Pharmacologic targeting of the dopamine D2 receptor impacts the efficacy of immune checkpoint blockade in melanoma.

[BACKGROUND] Immune checkpoint inhibitors (ICIs) have been successful in treating advanced melanoma, yet, the 10-year melanoma-specific survival is only 52%. Our prior work using genetic linkage analysis revealed that the murine prolactin (PRL) locus associates with ICI response in C57BL/6 (B6)-syngeneic B16F0 melanoma. This was validated in F1 crosses of B6 with Collaborative Cross mice selected as potential non-responders or responders in the PRL locus and directly by coadministration of PRL with ICIs which slowed B16F0 growth compared with ICIs alone. This study uses Food and Drug Administration (FDA)-approved drugs that act on the dopamine D2 receptor (D2R), which inhibits PRL release from the pituitary, and suggests the potential of this receptor as a target to improve ICI outcomes.

[METHODS] We used FDA-approved D2R agonist bromocriptine (BRC) and D2R antagonist metoclopramide (MCP) to lower and raise systemic PRL, respectively in vivo. PRL-locus ICI responder and non-responder animal models were employed to assess the effect of D2R modulation with ICIs on tumor growth. Immunohistochemistry, single-cell and bulk RNA sequencing, and flow cytometry were used to explore the mechanism by which pharmacologic D2R targeting impacts antitumor immunity.

[RESULTS] BRC accelerated B16F0 growth and diminished intratumoral CD8+ T cell infiltration with ICIs in a PRL-locus ICI responder model. Conversely, MCP with ICIs slowed both B16F0 and MEL11443 growth and increased intratumoral CD8+ T cell infiltration in B6 mice, a PRL-locus ICI non-responder model. Bulk RNA sequencing revealed that MCP enhanced intratumoral immune-mediated processes based on biological sex. Single-cell RNA sequencing uncovered enhanced activity of intratumoral CD8+ T cells in a PRL-locus ICI responder compared with the B6 PRL-locus ICI non-responder model. Strikingly, MCP directly enhanced major histocompatibility complex expression on B16F0 and MEL11443 cells, increased antigen-specific CD8+ T cell activation, and modulated bone marrow-derived macrophage polarization in vitro, suggesting an additional mechanism independent of PRL in promoting antitumor immunity.

[CONCLUSIONS] These findings demonstrate that pharmacologic modulation of D2R impacts the efficacy of immune checkpoint blockade in murine melanoma and raise the possibility of using FDA-approved D2R targeting therapies as cotherapeutics to overcome ICI resistance in patients with advanced melanoma.