Mesoblast Completes Patient Dosing in Pivotal Stem Cell Trial for Chronic Low Back Pain

What This Article Covers

Australian-American biotech Mesoblast announced on August 16, 2026 that it has finished dosing all 350 patients in MSB-DR004, its pivotal, confirmatory Phase 3 trial testing a single injection of the allogeneic mesenchymal stromal cell therapy rexlemestrocel-L for chronic low back pain (CLBP) caused by degenerative disc disease. The milestone moves one of the most closely watched cell-therapy programs in musculoskeletal medicine a step closer to a potential FDA filing, though the actual efficacy readout is not expected until roughly mid-2027 and the product remains unapproved.

A Major Enrollment Milestone, Not Yet a Result

MSB-DR004 is a randomized, controlled trial in which patients receive either a single intra-discal injection of rexlemestrocel-L or a sham (saline) injection, then are followed for 12 months to assess durable pain relief. Completing treatment of all 350 randomized patients across roughly 40 U.S. sites is, by design, an enrollment and dosing milestone rather than a data readout — no comparative pain, function, or opioid-use results from this trial have been reported yet. Mesoblast CEO Silviu Itescu called it "a momentous milestone," and the company says the last-treated patient's 12-month follow-up visit will determine the primary-endpoint readout timing.

What Rexlemestrocel-L Is and How It's Given

Rexlemestrocel-L is a second-generation, STRO-3-immunoselected mesenchymal precursor cell product manufactured from the bone marrow of healthy adult donors — an "off-the-shelf" allogeneic cell therapy rather than a patient's own cells. In this indication it is injected directly into the affected intervertebral disc in a single outpatient procedure, with the therapeutic hypothesis being that the cells modulate local inflammation and may help preserve disc integrity in patients whose back pain has not resolved with conservative care.

Why the FDA Is Already Engaged

This is not rexlemestrocel-L's first outing in low back pain. An earlier Phase 3 trial (often referred to as MSB-DR003) enrolled 404 patients and, in the pre-specified subgroup with shorter symptom duration, showed pain reduction on a visual analog scale of roughly -37 points at both 12 and 24 months versus a much smaller effect in saline controls (p<0.0001 in that subgroup), along with a meaningful reduction in opioid use among treated patients who were on opioids at baseline (roughly 40–46% achieving a combined pain-and-opioid-reduction response versus about 12% of controls, p=0.004 in the relevant analysis). Company materials describe no safety concerns over 24 months of follow-up in that trial. Based on that data package, the FDA granted the program Regenerative Medicine Advanced Therapy (RMAT) designation, which allows rolling submission of a Biologics License Application and eligibility for priority review. In a Type B meeting reported in January 2026, the agency reportedly acknowledged that "effects on pain intensity favored the active arm" and indicated that a clinically meaningful 12-month pain reduction, along with well-controlled opioid-reduction data, could support an efficacy claim — feedback that shaped the design of the current confirmatory trial.

Important Caveats

Several caveats are essential here. First, in the earlier trial, the effect was strongest in a pre-specified subgroup of patients with shorter-duration low back pain; the composite pain-and-function outcome did not reach statistical significance across the full study population, and benefit appeared to fade in patients with longer-standing, more fibrotic disc disease. Second, chronic low back pain trials — cell therapy or otherwise — have a well-documented history of large placebo/sham responses, which is precisely why the FDA and outside commentators have flagged the need for tight site controls and consistent patient selection in MSB-DR004. Third, and most importantly, rexlemestrocel-L is not FDA-approved for this or any low back pain indication; MSB-DR004's own primary-endpoint data are roughly a year away, and a BLA filing, review, and possible approval would follow only after that. Mesoblast does already market a related allogeneic MSC product, Ryoncil (remestemcel-L), which the FDA approved in December 2024 for pediatric steroid-refractory acute graft-versus-host disease — a track record that lends some manufacturing and regulatory credibility to the platform, but is a distinct product and distinct indication from rexlemestrocel-L in CLBP.

What It Means for Patients

For the roughly 7 million people in the U.S. that Mesoblast estimates live with chronic low back pain tied to degenerative disc disease — a population for which options beyond physical therapy, injections, opioids, or spinal fusion surgery are limited — a validated single-injection cell therapy with a meaningful, durable effect on pain and opioid use would be a significant addition to the treatment landscape. But patients and clinicians should treat this as a trial-progress update, not a treatment availability announcement: there is no product to prescribe today, the definitive efficacy data are pending, and prior results were strongest in a specific patient subgroup rather than uniformly across all CLBP patients.

Bottom Line

Mesoblast's completion of dosing in its 350-patient confirmatory Phase 3 trial is a genuine, verifiable step forward for a stem cell therapy program that already carries FDA RMAT designation and encouraging — though subgroup-dependent — prior efficacy signals in chronic low back pain. It is not, however, a result: the actual head-to-head data against sham injection are not expected until around mid-2027, and any approval would come only after that readout and a subsequent FDA review. Patients with degenerative disc disease should watch this program with cautious optimism rather than treat it as an available option.