To compare the effect of high- and low-efficacy treatments on disability outcomes in patients with recently active and inactive secondary progressive multiple sclerosis (SPMS).

In SPMS, reduction in the rates of disability accrual after starting treatment has largely been limited to patients with ongoing inflammatory activity. A delayed treatment effect, termed therapeutic lag, may obscure therapeutic benefits in SPMS.

Patients treated with high- (natalizumab, alemtuzumab, mitoxantrone, ocrelizumab, rituximab, cladribine, fingolimod) or low-efficacy (interferon β, glatiramer acetate, teriflunomide) therapies after SPMS onset were selected from MSBase and OFSEP, two large observational cohorts. Individualised estimates of therapeutic lag were calculated for each patient. Only events that occurred after the estimated therapeutic lag period were included in the analysis. Propensity score matching was used to select groups with comparable baseline characteristics. Disability and relapse outcomes were compared in paired, pairwise-censored analyses adjusted for visit density.

Of 7359 patients with SPMS, 1000 patients fulfilled the criteria for study inclusion (510 active SPMS, 490 inactive SPMS). For the relapse outcomes, patients with active SPMS treated with high-efficacy DMTs experienced lower probabilities of relapses than low-efficacy DMTs (hazard ratio [HR] 0.7 [95%CI 0.5-0.9], p=0.006).  Patients with inactive SPMS had similar probabilities of relapses in the high and low efficacy DMT groups (0.8 [0.6-1.2], p=0.39). No difference in the risk of 6-month sustained disability accumulation, or proportion of patients reaching EDSS>=7, was observed between groups when accounting for therapeutic lag.

The risk of disability accumulation in SPMS seems to be comparable in patients treated with high- and low- efficacy DMT. High efficacy DMT is superior to low efficacy therapy in reducing relapse activity in patients with active SPMS, but not those with inactive SPMS. Pre-treatment inflammatory activity, clinical or radiological, is a treatable target in SPMS which may benefit from higher-efficacy anti-inflammatory therapies.