Preclinical Models Of Breast Cancer Research Articles

Inhibition of lysine acetyltransferase KAT6 in ER+HER2- metastatic breast cancer: a phase 1 trial.

Inhibition of histone lysine acetyltransferases (KATs) KAT6A and KAT6B has shown antitumor activity in estrogen receptor-positive (ER+) breast cancer preclinical models. PF-07248144 is a selective catalytic inhibitor of KAT6A and KAT6B. In the present study, we report the safety, pharmacokinetics (PK), pharmacodynamics, efficacy and biomarker results from the first-in-human, phase 1 dose escalation and dose expansion study (n = 107) of PF-07248144 monotherapy and fulvestrant combination in heavily pretreated ER+ human epidermal growth factor receptor-negative (HER2-) metastatic breast cancer (mBC). The primary objectives of assessing the safety and tolerability and determining the recommended dose for expansion of PF-07248144, as monotherapy and in combination with fulvestrant, were met. Secondary endpoints included characterization of PK and evaluation of antitumor activity, including objective response rate (ORR) and progression-free survival (PFS). Common treatment-related adverse events (any grade; grades 3-4) included dysgeusia (83.2%, 0%), neutropenia (59.8%, 35.5%) and anemia (48.6%, 13.1%). Exposure was approximately dose proportional. Antitumor activity was observed as monotherapy. For the PF-07248144-fulvestrant combination (n = 43), the ORR (95% confidence interval (CI)) was 30.2% (95% CI = 17.2-46.1%) and the median PFS was 10.7 (5.3-not evaluable) months. PF-07248144 demonstrated a tolerable safety profile and durable antitumor activity in heavily pretreated ER+HER2- mBC. These findings establish KAT6A and KAT6B as druggable cancer targets, provide clinical proof of concept and reveal a potential avenue to treat mBC. clinicaltrial.gov registration: NCT04606446 .

Oral estrogen receptor PROTAC® vepdegestrant (ARV-471) is highly efficacious as monotherapy and in combination with CDK4/6 or PI3K/mTOR pathway inhibitors in preclinical ER+ breast cancer models.

Estrogen Receptor (ER) alpha signaling is a known driver of ER-positive (ER+)/human epidermal growth factor receptor 2 negative (HER2-) breast cancer. Combining endocrine therapy (ET) such as fulvestrant with CDK4/6, mTOR or PI3K inhibitors is now a central strategy for the treatment of ER+ advanced breast cancer. However, suboptimal ER inhibition and resistance resulting from ESR1 mutation dictates that new therapies are needed. A medicinal chemistry campaign identified vepdegestrant (ARV-471), a selective, orally bioavailable, potent small molecule PROteolysis-TArgeting Chimera (PROTAC®) degrader of ER. We used biochemical and intracellular target engagement assays to demonstrate the mechanism of action of vepdegestrant, and ESR1 wild-type and mutant ER+ preclinical breast cancer models to demonstrate ER degradation-mediated tumor growth inhibition. Vepdegestrant induced ≥90% degradation of wild-type (WT) and mutant ER, inhibited ER-dependent breast cancer cell line proliferation in-vitro and achieved significant tumor growth inhibition (TGI) (87-123%) in MCF7 orthotopic xenograft models, better than the ET agent fulvestrant (31-80% TGI). In the hormone-independent ER Y537S patient derived xenograft (PDX) breast cancer model ST941/HI, vepdegestrant achieved tumor regressions and was similarly efficacious in the ST941/HI/PBR palbociclib-resistant model (102% TGI). Vepdegestrant induced robust tumor regressions in combination with each of the CDK4/6 inhibitors palbociclib, abemaciclib, and ribociclib, the mTOR inhibitor everolimus, and the PI3K inhibitors alpelisib and inavolisib. Vepdegestrant achieved greater ER degradation in-vivo compared to fulvestrant, which correlated with improved tumor growth inhibition, suggesting vepdegestrant could be a more effective backbone ET for patients with ER+/HER2- breast cancer.

Syringeable Near-Infrared Light-Activated In Situ Immunogenic Hydrogel Boosts the Cancer-Immunity Cycle to Enhance Anticancer Immunity.

Effective anticancer immunity depends on properly activating multiple stepwise events in the cancer-immunity cycle. An immunologically "cold" tumor microenvironment (TME) engenders immune evasion and refractoriness to conventional checkpoint blockade immunotherapy. Here, we combine nanoparticle formulations and an in situ formed hydrogel scaffold to treat accessible tumors locally and to stimulate systemic immunity against metastatic tumor lesions. The nanoparticles encapsulate poly(ε-caprolactone)-derived cytotoxic chemotherapy and adjuvant of Toll-like receptor 7/8 through a reactive oxygen species (ROS)-cleavable linker that can be self-activated by the coassembled neighboring photosensitizer following near-infrared (NIR) laser irradiation. Further development results in syringeable, NIR light-responsive, and immunogenic hydrogel (iGEL) that can be implanted peritumorally and deposited into the tumor surgical bed. Upon NIR laser irradiation, the generated ROS induces iGEL degradation and bond cleavage in the polymer-drug conjugates, triggering the immunogenic cell death cascade in cancer cells and spontaneously releasing encapsulated agents to rewire the cancer-immunity cycle. Notably, upon application in multiple preclinical models of melanoma and triple-negative breast cancer, which are aggressive and refractory to conventional immunotherapy, iGEL induces durable remission of established tumors, extends postsurgical tumor-free survival, and inhibits metastatic burden. The result of this study is a locally administrable immunogenic hydrogel for triggering host systemic immunity to improve immunotherapeutic efficacy with minimal off-target side effects.

An Anti-VEGF-B Antibody Reduces Abnormal Tumor Vasculature and Enhances the Effects of Chemotherapy

The vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs) are key regulators of blood vessel formation, including in tumors, where their deregulated function can promote the production of aberrant, leaky blood vessels, supporting tumor development. Here we investigated the VEGFR1 ligand VEGF-B, which we demonstrate to be expressed in tumor cells and in tumor stroma and vasculature across a range of tumor types. We examined the anti-VEGF-B-specific monoclonal antibody 2H10 in preclinical xenograft models of breast and colorectal cancer, in comparison with the anti-VEGF-A antibody bevacizumab. Similar to bevacizumab, 2H10 therapy was associated with changes in tumor blood vessels and intra-tumoral diffusion consistent with normalization of the tumor vasculature. Accordingly, treatment resulted in partial inhibition of tumor growth, and significantly improved the response to chemotherapy. Our studies indicate the importance of VEGF-B in tumor growth, and the potential of specific anti-VEGF-B treatment to inhibit tumor development, alone or in combination with established chemotherapies.

Abstract PO1-04-07: Disitamab vedotin, a clinical stage HER2-directed antibody-drug conjugate, shows potent antitumor activity as a monotherapy and in combination with tucatinib in preclinical breast cancer models

Abstract Disitamab vedotin (DV, RC48-ADC) is an antibody-drug conjugate (ADC) that targets cancers expressing HER2. DV consists of an anti-HER2 monoclonal antibody, disitamab, conjugated with the microtubule-disrupting agent monomethyl auristatin E (MMAE) via a cleavable vedotin linker. DV has multimodal antitumor mechanisms of action that include direct cytotoxicity of HER2-expressing cancer cells and bystander effect-based cytotoxicity of neighboring cells, both of which are mediated by the intracellular release of MMAE. Released MMAE can also induce immunogenic cell death, which promotes immune cell recruitment to the tumor. In addition, DV stimulates Fc-gamma receptor mediated antibody-dependent cellular cytotoxicity, which can lead to target cell death. DV also inhibits HER2-activated downstream signaling pathways, further blocking cell growth, survival, and proliferation. Previously, we showed the cytotoxic activity of DV against a panel of breast cancer cell lines with varying levels of HER2 expression, including the HER2-low range. Here, we sought to further characterize the preclinical antitumor activity of DV in patient-derived 3D models and patient-derived xenographs (PDX). In cultured breast cancer cells, DV was more potent and internalized to a greater magnitude than the HER2-directed ADCs trastuzumab emtansine (T-DM1) and trastuzumab deruxtecan (T-DXd). In patient-derived 3D models, DV had superior activity compared to T-DXd. We further explored whether dual HER2 targeting with DV in combination with the HER2-selective oral TKI tucatinib improved the antitumor outcomes. In PDX models with varying HER2 IHC levels and models refractory to T-DXd, DV as a monotherapy and in combination with tucatinib showed significant tumor growth inhibition. This result is consistent with previous in vitro results that showed increased cytotoxicity of the combination of DV and tucatinib in cell lines with a wide range of HER2 expression, which may be mechanistically attributed to elevated HER2 cell surface levels upon treatment with tucatinib. Overall, these findings provide scientific rationale to explore DV in HER2-positive and HER2-low breast cancer patients as a monotherapy or in combination with tucatinib. Citation Format: Kelsi Willis, Renee Hein, Katie Snead, Robert Thurman, Anita Kulukian. Disitamab vedotin, a clinical stage HER2-directed antibody-drug conjugate, shows potent antitumor activity as a monotherapy and in combination with tucatinib in preclinical breast cancer models [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 PO1-04-07.

Abstract PO1-29-01: Single agent and combinatorial efficacy with imipridone ONC206 via inhibition of mitochondrial function in preclinical models of chemorefractory triple negative breast cancer

Abstract Breast cancer remains a major global health concern, with triple-negative breast cancer (TNBC) being one of the most aggressive subtypes associated with limited targeted therapies. Chemotherapy resistance in TNBC patients poses a significant clinical challenge and is associated with poor progression-free and overall survival. Thus, exploring novel approaches to improve chemotherapeutic efficacy is of paramount importance. We previously demonstrated a role for mitochondrial fusion in supporting oxidative phosphorylation (oxphos) activity and cell survival in residual TNBC (PMID: 30996079 and 36813854) cells refractory to chemotherapy treatments. Based on these observations, we sought to inhibit mitochondrial function to improve chemotherapeutic efficacy. We used ONC206, a novel agonist of mitochondrial serine protease, ClpP (Caseinolytic Mitochondrial Matrix Peptidase Proteolytic Subunit) to induce mitochondrial proteotoxic stress. ONC201, a parent molecule of ONC206, has shown efficacy in TNBC as a single agent or when combined with a MEK inhibitor in vitro (PMID: 34680527). We demonstrate treatment of TNBC cell lines with ONC206 results in reduced mitochondrial fusion and OXPHOS. Furthermore, ONC206 co-treatment enhanced chemo-sensitivity in vitro. Based on these promising findings, we tested this in orthotopic patient-derived xenograft (PDX) mouse preclinical trials. Three PDX models of TNBC were tested, BCM-2665, BCM-5471, and BCM-0002. ONC206 as a single agent (twice weekly for 4 weeks, oral gavage, 100mg/kg) significantly decreased tumor growth in all three PDX models. Remarkably, treatment ONC206 achieved nearly complete tumor regression as a single agent or when combined with carboplatin in BCM-2665. Regressed tumors did re-grow, but this was significantly delayed when combined with carboplatin. Treatment with ONC206 in BCM-5471 resulted in a modest but significant reduction in tumor growth rate as a single agent. However, ONC206 did not improve carboplatin (weekly, i.p., 50mg/kg) nor docetaxel (weekly, i.p., 20mg/kg) efficacy in this model. ONC206 treatment of BCM-0002 resulted in prolonged tumor stasis and a slight enhancement of docetaxel and carboplatin efficacy. Interestingly, the efficacy of ONC206 in these PDX models correlates with their relative mRNA expression of ClpP. The mice demonstrated tolerance to ONC206 as a single agent or when combined with chemotherapy during the course of treatment. Our study underscores the potentially crucial role of mitochondria in driving chemotherapy resistance in TNBC and provides novel insights into mitochondria-targeted therapeutic approaches. We report a novel targeting option for TNBC and anticipate that these findings will stimulate further research into innovative combination therapies for TNBC patients, and may lead us to a viable inhibitor. Citation Format: Lily Baek, Lacey Dobrolecki, Stefanie Faucher, Christina Sallas, Nia Griffith, Varun Prabhu, Bora Lim, Michael Lewis, Gloria Echeverria. Single agent and combinatorial efficacy with imipridone ONC206 via inhibition of mitochondrial function in preclinical models of chemorefractory triple negative 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 PO1-29-01.

Abstract PO2-25-01: Tumor Derived Chitinase-3 like 1 Induces Neutrophil Extracellular Traps that Promote T cell Exclusion and Resistance to Immunotherapies in Breast Cancer

Abstract Breast cancer remains the most prevalent cancer and a major cause of mortality affecting women worldwide. Chemotherapy along with radiation and surgery have long been the standard of care for breast cancer patients. However, devastating side effects, resistance to treatments and relapse are major concerns for patients and physicians. With the rise of immunotherapies as a treatment option for various cancers, there is a critical need to understand the mechanisms through which cancers evade the immune system. Targeting such mechanisms holds the key to improve patient response to immunotherapy, especially in breast cancer where efficacy rates remain low. In a subset of poor outcome Triple Negative Breast Cancers, cytotoxic T cells are excluded from the tumor nest and restricted to the surrounding stroma, a phenomenon known as stromal restriction. Our data from genetically engineered mouse models and human tumors identified that the secreted cytokine Chitinase-3 like 1 (Chi3l1) promotes stromal restriction of T cells in several tumor types including breast, lung and colon. Chi3l1 ablation in pre-clinical models of breast cancer results in increased T cell infiltration into the tumor nest which delayed mammary tumorigenesis and improved response to immunotherapy. We further demonstrate that Chi3l1 promotes T cell exclusion through the direct induction and deposition of neutrophil extracellular traps (NETs) which form a barrier that restricts T cell entry. Neutrophil depletion or pharmacological disruption of NET formation abrogates Chi3l1-induced T cell exclusion and histologically phenocopies Chi3l1 ablation. Thus, our results indicate that Chi3l1 is a major immunosuppressive cytokine that promotes breast cancer progression by inducing NET formation and inhibiting T cell infiltration. Given the importance of T cell infiltration in immune elimination of nascent tumors, the future targeting of Chi3l1 should improve immunotherapy efficacy and outcomes in patients with tumors characterized by a T cell excluded microenvironment. Citation Format: Tarek Taifour, Sherif Samer Attalla, Dongmei Zuo, Yu Gu, Virginie Sanguin-Gendreau, Hailey Dall-Proud, Emilie Solymoss, Tung Bui, Hellen Kuasne, Vasilios Papavasiliou, Chun Geun Lee, Suchitra Kamle, Peter Siegel, Jack Elias, Morag Park, William Muller. Tumor Derived Chitinase-3 like 1 Induces Neutrophil Extracellular Traps that Promote T cell Exclusion and Resistance to Immunotherapies 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 PO2-25-01.

Abstract PO3-25-09: OKI-219, a PI3KαH1047R-mutant-selective inhibitor demonstrates efficacy as a single agent and drives combination responses with standard of care therapies in pre-clinical PI3KαH1047R mutant breast cancer models

Abstract Mutations in PI3Kα lead to constitutive activation of the PI3K/AKT/mTOR pathway and are highly prevalent in cancer. PI3KαH1047R is the most common amino acid substitution mutation and represents approximately one-third of all PI3Ka mutations, with 15% of breast cancers having a PI3KαH1047R mutation. In breast cancer, PI3KαH1047R mutations are associated with resistance to targeted therapies, including HER2- and estrogen receptor (ER)- targeted agents. Although targeting PI3Kα in cancer is a therapeutically proven strategy, with the currently approved drug alpelisib showing clinical efficacy alone or in combination with other therapies, treatment with non-mutant selective PI3Kα inhibitors such as alpelisib is associated with significant toxicities such as hyperglycemia, due to on-target inhibition of the wild-type enzyme. This often leads to dose modification or discontinuation. Therefore, there is a significant need to develop new PI3Kα-targeted therapies that can avoid or minimize on-target toxicity and improve the safety and clinical benefit for patients. OKI-219 is an orally bioavailable, PI3KαH1047R mutant-selective inhibitor, with ~100-fold cellular selectivity for the H1047R mutation over wild-type. OKI-219 demonstrates selectivity across a panel of 412 kinases, with no other kinases showing >30% inhibition at 1 µM. In vitro, OKI-219 drives decreased phosphorylated AKT (pAKT) and decreased proliferation specifically in PI3KαH1047R mutant cell lines across several tumor types, including breast cancer. In vivo, as a monotherapy, OKI-219 demonstrates dose dependent antitumor activity in multiple human tumor xenograft models, including the H1047R mutant breast cancer CDX models T47D and HCC1954 and several PDX models, with regression rates up to 100% at doses that show no sign of the metabolic dysfunction (increased glucose or insulin) associated with wild-type protein inhibition. The in vivo antitumor activity correlates with pathway inhibition, measured by decreased pAKT in tumors. Moreover, OKI-219 dosed in combination with the selective estrogen receptor degraders (SERDs) fulvestrant, elacestrant or camizestrant showed significant combination benefit leading to tumor regressions of up to 100% in the ER+ H1047R mutant breast cancer model xxT47D, at doses where no regressions were observed with single agent treatment. Dosing OKI-219 in combination with HER2-inhibitors, such as tucatinib, led to tumor regressions in the ER-HER2+ and H1047R mutant breast cancer CDX model HCC1954, also at doses where no regressions were observed with single agent treatment. These data indicate that OKI-219 offers improved efficacy and a wider therapeutic window compared to non-mutant selective PI3Kα inhibitors. OKI-219 is advancing into clinical trials. Citation Format: Molly A. Taylor, Qian Zhao, David Mareska, Maria Hoh, Yevgeniy Izrayelit, Kevin Litwiler, Mark L Boys, Rich Woessner, Duncan Walker, James D Winkler, Jennifer Diamond. OKI-219, a PI3KαH1047R-mutant-selective inhibitor demonstrates efficacy as a single agent and drives combination responses with standard of care therapies in pre-clinical PI3KαH1047R mutant breast cancer models [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 PO3-25-09.

Abstract PO2-04-09: Combination of the PLK1 Inhibitor Onvansertib and the PI3Kα Inhibitor Alpelisib Overcomes Palbociclib Resistance in PIK3CA-mutated HR+ Breast Cancer

Abstract In the 1st line setting of hormone receptor-positive HER2 negative (HR+/HER2-) metastatic breast cancer, cyclin dependent kinase 4 and 6 (CDK4/6) inhibitors (palbociclib, abemaciclib and ribociclib) are typically used in combination with endocrine therapy. Though initially effective, acquired resistance to either CDK4/6 inhibitors or endocrine therapy eventually leads to disease relapse in most patients. Activating mutations in PIK3CA, the gene encoding alpha catalytic subunit of phosphatidylinositol-4,5-bisphosphat 3-kinase (PI3K) are found in approximately 40% of HR+ breast cancer patients and have been implicated in mediating resistance to CDK4/6 inhibitors. The PI3Kα inhibitor alpelisib is currently approved for PI3KCA-mutant HR+ breast cancer after progression on CDK4/6 inhibitors. However, drug-associated resistance mechanisms may limit its clinical benefit. Synergistic combination therapies have the potential to improve efficacy and overcome resistance mechanisms. Polo-like kinase 1 (PLK1) is a serine-threonine-protein kinase, key cell cycle regulator that controls G2/M entry and mitotic progression, and that has been shown to mediate resistance to palbociclib in HR+ breast cancer. Onvansertib is an oral and highly specific PLK1 inhibitor currently on clinical development in solid tumors and hematological malignancies. Here we evaluated the potential of onvansertib to increase the activity of alpelisib in PIK3CA-mutant HR+ breast cancer preclinical models. The combination of onvansertib and alpelisib synergistically inhibited cell viability and/or colony formation in three PI3KCA-mutant HR+ breast cancer cell lines, MCF-7 (PIK3CA E545K), EFM-19 (PIK3CA H1047L), and T-47D (PIK3CA 1047R). A significant increase in apoptosis was observed in cells treated with the combination compared to either agent alone. We next tested the combination of onvansertib and alpelisib in patient-derived xenograft (PDX) models. For this purpose, palbociclib-resistant PIK3CA-mutant HR+ breast cancer PDXs were established from primary breast tumor (PDX HBCx-86, PIK3CA E545K) or from metastatic bone biopsies of patients who had progressed on endocrine therapy plus palbociclib (HBCx-180, PIK3CA H1047R) or on PI3Kα inhibitor (HBCx-134palboR31, PIK3CA H1047R). PDX tumors were grafted subcutaneously in nude mice and mice were treated with vehicle, onvansertib (45 mg/kg, oral, 5 days a week), alpelisib (25mg/kg, oral, 5 days a week) or the combination. The combination was well tolerated and showed superior anti-tumor activity than the single agents in the 3 PDX models. In the HBCx-86 PDX, although none of the single agents showed activity, the combination induced potent tumor growth inhibition. In the metastasis-derived PDX HBCx-134palboR31, the combination treatment induced pronounced tumor regression in 62% of mice (5/8), with complete response in 25% (2/8), while mice treated with the single agents showed tumor progression. Finally, in the HBCx-180 PDX, established from a patient with primary resistance to palbociclib, the efficacy of the combination was greater and more durable, with significant survival advantage, compared to the monotherapies. Collectively, our preclinical findings suggest that co-targeting PLK1 and PI3Kα with onvansertib and alpelisib respectively, may constitute a promising therapeutic combination strategy for patients with PIK3CA-mutant HR+ breast cancer failing to respond to first-line standard of care therapies. Citation Format: Sreeja Sreekumar, Pierre Painsec, Davis Klein, Dalia Gonzalez, Laura Sourd, Elodie Montaudon, Tod Smeal, Elisabetta Marangoni, Maya Ridinger. Combination of the PLK1 Inhibitor Onvansertib and the PI3Kα Inhibitor Alpelisib Overcomes Palbociclib Resistance in PIK3CA-mutated HR+ 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 PO2-04-09.

Abstract PO3-18-06: First-in-human phase 1A study of RGT-419B, a next generation CDK4 inhibitor, in patients (pts) with hormone receptor positive (HR+) HER2- advanced/metastatic breast cancer (ABC) who progressed on prior CDK4/6 inhibitors (CDK4/6i)

Abstract Introduction: RGT-419B is a novel next generation CDK inhibitor. It has high potency against CDK4 with additional activity against CDK2 and selectivity against CDK6 to overcome resistance and reduce hematologic toxicity. In addition to anti-cancer activity in preclinical breast cancer models that are resistant to approved CDK4/6 inhibitors, we report the interim data from the first-in-human, multicenter trial of RGT-419B as a single agent in post-menopausal pts with HR+ HER2- ABC. Methods: Eligible pts received ≥2 lines of treatment and progressed on endocrine therapy (ET) and at least 1 line of an FDA approved CDK4/6i. Prior treatments of fulvestrant, other targeted agents and/or chemotherapy were allowed. Dose escalation of RGT-419B as oral (PO) monotherapy was administered in continuous 28-day cycles in 4 cohorts (25, 75,150 mg QD and 150 mg BID). A standard 3+3 design was employed to determine the maximum tolerated dose (MTD)/recommended dose for expansion (RDE). The primary objective was to assess safety and tolerability. The secondary objectives included pharmacokinetic (PK) assessment and exploratory efficacy. Results: At time of abstract submission, 12 eligible post-menopausal pts with HR+/HER2- ABC received RGT-419B. The median age was 64.8y (range 50-80y). All had prior palbociclib + ET (2 pts had abemaciclib or ribociclib after palbociclib); a majority received fulvestrant (67%) and prior chemotherapy (50%). Across the 4 cohorts (12 pts), the most observed treatment-emergent adverse events (TEAEs; all causality) with RGT-419B were nausea, reduced white blood cell counts (neutrophils and/or lymphocytes) and diarrhea. At data cutoff on June 30th 2023, there have been no CTCAE Grade 3 or higher treatment related AE (TRAEs). No ocular toxicity has been observed. No pts have discontinued treatment due to RGT-419B toxicity. Preliminary PK analyses for RGT-419B showed a dose-dependent increase of exposure at steady state with long half-life. Tumor assessments (RECIST v1.1) of pts in the first 3 cohorts treated with RGT-419B monotherapy (25-150 mg QD) showed partial responses in 2 pts (7 pts with measurable disease), 28.6% partial response (PR) (RECIST v. 1.1) and clinical benefit rate (CBR) 44%. Updated safety, PK and efficacy data will be presented. Conclusions: RGT-419B, with potent selective CDK4 activity, CDK2 activity and CDK6 selectivity, demonstrated a favorable safety and PK profile in an ongoing phase I study, with no grade 3 or higher TRAEs observed thus far. RGT-419B administered as once daily monotherapy also demonstrated preliminary evidence of efficacy as well. Dose expansions of RGT-419B as a single agent and in combination with ET in pts with HR+/HER2- ABC following prior CDK4/6i progression are planned. Citation Format: Seth Wander, Dave Valacer, Richard Zuniga, Heather Yeckes-Rodin, Christopher Perkins, Lisa Ge, Lili Yao, Jing Lin, Dawn Begley, Feifei Sun, Andie Zheng, Jing Han, Zhi (Julie) Xie, Aditya Bardia, Kevin Kalinsky. First-in-human phase 1A study of RGT-419B, a next generation CDK4 inhibitor, in patients (pts) with hormone receptor positive (HR+) HER2- advanced/metastatic breast cancer (ABC) who progressed on prior CDK4/6 inhibitors (CDK4/6i) [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 PO3-18-06.

Abstract PO1-04-14: PLK1 Inhibitor Onvansertib Extends the Response and Overcomes Resistance to Paclitaxel in Palbociclib-resistant HR+ Breast Cancer Patient-derived Xenografts

Abstract Paclitaxel is a first-line chemotherapy for hormone receptor positive (HR+) metastatic breast cancer patients after progression on a combination of endocrine and CDK4/6 inhibitor therapy. Response rate to paclitaxel ranges between 20-40%, and most patients progress due to intrinsic or acquired resistance, leaving them with limited treatment choices. Therapeutic strategies to overcome paclitaxel resistance and extend its clinical benefit are urgently needed. Paclitaxel-resistant tumors have increased expression of polo-like kinase 1 (PLK1), a serine-threonine-protein kinase that regulates mitosis and cell cycle progression. PLK1 has also been shown to mediate resistance to CDK4/6 inhibitor palbociclib in HR+ breast cancer. Onvansertib is an orally bioavailable, highly potent, and selective PLK1 inhibitor in clinical development. Onvansertib exhibited synergy in vitro and showed potent anti-tumor activity in vivo in combination with paclitaxel in ovarian cancer and triple negative breast cancer (TNBC) preclinical models. A phase 1b/2 clinical trial is ongoing to evaluate the safety and efficacy of onvansertib plus paclitaxel in advanced TNBC (NCT05383196). Here we investigated whether the potential of onvansertib and paclitaxel combination could be extended to HR+ breast cancer. Co-treatment with onvansertib and paclitaxel synergistically inhibited the viability of HR+ breast cancer cell lines (MCF-7, T-47D, EFM-19, CAMA-1, ZR-75-1). Compared to the single agents, the combination induced increased apoptosis. The combination was further tested in vivo in patient-derived xenograft (PDX) models. To this end, six HR+ breast cancer PDXs with intrinsic or acquired resistance to palbociclib were established from primary breast tumors (n=1) or metastatic bone biopsies (n=5). The PDX tumors were engrafted subcutaneously in nude mice and the effect of onvansertib (45 mg/kg, oral, 5 days a week) and paclitaxel (15-25mg/kg, IP, once a week) was investigated as monotherapy and in combination. The combination of onvansertib and paclitaxel was tolerated with minimal toxicity and showed enhanced anti-tumor activity compared to either agent alone in all 6 PDX models. Paclitaxel and onvansertib monotherapies had little to no anti-tumor activity in 4 out of 6 PDX models (HBCx124palboR25, HBCx-139 palboR5, HBCx-202 and HBCx-131). Importantly, the combination of onvansertib and paclitaxel induced strong anti-tumor activity in these 4 models. Tumor regression was observed in the 2 PDX models with acquired resistance to palbociclib, HBCx-124palboR25 and HBCx-139palboR5, with 62% (5/8 mice) and 54% (6/11 mice) complete response rates, respectively. The combination showed tumor stasis in HBCx-131 and induced tumor regression in HBCx-202, established from patient resistant to palbociclib. In 2 of the 6 PDXs, paclitaxel exhibited anti-tumor activity, inducing tumor regression in HBCx-137paboR26 and tumor stasis in HBCx-86. In comparison, the combination of onvansertib and paclitaxel had greater activity, inducing tumor regression in both models with a higher rate of complete response than the monotherapy. Complete response rates for paclitaxel versus the combination were 62% and 100% in the HBCx-137palboR26 model and 0% and 67% in the HBCx-86 model. Notably, the antitumor activity of the onvansertib and paclitaxel combination was very durable, showing a robust delay in tumor relapse after stopping therapy. Together, our data strongly support that combining paclitaxel with the PLK1 inhibitor onvansertib extends its benefit and overcomes paclitaxel resistance, and represents a promising therapeutic strategy for HR+ breast cancer patients after progression on a combination of endocrine and CDK4/6 inhibitor therapy. Citation Format: Maya Ridinger, Pierre Painsec, Sreeja Sreekumar, Dalia Gonzalez, Davis Klein, Laura Sourd, Elodie Montaudon, Paul Cottu, Tod Smeal, Elisabetta Marangoni. PLK1 Inhibitor Onvansertib Extends the Response and Overcomes Resistance to Paclitaxel in Palbociclib-resistant HR+ Breast Cancer Patient-derived Xenografts [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 PO1-04-14.

Abstract PO2-24-10: Anti-Tumorigenic Immuno-Microenvironment by AB-329, a Potent and Selective AXL inhibitor, combined with paclitaxel in Triple-Negative Breast Cancer Preclinical Model

Abstract BACKGROUND The Gas6/AXL signaling pathway plays a substantial role in cancer cell survival, proliferation, immunosuppression, and metastasis. Elevated AXL expression is associated with advanced cancer stages and decreased overall survival rates, underscoring its potential as a therapeutic target for metastatic cancers. We investigated this possibility by utilizing AB-329, a selective small molecule AXL kinase inhibitor, to treat triple-negative breast cancer (TNBC). TNBC, typified by the lack of ER, PR, and HER2 expression, frequently displays mesenchymal characteristics and carries a significant risk of metastasis and an immunosuppressive tumor microenvironment. METHODS We assessed AXL protein expression in 28 human and 11 mouse breast cancer cell lines using Western blotting. The Microarray database was utilized to examine AXL gene expression in 40 human breast cancer cells. The Sulforhodamine B proliferation and soft agar assays measured the antiproliferative effect of AB-329 in TNBC cell lines. The in vitro anti-metastatic impact of AB-329 was evaluated through migration and invasion assays. We appraised the anti-tumor effects of AB-329 in combination with paclitaxel using 4T1 and E0771-LMB mouse TNBC xenograft models. We used FACS and CIBERSORTx to perform immune cell type profiling in E0771-LMB xenograft tumor tissues. To understand AB-329’s immunomodulatory effects in tumors, single-cell RNA sequencing was conducted in a humanized mouse model with SUM149 human TNBC xenografts. RESULTS Microarray data revealed higher AXL expression in TNBC cells compared to non-TNBC cells. Western blotting supported this, showing higher AXL protein expression, particularly in basal and mesenchymal subtypes of TNBC cells, compared to luminal breast cancer cells. AB-329 presented moderate antiproliferative effects as a monotherapy in the TNBC cell lines tested, with half-maximal inhibitory concentrations exceeding 5 µM. Nevertheless, combining AB-329 with paclitaxel resulted in significant growth inhibition in all tested TNBC cell lines (p < 0.05). Importantly, AB-329 alone could inhibit TNBC migration and invasion at a concentration of 1 µM (p < 0.01) and showed enhanced inhibition rates when paired with paclitaxel (p < 0.05). The combination of AB-329 and paclitaxel considerably reduced tumor growth in the 4T1 and E0771-LMB xenograft models compared to paclitaxel or AB-329. Total RNA sequencing and CIBERSORTx analysis indicated that the AB-329 and paclitaxel combination increased Natural Killer (NK) cells’ infiltration into E0771-LMB mouse TNBC xenograft tumors compared to single agent-treated tumors. A nearly 2.5-fold increase in NKT cell infiltration was also detected with AB-329 treatment compared to Control in SUM149 human TNBC xenografts. CONCLUSIONS Our preclinical data suggest that AB-329, an AXL inhibitor, could synergize effectively with paclitaxel in TNBC treatment, notably inhibiting tumor cell proliferation and migration while enhancing NK cell infiltration. Intriguingly, we observed an increased infiltration of NKT cells into tumors following AB-329 treatment, which may promote an anti-tumorigenic immuno-microenvironment. These results underscore the necessity for further investigation into the role of AB-329 in modulating the tumor immuno-microenvironment through AXL pathway inhibition and accentuate the potential for combining AB-329 with immunotherapy. Citation Format: Dileep Reddy Rampa, Jon Fuson, Huey Liu, Max Pan, Naoto Ueno, Jangsoon Lee. Anti-Tumorigenic Immuno-Microenvironment by AB-329, a Potent and Selective AXL inhibitor, combined with paclitaxel in Triple-Negative Breast Cancer Preclinical Model [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 PO2-24-10.

Targeting CXCR4/CXCL12 axis via [177Lu]Lu-DOTAGA.(SA.FAPi)2 with CXCR4 antagonist in triple-negative breast cancer.

Radiopharmaceutical therapies targeting fibroblast activation protein (FAP) have shown promising efficacy against many tumor types. But radiopharmaceuticals alone in most cases are insufficient to completely eradicate tumor cells, which can partially be attributed to the protective interplay between tumor cells and cancer-associated fibroblasts (CAFs). The C-X-C chemokine receptor type 4/C-X-C motif chemokine 12 (CXCR4/CXCL12) interaction plays an important role inorchestrating tumor cells and CAFs. We hereby investigated the feasibility and efficacy of [177Lu]Lu-DOTAGA.(SA.FAPi)2, a FAP-targeting radiopharmaceutical, in combination with AMD3100, a CXCR4 antagonist, in a preclinical murine model of triple-negative breast cancer (TNBC). Public database was first interrogated to reveal the correlation between CAFs' scores and the prognosis of TNBC patients, as well as the expression levels of FAP and CXCR4 in normal tissues and tumors. In vitro therapeutic efficacy regarding cell proliferation, migration, and colony formation was assessed in BALB/3T3 fibroblasts and 4T1 murine breast cancer cells. In vivo therapeutic efficacy was longitudinally monitored using serial 18F-FDG, [18F]AlF-NOTA-FAPI-04, and [68Ga]Ga-DOTA-Pentixafor PET/CT scans and validated using tumor sections through immunohistochemical staining of Ki-67, α-SMA, CXCR4, and CXCL12. Intratumoral abundance of myeloid-derived suppressive cells (MDSCs) was analyzed using flow cytometry in accordance with the PET/CT schedules. Treatment toxicity was evaluated by examining major organs including heart, lung, liver, kidney, and spleen. CAFs' scores negatively correlated with the survival of TNBC patients (p < 0.05). The expression of CXCR4 and FAP was both significantly higher in tumors than in normal tissues. The combination of [177Lu]Lu-DOTAGA.(SA.FAPi)2 and AMD3100 significantly suppressed cell proliferation, migration, and colony formation in cell culture, and exhibited synergistic effects in 4T1 tumor models along with a decreased number of MDSCs. PET/CT imaging revealed lowest tumor accumulation of 18F-FDG and [18F]AlF-NOTA-FAPI-04 on day 13 and day 14 after treatment started, both of which gradually increased at later time points. A similar trend was observed in the IHC staining of Ki-67, α-SMA, and CXCL12. The combination of [177Lu]Lu-DOTAGA.(SA.FAPi)2 and AMD3100 is a feasible treatment against TNBC with minimal toxicity in main organs.

Abstract LB174: OKI-219 is an inhibitor of PI3Kα H1047R that has brain penetrance and anti-tumor activity in a preclinical intracranial model of metastatic breast cancer

Abstract Background: Dysregulation of PI3K signaling is a common driver of solid tumors, with the PI3KαH1047R activating mutation found in 4% of all cancers. Because many cancers, including breast and lung cancers, can metastasize to the CNS, brain penetration should be a key feature of new therapies. The FDA-approved PI3K inhibitor alpelisib lacks CNS activity, and brain-penetrant alternatives such as buparlisib and paxalasib were not approved due to toxicity associated with a lack of mutant selectivity. OKI-219 is a potent, highly selective PI3KαH1047R inhibitor that has shown preclinical anti-tumor activity in multiple cancer models, both as a monotherapy and in combination with HER2- and ER-mediated therapeutics. Here, we demonstrate its brain penetrance across multiple animal species and anti-tumor activity in an intracranial mouse model of PI3KαH1047R mutant breast cancer. Materials and methods: Brain penetrance in mouse, rat, dog, and monkey was quantified by the ratio of unbound drug concentrations in brain to plasma (Kp,uu). Plasma, brain, and cerebrospinal fluid (CSF) concentrations of OKI-219 were measured by LC/MS, and protein binding in plasma, CSF, and brain homogenate were measured by rapid equilibrium dialysis. Additionally, free drug levels in the brain were directly measured by microdialysis in rats. Mice were implanted subcutaneously and intracranially with PI3KαH1047R xxT47D-luc tumors and tumor growth was measured by calipers (subcutaneous tumors) and luminescence (intracranial tumors). Results: At the Cmax of a pharmacologically active 10 mg/kg PO dose, OKI-219 had a Kp,uu of 0.34, 0.61, and 0.58 in rat, dog, and monkey CSF, respectively. Additionally, OKI-219 had an average Kp,uu of 0.67 over 24h post-dose as measured by brain microdialysis in rats at 30 mg/kg, with a dose-proportionate increase in brain free drug levels at 300 mg/kg. After 21 days of treatment, OKI-219 induced a statistically significant dose-dependent anti-tumor effect on both flank and brain xxT47D-luc tumors. Conclusion: Here we show that OKI-219 had consistently good brain penetrance across multiple species by multiple methods of measuring free drug levels in the brain. We further demonstrated that OKI-219 had anti-tumor activity against xxT47D-luc tumors in an intracranial mouse model that is similar to activity in the same tumor when implanted subcutaneously, demonstrating that the brain levels achieved were pharmacologically active. These data indicate the potential for activity of OKI-219 in CNS disease settings, including metastatic brain cancer, which are not currently addressed with existing PI3K pathway inhibitors. OnKure has initiated the PIKture Phase I clinical study (OKI-219-101) to evaluate OKI-219 in patients with PI3KαH1047R-mutated solid tumors. Citation Format: Alexander A. Strait, Molly A. Taylor, Kevin S. Litwiler, Qian Zhao, David A. Mareska, Mark L. Boys, Yevgeniy Izrayelit, Richard Woessner, James D. Winkler. OKI-219 is an inhibitor of PI3Kα H1047R that has brain penetrance and anti-tumor activity in a preclinical intracranial model of metastatic breast cancer [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 LB174.

IRX4204 Induces Senescence and Cell Death in HER2-positive Breast Cancer and Synergizes with Anti-HER2 Therapy.

Rexinoids, agonists of nuclear retinoid X receptor (RXR), have been used for the treatment of cancers and are well tolerated in both animals and humans. However, the usefulness of rexinoids in treatment of breast cancer remains unknown. This study examines the efficacy of IRX4204, a highly specific rexinoid, in breast cancer cell lines and preclinical models to identify a biomarker for response and potential mechanism of action. IRX4204 effects on breast cancer cell growth and viability were determined using cell lines, syngeneic mouse models, and primary patient-derived xenograft (PDX) tumors. In vitro assays of cell cycle, apoptosis, senescence, and lipid metabolism were used to uncover a potential mechanism of action. Standard anti-HER2 therapies were screened in combination with IRX4204 on a panel of breast cancer cell lines to determine drug synergy. IRX4204 significantly inhibits the growth of HER2-positive breast cancer cell lines, including trastuzumab and lapatinib-resistant JIMT-1 and HCC1954. Treatment with IRX4204 reduced tumor growth rate in the MMTV-ErbB2 mouse and HER2-positive PDX model by 49% and 44%, respectively. Mechanistic studies revealed IRX4204 modulates lipid metabolism and induces senescence of HER2-positive cells. In addition, IRX4204 demonstrates additivity and synergy with HER2-targeted mAbs, tyrosine kinase inhibitors, and antibody-drug conjugates. These findings identify HER2 as a biomarker for IRX4204 treatment response and demonstrate a novel use of RXR agonists to synergize with current anti-HER2 therapies. Furthermore, our results suggest that RXR agonists can be useful for the treatment of anti-HER2 resistant and metastatic HER2-positive breast cancer.

Abstract 5710: The brain penetrant CDK4/6 inhibitor, PRT3645, is highly effective in combination with other targeted therapies in preclinical models of breast cancer, CRC and NSCLC

Abstract Cell cycle deregulation is a hallmark of cancer and the hyperactivation and overexpression of cyclin-dependent kinase (CDK)s are often drivers of cancer pathogenesis. CDK4/6 are critical mediators of cellular transition into S phase and important for cancer initiation, growth, and survival. Previously we reported a novel brain penetrant CDK4/6 inhibitor, PRT3645, that exhibits single digit nanomolar biochemical potency against CDK4/6 and &amp;gt;2000-fold selectivity against other CDKs. PRT3645 inhibits cellular phosphorylation of Rb with nanomolar potency. Furthermore, PRT3645 exhibits favorable in-vivo safety pharmacology and ADME properties, including increased brain penetration, and demonstrates oral bioavailability across rodents, dog and non-human primates. Here, we explored the therapeutic potential of PRT3645 in tumor models harboring actionable genomic alterations but lacking effective targeted treatments. Specifically, we explored the combination of PRT3645 with a selective estrogen receptor degrader (SERD), an approved treatment for patients with HR+/HER2-, ESR1-mutated advanced or metastatic breast cancer. In an ESR1 mutated breast cancer PDX model, PRT3645 not only exhibited monotherapy activity but also induced significant tumor regression in combination with a SERD through inhibiting ER-mediated signaling. We also investigated PRT3645 single agent activity and combination with a clinically validated MEK inhibitor in a BRAF class III mutant colorectal CDX model. PRT3645 was well-tolerated and demonstrated significant anti-tumor efficacy, which was further enhanced by combination therapy. To further explore comprehensive inhibition of cell cycle progression and overcome potential resistance to CDK4/6 inhibition, we investigated dual inhibition of CDK4/6 and CDK2. CDK2 is a crucial player in regulating the cell cycle by controlling the late G1/S transition, promoting DNA replication, and contributing to the DNA repair processes. Additionally, CDK2 activation has been identified as a potential mechanism of resistance to CDK4/6 inhibition. We evaluated CDK2 and CDK4/6 dual inhibition in CDKN2A loss or CCNE amplified cancer cell lines and observed enhanced proliferation suppression, cell cycle arrest and senescence induction, underscoring the potential of this combination for improved therapeutic efficacy across various cancer types. In summary, PRT3645 displays an excellent balance of potency, selectivity, pharmacokinetic parameters across species as well as brain penetrance. In preclinical studies, PRT3645 was highly efficacious when combined with other targeted therapies, offering potential benefits across a wide spectrum of cancer types. In addition, the concept of dual inhibition of CDK4/6 and CDK2 presents a rational and promising approach to enhance the efficacy of cancer therapy. Citation Format: Yue Zou, Kirsten Gallagher, Srijita Dhar, Andrew Busking, Sarah Pawley, Ryan Holmes, Xiaowei Wu, Min Wang, Joseph Rager, Tom Emm, Stefan Ruepp, Miles Cowart, Andrew Combs, Kris Vaddi, Sandy Geeganage, Peggy Scherle, Sang Hyun Lee. The brain penetrant CDK4/6 inhibitor, PRT3645, is highly effective in combination with other targeted therapies in preclinical models of breast cancer, CRC and NSCLC [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 5710.

Abstract 3370: Anti-tumor activity of Debio 0123 in combination with sacituzumab govitecan in preclinical models of breast cancer

Abstract Background: Debio 0123 is a WEE1 inhibitor that has previously shown significant preclinical efficacy in combination with DNA damaging agents and is currently being investigated in phase 1 clinical studies either as a monotherapy or in combination with different standard therapies. Sacituzumab govitecan (SG) is a TROP2 directed antibody-drug-conjugate (ADC) carrying an SN38 payload that has recently gained approval in TNBC and ER+ve breast cancer after demonstrating improvements in survival versus standard therapies. Despite these promising data some patients still do not respond to SG treatment, develop resistance or experience toxic side effects. Herein we present preclinical data that demonstrates Debio 0123 improves tumor response to SG in breast cancer models and provides sustained complete responses and anti-metastatic activity. Methods: Synergy between Debio 0123 and SG was evaluated in breast cancer cell lines, with varying levels of TROP2 expression, in vitro using standard cytotoxicity assay in a matrix combination format. Efficacy of Debio 0123 and SG as monotherapies or in combination was further assessed in vivo, in both TROP2high and TROP2low subcutaneous breast cancer xenograft models at different doses. Results: In vitro, Debio 0123 in combination with SG demonstrated synergistic tumor cell killing in all 4 breast cancer cell lines tested, at all concentrations. In vivo, TROP2high MDA-MB-468 breast cancer xenograft tumors treated with Debio 0123 at 30mg/kg QD resulted in minimal anti-tumor activity as a monotherapy but completely regressed 100% of the tumors in combination with SG. Furthermore, 40% of these complete regressions were sustained for up to 100 days post-treatment initiation. These data were then further replicated at a reduced dose of SG. In the TROP2low MDA-MB-231luc model, Debio 0123 significantly improved response to SG resulting in a 2-fold improvement in tumor growth inhibition. Additionally, combination treatment significantly inhibited metastasis of these tumors to the lung and lymph nodes as monitored by bioluminescence. Collectively, these results lead to a significant improvement in animal survival as a combination compared to either treatment as a monotherapy. Conclusion: These results demonstrate that Debio 0123 synergizes with, and significantly improves breast cancer tumor cell response to, SG regardless of TROP2 expression levels (both in vitro and in vivo), leading to complete regressions and inhibition of metastasis. Collectively these data support Debio 0123 in combination with SG could be a potential therapeutic approach for improving responses in patients with breast cancer. Citation Format: Luke Piggott, Esteban Rodrigo Imedio, Francesco Staehli. Anti-tumor activity of Debio 0123 in combination with sacituzumab govitecan in preclinical models of 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 3370.

Abstract 3959: Unraveling the anti-tumor partnership of CD169+ macrophages and CD8+ T cells in a preclinical breast cancer model

Abstract In this study, we describe both the individual and the cooperative mechanisms by which CD169+ macrophages (MP) and CD8+ T cells support an anti-tumor immune response against breast cancer tumors. Breast cancer is the second leading cause of death in women of all ages each year, thus a deeper understanding of the molecular determinants that promote tumor development and suppress the anti-tumor immune response is still greatly needed. Here we use high dimensional flow cytometry, functional assays, and new mouse models to investigate the MP - T cell anti-tumor partnership. With our collaborators, we developed a preclinical mouse model of breast cancer by genetically engineering mammary stem cells to harbor three cancer driver mutations that are frequently associated in human patients: functional loss of the tumor suppressor genes p53 and mixed-lineage leukemia protein 3, and constitutive activation of the PI3 kinase. To characterize the immune cells in this tumor model, we developed a powerful 30-color high dimensional spectral flow cytometry panel to track and phenotype most known immune cell subsets. We found that CD169+ MP rapidly infiltrated and outnumbered all other populations during tumor growth, suggesting that they play an important role in shaping the tumor microenvironment. Surprisingly, although current literature describes breast cancer MP to be highly immunosuppressive, our phenotypic analysis showed only low expression of the pro-tumor MP markers TREM2 and CD206, but extremely high expression of the anti-tumor markers FOLR2 and CD64. To further investigate if these MP promoted or prevented tumor growth, we selectively depleted CD169+ MP in tumor-bearing mice and observed significantly accelerated tumor growth compared to the non-depleted control group, suggesting that these MP promoted anti-tumor responses. MP depletion also affected tumor-infiltrating CD8+ T cells, which produced lower effector functions (e.g., secretion of IFNγ, CCL3, TNF), indicating that MP may also boost anti-tumor CD8+ T cell responses. To further test if CD8+ T cells control tumor growth, we depleted either CD8+ or CD4+ T cells in tumor-bearing mice and observed that only CD8+ but not CD4+ T cell depletion accelerated tumor growth compared to the control group. MP have been documented to provide type I IFN signals to CD8+ T cells via the triggering of the STING pathway. To explore this mechanism in our model, we created a CD169creSTINGflox/KO mouse in which MP lack functional STING. Tumors in these mice grew much faster compared to those in the control group, consistent with the hypothesis that STING sensing occurs in tumor MP and promotes anti-tumor effector CD8+ T cell responses. Overall, our data provide evidence for an unexpected anti-tumor role for CD169+ MP in breast tumors, presumably through enhancing anti-tumor effector CD8+ T cell responses. Citation Format: Nicole Couturier, Marie Boutet, Kenta Nishitani, Wenjun Guo, Gregoire Lauvau. Unraveling the anti-tumor partnership of CD169+ macrophages and CD8+ T cells in a preclinical breast cancer model [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 3959.

Abstract 1187: Sensitization to chemo-immunotherapy by targeting TGF-β in preclinical triple negative breast cancer models

Abstract Metastatic triple-negative breast cancer (mTNBC) is refractory to anti-PD-(L)1 immunotherapy. In order to sensitize it, preclinical data show the value of adding chemotherapy treatment for its ability to induce immunogenic cancer cell death (Kroemer et al, 2013). This would enable the recruitment of cytotoxic T-CD8+ lymphocytes (CTL) and thus initiate an antitumor immune response to sensitize tumors to immunotherapy. Nevertheless, the results of clinical trials using chemo-immunotherapy combinations are divergent (IMpassion 130 and 131). The IMpassion 130 trial demonstrated the efficacy of nab-paclitaxel and atezolizumab (anti-PD-L1) in PD-L1+ patients, but unfortunately, IMpassion 131 with paclitaxel did not replicate these results. The choice of chemotherapy to combine with immunotherapy appears to be crucial for inducing an antitumor immune response. Optimizing chemo-immunotherapy combinations is a major challenge in the management of TNBC. The initial aim of this project was to evaluate the immune effects of an experimental chemotherapy doublet, based on two drugs used in mTNBC: cisplatin (CDDP) and eribulin. The idea was to find an alternative to the use of taxanes in combination with anti-PD-(L)1.The effect of the two molecules, alone or in combination, was evaluated on immunogenic death stigmata, modulation of effector or immunosuppressive immune populations and stromal populations, and sensitization to anti-PD-L1 immunotherapy in the immunologically "cold" model: 4T1. The in vivo studies has been realized by generation of orthotopic 4T1 tumors, the analysis of the immune infiltration was performed by RT-qPCR, IHC and flow cytometry. In vitro, our results indicate that CDDP, and more intensely in combination with eribulin, significantly induces the various immunogenic death stigmas. In vivo, the therapeutic effect of chemotherapies is limited, with a superior effect of CDDP and the combination. Nevertheless, CTL recruitment and functionality induced by CDDP are significantly enhanced by eribulin. Despite these data pointing to a potential therapeutic synergy with anti PD-L1 immunotherapy, chemo-immunotherapy treatment fails to induce a superior treatment effect. The significant increase in a TGF-β signature induced by the combination in parallel with CTL recruitment explains this resistance. The results show that inhibition of TGF-β, combined with chemo-immunotherapy in the 4T1 model, leads to a reduction in tumor immunosuppression and intratumoral fibrosis, as well as an increase in CTL infiltration and cytotoxicity, enabling tumor sensitization. Overall, this project highlights the benefits of using an experimental doublet of CDDP and eribulin-based chemotherapies. This project demonstrates that management of chemo-induced immunosuppression mechanisms is necessary to overcome tumor resistance to immunotherapy. Citation Format: Laura Kalfeist, Fanny Ledys, Stacy Petit, Samia Kada-Mohammed, Mickael Rialland, François Ghiringhelli, Emeric Limagne, Sylvain Ladoire. Sensitization to chemo-immunotherapy by targeting TGF-β in preclinical triple negative breast cancer models [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 1187.

Abstract 1869: The AI-powered KisoSeek™ platform identifies functional anti-TROP2 heavy-chain antibodies with potent anti-cancer activity

Abstract Functional heavy-chain antibodies (HCAbs) are promising therapeutic agents for targeted cancer therapy. Since they do not have a light chain, HCAbs have a small antigen-binding domain that can unlock new functionalities by reaching epitopes that are inaccessible to classical antibodies. HCAbs can be obtained by immunizing camelids, but this approach is expensive, time-consuming, and results in a limited antibody repertoire. Previously, we engineered the KisoMouse™- a transgenic mouse with multi-camelid species VHH/VH genes and the complete alpaca D and J gene segments integrated into the mouse IgH locus, along with a functional loss of the heavy chain CH1 domain to eliminate light chain pairing. KisoMouse™ are healthy and generate a robust serum HCAb response against a diverse range of immunogens. Trophoblast cell surface antigen-2 (TROP2) is a membrane protein overexpressed in numerous solid tumor types such as breast, colon, and lung cancers. TROP2 is associated with tumor aggressiveness and decreased survival and has emerged as an attractive therapeutic target. An anti-TROP2 antibody-drug conjugate has been approved for breast and bladder cancer, but challenges related to payload toxicity and resistance highlight the need for alternative therapeutics. We had previously identified an anti-TROP2 HCAb with high potency in in vivo preclinical models of human breast and colorectal cancer. Conventional antibody discovery approaches are resource-intensive and can fail to identify rare antibodies. To overcome such limitations, we developed the KisoSeek™ platform, a proprietary AI-powered toolkit that categorizes antibodies in an immune response based on their structural and biophysical features. With KisoSeek™, we analyzed the entire antibody repertoire generated from KisoMouse™ immunized with TROP2 and rapidly identified HCAbs with similar features as the high potency reference antibody described above. We selected three additional antibodies based on their in vitro TROP2 affinity, competitiveness for the reference antibody’s epitope, and yield in CHO cells. These candidates showed comparable potent efficacy in inducing regression in human breast and colorectal xenograft models, akin to the original reference antibody. In summary, the KisoSeek™ platform identified novel anti-TROP2 HCAbs out of the KisoMouse™ antibody repertoire that were similar to our original reference antibody. This demonstrates how KisoMouse™ and KisoSeek™ are two powerful complementary platforms for novel clinical antibody discovery. By categorizing the entire antibody repertoire and identifying antibodies with similar characteristics as known functional antibodies, KisoSeek™ has the potential to leverage existing discovery efforts to uncover many more rare yet potent therapeutic antibodies - the proverbial needles in a haystack. Citation Format: Sophie E. Cousineau, Amit Subedi, Richard Wargachuk, Juliana The, Xiaowei Wang, Wenyang Hou, Amogh Nair, Wansu Qiu, Alex Zhou, Tiffany Cheng, Morteza Babaie, Benyamin Gojogh, Marinieve Montero, Hiba Zahreddine, Elijus Undzys, Lucy Lai, Gordon Ngan, Luis Da Cruz, David Young. The AI-powered KisoSeek™ platform identifies functional anti-TROP2 heavy-chain antibodies with potent anti-cancer activity [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 1869.