Citations

Abstract Background: Endocrine therapies remain central to estrogen receptor alpha-positive (ERα+) breast cancer treatment, yet their predominantly cytostatic effects highlight the need for combinatorial approaches that promote tumor cell death. We therefore tested BH3 mimetics as single agents and in combination with (Z)-endoxifen (ENDX) across endocrine sensitive and endocrine resistant ERα+ breast cancer models. Methods: Six BH3 mimetics were tested in MCF7 and T47D cells and matched ENDX-resistant and fulvestrant-resistant derivatives. ENDX combinations were assessed using 2D proliferation and synergy assays, 3D RASTRUM® bioprinted cultures, migration assays, and in vivo MCF7 xenograft studies. Results: BH3 mimetics showed broad anti-proliferative activity as single agents in endocrine sensitive and resistant models. ENDX combinations were predominantly additive in 2D assays, with limited synergy confined to specific dose windows. In 3D culture, selected combinations of an MCL1 inhibitor (MCL1i) or navitoclax with ENDX produced greater suppression than single agents and, in several conditions, reduced growth below baseline, consistent with loss of viability. Combination treatment also enhanced suppression of migration under select concentrations. In MCF7 xenograft studies, ENDX robustly suppressed tumor growth; however, adding MCL1i or navitoclax did not provide additional antitumor benefit at the doses, schedules, and timeframes tested. Conclusion: BH3 mimetics show activity in ERα+ breast cancer models in vitro , including endocrine resistant models. Although ENDX combination benefit did not translate in vivo under the conditions tested, these data support further study of optimized BH3 mimetic combinations, including evaluation of dose, ratio, schedule, tumor target engagement, and other clinically advanced agents.

Abstract Introduction: Epoxy fatty acid (EpFA) mechanisms of hormone therapy resistance (HTR) in ER+HER2- breast cancer (BC) are poorly understood. EETs are EpFAs that contribute to BC progression and HTR by promoting oxidative phosphorylation (OXPHOS). However, it remains unclear how EETs contribute to HTR. In this study, two fulvestrant resistant (FR) subclones of the letrozole resistant (LR) MCF-7AC1 line were isolated (termed LR/FR) and were incidentally resistant to the CDKi palbociclib (PR) (termed MCF-7AC1 LR/FR/PR or multiply resistant - MCF-7AC1 MR). These HTR cell lines exhibited CYP3A4 upregulation (2.3- to 4.3-fold) and increased cellular (±)14,15-EET levels, which were suppressed by biguanide treatment (P<0.05). Structure activity studies led to the design of hexyl cuban-1-yl biguanide (HCB) and fluorinated HCB derivatives C5F2-HCB and C6F3-HCB, which inhibited CYP3A4-mediated EET biosynthesis and growth of the HTR lines. HCB fluorination improved pharmacokinetics (PK) and toxicity. Methods: HCB and fluorinated HCBs were tested for impact on EET biosynthesis, OXPHOS, PK, and growth inhibition and reversal of hypoxia in the MCF-7AC1 MR tumor model. Results: Fulvestrant selection for MCF-7AC1 MR cell lines resulted in undetectable cyclin D1 and estrogen receptor (ERα) expression, while cyclin E1, CDK4, CDK6, CYP3A4, and c-MYC were upregulated (P<0.001). MCF-7 MR cells demonstrated 1.8, 1.4, and 1.2- fold higher levels of (±)8,9, (±)11,12, and (±)14,15-EET compared to parental MCF-7 cells (P<0.05). The most potent fluorinated HCB, C6F3-HCB, suppressed cellular levels of (±)8,9-EET, (±)11,12-EET, and (±)14,15-EET by 53-60% (P<0.029). The MCF-7AC1 MR cells were more potently inhibited by HCBs and fluorinated derivatives compared to the MCF-7 cell line. Comparing an MCF-7 MR line vs. MCF-7, HCB exhibited IC50 values of 4.1 vs. 7.0 uM, C5F2-HCB 10 vs. 25 uM, and C6F3-HCB 5.4 vs. 12 uM (P values all <0.05). C5F2-HCB exhibited the best PK and treatment of the MCF-7AC1 MR xenograft model showed reversal of hypoxia (P<0.05) and transient inhibition by C5F2-HCB (P=0.025 at 20 days). P/F also transiently inhibited this model (P=0.006 at 20 days). C5F2-HCB and P/F showed synergy in vitro (CI=0.46) leading to testing of C5F2-HCB/P/F in vivo enabled by acceptable toxicity of the triplet in mice. This combination showed synergy in vivo vs. standard of care palbociclib/fulvestrant by nonparametric statistical tests (Mann Whitney; P=0.0389; Kolmogorov-Smirnov; P=0.0473), providing a pathway for possible clinical development of the combination. Conclusions: MR BC cell lines exhibit upregulation of CYP3A4 and biosynthesis of EETs, indicating a role for EETs in HTR. The C5F2-HCB/P/F exhibits activity against the highly resistant MCF-7 MR xenograft with acceptable toxicity indicating an avenue for clinical development. Citation Format: Zhijun Guo, Jianxun Lei, Christian T. Wells, Allison M. Makovec, Andrew C. Yates, Swaathi Jayaraman, John R. Hawse, Joshua A. McCarra, Qing Cao, Michael J. Pryzbilla, Brenda L. Koniar, Beverly J. Norris, Craig M. Florey, Robert J. Schumacher, Michael A. Farrar, Kaylee L. Schwertfeger, Elizabeth A. Ambrose, Henry Wong, Gunda I. Georg, Antonino D'Assoro, Matthew P. Goetz, David A. Potter. Fluorinated cuban-1-yl biguanides overcome hormone therapy resistance to standard of care fulvestrant, and palbociclib [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(8_Suppl):Abstract nr LB196.

Abstract Background: Pyrimidine synthesis is a therapeutic vulnerability in cancer but inhibition results in toxicity to normal cells. Differential targeting of CTP synthase 1 (CTPS1), sparing the CTPS2 isoenzyme, is a therapeutic strategy in lymphoid cancers, supported by human genetic studies, that can potentially avoid toxicity; however, CTPS2 activity may compensate in epithelial tumours. Loss of CTPS2 protein expression characterises 25% of solid tumours and is predicted to expose a synthetic lethal vulnerability to CTPS1 inhibition. Dencatistat (STP938) is a first in class oral CTPS1 inhibitor showing >1,300-fold selectivity over CTPS2. In preclinical studies, CTPS2 expression was confirmed as a key modulator of response to CTPS1 inhibition, and dencatistat induced tumour regression in an in vivo model of CTPS2 negative ovarian cancer. Study design: The study comprises a standard 3+3 phase 1a dose escalation design, in which intra-patient dose escalation is encouraged, followed by a phase 1b safety expansion. The dose escalation study is recruiting participants with high grade advanced tumours who have exhausted standard of care therapy (excluding haematological cancers, salivary gland tumours and low grade tumours of any tissue). In addition, participants with high grade serous ovarian cancer whose tumours lack expression of CTPS2 (per central laboratory testing by immunohistochemistry) are eligible for backfill into cleared cohorts during dose escalation. The recommended phase 2 dose (RP2D) will be nominated based on safety, tolerability, target engagement and early signs of clinical efficacy. The phase 1b safety expansion study will recruit participants with CTPS2 negative ovarian, lung or endometrial cancer who will receive treatment at the RP2D. Standard inclusion criteria apply; patients with ECOG performance score >2 (>1 for phase 1a in France) or active/symptomatic CNS metastases are not eligible. The primary objectives of the study are to evaluate the safety and tolerability of single agent dencatistat and to determine the RP2D for further development. Secondary objectives comprise evaluation of pharmacokinetic properties and preliminary clinical activity. Exploratory endpoints include evaluation of target engagement, molecular features associated with response, and determination of drug metabolites. The study opened to enrolment in the UK, France and USA in September 2024 and is currently in dose escalation (NCT06297525). The phase 1b study is expected to commence mid-2026 and will include additional centres in Spain. Citation Format: Donna M. Graham, David Sommerhalder, Douglas Orr, Rowan Miller, Neeharika Makani, David O'Malley, Ira Winer, Philip A. Beer, Karen Smith, Maureen Higgins, John R. Hawse, Xiyin Wang, Saravut J. Weroha, Christophe Massard, Patricia Roxburgh. Lack of CTPS2 expression exposes synthetic lethal vulnerability to selective CTPS1 inhibition: A phase 1 dose escalation study of dencatistat (STP938) with safety expansion in biomarker selected cohorts [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(8_Suppl):Abstract nr CT075.

Abstract Background Central precocious puberty (CPP), which is traditionally defined as the development of secondary sexual characteristics before age 8 years in girls and age 9 years in boys, results from the premature activation of the hypothalamic–pituitary–gonadal (HPG) axis. CPP can be associated with short adult stature, adverse psychosocial outcomes, and increased cardiometabolic and cancer risks in adulthood. Gonadotropin-releasing hormone (GnRH) agonists can effectively suppress premature activation of the HPG axis and have the potential to increase adult height as well as improve psychosocial and long-term health outcomes among patients with CPP. However, as secular trends have continued to shift toward earlier age of pubertal onset, some subpopulations of children with CPP, as it is currently defined, may not require the same extent of diagnostic evaluation and treatment. Objective Develop evidence-based recommendations related to the diagnosis and treatment of CPP. Methods A multidisciplinary panel of clinical experts, along with experts in guideline methodology and systematic literature review, used the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) approach to address 10 clinical questions related to the diagnosis and treatment of CPP. Systematic reviews of health-related benefits and harms were conducted for each clinical question. The guideline development panel (GDP) also used the GRADE evidence-to-decision (EtD) framework to address stakeholder values and preferences, costs and required resources, cost-effectiveness, acceptability, feasibility, and potential impacts on health equity. Results In girls with thelarche (Tanner stage B2) between ages 7.0 and 8.0 years, the GDP suggests watchful waiting via periodic physical examinations (every 4-6 months) rather than immediately performing evaluation with laboratory testing or radiologic imaging. In addition, the GDP suggests that all girls with breast development (ie, Tanner stage B2) before age 7 years should first be observed for 4 to 6 months to differentiate unsustained or slowly progressive puberty vs rapidly progressive puberty. These recommendations are largely based on evidence that girls with slowly progressive puberty attain a normal adult height without treatment. When hormonal evaluation is performed to confirm central (GnRH-dependent) activation as the cause of precocious puberty, the GDP suggests starting the evaluation with ultrasensitive basal luteinizing hormone (LH) concentration rather than routine GnRH/GnRH agonist (GnRHa) stimulation testing for all patients. While brain magnetic resonance imaging has been a traditional part of CPP evaluation, the GDP suggests that it should not be routinely performed in girls ages 6.0 to 8.0 years and boys ages 8.0 to 9.0 years without central nervous system (eg, neuro-ophthalmologic) symptoms, largely based on a low prevalence of pathologic intracranial findings in these age groups. The GDP suggests against routine genetic testing for patients with CPP, although they judged that genetic testing (eg, MKRN3 sequencing) should be considered for patients with familial CPP through a shared decision-making process. The GDP suggests GnRHa treatment for many children with CPP, although available evidence suggests that some patient subgroups (eg, older girls with slowly progressive CPP) may be less likely to receive a net benefit with this treatment. Rather than always starting GnRHa treatment with a monthly injectable formulation, the GDP suggests that treatment should be initiated with the formulation (such as a longer-acting formulation) that is anticipated to be used long-term. The GDP suggests against routine addition of growth hormone therapy to increase adult height. They also suggest against the routine biochemical testing (eg, LH, sex steroids) to monitor pubertal suppression while receiving GnRHa, instead reserving biochemical testing to confirm clinically suspected treatment failure. Finally, the GDP suggests against routinely continuing GnRHa treatment beyond chronologic age 10.0 to 11.0 years (girls) or 11.0 to 12.0 years (boys) and/or bone age 11.0 to 12.0 years (girls) or 12.0 to 13.0 years (boys). Conclusion These clinical recommendations were developed to address important uncertainties in the diagnosis and treatment of children with CPP. They are based on the best available scientific evidence regarding clinical outcomes judged to be most important to patients and families. The GDP's overarching goal was to suggest diagnostic and therapeutic strategies that will most likely provide net clinical benefits while simultaneously considering important contextual factors such as cost and feasibility. The guideline-development process highlighted important knowledge gaps and the substantial need for additional research.

Abstract Breast and ovarian cancers are major causes of cancer mortality in women with frequent relapse after curative intent treatment, underscoring the need for novel therapeutics. To this end, we identified CTPS1 as an essential gene in multiple breast and ovarian cancer models, including those resistant to chemotherapy and PARP inhibitors. CTPS1 and CTPS2 are responsible for de novo synthesis of cytidine triphosphate (CTP) which is required for DNA/RNA polymerization, phospholipid synthesis and protein glycosylation. We found CTPS1 to be the predominant isoform expressed in cancer cells, with significantly higher levels in advanced and resistant disease models compared to normal tissue, benign lesions and treatment naïve cancers. Using CTPS1 siRNAs and CTPS1-dTAG (degradation tag) cell lines, we discovered that depletion of CTPS1 induces S-phase cell cycle arrest followed by apoptosis. Leveraging a first-in-class, highly selective, and orally bioavailable CTPS1 inhibitor (STP938) developed by Step Pharma, we identified nanomolar IC50 values across a panel of cell lines, both in 2D and 3D culture systems, and potent anti-neoplastic activity ex vivo and in vivo using patient-derived xenograft (PDX) models. STP938 also synergized with standard-of-care chemotherapy agents and PARP inhibitors, including in therapy-resistant models. Given the essentiality of CTP for biosynthetic processes, we performed bulk and single cell RNA-seq, total and phospho-proteomics, metabolomics, and lipidomic strategies to examine changes induced by acute and chronic STP938 exposure. This revealed that cancer cells are able to overcome pharmacologic CTPS1 inhibition by rewiring of pyrimidine and purine synthesis pathways, upregulation of the pentose phosphate pathway, and extensive remodeling of lipid synthesis pathways including shifts in PC/PE content, increased lysophospholipid production, and altered mitochondrial lipid composition (decreased cardiolipin and increased PG). To identify genetic drivers of STP938 resistance and synthetic lethal vulnerabilities, we performed a genome-wide CRISPR knockout screen together with a PRISM drug sensitivity screen. Genes regulating dNTP biosynthesis, S-phase progression, and RNA processing were identified as primary escape mechanisms, while DHODH inhibitors, purine and nucleoside analogs, and mitosis modifiers were implicated as synthetic lethal opportunities. Notably, we discovered that 25% of ovarian tumors lack CTPS2 protein expression, suggesting near complete reliance on CTPS1 for survival. Thus, a Phase 1a/b clinical trial of STP938 (NCT06297525) has been initiated, including an expansion cohort for patients with CTPS2-null ovarian tumors. These laboratory discoveries and ongoing clinical efforts aim to enable broader uptake of this novel therapeutic approach to improve long-term outcomes in patients with CTPS1-dependent disease. Citation Format: Xiyin Wang, Michael Emch, Lauren Voll, Rebecca Epp, Esther Rodman, Noa Odell, Hannah Smith, Nicole Pearson, Xiaonan Hou, Matthew Goetz, Scott Kaufmann, S. John Weroha, Phillip Beer, John Hawse. CTPS1 as a metabolic vulnerability and novel therapeutic target in breast and ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4560.

Abstract Breast cancer accounts for nearly a quarter of all cancers in women with an estimated 2.3 million new diagnoses and over 670,000 deaths annually. Triple-negative breast cancer (TNBC) is an aggressive subtype characterized by lack of estrogen receptor alpha (ERα) and progesterone receptor (PR) expression in the absence of HER2 amplification. TNBC accounts for ∼15% of all incident breast cancers and is associated with poor patient outcomes. Problematically, TN tumors exhibit vast inter-patient molecular heterogeneity making the identification of novel therapeutic vulnerabilities difficult and the development of alternative treatment strategies problematic. We and others have demonstrated that estrogen receptor beta (ERβ) is expressed in about 20% of TN tumors and potently suppresses proliferation, invasion, and migration in TNBC cells in vitro and in vivo. We thus hypothesized that endocrine therapies that selectively activate ERβ would be an effective treatment approach for this subset of patients. We also anticipated that some tumors would display de novo or acquired resistance to ERβ targeted therapies despite ERβ protein expression. We therefore sought to define mechanisms of resistance to ERβ targeted therapy and to identify synthetic lethal vulnerabilities. We generated the first models of ERβ resistant TNBC through chronic exposure to estradiol or an ERβ specific agonist (LY500307) and through a CRISPR genome wide knockout screen. We identified the LncRNA XIST as one of the most upregulated transcripts in multiple models of ERβ resistant disease, and knockdown of XIST expression was shown to resensitize resistant cell to estradiol and LY500307 treatment. XIST was not found to be directly regulated by ERβ in TNBC further suggesting that its overexpression is a bona fide mechanism of resistance to ERβ mediated suppression of proliferation. The phenomena of chronic ligand exposure leading to activation of XIST expression in ERβ+ cells is not shared with ERα suggesting that the mechanism by which XIST expression is elevated during the development of resistance is distinct between the two sister receptors. We further characterized XIST expression in patient samples of TNBC and found that XIST expression varies, but largely exhibits a bimodal distribution with most tumors displaying either low or high transcript levels. In conclusion, we have identified the X-inactivating transcript as a driver of resistance to ERβ targeted therapies in TNBC. Future studies aimed at uncovering the consequences of XIST over-expression on global gene expression, functionality of ERβ, and aggressiveness of TNBC are ongoing. Citation Format: Michael Emch, Kirsten Aspros, Matthew P. Goetz, John Hawse. The LncRNA XIST mediates resistance to estrogen receptor beta targeted therapies in triple negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2293.

Abstract Breast cancer is the most common female malignancy in the U.S. and the leading cause of cancer-related deaths in women worldwide. Nearly 80% of breast tumors express estrogen receptor alpha (ERα) which drives tumor progression. While endocrine therapies and CDK4/6 inhibitors have increased overall survival, many patients experience recurrence of metastatic disease that is largely refractory to additional lines of endocrine therapy. Metastatic disease remains incurable, highlighting the need for alternative treatment approaches. Through a genome-wide CRISPR knockout screen, we identified Lemur Tail Kinase 2 (LMTK2) as the top hit exhibiting synthetic lethality in the presence of the endocrine therapy, endoxifen, in both endocrine-sensitive and -resistant cell line models. Using publicly available patient tumor databases we identified LMTK2 copy number gain/amplification in 67% of ERα+ human metastatic breast tumors, while only 1% of primary tumors harbored these genomic alterations. Elevated LMTK2 expression in early-stage primary breast tumors was associated with worse recurrence-free survival and overall survival. We discovered that LMTK2 mRNA and protein levels were substantially elevated in nearly all cell-line and patient-derived models of endocrine and CDK4/6i resistance. Utilizing siRNAs and dox-inducible shRNA expressing cell lines, we found that knockdown of LMTK2 enhanced the efficacy of multiple endocrine therapies in treatment naïve models and resensitized resistant models to their respective treatment. LMTK2 overexpression was found to be sufficient for conferring resistance to all standard of care therapies tested including endocrine agents, CDK4/6i, and PI3K inhibitors. RNAseq and global proteomics of LMTK2 overexpressing and knockout models revealed the PI3K/AKT pathway and estrogen signaling as highly regulated pathways by LMTK2. Confirmatory western blot studies demonstrated that LMTK2 overexpression results in dramatic increases in AKT and ERα phosphorylation and activity, while knockout of LMTK2 diminishes these effects. Attempts to completely ablate LMTK2 in endocrine/CDK4/6i-resistant models via CRISPR were not successful, and continuous siRNA or shRNA-mediated knockdown was lethal in resistant cell lines, confirming its essentiality in this setting. In summary we have uncovered LMTK2 as a novel mediator of endocrine/CDK4/6i resistance that is mechanistically linked to the PI3K/AKT pathway among others. Genetic manipulation of LMTK2 has validated it as an ideal therapeutic target for both advanced and newly diagnosed forms of ERα+ breast cancer. Citation Format: Anna Detry, Sofia Marigliano, Jenna Heinen, Adam Bass, Calley Jones, Rajeev Muthyala, Matthew Goetz, John Hawse. LMTK2: A novel driver of endocrine resistance in ERα+ breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 443.

Abstract Background: Endocrine therapy (ET) with aromatase inhibitors (AIs) plus ovarian function suppression (OFS) is a standard endocrine strategy for premenopausal women with ER+/HER2- breast cancer. However, poor tolerability to AI+OFS limits its use, leaving tamoxifen as the only alternative. Prospective studies demonstrated that premenopausal women treated with neoadjuvant ET whose tumors exhibit Ki-67 ≤ 10% following 4 weeks of therapy achieve 5-year distant disease-free survival > 96%. Problematically, only 41% of patients reach this threshold with tamoxifen compared to 78% with AI+OFS (Nitz JCO 2022). (Z)-endoxifen (ENDX) is a potent selective estrogen receptor modulator that dually targets ERα and PKCβ1. At plasma concentrations of 3-5 ng/mL, it inhibits ERα; at ≥ 500 ng/mL, it targets PKCβ1, leading to AKT suppression. The EVANGELINE trial aims to examine endocrine sensitivity in terms of Week 4 Ki-67 in premenopausal women whose baseline Ki-67 > 10%, and objective response rate after 24 weeks of ENDX + goserelin in premenopausal women with baseline Ki-67≤ 10%. Methods: EVANGELINE (NCT05607004) is a multicenter, open-label, Phase 2 study comprised of three parts. Eligible patients are premenopausal women with Stage IIA/IIB ER+/HER2- breast cancer. • Part 1 (PK Run-in): assessed 40mg vs 80mg ENDX (+/- OFS) in 22 patients. • Part 2: patients with baseline Ki-67 >10% were randomized to 40 mg ENDX + goserelin or exemestane + goserelin for 6 months with the primary objective being 4-week ESD rate. Patients with a baseline Ki-67 ≤10% were assigned to a single-arm cohort of 40 mg ENDX monotherapy (no OFS) to evaluate the 24-week endocrine sensitivity disease rate. • Part 3 (replaces Part 2): all patients are treated with ENDX 40mg daily + goserelin every 28 days. A two stage Phase II design was chosen to assess outcomes as follows: Part 3a: Up to 45 patients with baseline Ki-67> 10% to assess Week 4 Ki-67≤ 10% rate is at least 65%. If 9 or more of these patients have a Week 4 Ki-67>10%, enrollment will be stopped due to futility. Otherwise, enrollment will continue. Part 3b: Up to 20 patients enrolled with baseline Ki-67≤ 10% to assess Week 24 objective response rate. Ki-67 levels are determined by central laboratory and images are centrally reviewed. Secondary objectives include safety, tolerability, surgical outcomes, and biomarker analysis using paired biopsies (baseline, week 4) and surgical specimens. Results: The PK run-in (completed Fall 2024) enrolled 22 patients and assessed ENDX doses (40 mg daily vs 80 mg daily (+/-OFS)). The Week 4 Ki-67 ≤ 10% rate with ENDX was 86% and did not differ according to dose. Early safety, PK, efficacy and compliance data supported the selection of 40 mg ENDX daily as the optimal dose. Enrollment in Part 2 began in May 2025. Part 3 will replace Part 2 with simplification of the trial design. Conclusions: EVANGELINE is the first trial to evaluate (Z)-endoxifen + OFS as neoadjuvant therapy in premenopausal ER+/HER2- breast cancer. By targeting both ER and PKC pathways, ENDX may offer a well-tolerated alternative to AI-based regimens and expand endocrine therapy options for this population. Clinical Trial Registration: NCT05607004 Citation Format: M. P. Goetz, V. J. Suman, C. Lopez, H. Erickson, L. Beaulieu, S. S. Hammer, L. Mina, R. Leon-Ferre, K. N. Hunt, M. Piltin, A. Degnim, J. N. Ingle, J. C. Boughey, B. A. Sarah, J. M. Reid, M. Schellenberg, J. R. Hawse, P. Advani, K. Giridhar, F. Batalini, D. Flora, J. M. Jones, N. A. Bagegni, J. Jakub, V. Abramson, V. Dabak, W. J. Irvin, E. Sima, K. Seema, S. C. Quay. A Phase 2 Trial of (Z)-endoxifen + Goserelin as Neoadjuvant Treatment for Premenopausal Women with ER+, HER2-, Breast Cancer (EVANGELINE) [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS5-12-16.

Abstract Breast cancer is the most common female malignancy in the U.S. and the leading cause of cancer-related deaths in women worldwide. Nearly 80% of breast tumors express estrogen receptor alpha (ERα) which drives tumor progression. While CDK4/6 inhibitors in combination with endocrine therapy improve overall survival in the metastatic setting and disease-free survival in the adjuvant setting, many patients eventually relapse/progress with resistant forms of disease. In the post-CDK4/6 inhibitor setting, second line endocrine therapy responses are limited, highlighting the need for alternative treatment approaches. Through a genome-wide CRISPR knockout screen, we identified Lemur Tail Kinase 2 (LMTK2) as the top hit exhibiting synthetic lethality in the presence of the tamoxifen metabolite, endoxifen, in both endocrine-sensitive and -resistant cell line models. We further studied multiple other endocrine resistant models and identified that LMTK2 mRNA and protein expression were both increased in ERα cell lines resistant to estrogen deprivation, tamoxifen, fulvestrant, abemaciclib and palbociclib, as well as in endocrine resistant patient derived models. We then studied LMTK2 in patient samples and identified copy number gain/amplification in 67% of ERα+ human metastatic breast tumors, while only 1% of primary tumors harbored these genomic alterations. From a prognostic standpoint, elevated tumoral LMTK2 expression was associated with worse recurrence-free survival and overall survival in early-stage breast cancer. Through proteomic studies, we identified the MET-PI3K-AKT pathway as overactivated in LMTK2 overexpressing cells. Unlike activating PIK3CA mutations that drive AKT signaling, LMTK2 was found to employ an alternative mechanism to activate AKT signaling by inhibiting PP1α, an AKT-inhibitory phosphatase. Further, LMTK2 overexpression was sufficient to drive endocrine and CDK4/6 inhibitor resistance in vitro and resulted in dramatic increases in AKT and ERα phosphorylation and activity. While knockout of LMTK2 in treatment naïve ERα+ breast cancer cells neither altered AKT/ERα phosphorylation/activity nor endocrine/CDK4/6 inhibitor responsiveness, attempts to knockout LMTK2 in endocrine/CDK4/6 inhibitor-resistant models failed, further confirming its essentiality in this setting. In summary we have uncovered LMTK2 as a novel biomarker of endocrine/CDK4/6 inhibitor resistance that is mechanistically linked to the AKT pathway. LMTK2 may be a novel drug target for the effective treatment of resistant and advanced forms of ERα+ breast cancer. Citation Format: A. M. Detry, S. Marigliano, J. Heinen, A. Bass, C. Jones, R. S. Muthyala, M. P. Goetz, J. R. Hawse. Lmtk2: a novel driver of endocrine resistance in ER+ breast cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-11-01.