Targeted Macrophage Modulation as a Disease-Modifying Approach in Canine Osteoarthritis: The Efficacy of EF-M2 (ImmutalonTM) in a Double-Blind Placebo-Controlled Study

Osteoarthritis is a prevalent and disabling condition in companion dogs, yet existing treatments are primarily symptomatic and limited by safety concerns. EF-M2, a defined derivative of vitamin D-binding protein, selectively biases macrophages toward an anti-inflammatory phenotype in vitro. We conducted a randomised, double-blind, placebo-controlled trial (IMPAWS-OA-1) in 60 client-owned dogs with naturally occurring hip or elbow osteoarthritis. Animals were allocated to subcutaneous EF-M2 (0.1 µg/kg) given thrice weekly or twice weekly, or to saline placebo for four weeks, followed by four weeks off-drug. The primary endpoint was change in Canine Brief Pain Inventory–Pain Severity Score (CBPI-PSS) at Day 28. EF-M2 produced dose–frequency-dependent benefits: LS-mean ΔPSS was −2.11 for thrice weekly, −1.42 for twice weekly, and −0.54 for placebo (arm effect p < 0.001). Objective measures showed parallel improvements in peak vertical force and accelerometery. Serum biomarkers confirmed macrophage repolarisation (ARG1/iNOS ratio, IL-10 increase, TNF-α decrease), correlating with clinical response. Adverse events were infrequent and mild, with no excess over placebo. In conclusion, EF-M2 achieved clinically meaningful pain relief, functional gains, and biomarker shifts without safety signals, establishing first-in-species proof that targeted macrophage modulation may be a viable disease-modifying approach for canine osteoarthritis.