Both cladribine and alemtuzumab may effect MS via B-cell depletion.

OBJECTIVE: To understand the efficacy of cladribine (CLAD) treatment in MS through analysis of lymphocyte subsets collected, but not reported, in the pivotal phase III trials of cladribine and alemtuzumab induction therapies.

METHODS: The regulatory submissions of the CLAD Tablets Treating Multiple Sclerosis Orally (CLARITY) (NCT00213135) cladribine and Comparison of Alemtuzumab and Rebif Efficacy in Multiple Sclerosis, study one (CARE-MS I) (NCT00530348) alemtuzumab trials were obtained from the European Medicine Agency through Freedom of Information requests. Data were extracted and statistically analyzed.

RESULTS: Either dose of cladribine (3.5 mg/kg; 5.25 mg/kg) tested in CLARITY reduced the annualized relapse rate to 0.16-0.18 over 96 weeks, and both doses were similarly effective in reducing the risk of MRI lesions and disability. Surprisingly, however, T-cell depletion was rather modest. Cladribine 3.5 mg/kg depleted CD4+ cells by 40%-45% and CD8+ cells by 15%-30%, whereas alemtuzumab suppressed CD4+ cells by 70%-95% and CD8+ cells by 47%-55%. However, either dose of cladribine induced 70%-90% CD19+ B-cell depletion, similar to alemtuzumab (90%). CD19+ cells slowly repopulated to 15%-25% of baseline before cladribine redosing. However, alemtuzumab induced hyperrepopulation of CD19+ B cells 6-12 months after infusion, which probably forms the substrate for B-cell autoimmunities associated with alemtuzumab.

CONCLUSIONS: Cladribine induced only modest depletion of T cells, which may not be consistent with a marked influence on MS, based on previous CD4+ T-cell depletion studies. The therapeutic drug-response relationship with cladribine is more consistent with lasting B-cell depletion and, coupled with the success seen with monoclonal CD20+ depletion, suggests that B-cell suppression could be the major direct mechanism of action.