Microenvironment-driven IGF-1R/PI3K signaling underlies acquired resistance to CSF1R inhibition in gliomas (CGH). Mus musculus

Macrophages accumulate with glioblastoma multiforme (GBM) progression, and can be acutely targeted via inhibition of colony stimulating factor-1 receptor (CSF-1R) to regress high-grade tumors in animal models. However, whether and how resistance emerges in response to sustained CSF-1R blockade is unknown. Here, we investigate whether long-term CSF-1R inhibition can stably regress GBM in preclinical trials. We show that while overall survival is significantly prolonged, tumors recur eventually in >50% of mice. Upon isolation and transplantation of recurrent tumor cells into naïve animals, gliomas re-establish sensitivity to CSF-1R inhibition, indicating that resistance is microenvironment-driven. PI3K pathway activity was elevated in recurrent GBM, driven by macrophage-derived IGF-1 and tumor cell IGF-1R. Consequently, combining IGF-1R or PI3K blockade with continuous CSF-1R inhibition in recurrent tumors significantly prolonged overall survival. By contrast, monotherapy with IGF-1R or PI3K inhibitors in rebound or treatment-naïve tumors was less effective, indicating the necessity of combination therapy to expose PI3K signaling-dependency in recurrent disease. Our findings thus reveal a potential therapeutic approach for treating resistance to CSF-1R inhibitors in the clinical setting. Overall design: 3 rebound neurosphere cell lines were assayed against 3 references to normal mouse liver