Stem Cells for Stroke

Acute ischemic stroke causes rapid, often permanent neuronal loss, and the appeal of stem cell therapy here is straightforward: an intravenous infusion of cells given soon after stroke onset might reduce inflammation and support tissue repair during the critical early recovery window. Athersys developed the most extensively tested product for this indication, an allogeneic bone-marrow-derived adult stem cell product called MultiStem, and its MASTERS clinical trial program is the largest and most rigorous stroke stem cell dataset available.

MASTERS-1, the Phase 2 trial, was a randomized, double-blind, placebo-controlled study run across 33 centers in the US and UK, published in Lancet Neurology in 2017 [1]. Of 134 enrolled patients, 126 received treatment, 65 with MultiStem at either 400 million or 1.2 billion cells and 61 with placebo, administered intravenously within 24 to 48 hours of stroke onset [1]. The trial found no dose-limiting toxicity and similar overall adverse event rates between groups, roughly 99% in the treated group versus 97% in placebo, indicating the infusion itself was well tolerated [1]. On its primary endpoint, global stroke recovery at day 90, the trial found no significant benefit: the odds ratio was 1.08, with a 95% confidence interval of 0.55 to 2.09 and a p-value of 0.83, statistically indistinguishable from no effect [1]. A post-hoc subgroup analysis suggested patients treated within 36 hours of stroke onset, rather than the full 24-48 hour window, showed a more favorable signal, but this was not a pre-specified primary comparison and should be read as hypothesis-generating rather than confirmatory [1].

A related trial testing MultiStem in a broader 18-to-36-hour treatment window enrolled 206 participants and also found no improvement in short-term functional outcomes at 90 days: an excellent outcome was achieved by 11.5% of the treated group versus 9.8% of placebo, a difference that was not statistically significant (p=0.90) [2]. Combined with the MASTERS-1 primary result, this represents two separate randomized trials failing to show a clear functional benefit from allogeneic adult stem cell infusion in acute ischemic stroke.

Athersys pursued a Phase 3 trial, MASTERS-2, registered as NCT03545607, narrowing the treatment window and refining the protocol based on the earlier post-hoc signal, enrolling roughly 300 patients across North America and Europe with the primary endpoint measured as a shift in modified Rankin Scale score at three months [3]. The FDA accepted modifications to the trial's endpoints and design as the program proceeded, reflecting ongoing uncertainty about the best way to detect a treatment effect in this population [3]. Public information on final MASTERS-2 outcomes is limited as of this writing; this page will be updated once mature published results are available.

Stroke is a useful contrast case for this encyclopedia because it shows what happens when a well-designed, adequately powered, multi-center randomized trial simply does not find the hoped-for effect. Unlike knee osteoarthritis, where large meta-analyses find a small but real benefit over placebo, or diabetes, where a single program shows a dramatic efficacy signal, the acute ischemic stroke data for allogeneic MSC-type products currently points toward a null or, at best, an unproven result on the primary endpoint that matters most to patients and clinicians: functional recovery.

Given a large randomized Phase 2 trial with a clearly negative primary endpoint and no confirmed positive Phase 3 result available at this time, stem cell therapy for acute ischemic stroke sits in tier 4 of our evidence framework: tested in rigorous human trials, but the strongest completed trial did not demonstrate the claimed benefit. Any clinic marketing stem cell infusion specifically for stroke recovery outside a registered, IRB-approved trial is making a claim well ahead of what the highest-quality available evidence supports.