Drug-free biodegradable nanoparticles alleviate myeloid cell-induced immunosuppression and inhibit metastasis.

e14555 Background: Metastasis is responsible for most cancer-related deaths and depends on the formation of a metastatic niche at distal sites. The metastatic niche consists primarily of immunosuppressive myeloid cells, such as neutrophils and monocytes. These cells are recruited to distal sites in response to cancer-induced immune dysregulation and promote the recruitment and survival of disseminated tumor cells. We have employed drug-free poly(lactide-co-glycolide) (PLGA) nanoparticles, ONP-302, to modulate myeloid cell phenotypes and trafficking to disrupt the formation of the metastatic niche. The anti-metastatic efficacy of ONP-302 nanoparticles was evaluated in a translationally relevant mouse model of orthotopic triple-negative breast cancer. Methods: ONP-302 nanoparticles were evaluated in the syngeneic 4T1 model of triple-negative breast cancer in female BALB/c or RAG1KO BALB/c mice. ONP-302 nanoparticles intravenously injected into tumor-bearing mice every 3 days starting 1 day after tumor inoculation. Lungs were extracted 14 and 21 days after inoculation for analysis by single cell RNA sequencing and quantification of metastases by luminescent imaging, respectively. In resection studies, primary tumors were resected 11 days after inoculation and tissues were extracted 42 days after resection for quantification of metastases. Results: ONP-302 nanoparticles significantly reduced primary tumor growth and completely inhibited the formation of lung metastases. ONP-302 was also effective at suppressing metastasis when applied following surgical resection. In mice which received ONP-302 after surgical resection, lung metastases were undetectable up to 42 days after resection, while 44% of control mice developed metastases. These findings suggest that ONP-302 targets both primary tumor growth and metastatic colonization. Single cell RNA sequencing was employed to analyze ONP-302 modulation of the lung metastatic niche. The lungs of ONP-302 treated mice had lower neutrophil infiltration and a higher proportion of monocytes and dendritic cells compared to control mice. Gene expression pathways associated with inflammatory interferon and tumor necrosis factor responses were upregulated in neutrophils, monocytes, and dendritic cells from ONP-302 treated mice. The upregulation of these pathways suggests that ONP-302 treatment prevents the formation of an immunosuppressive niche and promotes anti-tumor cytotoxicity. Adaptive immunity was essential for ONP-302 efficacy, as T and B cell deficient RAG1KO mice demonstrated complete loss of ONP-302 efficacy both in slowing primary tumor growth and preventing metastasis. Conclusions: Our findings demonstrate that our drug-free PLG nanoparticles, ONP-302, impede cancer metastasis as a monotherapy by inhibiting the accumulation of immunosuppressive myeloid cells.