Researchers have made progress in using CRISPR gene editing to treat aggressive blood cancers by modifying donor stem cells. In a clinical trial involving 30 patients, scientists removed a protein called CD33 from the stem cells. This modification may allow cancer treatments targeting CD33 to attack the disease without harming the healthy blood cells needed for recovery after a transplant. The study, published in Nature Medicine, was conducted at Siteman Cancer Center and 14 other medical centers in the U.S. and Canada. This approach addresses a major challenge in using CAR-T cell therapy for certain blood cancers. While CAR-T therapy has shown promise in treating aggressive leukemias and lymphomas, it has had limited success in diseases like acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). This is because the proteins on cancer cells in these diseases are also found on healthy blood cells, including donor stem cells used in transplants. If CAR-T cells are designed to target these shared proteins, they may accidentally destroy healthy cells along with cancer cells. In the trial, patients with AML and MDS received donor stem cells that had been genetically modified to remove CD33. The goal was to create blood cells that would not be targeted by cancer treatments designed to attack CD33. The modified stem cells, called tremtelectogene empogeditemcel (trem-cel), were developed by Vor Biopharma, which funded the study. After transplantation, patients also received gemtuzumab ozogamicin, a drug that targets CD33 and delivers an anti-cancer agent directly to cells with the protein. The results showed that all 30 patients achieved engraftment by day 28, meaning the transplanted stem cells successfully reached the bone marrow and began producing blood cells. Platelet levels returned to normal by day 16 on average, which is similar to standard stem cell transplant outcomes. Nineteen patients received gemtuzumab ozogamicin, and their blood cell counts remained stable, suggesting the gene-edited stem cells protected them from the usual side effects of this treatment, such as severe drops in blood cell levels. Side effects during the trial were similar to those seen in standard stem cell transplants, including anemia, low platelet counts, infections, and graft-versus-host disease. Seven patients died during the study—four due to cancer progression and three from transplant-related complications such as kidney failure, liver toxicity, and sepsis. The findings suggest that combining CD33-deleted stem cell transplants with CD33-targeted therapies could allow more aggressive cancer treatment while preserving the healthy blood cells needed for recovery. The study was funded by Vor Biopharma, and several researchers involved were employees of the company at the time.