HER2-targeted Therapy Research Articles

Implementation and Evaluation of a Breast Cancer Disease Model Using Real-World Claims Data in Germany from 2010 to 2020.

Breast cancer is the leading cause of cancer-related mortality among women in Germany and worldwide. This retrospective claims data analysis utilizing data from AOK Baden-Wuerttemberg, a major statutory German health insurance provider, aimed to construct and assess a real-world data breast cancer disease model. The study included 27,869 female breast cancer patients and 55,738 age-matched controls, analyzing data from 2010 to 2020. Three distinct breast cancer stages were analyzed: Stage A (early breast cancer without lymph node involvement), Stage B (early breast cancer with lymph node involvement), and Stage C (primary distant metastatic breast cancer). Tumor subtypes were estimated based on the prescription of antihormonal or HER2-targeted therapy. The study established that 77.9% of patients had HR+ breast cancer and 9.8% HER2+; HR+/HER2- was the most common subtype (70.9%). Overall survival (OS) analysis demonstrated significantly lower survival rates for stages B and C than for controls, with 5-year OS rates ranging from 79.3% for stage B to 35.4% for stage C. OS rates were further stratified by tumor subtype and stage, revealing varying prognoses. Distant recurrence-free survival (DRFS) analysis showed higher recurrence rates in stage B than in stage A, with HR-/HER2- displaying the worst DRFS. This study, the first to model breast cancer subtypes, stages, and outcomes using German claims data, provides valuable insights into real-world breast cancer epidemiology and demonstrates that this breast cancer disease model has the potential to be representative of treatment outcomes.

Clinicopathologic and Molecular Characteristics of HER2 (ERBB2)-Altered Non–Small Cell Lung Cancer: Implications for Precision Medicine

The heterogeneous relationship between protein expression, amplification, and mutations in human epidermal growth factor receptor 2 (HER2) in non–small cell lung cancer (NSCLC) and the optimal methods for detecting these alterations remain unclear. We aimed to elucidate the clinicopathological and molecular characteristics of HER2-altered NSCLC and investigate practical approaches for identifying patients who might benefit from HER2-targeted therapies. Using next-generation sequencing data from 1680 individuals, we searched for patients with HER2-altered NSCLCs, including amplifications and mutations. Clinicopathological data and tissue slides were reviewed. Immunohistochemistry (IHC) and silver in situ hybridization were performed according to the American Society of Clinical Oncology/College of American Pathologists guidelines. Our analysis identified 89 (5.3%) patients with HER2-altered NSCLCs, comprising 30 (1.8%) with amplification and 59 (3.6%) mutations, and they were compared with 165 control patients. Of the 59 HER2-mutated cases, 52 harbored tyrosine kinase domain (TKD) mutations, primarily HER2 exon 20 insertions. HER2 TKD alterations were associated with younger age, female sex, nonsmoking status, adenocarcinoma with a micropapillary pattern, lung-to-lung metastasis, and poor overall survival. The 33 patients with TKD mutations and 3 with non-TKD point mutations showed incomplete or complete membranous HER2 immunoreactivity (1+ and 2+, 61.07%). Six patients exhibiting amplifications had an IHC score of ≤2+ despite their high copy numbers and concomitantly displayed other actionable EGFR, KRAS, SMARCA4, and other HER2 mutations. These HER2-altered NSCLCs with molecular coalterations showed heterogeneous patterns through HER2 IHC and silver in situ hybridization. Therefore, next-generation sequencing should be used to identify HER2 mutations in patients with NSCLC who present with concomitant alterations. In addition, the above clinicopathological characteristics and HER2 IHC results can be valuable determinants for identifying patients with HER2-altered NSCLC. These insights hold promise for the development of more effective diagnostic and therapeutic strategies for this complex subset of NSCLC patients.

Abstract CT289: A phase II study of Tucatinib and Ado-trastuzumab Emtansine (T-DM1) in patients with HER2-positive metastatic solid tumors and metastases to brain (TUCATEMEB)

Abstract Background: HER2 overexpression or amplification is a validated prognostic and predictive biomarker for treatments in several tumor types. The development of brain metastases is an unfortunate and common complication in patients with metastatic cancer and the incidence of brain metastases continues to grow as advances in cancer treatment extend the survival. Unfortunately many of the clinical trials still exclude patients with brain metastases. Tucatinib is a selective small molecule HER2 inibitor that has shown intracranial activity and it is approved with trastuzumab and capecitabine for treatment of patients with breast cancer including with active brain metastases. Tucatinib showed significant preclinical and clinical antitumor activity in combination with other HER2-targeted therapies, including T-DM1 in breast and other cancers. We hypothesize that the combination of tucatinib and T-DM1 will be a safe and effective treatment in HER2-positive (+) metastatic solid tumors that have metastasized to the brain. To test this hypothesis, we are conducting a Phase II Study of Tucatinib and T-DM1 in Patients with HER2+ Metastatic Solid Tumors and Metastases to Brain (TUCATEMEB). Methods: Eligible adult patients with HER2-positive solid tumors and active brain metastases (with at least one untreated brain lesion ≥0.5 cm and <3.0 cm) will be enrolled in the study. HER2 positivity is defined as HER2 overexpression by immunohistochemistry 3+ or 2+ and fluorescence in situ hybridization positive and/or HER2 amplification by in situ hybridization or next-generation sequencing and/or activating ERBB2 mutation. Patients will receive the study treatment in 21-day cycles: Tucatinib 300 mg orally twice daily; T-DM1 3.6 mg/kg intravenously on Day 1. The primary objective is to determine the intracranial antitumor activity of the tucatinib and T-DM1 combination per the modified RECIST in patients with HER2+ metastatic solid tumors and brain metastases. The secondary objectives are to determine the intracranial antitumor activity per the RANO-BM criteria; to evaluate the safety and tolerability, the duration of intracranial response, the overall survival, the progression free survival, the systemic antitumor activity and clinical benefit rate in patients with HER2+ metastatic solid tumors and brain metastases. Exploratory objectives are to assess predictive and pharmacodynamic markers of response and to corelate them with clinical and molecular characteristics. The efficacy endpoint will be monitored using the Bayesian optimal phase 2 BOP2 design and safety and tolerability will be monitored based on Bayesian posterior probability. Restaging brain MRI and systemic scans will be performed every 6 weeks for the first 12 weeks, and every 9 weeks thereafter. The trial opened in May 2023 and is enrolling patients (NCT05673928). Citation Format: Ecaterina E. Dumbrava, Amber M. Kennon, Rashmi K. Murthy, Emma J. Montazari, Hussein Tawbi, Jing Li, Frederick F. Lang, Gisela Sanchez, Jianbo Wang, Isabella C. Glitza, Barbara J. O'Brien, Thomas Beckham, Todd Swanson, Martin C. Tom, Uyen M. Vu, Tanisha T. Darko, Tiantian Cai, Komal Shah, Funda Meric-Bernstam, Jordi Rodon. A phase II study of Tucatinib and Ado-trastuzumab Emtansine (T-DM1) in patients with HER2-positive metastatic solid tumors and metastases to brain (TUCATEMEB) [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 CT289.

Abstract LB061: Aurora kinases inhibition enhances neoadjuvant chemotherapy response in triple negative breast cancer

Abstract Triple-Negative Breast Cancer (TNBC) is a type of breast cancer without expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor (HER2). TNBC accounts for around 15-20% of all breast cancers, and it remains the most challenging subtype to treat because of its aggressive phenotype and limited treatment strategies. Since TNBC does not respond to endocrine and HER2-targeted therapy, chemotherapy and anti-PD-L1 immunotherapy remain the major treatment strategies so far. However, not all TNBC patients respond to chemotherapy, suggesting that TNBC is a complicated disease. Identifying biomarkers to further predict and subdivide TNBC patients with distinct chemotherapy responses and outcomes is very important. To profile the protein expression and phosphorylation alterations, comprehensive proteogenomics was deployed to probe the molecular basis for differential response to neoadjuvant chemotherapy for TNBC. From a Reverse Phase Protein Array (RPPA) on TNBC cell lines MDA-MB-468 cells and MDA-MB-231 cells treated with Docetaxel, Paclitaxel, Epirubicin, and Doxorubicin, we found only a short list of protein changes after chemotherapy. Among them, the most significant changes in protein are phosphor-AURKA/B/C, AURKA, AURKB, and PLK1, which fall into the mitotic kinase group and are confirmed by western blot analysis. In addition, no significant change in the mRNA level was found after chemotherapy treatment, suggesting a post-transcription level molecular mechanism regulates the upregulation of AURKA and AURKB protein. In fact, the protein level of AURKB decreased more significantly with Cycloheximide (CHX) treatment alone compared with the co-treatment of CHX and Paclitaxel, indicating Paclitaxel extended the lifetime of Aurora Kinases protein and increased the protein stability. Moreover, AURKB or AURKA overexpression in TNBC cells renders them resistant to Paclitaxel treatment and attenuates the apoptosis effect triggered by chemotherapy treatment, suggesting Aurora Kinases could mediate the chemo-resistance in TNBC. To overcome the resistance to chemotherapy, we treated resistant TNBC cells with the combination of Aurora kinase inhibitors and anthracycline/taxane chemotherapy and found that Aurora kinase inhibition significantly enhanced the chemotherapy effect. In summary, we found that Aurora Kinases upregulation by treatment with Taxanes-type and Anthracyclines-type chemotherapy could render chemotherapy resistance to TNBC cells. Citation Format: Jia Xu, Chang Liu, Bohan Ning, Ali Cem Kucukdagli, Yunjia Chen. Aurora kinases inhibition enhances neoadjuvant chemotherapy response in triple negative 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 LB061.

Abstract LB365: Generation of a novel severely immunodeficient Sprague Dawley rat model for xenograft studies

Abstract The recombinant-activating gene 2 (Rag2) plays a critical role in the recombination of the immunoglobulin and T cell receptor genes during the maturation of T and B cells. Loss of Rag2 protein leads to a lack of functional T and B cells, which eliminates the adaptive immune response. However, Rag2 deficiency alone may not guarantee the success of xenograft implantation, due to NK cells and other innate cells remaining. Il2rg is a key component of several cytokine receptors involved in differentiation and/or function of numerous adaptive and innate immune cells, including NK cells, mast cells, basophils, and dendritic cells. To generate a more severely immunodeficient rat model, both the Rag2 and Il2rg genes were knocked out in Sprague Dawley (SD) rats; this model was termed B-SDG rats. The immune profile and organ weights of thymus and spleen were investigated in B-SDG rats in comparison with Rag2-/- and wild-type SD rats. The spleen weights of both Rag2-/- and B-SDG rats were similar, but significantly reduced compared to wild-type SD rats. Notably, the thymus weights of B-SDG male rats were significantly smaller than those of Rag2-/- males. The immune cell profiles in thymus, spleen and blood were next evaluated by flow cytometry. A complete lack of B and T cells was observed in both Rag2-/- and B-SDG rats. B-SDG rats lacked CD161+ NK cells in the spleen and blood, which were maintained in Rag2-/- rats. Dendritic cells, monocytes, macrophages, and neutrophils were detected in all strains. In vivo xenograft implantation with human lung, bladder, breast, pancreas, acute monocytic and myelomonocytic leukemia tumor cell lines was next evaluated in B-SDG rats, which demonstrated successful engraftment. Furthermore, several efficacy studies were performed in B-SDG rats, including cisplatin chemotherapy on human breast invasive ductal carcinoma cell (HCC1954) and human large cell lung cancer cell (NCI-H460) xenografts, and a HER2-targeting ADC therapy was tested in B-SDG rats harboring human non-small cell lung cancer tumors (NCI-H1975). Taken together, these data confirm that B-SDG rats provide a powerful immunodeficient model for preclinical in vivo evaluation of novel drug candidates. Citation Format: Yanhui Nie, Meiqi Zhang, Wanling Zhong, Jia Feng, Bahetiyaer Huwatibieke, Zhaoxue Yu. Generation of a novel severely immunodeficient Sprague Dawley rat model for xenograft studies [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 LB365.

Development and Validation of MRI Radiomics Models to Differentiate HER2-Zero, -Low, and -Positive Breast Cancer.

BACKGROUND. Breast cancer HER2 expression has been redefined using a three-tiered system, with HER2-zero cancers considered ineligible for HER2-targeted therapy, HER2-low cancers considered candidates for novel HER2-targeted drugs, and HER2-positive cancers treated with traditional HER2-targeted medications. OBJECTIVE. The purpose of this study was to assess MRI radiomics models for a three-tiered classification of HER2 expression of breast cancer. METHODS. This retrospective study included 592 patients with pathologically confirmed breast cancer (mean age, 47.0 ± 18.0 [SD] years) who underwent breast MRI at either of a health system's two hospitals from April 2016 through June 2022. Three-tiered HER2 status was pathologically determined. Radiologists assessed the conventional MRI features of tumors and manually segmented the tumors on multiparametric sequences (T2-weighted images, DWI, ADC maps, and T1-weighted delayed contrast-enhanced images) to extract radiomics features. Least absolute shrinkage and selection operator analysis was used to develop two radiomics signatures, to differentiate HER2-zero cancers from HER2-low or HER2-positive cancers (task 1) as well as to differentiate HER2-low cancers from HER2-positive cancers (task 2). Patients from hospital 1 were randomly assigned to a discovery set (task 1: n = 376; task 2: n = 335) or an internal validation set (task 1: n = 161; task 2: n = 143); patients from hospital 2 formed an external validation set (task 1: n = 55; task 2: n = 50). Multivariable logistic regression analysis was used to create nomograms combining radiomics signatures with clinicopathologic and conventional MRI features. RESULTS. AUC, sensitivity, and specificity in the discovery, internal validation, and external validation sets were as follows: for task 1, 0.89, 99.4%, and 69.0%; 0.86, 98.6%, and 76.5%; and 0.78, 100.0%, and 0.0%, respectively; for task 2, 0.77, 93.8%, and 32.3%; 0.75, 92.9%, and 6.8%; and 0.77, 97.0%, and 29.4%, respectively. For task 1, no nomogram was created because no clinicopathologic or conventional MRI feature was associated with HER2 status independent of the MRI radiomics signature. For task 2, a nomogram including an MRI radiomics signature and three pathologic features (histologic grade of III, high Ki-67 index, and positive progesterone receptor status) that were independently associated with HER2-low expression had an AUC of 0.87, 0.83, and 0.80 in the three sets. CONCLUSION. MRI radiomics features were used to differentiate HER2-zero from HER2-low cancers or HER2-positives cancers as well as to differentiate HER2-low cancers from HER2-positive cancers. CLINICAL IMPACT. MRI radiomics may help select patients for novel or traditional HER2-targeted therapies, particularly those patients with ambiguous results of immunohistochemical staining results or limited access to fluorescence in situ hybridization.

The use of targeted therapies in gastrointestinal oncology

In recent years, several drugs have been approved that specifically target molecular changes in tumour cells. For patients with gastrointestinal cancer, this has contributed to a significant improvement in prognosis. This article provides an overview of the currently available treatment options and the underlying biomarkers of their mechanisms of action.The evaluation of biomarkers and the use of targeted therapeutics have now become standard care in gastrointestinal oncology. Beyond the molecular-targeted therapy options already approved in the European Union, there is a multitude of additional drugs and biomarkers available, which can also be used outside of formal approval (so-called off-label use). Examples of this are also discussed in this overview (e.g., HER2-targeted therapy for cholangiocarcinoma, the use of KRASG12C inhibitors, or checkpoint inhibition in microsatellite unstable ductal pancreatic carcinoma).The question whether the use of one of these therapeutics represents a possible treatment option for patients with gastrointestinal cancers is typically discussed in a Molecular Tumour Board after undergoing guideline-appropriate therapies.

472 Utility of [89Zr]Trastuzumab-PET/MRI Imaging for Quantitative Assessment of Tumor Heterogeneity In HER2+ Breast Cancer

OBJECTIVES/GOALS: This study was performed to explore the capabilities of simultaneous [89 Zr]trastuzumab-PET/MRI acquisition in a cohort of metastatic HER2+ breast cancer. The insights derived provide additional noninvasive characterization and precise intratumoral analysis tools for healthcare providers. METHODS/STUDY POPULATION: A total of 13 patients, aged between 40 and 70, diagnosed with HER2-positive breast cancer, were selected to participate in this study. Whole-body [89 Zr]trastuzumab-PET/MR imaging was performed 5 ± 1 days post-injection of the radiopharmaceutical during ongoing HER2-directed therapy. Concurrently acquired T1-weighted MRI facilitated the identification of normal organ and tumor regions of interest, which were further analyzed for mean ADC and mean standardized uptake value. Multiparametric intratumoral habitat analysis was performed. Utilizing the median metric values, tumors were evaluated for heterogeneity, specifically assessing high and low HER2 expression through an image processing framework in conjunction with ADC metrics. Long-term treatment response evaluation is ongoing. RESULTS/ANTICIPATED RESULTS: Initial analysis indicate all tumors exhibited higher overall uptake of [89 Zr]trastuzumab across various sites including the bone (p=0.019), brain (p=0.014), and breast (p=0.069), when compared to corresponding normal organs. Additionally, increased ADCmean values were observed in all regions besides brain tumors (bone: p=0.002, brain: p=0.5, breast: p=0.03, juxtapulmonary: p=0.037), indicating distinct patterns of cellularity. Notably, one of five patients with a breast lesion, who exhibited a complete response to HER2-targeted therapy, exhibited the highest breast lesion SUVmean. Brain and lymph node lesions demonstrated intratumoral heterogeneity of HER2 expression. Qualification of multi parametric maps is anticipated to inform on intratumoral heterogeneity DISCUSSION/SIGNIFICANCE: Despite limitation in clinical applications of quantitative approaches due to lack of standardization of processing, initial investigations, in combining molecular imaging of HER2 and quantitative MRI demonstrate potential in characterizing metastatic HER2+ breast cancer for intratumoral classification and therapeutic stratification.

HER2-Positive Metastatic Colorectal Cancer.

Targeted treatment strategies are available for human epidermal growth factor receptor 2 (HER2)-positive (amplified and/or overexpressed) metastatic colorectal cancer (mCRC), and HER2 testing is indicated in patients with mCRC. At present, standard of care first-line treatment for those with HER2-positive mCRC remains chemotherapy in combination with epidermal growth factor receptor (EGFR) inhibitors or bevacizumab, depending on RAS/BRAF mutational status and tumor sidedness. HER2-targeted agents should be considered for those with RAS/BRAF wild-type disease in subsequent-line treatment and in first-line treatment for patients not appropriate for intensive therapy. While the choice of anti-HER2 therapy is empiric given lack of head-to-head comparisons, the combination of trastuzumab plus tucatinib has received FDA accelerated approval for use in this setting and is generally the authors' preference. Trastuzumab plus lapatinib, trastuzumab plus pertuzumab, and trastuzumab deruxtecan (T-DXd) also have evidence of efficacy in this setting. As T-DXd has demonstrated activity following treatment with other HER2-targeted regimens and carries an increased risk of high-grade toxicities, the authors favor reserving it for use after progression on prior anti-HER2 therapy. HER2-targeted therapies that inhibit signal transduction appear to have limited activity in those with RAS mutations, including trastuzumab-containing regimens. However, the antibody drug conjugate T-DXd has some data showing efficacy in this setting, and the authors would consider T-DXd in subsequent-line therapy for HER2-positive, RAS-mutated mCRC. Several areas of uncertainty remain regarding how to best utilize HER2-targeted therapies in mCRC. These include the optimal sequence of anti-HER2 therapies with chemotherapy and anti-EGFR therapies, the optimal combination partners for anti-HER2 therapies, and the incorporation of predictive biomarkers to guide use of anti-HER2 therapies. Results of ongoing studies may thus alter the treatment paradigm above in the coming years.

Abstract 3815: Clinical efficacy and safety of HER2 targeted therapy in patients with metastatic colorectal cancer: A systematic review and meta-analysis

Abstract Background: Therapies targeting HER2 have shown clear benefit in patients with breast or gastric cancer,; and their use in metastatic colorectal cancer (mCRC) has been increasingly explored in recent years. HER2 overexpressed or amplified (HER2-positive) mCRC is seen in 2-8% of all mCRC cases and is enriched in RAS wild-type (WT) tumors, representing a patient population that may benefit from anti-HER2 therapies. Although, several single-arm and some non-comparative randomized clinical trials have now tested the use of diverse dual anti-HER2 therapies in mCRC, no summative analysis have been performed. We conducted a systematic review and meta-analysis to evaluate the overall clinical benefit (efficacy and safety) of dual-HER2 inhibition in patients with RAS-WT HER2-positive mCRC. Methods: A systematic review of available literature was performed for clinical trials involving patients with mCRC treated with HER2 targeting therapy. Two independent reviewers selected studies and extracted data from PubMed and abstracts published at major international oncology meetings during the past 5 years. Case reports and other studies not meeting inclusion criteria were excluded, therefore a total of 13 studies were used for analysis. Patients with RAS-mutant tumors within each trial were excluded due to known limited benefit of dual HER2 inhibition in this subset. Inverse variance method with random-effects model was used for meta-analysis to determine the overall effect of the therapy on overall response rate (ORR), disease control rate (DCR), progression free survival (PFS) and treatment-related adverse events (TRAE). Results: Systematic review identified 9 study cohorts which were utilized in the meta-analysis, encompassing a total of 279 patients with mCRC who received Trastuzumab (T) in combination with either Pertuzumab (P), Tucatinib (Tu), Pyrotinib (Py), or Lapatinib (L). Collectively, dual HER2 inhibition showed an ORR of 35.1% (95% CI: 29.4-41.0%), a DCR of 68.3% (95% CI: 62.5-73.9%), and a PFS of 5.2 months (95% CI: 4.4-6.0 months). Pooled TRAE of grade ≥ 3 across studies was 29.9% (95% CI: 18.8-42.4%) but displayed high inter-study heterogeneity (p<0.0001, I2 = 79%), mainly driven by one trial (Pyro-HER2; TRAE = 75.0%, 95% CI: 50.9-91.3%). Conclusions: Overall, this comprehensive meta-analysis confirms that dual HER2 targeting therapies yield meaningful clinical efficacy, with favorable safety profiles, and improve outcomes in patients with RAS-WT HER2-positive mCRC. Citation Format: Oscar E. Villarreal, Lianchun Xiao, Joelle Allam, Alexey Sorokin, Ying Yuan, Scott Kopetz, Kanwal Pratap Raghav. Clinical efficacy and safety of HER2 targeted therapy in patients with metastatic colorectal cancer: A systematic review and meta-analysis [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 3815.

Abstract 2014: Tumor glycosylation impacts the efficacy of HER2 targeting therapies

Abstract Cancer progression remains a major obstacle to the successful treatment of cancer. In HER2+ breast cancer, targeting therapies against HER2 have revolutionized the treatment landscape. However, predictive factors for response are largely unknown and a significant number of patients develop resistance. An extensive body of work has revealed a plethora of mechanisms of resistance. However, a key aspect of cancer cells has been largely overlooked: their aberrant glycosylation profile. Aberrant glycosylation of proteins is a hallmark in cancer and has been linked to multiple processes such as invasion, angiogenesis and modulation of the immune response. Yet the influence of altered glycosylation on the efficacy of HER2-targeted therapies remains unknown. Therefore, here, we address this question using clinical data, 3D co-culture systems and advanced live microscopy. Bioinformatics analysis of SPY-2 clinical trial data reveals an association of specific glycogenes with response to HER2-targeting therapies in breast cancer patients. We validated these findings in resistance cell models generated in-vitro and in-vivo using gain and loss of function assays. Furthermore, to provide a more relevant cell model system, we implement the use of 3D co-cultures of cancer spheroids and immune cells. Thus, our research addresses a largely unexplored area in cancer biology combining clinical data from tumor biopsies, trastuzumab-resistant cancer cell models and, live imaging of 3D heterotypic cultures. This work holds the potential to identify new markers of response to therapies and highlights the importance of the interplay of cancer cells and their microenvironment on the efficacy of current treatments. Citation Format: Ana Ruiz-Saenz, Roos Vincken, Denise Wolf, Veronica Steri, Laura van't Veer, John Martens, Danny Huylebroeck, Mark M. Moasser. Tumor glycosylation impacts the efficacy of HER2 targeting therapies [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 2014.

Abstract 6354: The HER2/4-1BB bispecific antibody, YH32367 (ABL105) demonstrates optimal efficacy and safety for HER2-expressing tumors and exhibits synergistic efficacy when combined with anti-PD-L1

Abstract Overexpression/amplification of the human epidermal growth factor receptor 2 (HER2) is associated with clinically aggressive subtypes in various cancers. Currently, new HER2-targeted therapy like trastuzumab deruxtecan have improved outcomes not only for HER2-positive tumors but also for HER2-low tumors. However, curing cancer patients who develop resistance to current HER2-targeted therapies or experience unwanted side effects remains a significant challenge. Therefore, there is a pressing need for more advanced treatments, and one promising approach involves combination with immunotherapy. 4-1BB (CD137) is a key costimulatory receptor expressed on activated T-cells and natural killer cells, making it a promising therapeutic target in cancer. This study presents the anti-tumor efficacy of the HER2/4-1BB bispecific antibody, YH32367 (ABL105), against both HER2-positive and HER2-low tumors. Additionally, the study suggests that YH32367 could be a viable treatment option for combination with anti-PD-1 antibody (Ab).​ YH32367 has been designed to overcome the challenges with HER2 resistance via tumor-directed 4-1BB agonism. YH32367 induced IFN-γ secretion and thereby eliciting tumor cell death in co-culture condition with hPBMC and HER2-expressing tumor cells. YH32367 showed superior efficacy in eradicating tumors and triggered long-term anti-tumor immunity in MC38/hHER2-bearing h4-1BB KI mouse models, compared to benchmark 4-1BB/HER2-bispecific molecule and anti-HER2 Ab. Even in HER2-low tumor, YH32367 was more effective than benchmark 4-1BB/HER2-bispecific molecule. Additionally, when YH32367 was administered in combination with anti-PD-1 Ab to h4-1BB KI mice bearing HER2-low tumors, a synergistic effect was observed, surpassing the effects of anti-PD-1 Ab alone or anti-HER2 Ab-drug conjugate. The potent anti-tumor efficacy of YH32367 was also verified in HER2 expressing tumor-bearing hPBMC humanized mice. In terms of toxicity, YH32367 demonstrates a favorable safety profile, which was confirmed through a 4-week GLP toxicity study using cynomolgus monkeys at doses of 10, 30, and 100 mg/kg. In summary, YH32367 has demonstrated significant anti-tumor effects, impacting tumor immunity in h4-1BB KI mice with HER2-low and HER2-positive tumors. Furthermore, its combination with anti-PD-1 Ab has shown synergistic efficacy, indicating that YH32367 may offer treatment options not only as a monotherapy but also in combination therapy for HER2 expressing tumors. The phase 1 dose escalation study is currently underway (NCT05523947), and a dose optimization study to determine the recommended phase 2 dose (RP2D) is scheduled for the second half of 2024. Citation Format: Eunjung Lee, Young Bong Park, Minji Choi, Jangwoo Shin, Kyong Bae Kim, Ju Young Park, Junhwan Kim, Kyoung Kyu Ahn, Yangsoon Lee, Kyeongsu Park, Wonjun Son, Donghoon Yeom, Jaehyun Eon, Hanbyul Lee, Hanbyul Jung, Jonghwa Won, Se-Woong Oh, Yeul Hong Kim. The HER2/4-1BB bispecific antibody, YH32367 (ABL105) demonstrates optimal efficacy and safety for HER2-expressing tumors and exhibits synergistic efficacy when combined with anti-PD-L1 [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 6354.

Abstract 4190: Patient-derived xenograft (PDX) models with differential HER2 expression for preclinical evaluation of HER2-targeted therapies

Abstract Human epidermal growth factor receptor 2 (HER2) is a critical therapeutic target in gastric cancer. However, the varying expression levels of HER2 in patients present a significant challenge in selecting the appropriate HER2-targeted therapies. Patient-derived xenograft (PDX) models offer a valuable tool to address this clinical concern. We established a panel of PDX models derived from gastric cancer patients, each representing a distinct range of HER2 expression levels. These models were subsequently treated with HER2-targeted drugs, including Trastuzumab (Herceptin), Margetuximab, and Enhertu. Our results demonstrated differential drug responses corresponding to the varying HER2 expression levels in the PDX samples. Trastuzumab, Margetuximab, and Enhertu exhibited distinct efficacies across the HER2 expression spectrum. This underscores the significance of selecting the right therapeutic approach based on the HER2 status of individual patient. Our collection of PDX models with differential HER2 expression levels provides a robust platform for preclinical evaluation of HER2-targeted therapies. This approach not only addresses the clinical challenge of HER2 heterogeneity but also facilitates a more personalized and effective treatment strategy for gastric cancer patients. Citation Format: Xiaoliu Yang, Shiying Guo, Huiyi Wang, Xu Su, Jing Zhao, Xiang Gao, Cunxiang Ju. Patient-derived xenograft (PDX) models with differential HER2 expression for preclinical evaluation of HER2-targeted therapies [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 4190.

Abstract 4661: A dual degrader of HER2 and EGFR obliterates oncogenic signaling, overcomes therapy resistance, and inhibits metastatic lesions in HER2-positive breast cancer models

Abstract Background: HER2 is an oncogenic receptor tyrosine kinase (RTK) implicated in several types of human cancer. It is strongly expressed in about 20% of breast cancer (BC), known as HER2-positive BC (HER2+ BC), due to gene amplification. HER2 amplification or overexpression is a strong predictor of poor disease prognosis. HER2-targeted therapies are the linchpin of treating HER2+ BC. However, drug resistance is common. The main resistance mechanism is unknown. We exploited cell lines and mouse models (orthotopic patient-derived xenografts, and metastatic lesions in the brain and lung of therapy-resistant HER2+ BC) for experiments of genetic and pharmacologic depletion of HER2 and its family member EGFR, analysis of RTKs and no-RTKs, and evaluation of cell and tumor growth, to delineate the mechanism of HER2+ BC resistance to HER2 inhibitors and assess the therapeutic activity of PEPDG278D. PEPDG278D, a recombinant human protein, is the first-in-class dual degrader of HER2 and EGFR. Results: Trastuzumab-resistant HER2+ BC cells and tumors are highly vulnerable to targeted degradation of HER2 and EGFR. Trastuzumab is the most commonly used HER2 inhibitor in the clinic. Targeted degradation of HER2 and EGFR by PEPDG278D, which binds to their extracellular domains, causes extensive inhibition of oncogenic signaling proteins in HER2+ BC cells resistant to trastuzumab. This is accompanied by strong growth inhibition of cultured cells, orthotopic patient-derived xenografts, and metastatic lesions in the brain and lung of trastuzumab-resistant HER2+ BC. siRNA knockdown shows that eliminating both HER2 and EGFR is important to maximize therapeutic outcome. Collectively, our results reveal the key vulnerability of trastuzumab-resistant HER2+ BC and shows a promising strategy that overcomes trastuzumab resistance by targeting this vulnerability. Conclusions & Therapeutic Relevance: This study identifies a key therapeutic vulnerability of drug-resistant HER2+ BC and shows that an agent that targets this vulnerability is highly effective in overcoming drug resistance in this disease. The findings provide new insights and innovations to advance treatment of drug-resistant HER2+ BC that remains an unmet problem. Citation Format: Lu Yang, Arup Bhattacharya, Darrell Peterson, Yun Li, Xiaozhuo Liu, Elisabetta Marangoni, Valentina Robila, Yuesheng Zhang. A dual degrader of HER2 and EGFR obliterates oncogenic signaling, overcomes therapy resistance, and inhibits metastatic lesions in HER2-positive 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 4661.

Abstract 4113: AST-301,a pDNA-based cancer vaccine encoding HER2-ICD, enhances anti-tumor effect of HER2-ADC in a HER2-expressed gastric cancer xenograft model

Abstract Background: In gastric cancer, the HER2 overexpression (6% to 29.5% of gastric cancers) is considered to increase proliferation activity and suppress apoptosis of the cancer cells. However, HER2-targeting therapy in gastric cancer has not been fully evaluated. Currently, new approaches with HER2-tageted antibody-drug conjugates (ADCs) have led to promising results in HER2-related solid tumors. ADCs may have immune-modulating properties and that the payloads delivered by ADCs may induce immunogenic cell death. Based on this rationale, ADC and cancer vaccine combination would be potentially synergistic to improve clinical outcomes. This study was conducted to evaluate an synergistic effect and immune response of the combination of AST-301 and HER2-ADC in HER2-expressed gastric cancer xenograft model. AST-301 phase 1 study (PN109) was completed and phase 2 randomized-control trials (CornerStone-001, CornerStone-003) have been conducted in breast cancer and gastric cancer. Methods: NCI-N87 cells were mixed with the same amount of matrigel at 1x107 cells to formation tumors in the flanks of athymic BALB/c nude mice. After the tumor size of mice reached 100-200 mm3, all of them were randomly classified into four groups (n=7/group) and drug administration was initiated. AST-301 (100 μg/animal, intradermal injection, mixed with rhuGM-CSF as an immune adjuvant) was administered once a week to a total of fourth. LCB01 (1 mg/kg, intravenous injection) was administered to once. Enhertu (1 or 3 mg/kg, intravenous injection) was administered to twice for four weeks. The relative tumor size and TGI rate at day 25 were key endpoints of efficacy. To evaluate the mechanism underlying the enhanced anti-tumor effect due to the combined administration of AST-301 and HER2-ADCs, immune-cell profiling was also conducted using FACS analysis. Results: Our previous study has shown that AST-301 has anti-tumor effects in human gastric cancer xenograft athymic BALB/c nude mouse model. It was showed that adding AST-301 to LCB01 or Enhertu inhibited tumor growth based on a TGI rate at days 25; AST-301 combining with LCB01 vs LCB01 mono (52 % vs 43 %), AST-301 combining with Enhertu vs Enhertu mono (42 % vs 24 %).As an explorative endpoint, it was observed that myeloid-derived suppressor cells (MDSCs) elevation was modestly inhibited in tumor site in AST-301 mono or certain combination regimen adding AST-301. It was remarkable (or observed or captured) that MDSCs elevation was inhibited in tumor site in the combination groups of all ADCs with AST-301 compared to normal saline groups. Conclusion: With combining AST-301 and HER2 ADCs, it was demonstrated that anti-tumor effect was likely synergistic in the athymic mouse model. Phase 2 study of AST-301 in HER2-expressed gastric cancer has been on going (CornerStone-003, NCT 05771584). Citation Format: Jinho Kang, Hyo-Hyun Park, Jee Hyun Choi, Jinback Lim, Seong-Yong Jang, Min-Ah Kim, Myeong-Kyu Park, Yun-Hee Park, Soyeon Lim, Chul-Woong Chung, Jin Kyeong Choi, Eunkyo Joung, Ashley Yongmin Kim, Hun Jung. AST-301,a pDNA-based cancer vaccine encoding HER2-ICD, enhances anti-tumor effect of HER2-ADC in a HER2-expressed gastric cancer xenograft 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 4113.

Abstract 5914: Non-muscle myosin IIA as a promising therapeutic target in breast cancer

Abstract We aimed to determine the role of non-muscle myosin IIA (NMIIA) in breast cancer using various cancer cell lines including estrogen receptor (ER)+, HER2+ and triple negative breast cancers (TNBC). Myosin heavy chain 9 (MYH9) gene encodes for the heavy chain of the hexameric protein NMIIA. We have recently identified that NMIIA serves as a binding partner for HER3 following the inhibition of the HER family member compared to the vehicle control treatment, as observed in HER3 immunoprecipitates. We checked the basal protein and mRNA levels of NMIIA in different breast cancer cell lines and found higher levels of NMIIA in both HER2+ and TNBC cell lines compared to ER+ cell lines. These data correlate with clinical data of primary breast cancer patients with high MYH9 being less likely in ER+ breast cancer patients and more likely in the PAM50 subtypes basal and HER2 enriched breast cancer patients. By employing a pool of four siRNA targeting MYH9, our data indicated a reduction in proliferation of HER2+ and TNBC cell lines when compared to the control siRNA. We have previously found that loss of MYH9 sensitizes to the HER family inhibitor neratinib treatment as assessed by cell proliferation, colony formation on matrigel, cell migration and invasion. We aimed to determine if pharmacological inhibition of the NMIIA pathway would sensitize to neratinib. RhoA-associated kinase (ROCK) serves as a master regulator of NMIIA activity as it directly phosphorylates the regulatory light chain (RLC) of NMIIA. Pharmacological inhibition of ROCK-NMIIA pathway using the ROCK inhibitor GSK269962A alone or in combination with neratinib reduced the cell proliferation in HER2+ cell lines BT474 and MDA-MB-453 as indicated by MTT and cell proliferation assay. It is well known that HER2-targeted therapies cause the upregulation of HER3. Given our identification of NMIIA as one of the interacting partners of HER3, our focus will be on investigating the NMIIA-HER3 signaling axis to elucidate its contribution to an adaptive response to HER2 inhibition and potential treatment resistance. Furthermore, we will explore strategies to target NMIIA in breast cancer. Citation Format: Wasim Feroz, Rosalin Mishra, Mary Kate Kilroy, Joan T. Garrett. Non-muscle myosin IIA as a promising therapeutic target in 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 5914.

Abstract 3705: Development and analytical validation of a novel assay for HER2 assessment on circulating tumor cells using Parsortix® isolation and BioView imaging technologies

Abstract Background. HER2 testing in tissue biopsies is extensively performed in breast cancer (BC) patients. However, tissue biopsy is invasive and repeated examination is impractical. Additionally, lack of concordance between primary tumor and metastatic sites are often observed. Liquid biopsy, on the other hand, enables less invasive and recurrent sample collection for ongoing monitoring. In this study, we developed an end-to-end Research Use Only assay for concomitant evaluation of HER2 gene amplification and protein overexpression in circulating tumor cells (CTCs). Methods. Blood was drawn from healthy volunteers into Streck Cell-Free DNA tubes, spiked with SKBR3 (HER2+) and Hs 578T (HER2-) breast cancer cells and processed on ANGLE's Parsortix® PC1 System, a microfluidic device that captures CTCs from metastatic breast cancer patients’ blood based on their size and deformability, without relying on specific markers. Captured cells were harvested and spun onto positively charged slides and stained with the Portrait HER2 immunofluorescence (IF) panel, consisting of a nuclear dye, epithelial and mesenchymal markers, leukocytes’ markers, and an antibody targeting HER2. Slides were then de-stained and subjected to HER2 FISH using a commercially available kit. All slides were imaged twice using BioView Allegro Plus, a platform equipped with artificial intelligence for automated imaging, CTC candidate identification and reporting. First, an automated scan after IF was performed to identify CTCs based on marker expression and morphology profiles. Second, an automated relocation of the CTCs based on IF results and reimaging using higher magnification to ensure precise analysis of the FISH signals in single cells. The workflow is being implemented in a cohort of BC patients. Results. Following IF, HER2 protein expression in SKBR3 cells was significantly higher (p<0.0001) than that of HER2-negative cells, with 100% analytical specificity and sensitivity. Following FISH, 100% of the cells were de-stained and presented sharp HER2/CEP17 foci, with 96% of the SKBR3 cells and only 4% of the Hs 578T cells displaying HER2 amplification. Additionally, in a small cohort of metastatic BC patients’, 100% of the identified CTCs are retained and evaluable from IF to FISH. Conclusions. An assay integrating HER2 identification by IF and FISH to characterize CTCs in metastatic BC patients was successfully developed. The assay enables the identification of both HER2 protein and gene copy information from each target cell with minimal cell loss, with the potential for minimally invasive patient monitoring during treatment and better classification of patients who may qualify and benefit from HER2-targeted therapies. An added value of the assay is the practicality of utilizing preserved blood samples, allowing streamline batch shipping of clinical samples. Citation Format: Mariacristina Ciccioli, Alex Young, Laila Vlietinck, Chassidy Johnson, Anne-Sophie Pailhes-Jimenez. Development and analytical validation of a novel assay for HER2 assessment on circulating tumor cells using Parsortix® isolation and BioView imaging technologies [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 3705.

Abstract 2619: A novel EGFR × HER3-targeting bispecific antibody drug-conjugate, BCG019, demonstrates robust anti-tumor efficacy in preclinical evaluation

Abstract EGFR and HER3 are receptor tyrosine kinases that are highly expressed in multiple epithelial tumors. EGFR point mutations and small insertions within the kinase domain are common in lung cancer. HER3 is overexpressed in patients who are resistant to EGFR-TKI therapy, and is one of the common resistance mechanisms of EGFR and HER2-targeted therapy. We hypothesize that simultaneous targeting of EGFR and HER3 with a bispecific ADC will have the potential to overcome the resistance caused by HER3 after EGFR-targeted therapy and obtain better efficacy than the combination of EGFR monotherapy and HER3 monotherapy. We first generated fully human bispecific antibodies (bsAbs) targeting EGFR and HER3 using RenLite® mice, which contain the full human heavy chain variable domain with a common human kappa light chain to facilitate bispecific antibody assembly. The anti-EGFR × HER3 bsAb demonstrated reactivity to human and cynomolgus monkey antigens and binding to multiple cancer cell lines, including NSCLC, gastric, pancreatic, colorectal, and breast cancer cell lines. The anti-EGFR × HER3 bsAb also showed high internalization activity in cell lines expressing EGFR and HER3. The anti-EGFR × HER3 bsAb was then conjugated with vc-MMAE (valine-citrulline-monomethyl auristatin E) for proof-of-concept studies, as well as BLD1102, Biocytogen’s novel, proprietary linker/payload system composed of a DNA Topo I inhibitor payload (BCPT02) and highly hydrophilic protease-cleavable linker, to generate EGFR- and HER3-targeting bispecific ADC candidates BCG019. In vivo efficacy of the bsADC candidates were evaluated in cell-derived NSCLC and gastric cancer xenografts and in patient-derived gastric and colorectal xenograft models. Both bsADCs showed superior anti-tumor activity when compared to benchmark ADCs. Collectively, these results suggest that our novel anti-EGFR × HER3 bsADC has therapeutic potential for EGFR and HER3 co-expressing tumors. Citation Format: Zhuolin Li, Chengzhang Shang, Gao An, Chaoshe Guo, W. Frank An, Yi Yang. A novel EGFR × HER3-targeting bispecific antibody drug-conjugate, BCG019, demonstrates robust anti-tumor efficacy in preclinical evaluation [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 2619.

Abstract 7391: Beyond traditional staining: A virtual-IHC digital assay for HER2 score prediction from routine H&E images

Abstract Background: In breast cancer diagnostics, the accurate assessment of HER2 status is essential for treatment stratification. Conventionally, HER2 immunohistochemistry (IHC) scoring necessitates specialized staining protocols, which are resource- and time-intensive. This study presents a virtual IHC (vIHC) digital assay to predict HER2 scores directly from hematoxylin and eosin (H&E) stained images, which are routinely acquired for all specimens. This methodology aims to accelerate standard practices by reducing the need for additional IHC staining, thereby decreasing laboratory workload, costs, and turnaround time. It also presents an opportunity to screen a wider array of samples promptly, potentially identifying candidates for HER2-targeted therapy that might otherwise be delayed or overlooked. Methods: Serially sectioned breast core needle biopsy samples were stained with H&E and HER2-IHC following Leica-Bond staining protocols. Stained slides were imaged using the Hologic GeniusTM Digital Diagnostics System with Research Use Only software for Whole Slide Imaging. From this initial image dataset, 240 regions of interest (ROIs) were manually selected in order to create a dataset that uniformly covers: (i) all HER2-IHC scores (0-3), (ii) a wide range of HER2 stain intensities, and (iii) expression patterns. The IHC scores were assigned to each selected ROI by a pathology expert, following the scoring recommendations of ASCO/CAP American Society of Clinical Oncology and College of American Pathologists, with the caveat that ductal carcinoma in situ was also given a score for the purpose of training the model. These ROIs were then extracted from the images of the serially sectioned H&E-stained slides using Reveal Bio’s proprietary co-registration digital assay. A predictive digital assay was developed at Reveal Biosciences to classify the 240 H&E-stained breast tissue ROIs into four HER2 IHC score categories: 0, 1, 2, and 3. For model evaluation, cross-validation was performed 10 times by randomly splitting the dataset of H&E ROIs into 80% for training MIL and 20% for testing the model, followed by averaging the performance metrics across the 10 dataset splits. The performance metrics included classification accuracy, precision, recall, and F1 scores for each HER2 score category. Results: The vIHC digital assay for predicting the HER2 scores directly from H&E stained breast tissue images achieved the average classification accuracy of 93% across the four HER2 score categories on the test set of ROIs. This performance demonstrates that the model is capable of successfully distinguishing between the HER2 score categories directly from H&E images without the need for IHC staining. The results suggest that the vIHC digital assay could be a reliable and cost-effective tool for preliminary breast cancer screening, providing a valuable adjunct to traditional histopathological analysis. Citation Format: Misagh Naderi, Nikola Mitic, Nikola Spasic, Djordje Cikic, Kristen Ruf, Nikolaus Wagner, Igor Mihajlovic, Sinisa Todorovic, Ray Jenoski, Sid Mayer, Michael Quick, Claire Weston. Beyond traditional staining: A virtual-IHC digital assay for HER2 score prediction from routine H&E images [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 7391.

Abstract 6506: SOS1 inhibition in combination with HER2 inhibition in models of HER2-driven cancer

Abstract Cancers driven by amplifications or mutations of the gene ERBB2 (coding for the receptor tyrosine kinase (RTK) HER2) have been the focus of HER2-targeting antibodies as well as small molecule inhibitors. Despite initial efficacy, resistance to these HER2-targeted therapies inevitably develops. One strategy to tackle this resistance is to identify rational combinations to address common pathways/factors that contribute to resistance. One such common mechanism of resistance is reactivation of the Mitogen-activated protein kinase (MAPK) pathway. SOS1 is a guanine nucleotide exchange factor (GEF) which regulates the MAPK pathway by catalyzing the exchange of RAS-GDP (inactive) to RAS-GTP (active). SOS1 is activated by multiple different RTKs - upon RTK-ligand binding, autophosphorylation of the RTK recruits the GRB2-SOS1 protein complex to the plasma membrane, enabling the complex to bind and activate RAS-GDP. Given SOS1’s role downstream of RTKs and upstream of the MAPK pathway, we sought to investigate the potential of a SOS1 inhibitor (SOS1i) in combination with a HER2 inhibitor (HER2i) in preclinical models of cancer driven by HER2 mutation or gene amplification. Using in vitro and in vivo models we found evidence of added benefit of SOS1i in combination with HER2i, as well as enhanced inhibition of MAPK signaling, as seen by decrease in ERK activation and expression of MAPK effector genes such as DUSP6. Taken together, we propose that SOS1 inhibition combined with HER2 inhibition may be a rational strategy for deeper and more prolonged responses to HER2 inhibitor treatment by preventing resistance via the MAPK pathway. Citation Format: Robyn L. Schenk, Kaja Kostyrko, Johannes Popow, Christina Gewinner, Anke Baum, Christoph Albrecht, Melanie Hinkel, Robin Jacob, Tom Madensky, Regina Ruzicka, Eva Strauss, Hengyu Lyu, Anastasia M. Lopez, Ningping Feng, Christopher P. Vellano, Joseph R. Marszalek, Timothy P. Heffernan, Mark Pearson, Marco H. Hofmann. SOS1 inhibition in combination with HER2 inhibition in models of HER2-driven 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 6506.