A groundbreaking stem cell therapy known as CALEC has demonstrated a remarkable 92% success rate in regenerating corneas and restoring vision for patients with previously untreatable eye injuries. Although still in the experimental stage, the procedure shows significant promise, particularly for those suffering from severe corneal damage.

In an expanded clinical trial, the CALEC therapy was tested on 14 patients with blinding corneal injuries. The results, published on March 4, 2025, in Nature Communications, revealed that the treatment was not only feasible and safe but also showed a high proportion of complete or partial success. The patients were followed for 18 months, and the trial’s outcomes were hailed as a major advancement in regenerative medicine for eye injuries.

Developed by Mass Eye and Ear, part of the Mass General Brigham healthcare system, CALEC (cultivated autologous limbal epithelial cells) is a stem cell-based treatment aimed at regenerating the cornea. The process begins by harvesting stem cells from the patient’s healthy eye through a small biopsy. These stem cells are then cultured into a tissue graft over the course of two to three weeks. Afterward, the graft is surgically transplanted onto the damaged cornea.

Dr. Ula Jurkunas, the principal investigator of the study and associate director of the Cornea Service at Mass Eye and Ear, explained, “Our first trial in four patients showed that CALEC was safe and the treatment was possible. Now, we have this new data supporting that CALEC is more than 90% effective at restoring the cornea’s surface, making a meaningful difference for individuals with cornea damage that was once considered untreatable.”

In this new phase 1/2 clinical trial, CALEC completely restored the cornea in 50% of participants at their three-month follow-up. By 12 months, the complete success rate increased to 79%, and by 18 months, it was 77%. Overall, 93% of patients showed either complete or partial success by the 12-month mark, with 92% achieving similar outcomes by 18 months. The study also showed significant improvements in visual acuity across all 14 participants.

Three patients required a second CALEC transplant, with one ultimately achieving complete success. The safety profile of CALEC was promising, with no serious adverse events occurring in either the donor or recipient eyes. A minor bacterial infection, which was related to chronic contact lens use, was reported in one participant but was resolved quickly. Other adverse events were minor and self-resolving.

CALEC is still an experimental procedure and is not yet available at Mass Eye and Ear or any U.S. hospital. Additional research and clinical trials will be needed before the treatment can be submitted for federal approval.

The cornea, the clear outer layer of the eye, is essential for focusing light and maintaining clear vision. The limbus, the outer border of the cornea, contains stem cells known as limbal epithelial cells, which help repair and maintain the corneal surface. However, severe injuries to the cornea—such as those caused by chemical burns, infections, or trauma—can deplete these cells, resulting in limbal stem cell deficiency. This condition can permanently damage the corneal surface, making it impossible to perform a standard corneal transplant.

For individuals with such injuries, the result is often chronic pain and vision impairment. This challenge led Dr. Jurkunas, in collaboration with Dr. Reza Dana (director of the Cornea Service at Mass Eye and Ear), to explore a new approach for regenerating limbal epithelial cells. Nearly two decades of preclinical research, combined with efforts from collaborators at Dana-Farber Cancer Institute and Boston Children’s Hospital, eventually led to the development of CALEC grafts that meet the rigorous quality criteria required for human transplantation.

The first patient treated with CALEC in this clinical trial was in 2018, after the therapy was approved by the U.S. Food and Drug Administration (FDA) and Mass General Brigham Institutional Review Board (IRB). The successful completion of the trial was a collaborative effort involving Dr. Jurkunas’ surgical team and the cell manufacturing facility at Dana-Farber.

While CALEC has shown impressive results, the therapy currently has limitations. As an autologous (patient-specific) therapy, it can only be used in cases where the patient has one healthy eye from which stem cells can be harvested. The goal for future advancements is to develop an allogeneic (donor-based) manufacturing process, using limbal stem cells from cadaveric donor eyes. This approach could significantly expand the treatment’s reach, allowing patients with damage to both eyes to benefit from the therapy.

Dr. Jerome Ritz, a collaborator on the study, said, “Our future hope is to set up an allogeneic manufacturing process starting with limbal stem cells from a normal cadaveric donor eye. This would hopefully expand the use of this approach and make it possible to treat patients who have damage to both eyes.”

Future studies will include larger patient cohorts, longer follow-ups, and randomized-controlled trials to further assess the efficacy and safety of CALEC. Mass General Brigham’s Gene and Cell Therapy Institute is actively working to transition these scientific discoveries into real-world, life-changing treatments.

If these trials continue to show success, CALEC could revolutionize the treatment of corneal injuries, offering hope for patients with conditions that were once considered untreatable. With ongoing research and future advancements, CALEC holds the potential to not only restore vision but also improve the quality of life for many individuals facing severe eye trauma.

By Impact Lab