OBJECTIVE: Through binding to folate receptor-β (FR-β), the new (99m)Tc-EC20 
(Etarfolatide) imaging technique detects activated but not resting macrophages 
in vivo. The goal of this study was to investigate macrophage-related 
inflammation in osteoarthritis (OA).
METHODS: Twenty-five individuals (50 knees) with symptomatic OA of at least one 
knee underwent SPECT-CT imaging of both knees and planar imaging of the whole 
body after injection of Etarfolatide. Scans and knee radiographs were scored 
blinded to clinical information including knee and other joint site pain 
severity. Measures of association controlled for age, gender, body mass index 
(BMI) and employed repeated measures to adjust for correlation between knees.
DESIGN: Activated macrophages were present in the majority (76%) of knees. The 
quantity of knee-related macrophages was significantly associated with knee pain 
severity (R = 0.60, P < 0.0001) and radiographic knee OA severity including 
joint space narrowing (R = 0.68, P = 0.007), and osteophyte (R = 0.66, 
P = 0.001). Macrophages were also localized to joints commonly affected by OA 
including hand finger joints (12%), thumb bases (28%), shoulders (26%), great 
toes (18%) and ankles (12%). The presence of joint pain at fingers, wrists, 
ankles and great toes was significantly positively associated with presence of 
activated macrophages at these sites (P < 0.0001-0.04).
CONCLUSIONS: This study provides the first direct in vivo evidence for 
macrophage involvement in OA in a substantial proportion of human knees. The 
association of quantity of activated macrophages with radiographic knee OA 
severity and joint symptoms suggests that drugs targeting macrophages and 
macrophage-associated inflammatory pathways may have the potential to be both 
symptom and structure modifying.