Background F-18 fluorodeoxyglucose (FDG) positron emission tomography (PET) can be used to image synovial inflammation in patients with rheumatoid arthritis (RA). Tocilizumab (TCZ) binds soluble as well as membrane bound interleukin-6 (IL-6) receptors, hindering IL-6 from exerting its pro-inflammatory effects. After the initiation of the TCZ therapies, the serum CRP level and ESR are usually decreased, which lead to the difficulty of the evaluation of the treatment responses.

Objectives One of the merits of PET is quantitative measurement of metabolic activity. Therefore, in this study, for the assessment of the activity of synovitis, we measured standardized uptake value (SUVs) of the affected joints at baseline and after treatment, compared them in themselves and with the clinical findings. In addition, we evaluated if FDG PET would be able to detect a clinical response in the individual joints of RA patients undergoing anti-IL-6 therapies.

Methods FDG-PET and the clinical assessments were performed, at baseline (0M) and 3 months (3M) after the initiation of the TCZ therapies. The clinical assessment included ESR, CRP and MMP-3. Activity of inflammation was evaluated using DAS28-ESR(4), SDAI and CDAI. PET images were interpreted by experienced nuclear physician and increased FDG uptake in bilateral shoulder, elbow, wrist, hip, knee, and ankle joints were recorded. Therapeutic response was evaluated by the changes in the sum of the maximal SUV (SUVmax-sum) of all measured joints and the clinical findings. ΔSUV were defined as follows; ΔSUV=SUVmax-sum(0M) - SUVmax-sum(3M)_. In the following result section, Δ means the difference in the value between at baseline and after the treatment.

Results Eleven (3 men, 8 women; average age: 60.3 (30-82) years) who underwent anti-IL-6 therapies were assessed. The average disease duration of these patients was 17.7 (2-52) years. At baseline, the average of the measured parameters were below; DAS28-ESR(4) 4.89±1.71 (2.49-8.88), SDAI 21.9±20.3 (3.21-75.1), CDAI 20.0±18.9 (2.80-68.0), ESR (mm/h) 70.5±33.7 (16-122), CRP (mg/dl) 1.90±2.02 (0.11-7.10), MMP-3 (ng/ml) 290.5±249.1(28.4-706.8), Tender joint count of 44 joints (TJC) 5.9±10.3 (0-36), Swollen joint count of 44 joints (SJC) 6.9±7.0 (1-24), SUVmax-sum 26.43±9.52 (18.78-53.61). After treatment, the average of observed value were below; DAS28-ESR(4) 2.94±1.56 (1.25-6.28), SDAI 9.1±12.0 (0.71-40.9), CDAI 9.0±11.5 (0.70-39.0), ESR (mm/h) 17.0±15.43 (5-55), CRP (mg/dl) 0.19±0.56 (0.01-1.90), MMP-3 (ng/ml) 139.0±184.1 (32.1-600.7), TJC 3.9±7.4 (0-25), SJC 3.2±5.0 (0-17), SUVmax-sum 21.42±6.98 (14.69-39.59). All disease parameters except for MMP-3 and TJC were significantly decreased after the TCZ treatment compared to the baseline (p<0.01). The ΔSUV significantly correlated with the ΔMMP-3 (r=0.655, p=0.029) and the ΔTJC (r=0.625, p=0.040). At baseline, there were 21 tender joints and 20 swollen joints of all the FDG-PET affected joints. The SUVmax of these joints were significantly decreased after 3M TCZ therapies (p<0.001).

Conclusions The FDG-uptakes of the inflamed RA joints decreased 3 months after the initiation of TCZ treatment. TCZ might exert rapid effects on the synovitis of RA patietnts. FDG-PET can be used to evaluate the response of RA patients to the TCZ therapy.

Disclosure of Interest None Declared