High dose acetaminophen re-polarizes CD11b+ cells in the tumor microenvironment towards an activated macrophage phenotype.

OBJECTIVE: High dose acetaminophen (AAP) has demonstrated promising results in early phase clinical trials for treatment of advanced malignancies. When administered concurrently with the CYP2E1 inhibitor fomepizole, AAP can be dose-escalated 100-fold relative to standard dosing without liver toxicity and without compromising anti-tumor activity. METHODS AND ANALYSIS: The current study used a 23-plex flow cytometry panel to study AAP-induced changes in the tumor immune microenvironment of syngeneic mouse breast tumors. Effects of AAP on macrophage function and antigen presentation, alone and in combination with PD-1 antibodies, were evaluated. RESULTS: Findings demonstrated that the vast majority of CD45+ cells in the triple negative breast cancer model are CD11b+ cells of the innate immune system. The CD11b+ cells in the tumor micro-environment of vehicle-treated mice were mostly comprised of GR1+ myeloid derived suppressor cells. On the other hand, the CD11b+ cells in the high dose AAP-treated mice were mostly of the activated macrophage phenotype (F4/80+CD80+MHCII+). In vitro studies were performed to better understand AAP effects on macrophage function. It was demonstrated that AAP enhances phagocytosis as well as antigen presentation by macrophages to antigen-reactive T-cells. These effects are amplified when PD-1 antibodies are combined with AAP in the in vitro antigen presentation assay. Furthermore, AAP has synergistic anti-tumor activity in vivo when combined with PD-1 antibody therapy, an effect that is macrophage dependent. CONCLUSION: The present study demonstrates profound AAP-induced changes in the tumor immune microenvironment including the differentiation of CD11b+ cells towards an activated "M1" macrophage phenotype.