Cladribine tablets in Relapsing-Remitting Multiple Sclerosis preferentially target B-cells

Recently it has been shown that treatments targeting B cells in multiple sclerosis (MS) are effective in controlling disease activity. B cells contribute to the pathogenesis of MS via antigen presentation, T cell activation, and antibody production. In the chronic progressive cladribine trial, some patients treated with cladribine had a significant decline in oligoclonal band number. However, the mode of action of cladribine tablets (CladT) on peripheral immune cells and its biological activity within the CNS remains to be determined further. The CladB study is a longitudinal prospective investigation of CladT treatment in relapsing-remitting MS (RRMS). Blood was sampled at Day 0, 1, 5, then once a week for 8 weeks, fortnightly up to 24 weeks, and once a month till 96 weeks for immune cells. This was compared to a historical cohort of alemtuzumab treated samples for one month. Paired cerebrospinal fluid (CSF) and blood were also taken at Day 0, 48 and 96 weeks after initiating CladT for Kappa and Lambda-free light chain (кFLC, λFLC) index, oligoclonal bands (OCBs), immunoglobulin indices, inflammatory mediators and neurofilament light chain (NfL). Participants also underwent clinical and magnetic resonance imaging brain assessments. Ten participants (3 male, 7 female, mean age 35.9 ± 10.5 (SD) and Expanded disability Status Scale 2.5 (range 0-6) at baseline were enrolled. B cells, in particular memory B cells, were heavily depleted by CladT. Alemtuzumab, conversely rapidly depleted both T and B cells. Although still present, reduction in OCB numbers were observed in 4/10 participants and кFLC index reduced from mean 164.5 ± 227.1 (SD) at baseline to 71.3 ± 84.7 at 48 weeks (p=0.002) and 64.4 ± 67.3 at 96 weeks (p=0.01). This coincided with reduction in IgG index [1.1 ± 0.5 (SD) at baseline, 0.8 ± 0.4 (p=0.014) at 48 weeks and 0.8 ± 0.3 (P=0.02) at 96 weeks] and CSF CXCL-13 [88.6± 68.4 (SD) pg/mL, 39.4 ± 35.2 mg/mL (p=0.037) at 48 weeks and 19.1 ± 11.7pg/ml at 96 weeks (p=0.027)]. CSF NfL levels were reduced at 48 weeks only (p=0.01). In conclusion, our study supports the view that CladT treatment works primarily by depleting memory B-cells and antibody-secreting cell precursors in RRMS leading to sustained effects on intrathecal antibody production and total IgG associated with a reduction in the B-cell chemoattractant CXCL-13 in the CSF. ### Competing Interest Statement All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf and declare: all authors had financial support from Merck for the submitted work; no financial relationships with any organizations that might have an interest in the submitted work in the previous three years; no other relationships or activities that could appear to have influenced the submitted work. ### Funding Statement Merck Serono Ltd., Feltham, UK, an affiliate of Merck KGaA, Darmstadt, Germany. ### 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: London - Stanmore Research Ethics Committee. 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 Anonymised data are available from the authors upon reasonable request.