In vivo multimodal imaging of hyaluronan-mediated in fl ammatory response in articular cartilage

Objective: One driving factor in the progression to posttraumatic osteoarthritis (PTOA) is the perpetu- ation of the inflammatory response to injury into chronic inflammation. Molecular imaging offers many opportunities to complement the sensitivity of current imaging modalities with molecular specificity. The goal of this study was to develop and characterize agents to image hyaluronan (HA)-mediated in- flammatory signaling. Design: We developed optical (Cy5.5-P15-1) and magnetic resonance contrast agents (Gd-DOTA-P15-1) based in a hyaluronan-binding peptide (P15-1) that has shown anti-inflammatory effects on human chondrocytes, and validated them in vitro and in vivo in two animal models of PTOA. Results: In vitro studies with a near infrared (NIR) Cy5.5-P15-1 imaging agent showed a fast and stable localization of Cy5.5-P15-1 on chondrocytes, but not in synovial cells. In vivo NIR showed significantly higher retention of imaging agent in PTOA knees between 12 and 72 h (n ¼ 8, Cohen's d > 2 after 24 h). NIR fluorescence accumulation correlated with histologic severity in cartilage and meniscus (r between 0.37 and 0.57, P < 0.001). By using in vivo magnetic resonance imaging with a Gd-DOTA-P15-1 contrast agent in 12 rats, we detected a significant decrease of T1 on injured knees in all cartilage plates at 48 h (?15%, 95%-confidence interval (CI) ¼ [-18%,-11%]) while no change was observed in the controls (?2%, 95%-CI ¼ [-5%,þ1%]). Conclusions: This study provides the first in vivo evidence that hyaluronan-related inflammatory response in cartilage after injury is a common finding. Beyond P15-1, we have demonstrated that mo- lecular imaging can provide a versatile technology to investigate and phenotype PTOA pathogenesis, as well as study therapeutic interventions.