Relapse rates and enhancing lesions in a phase II trial of natalizumab in multiple sclerosis

Background: Natalizumab, a humanized monoclonal IgG4 antibody, is an α4-integrin antagonist, which inhibits the migration of inflammatory cells into the central nervous system, a key pathogenic mechanism in multiple sclerosis (MS). In a six month, phase II clinical trial of patients with relapsing MS, natalizumab significantly reduced the formation of new gadolinium-enhanced (Gd+) lesions and the number of clinical relapses. Objective: To investigate the relationship of historical relapse rate and new Gd+ lesion number with subsequent MS disease activity and natalizumab treatment in the phase II study. Methods: Patients who participated in the phase II study were stratified into subgroups according to: (i) the number of relapses in the two years prior to entry into the study: 2 relapses (n = 108), 3 relapses (n = 57), and >3 relapses (n = 48); (ii) the number of new Gd+ lesions at baseline (Month 0): 0 (n = 129), 1-2 (n = 50), and >2 (n = 33). Relapses and new Gd+ lesions during the treatment phase of the trial were determined and compared for each subgroup. Results: Both the prestudy relapse rate and number of new Gd+ lesions at baseline were related to the subsequent risk of a relapse; baseline number of Gd+ lesions was related to the likelihood of subsequent new Gd+ lesion formation. There was a lower proportion of subjects with an on-study relapse and fewer new Gd+ lesions in all natalizumab-treated subgroups when compared with their placebo counterpart; the difference was most apparent in the subgroups of patients with >3 relapses in the two years prior to study entry and >2 new Gd+ lesions at Month 0. Conclusions: There was a lower proportion of subjects with an on-study relapse in natalizumab-treated patients, particularly in those with a more active disease at study entry. Larger ongoing phase III studies will allow more definitive investigation of these preliminary subgroup findings.