Endocrine-resistant Breast Cancer Research Articles

Abstract PO2-27-04: Unravelling the role of FGFR1 in innate immune sensing and endocrine resistance in breast cancer

Abstract Fibroblast Growth Factor Receptor 1 (FGFR1) has emerged as a critical player in various cancer types, including breast cancer. FGFR1 alterations, including gene amplification, overexpression, and activating mutations, have been identified in subsets of breast cancers, contributing to aberrant signaling pathways that promote tumor growth, angiogenesis, and metastasis. Our current research endeavors are directed towards unravelling the intricate mechanistic underpinnings of FGFR1's dual impact on breast tumor progression, specifically its contribution to endocrine resistance and its regulatory role in innate immune sensing and subsequent induction of Type 1 interferons (IFNs) Our investigative efforts have previously established a direct association between FGFR1 and endocrine resistance in hormone receptor positive T47D breast cancer cells. Moreover, a remarkable and distinct inverse correlation linking FGFR1 expression to the Type 1 IFN response was observed. We noticed that in vitro stimulation of T47D cells with Type 1 interferons induces the expression of FGFR1 and ESR1, thereby suppressing the interferon-stimulated genes. Interestingly, we observed that the FGFR1-mediated suppression of the Type 1 interferon response is associated with intracellular nucleic acid sensors, specifically toll-like receptors TLR7 and TLR9. Furthermore, we found that the inhibition of FGFR kinase enhances the expression of IFN-alpha, IFN-beta, TLR7, and TLR9 in T47D cells when stimulated with the TLR7 agonist loxoribine. These results strongly suggest that FGFR1 functions as an innate checkpoint and may be exploited by tumor cells to dampen Type 1 interferon responses, thereby inhibiting antitumor immunity in hormone receptor-positive breast cancer. Citation Format: Sushil Dhakal, Torbjørn Amundsen Lien, Jens Henrik Norum, Helga Bergholtz, Helene Midtun Flatekvål, Therese Sørlie. Unravelling the role of FGFR1 in innate immune sensing and endocrine resistance in 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 PO2-27-04.

Abstract PO5-23-08: Adenylosuccinate lyase is essential for proliferation and mitochondrial function of endocrine therapy resistant breast cancer cells

Abstract Eighty percent of breast cancers express estrogen receptor alpha (ERα, ESR1) at the time of diagnosis. Endocrine therapy (ET) is the standard treatment that either block estrogen mediated ER activation (such as tamoxifen, fulvestrant) or suppress estrogen synthesis (such as exemestane, letrozole, or anastrozole). While ET is initially effective, emergence of ET-resistance is common and possess a major clinical challenge. Hence, there is an ongoing need to develop new treatments for ET-resistant-ERα positive breast cancer. In this study we investigated the role of adenylosuccinate lyase (ADSL), an enzyme of the de-novo purine biosynthesis pathway as a potential target of ET-resistant breast cancer. The protein level of ADSL in two independent ET-resistant cell line model LCC9 and T47D-4HT cells were significantly higher in comparison to its parental ET-sensitive cell lines, MCF7 and T47D cells, respectively. Transient knockdown of ADSL using two independent siRNA in both LCC9 and T47D-4HT cells results in a significant decrease of cell growth, colony, and spheroid formation. Notably, reduced ADSL levels in LCC9 cells prevented the cells from progressing from G1 to S phase of cell cycle with concurrent elevation of cyclin D1/D3, CDK2/4, and cyclin E. On the other hand, in T47D-4HT cells, ADSL reduction caused cells to accumulate in S-phase of cell cycle and showed lower levels of total cyclin D1 protein. Mechanistically, in LCC9 cells (but not in T47D-4HT cells), ADSL depletion induced DNA replication stress which activates (phosphorylate) ataxia-telangiectasia-mutated-and-Rad3-related kinase (ATR) and its major downstream effector checkpoint kinase 1 (Chk1) that in turn failed to de-phosphorylate the inhibitory-phospho-groups of cyclin-dependent kinase 2 (CDK2). In addition, ADSL depletion perturbed the mitochondrial function in both LCC9 and T47D-4HT cells. Oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) was lower in both the cells along with the mitochondrial membrane potential. Pertinently, lower concentration of AICAR (5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside), a product of ADSL enzyme function, partially rescued the cell growth and restored mitochondrial membrane potential in ADSL-depleted LCC9 cells. To further understand the global effects of ADSL depletion we performed RNA-sequencing of LCC9 and T47D-4HT cells after ADSL depletion and compared with respective control siRNA transfected cells followed by gene set enrichment analysis. Intriguingly, diverse effect was observed in LCC9 and T47D-4HT cells as very few genes were found to be commonly regulated between these two cell lines. Analysis of clinical data sets revealed high level of ADSL transcript was associated with adverse progression free survival and overall survival in breast cancer patients treated with endocrine therapies. Overall, our findings demonstrate that ADSL-mediated de novo purine synthesis is critical for cellular growth, proliferation, and mitochondrial function of endocrine therapy-resistant breast cancer cells. Therefore, targeting ADSL is a novel potential therapeutic approach for ERα positive ET- resistant breast cancer. Citation Format: Anil Yadav, Lu Jin, Robert Clarke, Surojeet Sengupta. Adenylosuccinate lyase is essential for proliferation and mitochondrial function of endocrine therapy resistant breast cancer cells [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 PO5-23-08.

Abstract PO3-26-11: Enhancing endoplasmic reticulum stress for treating endocrine therapy resistant breast cancers driven by mutant estrogen receptors

Abstract Background: Estrogen receptor alpha (ERα) mutations are common (30-40%) in metastatic endocrine therapy-resistant breast cancers (ETR-BC), enable resistance to endocrine therapies and are the molecular drivers of ETR-BC. We had previously shown that an oligobenzamide, ERX-41, could enhance endoplasmic reticulum stress in ETR-BCs driven by mutant (MT) ERα, resulting in cancer cell death in vitro and and in vivo. To enable clinical translation of ERX-41, we performed lead optimization, followed by preclinical and IND-enabling studies. Methods: Over 2000 oligo-benzamides were designed, synthesized and tested in multiple BC models including those that express WT-ERα (MCF7, and ZR75) and BC models with acquired resistance (MCF7-Tam, and MCF7-LTLT) and engineered models that express MT-ERα (MCF7-ERα-D538G, MCF7-ERα-Y537S, ZR75-ERα-D538G, ZR75-ERα-Y537S). For our lead compound, mechanistic validation studies were performed using CRISPR LIPA mutants, RT-qPCR and Western blotting. Explants, organoids, cell line-(CDX) and patient-derived (PDX) xenografts were used to test the ex vivo and in vivo effectiveness of our lead compound as a monotherapy and in combination with abemaciclib. Results: Testing of >2000 synthesized oligo-benzamides identified a lead compound, ERX-315, that had broad and potent activity (IC50 between 20-100 nM) against both WT and MT (mutant) ERα-driven BC cells in in vitro assays. CRISPR KO of LIPA (which encodes lysosomal acid lipase (LAL) in BC cells abrogated cytotoxic response to ERX-315, validating LIPA as the molecular target of ERX-315. Ultrastructural and molecular studies confirm that ERX-315 induces significant endoplasmic reticulum stress, leading to a shutdown of de novo protein synthesis and apoptotic cell death in BC. Importantly, ERX-315 does not induce endoplasmic reticulum stress or cell death in normal cells and is non-toxic in animal models. We have shown that this capacity of ERX-315 to induce endoplasmic reticulum stress is unique among drugs targeting ERα, including selective ERα modulators and degraders, such as GDC-0180, AZD-9496 and fulvestrant. ERX-315 has potent anti-proliferative activity against MT-ERα-driven BC, as seen in genetically modified cell lines both grown as monolayer or spheroids in vitro, patient derived explants (PDEs) ex vivo and cell line derived (CDX) and patient-derived (PDX) xenografts in vivo. The combination of ERX-315 and CDK4/6 inhibitor abemaciclib was synergistic in decreasing the proliferation of both endocrine therapy-sensitive and endocrine therapy-resistant BC cells, in vitro, in xenograft models in vivo, and in primary patient-derived explants ex vivo. We are currently optimizing both the synthesis of ERX-315 using GMP process for kilogram scale production and validated methods for pharmacokinetic studies. We have developed formulations for both intravenous and oral administration with favorable pharmacokinetic parameters and proven utility of the formulated ERX-315 against CDX and PDX tumors in vivo. Toxicity studies in dogs and rodents have demonstrated >8-fold therapeutic to toxicity window. A phase I clinical trial is planned to be open to enrollment by Q1 2024. Conclusions: We have identified a lead compound ERX-315, which represents a novel class of agent that induce catastrophic levels of ER stress resulting in cancer cell death and that can effectively work against multiple forms of ETR-BC, including those driven by MT-ERα. Preclinical studies, GMP manufacturing, formulation and IND-enabling studies are being completed in time for the commencement of the phase I clinical trial by Q1 2024. Citation Format: Suryavathi Viswanadhapalli, Karla Parra, Tae-Kyung Lee, Rahul Gopalam, Kara Kassees, Tanner Reese, Gaurav Sharma, Xihui Liu, Xue Yang, ChiaYuan Chen, Carlos Roggero, Liping Chen, Sautam Bhattacharya, Uday Pratap, Russell Hayward, Sharron Gargosky, Jung-Mo Ahn, Ganesh V. Raj, Ratna Vadlamudi. Enhancing endoplasmic reticulum stress for treating endocrine therapy resistant breast cancers driven by mutant estrogen receptors [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 PO3-26-11.

Abstract PO2-16-02: RBsig gene-expression signature in patients with endocrine resistant metastatic breast cancer treated with palbociclib and fulvestrant in the PYTHIA trial

Abstract Background: We have previously identified RBsig, a gene-expression signature of resistance to CDK4/6 inhibitors composed of E2F1/E2F2 dependent genes, which is associated with genetic loss of RB1. Although RBsig has been previously tested in vitro and in clinical trials with CDK4/6 inhibitors, further clinical validation is needed. Additionally, data on the correlation of RBsig with other prognostic/predictive biomarkers for CDK4/6 inhibitors such as serum Thymidine Kinase activity (sTKa), is lacking. Here, we aimed to investigate RBsig in patients (pts) treated with palbociclib (P) + fulvestrant (F) within the PYTHIA trial (IBCSG 53- 14/BIG 14-04; NCT02536742), a downstream trial of the AURORA molecular program (BIG 14-01; NCT02102165). Methods: PYTHIA is a biomarker discovery phase II trial which enrolled 122 pts with endocrine-resistant ER+ and HER2 - metastatic breast cancer (MBC) who received P+F at standard schedule and dose. We analyzed a subset of 38 pts enrolled in PYTHIA and successfully included in AURORA who had available RNAseq from tumor tissue samples (24 primary, 24 metastatic, 10 both). Ki67 by IHC was also available. The PYTHIA biomarker assessment included sTKa at baseline and during treatment (D15 and D28) measured using DiviTumÒ TKa (Biovica International). Evaluation of the prognostic role of RBsig was a pre-specified primary objective of PYTHIA. RBsig is composed of 87 genes and was calculated as previously described. Wilcoxon rank sum test was used to compare continuous RBsig values between groups defined by sTKa. Median progression free survival (mPFS) was estimated using Kaplan-Meier method for groups defined by dichotomizing RBsig at zero. Results: Median RBsig was higher in metastatic than in primary samples; in paired samples, RBsig increased in metastatic for 7/10 pts vs primary. When evaluated as continuous variables, RBsig correlated with tissue Ki67 both in primary (Spearman R 0.8) and in metastatic (R 0.68) samples. Additionally, RBsig was higher in Basal and Luminal B intrinsic subtypes in metastatic samples. RBsig was significantly higher in pts with high baseline sTKa (median cut-off) (Wilcoxon test, p=0.04 and p=0.03 in primary and metastatic samples respectively), with a similar direction, albeit not statistically significant, observed in the group of pts with no sTKa clearance at D15 (below limit of detection cut-off). When evaluated in the combined cohort, where RBSig values were taken from metastatic samples when available and from primary samples otherwise, higher RBsig values were associated with poorer PFS as compared to lower RBsig values [mPFS 5.5 months (95% CI 3.7, 14) vs 13 months (95% CI 8.6, 28); Log-rank p-value=0.004]. Conclusions: Higher RBsig was associated with higher tumor proliferation (measured by both tissue ki67 and sTKa), luminal B status, and shorter PFS on treatment with P+F. This analysis reinforces prior data on RBsig prognostic significance in pts treated with CDK4/6 inhibitors. Further investigation on the added value of RBsig as a novel biomarker for patients’ stratification, including in the early setting, is warranted. Citation Format: Matteo Benelli, Svitlana Tyekucheva, Thayane Crestani, Michail Ignatiadis, Patrick Neven, Marco Colleoni, Stéphanie Henry, Konstantinos Papadimitriou, Antonio Bernardo, Elena Seles, François Duhoux, Iain Macpherson, Alastair Thomson, David Mark Davies, Ilenia Migliaccio, Gabriele Zoppoli, Roswitha Kammler, Heidi De Swert, Barbara Ruepp, Philippe Aftimos, Jose Seoane, Dario Romagnoli, David Venet, Theodora Goulioti, David Cameron, Sherene Loi, Martine Piccart, Meredith Regan, Luca Malorni. RBsig gene-expression signature in patients with endocrine resistant metastatic breast cancer treated with palbociclib and fulvestrant in the PYTHIA trial [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 PO2-16-02.

Abstract PO5-05-09: Testing AI-Predicted Protein Motifs that Direct Constitutive Genomic AR Activity in Endocrine Resistant Breast Cancer

Abstract Testing AI-Predicted Protein Motifs that Direct Constitutive Genomic AR Activity in Endocrine-Resistant Breast Cancer Ashfia F. Khan 1,2, Anthony S. Peidl 1,2, Shaymaa Bahnassy 3, Henry Vo2, Micah B. Castillo 2, Sarah K Herzog4,5, Suzanne AW Fuqua4,6, Preethi Gunaratne 2, Xiaolian Gao 2, Subash C. Pakhrin7, Tasneem Bawa-Khalfe1,2 1 Center for Nuclear Receptors & Cell Signaling, University of Houston, Houston, TX 2 Department of Biology & Biochemistry, University of Houston, Houston, TX 3 Department of Oncology, Lombardi Comprehensive Cancer Center, Georgetown University, Washington, DC 4 Lester & Sue Smith Breast Center, Baylor College of Medicine, Houston, TX 5 Program in Integrative Molecular and Biomedical Sciences, Baylor College of Medicine, Houston, TX 6 Dan L Duncan Comprehensive Cancer Center, Baylor College of Medicine, Houston, TX 7 Department of Computer Science & Engineering Technology, University of Houston-Downtown, Houston, TX Background: Endocrine therapy (ET) remains the first-line treatment for hormone-receptor positive (HR+) breast cancer (BCa). Approximately 15–20% of HR+ BCa are intrinsically resistant to ET, and 30–40% of patients acquire resistance. Resistance to ET (ET-R) supports cancer progression with reduced disease-free survival and greater incidence of metastatic disease. Hence, therapeutic strategies for ET-R HR+ BCa remain an overarching challenge. The androgen receptor (AR) is emerging as an attractive alternative target for BCa subtypes, and elevated AR levels drive HR+ BCa progression. Targeting AR in HR+ BCa is proving difficult with preclinical studies showing conflicting results for AR antagonists. Yet clinical trials with several AR-targeting drugs are ongoing. Our recent report highlights a unique constitutively active modified AR population that drives HR+ BCa metastatic properties and is insensitive to AR inhibitors. Our current objectives are to 1) use a novel machine-learning model to predict AR modifications and 2) establish a strategy to identify patients with high modified AR levels. Methods: An advanced artificial intelligence (AI) tool and mid-throughput microfluidic peptide array technology were used to map modification domains on AR. SUMO post-translational modification of AR (SUMO-AR) was eliminated in HR+ BCa using CRISPR-Cas9 technology. RNA-seq was employed to identify a unique gene signature for SUMO-AR, and comparative bioinformatic analysis stratified patients with high versus low SUMO-AR. Results: A novel deep-learning AI platform SumoPred-PLM is trained to identify consensus, non-consensus, and SUMO2/3-specific motifs on AR. We verified SUMO2/3-specific sites on AR with a mid-throughput microfluidic peptide array. The identified SUMO2/3-acceptor site of AR is important for HR+ BCa cell pathophysiology; loss of this SUMO2/3-acceptor site impacts endogenous AR SUMOylation, cell morphology, and proliferation/apoptosis. Using both high and low SUMO-AR BCa lines, a unique SUMO-AR gene profile was established. Our SUMO-AR gene signature identifies HR+ BCa patients with greater susceptibility to metastatic progression. Conclusion: Our studies present a unique pipeline that incorporates deep-learning AI technology to identify vulnerable motifs in AR for future drug discovery. Drug screens are currently ongoing. In addition, we establish a SUMO-AR gene signature that stratifies HR+ BCa patients with high/low SUMO-AR and predicts disease progression. We expect the results could be utilized to identify responders to AR inhibitors in ongoing clinical trials. Citation Format: Ashfia Khan, Anthony Peidl, Shaymaa Bahnassy, Henry Vo, Micah Castillo, Sarah Herzog, Suzanne Fuqua, Preethi Gunaratne, Xiaolian Gao, Subash Pakhrin, Tasneem Bawa-Khalfe. Testing AI-Predicted Protein Motifs that Direct Constitutive Genomic AR Activity in Endocrine Resistant 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 PO5-05-09.

Abstract PO3-16-04: Distribution of intrinsic subtypes in endocrine-resistant and endocrine-sensitive breast cancer

Abstract Background Breast cancer (BC) is a heterogeneous disease that can be divided into intrinsic molecular subtypes. Among the four intrinsic subtypes, Luminal A and Luminal B are the most common and associated with the best outcome, whereas HER2-enriched and basal-like are less frequent. Tumors that are estrogen receptor α (ER)-positive and human epidermal growth factor receptor 2 (HER2)-negative (ER+/HER2-) are mostly found among the luminal subtypes. Despite that patients with ER+/HER2- tumors are offered adjuvant endocrine therapy, around one-third eventually develop endocrine resistance. The aim of this study was to explore the distribution of intrinsic subtypes in endocrine-resistant primary and relapse tumors, comparing these to endocrine-sensitive controls. Materials and methods Verified endocrine-resistant BC patients diagnosed in 2008-2012 were retrospectively collected at the Karolinska University Hospital, Stockholm, Sweden. Cases (N=70) were identified as patients with ER+/HER2- primary tumors and subsequent ER+/HER2- relapse tumors within 5 years and during endocrine therapy. Patients with primary ER+/HER2- tumors without progression or relapse at 10 years of follow-up were defined as controls (N=78), diagnosed in 2005-2006. Extracted RNA from formalin-fixed paraffin-embedded tumor tissue was analyzed by Affymetrix Clariom D Microarray and assessed by Transcriptome Analysis Console and R software. Results Among the controls, the distribution of Luminal A, Luminal B, and HER2-enriched subtypes was 65.4%, 34.6%, and 0%, respectively. In primary tumors, the distribution was in turn 55.4%, 43.1%, and 1.5%, and among relapse tumors 57.6%, 37.9%, and 4.5%, respectively. A total of 19 cases, accounting for 27.1%, switched intrinsic subtypes between the paired primary and relapse tumors. No tumors exhibited the basal-like subtype. Conclusion In this unique cohort, Luminal B and HER2-enriched subtypes were more common in primary and relapse tumors of verified endocrine-resistant patients than in primary tumors of endocrine-sensitive controls. Further, the HER2-enriched subtype, although sparse, was more frequently found in patients’ relapse tumors than in their corresponding primary tumors, and over one-fourth of cases switched subtypes during progression. These findings support the presence of subtype shifting in endocrine-resistant relapses and that endocrine-resistant tumors are more prone to be of Luminal B or HER2-enriched subtypes than endocrine-sensitive tumors. Citation Format: Caroline Schagerholm, Stephanie Robertson, Emelie Karlsson, Emmanouil Sifakis, Johan Hartman. Distribution of intrinsic subtypes in endocrine-resistant and endocrine-sensitive 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 PO3-16-04.

Abstract PO5-13-02: BRCA mutational profiles of endocrine-resistant breast cancer

Abstract Background Around 75% of breast cancer (BC) patients have tumors expressing the treatment-predictive biomarker estrogen receptor α (ER) and are consequently offered endocrine therapy. One-third of these patients develop endocrine resistance, a majority with tumors that retain ER expression. For most cases, the resistance mechanism is still unknown, but mutational activity is a suggested mechanism. Germline BRCA mutations are well-known in BC and are found in approximately 23% of patients with triple-negative BC and in 5% of patients with ER+ BC. However, less is certain about the somatic variants, though studies have shown prevalences of around 3% in BC. This study aimed to examine the somatic BRCA mutational profiles of endocrine-resistant paired primary and relapse tumors. Methods A retrospective cohort of verified endocrine-resistant BC patients was collected at Karolinska University Hospital in Stockholm, Sweden. Patients diagnosed in 2008-2012 with an ER+ and human epidermal growth factor receptor 2 (HER2) negative primary tumor and a following ER+ and HER2- relapse tumor within five years of ongoing endocrine therapy were included (N=63). DNA was extracted from archived formalin-fixed paraffin-embedded relapse and primary tumor tissue and analyzed by panel sequencing. Tumor-free lymph nodes were used as germline normal samples. Results The analysis showed somatic BRCA mutational activity in 38.1% of the patients. Variants were present in 20.3% of primary tumors and 30.4% of relapse tumors. BRCA1 mutations were present in 11.9% of primary tumors and 21.4% of relapse tumors. BRCA2 variations were, in turn, seen in 11.9% of primary tumors and 14.3% of relapse tumors. The frequency of patients that had alterations in both the primary and relapse tumors was 3.2% for BRCA1 and BRCA2 respectively. Conclusion This study of a unique endocrine-resistant BC cohort shows a high presence and varying distribution of somatic BRCA mutations in both primary and even higher in relapse tumors. These results suggest that mutational testing for possible further treatment decisions may be relevant for patients exhibiting endocrine resistance, especially in the advanced setting. Citation Format: Emelie Karlsson, Caroline Schagerholm, Stephanie Robertson, Hosein Toosi, Emmanouil Sifakis, Johan Hartman. BRCA mutational profiles of endocrine-resistant 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 PO5-13-02.

Abstract PO4-23-08: Estrogen therapy induces receptor-dependent DNA damage enhanced by PARP inhibition in ER+ breast cancer

Abstract Although clinical evidence indicates that treatment with estrogens elicits anti-cancer effects in ~30% of patients with advanced endocrine-resistant ER+ breast cancer, the underlying mechanism of action is unclear and this treatment remains under-utilized. ER+ breast cancer cells with acquired resistance to long-term estrogen deprivation (LTED) were used to model resistance to aromatase inhibitors (AIs) seen clinically. Genome-wide CRISPR/Cas9 knockout screening and transcriptomic profiling of LTED cells highlighted DNA Damage Response as a required pathway for therapeutic response to the estrogen 17b-estradiol. Tumor specimens from 2 patients with advanced ER+ breast cancer showed increased DNA damage response upon treatment with 17b-estradiol therapy compared to baseline. LTED cells treated with 17b-estradiol exhibited replication-dependent markers of DNA damage and the DNA damage response prior to apoptosis, while parental cells did not. Such DNA damage was partially driven by the formation of DNA:RNA hybrids (R-loops), and RNase H1-mediated prevention of R-loop formation abrogated 17b-estradiol-induced DNA damage. Pharmacological suppression of the DNA damage response via poly(ADP-ribose) polymerase (PARP) inhibition with olaparib enhanced 17b-estradiol-induced DNA damage. PARP inhibition synergized with 17b-estradiol to suppress growth and prevent tumor recurrence in both BRCA1/2-mutant and BRCA1/2-wild-type cell line and patient-derived xenograft models. We therefore conclude that 17b-estradiol-induced ER activity drives DNA damage and growth inhibition in endocrine-resistant breast cancer cells, and inhibition of the DNA damage response using drugs such as PARP inhibitors can exacerbate transcriptional stress and enhance therapeutic response to 17b-estradiol. This concept is being explored clinically in the PHOEBE trial testing the combination of 17b-estradiol and olaparib (NCT05900895). These findings also warrant further study of other DNA damage response inhibitors in advanced ER+ breast cancer. Moreover, these data indicate that PARP inhibitors may have applications beyond homologous recombination-deficient tumors. Citation Format: Nicole Traphagen, Gary Schwartz, Steven Tau, Alyssa Roberts, Amanda Jiang, Sarah Hosford, Jonathan Marotti, Abigail Goen, Bianca Romo, Anneka Johnson, Emily Duffy, Eugene Demidenko, Paul Heverly, Yaron Mosesson, Shannon Soucy, Fred Kolling, Todd Miller. Estrogen therapy induces receptor-dependent DNA damage enhanced by PARP inhibition in ER+ 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 PO4-23-08.

Endocrine Resistance in Breast Cancer: The Role of mTOR Signaling in Mediating Resistance to Selective Estrogen Receptor Modulators

Selective estrogen receptor modulators (SERMs) have been widely prescribed and effective as a first-line endocrine therapy to treat ER+ breast cancer. Tamoxifen, the most used SERM in the treatment of breast cancer, has been shown to be effectively anti-proliferative in breast tissue and has made a tremendous contribution to reducing breast cancer mortality. Vast experimental evidence from seven sources supports tamoxifen’s ability in repressing the expression of estrogen-responsive genes involved in cancer growth. The binding of tamoxifen to the estrogen receptor prevents the recruitment of coactivators to the complex and instead promotes the recruitment of corepressors and histone deacetylases, thus inhibiting transcriptional activation of target genes. However, the issue of endocrine resistance remains a predominant problem with this therapy. Sources have found that endocrine resistance can arise due to dysregulations in the mTOR signaling pathway. Experiments have revealed some hope regarding a mechanism by which we can re-sensitize the breast cancer cells to the therapy, notably by knocking down YAP/TAZ or PSAT1 in the mTOR pathway. Despite this discovery, endocrine resistance prevails due to irregularities in additional pathways. Therefore, subsequent research is crucial to identify more targets that, when knocked down, enable re-sensitization of resistant cells, restoring full therapeutic ability of SERMs in afflicted women.

Abstract LB051: Protein kinase C beta 1 (PKCβ1) is allosterically inhibited and paradoxically translocated to the membrane by z-endoxifen

Abstract The protein kinase C (PKC) family of enzymes are indispensable serine-threonine kinases involved in signal pathways implicated in many different cellular processes. Dysregulation of PKC isoforms may contribute to the development and progression of breast cancer. Z-Endoxifen (Z-ENDX), the active metabolite of tamoxifen (TAM) and a selective estrogen receptor modulator (SERM) exhibited potent antitumor activity in endocrine-resistant hormone receptor-positive breast and other gynecologic cancers. Mechanistically, we discovered that ENDX inhibits PKCβI and downstream AKT signaling. Based on insights from our recently determined crystal structure of PKCβI, we investigated the effects of ENDX on PKCβI in estrogen receptor-positive (ER+) breast cancer. In vitro FRET-based biochemical assays revealed that ENDX inhibits the kinase activity of conventional (PKCβI) as well as a novel (PKC;) PKC isoform with a similar IC50. However, the PKCβI catalytic domain was found to be less sensitive to ENDX compared to full-length PKCβI. We identified a multi-domain mechanism of PKC inhibition through an allosteric inhibitory model using X-ray crystallography. Since the spatial assembly of PKC is crucial for its kinase activity, we assessed the effects of ENDX on the intracellular localization of PKCβI using live cell confocal imaging. Sub-cellular tracking of YFP-tagged PKCβI in ER+ MCF7 cells paradoxically revealed that ENDX promoted the translocation of PKCβI to the plasma membrane in both a dose and time-dependent manner. Furthermore, co-treatment with inhibitors of PHLPP phosphatases, which regulate PKC, alleviated this translocation. Based on our prior data demonstrating greater antitumor activity of the Z vs E isomer of ENDX, we discovered that Z-ENDX inhibits PKCβI kinase activity more effectively than its E-isomer using in vitro kinase assays. Further, E-ENDX failed to promote PKCβI translocation to the membrane.Taken together, these results suggest that allosteric effects of Z-ENDX trigger PKCβI recruitment to the plasma membrane where it is normally active yet suppresses its kinase activity. These findings indicate that Z-ENDX induces a non-productive structure of the enzyme and "breaks" the well-known mechanism of PKC activation upon binding to the cell membrane. Moreover, Z-ENDX likely triggers PHLPP-mediated PKCβI dephosphorylation, and ultimately PKCβI degradation, suggesting that Z-ENDX represents a new mechanistic basis for targeting and downregulating PKCβI, and potentially other PKC family members, in cancer cells. This newly discovered function of Z-ENDX likely contributes to its superior anti-cancer effects when compared to TAM and other endocrine therapies. Citation Format: Sayantani Sarkar Bhattacharya, Huy V. Huynh, Jasmin K. Farmakes, Taylor L. Witter, Elizabeth S. Bruinsma, Swaathi Jayaraman, Anh T. Cong, John R. Hawse, Matthew P. Goetz, Matthew J. Schellenberg. Protein kinase C beta 1 (PKCβ1) is allosterically inhibited and paradoxically translocated to the membrane by z-endoxifen [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(7_Suppl):Abstract nr LB051.

Epigenetic Therapy Targets Endocrine-Resistant Breast Cancer

Epigenetic Therapy Targets Endocrine-Resistant Breast Cancer

Blockade of a novel MAP4K4-LATS2-SASH1-YAP1 cascade inhibits tumorigenesis and metastasis in luminal breast cancer

Novel components in the noncanonical Hippo pathway that mediate the growth, metastasis, and drug resistance of breast cancer (BC) cells need to be identified. Here, we showed that SAM and SH3 domain containing protein 1 (SASH1) expression is negatively correlated with mitogen-activated protein kinase kinase kinase kinase 4 (MAP4K4) expression in a subpopulation of patients with luminal-subtype BC. Downregulated SASH1 and upregulated MAP4K4 synergistically regulated the proliferation, migration, and invasion of luminal-subtype BC cells. The expression of LATS2, SASH1 and YAP1 and the phosphorylation of YAP1 were negatively regulated by MAP4K4, and LATS2 then phosphorylated SASH1 to form a novel MAP4K4-LATS2-SASH1-YAP1 cascade. Dephosphorylation of Yes1 associated transcriptional regulator (YAP1), YAP1/TAZ nuclear translocation and downstream transcriptional regulation of YAP1 were promoted by the combined effects of ectopic MAP4K4 expression and SASH1 silencing. Targeted inhibition of MAP4K4 blocked proliferation, cell migration and ER signaling both in vitro and in vivo. Our findings reveal a novel MAP4K4-LATS2-SASH1-YAP1 phosphorylation cascade, a noncanonical Hippo pathway that mediates ER signaling, tumorigenesis and metastasis in breast cancer. Targeted intervention with this noncanonical Hippo pathway may constitute a novel alternative therapeutic approach for endocrine-resistant BC.

Abstract 6576: Discovery of potent and selective novel small molecule drugs to target estrogen receptor mutants in endocrine-resistant breast cancer

Abstract Introduction: The acquisition of estrogen receptor alpha (ERα) gene (ESR1) mutations is a key driver for the development of resistance to current endocrine therapy in breast cancer. Clinical studies have shown that ESR1 mutations are frequently observed in metastatic ER-positive breast cancer patients and are associated with poor survival. It is well appreciated that activating ESR1 somatic mutations, especially Y537S and D538G, can drive estrogen-independent activities. In addition, these ESR1 mutations diminish the potency of the current standard-of-care agents (tamoxifen and fulvestrant) that bind ERα directly. Therefore, it is an unmet need to develop next-generation antiestrogens that inhibit ERα mutant signaling in breast cancer to improve patient survival. Here, we search for small molecule inhibitors against ERα mutants Y537S and D538G using DNA-encoded chemical library (DEL). Methods: ERα ligand binding domain (LBD) proteins corresponding to wild type (WT), Y537S and D538G mutants were expressed in E. coli and purified by Ni-NTA, anion exchange, and size exclusion chromatography. The ability of these purified proteins to bind ligands was tested in biochemical assays to validate their use in DEL selections. We conducted a DEL affinity selection using our in-house collection of 6 billion small molecule compounds against WT, Y537S and D538G mutants ER LBD proteins in the presence and absence of estradiol. Selection hits that enriched with these targets were resynthesized off-DNA and tested in biochemical and functional studies using CRISPR-Cas9 knock-in Y537S or D538G mutant breast cancer cells. Results: We have successfully purified microgram amounts of ERα LBD for WT, Y537S, and D538G proteins. In our biochemical assay, the SRC3 peptide binds to the WT ERα LBD in the presence of estradiol, whereas Y537S and D538G LBDs bind the SRC3 peptide in the absence of estradiol, consistent with these mutants constitutively binding to SRC3. Our multibillion small molecule collections of DELs can screen and identify several hits in WT and mutant ERα LBDs. To confirm the selection output, we synthesized off-DNA compounds and validated these in biochemical and cell-based studies. Compounds CDD-1274 and CDD-1802 dramatically decrease the WT and mutant ERα protein levels in multiple ER+ breast cancer cells. These compounds greatly reduce protein levels for GREB1, TFF1, c-MYC, E2F1, and survivin in wildtype and mutant breast cancer cells, and they increase levels of cleaved PARP, a marker for apoptosis in wildtype and mutant ER+ breast cancer cells but not in ER-negative breast cancer cells. Conclusion: Our study rapidly identified potent and novel small molecule drugs to target estrogen receptor mutants using our DNA-encoded chemical library platform. Acknowledgments: Funding sources (NIH/NCI R03 CA259664 and CPRIT RP220524 to MP). Citation Format: Anil Kumar Devakrishnan, Chandrashekhar Madasu, Suzanne A.W Fuqua, Martin M. Matzuk, Murugesan Palaniappan. Discovery of potent and selective novel small molecule drugs to target estrogen receptor mutants in endocrine-resistant breast cancer [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 6576.

Abstract 1985: Ablation of adenylosuccinate lyase retarded the cell proliferation and induces mitochondrial dysfunction in endocrine therapy resistant breast cancer cells

Abstract Approximately 75% of breast tumors express estrogen receptors (ER+); recurrent ER+ breast cancer is generally incurable. Endocrine therapy (ET) is the mainstay of treatment for ER+ breast cancer and works by targeting either ER activity (Tamoxifen or Fulvestrant) or availability of its ligand estrogen (Letrozole, Exemestane). Unfortunately, endocrine therapies are limited by the emergence of resistance; resistance to endocrine therapies (ET) remains a significant clinical challenge. We have explored the role of adenylosuccinate lyase (ADSL), an enzyme of the de-novo purine biosynthesis pathway, in ET-resistant breast cancer cells. Protein expression of ADSL was significantly higher in two different ET-resistant cells (LCC9 and T47D-4HT) when compared with their respective parental ET-sensitive cells (MCF-7 and T47D). siRNA mediated ablation of ADSL in LCC9 and T47D-4HT cells blocked cell proliferation and both colony and spheroid formation. Depletion of ADSL in LCC9 cells obstructed cell cycle progression from G1 to S phase and elevated the protein expression of cyclin D1/D3, CDK2/4, and cyclin E. In T47D-4HT cells, ADSL depletion led to an accumulation of cells in S-phase with reduced protein expression of cyclin D1. Notably, ADSL ablation in LCC9 cells lead to DNA replication stress, resulting in activation (phosphorylation) of ataxia-telangiectasia-mutated-and-Rad3-related kinase (ATR) and its key downstream effector checkpoint kinase 1 (Chk1), which further limited the phosphatase activity of cdc25A leading to deactivation (phosphorylation) of Cyclin-Dependent Kinase 2 (CDK2). ADSL knockdown reduced the mitochondrial membrane potential and impaired the oxygen consumption (OCR) and extracellular acidification (ECAR) rates in both LCC9 and T47D-4HT cells. Of note, addition of AICAR to ADSL-depleted LCC9 cells significantly increased cell proliferation, colony formation, and restored mitochondrial membrane potential. RNA sequencing data revealed the effect of ADSL depletion on enrichment of different pathways in LCC9 and TH47D-4HT cells. Importantly, p53 signaling was significantly enriched in LCC9 cells but not in T47D-4HT cells after ADSL knockdown (T47D cells have a mutated p53 gene). ER+ breast cancer patients treated with ET showed high ADSL expression that is associated with poor recurrence-free survival. In summary, our findings indicate that ADSL-mediated de novo purine synthesis is crucial for cell proliferation. Therefore, targeting ADSL may be a novel therapeutic approach for ER, ET-resistant breast cancer. Citation Format: ANIL KUMAR Yadav, Lu Jin, Robert Clarke, Surojeet Sengupta. Ablation of adenylosuccinate lyase retarded the cell proliferation and induces mitochondrial dysfunction in endocrine therapy resistant breast cancer cells [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 1985.

Abstract 3281: Silencing of necroptosis related genes in endocrine-resistant breast cancer cells causes PFKFB3 induced arrest of necroptosis and contributes to resistance

Abstract Approximately 70% of newly diagnosed breast cancer patients are estrogen receptor positive. Patients treated with approved treatment strategies may develop resistance to these therapies. Because of this problem in treatment, new treatment regimens need to emerge. Altered reprogramming of metabolism and an intense dependence on glycolysis are hallmarks of cancer, which is necessary to support the high energy demand of rapidly proliferating cancer cells. Rapidly dividing cancer cells are known to have increased energy requirements and altered reprogramming of glucose metabolism, leading to an increased reliance on glycolysis. The conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, a critical rate-limiting step in the glycolysis pathway, is greatly influenced by an allosteric activator produced by the enzyme PFKFB3 (6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3), thus making PFKFB3 an important regulator of glycolytic flux. Furthermore, PFKFB3 has been shown to be overexpressed in many human cancers, including breast cancer, and upregulated by estrogen. In our previous studies, we have shown that endocrine-resistant BC cells growth is strongly reduced by PFKFB3 targeting using PFKFB3 inhibitors, glucose uptake is reduced, triggered to necroptosis by RIPK1 and MLKL phosphorylation, and PFKFB3 inhibition reduces tumor size in mouse xenografts. In this study, we showed that silencing of RIPK1 and MLKL, genes involved in necroptosis, by siRNAs followed by inhibition of PFKFB3 by PFK158 did not induce necroptosis in endocrine-resistant BC cells. Furthermore, we further confirmed that phosphorylation of MLKL and RIPK occurs with PFKFB3 inhibition. In summary, this study reveals that necroptosis induced by PFKFB3 inhibition contributes to resistance by arresting necroptosis in endocrine-resistant BC cells after silencing of necroptosis-related genes. Citation Format: Brandon C. Jones, Tuna Onal, Surojeet Sengupta, Catherine M. Sevigny, Lu Jin, Paula R. Pohlmann, Ayesha Shajahan-Haq, Robert Clarke. Silencing of necroptosis related genes in endocrine-resistant breast cancer cells causes PFKFB3 induced arrest of necroptosis and contributes to resistance [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 3281.

The role of CXCL1 in crosstalk between endocrine resistant breast cancer and fibroblast

BackgroundER positive breast cancer is currently targeted using various endocrine therapies. Despite the proven therapeutic efficacy, resistance to the drug and reoccurrence of tumor appears to be a complication that many patients deal with. Molecular pathways underlying the development of resistance are being widely studied.Methods and resultsIn this study, using four established endocrine resistant breast cancer (ERBC) cell lines, we characterized CXCL1 as a secreted factor in crosstalk between ERBC cells and fibroblasts. Protein array revealed upregulation of CXCL1 and we confirmed the CXCL1 expression by real-time qRT-PCR and U-Plex assay. Co-culturing ERBC cells with fibroblasts enhanced the cell growth and migration compared to monoculture. The crosstalk of ERBC cells with fibroblasts significantly activates ERK/MAPK signaling pathway while reparixin, CXCR1/2 receptor inhibitor, attenuates the activity. Reparixin displayed the ERBC cell growth inhibition and the combination treatment with reparixin and CDK4/6 inhibitor (palbociclib and ribociclib) increased these inhibitory effect.ConclusionsTaken together, our study implicates CXCL1 as a critical role in ERBC growth and metastasis via crosstalk with fibroblast and cotargeting CXCR1/2 and CDK4/6 could potentially overcome endocrine resistant breast cancer.

Proton Pump Inhibitors and Cyclin-Dependent Kinase 4/6 Inhibitors in Patients With Breast Cancer

Abstract Background Proton pump inhibitors (PPIs) reduce the bioavailability of several anticancer drugs. The impact of PPIs co-administered with cyclin-dependent kinase 4 and 6 inhibitors is controversial. We aimed to clarify whether the concomitant use of PPIs impacts palbociclib and abemaciclib effectiveness in breast cancer treatment. Patients and Methods This multicenter, retrospective, observational study, conducted across 4 medical institutions in Japan, consecutively included patients with endocrine-resistant metastatic breast cancer, receiving palbociclib or abemaciclib between December 2017 and August 2022. Propensity score-matched analyses were performed. Treatment efficacy and safety with and without PPIs were compared. Progression-free survival and overall survival were estimated using the Kaplan-Meier method and compared using a log-rank test. A Cox proportional hazards model was used to estimate the hazard ratio. Results The study included 240 patients. After 1:1 matching, 112 patients were treated with and without PPIs. The median progression-free survival period was 1.2 years in the PPI group and 1.3 years in the non-PPI group (hazard ratio, 1.19; 95% CI, 0.70-2.02). The median overall survival period was 3.6 years in the PPI group, whereas it was not reached in the non-PPI group (hazard ratio, 1.23; 95% CI, 0.61-2.47). Consistent results were obtained for subgroups receiving palbociclib (n = 177) and abemaciclib (n = 63) without propensity score matching. Adverse event incidence and severity were similar in both groups. Conclusion The effectiveness of cyclin-dependent kinase 4/6 inhibitors is unlikely to be affected by concomitant PPI use. Future prospective pharmacokinetic studies are warranted.

Macrophages Promote Subtype Conversion and Endocrine Resistance in Breast Cancer

Background: The progression of tumors from less aggressive subtypes to more aggressive states during metastasis poses challenges for treatment strategies. Previous studies have revealed the molecular subtype conversion between primary and metastatic tumors in breast cancer (BC). However, the subtype conversion during lymph node metastasis (LNM) and the underlying mechanism remains unclear. Methods: We compared clinical subtypes in paired primary tumors and positive lymph nodes (PLNs) in BC patients and further validated them in the mouse model. Bioinformatics analysis and macrophage-conditioned medium treatment were performed to investigate the role of macrophages in subtype conversion. Results: During LNM, hormone receptors (HRs) were down-regulated, while HER2 was up-regulated, leading to the transformation of luminal A tumors towards luminal B tumors and from luminal B subtype towards HER2-enriched (HER2-E) subtype. The mouse model demonstrated the elevated levels of HER2 in PLN while retaining luminal characteristics. Among the various cells in the tumor microenvironment (TME), macrophages were the most clinically relevant in terms of prognosis. The treatment of a macrophage-conditioned medium further confirmed the downregulation of HR expression and upregulation of HER2 expression, inducing tamoxifen resistance. Through bioinformatics analysis, MNX1 was identified as a potential transcription factor governing the expression of HR and HER2. Conclusion: Our study revealed the HER2-E subtype conversion during LNM in BC. Macrophages were the crucial cell type in TME, inducing the downregulation of HR and upregulation of HER2, probably via MNX1. Targeting macrophages or MNX1 may provide new avenues for endocrine therapy and targeted treatment of BC patients with LNM.

Abstract B007: Vandetanib abrogates antiestrogen resistance in MAPK-driven ER+ breast cancer by halting key intracellular signaling pathways and multiple druggable kinases

Abstract Introduction: Vandetanib has been shown to abrogate ER+ breast cancer growth via inhibition of MAPK; we sought to elucidate the effects of Vandetanib on MAPK-driven endocrine-resistant breast cancer. Methods: Chronically treated MCF7-TAM (Tamoxifen), MCF7-EXE (Exemestane), and wild-type MCF7 ER+ breast cancer cell lines were treated with Vandetanib and/or Tamoxifen. Cell viability assays were conducted with CellTiter-Glo. Cells were treated with Vandetanib (10 uM) and vehicle for 24 hours, and mRNA sequencing was performed. To elicit TKI-specific proteomic changes, cells were treated with control or IC90 Vandetanib for 48 hours, and the signatures of kinase activity were determined using a multiplex inhibitor bead library coupled with mass spectrometry. Differential gene (DGE) and protein (DPE) expression analyses were performed to determine enriched and depleted transcripts and kinase activity with Vandetanib treatment. Gene set enrichment analysis was performed to determine the shared and unique pathways regulated by Vandetanib for each model. A Vandetanib response signature was compiled using the top 20 downregulated genes and analyzed with METABRIC and SCAN-B datasets to assess effects on survival in ER+ breast cancer patients. Results: MCF7-TAM and MCF7-EXE demonstrated lower Tamoxifen IC50 when treated with Vandetanib compared to Tamoxifen alone, which showed synergistic effects. Data analysis of mRNA libraries showed reduced tumor progression and signaling of MAPK, PI3K/AKT, RAS, CCND1, MYC, and EGFR across all three lines, as well as increased ERBB2 signaling, which may denote a potential adaptive bypass mechanism to Vandetanib treatment. Gene sets showed increased estrogen response, ESR1 activity, and sensitivity to tamoxifen across all three lines. Notable key players in PI3K/AKT and MAPK signaling such as TYRO3 were significantly downregulated at both the gene and protein levels. Additionally, JAK2 demonstrated dual upregulation and has been implicated with favorable prognosis due to associations with immune infiltration. The downregulated Vandetanib gene set conferred significantly improved OS (HR=0.75, p<0.0001, METABRIC; HR=0.87, p=0.034, SCAN-B) and RFS (HR=0.60, p<0.0001, METABRIC) in ER+ breast cancer patients from both datasets. Multiple druggable kinases such as RET, EGFR, FGFR2/3, CDK6, and SRC were depleted by Vandetanib treatment. Upregulated druggable kinases identified with Vandetanib treatment include MTOR, FGFR4, ERBB2, and JAK1. Conclusions: Vandetanib overcomes antiestrogen resistance in MAPK-driven models of ER+ breast cancer by halting key signaling pathways such as MAPK, PI3K/AKT, MYC, and ERBB2, among others. Upregulated druggable kinases may serve as targets to overcome resistance to Vandetanib. Downregulated targets with Vandetanib could serve as biomarkers to select patients most likely to benefit from Vandetanib treatment. Citation Format: Rasha T Kakati, Austin Whitman, Hyunsoo Kim, Philip M Spanheimer. Vandetanib abrogates antiestrogen resistance in MAPK-driven ER+ breast cancer by halting key intracellular signaling pathways and multiple druggable kinases [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr B007.

Combining lapatinib and palbociclib inhibits cell proliferation and invasion via AKT signaling pathway in endocrine-resistant breast cancer cells.

Endocrine therapy plays a critical role in patients with hormone receptor-positive breast cancer. Endocrine-resistant breast cancer cells exhibit more HER2 signaling proteins (pAKT and pERK) and mesenchymal biomarkers than wild-type cell lines. In head and neck squamous cell carcinoma, the combination of lapatinib and palbociclib demonstrated synergistic inhibitory effects on cell proliferation and suppressed ERK1/2 phosphorylation. The combination of lapatinib and palbociclib at half-maximal inhibitory concentrations resulted in an increasing cytotoxic effect on cell proliferation. Furthermore, invasion activity was significantly decreased when combining two drugs at nontoxic concentrations more than either single drug alone did. The combination also remarkably suppressed epithelial-mesenchymal transition transcription factors, such as Snail and pAKT, more than monotherapy. Combining drugs, particularly lapatinib and palbociclib for targeting endocrine-resistant breast cancer cells whose tumors overexpressed HER2 after resistance to hormonal therapy, demonstrated better antiproliferative, anti-invasive effects, and suppression of EMT protein and pAKT than a single drug. These results could be from the interruption of the EMT process via the AKT pathway. Thus, this study provides preliminary data for applying this combination to patients with endocrine-resistant breast cancer in further clinical trials.