An orally bioavailable inhibitor of colony stimulating factor 1 receptor (CSF1R; CSF-1R; CD115; M-CSFR), with potential immunomodulatory and antineoplastic activities. Upon administration, CSF1R inhibitor ABSK021 targets and binds to CSF1R, thereby blocking CSF1R activation and CSF1R-mediated signaling. This inhibits the activities of tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs), and prevents immune suppression in the tumor microenvironment (TME). This enhances antitumor T-cell immune responses and inhibits the proliferation of tumor cells. CSF1R, also known as macrophage colony-stimulating factor receptor (M-CSFR) and CD115 (cluster of differentiation 115), is a cell-surface receptor that plays major roles in tumor cell proliferation and metastasis.