A clinically acceptable strategy for sensitizing anti-PD-1 treatment by hypoxia relief

The hypoxic tumor microenvironment (TME) hinders the effectiveness of immunotherapy. Alleviating tumor hypoxia to improve the efficacy of immune checkpoint inhibitors (ICIs) represented by programmed cell death protein 1 (PD-1) antibody has become a meaningful strategy. In this study, we adopted three methods to alleviate hypoxia, including direct oxygen delivery using two different carriers and an indirect way involving HIF-1α inhibition. Both in vivo and in vitro experiments showed that liposomes modified with perfluorocarbon or hemoglobin (PFC@lipo or Hb@lipo) were able to efficiently load and release oxygen, relieving tumor hypoxia. However, the gas release behavior of PFC@lipo was uncontrollable, which might induce acute hyperoxia side effects during intravenous injection and reduce its biosafety. In contrast, whether administered locally or systemically, Hb@lipo revealed high animal tolerance, and was much safer than commercial HIF-1α inhibitor (PX-478), displaying prospects as a promising oxygen carrier for clinical practice. Pharmacodynamic experiments suggested that Hb@lipo helped PD-1 antibody break the therapeutic bottleneck and significantly inhibited the progression of 4 T1 breast cancer. But in CT26 colon cancer, the combination therapy failed to suppress tumor growth. After in-depth analysis and comparison, we found that the ratio of M1/M2 tumor associated macrophages (TAMs) between these two tumor models were dramatically different. And the lower M1/M2 ratio in CT26 tumors limited the anti-tumor effect of combination therapy. In this study, three methods for alleviating tumor hypoxia were compared from the perspectives of biosafety, efficacy and clinical applicability. Among them, Hb@lipo stood out, and its combined use with PD-1 antibody exhibit a distinct synergistic suppression effect on tumors with more M1 macrophages presented in the microenvironment. Our work provided a good reference for improving the efficacy of PD-1 antibody by alleviating tumor hypoxia.