A Stable RNA Vaccine Against the Regulatory Peptide Adrenomedullin Reduces Angiogenesis and Tumor Burden in a Subcutaneous Melanoma Model Without Inducing an Immunosuppressive Tumor Microenvironment

Adrenomedullin (AM) is a regulatory peptide that stimulates proliferation, migration, and invasion of melanoma cells, and promotes neovascularization within the tumor microenvironment, making it a compelling therapeutic target in melanoma and other cancers. As a continuation of our previous study on a metastatic tumor model, here we tested an mRNA vaccine encoding a fusion antigen comprising keyhole limpet hemocyanin (KLH) and mouse AM in a subcutaneous melanoma mouse model. In vitro synthesized mRNA was encapsulated in lipid nanoparticles (LNPs) and administered to C57BL/6J mice; empty LNPs served as negative controls. After a four-dose immunization schedule, B16-F10 melanoma cells were injected subcutaneously, followed by a fifth immunization. Mice were sacrificed once tumors reached humane endpoints. Immunization led to a significant increase in anti-AM IgG titers (<i>p</i> = 0.033) and CD8⁺ T cell (<i>p</i> = 0.049) numbers in treated mice. Tumor initiation was significantly delayed (<i>p</i> = 0.005) and subcutaneous tumor volume was reduced (<i>p</i> = 0.0004) compared to controls. A marked decrease in the area occupied by tumor blood vessels (<i>p</i> = 0.028) was also observed, with no signs of systemic toxicity or weight loss. In addition, there was no significant impairment of Ki67⁺ tumor cell proliferation nor changes in the tumor infiltration of CD4⁺, CD8⁺, FoxP3⁺ nor Arg1⁺ cells. The vaccine also proved highly stable at 4 °C, in the absence of cryoprotectants, for more than a month. In summary, we confirmed that a KLH-AM mRNA vaccine is very stable and can elicit humoral and cellular immune responses, inhibit angiogenesis, and delay tumor growth in subcutaneous melanoma, without inducing an immunosuppressive tumor microenvironment (TME), further supporting mRNA vaccines targeting AM as an attractive immunotherapeutic approach.