Shenqi Fuzheng injection modulates tumor fatty acid metabolism to downregulate MDSCs infiltration, enhancing PD-L1 antibody inhibition of intracranial growth in Melanoma

Background: Addressing brain metastases in cancer presents substantial challenges due to limited therapeutic options and high mortality rates. In clinical practice, the amalgamation of traditional Chinese medicine with other treatment modalities has exhibited noteworthy efficacy in managing disease progression and enhancing quality of life. Objective: To substantiate the regulatory effects of Shenqi Fuzheng Injection (SFI) on the microenvironment of melanoma brain metastases and appraise whether SFI augments the antitumour effects of immune checkpoint inhibitors, with a specific focus on investigating the mechanisms underlying SFI's actions. Methods: Initially, we established a B16-F10 brain transplant tumour model in C57BL/6 mice using a stereotaxic apparatus. The efficacy of the drug was evaluated through in vivo imaging technology, HE staining, and immunofluorescence. Mass Cytometry (CyTOF) and flow cytometry were employed to analyse the impact of SFI on immune cell subpopulations in the tumour microenvironment. Subsequently, transcriptome sequencing and metabolomics were utilised to examine the effects of SFI on melanoma-related genes and metabolism. Molecular docking, Western Blot, and ELISA assays were conducted to investigate the targets of SFI in intervening in melanoma fatty acid metabolism. Finally, the antitumour effects of SFI in combination with immune checkpoint inhibitors were scrutinised in the brain transplant tumour model. Results: The pharmacological findings demonstrated that SFI inhibits the growth of melanoma brain transplant tumours in a dose-dependent manner. CyTOF, flow cytometry, and immunofluorescence results revealed that SFI significantly diminishes the levels of Myeloid-Derived Suppressor Cells (MDSCs) and Regulatory T cells (Tregs) in the tumour microenvironment while enhancing the levels of CD8(+)T and CD4(+) T cells. Subsequently, tran-scriptomic and metabolomic findings, both in vitro and in vivo , indicate that SFI significantly inhibits the arachidonic acid metabolism process in melanoma cells. Molecular docking and biological experiments showed that SFI inhibits the expression of D6D and the activity of COX-2, leading to a reduction in downstream PGE2 production. Lastly, SFI significantly enhances the antitumour effects of PD-L1 antibody against intracranial melanoma. Conclusion: SFI improves the tumour immune microenvironment in melanoma by intervening in fatty acid metabolism, thereby reducing levels of MDSCs and Tregs while increasing levels of CD8+ T and CD4+ T cells. Ultimately, this augmentation leads to enhanced antitumour effects of the immune checkpoint inhibitor PD-L1 antibody.