PI3Kgamma Inhibitor Attenuates Immunosuppressive Effect of Poly(l‐Glutamic Acid)‐Combretastatin A4 Conjugate in Metastatic Breast Cancer

Abstract Vascular disrupting agents (VDAs) have great potential for cancer treatment. Poly( l ‐glutamic acid)‐combretastatin A4 conjugate (PLG‐CA4) is a novel class of VDAs. Though it has notable antitumor activity, it can induce host immune responses that promote tumor growth. Here, PLG‐CA4 induces the polarization of tumor‐associated macrophages (TAMs) toward the M2‐like phenotype in 4T1 metastatic breast cancer (Control 30% vs PLG‐CA4 53%; p < 0.05). Compared to the monotherapy of PLG‐CA4, inhibition of phosphoinositide 3‐kinase gamma (PI3Kγ) attenuates the immunosuppressive effect of PLG‐CA4 treatment by decreasing the number of M2‐like TAMs (2.0 × 10 4 to 1.5 × 10 4 per tumor) and potential enhancement of cytotoxic T lymphocyte (3.0 × 10 4 to 5.7 × 10 4 per tumor). Importantly, PI3Kγ inhibitor synergizing with PLG‐CA4 significantly extends the mean survival time from 52 days in monotherapy‐treated mice to 61.8 days. Additionally, the combination of PLG‐CA4 and PI3Kγ inhibitor improves the tumor therapeutic effect of NLG919, an inhibitor of immune checkpoint indoleamine 2,3‐dioxygenase (IDO). As far as it is known, this is the first demonstrated study that VDAs induce the reshaping of macrophages to the M2‐like phenotype. The findings also indicate a potential therapeutic strategy of the combination VDAs with an accurate immune modifier in the tumor to reverse the immune resistance.