For individuals battling some of the most aggressive forms of blood cancer, a stem cell transplant often represents the single remaining treatment option with the potential to achieve a lasting cure. Yet, even this high-stakes intervention comes with significant limitations. Cancers frequently return after transplantation, plunging patients and their medical teams into difficult scenarios where available follow-up options are exceptionally scarce. Addressing this critical vulnerability, a groundbreaking new clinical trial led by researchers at Washington University School of Medicine in St. Louis suggests that genetically modifying donor stem cells prior to transplantation could fundamentally alter the landscape of post-transplant care, making subsequent cancer treatments both considerably safer and potentially much more effective.
The innovative strategy centers on removing a specific, targeted protein from donor stem cells. By doing so, subsequent therapies designed to hunt down and eliminate that exact protein can aggressively attack remaining or returning cancer cells while completely sparing the healthy transplanted cells that are vital for patient survival. The comprehensive study was conducted collaboratively across the Siteman Cancer Center—based at Barnes-Jewish Hospital and WashU Medicine—alongside 14 other specialized medical and research centers spanning the United States and Canada. The promising results of this multi-site investigation have now been published in the prominent scientific journal Nature Medicine.
A Major Challenge for CAR-T Therapy
According to the study’s corresponding author, Dr. John F. DiPersio, the Virginia E. & Sam J. Golman Professor of Medicine at WashU Medicine, this advanced gene editing approach offers a practical pathway toward overcoming a massive obstacle that has historically restricted the application of chimeric antigen receptor T-cell, or CAR-T, therapy in specific hematologic malignancies.
While CAR-T cell therapy has achieved extraordinary success in treating certain aggressive blood cancers, its clinical efficacy has lagged significantly when applied to complex and hard-to-treat diseases such as acute myeloid leukemia, commonly known as AML, and myelodysplastic syndrome, referred to as MDS.
The underlying technical problem, as DiPersio explained, lies in cellular biology overlap. Many of the specific surface proteins found abundantly on AML and MDS cancer cells also appear naturally on healthy myeloid cells. Critically, these same shared proteins are present on the healthy donor stem cells utilized during a standard bone marrow or stem cell transplant. Consequently, if CAR-T cells are engineered and programmed to seek out and destroy one of these shared proteins, they face a devastating dual challenge: they may inadvertently execute and destroy healthy blood stem cells right alongside the malignant cancer cells they were intended to eradicate.
This collateral cellular damage can trigger a severe, life-threatening inflammatory response in the patient. Furthermore, it inherently weakens the potency of the cancer treatment itself. Because vast numbers of engineered CAR-T cells become heavily occupied and distracted by attacking healthy cellular targets throughout the body, fewer cells remain concentrated on hunting down the actual malignant cancer cells.
The conceptual framework required to bypass this clinical roadblock was originally formulated by Dr. Miriam Y. Kim, now an assistant professor of medicine at WashU Medicine. Kim initially embarked on this intricate line of investigation while working as a postdoctoral researcher at the University of Pennsylvania. She subsequently carried the research forward into the DiPersio laboratory before ascending to her current role as an independent investigator within the WashU Medicine Division of Oncology, where she also treats patients and serves as a dedicated research member at Siteman Cancer Center.
Removing CD33 From Healthy Stem Cells
Within the parameters of the new clinical trial, patients diagnosed with AML and MDS received donor stem cells that had undergone precise genetic modification to eliminate a specific surface protein designated as CD33. The overarching objective of this cellular engineering was to generate a fresh population of healthy blood cells that would be entirely immune and invulnerable to subsequent treatments specifically engineered to target and destroy CD33.
"We are encouraged by the results of this study showing that a CD33-deleted stem cell transplant looks very similar to the outcomes of standard stem cell transplantation," stated DiPersio, who additionally serves as the director of WashU Medicine’s Center for Gene and Cellular Immunotherapy. "In the future, we are hopeful we will be able to combine this with CD33-targeted immunotherapies, such as CAR-T cells, and improve treatment options for patients with these very aggressive blood cancers."
Demonstrating the real-world clinical application of this hypothesis, DiPersio and his collaborative research team previously documented a single compelling case involving a patient with high-risk AML who underwent a CD33-deleted stem cell transplant. When the aggressive cancer unfortunately resurfaced later on, the patient was treated using CD33-targeted CAR-T cells. Crucially, these CAR-T cells were manufactured directly from T cells supplied by the exact same donor who had provided the original modified stem cells.
That particular patient, confronting one of the most aggressive and treatment-resistant forms of AML, achieved a complete remission and remained entirely cancer-free for more than a year following the CAR-T treatment. Furthermore, normal blood cell production successfully resumed, and subsequent testing confirmed that all of the patient’s circulating blood cells completely lacked CD33. This vital finding demonstrated that the genetically engineered donor cells had permanently and successfully engrafted and established themselves within the patient’s bone marrow. DiPersio served as the senior author of that initial case study, which was published in October 2025 in the journal JCO Precision Oncology.
Shielding Healthy Blood Cells
The protein CD33 serves as an exceptionally attractive target for this specialized therapeutic strategy because its natural expression is largely restricted to blood-forming cells, sparing other vital tissues throughout the human body. Additionally, compelling scientific evidence indicates that CD33 is not strictly required for the normal physiological function of human blood stem cells, as individuals born with a natural genetic absence of the protein do not appear to suffer from related health complications or immunodeficiencies.
Following a successful transplant procedure utilizing CD33-deleted stem cells, the underlying medical theory dictates that any remaining cells in the body carrying the CD33 protein should consist primarily or exclusively of malignant cancer cells. Following this cellular clearing, a specialized CAR-T therapy or alternative immunotherapy directed squarely against CD33 can be administered to ruthlessly hunt down and destroy those remaining cancer cells while leaving the healthy, donor-derived blood cell population completely unharmed.
To evaluate this strategy rigorously, the phase 1/2 multicenter clinical trial enrolled 30 adult patients diagnosed with either AML or MDS who were categorized as facing a high clinical risk of disease relapse. Prior to the transplantation procedure, the donor stem cells were processed using advanced CRISPR gene-editing technology to cleanly remove the CD33 protein.
The resulting engineered cellular product, known scientifically as tremtelectogene empogeditemcel or simply trem-cel, was developed by Vor Biopharma, the corporate entity that provided financial funding for the clinical study.
Testing a CD33-Targeted Cancer Treatment
To thoroughly test whether these gene-edited stem cells possessed the resilience needed to withstand a therapy directed explicitly at CD33, trial participants were administered a specific maintenance treatment following their transplantation.
The pharmacological agent utilized, known as gemtuzumab ozogamicin, functions distinctively from a CD33-targeted CAR-T cell therapy. Instead, it is an engineered monoclonal antibody designed to recognize the CD33 protein and deliver a potent anti-cancer payload directly into any cell carrying that protein on its surface.
While gemtuzumab ozogamicin holds regulatory approval from the U.S. Food and Drug Administration for treating CD33-positive AML and is actively evaluated in clinical trials for CD33-positive MDS, its broader clinical utility has historically been hampered by severe dose-limiting side effects. These toxicities commonly include liver damage and profound destruction of healthy blood cells, often leaving patients vulnerable to dangerously plummeting levels of white blood cells, red blood cells, and platelets.
Gene-Edited Cells Successfully Engrafted
A primary milestone for the clinical trial was evaluating whether the genetically modified stem cells could successfully engraft within the patients. The results showed that all 30 participants achieved successful engraftment by day 28 post-transplant, meaning the transplanted stem cells successfully migrated to the bone marrow environment and initiated the production of new blood cells. Several patients reached this crucial physiological milestone even sooner, with normal platelet production returning by day 16, on average.
Importantly, these cellular recovery timelines closely mirrored what clinicians typically observe during standard, unmanipulated stem cell transplantation procedures.
Across the cohort, the average overall survival time observed during the trial stood at just over 14 months. A subset of 19 patients received at least one cycle of gemtuzumab ozogamicin as part of a carefully structured dose-escalation protocol, which allowed the research team to safely identify an optimal recommended dose for future investigations.
Across the various administered doses, patients successfully maintained their baseline blood cell counts. This key finding strongly indicates that the gene-edited transplant effectively shielded the patients from the severe, precipitous drops in blood cell levels that typically accompany the use of this maintenance therapy following conventional stem cell transplants.
Side Effects Remained Similar to Standard Transplants
The adverse events and side effects documented throughout the course of the clinical study were broadly consistent with those traditionally associated with standard allogeneic stem cell transplantation. Common complications included severe anemia, low platelet counts, persistent fevers, various infections, and graft-versus-host disease—a complex condition wherein the newly introduced donor immune cells mistakenly attack the recipient’s healthy tissues.
A total of seven patients passed away during the course of the investigation. Of those fatalities, four were directly attributed to the progressive advancement of their underlying cancer, while the remaining three deaths were linked to transplant-related complications, specifically including kidney failure, liver toxicity, and sepsis.
Reflecting on the broader implications of the findings, Dr. DiPersio emphasized that the clinical data establish a solid foundational framework for future therapeutic regimens. By successfully pairing CD33-deleted stem cell transplantation with subsequent CD33-targeted immunotherapies, researchers hope to empower physicians to attack aggressive blood cancers with unprecedented clinical aggression, all without simultaneously destroying the healthy donor cells required to rebuild and sustain the patient’s hematopoietic and immune systems.