2523 Background: Binding of CSF-1 to the CSF-1 receptor (CSF-1R) results in proliferation, differentiation, and migration of monocytes/macrophages. Intratumoral infiltration with macrophages correlates with increased invasiveness, growth, and immunosuppression. LY3022855 (LY) is a human IgG1 antibody (mAb) targeting CSF-1R. Methods: Eligible pts (ECOG 0-2) with advanced solid tumors were enrolled. Mandatory pre and post-treatment biopsies were obtained. LY was given on a 6-week cycle. Two escalation regimens (Part A: weight-based dosing; Part B: flat dosing) were investigated in a 3+3 design. Primary objective was to establish the safety and characterize the pharmacokinetics (PK) of LY. Secondary objectives were to establish recommended phase 2 dose (RP2D) and to characterize pharmacodynamics (PD). Results: As of Sept 6, 2016, 35 cancer pts (colorectal 14; lung 4; pancreas 3; others 14) were treated (29 in Part A; 6 in Part B) with median treatment duration 4 weeks (range 1-21). Common treatment-emergent adverse events (TEAEs) were fatigue (54%), hypoalbuminemia (40%), nausea (37%), AST increase (37%), anemia (34%), anorexia (34%), creatine kinase elevation (29%), and constipation (23%). Most common grade (G) 3/4 TEAEs were anemia (11%), fatigue (11%), ascites (9%), and lymphocyte count decrease (9%). 3/28 evaluable pts had DLTs: G3 left ventricular systolic dysfunction (1), G4 rhabdomyolysis and G4 acute renal failure (1), and G3 pancreatitis (1). Eight treatment unrelated deaths were reported. One pt (adenoid cystic carcinoma) had stable disease (~3 mo as of last visit), 19 pts had progressive disease, and 15 pts were non-evaluable for response assessment. PK profile of LY was consistent with IgG1 mAbs. An interim analysis following completion of Part A demonstrated a lack of relationship between weight and clearance, prompting evaluation of non-weight based dosing. PD analyses revealed dose-dependent increases in serum CSF-1 levels as well as suppression of circulating non-classical monocytes (CD14^dim CD16^bright), indicating biologic activity at studied doses. Conclusions: RP2D for LY monotherapy has been determined. Detailed PK and PD data will be presented. Clinical trial information: NCT01346358.