Enrollment milestones in cell therapy trials rarely carry the weight they do here: Mesoblast has now treated all 300 patients required for its pivotal MSB-DR004 trial of rexlemestrocel-L in chronic low back pain, locking in the statistical power needed to pursue a Biologics License Application and setting a hard clock toward a mid-2027 readout. That timeline is driven by the 12-month primary endpoint, meaning the last patient treated this month will complete follow-up around mid-2026, with data analysis and top-line reporting to follow. The path was deliberately de-risked. The FDA had already agreed on the 300-patient size, the sham-controlled design, and the 12-month pain reduction endpoint as an approvable indication, a concession earned in part because the predecessor MSB-DR003 trial met that same primary endpoint in its 404-patient population.
The mechanistic logic behind rexlemestrocel-L is worth keeping in focus. It is an allogeneic, STRO3-immunoselected stromal cell product co-administered with hyaluronic acid directly into the lumbar disc. The earlier Phase 3 MSB-DR003 data showed durable pain reduction persisting for at least two years from a single intra-discal injection, which is the central clinical claim Mesoblast is now trying to confirm. The confirmatory trial also carries a secondary focus on opioid cessation, a strategically important endpoint given that discogenic pain accounts for roughly half of all prescription opioid use in the U.S. If that signal replicates, it introduces a pharmacoeconomic argument that stretches well beyond standard pain management comparisons.
On the regulatory side, rexlemestrocel-L holds RMAT designation, which bundles the benefits of Breakthrough and Fast Track status and, critically, makes the product eligible for Priority Review at the BLA stage. Priority Review compresses the FDA’s standard 10-month review window to 6 months. Mesoblast is also running commercial manufacturing in parallel with the trial, a resource allocation that signals genuine confidence this won’t stall at the filing stage the way earlier programs did. The company’s only currently approved product, Ryoncil, received FDA approval in December 2024 for pediatric SR-aGvHD, giving the organization its first commercial-scale manufacturing validation just as this larger program reaches its pivotal moment.
The number to track now is not enrollment but attrition. A single-visit, single-injection cell therapy administered into the lumbar disc is an unusual ask of patients already managing chronic pain, and dropout rates through month 12 will determine whether the intent-to-treat population stays robust enough to meet the primary endpoint’s power assumptions. Any meaningful loss to follow-up is the clearest path to a failed readout despite a successful enrollment.
Jon Napitupulu is Director of Media Relations at The Clinical Trial Vanguard. Jon, a computer data scientist, focuses on the latest clinical trial industry news and trends.
