PROJECT SUMMARY Macrophages are a class of immune cells important in the prognosis and treatment of several diseases, including cancer. CD68 is the established clinical biomarker for pan-macrophage. Currently, there are no non- invasive probes specific for human or mouse CD68. We propose to develop novel probes for human and mouse CD68 for PET imaging of macrophages. This project involves phage display screening of a human Fab library (Aim 1a); binding of resulting “hits” to CD68 proteins (Aim 1b) validating in CD68-positive macrophages to select lead huFabs (Aim 1c); and evaluating the function of the resulting lead radiolabeled huFabs in appropriate mice models (Aim 2). From this work, we will produce a PET imaging toolkit for imaging human and mouse macrophages with high binding affinity, specificity, and pharmacokinetic properties suitable for same-day imaging. Our long-term goal is to apply our novel PET probes to quantify macrophage density in vivo in different diseases in humans and mice. Specifically, we envision that co-clinical trials will employ these probes for imaging human subjects and corresponding mice models of disease to monitor response to treatments that modulate macrophage activity.