OP0222 Non-Invasive Pet Imaging of B-Cells in RA Patients Initiating Rituximab Treatment
Authors: S. Bruijnen, M. Tsang-A-Sjoe, H. Raterman, T. Ramwadhdoebe, D. Vugts, G. Van Dongen, M. Huisman, O. Hoekstra, P.P. Tak, A. Voskuyl, C. van der Laken
Journal: Annals of the Rheumatic Diseases (2015)

<h3>Background</h3> In last decade rituximab (RTX), a B-cell targeted monoclonal antibody, has been introduced to treat rheumatoid arthritis (RA) patients but with variable response rates (30-50%). To increase treatment efficacy and reduce costs, treatment should be individualized to match the spectrum of RA. B-cell targeted therapy has been hypothesized to be most effective in a more B-cell mediated RA disease, possibly correlated to serological status (1). Whole body Positron Emission Tomography (PET) is capable to show B-cells targeting with high specificity in lymphoma patients using <sup>89</sup>Zr-RTX (2). In addition, our group previously demonstrated the potential of PET for non-invasive visualization of immunological targets in RA (3). <h3>Objectives</h3> To investigate the feasibility of non-invasive imaging of B-cells in RA patients who initiate RTX treatment by [<sup>89</sup>Zr]RTX PET-CT. <h3>Methods</h3> Anti-B cell therapy naïve RA patients (n=20; female18/20; age 53±11; 65% IgM RF/a-CCP +) with clinical arthritis in at least two hand joints, who were eligible for RTX treatment, were included. Directly after the first therapeutic RTX infusion (1000 mg, without methylprednisolone), 18MBq [<sup>89</sup>Zr]-10 mg RTX was administered. Whole body PET-CT and detailed images of wrists and hands (22 joints/patient) were acquired at 3, and in addition, in a subpopulation at 6 days post-injection (p.i.). Thereafter, RTX treatment was continued according to standard clinical protocol. Areas of enhanced uptake on PET were defined and quantified as maximum standardized uptake values (SUVmax). Wrist and hand joints were clinically assessed for swelling and tenderness. <h3>Results</h3> Visually, all patients showed at least one hand joint with increased focal tracer uptake (87/440 joints; mean/patient ± SD 4.4±4.9) (Figure 1) with distribution: metacarpophalangeal (n=44/200), proximal interphalangeal (n=22/200) and wrist joints (n=21/40). Interestingly, 66% of PET-positive joints (57/87) corresponded to clinical findings of arthritis, while PET additionally displayed possible subclinical disease activity in another 30 joints (34%). Quantitative analysis showed high mean SUVmax values of hotspots in hand joints (2.98±1.46) which were up to 4 times higher than maximum background uptake, but varying between patients (range SUVmax 1.2-8.0), regardless of serological status. Stability of joint uptake was found over time (3-6 days p.i.) while the tracer cleared from circulation, pointing at specific binding in joints. Whole body PET-CT also demonstrated tracer uptake in extra-articular tissues especially in liver, spleen and in 5/20 patients slightly enhanced uptake in at least one peripheral lymph node. <h3>Conclusions</h3> [<sup>89</sup>Zr]RTX PET-CT seems to be a sensitive tool for <i>in vivo</i> identification of B-cells in arthritic joints and extra-articular tissues in RA patients. Whether quantitative differences in uptake (articular and body distribution) correlate to clinical and RTX response data is currently investigated in a prospective study. <h3>References</h3> Isaacs JD et al; Ann Rheum Dis 2013. Muylle K et al; Ann Oncol 2008. van der Laken CJ et al; Arthritis Rheum 2008. <h3>Acknowledgements</h3> This study was financially supported by Hoffmann-La-Roche,The Netherlands <h3>Disclosure of Interest</h3> S. Bruijnen: None declared, M. Tsang-A-Sjoe: None declared, H. Raterman: None declared, T. Ramwadhdoebe: None declared, D. Vugts: None declared, G. Van Dongen: None declared, M. Huisman: None declared, O. Hoekstra: None declared, P. Tak Grant/research support from: P.P. Tak participated in this study from his position at the Academic Medical Center, Amsterdam, The Netherlands. GSK did neither sponsor this study nor supported this study otherwise. P.P.Tak also has affiliations at Cambridge and Ghent but his activities at these locations were not involved in the current study., A. Voskuyl: None declared, C. van der Laken: None declared