Researchers from Keio University School of Medicine and Keio University Regenerative Medicine Research Center in Japan transplanted neural stem/progenitor cells derived from induced pluripotent stem cells (iPSCs) into the injured spinal cords of four men with recent, complete cervical spinal cord injuries. The first-in-human Phase I study, published in Nature Medicine, primarily evaluated safety, following participants for up to four years after treatment.
Spinal cord injury affects more than 20 million people worldwide and often results in permanent paralysis because the adult spinal cord has only limited capacity to regenerate. Current treatments—including surgery and rehabilitation—can stabilize the injury and maximize remaining function but cannot rebuild the damaged neural circuits responsible for movement and sensation.
The transplanted cells were manufactured from clinical-grade iPSCs under strict quality controls before being differentiated into neural stem/progenitor cells. Approximately two million cells were injected directly into each patient’s spinal cord injury site two to four weeks after injury while patients received temporary immunosuppressive therapy to reduce the risk of rejection.
The trial met its primary objective. Researchers observed no tumor formation, abnormal cell growth, or other serious complications attributable to the transplanted cells during the initial 52-week study period or the subsequent long-term follow-up. Imaging studies likewise revealed no evidence of graft-related abnormalities, addressing one of the field’s greatest concerns regarding therapies derived from pluripotent stem cells.
Although safety was the principal endpoint, investigators also tracked neurological recovery. All four participants showed improvements in motor function, and two improved enough to advance from complete paralysis (American Spinal Injury Association Impairment Scale grade A) to grades C or D, indicating recovery of some voluntary movement below the injury level. Median motor scores improved by 13 points after one year, exceeding the recovery typically observed in a comparable historical patient registry, although the researchers caution that the small, uncontrolled study cannot establish that the stem cell treatment caused these gains.
The encouraging results build on years of preclinical research showing that transplanted neural stem cells can differentiate into neurons and supporting cells, promote remyelination, stimulate regrowth of damaged nerve fibers, and release molecules that support tissue repair. Animal studies have also suggested that the transplanted neurons can integrate into existing spinal cord circuits, although such integration cannot yet be directly confirmed in patients.
The investigators emphasize that many questions remain before the therapy could become a standard treatment. The study enrolled only four participants, lacked a placebo control, and included only men with recent cervical spinal cord injuries. Larger randomized clinical trials will be needed to determine whether the treatment consistently improves neurological recovery and to identify which patients are most likely to benefit. Researchers also plan to continue monitoring participants to assess the long-term safety of the transplanted cells.
Even with those caveats, the findings represent a significant advance for regenerative medicine. Rather than demonstrating a cure for paralysis, the study establishes that carefully manufactured iPSC-derived neural stem cells can be transplanted into the human spinal cord without the serious safety issues that have long challenged the field. That achievement provides a critical foundation for the next generation of clinical trials aimed at determining whether stem cell therapy can ultimately restore function after devastating spinal cord injuries.
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