Background/Purpose: The optimal way to monitor disease activity in patients with GCA on tocilizumab remains uncertain.  Acute phase reactants are not a reliable indicator of inflammation in these patients.  The study objective was to assess the value of FDG-PET scans to monitor disease in patients with GCA on tocilizumab.

Methods: Patients with GCA on tocilizumab for ≥ 6 months were selected from an ongoing prospective, observational cohort.  A comparator group of patients with GCA treated with glucocorticoid monotherapy was also selected.  All patients fulfilled the 2022 ACR/EULAR classification criteria for GCA.  At each study visit, patients underwent clinical, laboratory, and imaging assessments, including FDG-PET and non-invasive angiography.

Two independent readers reviewed all PET scans, blinded to clinical data.  PET scans were interpreted as active or inactive.  The PET Vascular Activity Score (PETVAS) was used to quantify arterial FDG uptake on a scale of 0-27.

 Fisher’s exact or Wilcoxon rank sum test were used to compare characteristics of patients with and without PET activity. 

Results: Thirty-six patients with GCA underwent FDG-PET imaging while on tocilizumab.  Five patients had persistent clinical disease activity despite tocilizumab, and FDG-PET scans were active in all 5 patients (PETVAS ranging 20-27).  Of the remaining 31 patients on tocilizumab, FDG-PET was performed during clinical remission after median tocilizumab treatment duration of 336 days (IQR 189-558), on a median prednisone dose of 3.5 mg/day (IQR 0-6), and with a median disease duration of 1070 days (IQR 535-1497).  FDG-PET was active in 16 of these patients (52%).  PETVAS was significantly lower during clinical remission in patients treated with tocilizumab compared to an additional 12 patients treated with glucocorticoid monotherapy [PETVAS 18 (IQR 15-20) vs 24 (IQR 17-26); p=0.02].

There were no differences between patients with and without PET activity during remission with respect to age, sex, disease duration, historical features of GCA, large vessel involvement by angiography, risk factors for atherosclerosis, tocilizumab treatment duration, acute phase reactants, or glucocorticoid dose.  Treatment was not changed based on PET activity, and no patient with or without PET activity developed angiographic progression over the follow-up period of 1.7 years (IQR 0.8-4).

Seventeen patients discontinued tocilizumab after a median treatment duration of 1.8 years (IQR 0.9-2.7).  Six of these patients (35%) subsequently had a clinical relapse a median of 1.3 years (IQR 0.5-2.9) after stopping tocilizumab.  Among the 6 patients who relapsed, there were no differences in whether PET scan was active (n=3) or inactive (n=3) during clinical remission (p=0.64) and no association with degree of PET activity by PETVAS.

Conclusion: Compared to glucocorticoid monotherapy, tocilizumab significantly reduces, but often does not eliminate, vascular inflammation in GCA.  FDG-PET has limited value to guide management decisions or inform prognostic risk for relapse when obtained during clinical remission in patients with GCA receiving tocilizumab.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/use-of-fdg-pet-to-monitor-disease-activity-in-patients-with-giant-cell-arteritis-on-tocilizumab/