Single Treatment Boiling Histotripsy Focused Ultrasound Ablation Neither Negates nor Enhances the Activity of α-CD40 in a Pancreatic Cancer Model

Objective: Pancreatic adenocarcinoma (PDAC) tumors are often unresectable and do not respond to standard anti-cancer treatments. Boiling histotripsy (BH) is a mechanical focused ultrasound ablation regimen that can target nonoperable tumors. BH is a non-invasive procedure that might synergize with immunotherapies being developed for PDAC. We reasoned that BH-mediated release of tumor antigen and damage-associated molecular patterns would augment tumor immunity and cooperate with α-CD40 therapy. Methods: We explored the efficacy of BH ablation either as monotherapy for PDAC or in combination with systemically administered αCD40 agonistic antibodies in controlling tumor outgrowth. We further assessed the changes in the tumor immune compartment and the ability of BH to release tumor antigens by flow cytometry. Results: A single BH treatment could not control tumor growth and had limited independent immunostimulatory properties. BH effectively liberated tumor antigen into the tumor microenvironment for acquisition by local phagocytes but did not promote its presence in the tumor-draining lymph nodes. BH did not activate either tumor- or lymph noderesident conventional dendritic cells. BH did not impede the ability of α-CD40 immunotherapy to reduce the tumor burden and promote the infiltration of M1-like macrophages and IFNγ+ CD8+ T cells to the tumor microenvironment, suggesting that antibody access to the ablated tissue was not obstructed. Conclusion: Our findings indicate that PDAC tumor progression cannot be halted by BH ablation using sub-total ablation, and that the BH treatment regimen utilized in this study has limited immunogenicity.