To investigate the distribution of radiolabelled interleukin-1 receptor antagonist (IL-1ra) in patients with rheumatoid arthritis (RA) and to assess whether this cytokine is suitable for scintigraphic visualization of synovitis.
In patients with active RA, scintigraphy was performed after a single i. v. dose of [(123)I]IL-1ra. Clearance and organ distribution of radiolabelled IL-1ra were studied. To assess whether radiolabelled IL-1ra targets the synovial IL-1 receptors, the scintigraphic images obtained with IL-1ra were compared with those obtained by the use of a non-specific control agent. In addition, autoradiography was performed in mice with antigen-induced arthritis that were injected with either radiolabelled IL-1ra or a size-matched, non-receptor-binding protein.
Radiolabelled IL-1ra allowed clear visualization of inflamed joints. Specificity in the detection of synovitis was high, whereas a number of painful and swollen joints were not visualized by scintigraphy. The procedure was well tolerated and [(123)I]IL-1ra was rapidly cleared from the circulation (t(1/2)alpha 11 min, t(1/2)beta 612 min) and excreted mainly in the urine. The definition of synovial contours by IL-1ra scintigraphy was not better than that observed with a non-specific agent. Although radiolabelled IL-1ra retained its affinity for IL-1 receptors, no binding to synovium was observed by autoradiography.
Radiolabelled IL-1ra allows the visualization of synovitis in patients with RA. However, neither the imaging nor the autoradiographic studies indicate that joint accumulation of radiolabelled IL-1ra is due to specific IL-1 receptor targeting. IL-1ra has proved its therapeutic value in RA, but with the dose schedule in this study it does not behave as a specific radiopharmaceutical that is suitable for scintigraphic detection of inflammation.