A multiprobe approach to monitoring therapy response in rheumatoid arthritis

ntroduction: Rheumatoid arthritis (RA) is an autoimmune disease leading to chronic synovial inflammation. Monitoring therapy response by molecular imaging may allow optimization of therapy regimens and/or improve therapeutic outcome. Etanercept is a soluble fusion protein consisting of human p75 TNF-α and the Fc portion of human IgG and is often given to RA patients if methotrexate is contra-indicated or withdrawn due to adverse events. Here, we focused on imaging three different targets that play a role in arthritis, namely fibroblast activation protein (FAP), macrophages, and integrin αvβ3, and on determining whether specific imaging of these targets could monitor response to etanercept in RA mice. Methods: Male DBA/1J mice with collagen-induced arthritis were treated with anti-TNFα therapy (etanercept 10 mg/kg, 3x/week, i.p.). SPECT/CT scans were acquired of treated and untreated mice at 1, 24 and 48 hours after injection of 111In-RGD2peptide (targeting integrin αvβ3; 1 ?g/mouse), 111In-anti-F4/80 antibody (targeting macrophages; 10 ?g/mouse), or 111In-28H1 antibody (anti-FAP; 50 ?g/mouse), respectively. Mice were dissected after the last scan to determine the biodistribution. Coinjection of an excess of RGD2with 111In-RGD2 or injection of 111In-labeled control antibodies (rat IgG2b or DP47GS) served as controls to determine nonspecific tracer uptake. Results: RA was imaged with 111In-28H1, 111In-anti-F4/80, and 111In-RGD2. Treatment with etanercept was successful as macroscopic scores of RA were lower in treated mice than in untreated mice (Figure 2). At day 6 after start of therapy, average RA scores were 3.99 ? 1.67 in untreated mice and 1.84 ? 1.08 in treated mice. Tracer uptake in joints correlated with RA score in biodistribution studies and quantitative SPECT; uptake decreased from 28?15%ID/g, 8?4%ID/g, 2?1%ID/g to 11?11%ID/g, 4?4%ID/g, 1?0%ID/g in treated and untreated mice for 111In-28H1, 111In-anti-F4/80, and 111In-RGD2, respectively (p<0.001). Joint uptake of 111In-28H1, 111In-anti-F4/80, and 111In-RGD2 is antigen/receptor-mediated as when competing uptake of these tracers with either antibody controls or excess of peptide, joint:blood values decreased. RA-to-blood ratios (in mice with arthritis score 2) were higher for 111In-28H1 (6?1, lowest population), 111In-anti-F4/80 (10?4), and 111In-RGD2(7?1) than for control 111In-DP47GS (1?1), 111In-rat IgG2b (1?0), or coinjection of excess RGD2, (4?0). Conclusions: The three tracers can be used to specifically monitor the response to therapy in RA at the molecular level