Modular iodinated carboxybetaine copolymers as charge-sensitive contrast agents for the detection of cartilage degradation

Accurately assessing cartilage tissue degradation is a big challenge in osteoarthritis (OA) research, as histology only provides information about a 2D tissue section, and currently available contrast agents for tomographic evaluation suffer from low specificity. In this study, we present a modular platform based on zwitterionic carboxybetaine (CBAA) to create multivalent polymeric contrast agents for x-ray computed tomography (CT) with high specificity towards the anionic glycosaminoglycans in the cartilage tissue. By copolymerizing CBAA with different ratios of anionic and cationic iodinated comonomers, we created a library of polymers with net charges ranging from strongly anionic to strongly cationic. The polymers were applied onto osteochondral plugs with different degradation states and the resulting CT images compared to histological stainings. In healthy tissues, the bulk contrast enhancement was strongly correlated with polymer charge, with cationic polymers reaching a 2-fold stronger contrast compared to established small molecule contrast agents. While a further increase in cationic charge slowed the penetration, it increased the polymer's specificity, thereby enabling the most cationic polymer C40 (40 mol% cationic iodinated comonomer) to discriminate accurately between tissues treated with IL-1β for 0, 1, 2 and 3 weeks. Moreover, this polymer also showed a strong local specificity, visualizing local differences in GAG distribution with significantly increased accuracy compared to the controls. Our polymer contrast agents show the importance of multivalency and charge control for the accurate, volumetric detection of GAGs in the cartilage tissue and paves the way towards new contrast agents in- and outside of the clinic.