CSF-1R Inhibitors

Colony-Stimulating Factor 1 Receptor (CSF-1R), also known as c-Fms, is a crucial receptor tyrosine kinase (RTK) predominantly expressed on cells of the myeloid lineage, including monocytes, macrophages, and osteoclasts. Its activation by its ligands, CSF-1 (M-CSF) and IL-34, is indispensable for the survival, proliferation, differentiation, and functional maturation of these cells. Upon ligand binding, CSF-1R undergoes dimerization and autophosphorylation, initiating a cascade of downstream signaling events involving the PI3K/Akt, MAPK/ERK, and STAT pathways. These pathways orchestrate gene expression programs vital for macrophage biology, including their polarization, phagocytic activity, and cytokine production. Dysregulated CSF-1R signaling is implicated in a wide array of diseases. In cancer, tumor-associated macrophages (TAMs) are abundant in the tumor microenvironment, where they promote tumor growth, angiogenesis, invasion, metastasis, and immunosuppression. CSF-1R signaling is critical for the recruitment, survival, and pro-tumorigenic functions of TAMs, making its inhibition a strategy to deplete TAMs or reprogram them towards an anti-tumor phenotype.

Macrophages also play central roles in chronic inflammation, and CSF-1R signaling contributes to inflammatory conditions such as rheumatoid arthritis, inflammatory bowel disease, and multiple sclerosis. Furthermore, macrophages are key drivers of fibrosis in organs like the liver, kidney, and lung. In neurodegenerative diseases, microglia, the resident macrophages of the central nervous system, express CSF-1R, and their dysregulated activity is implicated in conditions like Alzheimer's, Parkinson's, and amyotrophic lateral sclerosis. Lastly, osteoclasts, derived from monocytes, are responsible for bone resorption, and their activity is tightly regulated by CSF-1R. Significant progress has been made in developing CSF-1R inhibitors, particularly in oncology. Pexidartinib (Turalio), an oral, selective CSF-1R inhibitor, received approval in 2019 for tenosynovial giant cell tumor (TGCT), a rare, locally aggressive tumor driven by CSF-1R overexpression. Other CSF-1R inhibitors are in various phases of clinical trials (I-III) for solid tumors (e.g., pancreatic, breast, glioblastoma) and hematological malignancies, often in combination with chemotherapy or immunotherapy.

These include monoclonal antibodies like emactuzumab and cabiralizumab, and small molecule inhibitors such as BLZ945 and axitinib (a multi-kinase inhibitor with CSF-1R activity). Extensive preclinical research continues on novel small molecule inhibitors and antibodies aimed at modulating macrophage function across diverse disease settings. CSF-1R inhibitors are classified by their chemical nature (small molecule kinase inhibitors, monoclonal antibodies that are either ligand-blocking or receptor-blocking) and their selectivity (selective CSF-1R inhibitors versus multi-kinase inhibitors that also target CSF-1R, such as sunitinib or cabozantinib). Key research tools include PLX3397 (a precursor to pexidartinib), BLZ945, and GW2580. Challenges include managing on-target toxicities associated with systemic CSF-1R inhibition, given the widespread physiological roles of macrophages (e.g., liver toxicity, hair discoloration, increased serum creatinine). The plasticity of macrophages in the tumor microenvironment also means that complete depletion may not always be beneficial, suggesting that macrophage reprogramming could be a more effective strategy. Combination therapy is a particularly promising avenue, with CSF-1R inhibitors being explored alongside immunotherapies (e.g., PD-1/PD-L1 inhibitors) to overcome immunosuppression in the tumor microenvironment, and with conventional chemotherapies or targeted therapies to enhance anti-tumor responses.

They are also being investigated in combination with anti-fibrotic agents.