NK cell-triggered CCL5/IFNγ-CXCL9/10 axis underlies the clinical efficacy of neoadjuvant anti-HER2 antibodies in breast cancer.

BACKGROUND: The variability in responses to neoadjuvant treatment with anti-HER2 antibodies prompts to personalized clinical management and the development of innovative treatment strategies. Tumor-infiltrating Natural Killer (TI-NK) cells can predict the efficacy of HER2-targeted antibodies independently from clinicopathological factors in primary HER2-positive breast cancer patients. Understanding the mechanism/s underlying this association would contribute to optimizing patient stratification and provide the rationale for combinatorial approaches with immunotherapy. METHODS: We sought to uncover processes enriched in NK cell-infiltrated tumors as compared to NK cell-desert tumors by microarray analysis. Findings were validated in clinical trial-derived transcriptomic data. In vitro and in vivo preclinical models were used for mechanistic studies. Findings were analysed in clinical samples (tumor and serum) from breast cancer patients. RESULTS: NK cell-infiltrated tumors were enriched in CCL5/IFNG-CXCL9/10 transcripts. In multivariate logistic regression analysis, IFNG levels underlie the association between TI-NK cells and pathological complete response to neoadjuvant treatment with trastuzumab. Mechanistically, the production of IFN-É£ by CD16+ NK cells triggered the secretion of CXCL9/10 from cancer cells. This effect was associated to tumor growth control and the conversion of CD16 into CD16-CD103+ NK cells in humanized in vivo models. In human breast tumors, the CD16 and CD103 markers identified lineage-related NK cell subpopulations capable of producing CCL5 and IFN-É£, which correlated with tissue-resident CD8+ T cells. Finally, an early increase in serum CCL5/CXCL9 levels identified patients with NK cell-rich tumors showing good responses to anti-HER2 antibody-based neoadjuvant treatment. CONCLUSIONS: This study identifies specialized NK cell subsets as the source of IFN-É£ influencing the clinical efficacy of anti-HER2 antibodies. It also reveals the potential of serum CCL5/CXCL9 as biomarkers for identifying patients with NK cell-rich tumors and favorable responses to anti-HER2 antibody-based neoadjuvant treatment.

AXL - a new player in resistance to HER2 blockade.

HER2 is a driver in solid tumors, mainly breast, oesophageal and gastric cancer, through activation of oncogenic signaling pathways such as PI3K or MAPK. HER2 overexpression associates with aggressive disease and poor prognosis. Despite targeted anti-HER2 therapy has improved outcomes and is the current standard of care, resistance emerge in some patients, requiring additional therapeutic strategies. Several mechanisms, including the upregulation of receptors tyrosine kinases such as AXL, are involved in resistance. AXL signaling leads to cancer cell proliferation, survival, migration, invasion and angiogenesis and correlates with poor prognosis. In addition, AXL overexpression accompanied by a mesenchymal phenotype result in resistance to chemotherapy and targeted therapies. Preclinical studies show that AXL drives anti-HER2 resistance and metastasis through dimerization with HER2 and activation of downstream pathways in breast cancer. Moreover, AXL inhibition restores response to HER2 blockade in vitro and in vivo. Limited data in gastric and oesophageal cancer also support these evidences. Furthermore, AXL shows a strong value as a prognostic and predictive biomarker in HER2+ breast cancer patients, adding a remarkable translational relevance. Therefore, current studies enforce the potential of co-targeting AXL and HER2 to overcome resistance and supports the use of AXL inhibitors in the clinic.

Delivery of miR-200c-3p Using Tumor-Targeted Mesoporous Silica Nanoparticles for Breast Cancer Therapy.

Despite advances in breast cancer treatment, it remains the leading cause of cancer-related death in women worldwide. In this context, microRNAs have emerged as potential therapeutic targets but still present some limitations for in vivo applications. Particularly, miR-200c-3p is a well-known tumor suppressor microRNA that inhibits tumor progression and metastasis in breast cancer through downregulating ZEB1 and ZEB2. Based on the above, we describe the design and validation of a nanodevice using mesoporous silica nanoparticles for miR-200c-3p delivery for breast cancer treatment. We demonstrate the biocompatibility of the synthesized nanodevices as well as their ability to escape from endosomes/lysosomes and inhibit tumorigenesis, invasion, migration, and proliferation of tumor cells in vitro. Moreover, tumor targeting and effective delivery of miR-200c-3p from the nanoparticles in vivo are confirmed in an orthotopic breast cancer mouse model, and the therapeutic efficacy is also evidenced by a decrease in tumor size and lung metastasis, while showing no signs of toxicity. Overall, our results provide evidence that miR-200c-3p-loaded nanoparticles are a potential strategy for breast cancer therapy and a safe and effective system for tumor-targeted delivery of microRNAs.

HOPE (SOLTI-1903) breast cancer study: real-world, patient-centric, clinical practice study to assess the impact of genomic data on next treatment decision-choice in patients with locally advanced or metastatic breast cancer.

Background: Metastatic breast cancer (mBC) causes nearly all BC-related deaths. Next-generation sequencing (NGS) technologies allow for the application of personalized medicine using targeted therapies that could improve patients' outcomes. However, NGS is not routinely used in the clinical practice and its cost induces access-inequity among patients. We hypothesized that promoting active patient participation in the management of their disease offering access to NGS testing and to the subsequent medical interpretation and recommendations provided by a multidisciplinary molecular advisory board (MAB) could contribute to progressively overcome this challenge. We designed HOPE (SOLTI-1903) breast cancer trial, a study where patients voluntarily lead their inclusion through a digital tool (DT). The main objectives of HOPE study are to empower mBC patients, gather real-world data on the use of molecular information in the management of mBC and to generate evidence to assess the clinical utility for healthcare systems. Trial design: After self-registration through the DT, the study team validates eligibility criteria and assists patients with mBC in the subsequent steps. Patients get access to the information sheet and sign the informed consent form through an advanced digital signature. Afterwards, they provide the most recent (preferably) metastatic archival tumor sample for DNA-sequencing and a blood sample obtained at the time of disease progression for ctDNA analysis. Paired results are reviewed by the MAB, considering patient's medical history. The MAB provides a further interpretation of molecular results and potential treatment recommendations, including ongoing clinical trials and further (germline) genetic testing. Participants self-document their treatment and disease evolution for the next 2 years. Patients are encouraged to involve their physicians in the study. HOPE also includes a patient empowerment program with educational workshops and videos about mBC and precision medicine in oncology. The primary endpoint of the study was to describe the feasibility of a patient-centric precision oncology program in mBC patients when a comprehensive genomic profile is available to decide on a subsequent line of treatment. Clinical trial registration: www.soltihope.com, identifier NCT04497285.

miR-146a-5p Promotes Angiogenesis and Confers Trastuzumab Resistance in HER2+ Breast Cancer.

Trastuzumab treatment has significantly improved the prognosis of HER2-positive breast cancer patients. Despite this, resistance to therapy still remains the main clinical challenge. In order to evaluate the implication of microRNAs in the trastuzumab response, we performed a microRNA array in parental and acquired trastuzumab-resistant HER2-positive breast cancer cell lines. Our results identified miR-146a-5p as the main dysregulated microRNA. Interestingly, high miR-146a-5p expression in primary tumor tissue significantly correlated with shorter disease-free survival in HER2-positive breast cancer patients. The gain- and loss-of-function of miR-146a-5p modulated the response to trastuzumab. Furthermore, the overexpression of miR-146a-5p increased migration and angiogenesis, and promoted cell cycle progression by reducing CDKN1A expression. Exosomes from trastuzumab-resistant cells showed a high level of miR-146a-5p expression compared with the parental cells. In addition, the co-culture with resistant cells' exosomes was able to decrease in sensitivity and increase the migration capacities in trastuzumab-sensitive cells, as well as angiogenesis in HUVEC-2 cells. Collectively, these data support the role of miR-146a-5p in resistance to trastuzumab, and demonstrate that it can be transferred by exosomes conferring resistance properties to other cells.

High VEGFR3 Expression Reduces Doxorubicin Efficacy in Triple-Negative Breast Cancer.

Due to the lack of specific targets, cytotoxic chemotherapy still represents the common standard treatment for triple-negative breast patients. Despite the harmful effect of chemotherapy on tumor cells, there is evidence that treatment could modulate the tumor microenvironment in a way favoring the propagation of the tumor. In addition, the lymphangiogenesis process and its factors could be involved in this counter-therapeutic event. In our study, we have evaluated the expression of the main lymphangiogenic receptor VEGFR3 in two triple-negative breast cancer in vitro models, resistant or not to doxorubicin treatment. The expression of the receptor, at mRNA and protein levels, was higher in doxorubicin-resistant cells than in parental cells. In addition, we confirmed the upregulation of VEGFR3 levels after a short treatment with doxorubicin. Furthermore, VEGFR3 silencing reduced cell proliferation and migration capacities in both cell lines. Interestingly, high VEGFR3 expression was significantly positively correlated with worse survival in patients treated with chemotherapy. Furthermore, we have found that patients with high expression of VEGFR3 present shorter relapse-free survival than patients with low levels of the receptor. In conclusion, elevated VEGFR3 levels correlate with poor survival in patients and with reduced doxorubicin treatment efficacy in vitro. Our results suggest that the levels of this receptor could be a potential marker of meager doxorubicin response. Consequently, our results suggest that the combination of chemotherapy and VEGFR3 blockage could be a potentially useful therapeutic strategy for the treatment of triple-negative breast cancer.

Trastuzumab deruxtecan in patients with central nervous system involvement from HER2-positive breast cancer: The DEBBRAH trial.

BACKGROUND: Trastuzumab deruxtecan (T-DXd) has shown durable antitumor activity in pretreated patients with HER2-positive advanced breast cancer (ABC), but its efficacy has not yet been evaluated in patients with active brain metastases (BMs). DEBBRAH aims to assess T-DXd in patients with HER2-positive or HER2-low ABC and central nervous system involvement. METHODS: This ongoing, five-cohort, phase II study (NCT04420598) enrolled patients with pretreated HER2-positive or HER2-low ABC with stable, untreated, or progressing BMs, and/or leptomeningeal carcinomatosis. Here, we report findings from HER2-positive ABC patients with non-progressing BMs after local therapy (n = 8; cohort 1), asymptomatic untreated BMs (n = 4; cohort 2), or progressing BMs after local therapy (n = 9; cohort 3). Patients received 5.4 mg/kg T-DXd intravenously once every 21 days. The primary endpoint was 16-week progression-free survival (PFS) for cohort 1 and intracranial objective response rate (ORR-IC) for cohorts 2 and 3. RESULTS: As of October 20, 2021, 21 patients received T-DXd. In cohort 1, 16-week PFS rate was 87.5% (95%CI, 47.3-99.7; P < .001). ORR-IC was 50.0% (95%CI, 6.7-93.2) in cohort 2 and 44.4% (95%CI, 13.7-78.8; P < .001) in cohort 3. Overall, the ORR-IC in patients with active BMs was 46.2% (95%CI, 19.2-74.9). Among patients with measurable intracranial or extracranial lesions at baseline, the ORR was 66.7% (12 out of 18 patients; 95%CI, 41.0-86.7), 80.0% (95%CI, 28.4-99.5) in cohort 1, 50.0% (95%CI, 6.7-93.2) in cohort 2, and 66.7% (95%CI, 29.9-92.5) in cohort 3. All responders had partial responses. The most common adverse events included fatigue (52.4%; 4.8% grade ≥3), nausea (42.9%; 0% grade ≥3), neutropenia (28.6%; 19% grade ≥3), and constipation (28.6%; 0% grade ≥3). Two (9.5%) patients suffered grade 1 interstitial lung disease/pneumonitis. CONCLUSIONS: T-DXd showed intracranial activity with manageable toxicity and maintained the quality of life in pretreated HER2-positive ABC patients with stable, untreated, or progressing BMs. Further studies are needed to validate these results in larger cohorts.

Triple-Negative PAM50 Non-Basal Breast Cancer Subtype Predicts Benefit from Extended Adjuvant Capecitabine.

PURPOSE: Predictive biomarkers for capecitabine benefit in triple-negative breast cancer (TNBC) have been recently proposed using samples from phase III clinical trials, including non-basal phenotype and biomarkers related to angiogenesis, stroma, and capecitabine activation genes. We aimed to validate these findings on the larger phase III GEICAM/CIBOMA clinical trial. EXPERIMENTAL DESIGN: Tumor tissues from patients with TNBC randomized to standard (neo)adjuvant chemotherapy followed by capecitabine versus observation were analyzed using a 164-gene NanoString custom nCounter codeset measuring mRNA expression. A prespecified statistical plan sought to verify the predictive capacity of PAM50 non-basal molecular subtype and tested the hypotheses that breast tumors with increased expression of (meta)genes for cytotoxic cells, mast cells, endothelial cells, PDL2, and 38 individual genes benefit from adjuvant capecitabine for distant recurrence-free survival (DRFS; primary endpoint) and overall survival. RESULTS: Of the 876 women enrolled in the GEICAM/CIBOMA trial, 658 (75%) were evaluable for analysis (337 with capecitabine and 321 without). Of these cases, 553 (84%) were profiled as PAM50 basal-like whereas 105 (16%) were PAM50 non-basal. Non-basal subtype was the most significant predictor for capecitabine benefit [HRcapecitabine, 0.19; 95% confidence interval (CI), 0.07-0.54; P < 0.001] when compared with PAM50 basal-like (HRcapecitabine, 0.9; 95% CI, 0.63-1.28; P = 0.55; Pinteraction<0.001, adjusted P value = 0.01). Analysis of biological processes related to PAM50 non-basal subtype revealed its enrichment for mast cells, extracellular matrix, angiogenesis, and features of mesenchymal stem-like TNBC subtype. CONCLUSIONS: In this prespecified correlative analysis of the GEICAM/CIBOMA trial, PAM50 non-basal status identified patients with early-stage TNBC most likely to benefit from capecitabine.

SOLTI-1904 ACROPOLI TRIAL: efficacy of spartalizumab monotherapy across tumor-types expressing high levels of PD1 mRNA.

Improved selection of cancer patients who are most likely to respond to immune checkpoint inhibitors remains an unmet clinical need. Recently, a positive correlation between levels of PD1 mRNA and clinical outcome in response to PD1 blockade across diverse tumor histologies has been confirmed in several datasets. ACROPOLI is a parallel cohort, non-randomized, phase II study that aims to evaluate the efficacy of the anti-PD1 immune checkpoint inhibitor spartalizumab as monotherapy in metastatic patients with solid tumors that express high levels of PD1 (cohort 1; n = 111). An additional cohort of 30 patients with tumors expressing low levels of PD1, where PD1/PD-L1 antibodies in monotherapy are standard treatment, will also be included (cohort 2). Primary end point is overall response rate in cohort 1. Trial registration number: NCT04802876 (ClinicalTrials.gov).

Immunological Landscape of HER-2 Positive Breast Cancer.

Understanding the biological aspects of immune response in HER2+ breast cancer is crucial to implementing new treatment strategies in these patients. It is well known that anti-HER2 therapy has improved survival in this population, yet a substantial percentage may relapse, creating a need within the scientific community to uncover resistance mechanisms and determine how to overcome them. This systematic review indicates the immunological mechanisms through which trastuzumab and other agents target cancer cells, also outlining the main trials studying immune checkpoint blockade. Finally, we report on anti-HER2 vaccines and include a figure exemplifying their mechanisms of action.

Targeting HER2-AXL heterodimerization to overcome resistance to HER2 blockade in breast cancer.

Anti-HER2 therapies have markedly improved prognosis of HER2-positive breast cancer. However, different mechanisms play a role in treatment resistance. Here, we identified AXL overexpression as an essential mechanism of trastuzumab resistance. AXL orchestrates epithelial-to-mesenchymal transition and heterodimerizes with HER2, leading to activation of PI3K/AKT and MAPK pathways in a ligand-independent manner. Genetic depletion and pharmacological inhibition of AXL restored trastuzumab response in vitro and in vivo. AXL inhibitor plus trastuzumab achieved complete regression in trastuzumab-resistant patient-derived xenograft models. Moreover, AXL expression in HER2-positive primary tumors was able to predict prognosis. Data from the PAMELA trial showed a change in AXL expression during neoadjuvant dual HER2 blockade, supporting its role in resistance. Therefore, our study highlights the importance of targeting AXL in combination with anti-HER2 drugs across HER2-amplified breast cancer patients with high AXL expression. Furthermore, it unveils the potential value of AXL as a druggable prognostic biomarker in HER2-positive breast cancer.

Oncolytic viruses: A new immunotherapeutic approach for breast cancer treatment?

Immunotherapy has revolutionized the oncology field during the last years, mainly with the introduction of immune checkpoint inhibitors in the clinical routine. Despite the recent approval of these drugs for the treatment of triple-negative breast cancer, most breast cancer patients cannot benefit from immunotherapy as most tumors are not considered immunoreactive. Therefore, new strategies must be developed to bring immunotherapy closer to breast cancer patients. The introduction of oncolytic viruses in the immuno-oncology field has shown promising results in cancer treatment, including breast cancer. However, a better understanding of their mechanisms of action, increase evidence of safety and efficacy, and the implications of its use as a systemic therapy must be examined in more depth. This review provides a summary of oncolytic virotherapy in the context of breast cancer, both in the pre-clinical and clinical setting.

LCOR mediates interferon-independent tumor immunogenicity and responsiveness to immune-checkpoint blockade in triple-negative breast cancer.

Ligand-dependent corepressor (LCOR) mediates normal and malignant breast stem cell differentiation. Cancer stem cells (CSCs) generate phenotypic heterogeneity and drive therapy resistance, yet their role in immunotherapy is poorly understood. Here we show that immune-checkpoint blockade (ICB) therapy selects for LCORlow CSCs with reduced antigen processing/presentation machinery (APM) driving immune escape and ICB resistance in triple-negative breast cancer (TNBC). We unveil an unexpected function of LCOR as a master transcriptional activator of APM genes binding to IFN-stimulated response elements (ISREs) in an IFN signaling-independent manner. Through genetic modification of LCOR expression, we demonstrate its central role in modulation of tumor immunogenicity and ICB responsiveness. In TNBC, LCOR associates with ICB clinical response. Importantly, extracellular vesicle (EV) Lcor-messenger RNA therapy in combination with anti-PD-L1 overcame resistance and eradicated breast cancer metastasis in preclinical models. Collectively, these data support LCOR as a promising target for enhancement of ICB efficacy in TNBC, by boosting of tumor APM independently of IFN.

mTOR Inhibition and T-DM1 in HER2-Positive Breast Cancer.

In patients with trastuzumab-resistant HER2-positive breast cancer, the combination of everolimus (mTORC1 inhibitor) with trastuzumab failed to show a clinically significant benefit. However, the combination of mTOR inhibition and the antibody-drug conjugate (ADC) trastuzumab-emtansine (T-DM1) remains unexplored. We tested T-DM1 plus everolimus in a broad panel of HER2-positive breast cancer cell lines. The combination was superior to T-DM1 alone in four cell lines (HCC1954, SKBR3, EFM192A, and MDA-MB-36) and in two cultures from primary tumor cells derived from HER2-positive patient-derived xenografts (PDX), but not in BT474 cells. In the trastuzumab-resistant HCC1954 cell line, we characterized the effects of the combination using TAK-228 (mTORC1 and -2 inhibitor) and knockdown of the different mTOR complex components. T-DM1 did not affect mTOR downstream signaling nor induct autophagy. Importantly, mTOR inhibition increased intracellular T-DM1 levels, leading to increased lysosomal accumulation of the compound. The increased efficacy of mTOR inhibition plus T-DM1 was abrogated by lysosome inhibitors (chloroquine and bafilomycin A1). Our experiments suggest that BT474 are less sensitive to T-DM1 due to lack of optimal lysosomal processing and intrinsic resistance to the DM1 moiety. Finally, we performed several in vivo experiments that corroborated the superior activity of T-DM1 and everolimus in HCC1954 and PDX-derived mouse models. In summary, everolimus in combination with T-DM1 showed strong antitumor effects in HER2-positive breast cancer, both in vitro and in vivo. This effect might be related, at least partially, to mTOR-dependent lysosomal processing of T-DM1, a finding that might apply to other ADCs that require lysosomal processing. IMPLICATIONS: Inhibition of mTOR increases the antitumor activity of T-DM1, supporting that the combination of mTOR inhibitors and antibody-drug conjugates warrants clinical evaluation in patients with HER2-positive breast cancer.

First Nationwide Molecular Screening Program in Spain for Patients With Advanced Breast Cancer: Results From the AGATA SOLTI-1301 Study.

BACKGROUND: The SOLTI-1301 AGATA study aimed to assess the feasibility of a multi-institutional molecular screening program to better characterize the genomic landscape of advanced breast cancer (ABC) and to facilitate patient access to matched-targeted therapies in Spain. METHODS: DNA sequencing of 74 cancer-related genes was performed using FFPE tumor samples in three different laboratories with three different gene panels. A multidisciplinary advisory board prospectively recommended potential targeted treatments. The primary objective was to determine the success of matching somatic DNA alteration to an experimental drug/drug class. RESULTS: Between September 2014 and July 2017, 305 patients with ABC from 10 institutions were enrolled. Tumor sequencing was successful in 260 (85.3%) patients. Median age was 54 (29-80); most tumors were hormone receptor-positive/HER2-negative (74%), followed by triple-negative (14.5%) and HER2-positive (11.5%). Ninety-seven (37%) tumor samples analyzed proceeded from metastatic sites. Somatic mutations were identified in 163 (62.7%) patients, mostly in PIK3CA (34%), TP53 (22%), AKT1 (5%), ESR1 (3%), and ERBB2 (3%) genes. Significant enrichment of AKT1 mutation was observed in metastatic versus primary samples (9% vs. 2%; p=0.01). Genome-driven cancer therapy was recommended in 45% (n=116) of successfully screened patients, 11% (n=13) of whom finally received it. Among these patients, 46.2% had a PFS of ≥6 months on matched therapy. CONCLUSIONS: AGATA is the first nationwide molecular screening program carried out in Spain and we proved that implementing molecular data in the management of ABC is feasible. Although these results are promising, only 11% of the patients with genome-driven cancer therapy received it.

Molecular profiling of advanced solid tumours. The impact of experimental molecular-matched therapies on cancer patient outcomes in early-phase trials: the MAST study.

INTRODUCTION: Molecular-matched therapies have revolutionized cancer treatment. We evaluated the improvement in clinical outcomes of applying an in-house customized Next Generation Sequencing panel in a single institution. METHODS: Patients with advanced solid tumors were molecularly selected to receive a molecular-matched treatment into early phase clinical trials versus best investigators choice, according to the evaluation of a multidisciplinary molecular tumor board. The primary endpoint was progression-free survival (PFS) assessed by the ratio of patients presenting 1.3-fold longer PFS on matched therapy (PFS2) than with prior therapy (PFS1). RESULTS: Of a total of 231 molecularly screened patients, 87 were eligible for analysis. Patients who received matched therapy had a higher median PFS2 (6.47 months; 95% CI, 2.24-14.43) compared to those who received standard therapy (2.76 months; 95% CI, 2.14-3.91, Log-rank p = 0.022). The proportion of patients with a PFS2/PFS1 ratio over 1.3 was significantly higher in the experimental arm (0.33 vs 0.08; p = 0.008). DISCUSSION: We demonstrate the pivotal role of the institutional molecular tumor board in evaluating the results of a customized NGS panel. This process optimizes the selection of available therapies, improving disease control. Prospective randomized trials are needed to confirm this approach and open the door to expanded drug access.

Fertility and breast cancer: A literature review of counseling, preservation options and outcomes.

Fertility preservation is an important issue in breast cancer patients undergoing oncological treatment. Fertility counseling is a crucial need given the physical and psychological stress experienced by patients. Cryopreservation of mature oocytes is currently the standard fertility-preserving procedure. Other options such as ovarian tissue preservation or gonadal protection during chemotherapy are still experimental, but have proven effectiveness. Prompt referral to a fertility unit is highly recommended in order to ensure quality of care. In this article, we focus on the different strategies to preserve fertility in breast cancer patients, assessing also the safety of pregnancy and breastfeeding after cancer. A systemic literature review was performed for research articles published in English in PubMed, or as abstracts from the European Society for Medical Oncology (ESMO), San Antonio Breast Cancer Symposium (SABCS) and American Society of Clinical Oncology (ASCO) annual meetings, using the search terms "breast cancer" and "fertility".

Oral Selective Estrogen Receptor Degraders (SERDs) as a Novel Breast Cancer Therapy: Present and Future from a Clinical Perspective.

Estrogen receptor-positive (ER+) is the most common subtype of breast cancer. Endocrine therapy is the fundamental treatment against this entity, by directly or indirectly modifying estrogen production. Recent advances in novel compounds, such as cyclin-dependent kinase 4/6 inhibitors (CDK4/6i), or phosphoinositide 3-kinase (PI3K) inhibitors have improved progression-free survival and overall survival in these patients. However, some patients still develop endocrine resistance after or during endocrine treatment. Different underlying mechanisms have been identified as responsible for endocrine treatment resistance, where ESR1 gene mutations are one of the most studied, outstanding from others such as somatic alterations, microenvironment involvement and epigenetic changes. In this scenario, selective estrogen receptor degraders/downregulators (SERD) are one of the weapons currently in research and development against aromatase inhibitor- or tamoxifen-resistance. The first SERD to be developed and approved for ER+ breast cancer was fulvestrant, demonstrating also interesting activity in ESR1 mutated patients in the second line treatment setting. Recent investigational advances have allowed the development of new oral bioavailable SERDs. This review describes the evolution and ongoing studies in SERDs and new molecules against ER, with the hope that these novel drugs may improve our patients' future landscape.