Endocrine Therapy In ER Research Articles

Resistance Mechanisms to Combined CDK4/6 Inhibitors and Endocrine Therapy in ER+/HER2- Advanced Breast Cancer: Biomarkers and Potential Novel Treatment Strategies.

Simple SummaryAdvanced stage breast cancer is incurable, and a leading cause of cancer-related death in women. Recently, novel treatment options have improved clinical outcomes of breast cancer patients. Combined CDK4/6 inhibitor and endocrine therapy have been shown to be highly effective, improving both progression-free and overall survival, but resistance inevitably develops and disease progression occurs. Identification of clinically useful biomarkers predicting therapeutic response/failure and resistance mechanisms are essential for optimal patient stratification and the development of novel therapeutic strategies for patients who have progressed on combined CDK4/6 inhibitor and endocrine therapy. However, finding a universal single predictive biomarker is unlikely mainly due to the heterogenicity of the cancers and the associated resistance mechanisms. Here, we review the current state of knowledge on resistance mechanisms to combined CDK4/6 inhibitor and endocrine therapy and associated predictive molecular biomarkers, including the use of liquid biopsy.The introduction of CDK4/6 inhibitors (CDK4/6i) in combination with endocrine therapy (ET) has revolutionized the treatment landscape for patients with estrogen receptor-positive (ER+) advanced breast cancer (ABC) and has become the new standard treatment. However, resistance to this combined therapy inevitably develops and represents a major clinical challenge in the management of ER+ ABC. Currently, elucidation of the resistance mechanisms, identification of predictive biomarkers, and development of novel effective combined targeted treatments to overcome the resistance are active areas of research. Given the heterogeneity of the resistance mechanisms towards combined CDK4/6i and ET, identification of a single universal predictive biomarker of resistance is unlikely. Novel approaches are being explored, including examination of multiple genetic alterations in circulating cell-free tumor DNA in liquid biopsies from ABC patients with disease progression on combined CDK4/6i and ET treatment. Here, we review the molecular basis of the main known resistance mechanisms towards combined CDK4/6i and ET and associated potential biomarkers. As inhibiting key molecules in the pathways driving resistance may play an important role in the selection of therapeutic strategies for patients who experience disease progression on combined CDK4/6i and ET, we also review preclinical and early phase clinical data on novel combination therapies for these patients.

MINDY1 promotes breast cancer cell proliferation by stabilizing estrogen receptor \u03b1

Breast cancer is the most commonly diagnosed malignant tumor among females. Estrogen receptor α (ERα) is initially expressed in 70% of breast cancers and is a well-known target of endocrine therapy for ERα-positive breast cancer. In the present study, we identified MINDY1, a member belongs to the motif interacting with Ubcontaining novel DUB family (MINDY), as a potential deubiquitylase of ERα in breast cancer. There was a positive correlation between ERα and MINDY1 protein levels in human breast cancer tissues. We found that high expression of MINDY1 was associated with poor prognosis. MINDY1 interacted with ERα, thereby mediating the deubiquitination of ERα and increased its stability in a deubiquitylation activity-dependent manner. MINDY1 depletion significantly decreased the ERα protein level and ERα signaling activity in breast cancer cells. Specifically, MINDY1 associated with the N-terminal of ERα via its catalytic domain, thus inhibiting K48-specific poly-ubiquitination process on ERα protein. In addition, MINDY1 depletion led to growth inhibition and cell cycle arrest of ERα-positive breast cancer cells. Finally, overexpression of ERα could rescue the MINDY1 depletion-induced growth inhibition both in vitro and in vivo, suggesting that MINDY1 promotes breast carcinogenesis through increasing ERα stability. Overall, our study proposed a novel post-translational mechanism of ERα in supporting breast cancer progression. Targeting the MINDY1 may prove to be a promising strategy for patients with ERα-positive breast cancer.

Abstract 1414: Effect of E7090, an FGFR1-3 selective inhibitor, on resistance to a CDK4/6 inhibitor and endocrine therapy in ER(+)/HER2(-) breast cancer preclinical models

Abstract Introduction: The fibroblast growth factor receptor (FGFR) signaling has a crucial role in proliferation, survival, migration, and drug resistance of cancer cells, as well as tumor angiogenesis. Moreover, gene alterations of FGF/FGFR were reported after treatment of a CDK4/6 inhibitor and endocrine therapy (ET) in ER(+)/HER2(-) breast cancer (BC) patients, suggesting FGF/FGFR signaling mediates resistance to these therapies. E7090, an orally available FGFR1-3 selective inhibitor, is currently under evaluation in a Phase 2 study in patients with unresectable advanced or metastatic cholangiocarcinoma with FGFR2 gene fusion (NCT04238715), and a Phase 1 study as a monotherapy or in combination with fulvestrant (FUL) or exemestane for ER(+)/HER2(-) BC patients (NCT04572295). In this study, we present the roles of FGF/FGFR signaling in resistance to a CDK4/6 inhibitor and/or ET, and activities of E7090 on drug resistance in preclinical ER(+)/HER2(-) BC cell lines and PDx models. Results: Antitumor activities of E7090 (25 or 50mg/kg, Q1Dx14 or x21, p.o.) were evaluated using five ER(+)/HER2(-) BC PDx models, OD-BRE-0438, -0450, -0188, -0704, and IM-BRE-556 with or without prior treatment of FUL (5 mg/mouse, Q7Dx2, s.c.)+palbociclib (PAL, 100mg/kg, Q1Dx14, p.o.). Among them, OD-BRE-0438 and -0704 showed higher sensitivities to E7090 with prior FUL+PAL than without it, and E7090 showed tumor regression in the OD-BRE-0438 model only with prior FUL+PAL. In these two models, FUL+PAL treatment upregulated expressions of several FGF ligands mRNA (qPCR) and FGFRs protein (IHC) in tumors. To evaluate the activities of FGF/FGFR signaling on sensitivities to FUL or FUL+PAL, in vitro cell proliferation assay using ER(+)/HER2(-) BC cell lines, MCF-7, ZR-75-1, and HCC1428 were performed in the presence of FGF2 and FGF10, and decreased sensitivities to FUL or FUL+PAL were observed. Furthermore, co-treatment with E7090 restored the sensitivity to those drugs in cultured cell lines. In consistent with in vitro results, FUL in combination with E7090 showed a combination antitumor activity in the OD-BRE-0438 model, in which expressions of several FGF ligands mRNA and FGFR2 protein in tumors showed a tendency of upregulation by FUL. Conclusion: In ER(+)/HER2(-) BC PDx models, FGFs/FGFRs expression in tumors were induced by treatment of FUL and/or PAL, and antitumor activity of E7090 was enhanced with prior FUL+PAL. In vitro study using cultured ER(+)/HER2(-) BC cell lines indicated that sensitivity against FUL or FUL+PAL was decreased by FGFR activation. Furthermore, a combination antitumor activity of E7090 and FUL was observed in the OD-BRE-0438 model. These data suggest that activation of FGF/FGFR signaling confer the resistance to a CDK4/6 inhibitor and/or ET, and E7090 shows antitumor activity against ER(+)/HER2(-) BC with prior a CDK4/6 inhibitor and/or ET. Citation Format: Satoshi Kawano, Sayo Fukushima, Kyoko Nishibata, Ryu Gejima, Yuki Niwa, Yasuhiro Funahashi, Saori Watanabe Miyano. Effect of E7090, an FGFR1-3 selective inhibitor, on resistance to a CDK4/6 inhibitor and endocrine therapy in ER(+)/HER2(-) breast cancer preclinical models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1414.

Impact of endocrine therapy on overall survival in ER negative/PR positive locoregional breast cancer: An analysis of the National Cancer Database.

545 Background: Breast cancer expressing PR, but not ER (ER-/PR+) are uncommon, comprising 2–8% of breast cancers, with less known about their characteristics and responsiveness to therapy. The role of adjuvant endocrine therapy (ET) in ER-/PR+ locoregional breast cancer is unclear as these patients have been largely excluded from prospective clinical trials. Despite a lack of data patients are often treated with adjuvant ET due to perceived possible benefit. We used the National Cancer Data Base (NCDB) to assess role of adjuvant ET in ER-/PR + breast cancer. Methods: Using the NCDB, we included adults ≥ 18 and ≤ 70 years old in order to minimize the non-cancer related deaths. We selected only female patients with stage I, II, and III. Patients have received definitive surgery (lumpectomy with radiation or mastectomy) with negative margins. Systemic therapy (ST) was defined as receipt of chemotherapy and/or immunotherapy. We excluded those who had unknown ST status, unknown ET status, unknown HER 2 status, who did not receive definitive surgery and those whose survival time was missing. Patients were stratified into four groups based on HER2 status and receipt of ET. Both Multivariable Cox proportional hazards regression modeling was utilized to determine predictors of overall survival (OS). A propensity score matched cohort was developed based on relevant demographic and clinical factors. The primary endpoint assessed was OS. All analyses were performed using SAS 9.4. Results: We identified 5344 patients (74% were Caucasian, 20% were African American and 6% were others) with ER-/PR+(74% were HER2 - and 26% were HER2 +) locoregional breast cancer (51% were Stage I, 38 % were stage II and 11% were stage III). Grade 1 cancer was seen in 2%, grade 2 in 18%, and majority being grade 3 in 80%. Of which 3093 (58%) patients received ET and 4462 (83%) received ST. Majority of patients were in age group 50-70 comprising of 69% patients, 8 % in age 18—39, 23% in age 40-49 with Charlson-Deyo Score of 0 in 83%, 1 in13%, 2-3 in 4%. In a propensity matched cohort (N=3980), ET was not significantly associated with OS among HER2 negative (HER2-) patients (HR=1.05, 95% CI 0.86-1.28, p=0.63). In HER2+ patient ET was associated with significantly improved OS (HR=0.65, 95% CI 0.42-0.99, p=.047). Conclusions: Receiving ET was not associated with improved OS in locoregional ER-/PR+/HER2- breast cancer based on our study using a propensity matched cohort in the NCDB. However, was frequently administered. Interestingly, improved OS was seen in locoregional ER-/PR+/HER2+ breast cancer with adjuvant ET.

Phase 1 study of OKI-179, an oral class 1-selective depsipeptide HDAC inhibitor, in patients with advanced solid tumors: Final results.

3075 Background: OKI-179 is a novel, oral pro-drug analog of largazole, a compound in the romidepsin-depsipeptide class of natural products. OKI-006, the active metabolite of OKI-179, inhibits HDAC 1,2,3 (IC50 = 1.2, 2.4, 2.0 nM, respectively), with no significant inhibition of Class IIa HDACs and has shown promising activity in preclinical models of solid tumors. We conducted a first-in-human dose escalation study of OKI-179 in patients with advanced solid tumors. Methods: Patients with advanced solid tumors, ECOG ≤1, normal QTc, and disease refractory to or with no available standard therapy options were treated with OKI-179 with either intermittent dosing (once daily for 4 days on 3 days off) or continuous dosing (once daily). Dose escalation was conducted using a standard 3+3 design. Pharmacokinetic (PK) and pharmacodynamic (PD) testing was performed at various time points after dosing. Results: As of Feb 4, 2021, 26 patients (19 female, 7 male) were enrolled with mean age of 63 (range 41-83). Patients received a median of 5 (range 1-11) prior lines of therapy and most common tumor types included pancreatic (N = 5), breast (N = 4), lung (N = 4), and ovarian cancer (N = 4). Twenty patients were treated in intermittent dosing cohorts from 30-450 mg. One DLT (Grade 2 [G2] thrombocytopenia) occurred in the 450 mg cohort which was expanded to 6 patients without subsequent DLTs. Six patients were treated in 2 continuous dosing cohorts of 200 mg and 300 mg. Two of 3 patients in the 300 mg cohort had DLTs of G3-4 thrombocytopenia and no DLTs were observed in 3 patients treated at 200 mg PO daily. The most common adverse events (AEs) were nausea (62%), fatigue (42%), anemia (39%), anorexia (27%), and vomiting (23%). These AE’s were G1-2 except for G3 anemia (12%), G3 fatigue (12%), and G3 anorexia (4%). No other G4-5 treatment-related AEs occurred. Median time on study was 79 days and best response was stable disease (SD) in 10 of 24 patients evaluable for efficacy (42%). Prolonged SD was observed in patients with platinum-resistant serous ovarian cancer (446 days) and adenoid cystic nasopharyngeal carcinoma (256 days). OKI-006 achieved consistent exposure with Cmax > 2,000 ng/ml and AUC > 8,000 hr*ng/ml, well above the targeted exposure for efficacy based on pre-clinical studies in murine models. Tmax was 2 hours and T1/2 was 6-8 hours. OKI-179 treatment resulted in > 3X increased T cell histone H3K9 and H3K27 acetylation within circulating PBMCs at doses of 180 - 450 mg. Conclusions: OKI-179 has a manageable safety profile, with thrombocytopenia being the on-target DLT. It has a favorable PK profile and demonstrated on-target PD effects at tolerable doses. The MTD and RP2D for OKI-179 was 450 mg daily for intermittent dosing and 200 mg daily for continuous dosing. Phase 2 studies are being designed, with a focus on combination with endocrine therapy in ER+ breast cancer and in NRAS-mutant melanoma. Clinical trial information: NCT03931681.

Prognostic Outcomes of Signet Ring Cell Carcinoma of the Breast

BackgroundSignet ring cell breast carcinoma (SRCBC) is a rare variant of invasive lobular carcinoma and there are no large series characterizing its long-term prognosis. Materials and MethodsThe NCDB was queried from 2004-2016 to identify SRCBC patients. Patients were excluded if they had non-invasive tumors, multiple malignancies, or incomplete surgical data. Univariate analysis was performed utilizing chi-squared and Fischer's Exact tests. Kaplan-Meier and Cox proportional hazard models were used for survival analysis. Results324 patients met inclusion criteria. Patients were mostly White (75.3%), ≥50 years of age (88.2%), female (98.5%), and had a low Charlson-Deyo score (82.7%). 34.5% had Stage IV disease and 78.1% had ER+ tumors. In patients with non-Stage IV disease, 91.5% received surgery: 49.5% had lumpectomy and 50.5% underwent mastectomy. Radiation therapy was used in 40.7% (71.4% with lumpectomy and 35.8% with mastectomy) and 50% received chemotherapy. Significant differences in unadjusted overall survival were seen at 5 and 10 years based on stage (P < 0.001). On multivariate analysis, ER+ patients showed an improved survival (HR 0.5, P < 0.01) but there was no difference in survival if ER+ patients received endocrine therapy (ET) (HR 0.9, P = 0.57). Non-metastatic patients who underwent surgery had improved overall survival compared to those that did not (HR 0.5, P = 0.02), but there was no survival difference based upon type of breast operation (P = 0.8). ConclusionSRCBC frequently presents at an advanced stage. While ER+ patients appear to have improved survival, there was no clear survival benefit to receiving ET in ER+ patients.

Outcomes of HER2-positive non-metastatic breast cancer patients treated with anti-HER2 therapy without chemotherapy.

Anti-HER2 therapy delivered in the adjuvant setting for breast cancer is given in conjunction with cytotoxic chemotherapy. For HER2-positive (HER2+) patients who cannot tolerate chemotherapy, there is no randomized data regarding the role of anti-HER2 therapy without chemotherapy. The National Cancer Database (NCDB) was queried for non-metastatic breast cancer patients with estrogen receptor-positive (ER+) and HER2+ breast cancer who received surgery and endocrine therapy, without chemotherapy from 2013 to 2016. Outcomes were compared between endocrine therapy alone (ET) or endocrine therapy with anti-HER2 therapy (ET + aHER2). Univariate and multivariate Cox-proportional hazards models were used to analyze the association between clinical characteristics and survival outcomes between groups. Propensity score matching (PSM) was performed to account for differences between the two groups. Of all patients with non-metastatic ER+/HER2+ breast cancer, we identified 9458 (20.5%) who did not receive chemotherapy. Of the 6741 patients who received ET, 17.2% also received aHER2 therapy. Median follow-up was 31.7months (IQR 21.1-42.1). In the aHER2 group (vs. ET), there were more patients with older age, higher stage, node positivity, poorly or undifferentiated disease, lymphovascular invasion, lobular cancer, and Medicare insurance. Compared to the ET cohort, ET + aHER2 was not significantly associated with improved OS on multivariate analysis (HR 0.88 95% CI 0.68-1.15) or after propensity score matching (HR 0.80 95% CI 0.57-1.11). There is no significant difference in survival with the addition of HER2 therapy to endocrine therapy in ER+/HER2+ non-metastatic breast cancer patients who do not receive chemotherapy. To our knowledge, this is the largest series investigating this question.

MCM3 upregulation confers endocrine resistance in breast cancer and is a predictive marker of diminished tamoxifen benefit

Resistance to endocrine therapy in estrogen receptor-positive (ER+) breast cancer is a major clinical problem with poorly understood mechanisms. There is an unmet need for prognostic and predictive biomarkers to allow appropriate therapeutic targeting. We evaluated the mechanism by which minichromosome maintenance protein 3 (MCM3) influences endocrine resistance and its predictive/prognostic potential in ER+ breast cancer. We discovered that ER+ breast cancer cells survive tamoxifen and letrozole treatments through upregulation of minichromosome maintenance proteins (MCMs), including MCM3, which are key molecules in the cell cycle and DNA replication. Lowering MCM3 expression in endocrine-resistant cells restored drug sensitivity and altered phosphorylation of cell cycle regulators, including p53(Ser315,33), CHK1(Ser317), and cdc25b(Ser323), suggesting that the interaction of MCM3 with cell cycle proteins is an important mechanism of overcoming replicative stress and anti-proliferative effects of endocrine treatments. Interestingly, the MCM3 levels did not affect the efficacy of growth inhibitory by CDK4/6 inhibitors. Evaluation of MCM3 levels in primary tumors from four independent cohorts of breast cancer patients receiving adjuvant tamoxifen mono-therapy or no adjuvant treatment, including the Stockholm tamoxifen (STO-3) trial, showed MCM3 to be an independent prognostic marker adding information beyond Ki67. In addition, MCM3 was shown to be a predictive marker of response to endocrine treatment. Our study reveals a coordinated signaling network centered around MCM3 that limits response to endocrine therapy in ER+ breast cancer and identifies MCM3 as a clinically useful prognostic and predictive biomarker that allows personalized treatment of ER+ breast cancer patients.

Abstract 60: Semaphorin 7a promotes cell survival and resistance to endocrine therapy in ER+ breast cancer

Abstract Estrogen receptor positive (ER+) breast cancers comprise the largest percentage, ~70%, of all breast cancers diagnosed in the Unites States. Despite the development of effective anti-estrogen based therapies, such as tamoxifen, aromatase inhibitors, and fulvestrant, 1 in 5 women with an ER+ tumor will experience recurrence within 10 years of diagnosis. Thus, the largest number of breast cancer-related deaths each year are attributed to ER+ breast cancers. Additionally, metastatic ER+ breast tumors uniformly develop resistance to anti-estrogen-based therapies, including combination targeted therapies with CDK4/6 and mTOR inhibitors, which represent some of the best therapeutic advances of the last five years and reflect our increasing understanding of cross-targeting to overcome acquired resistance. Our studies have shown that Semaphorin 7a expressing tumors are resistant to ER targeting therapies, including tamoxifen and fulvestrant. Furthermore, Semaphorin 7a expressing tumors do not respond to CDK4/6 inhibition. We have utilized luciferase reporter assays to show that Semaphorin 7a is initially regulated by ER; we then utilized overexpression studies to show that sustained upregulation of Semaphorin 7a results in downregulation of ER in xenograft models. This downregulation of ER is accompanied by upregulation of cell survival signaling mediated by pro-survival kinase Akt. In TCGA and METABRIC datasets, Semaphorin 7a expression is associated with significant decreases in overall survival. Furthermore, using KmPlot analysis, we have shown that Semaphorin 7a expression in patients treated with endocrine therapy predicts for poor survival rates. Thus, we sought to determine whether Semaphorin 7a expressing tumors would be susceptible to PI3K/Akt targeted therapies. We observe that Semaphorin 7a expressing tumors are sensitive to PI3K/Akt inhibition, as well as to inhibition of the ER target gene Bcl2. Thus, we suggest that patients with high Semaphorin 7a expression be considered as candidates for the addition of Venetoclax, an FDA approved Bcl2 targeted therapy, to their current therapeutic regimen. As Venetoclax is in Phase II trials in combination with fulvestrant for advanced CDK4/6 inhibitor refractory breast cancer, we believe our results could be rapidly translated to the clinical setting and will improve outcomes for the thousands of ER+ breast cancer patient that develop resistance to endocrine therapy. Citation Format: Traci Renae Lyons. Semaphorin 7a promotes cell survival and resistance to endocrine therapy in ER+ breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 60.

Camouflaged, activatable and therapeutic tandem bionanoreactors for breast cancer theranosis

The one-pot cascade reaction of naturally occurring enzymes is exciting for highly selective complex reaction and biodegradable approaches. Tamoxifen is the main drug against breast cancer for decades and induces an anticancerous effect upon metabolic activation by cytochrome P450 (CYP450). Herein, bi-enzymatic nanoreactors (NRs) are developed as a multimodality platform for smart action against breast tumors. CYPBM3 of Bacillus magaterium (CYP) is co-confined with glucose oxidase (GOx) where GOx produces H2O2 in the presence of glucose that elicits the CYP-mediated transformation of tamoxifen. The scintillating and mesoporous LaF3:Tb as nanocarrier showed advantages like a wide range of pore size and positive surface charge for efficient loading of enzyme couple, while the smallest pores were available for substrate/product diffusion. The obtained NRs were camouflaged with human serum albumin (HSA) to overcome premature enzyme leaching and provide active stealth properties. The nanocomposite was characterized for physicochemical properties and glucose-mediated sequential catalysis. The in vitro studies demonstrated the cell internalization of NRs in both ER+ and triple-negative breast cancer cell lines and showed significant cytotoxicity. The developed NRs not only improve the outcomes of endocrine therapy in ER+ cells but also synergistically act with oxidation therapy for enhanced therapeutic effect. Importantly, inhibition of triple-negative cells was also achieved. Thus, the development of the new multimodal nanomedicine of the present work should afford new tools towards the theranosis of breast cancer with minimized adverse effects.

A phase III trial of nivolumab with neoadjuvant chemotherapy and adjuvant endocrine therapy in ER+/HER2- primary breast cancer: CheckMate 7FL.

TPS604 Background: Patients (pts) diagnosed with primary estrogen receptor-positive (ER+), human epidermal growth factor 2-negative (HER2−) breast cancer (BC) of high grade and/or low ER expression are at increased risk of relapse, despite current standard of care (SoC). Promising data assessing programmed death-1 (PD-1) inhibition coupled with neoadjuvant chemotherapy for pts with high-risk ER+, HER2− BC noted improved pathologic complete response (pCR), which is identified as a valid surrogate endpoint for long-term clinical outcomes. Methods: CheckMate 7FL (NCT04109066) is a randomized, double-blind, placebo-controlled, multicenter, global phase 3 study evaluating nivolumab (NIVO) vs placebo (PBO) in combination with neoadjuvant chemotherapy and adjuvant endocrine therapy (ET) in pts with high-risk, ER+, HER2− primary BC. Eligible pts are male or female, aged ≥18 years with newly diagnosed grade 2 with ER expression of 1–9%, or grade 3, T1c-2, cN1-2 (tumor size ≥2 cm) or T3-T4, cN0-cN2 ER+, HER2− BC. Pts eligible for neoadjuvant chemotherapy and surgery, with adequate organ function, ECOG PS of 0 or 1, and tissue available for biomarker assessments will be enrolled. Approximately 1200 pts will be randomized 1:1 to NIVO or PBO, stratified by programmed death ligand 1 (PD-L1) expression, tumor grade (2 or 3), axillary nodal status (+ or −), and anthracycline + cyclophosphamide schedule (Q3W or Q2W). In the neoadjuvant phase, pts will receive NIVO 360 mg Q3W or PBO + paclitaxel 80 mg/m2 QW for four 3-week cycles, followed by NIVO 360 mg Q3W (or 240 mg Q2W) or matching PBO in combination with either doxorubicin 60 mg/m2 or epirubicin 90 mg/m2 and cyclophosphamide 600 mg/m2 Q3W or Q2W for 4 cycles. Pts will undergo surgery after completion of the neoadjuvant phase. Following surgery, pts will enter the adjuvant phase and receive NIVO 480 mg Q4W or PBO for 7 cycles + investigator’s choice of ET per local SoC. Primary endpoints are pCR (ypT0/is, ypN0) and event-free survival. Secondary endpoints include overall survival, disease-free survival, distant-metastasis-free survival, safety, pCR (ypT0 ypN0 and ypT0/is) rates, overall response rates, residual cancer burden, and quality of life. Interim analyses are planned. The study is currently enrolling. Clinical trial information: NCT04109066 .

SAT-119 Targeting Glutamate Metabolism and Signaling in ER+, Endocrine Therapy-Resistant Breast Cancer

Estrogen receptor-positive (ER+) breast cancer is the most commonly diagnosed form of this malignancy. Aromatase inhibitors and selective estrogen receptor modulators or degraders (SERMS, SERDs) can be highly effective in treating ER+ breast cancer, but de novo and acquired resistance to these interventions is a persistent clinical problem. Endocrine therapy resistant breast cancer cells rewire their metabolism to support cellular demands associated with rapid proliferation and/or increased invasion and metastasis. An important feature of this metabolic flexibility is conversion of glutamine to glutamate, an amino acid integral to protection of cells from oxidative stress. Consistent with this, we show multiple cellular models of ER+, endocrine resistant breast cancer cells markedly increase glutamate release and upregulate expression of essential glutamine/glutamate metabolic enzymes and transporters, including the glutamate/cystine antiporter xCT, glutamate dehydrogenase (GLUD1/2), and/or the glutamine importer SLC1A5. Riluzole (RIL) is FDA-approved for the treatment of amyotrophic lateral sclerosis (ALS), and has several proposed mechanisms of action, including suppression of glutamate release and increased glutamate uptake. We show ER+, endocrine responsive and resistant breast cancer cells are growth-inhibited by RIL. This is due to an increase in cell death, particularly in endocrine resistant breast cancer cells, and cell cycle arrest. Interestingly, histologic subtype confers a different cell cycle arrest profile, with invasive ductal cancer (IDC) models arresting in G1 but invasive lobular cancer (ILC) models arresting in G2/M. Isobologram analysis of RIL plus SERMs or SERDs shows additive-to-synergistic activity in a subset of ER+ cell line models, and preliminary studies show combination activity in patient-derived explants (PDEs). Mechanistically, we tested whether signaling through metabotropic glutamate receptors (mGluRs, GRMs) and/or cystine import contribute to RIL’s growth-inhibitory phenotype. Antagonists of mGluRs/GRMs don’t phenocopy the effects of RIL, suggesting extracellular glutamate signaling through these receptors is not a key mechanism. Rescue experiments with β-mercaptoethanol to promote cystine uptake through transporters other than xCT show partial reversal of RIL-mediated cell cycle arrest in some cells, suggesting xCT may contribute to RIL-induced growth inhibition. In summary, we show RIL may be a viable addition to endocrine therapy in ER+ breast cancer. Ongoing studies will test additional mechanism(s) by which RIL may attenuate the growth of ER+ breast cancer models in vitro, including inhibition of protein kinase C and casein kinase 1 delta. We are further testing RIL efficacy alone and in combination with a SERD in primary tumors and lung metastases in a ER+ patient-derived xenograft (PDX) model.

134TiP A phase III trial of nivolumab with neoadjuvant chemotherapy and adjuvant endocrine therapy in ER+/HER2− primary breast cancer: CheckMate 7FL

Patients (pts) diagnosed with primary estrogen receptor-positive (ER+), human epidermal growth factor 2-negative (HER2−) breast cancer (BC) of high grade and/or low ER expression are at increased risk of relapse, despite current standard of care (SoC). Promising data assessing programmed death-1 (PD-1) inhibition coupled with neoadjuvant chemotherapy for pts with high-risk ER+, HER2− BC noted improved pathologic complete response (pCR), which is identified as a valid surrogate endpoint for long-term clinical outcomes.

Abstract OT2-04-03: Nivolumab with neoadjuvant chemotherapy and adjuvant endocrine therapy in ER+/HER2- primary breast cancer: CheckMate 7FL

Abstract Background: Estrogen receptor-positive (ER+), human epidermal growth factor receptor 2-negative (HER2−) primary breast cancer (BC) includes a group of heterogeneous tumors, spanning from biologically indolent, ER signaling-driven to aggressive, endocrine-resistant tumors. Patients (pts) diagnosed with ER+, HER2− BC of high grade and/or low ER expression are at increased risk of relapse, despite current standard of care (SoC): chemotherapy and endocrine treatment (ET) in the neo- and adjuvant settings, respectively. New treatment options to increase cure rates are needed. Increased understanding of ER+, HER2− BC immune biology show an enrichment of cases with tumor cell recognition by the immune system and immune suppression mediated via programmed-death-1 (PD-1) signaling in pts with high-risk disease. Such features, linked with poorer prognosis and potential ET resistance, may be addressed through PD-1 inhibition combined with current SoC for pts with high-risk ER+, HER2− BC. Promising data assessing PD-1 inhibition coupled with neoadjuvant chemotherapy for pts with high-risk ER+, HER2− BC noted improved pathologic complete response (pCR) which is identified as a valid surrogate endpoint for long-term clinical outcomes. Trial Design: CheckMate 7FL is a randomized, double-blind, placebo-controlled, multicenter, global phase 3 study evaluating nivolumab (NIVO) vs placebo (PBO) in combination with neoadjuvant chemotherapy and adjuvant ET in pts with high-risk, ER+, HER2− primary BC. Eligible pts are male or female, aged ≥18 years with newly diagnosed grade 2 with ER expression of 1-9%, or grade 3, T1c-2, cN1-2 (tumor size &amp;gt;2 cm) centrally confirmed ER+, HER2− BC. Pts eligible for neoadjuvant chemotherapy and surgery, with adequate organ function, ECOG PS of 0 or 1, and tissue available for biomarker assessments will be enrolled. Pts with history of ipsilateral invasive BC regardless of treatment, ipsilateral ductal carcinoma in situ treated with radiotherapy, contralateral invasive BC at any time, grade ≥1 peripheral neuropathy, or those with prior treatment for the currently diagnosed BC are excluded. Approximately 1200 pts will be randomized 1:1 to NIVO or PBO, stratified by programmed death ligand 1 (PD-L1) expression (+ or −), tumor grade (2 or 3), axillary nodal status (+ or −), and anthracycline + cyclophosphamide schedule (Q3W or Q2W). Node negative pts will be capped at 20% of the intent-to-treat population. In the neoadjuvant phase, pts will receive NIVO 360 mg Q3W or PBO plus paclitaxel 80 mg/m2 QW for four 3-week cycles, followed by NIVO 360 mg Q3W (or 240 mg Q2W) or matching PBO in combination with either doxorubicin 60 mg/m2 or epirubicin 90 mg/m2 and cyclophosphamide 600 mg/m2 Q3W (or Q2W, respectively, dose-dense schedule option) for 4 cycles. Pts will undergo surgery after completion of the neoadjuvant phase. Following surgery, pts will enter the adjuvant phase and receive NIVO 480 mg Q4W or PBO for 7 cycles plus investigator’s choice of ET per local SoC. Primary endpoints are pCR, defined as no histologic evidence of invasive tumor cells in breast and axillary lymph nodes as well as non-axillary sentinel node (ypT0/is, ypN0) as determined by local pathologist, and event-free survival. Secondary endpoints include overall survival, disease-free survival, distant-metastasis-free survival, safety, pCR (ypT0 ypN0 and ypT0/is) rates, overall response rate, residual cancer burden, and quality of life. pCR rates will be compared using a 2-sided stratified Cochran-Mantel-Haenszel test with alpha = 0.01. Event-free survival will be compared using a stratified 2-sided log-rank test with adjusted alpha at each interim analysis and at final analysis. Recruitment worldwide is expected to begin in November 2019. For further information, please contact Kristen.Letrent@bms.com. Citation Format: Sherene Loi, Heather McArthur, Nadia Harbeck, Lajos Pusztai, Suzette Delaloge, Kristen Letrent, Tian Chen, Bin Li, Kay Tatsuoka, Dimitrios Zardavas, Giuseppe Curigliano. Nivolumab with neoadjuvant chemotherapy and adjuvant endocrine therapy in ER+/HER2- primary breast cancer: CheckMate 7FL [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr OT2-04-03.

Abstract P5-06-06: Clinically significant changes of receptors status (ER, PR and HER2) after receiving neoadjuvant chemotherapy (NACT) in breast cancer (BC) predicts the prognosis and guides the choice of the optimal adjuvant therapy (AT): Retesting of the receptor status should be mandatory

Abstract Background: NACT is a standard option for BC (T2-4, N0-3, M0) and certain BC phenotypes. The choice of AT based on the pre-NACT biomarkers status may not be optimum for individual patient because dynamic phenotypic changes induced by NACT may alter the response to treatment. Aim: The aim of this study is to determine the incidence of changes in the receptor status (ER, PR and HER2) and other proliferation biomarkers (Ki67 and SPAG5) before and after NACT and to assess the clinical significance of such changes. Methods Immunohistochemistry staining of ER, PR, HER2, Ki67 and SPAG5 in pre and post NACT tumors tissues from a consecutive series of 850 of BC (T2-4, N0-3, M0) treated at the Nottingham University Hospital (NUH) from 2000 to 2018, have been centrally evaluated according to ASCO guidelines. All cases with conversion in HER2 status had also been retested by HER2-FISH. The results were validated in an external cohort of 250 cases. Treatment options and patients characteristics are the same between the centres: (68%) received anthracycline plus Taxane (AC+T) NACT and 32% of patients have received NACT Anthracycline only (AC). Neoadjuvant HER2 targeting agents (Trastuzumab) or (Trastuzumab + Petruzumab) had been prescribed to 16% of patients in addition to AC+T followed by adjuvant Trastuzumab (total=18 cycles). All pre-NACT ER+ patients were given at least 5 year of adjuvant endocrine therapy (ET). In 2013 NUH started a prospective audit of retesting of receptor status in all post NACT surgical tumour samples. The results of the tests were presented to the weekly tumour board meeting and any change in the receptor status (ER and HER2) from negative (in the pre NACT core biopsies) to positive (in the post NACT surgical specimen) being considered for additional AT (ET for ER+ and Trastuzumab for HER2+ cases).The primary end points for this study are the % changes of biomarker changes and the disease free survival (DFS). The median follow up was 72 months. Results In pre NACT core biopsies 32% and 68% were HER2+ and HER2-; respectively. Twelve percent (12%) of the pre NACT HER2- tumours had a conversion to HER2+ in the post NACT surgical specimens. In this group of patients who subsequently received adjuvant Trastuzumab, 95% 3-year DFS was reported; which was similar to those patients who achieved pCR (3-year DFS; 90%) and was superior to cases which remained HER- in post NACT specimens (3-year DFS; 41%); (p&amp;lt;0.0001). Furthermore, similar group of patients with pre NACT HER- tumour before the 2013 audit, who did not receive Trastuzumab for the change to post NACT HER2+ receptors, has inferior 3-year DFS to those received adjuvant Trastuzumab based on the conversion (HR (95% CI)= 7.40 (1.04-52.86); p=0.046). In pre NACT HER2+ BC, 20% of cases had been converted into HER2- in the post NACT surgical specimens. These patients had better 3y-DFS (94%) compared to those who remained HER2+ in post NACT specimens (3y-DFS=70%; HR (95% CI)= 0.86 (0.77-0.97); p=0.01). Furthermore those patients who received neoadjuvant HER2 targeting therapy had statistically higher incidence of post HER2- conversion (p=0.005) and lower level of post NACT proliferation markers (Ki67 and SPAG5); p=0.01. In pre NACT ER+/PR+ patients, those who has converted into ER+/PR- in post NACT specimens had shorter 5-year DFS (40%) in spite of receiving ET, compared to those who remained ER+/PR+ in post NACT specimen [5-DFS= 72%]; HR (95% CI)= 1.98 (1.18-3.31); p=0.009). Conclusion: To our knowledge, this is the first report, which showed the significant clinical benefit of adjuvant therapy, based on the re-testing of the standard receptors status. Citation Format: Tarek M. a. Abdel-Fatah, Rebekah Webb, Jennifer Walker, Jun Lim, A. Graham Pockley, Graham Ball, Matthew Griffiths, Ian Ellis, Emad Rakhah, Zsolt Hodi, Andrew Lee, Arlene Chan, Stephen Chan. Clinically significant changes of receptors status (ER, PR and HER2) after receiving neoadjuvant chemotherapy (NACT) in breast cancer (BC) predicts the prognosis and guides the choice of the optimal adjuvant therapy (AT): Retesting of the receptor status should be mandatory [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P5-06-06.

Abstract OT1-04-04: EMERALD: A randomized, open-label, phase 3 trial to evaluate the efficacy and safety of elacestrant (RAD1901), a novel oral selective estrogen receptor degrader (SERD), vs investigator’s choice of endocrine therapy for ER+/HER2- advanced breast cancer following CDK4/6 inhibitor therapy

Abstract Background: Estrogen receptor-positive (ER+) breast cancer (BC) comprises ~70% of all BC and advanced/metastatic ER+ disease (mBC) remains a major clinical challenge. The addition of CDK4/6i to endocrine therapy (ET) has improved progression-free survival (PFS); however, novel treatments are needed after progression. Putative mechanisms of endocrine resistance, such as ESR1 mutations (mESR1), also indicate the need for additional therapies. Elacestrant, an oral SERD, demonstrated anti-tumor activity in preclinical models of ER+ BC, including models resistant to CDK4/6 inhibitors (CDK4/6i) and models with mESR1. An interim evaluation of a phase 1 trial (NCT02338349) of elacestrant in heavily pretreated patients (pts) with mBC, demonstrated an overall response rate (ORR) of 27% with a PFS of 5.4 mo (Bardia, SABCS, 2017). Responses were seen in pts with prior CDK4/6i and with wild-type (WT) or mESR1. Methods: This is a multicenter, international, randomized, open-label, active-controlled phase 3 trial for post-menopausal women or men with mBC. Pts must have received 1-2 prior lines of ET, ≤1 line of chemotherapy for mBC, and have documented progression on a CDK4/6i. Pts with measurable disease (RECIST v1.1) or bone-only disease are eligible. Pts are randomized 1:1 to elacestrant (400 mg orally daily) or investigator’s choice of fulvestrant or an aromatase inhibitor. Stratification factors include ESR1 mutation status (detected by ctDNA), prior fulvestrant treatment and presence of visceral disease. The co-primary endpoints are PFS by blinded independent review committee (IRC) in pts with mESR1 and in all pts (mESR1 or mESR1 not detected). Secondary endpoints include: overall survival; PFS by investigator review; ORR, duration of response, and clinical benefit rate; safety; pharmacokinetics; and quality of life. Approximately 466 pts will be enrolled to detect 340 PFS events in all pts (power ≥90%, hazard ratio (HR) = 0.667) and 160 PFS events in the mESR1 subset (power ≥80%, HR = 0.610), overall α level at 2-sided 5% using the Hochberg procedure. The EMERALD study is open for enrollment. NCT03778931 Citation Format: Philippe Aftimos, Aditya Bardia, Virginia G Kaklamani, Janice Lu, Teeru Bihani, JungAh Jung, Alfred T Anderson-Villaluz, Maureen G Conlan, Javier Cortes. EMERALD: A randomized, open-label, phase 3 trial to evaluate the efficacy and safety of elacestrant (RAD1901), a novel oral selective estrogen receptor degrader (SERD), vs investigator’s choice of endocrine therapy for ER+/HER2- advanced breast cancer following CDK4/6 inhibitor therapy [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr OT1-04-04.

Abstract ES5-1: Neoadjuvant therapy for hormone receptor-positive breast cancer: challenges and advances

Abstract Neoadjuvant therapy in breast cancer has evolved from a modality able to facilitate surgery into a sophisticated treatment approach, where patterns of response influence the extent of surgery (i.e. breast and axilla) and inform treatment decisions in the adjuvant setting. This is particularly true for patients diagnosed with HER2-positive or triple-negative breast cancer (TNBC). In the subset of estrogen receptor-positive (ER+) disease, the neoadjuvant treatment model has proven to be a successful clinical research tool. Studies investigating pharmacodynamic markers (i.e., Ki67) demonstrated the differences in the relative effectiveness of tamoxifen versus aromatase inhibitor and, more recently, neoadjuvant studies with endocrine therapy plus CDK4/6 inhibitors demonstrated the efficacy of CDK4/6 inhibitors in potentiating the effects of endocrine therapy through a profound suppression of Ki67. However, in clinical practice neoadjuvant endocrine therapy remains underutilized. Routinely, ER+ candidates for neoadjuvant therapy are treated with chemotherapy with minimal attention to disease biology. By contrast, disease biology plays a major role in the adjuvant setting, and the “old notion” of disease burden predicting chemotherapy benefit is fading away. The disconnect between the choices of systemic therapy in the neoadjuvant versus adjuvant setting in ER+ breast cancer has multiple roots: 1) Physicians and patients may expect higher response rates to neoadjuvant chemotherapy; 2) genomic biomarkers to guide chemotherapy treatment decisions were developed using material from surgical specimens; 3) there are limited data with neoadjuvant endocrine therapy in premenopausal women; and 4) the objectives with neoadjuvant endocrine therapy are not always clear. While additional clinical studies are warranted, the available clinical data suggest neoadjuvant endocrine therapy is an appropriate treatment approach for patients with ER+ disease. When selecting patients by ER+ status only, response rates to neoadjuvant endocrine therapy are similar or higher than chemotherapy. Clinical studies have successfully used diagnostic biopsies for genomic profiling, and if anything, the quality of FFPE material should be superior due to pre-analytical variables (e.g., reduced ischemic time). Additionally, neoadjuvant endocrine therapy for ER+ breast cancer and favorable disease biology fulfills the promise of a tailored treatment approach and avoids unnecessary use of cytotoxic therapy. When selecting neoadjuvant endocrine therapy for patients, it is important to understand some key points regarding this treatment approach. The maximum treatment response is not expected before four to six months of therapy, and the interpretation of pathologic response should not be confounded by data from HER2+ and TNBC. Residual disease after neoadjuvant endocrine therapy is not a predictor of survival outcomes. And, lastly, if the patient was not deemed as an ideal candidate for neoadjuvant chemotherapy at baseline, the extent of residual disease should not be used a predictor of chemotherapy benefit. There are, however, caveats and extensive disease at the time of surgery (e.g., four or more positive nodes) might justify a course of adjuvant chemotherapy. In summary, neoadjuvant endocrine therapy should not be a treatment of exception for elderly patients when trying to avoid the toxicities of chemotherapy. Neoadjuvant endocrine therapy is an effective treatment modality with proven clinical utility for postmenopausal patients with ER+ breast cancer. Citation Format: O Metzger. Neoadjuvant therapy for hormone receptor-positive breast cancer: challenges and advances [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr ES5-1.

Factors associated with prolonged overall survival in patients with postmenopausal estrogen receptor-positive advanced breast cancer using real-world data: a follow-up analysis of the JBCRG-C06 Safari study

BackgroundAssessing survival risk is important for discussing treatment options with estrogen receptor-positive (ER+) advanced breast cancer (ABC) patients. However, there are few reports from large-scale databases on the survival risk factors in ER+ ABC. The Safari study (UMIN000015168) was a retrospective, multicenter cohort study involving 1072 Japanese patients receiving fulvestrant 500 mg mostly as a second- or later-line endocrine therapy for ER+ ABC. The follow-up data after the Safari study were examined, focusing on any relationship between clinicopathological factors and overall survival (OS) in ER+ ABC patients.MethodsOS in patients with ER+ ABC was analyzed by univariate and multivariate analyses with a Cox proportional hazards model in this study.ResultsA total of 1031 cases were evaluable for OS analysis. Multivariate analysis showed that younger age (< 60 years), longer time from ABC diagnosis to fulvestrant use (≥ 3 years), no prior palliative chemotherapy before fulvestrant use, and progesterone receptor (PgR) negativity (PgR−) were significantly correlated with prolonged OS (median 7.0 years). For cases with histological or nuclear grade data, lower histological or nuclear grades were also correlated with longer OS. In recurrent metastatic cases, long disease-free interval (DFI) was not correlated with longer OS.ConclusionsIn ER+ ABC patients whose treatment history included fulvestrant, younger age, longer time from ABC diagnosis to fulvestrant use, no prior palliative chemotherapy use, PgR−, and lower histological or nuclear grade correlated positively with prolonged OS.

CDC20 expression in oestrogen receptor positive breast cancer predicts poor prognosis and lack of response to endocrine therapy.

Endocrine therapy is the standard treatment for oestrogen receptor positive (ER+) breast cancer. Despite its efficacy, around half of patients will develop resistance to this treatment and eventually relapse. Identification of effective and reliable biomarkers to predict the efficacy of endocrine therapy is of crucial importance in the management of ER+ breast cancer. Emerging evidence has revealed that the cell division regulator CDC20 exhibits an oncogenic function and plays important roles in tumourigenesis and progression of solid tumours. In this study, we investigated the prognostic and predictive role of CDC20 in early ER+ breast cancer patients. The biological and clinical impact of CDC20 expression was assessed in large clinical annotated cohort of ER+ breast cancer with long-term follow-up at the mRNA level, using METABRIC and KM-Plotter datasets, and the protein level using immunohistochemistry on patients presenting at Nottingham. CDC20 expression was correlated with clinico-pathological parameters, molecular subtypes, clinical outcome and efficacy of endocrine therapy. High CDC20 mRNA expression was associated with poor clinico-pathological parameters including large tumour size and high tumour grade (P < 0.0001) in patients with ER+ breast cancer. High CDC20 mRNA expression was significantly associated with poor patient outcome (P < 0.0001). Importantly, high CDC20 expression was correlated with poor response to endocrine treatment in patients who treated with hormonal therapy only (P < 0.01). In multivariate analysis, CDC20 mRNA was an independent predictor of poor clinical outcome after treatment with endocrine therapy (P = 0.02). CDC20 is a candidate biomarker for a subgroup of ER+ breast cancer characterised by poor clinical outcome. This study shows that the CDC20 could act as potential predictive biomarker of poor response to endocrine therapy in ER+ breast cancer.

Abstract 4827: The therapeutic superiority of neratinib in combination with trastuzumab compared to pertuzumab plus trastuzumab in HER2-positive in vivo breast cancer models

Abstract Neoadjuvant clinical trials in HER2+ breast cancer showed that lapatinib (L) plus trastuzumab (T), combined with endocrine therapy for ER+ tumors, achieved meaningful complete pathologic response rates without chemotherapy. The irreversible pan-HER kinase inhibitor neratinib (N) has shown greater potency compared to L in the preclinical setting. However, the efficacy of N in combination with T (N+T) and how it compares to pertuzumab (P) +T (without chemotherapy) has not been well studied. We hypothesize that dual HER2 inhibition using N+T will be highly efficacious due to more complete blockade of the HER pathway, with comparable or better potency than P+T. Here, we evaluate the therapeutic efficacy and molecular mechanisms of N, P, and T, either alone or in combination, in cell- and patient-derived xenograft (PDX) models. Immunodeficient mice bearing BT474-AZ cell (ER+/HER2+), and BCM-3963 PDX tumors (ER-/HER2+, wild-type PIK3CA) were randomized to vehicle, N, T, P, N+T, or P+T, with simultaneous estrogen deprivation in BT474-AZ xenograft model. Study endpoints included: (i) treatment outcome - time to tumor regression (TTR) and progression (TTP) (tumor halving/doubling over baseline, respectively), and rate and time to complete response (CR and TCR, respectively); and (ii) biomarker analysis - immunohistochemistry (IHC) and western blot (WB) analysis of tumors harvested 2-4 days post-treatment to assess key biomarkers. In the BT474-AZ model, while tumor regression was observed in 100% of N, P, T, N+T, and P+T treated mice, the tumors treated with N+T regressed faster compared to P (p&amp;lt;0.001), T (p=0.004), and P+T (p=0.044). Further, N+T was superior to N (p=0.018), and T (p=0.007) alone in achieving accelerated CR. In the BCM-3963 model, which was refractory to T, P, or T+P, while CR was achieved in 100% of N and N+T treated mice, the combination of N+T accelerated the attainment of CR compared to N alone (p=0.026). IHC analysis of short-term treated tumors showed that Ki67, pAKT, and pMAPK levels were significantly inhibited by N and N+T, but not by T, P, or P+T. Compared to P+T, N and N+T more potently inhibited Ki67, suggesting the superiority of N-containing regimens in suppressing tumor cell proliferation. Likewise, WB analysis showed that N and N+T markedly inhibited pHER2 (Y1248), pEGFR (Y1068), pAKT (S473), pERK, and pS6 levels, compared to P+T, suggesting a more potent blockade of the HER pathway by N-containing regimens, especially after short-term treatment. In the BT474-AZ model, short-term N+T treatment yielded greater inhibition of pHER2 (Y1248) and survivin levels, compared to N alone. These preclinical findings establish the efficacy of combining N with T for HER2+ breast cancer and support further clinical testing to investigate the efficacy of N+T without chemotherapy in the neoadjuvant setting for patients with HER2+ breast cancer. Citation Format: Jamunarani Veeraraghavan, Vidyalakshmi Sethunath, Martin J. Shea, Tamika Mitchell, Resel Pereira, Lanfang Qin, Sarmistha Nanda, Carmine De Angelis, Kristina Goutsouliak, Irmina Diala, Alshad S. Lalani, Sepideh Mehravaran, Susan G. Hilsenbeck, Chandandeep Nagi, Carolina Gutierrez, Mothaffar F. Rimawi, C. Kent Osborne, Rachel Schiff. The therapeutic superiority of neratinib in combination with trastuzumab compared to pertuzumab plus trastuzumab in HER2-positive in vivo breast cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4827.