<h3>Background:</h3> Temporal artery biopsy (TAB) nowadays is still the gold-standard for the diagnosis of giant cell arteritis (GCA). Different patterns of vessels' wall inflammation have been described and media inflammation seems to persist in some patients in clinical remission notwithstanding long-term glucocorticoids (GCs) therapy[1]. Tocilizumab (TCZ) is the most efficacious steroid-sparing agent used in both relapsing and new-onset GCA, but the optimal treatment duration remains unknown. In clinical practice, evaluation of disease activity with TCZ treatment is partially invalidated by the fact that inflammatory markers, particularly C-reactive protein (CRP), are affected by the biologic activity of the drug itself and commonly test negative. <h3>Objectives:</h3> To assess the residual degree of vessel inflammation in GCA patients treated with TCZ, in addition to GCs, by mean of repeating histological evaluation of the temporal artery and large vessel imaging, along with the clinical evaluation. <h3>Methods:</h3> We performed a second biopsy in a single cohort of GCA patients treated with GCs and TCZ, who were followed prospectively between December 2017 and December 2022. At baseline TAB and positron emission tomography/computed tomography (PET/CT) were performed in all patients. All patients were started on GCs and TCZ was added on physician's discretion. The second biopsies were performed by the same vascular surgeon in the contralateral temporal artery with respect to the already biopsied one after a minimum TCZ treatment duration of 6 months. PET/CT scans were also repeated at the time of the second biopsy. All the histological samples were reviewed by a single pathologist and classified into 4 patterns: transmural inflammation (TMI), inflammation limited to adventitia (ILA), vasa vasorum vasculitis (VVV), and small vessel vasculitis (SVV). PET/CT showing vascular fluorodeoxyglucose (FDG) uptake ≥2 according to the Meller scale in at least one vascular district was considered consistent with active vasculitis. Clinical active disease was defined as one or more of the following: recurrence of signs or symptoms of GCA or polymyalgia rheumatica; CRP &gt;10 mg/L, or erythrocyte sedimentation rate (ESR) &gt; 40 mm/h if these were considered by the investigator to be due to GCA; the necessity for introduction or increase in GCs dose. <h3>Results:</h3> 13 GCA biopsy-positive patients were included. Baseline clinical features are reported in Table 1. The histopathological patterns at first and second biopsies are shown in Figure 1. 9/13 (69.2%) patients showed active large vessel vasculitis (LVV) at baseline. TCZ was introduced from disease onset in 3/13 (23.1%) patients, and in 10/13 (76.9%) at relapse. At second biopsy, the median (IQR) disease duration was 3.7 (2.2-4.3) years. The median (IQR) treatment with TCZ was 2.4 (1.2-3.9) years, and 10/13 (76.9%) patients were still on low doses of GCs. A total of 5/13 (38.5%) TABs still presented an inflammatory infiltrate at the second biopsy. 4/9 (44.4%) with TMI at first biopsy still had some form of inflammation at second biopsy: 2 (22.2%) ILA, 1 (11.1%) VVV, and 1 (11.1%) persistent TMI. 1/3 (66.7%) patient with ILA at first biopsy presented SVV at second one. PET/CT was repeated in 9/13 (69.2%) patients; 1/9 (11.1%) was consistent with active LVV. 3/13 (23.1%) patients were considered clinically active at second TAB: of those only the patient with persistent TMI, also showed an active PET/CT. <h3>Conclusion:</h3> Our data suggest that despite treatment with TCZ some inflammation may persist at TAB. The clinical relevance of inflammatory infiltrates other than TMI is still debated, however they might indicate different stages of the disease course. Further molecular studies and larger cohort studies may clarify the clinical significance of our observation. <h3>REFERENCES:</h3> [1] Cavazza A, et al. Am J Surg Pathol. 2014 Oct;38:1360-70. <h3>Acknowledgements:</h3> <b>NIL.</b> <h3>Disclosure of Interests:</h3> <b>None declared.</b>

OP0233 CAN TOCILIZUMAB TURN OFF INFLAMMATION IN GIANT CELL ARTERITIS?