Abstract 3528: Combination therapy with HIF1α/c-MET peptide vaccine and bone-modifying agents is a promising strategy for suppressing bone metastases though a synergistic effect in mouse model of metastatic triple-negative breast cancer

Abstract Metastatic triple-negative breast cancer (mTNBC) remains incurable due to its aggressiveness and high metastatic potential, particularly to bones. Bone metastases activate osteoclasts, resulting in severe pain and bone destruction. While bone-modifying agents such as denosumab (anti-RANKL) and bisphosphonate (zoledronic acid; ZA) have been commonly used to inhibit the activity of osteoclasts, they have limitations such as high cost and jaw osteonecrosis. Due to the high immunogenicity and hypoxic tumor microenvironment (TME) of TNBC, immunotherapies such as immune checkpoint inhibitors (ICIs) and cancer vaccines are widely explored as treatment options. In our previous study, we demonstrated that HIF1α/c-MET peptide vaccine combined with anti-PD-1 effectively inhibited bone metastases. However, in clinical practice, many patients exhibit low response rates due to the heterogeneity of TNBC and its diverse immune evasion mechanism, highlighting the need for new strategies to reverse the TME. The synergistic effects of bone-modifying agents, such as denosumab or ZA have not yet been explored. This study aimed to evaluate whether the combination therapy of HIF1α/c-MET peptide vaccine with bone-modifying agents will provide a synergistic effect for suppressing bone metastases. We established the bone metastatic model by intracardiac inoculation of M6-bone cells in C3(1)Tag mouse. Mice were vaccinated with HIF1α/c-MET peptide vaccine weekly for three times and treated anti-PD-1 or anti-RNAKL or ZA. Control group received PBS vaccination and treated with IgG. Weekly In vivo imaging showed that combination therapy with HIF1α/c-MET peptide vaccine and bone-modifying agents reduced metastases. Through micro-CT and bone volume-to-total volume analysis, bone metastasis and destruction were reduced in the combination therapy of peptide vaccine with bone-modifying agents compared to the anti-PD-1 combination group. To understand the TME changes more comprehensively, immunofluorescence staining of HIF1α and c-MET is ongoing to confirm the inhibitory effects of the peptide vaccine on targeted oncogenic protein. Osteoclast activity is also being quantitatively assessed by analyzing the ratio of osteoclast surface to bone surface using tartrate-resistant acid phosphatase (TRAP) staining. To determine whether combination therapy can modulate the TME, tumor infiltrating lymphocytes (TILs) are being evaluated by assessing the expression of CD4 and CD8. Although further analysis of TME is necessary, we propose that the combination therapy of HIF1α/c-MET peptide vaccine with bone-modifying agents may provide a promising strategy for suppressing bone metastases in mTNBC. Citation Format: JinHwa Hong, Soon Young Lim, Hye Yeon Rho, Dong Ge Ra Mi Moon, Ju Won Kim, Jiwon Lee, Kyong Hwa Park. Combination therapy with HIF1α/c-MET peptide vaccine and bone-modifying agents is a promising strategy for suppressing bone metastases though a synergistic effect in mouse model of metastatic triple-negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3528.