Estrogen Receptor-positive Metastatic Breast Research Articles

Abstract PO4-15-04: Differential genomic profiling of progesterone receptor (PR) negative, estrogen receptor (ER) positive HER2-negative metastatic breast cancer (MBC) through circulating tumor DNA

Abstract Background: MBC is usually classified based on ER and HER2 status. Although the absence of PR expression is commonly associated with a poorer response to endocrine therapy (ET) and an unfavorable prognosis, data regarding the underlining molecular features that occur in this subgroup are still unclear. The aim of this study was to investigate genomic differences between ER/PRpos HER2neg (PRpos) and ERpos PR/HER2neg (PRneg) MBC through circulating tumor DNA (ctDNA) profiling in a large multicenter consortium. Methods: This retrospective study analyzed a cohort of 1089 patients at Weill Cornell Medicine, Northwestern University, Massachusetts General Hospital, and Washington University in St. Louis with HER2neg MBC and ctDNA testing by Guardant360. Oncogenic pathway status (i.e., RTK, RAS, RAF, MEK, NRF2, ER, WNT, MYC, P53, cell cycle, notch, and PI3K) was defined based on previous research (Sanchez-Vega et al. Cell, 2018). Associations across single nucleotide variations (SNVs), copy number variations (CNVs), pathway classification, and ER/PR status were tested by multinomial logistic regression and corrected for significant clinical characteristics (i.e., lines of treatment, metastatic sites). Prognosis was analyzed through Cox regression for overall survival (OS) defined from time of ctDNA collection. Results: Among the 1089 analyzed patients, PRpos was the most represented subtype (N:580, 53%), followed by PRneg (N:300, 28%) and Triple Negative (TNBC) (N:209, 19%). In the hormone receptor positive subgroup, bone (N:663, 75%), liver (N:323, 37%) and lymph nodes (N:314, 36%) were the main sites of distant involvement. The TNBC group was exposed to significantly fewer lines of therapy (RRR:0.37, P= 0.005 for ≥5 lines). As compared to PRneg, TNBC had less frequent and PRpos had more frequent bone involvement (RRR:0.44, P< 0.001 and RRR:1.80, P< 0.001, respectively) and less frequent liver involvement (RRR:0.64, P=0.019 and RRR:0.71, P=0.020, respectively for TNBC and PRpos). After multivariable analysis, a significant difference with respect to PRneg was observed in SNVs for the PI3K and P53 pathways for TNBC (RRR 0.49 P = 0.013 and RRR 4.06 P < 0.001, respectively) but not for PRpos. On a single gene level, a lower incidence in CDH1 SNVs (RRR 0.09 P = 0.036) and a higher incidence of ESR1 SNVs (RRR 0.09 P = 0.036) was present in PRpos compared to PRneg. TNBC, on the other hand, was characterized by a lower prevalence of KRAS SNVs (RRR 0.09 P = 0.036) and PIK3CA SNVs (RRR 0.25 P < 0.001) and a higher prevalence of TP53 SNVs (RRR 0.09 P < 0.001). No ESR1mutations were detected in TNBC. Median OS was 15 months (mo) for TNBC, 21 mo for PRneg and 31 mo for PRpos (P < 0.001). The differential prognostic impact of ctDNA features on OS was then analyzed across the PRpos and PRneg subpopulations. ESR1 SNVs was prognostic for both subgroups in univariable analysis (HR 2.19 P < 0.001 and HR 1.96 P = 0.001 for PRpos and PRneg, respectively). In multivariable analysis corrected for lines of therapy, ESR1 SNVs retained its significance only in the PRpos subgroup (HR 1.41 P = 0.021) together with NF1 SNVs (HR 2.04 P = 0.034). On the other hand, TP53 SNVs had a significant impact on OS in the PRneg subgroup (HR 2.02 P = 0.014). Conclusions: Our study confirmed the prognostic impact of PR status in ER positive, HER2 negative MBC. Moreover, it suggested a differential profile in terms of ctDNA-detectable genomic features and their impact on survival, including well-established markers such as ESR1 SNVs, with a potential relevance for disease subtyping and future algorithms for drug development trials. Citation Format: Lorenzo Gerratana, Andrew Davis, Arielle Medford, Carolina Reduzzi, Katherine Clifton, Emily Podany, Whitney Hensing, Marko Velimirovic, Ami N. Shah, Lorenzo Foffano, Laura Munoz Arcos, Eleonora Nicolò, Charles S. Dai, Jennifer Keenan, Amir Behdad, Seth Wander, William Gradishar, Cynthia Ma, Fabio Puglisi, Aditya Bardia, Massimo Cristofanilli. Differential genomic profiling of progesterone receptor (PR) negative, estrogen receptor (ER) positive HER2-negative metastatic breast cancer (MBC) through circulating tumor DNA [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-15-04.

Abstract PO5-19-05: A Phase II Trial of Stereotactic Body Radiation Therapy and Fluoroestradiol Positron Emission Tomography in Patients with Oligoprogressive Estrogen Receptor Positive Metastatic Breast Cancer

Abstract Background: The term oligoprogression (OP) refers to a clinical scenario in which patients with diffuse metastatic disease on systemic therapy have a limited number of metastases that have progressed or are new whereas the majority of metastases are stable or improved. OP disease is increasingly being encountered in clinical practice due to improvements in systemic therapy. Treating OP disease with local ablative therapies may therefore prolong the time to more diffuse progression necessitating a change in systemic therapy and may therefore lead to improved overall survival in these patients. Only one study has evaluated the role of SBRT in patients with OP MBC. This trial accrued 47 patients with OP MBC, with 2/3 having triple negative disease. There was no difference in PFS between patients that received SBRT versus those that did not (4.2 months vs. 4.4 months, p=0.2). Whether ablative therapies are beneficial in subtypes of breast cancer that have many effective systemic therapies available, such as ER+ breast cancer, remains an open question and an unmet clinical need. Design: Phase II study for patients with OP ER+ MBC (1-4 new and/or progressing metastatic lesions). Eligible patients will receive stereotactic body radiation therapy (SBRT) to the OP lesions. SBRT will be delivered to each lesion in 3-5 fractions. Each patient’s systemic therapy regimen will be held during study therapy and will resume upon completion of study therapy. Patients will then be restaged at 12 weeks post-SBSRT. Patients that have at least stable disease at that time point will continue on their systemic therapy and then be re-staged 12 weeks later (24 weeks after SBRT). This study will also study the role of ER-targeted positron emission tomography (PET) imaging with 16α-18F-Fluoro-17β-Fluroestradiol (FES) in the OP ER+ patient population with FES PET scans obtained at baseline, and at each of the 2 follow-up imaging timepoints. A key secondary hypothesis is that the use of FES PET in addition to standard imaging at baseline and in follow-up will help confirm patients have OP disease and will help assess for new lesions on subsequent restaging. Eligibility: Key inclusion criteria: age≥18 yo; histologically confirmed ER+/HER2- metastatic breast cancer; the presence of metastatic breast cancer at the time of study entry with progression in 1-4 lesions (including new lesions); SBRT must be feasible for all progressing lesions. Key Exclusion Criteria: >2 lines of systemic therapy for metastatic disease; intracranial disease progression Objective: To determine whether using SBRT to treat OP lesions allows ER+ breast cancer patients to continue on their current systemic therapy for at least 24 weeks post SBRT. Objective: To assess whether FES-PET increases the number of lesions found prior to SBRT; To determine the impact of SBRT on patient quality of life; time to next line systemic therapy; PFS Statistical Methods Simon 2 stage optimal design with α=0.05 and 1-β=0.80. The null hypothesis is that the proportion of patients that remain on their original systemic therapy ≥24 weeks (2 restaging scans) post-treatment is 20%. The alternative hypothesis is that the proportion of patients that remain on their original systemic therapy after 2 restaging scans is 50%. In stage 1, 8 patients that proceed to SBRT will be enrolled. The study will be terminated if only 0 or 1 subjects remain on original systemic therapy after 2 restaging scans. However, if ≥2 patients remain on original systemic therapy after 2 restaging, then an additional 10 patients would be enrolled for a total of 18 patients. The null hypothesis will be rejected if ≥6 patients remain on their original systemic therapy after 2 restaging scans. Contact Information: Jose G. Bazan (jbazan@coh.org) Funding Source: City of Hope Comprehensive Cancer Center Citation Format: Jose Bazan, Joanne Mortimer, Yun-Rose Li. A Phase II Trial of Stereotactic Body Radiation Therapy and Fluoroestradiol Positron Emission Tomography in Patients with Oligoprogressive Estrogen Receptor Positive Metastatic Breast Cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-19-05.

Abstract ED10-01: PI3K past, present, and future

Abstract The PI3K pathway has long remained a fertile ground for drug discovery in cancer, including in estrogen receptor positive metastatic breast cancer. It has also remained a pathway difficult to drug effectively due to on target toxicities. New strategies to overcome these toxicities have manifested in novel drug types and management strategies for patients. Here I discuss the PI3K pathway in signaling and oncogenesis, current therapies inhibiting PI3K, mechanisms limiting efficacy, and increasing the PI3K therapeutic window with a focus on mutant selective PI3K inhibitors. Finally, I will expound on the translational future of the PI3K field, highlighting advances in biomarker discovery, tumor correlative analysis, and patient advocacy. Citation Format: N. Vasan. PI3K past, present, and future [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr ED10-01.

CDK4/6 Inhibitor Efficacy in ESR1-Mutant Metastatic Breast Cancer

BackgroundIn estrogen receptor–positive metastatic breast cancer, ESR1 mutations (ESR1m) are a common mechanism of acquired resistance to aromatase inhibitors (ArIh). However, the impact ESR1 alterations have on CDK4/6 inhibitor (CDK4/6i) sensitivity has not been established. Analyses of CDK4/6i trials suggest that the endocrine therapy partner and specific ESR1 allele may affect susceptibility. We analyzed a real-world data set to investigate CDK4/6i efficacy in ESR1m metastatic breast cancer and associated clinical factors.MethodsESR1m were identified by analysis of circulating-tumor deoxyribonucleic acid. The GuardantINFORM database contains genomic information from tumors linked with claims data. Patients who started a CDK4/6i within 30 days of sequencing were categorized as having ESR1m or non-ESR1–mutant (non-ESR1m) breast cancer. Data were analyzed to determine the real-world time-to-next-treatment, defined as the start of a breast cancer treatment to initiation of the subsequent treatment.ResultsOne hundred forty-five patients with ESR1m and 612 with non-ESR1m metastatic breast cancer were analyzed. ESR1m and non-ESR1m tumors had similar real-world time-to-next-treatment on CDK4/6i regimens (hazard ratio, 1.02; 95% confidence interval, 0.82 to 1.23). Duration on therapy in the first-line and second-line plus treatment settings were comparable regardless of ESR1 status. We stratified treatment duration by concurrent endocrine therapy, and patients with ESR1m had worse outcomes on ArIh but comparable real-world time-to-next-treatment on fulvestrant.ConclusionsThese data suggest ESR1 variants are not associated with pan-CDK4/6i resistance and are consistent with the hypothesis that CDK4/6 blockade combined with a selective estrogen receptor degrader is potentially an effective option for ESR1m metastatic breast cancer.

Abstract 434: Understanding metabolic heterogeneity of estrogen receptor positive metastatic breast tumors using spatial transcriptomics and geographic information systems approaches

Abstract The tumor microenvironment consists of numerous cell populations showing diverse metabolic preferences, causing metabolic heterogeneity and therapy resistance. Fulvestrant, a clinically approved selective estrogen receptor (ER) degrader, is the standard of care for ER-positive breast cancer, yet resistance has been reported among metastatic breast cancer patients. In our previous studies, we showed such resistance is attributable to metabolic heterogeneity in the tumor microenvironment. Despite metabolic heterogeneity posing a common challenge in cancer research, current methods, such as histology and Magnetic Resonance Imaging, cannot explore nutrient uptake at transcript-level resolution with spatial context, making it difficult to understand the heterogeneity that affects cellular growth and metabolism. Visium Spatial Transcriptomics, a method that shows regional gene expression, is emerging as a powerful spatial analysis tool, and we integrated this method with geographic information systems to investigate the tumor microenvironment heterogeneity and highlight the importance of cell-microenvironment interactions. We developed a computational pipeline to visualize hotspots of spatially relevant genes using breast cancer metastatic samples from xenograft models and human patients. This pipeline integrates Visium tools and geospatial methods for processing transcriptome profiling data. We analyzed the degree of local spatial association based on Local Moran’s I to estimate regions with significant co-localization of single and multiple gene expressions within a tumor. We then leveraged the co-localized areas in the process of biomarker identification and revealing biological pathways. We also used unsupervised clustering of the complete transcriptome profiles to test the resemblance between identified clusters and hotspots as well as any disruption in the spatial structure of clusters in response to therapy or metastasis. Gene set enrichment analysis of these clusters revealed several glucose or lipid metabolism pathways, and mapping of the genes corresponding to each pathway showed mutually exclusive gene expression patterns in the sample. We also observed minimal co-localization of two metabolic pathways but extensive co-localization between metabolic genes and ER target genes, suggesting distinct metabolic preferences and spatial distributions of endocrine therapy resistant cell populations. Altogether, our analysis pipeline established a computational framework for characterizing the link between tumor microenvironment, local metabolism, and changes in transcriptional response to external stimuli. Our findings support the development of strategies to target different endocrine resistant regions using a combination of ER and metabolic signaling inhibitors. Citation Format: Jin Young Yoo, Sabrina Akter, Nandana Varma, Atharva Jain, Aiman Soliman, Zeynep Madak Erdogan. Understanding metabolic heterogeneity of estrogen receptor positive metastatic breast tumors using spatial transcriptomics and geographic information systems approaches [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 434.

Abstract 2407: Detection of hotspot PIK3CA mutations in primary tumors and plasma-cfDNA of breast cancer patients using a novel highly sensitive CE-IVD kit

Abstract Introduction: Detection of PIK3CA mutations in primary tumors (FFPEs) and liquid biopsy samples is of increasing importance for treatment decisions in breast cancer. Oncolipsy PIK3CA kit (Pharmassist Ltd, Greece) is based on the combination of allele-specific priming, asymmetric PCR, and melting curve analysis. The CE-IVD kit can detect the presence of four PIK3CA hotspot mutations (E542K, E545K, E545Q, H1047R). The aim of the present study was to: a) evaluate the performance of this novel CE-IVD kit both in FFPEs and plasma-cfDNA samples from patients with metastatic breast cancer (MBC) and b) directly compare the derived results using the Oncolipsy PIK3CA kit with the cobas® PIK3CA mutation test (CE-IVD, Roche Diagnostics, Germany) in identical gDNAs isolated from FFPEs samples. Patients and methods: We analyzed 42 primary tumors (FFPEs) and 29 plasma-cfDNA samples from patients with estrogen receptor (ER) positive metastatic breast cancer. Genomic DNA was isolated from FFPEs using the QIAamp DNA FFPE Tissue Kit (Qiagen, Hilden, Germany), while cfDNA was extracted from 2.00 mL of plasma using the QIAamp DSP cNA Kit (Qiagen, Hilden, Germany). PIK3CA hotspot mutations were detected in the LightCycler® 2.0 (IVD instrument, Roche Diagnostics, Germany) using the highly sensitive Oncolipsy PIK3CA kit (analytical sensitivity 0.05%). A direct comparison study was performed using the cobas® PIK3CA Mutation Test on the cobas z 480 analyzer (IVD instrument, Roche Diagnostics, Germany). Results: In primary tumors the E545K mutation was detected in 22/42 (52.4%), the E542K and H1047R mutations were detected in 11/42 (26.2%) and 4/42 (9.5%) of primary tumor samples, respectively, while the E545Q mutation was not detected in any FFPE sample tested. At least one PIK3CA mutation was detected in 14/29 (48.3%) plasma-cfDNA samples. More specifically the E545K mutation was detected in 11/29 (38%), the H1047R mutation in 3/29 (10.4%) and the E542K mutation in 2/29 (6.9%) while the E545Q mutation was not detected in any plasma-cfDNA sample tested. In FFPEs, as expected, PIK3CA mutation frequency was higher compared to plasma-cfDNA. Direct comparison of the results derived using these two commercially available kits, the Oncolipsy CE-IVD kit and the cobas PIK3CA mutation test, using the same gDNA isolated from 42 FFPEs revealed a concordance of 34/42 (81%). Conclusions: The commercially available Oncolipsy PIK3CA CE-IVD kit is highly sensitive and specific for the detection of four PIK3CA hotspot mutations in primary tumors and plasma-cfDNA. Acknowledgement: This study has been financially supported by the European Union and Greek national funds through the Operational Program Competitiveness, Entrepreneurship and Innovation, under the call RESEARCH - CREATE - INNOVATE (project code: T1RCI-02935). Citation Format: Stavroula Smilkou, Dimitra Stergiopoulou, Eleni Tzanikou, Loukas Kaklamanis, Ioanna Balgkouranidou, Helena Linardou, Flora Zagouri, Evangelia Razi, Stylianos Kakolyris, Amanda Psyrri, Christos Papadimitriou, Ioanna Koukli, Athina Markou, Evi Lianidou. Detection of hotspot PIK3CA mutations in primary tumors and plasma-cfDNA of breast cancer patients using a novel highly sensitive CE-IVD kit [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2407.

Comparison of treatment discontinuation rates between CDK4/6 inhibitors.

2 Background: Abemaciclib and Palbociclib are both CDK4/6 inhibitors and its combination use with hormone drugs prolonged progression-free survival in estrogen receptor positive metastatic breast cancer. Both are not yet directly compared in terms of efficacy, adverse events, and discontinuation rates due to the adverse events. MonarchE trial evaluated adverse events of Abemaciclib as follows; 81.3% had diarrhea and 41.3% had neutropenia, whereas PALOMA-3 trial showed 92% of neutropenia (74% had Grade3 neutropenia) in patients receiving Palbociclib. Of the patients who received CDK4/6 inhibitors in our institutions, we compared the difference of rates in adverse events, and we discussed the importance of acknowledging the risks between the two drugs. Methods: We reviewed a total of 169 patients comprising 53 with Abemaciclib and 116 with Palbociclib from 2017 to 2023 at two institutions as 1st – 3rd line treatments. The year of prescription, treatment line, age and the adverse event which led to treatment discontinuation was analyzed. Results: All patients were women, and the median age was 57 (29-84). The patients who received treatment as 1st, 2nd, 3rd lines were 98, 49, and 22, respectively. Twenty-six patients received cytotoxic chemotherapy before CDK4/6 inhibitors. Patients diagnosed as de-novo stage IV patients were 108, while 61 had recurrent tumors. Treatment discontinuation due to adverse events of CDK4/6 inhibitors were 41 (24%) in which 14 (8.3%) had diarrhea and 20 (12%) had neutropenia. We conducted a multivariate analysis setting the endpoint as any adverse events resulting to treatment discontinuation. Abemaciclib (HR=1.20 p=0.01), the first prescription date (2017-19 vs 2020-23, HR:0.84 p=0.02), and patients with metastatic breast cancer at first diagnosis (HR:1.15 p=0.039) significantly affected adverse events leading to treatment discontinuation. Patients receiving Abemaciclib discontinuedecause of diarrhea more than patients receiving Palbociclib (p<0.001 HR:1.28), whereas patients receiving Palbociclib discontinued treatment because of neutropenia more than patients receiving Abemaciclib (HR=1.13 p=0.036). Conclusions: The type of CDK4/6 inhibitor, de-novo Stage IV or recurrent breast cancer, and the year of treatment initiation led to differences in safety of CDK4/6 inhibitor treatment. Abemaciclib leads to treatment discontinuation more than Palbociclib with the most reported adverse event; diarrhea. In addition, the risk of adverse events differs from the year of treatment initiation, which suggests that management of adverse events is improving over time. Thus, it is crucial to accomplish the management of adverse events when starting Abemaciclib which has higher treatment discontinuation rates due to adverse events.

Abstract GS3-07: GS3-07 Clonal evolution and mechanisms of acquired resistance to CDK4/6 inhibitors in ER-wild type and ER-mutant breast cancer

Abstract Background Despite the remarkable activity of CDK4/6 inhibitors (CDK4/6i) in the treatment of estrogen receptor positive (ER+) metastatic breast cancer (BC), most patients eventually develop resistance to these drugs. The ctDNA analysis of the PALOMA-3 trial showed that the estrogen receptor (ER) mutation Y537S is a potential mechanism of acquired resistance to the combination of endocrine therapy (ET) with CDK4/6i. To date, the role of the ER mutations in the clonal evolution and the mechanisms of acquired resistance to CDK4/6i is unknown. Moreover, it is not known if the development of resistance to CDK4/6i in the presence or absence of ER mutations is due to the expansion of pre-existing resistant clones or to the de novo acquisition of resistance mechanisms. Methods To explore the clonal evolution and the mechanisms of resistance to CDK4/6i in ER-wild type (ER-WT) and ER-mutant (ER-Mut) BC, we transduced doxycycline (DOX)-inducible Y537S ER-Mut MCF7 cells with the ClonTracer library, a high-complexity DNA barcode library, and cultured the barcoded cells without DOX (MCF7), or with DOX to induce the expression of the Y537S ER mutation (MCF7-YS). To develop Palbociclib (Palbo)-resistant (PDR) and Abemaciclib (Abema)-resistant (ABR) cell models, the barcoded MCF7 and MCF7-YS cells were passaged in culture with increasing concentrations of Palbo and Abema until the acquisition of resistance. The clonal dynamics and the molecular characteristics of the PDR and ABR models were investigated by barcode sequencing, whole-exome sequencing (WES), bulk and single cell RNA sequencing (RNAseq) and protein analyses. Finally, using an ER-Mut barcoded mice model, we compared the in vitro clonal evolution of ER-Mut CDK4/6i-resistant cells with the in vivo clonal evolution of ER-Mut metastases. Results The analysis of the barcodes revealed that during the acquisition of resistance to either Palbo or Abema there is a strong clonal selection of pre-existing resistant clones. The PDR clones were different in the presence of the Y537S mutation versus WT-ER. In contrast, the clones enriched in the ABR cells were comparable between WT and mutant ER. Furthermore, the ER mutations led to decreased diversity of the enriched clones in the PDR but not in the ABR cells. Interestingly, the barcodes enriched in the PDR and ABR models did not overlap. Unsupervised analyses showed that the samples clustering based on the barcodes fractions and the mutations were similar, suggesting that the clonal selection was driven by cellular populations with specific mutational landscapes. All the ER-WT and ER-Mut resistant models had different transcriptional profiles and by single-cell RNAseq showed various degrees of intra-sample heterogeneity. At the protein level, the PDR and the ABR cells displayed downregulation of ER, Rb and p27 and upregulation of p21. In the ER-Mut conditions Cyclin D1 was upregulated in the PDR cells, while Cyclin E was upregulated in the ABR cells. Finally, the barcode sequencing of the mice metastases revealed that the clonal selection in ER-Mut metastases and in ER-Mut CDK4/6i-resistant cells is different. Conclusion Our study suggests that the development of resistance to CDK4/6i is due to the selection of pre-existing resistant clones. We also demonstrate that the expression of the Y537S ER mutation impacts the clonal evolution and the mechanisms of acquired resistance to Palbo but not to Abema. Finally, we show that the clonal evolution and mechanisms are disparate in Palbo and Abema resistance. These results support the addition of a third drug to CDK4/6i and ET, early in treatment, to delay the selection of pre-existing resistant clones and prolong the response to treatment and highlight differences between Palbo and Abema. Citation Format: Cristina Guarducci, Simona Cristea, Avery Feit, Sergey Naumenko, Agostina Nardone, Wen Ma, Douglas Russo, Gabriella Cohen Feit, Ariel Feiglin, Francisco Hermida-Prado, Shira Sherman, Myles Brown, Franziska Michor, Rinath Jeselsohn. GS3-07 Clonal evolution and mechanisms of acquired resistance to CDK4/6 inhibitors in ER-wild type and ER-mutant 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 GS3-07.

200 (PB-113) Poster - Systemic therapies post progression on CDK4/6 inhibitors in patients with oestrogen receptor positive metastatic breast cancer: Real world data from Guy’s Cancer Centre

Background: There is no prospective data to guide treatment strategies after progression on cyclin dependent kinases 4 and 6 inhibitors (CDK4/6i) in oestrogen receptor positive (ER+) metastatic breast cancer. Here we present a single-institution experience to assess prescribing patterns and outcomes with subsequent therapies post CDK4/6i progression.

Abstract 1772: EGFR signaling as a mechanism of resistance to CDK4/6 inhibitors in Palbociclib-resistant ER+ breast cancer cells

Abstract Breast cancer driven by estrogen receptor (ER) is responsible for approximately 60-70% of cases among women. Selective inhibition of cyclin-dependent kinases 4 and 6 (CDK4/6) and ER signaling is now standard-of-care therapy for ER positive (ER+) metastatic breast cancer. CDK4/6 inhibitors, including palbociclib (PD), used in combination with endocrine therapy have shown improvement in progression-free survival compared to endocrine therapy alone in the metastatic setting. However, the inevitable development of acquired resistance significantly limits the efficacy of this targeted therapy. In ER+ breast cancer resistant cells, rewired signaling driven by different oncogenes, including the epidermal growth factor receptor (EGFR), overcomes the inhibition of CDK4/6 by PD, which, in turn, allows cell cycle progression contributing to PD-resistance. We investigated the dependency of ER+ breast cancer cell lines sensitive and resistant to PD on EGFR signaling in vitro. We evaluated the expression and activity of EGFR in two different matching pairs of ER+ palbociclib-sensitive (pS) and palbociclib-resistant (pR) breast cancer cell lines (MCF7 and T47D) and observed elevated EGFR expression in MCF7/pR cells. Increased EGFR expression in MCF7/pR cells correlated with elevated ER alpha phosphorylation at S118 and S167 sites, suggesting a direct cross-talk between EGFR and ER signaling in PD-resistant cells. Serum starvation significantly decreased EGFR activity, cdk2 and cyclin D1 expression in both MCF7 and T47D PD-resistant cells compared to the parental cell lines, confirming their dependency on the presence of growth factors. Notably, stimulation with EGF promoted ER phosphorylation and expression of cell cycle related targets: cdk2, cyclin E and wee1 in both MCF7 and T47D cell lines sensitive and resistant to PD. Furthermore, stimulation with estrogen promoted EGFR activation in MCF7/pS and MCF7/pR cells, indicating a direct cross-regulation between these molecular targets. Further, exposure to the EGFR inhibitor erlotinib dramatically inhibited the proliferation of MCF7/pR cells compared to the parental cells, suggesting that EGFR could be an important bypass signaling regulator contributing to resistance to PD. We found that dual treatment with erlotinib and PD significantly inhibited Rb phosphorylation in MCF7/pR cells improving the effect of PD on cell cycle arrest. Moreover, dual treatment of MCF7/pR cells attenuated ER expression suggesting the higher dependency of ER signaling on EGFR activity in resistant cells. Taken together, our findings suggest that EGFR signaling is a mechanism of resistance in ER+ cell lines with acquired resistance to PD in vitro and that EGFR targeting can be a promising approach to improve palbociclib efficacy in breast cancer patients. Citation Format: Nadiia Lypova, Lilibeth Lanceta, Susan M. Dougherty, Jason A. Chesney, Yoannis Imbert-Fernandez. EGFR signaling as a mechanism of resistance to CDK4/6 inhibitors in Palbociclib-resistant ER+ breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1772.

Abstract 4043: KSHN001034: An intramuscular prodrug of fulvestrant to treat estrogen-receptor (ER) positive advanced metastatic breast cancer

Abstract Purpose: Breast cancer (BC) remains a considerable health concern, with over 75% of total cases accounting for ER+ BC. Fulvestrant, a selective estrogen receptor degrader (SERD), is the primary drug of choice for treating advanced metastatic ER+, BC. Currently, Faslodex® (500 mg) is given as an intramuscular (IM) depot that achieves the steady-state plasma concentration of 24-28 ng/mL of fulvestrant from day 28 onwards. Additionally, clinical studies suggest that current Faslodex® (500 mg) achieves insufficient blockade of ERα receptor plausibly due to suboptimal physicochemical and pharmacokinetic (PK) properties. Hence, achieving higher fulvestrant plasma concentrations is a significant unmet need to improve efficacy and inhibit the development of drug resistance. To overcome these issues, we modified the physicochemical properties of the fulvestrant to enhance its solubility by developing a prodrug, KSHN001034, with the potential to deliver higher fulvestrant plasma exposures. Methods: KSHN001034 was evaluated for its physicochemical and ADME properties. Thermodynamic solubility and liver microsomal stability were performed. KSHN001034 was formulated as 100 mg/mL compared with Faslodex (50 mg/mL). Single-dose PK studies were performed using KSHN001034 in Sprague Dawley (SD) rats and Beagle Dogs post-IM administration for 7 and 14 days, respectively. The uterotrophic activity was investigated by determining changes in uterine weight in juvenile female SD rats for KSHN001034 (10 mg/kg, IP) and fulvestrant (25 mg/kg, SC). The ERα expression was also evaluated in rat uterus tissue via a simple western assay. Results: A remarkable improvement (~1100 fold) was achieved for KSHN001034 in aqueous solubility (1.112 mg/mL) vs. fulvestrant (<1 µg/mL). Significant improvement was observed in metabolic stability vs. fulvestrant. KSHN001034 showed significant improvement in PK exposure of fulvestrant upon IM administration in rats and dogs with higher AUC and Cmax compared to that of Faslodex. Relative bioavailability of fulvestrant post-KSHN001034 treatment was found to be >6-fold in rats and ~2.5 fold in dogs compared to Faslodex. Treatment of KSHN001034 antagonized estradiol (E2)-mediated uterine stimulation, exhibiting significant inhibition (85%) of uterotrophic activity vs. fulvestrant (73%). KSHN001034 reduced E2-induced uterus weight, supporting efficacy due to released fulvestrant as it is a true SERD. Additionally, KSHN001034 significantly inhibited the E2-induced ERα expression in rat uterus. Conclusions: Overall, KSHN001034 indicates the potential to deliver higher fulvestrant concentrations that would effectively block the ERα receptor resulting in improved efficacy and minimal emergence of resistance. Further, enhanced solubility and reduced injection volume would result in better patient compliance. Citation Format: Heta Pandya, Ganesh Sangle, Vishal Unadkat, Vishal Goswami, Parva Purohit, Chirag Mehta, Sonam Sinha, Ketan Variya. KSHN001034: An intramuscular prodrug of fulvestrant to treat estrogen-receptor (ER) positive advanced metastatic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 4043.

Results of the phase IIa RADICAL trial of the FGFR inhibitor AZD4547 in endocrine resistant breast cancer

We conducted a phase IIa, multi-centre, open label, single arm study (RADICAL; NCT01791985) of AZD4547 (a potent and selective inhibitor of Fibroblast Growth Factor Receptor (FGFR)-1, 2 and 3 receptor tyrosine kinases) administered with anastrozole or letrozole in estrogen receptor positive metastatic breast cancer patients who had become resistant to aromatase inhibitors. After a safety run-in study to assess safety and tolerability, we recruited 52 patients. The primary endpoint was change in tumour size at 12 weeks, and secondary endpoints were to assess response at 6 weeks, 20 weeks and every 8 weeks thereafter and tolerability of the combined treatment. Two partial responses (PR) and 19 stable disease (SD) patients were observed at the 12-week time point. At 28 weeks, according to centrally reviewed Response Evaluation Criteria in Solid Tumours (RECIST) criteria, five PR and 8 SD patients were observed in 50 assessable cases. Overall, objective response rate (5 PR) was of 10%, meeting the pre-specified endpoint. Fourteen patients discontinued due to adverse events. Eleven patients had retinal pigment epithelial detachments which was asymptomatic and reversible in all but one patient. Exploratory ribonucleic acid sequencing (RNA-Seq) analysis was done on patients’ samples: 6 differentially-expressed-genes could distinguish those who benefited from the addition of AZD4547.

Mutual exclusivity of ESR1 and TP53 mutations in endocrine resistant metastatic breast cancer

Both TP53 and ESR1 mutations occur frequently in estrogen receptor positive (ER+) metastatic breast cancers (MBC) and their distinct roles in breast cancer tumorigenesis and progression are well appreciated. Recent clinical studies discovered mutual exclusivity between TP53 and ESR1 mutations in metastatic breast cancers; however, mechanisms underlying this intriguing clinical observation remain largely understudied and unknown. Here, we explored the interplay between TP53 and ESR1 mutations using publicly available clinical and experimental data sets. We first confirmed the robust mutational exclusivity using six independent cohorts with 1,056 ER+ MBC samples and found that the exclusivity broadly applies to all ER+ breast tumors regardless of their clinical and distinct mutational features. ESR1 mutant tumors do not exhibit differential p53 pathway activity, whereas we identified attenuated ER activity and expression in TP53 mutant tumors, driven by a p53-associated E2 response gene signature. Further, 81% of these p53-associated E2 response genes are either direct targets of wild-type (WT) p53-regulated transactivation or are mutant p53-associated microRNAs, representing bimodal mechanisms of ER suppression. Lastly, we analyzed the very rare cases with co-occurrences of TP53 and ESR1 mutations and found that their simultaneous presence was also associated with reduced ER activity. In addition, tumors with dual mutations showed higher levels of total and PD-L1 positive macrophages. In summary, our study utilized multiple publicly available sources to explore the mechanism underlying the mutual exclusivity between ESR1 and TP53 mutations, providing further insights and testable hypotheses of the molecular interplay between these two pivotal genes in ER+ MBC.

Beyond First Line CDK4/6 Inhibitors (CDK4/6i) and Aromatase Inhibitors (AI) in Patients with Oestrogen Receptor Positive Metastatic Breast Cancer (ER+ MBC): The Guy’s Cancer Centre Experience

Purpose: The optimal treatment strategy following progression on first line (1L) CDK4/6i and AI in patients with ER+ MBC is uncertain [1]. We performed a single-institution retrospective review of patients who received 1L CDK4/6i to assess prescribing patterns and clinical responses to post-CDK4/6i treatment.

Abstract P2-01-08: Esr1 Y537 mutations are associated with increased baseline circulating tumor cells enumeration for patients with estrogen receptor positive metastatic breast cancer

Abstract Introduction: Estrogen Receptor (ER)-positive breast cancer is the most frequent breast cancer subtype. Endocrine therapy (ET) targeting the ER pathway including CDK4/6 inhibitors represents the main initial therapeutic approach. However, clinical resistance associated with progression of disease caused by ESR1 mutations is a recognized an important mechanism of ET resistance. ESR1 mutations, most often detected from liquid biopsies, have been consistently associated with a worse outcome and are being currently evaluated as a potential biomarker to guide therapeutic decisions. Here we reported a new finding on the association between ESR1 amino acid Y537 mutations in ctDNA and circulating tumor cells (CTC) in metastatic breast cancer (MBC). Methods: The study included 158 ER positive MBC patients enrolled under an IRB-approved trial (NU16B06) at Lurie Cancer Center, Northwestern University. The patients received systemic treatments in 2016-2020. Whole blood samples (7.5ml/each) were collected in EDTA tubes from patients who were longitudinally characterized for CTCs before therapy (baseline). CTCs enumeration were performed in FDA approved CELLTRACKS ANALYZERII® System (Menarini) by using CXC Kit contains antibodies targeting EpCAM antigen for capturing CTCs, anti-CK-PE which is specific for the intracellular protein cytokeratin in epithelial cells, DAPI for staining the cell nucleus, anti-CD45-APC is specific for leukocytes. The CTCs were classified based on morphology and correct phenotype as CK+, DAPI+ and CD45-. Plasma ctDNA was isolated using a Qiagen circulating nucleic acid kit, and then was analyzed using the Guardant360 next-generation sequencing (NGS)-based assay. In this study, only patients with ESR1 mutations were included for statistical analyses of correlation between the hotspot mutation with the endocrine therapy resistance by using Causal Inference approach. All statistical analyses were conducted Mann-Whitney U test by IBM SPSS version 23.0. Results: ESR1 mutations were detected in 40 out of 158 patients at baseline. ESR1 Y537(N/C/S) mutations were detected in 13 patients, among which there were 9 patients who only had the Y537(N/C/S) mutations and no other ESR1 hotspot mutations (Group 1) and there were 4 patients who had polyclonal ESR1 mutations. In the latter cohort, there were 17 ESR1 amino acid mutations detected in 31 patients (Group 2) including Q314, T347T, N359I, K362N, E380Q, V392I, G442R, F461I, S463P, S464, I487M, K520K, M528V, L536H, D538G, D545D and Q565. There were no any ESR1 mutations detected in the other 118 patients (Group 3). The CTC>5/7.5mL (Stage IV aggressive) were found in 66.7%, 38.7% and 29.3% patients in Group 1, Group 2 and Group 3 respectively. The median total CTCs was 26.0/7.5mL in MBC patients with ESR1 Y537(N/C/S) mutations (Group 1). The median of total CTCs was 2.0/7.5mL in the MBC patients with polyclonal ESR1 mutations but not on Y537 (N/C/S) (Group 2), and it was 1.0/7.5mL in the group of metastatic breast cancer without ESR1 mutations (Group 3). The median total CTCs in Group 1 was significantly higher than Group 2 (Mann-Whitney U, P<0.05) and Group 3 (P=0.009), which indicated that Y537(N/C/S) mutations were correlated with higher baseline CTCs. Conclusions: In this study, we demonstrated a correlation between presence of ESR1 Y537 mutations and increased CTCs (higher Stage IV aggressive cases) in patients with HR+ MBC, These preliminary data may suggest a critical role of ESR1 Y537 mutations in the metastatic process, also indicating that different ESR1 amino acids mutations may play different roles on disease progress. Larger validation studies are needed to confirm that evaluation of ESR1 Y537 variant together with CTCs numeration is an accurate tool to identify endocrine-refractory disease. Citation Format: Qiang Zhang, Jianhua Jiao, Paolo D'Amico, Andrew A. Davis, Weijun Qin, Lorenzo Gerratana, Saya L. Jacob, Youbin Zhang, Jeannine Donahue, Wenan Qiang, Ami N. Shah, Lisa Flaum, William Gradishar, Leonidas C. Platanias, Massimo Cristofanilli. Esr1 Y537 mutations are associated with increased baseline circulating tumor cells enumeration for patients with estrogen receptor positive metastatic 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 P2-01-08.

Abstract P1-13-02: Autophagy inhibition and senolytics to improve the response to fulvestrant + palbociclib in ER positive breast cancer

Abstract While anti-estrogens or aromatase inhibitors in combination with cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors are the current standard of care for estrogen receptor-positive (ER+) Her-2 negative metastatic breast cancer, relapse after remission continues to be a serious limitation to the effectiveness of these therapies. Although these combination therapies prolong progression-free survival compared to endocrine therapy alone, the growth-arrested state of residual tumor cells is clearly transient. Tumor cells that escape what might be considered a dormant or quiescent state and regain proliferative capacity often acquire resistance to further therapies. Our studies are based upon the observation that breast tumor cells arrested by Fulvestrant + Palbociclib enter into a state of autophagy/senescence from which a subpopulation ultimately escapes, potentially contributing to disease recurrence. ER+ MCF-7 breast tumor cells that were induced into autophagy/senescence by Fulvestrant + Palbociclib in vitro were exposed to the autophagy inhibitors, chloroquine and bafilomycin, prior to as well as after growth arrest. However, autophagy inhibition only moderately enhanced the response to therapy and, at best, delayed proliferative recovery. These findings were confirmed by genetic inhibition of autophagy. Despite clear entry into a state of senescence, the Bcl-xl inhibitor and senolytic ABT-263 (navitoclax) failed to promote tumor cell death; this is likely a consequence of high levels of survivin in the MCF-7 breast tumor cell line. However, the BET inhibitor, ARV-825, eliminated a large fraction of the senescent cell population and significantly delayed proliferative recovery. Cell death did not appear to be a consequence of increased apoptosis. Additionally, analysis of three database sets demonstrated that higher breast cancer expression of BRD4, a primary target of ARV-825, correlated with lower recurrence-free survival compared to patients with cancers expressing lower levels of BRD4. These studies indicate that administration of BET inhibitors, which are currently being investigated in multiple clinical trials, may potentially improve standard of care therapy in metastatic ER+ breast cancer patients by prolonging progression-free survival. Citation Format: Ryan Michael Finnegan. Autophagy inhibition and senolytics to improve the response to fulvestrant + palbociclib in ER positive 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 P1-13-02.

Monoclonal antibodies to activated CDK4: use to investigate normal and cancerous cell cycle regulation and involvement of phosphorylations of p21 and p27

ABSTRACT Cyclin-dependent kinase 4 (CDK4) is a master integrator that couples mitogenic/oncogenic signaling with the cell division cycle. It is deregulated in most cancers and inhibitors of CDK4 have become standard of care drugs for metastatic estrogen-receptor positive breast cancers and are being evaluated in a variety of other cancers. We previously characterized the T-loop phosphorylation at T172 of CDK4 as the highly regulated step that determines the activity of cyclin D-CDK4 complexes. Moreover we demonstrated that the highly variable detection of T172-phosphorylated CDK4 signals the presence or absence of the active CDK4 targeted by the CDK4/6 inhibitory drugs, which predicts the tumor cell sensitivity to these drugs including palbociclib. To date, the phosphorylation of CDK4 has been very poorly studied because only few biochemical techniques and reagents are available for it. In addition, the available ones including 2D-IEF separation of CDK4 modified forms are considered too tedious. The present report describes the generation, selection and characterization of the first monoclonal antibodies that specifically recognize the active CDK4 phosphorylated on its T172 residue. One key to this success was the immunization with a long phosphopeptide corresponding to the complete activation segment of CDK4. These monoclonal antibodies specifically recognize T172-phosphorylated CDK4 in a variety of assays, including western blotting, immunoprecipitation and, as a capture antibody, a sensitive ELISA from cell lysates. The specific immunoprecipitation of T172-phosphorylated CDK4 allowed to clarify the involvement of phosphorylations of co-immunoprecipitated p21 and p27, showing a privileged interaction of T172-phosphorylated CDK4 with S130-phosphorylated p21 and S10-phosphorylated p27. Abbreviations:2D: two-dimensional; CAK: CDK-activating kinase; CDK: cyclin-dependent kinase; HAT: Hypoxanthine-Aminopterin-Thymidine; FBS: fetal bovine serum; IP: immunoprecipitation; ID: immunodetection; mAb: monoclonal antibody; PAGE: polyacrylamide gel electrophoresis; PBS: phosphate buffer saline; pRb: retinoblastoma susceptibility protein; SDS: sodium dodecyl sulfate; DTT: dithiotreitol; TET: tetracyclin repressor; Avi: Avi tag; TEV: tobacco etch virus cleavage site; EGFP: enhanced green fluorescent protein; BirA: bifunctional protein biotin ligase BirA; IRES: internal ribosome entry site; HIS: poly-HIS purification tag; DELFIA: dissociation-enhanced lanthanide fluorescent immunoassay; 3-MBPP1: 1-(1,1-dimethylethyl)-3[(3-methylphenyl) methyl]-1H-pyrazolo[3,4-d] pyrimidin-4-amine; BSA: bovine serum albumin; ECL: Enhanced chemiluminescence

A Phase Ib Dose Escalation Trial of RO4929097 (a γ-secretase inhibitor) in Combination with Exemestane in Patients with ER + Metastatic Breast Cancer (MBC)

Preclinical studieshave demonstrated a complex cross-talk between Notch and estrogen signaling in ERα-positive breast cancer. Gamma-secretase inhibitors (GSIs) are investigational agents that block the cleavage and activation of Notch receptors. In animal models of endocrine-resistant breast cancer, combinations of tamoxifen and GSIs produce additive or synergistic efficacy, while decreasing the intestinal toxicity of GSIs. However, results of a clinical trial of a GSI-endocrine therapy combination in the metastatic setting have not been published to date, nor had the safety of such combinations been investigated with longer term treatment. We conducted a phase 1b dose escalation trial (NCT01149356) of GSI RO4929097 with exemestane in patients with ERα+, metastatic breast cancer (MBC) Study objectivesTo determine the safety, tolerability and maximum tolerated dose (MTD) or recommended phase 2 dose (RP2D) of RO4929097 when administered in combination with exemestane in patients with estrogen receptor positive metastatic breast cancer ResultsWe enrolled 15 patients with MBC. Of 14 evaluable patients, one had a partial response, 6 had stable disease and 7 progressive disease. Twenty % of patients had stable disease for ≥ 6 months. Common toxicities included nausea (73.3%), anorexia (60%), hyperglycemia (53.3%), hypophosphatemia (46.7%), fatigue (66.7%) and cough (33.0%). Grade 3 toxicities were uncommon, and included hypophosphatemia (13%) and rash (6.3%). Rash was the only DLT observed at 140 mg/d. Results suggest a possible recommended phase 2 dose of 90 mg/d. Ten patients with evaluable archival tissue showed expression of PKCα, which correlated with expression of Notch4. Mammospheres from a PKCα-expressing, endocrine-resistant T47D cell line were inhibited by a GSI-fulvestrant combination ConclusionsOur data indicate that combinations including endocrine therapy and Notch inhibitors deserve further investigation in endocrine-resistant ERα-positive breast cancer.

Abstract 742: The integration of a structure-function rule and a transcriptional signature to assign ESR1 fusion activity in metastatic breast cancer

Abstract Background: In-frame chimeric ESR1 fusion proteins can cause endocrine therapy (ET) resistance in estrogen receptor-positive (ER+) metastatic breast cancer (MBC) (PMC6171747). Here, we further investigated multiple new examples of ESR1 fusions in order to differentiate between transcriptionally active “driver” ESR1 fusions from inactive, likely “passenger” events. Methods: ESR1 fusions were identified by RNA-sequencing (RNA-seq) or whole genome sequencing in MBC (PMC5913625, PMC6872491 and unpublished). ESR1 fusion cDNA constructs were expressed in ER+ breast cancer cell lines. Cell proliferation and cell motility were assessed. RNA-seq followed by qPCR validation was conducted to examine individual transcriptional profiles. Results: All ESR1 fusions studied contained the first six exons of ESR1 (e6) fused in-frame to diverse partner gene sequences that replaced the ESR1 drug/ligand binding domain. Fusions with a transcription factor (TF) or coactivator (CoA) gene partner, for example ESR1-YAP1, ESR1-ARNT2-e18 and ESR1-SOX9 activated fulvestrant-resistant cell growth and hormone-independent cell motility. Other ESR1-e6 fusions, including ESR1-DAB2, ESR1-GYG1, ESR1-PCMT1 and ESR1-ARID1B did not show these phenotypes. We thus conclude that in-frame ESR1-e6 fusions arising from inter-chromosomal translocations with 3' TF/CoA partners are likely to be active. However, the active ESR1-PCDH11X fusion involves a proto-cadherin and is therefore an exception to this rule. This outlier emphasizes the need to establish additional approaches to determine ESR1 fusion activity. RNA-seq of T47D cells expressing active and inactive ESR1 fusions defined a gene signature associated with active ESR1 fusions, with activation of estrogen response and epithelial-to-mesenchymal transition (EMT) genes. This transcriptional signature was present in a patient-derived xenograft bearing the ESR1-YAP1 fusion, and thus potentially identifies the presence of an active ESR1 fusion protein. We subsequently identified two new ESR1 fusions involving recurrent TF/CoA partners, ESR1-ARNT2-e2 and ESR1-LPP. Both demonstrated tumor cell growth and motility activation, as predicted by the functional rule. The gene activation patterns were similar to the three other active fusions suggesting that despite marked diversity in the 3' partners, the transcriptional activities were similar and potentially diagnostic. Conclusion: The integration of the structure-function rule and the stereotypic transcriptional signature distinguishes pathogenic ESR1 fusions from non-active passenger events, thus prioritizing patients bearing ESR1 translocation-driven tumors for targeted therapeutic approaches. Citation Format: Xuxu Gou, Meenakshi Anurag, Jonathan T. Lei, Sinem Seker, Adrian V. Lee, Dan R. Robinson, Matthew J. Ellis. The integration of a structure-function rule and a transcriptional signature to assign ESR1 fusion activity in metastatic breast cancer [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 742.

Inhibition of CDK4/6 as Therapeutic Approach for Ovarian Cancer Patients: Current Evidences and Future Perspectives.

Simple SummaryAltered regulation of the cell cycle is a hallmark of cancer. The recent clinical success of the inhibitors of CDK4 and CDK6 has convincingly demonstrated that targeting cell cycle components may represent an effective anti-cancer strategy, at least in some cancer types. However, possible applications of CDK4/6 inhibitors in patients with ovarian cancer is still under evaluation. Here, we describe the possible biological role of CDK4 and CDK6 complexes in ovarian cancer and provide the rationale for the use of CDK4/6 inhibitors in this pathology, alone or in combination with other drugs. This review, coupling basic, preclinical and clinical research studies, could be of great translational value for investigators attempting to design new clinical trials for the better management of ovarian cancer patients.Alterations in components of the cell-cycle machinery are present in essentially all tumor types. In particular, molecular alterations resulting in dysregulation of the G1 to S phase transition have been observed in almost all human tumors, including ovarian cancer. These alterations have been identified as potential therapeutic targets in several cancer types, thereby stimulating the development of small molecule inhibitors of the cyclin dependent kinases. Among these, CDK4 and CDK6 inhibitors confirmed in clinical trials that CDKs might indeed represent valid therapeutic targets in, at least some, types of cancer. CDK4 and CDK6 inhibitors are now used in clinic for the treatment of patients with estrogen receptor positive metastatic breast cancer and their clinical use is being tested in many other cancer types, alone or in combination with other agents. Here, we review the role of CDK4 and CDK6 complexes in ovarian cancer and propose the possible use of their inhibitors in the treatment of ovarian cancer patients with different types and stages of disease.