for 5 additional patients to account for inevaluability during the dose escalation and expansion portions of the trial. The trial is currently open to enrollment at MSKCC. Contact Information: Dragoj@mskcc.org; Jhaverik@mskcc.org Citation Format: Joshua Drago, Elaine Walsh, Mithat Gonen, Michael Berger, Mark E. Robson, Sarat Chandarlapaty, Pedram Razavi, Komal Jhaveri. ctDNA-guided Adaptive Therapy Escalation in ER+ MBC: A Phase 1b Study with Letrozole, Palbociclib and Onapristone ER [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT2-01-11.

HER2 mutations are infrequent genomic events in biliary tract cancers (BTCs). Neratinib, an irreversible, pan-HER, oral tyrosine kinase inhibitor, interferes with constitutive receptor kinase activation and has activity in HER2-mutant tumours. SUMMIT is an open-label, single-arm, multi-cohort, phase 2, ‘basket’ trial of neratinib in patients with solid tumours harbouring oncogenic HER2 somatic mutations (NCT01953926). The primary objective of the BTC cohort was objective response rate (ORR). Among 25 treatment-refractory patients (11 cholangiocarcinoma, 10 gallbladder, 4 ampullary cancers), the ORR was 16%. The most common HER2 mutations were S310F (n = 11; 48%) and V777L (n = 4; 17%). Outcomes appeared worse for ampullary tumours or those with co-occurring oncogenic TP53 and CDKN2A alterations. Loss of amplified HER2 S310F and acquisition of multiple previously undetected oncogenic co-mutations were identified at progression in one responder. Neratinib demonstrated anti-tumour activity in patients with refractory BTC harbouring HER2 mutations. Future studies of rational combinations are warranted.