Glial activity load on PET (GALP) reveals persistent ‘smoldering’ inflammation in MS despite disease modifying treatment: [F-18]PBR06 study

Introduction Cortical grey (CoGM) and white matter (WM) microglial activation (MA) is involved in the pathogenesis of multiple sclerosis (MS). [F-18]PBR06 positron emission tomography (PET) targeting 18kilodalton-translocator protein (TSPO) can detect abnormal MA in MS. Aims and Objectives The goal of this study is to determine the effect of disease modifying treatment (DMT) efficacy on modulating the extent and clinical and radiological correlates of MA in MS patients. Methods Thirty [F-18]PBR06 PET scans were performed in 22 MS patients (13 RR, 9 SP, mean age 46±14 years, 15 females, median EDSS 3.5, mean T25FW 7.2±4.6s) and 8 healthy controls (HC). Individualized z-score maps of brain parenchymal MA were generated by voxel-by-voxel comparison between each subject’s PET SUVR images and a HC dataset. Logarithmically transformed ‘Glial activity load on PET’ scores (calculated as the sum of voxel-by-voxel z-scores ≥4 in CoGM and WM regions), ‘lnGALP’, were compared between MS subjects on DMT with high efficacy (HT; including rituximab, ocrelizumab, natalizumab and fingolimod, n=13) versus those on no or lower efficacy treatment (LT; including glatiramer acetate and interferons), and correlated with clinical measures and cortical thickness (measured using Freesurfer). p<0.05 was considered statistically significant. Results CoGM and WM lnGALP scores were higher in MS vs. HCs (10.0±1.5 vs. 7.5±1.5 and 9.8±1.5 vs. 6.6±2.4, both p<0.01) and were inversely correlated with cortical thickness across groups (r=-0.44 and - 0.48, both p<0.05, n=30). In HT-MS group, CoGM and WM lnGALP was significantly lower as compared to LT-MS group (9.1±1.0 vs. 11.3±1.1 and 9.1±1.3 vs. 10.8±1.4, p=0.000075 and 0.006) but remained abnormally higher than in HC group (p=0.006 and 0.02, respectively). Within HT-MS patients, CoGM lnGALP scores were higher in SP vs. RR subgroups (p=0.008), correlated positively with EDSS, T25FW, fatigue scores and serum GFAP levels (r=0.65,0.79, 0.75 and 0.67, all p<0.05), and inversely with cortical thickness (r=-0.66, p=0.01). Conclusions High-efficacy DMTs decrease, but do not normalize, CoGM and WM MA in MS patients. Such “residual” MA in CoGM is associated with clinical disability, symptom severity and cortical degeneration. Individualized mapping of TSPO-PET using [F-18]PBR06 can potentially serve as an imaging biomarker for evaluating emerging therapies targeting MA in MS patients who are worsening despite high-efficacy DMTs. ### Competing Interest Statement Tarun Singhal has received research support from Novartis Pharmaceuticals and Genzyme-Sanofi, consulting fees from Novartis Pharmaceuticals, Genentech, and EMD Serono, speaking fees from Tiziana Life Sciences, and research funding from Nancy Davis Foundation's "Race to Erase MS" program, Ann Romney Center for Neurologic Diseases, Harvard Neuro-Discovery Center, National Multiple Sclerosis Society, Department of Defense, and Water Cove Charitable Foundation. Eero Rissanen has received a fellowship grant from the Sigrid Juselius Foundation and research grant from the Sakari Alhopuro Foundation. Tanuja Chitnis has received compensation for consulting from Biogen, Novartis Pharmaceuticals, Roche Genentech and Sanofi Genzyme, and has received research support from the National Institutes of Health, National MS Society, US Department of Defense, Sumaira Foundation, Brainstorm Cell Therapeutics, EMD Serono, I-MAB Biopharma, Mallinckrodt ARD, Novartis Pharmaceuticals, Octave Bioscience, Roche Genentech and Tiziana Life Sciences. Rohit Bakshi has received consulting fees from Bristol-Myers Squibb and EMD Serono and research support from Bristol-Myers Squibb, EMD Serono and Novartis. Howard L. Weiner has received research support from Cure Alzheimer's Fund, EMD Serono Inc., Genentech Inc., National Institutes of Health, National Multiple Sclerosis Society, Sanofi Genzyme, and Verily Life Sciences, and has received payment for consulting from Genentech, Inc, IM Therapeutics, I-MAB Biopharma, MedDay Pharmaceuticals, Tiziana Life Sciences and vTv Therapeutics. ### Clinical Trial NCT02649985 ### Funding Statement This study was funded by Nancy Davis Foundation's "Race to Erase MS" program, Ann Romney Center for Neurologic Diseases, Harvard Neuro-Discovery Center, and Water Cove Charitable Foundation. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: IRB of Mass General Brigham gave ethical approval for this work. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors.