For patients 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 true cure. Yet, even this high-stakes medical intervention carries a heavy shadow: malignancies frequently return after transplantation, leaving hematologists and oncologists with drastically narrowed options for saving the patient’s life.
Now, results from a groundbreaking clinical trial led by researchers at Washington University School of Medicine in St. Louis suggest that a novel strategy of genetically modifying donor stem cells prior to transplantation could fundamentally change this dynamic. By altering the donor cells beforehand, scientists hope to make follow-up cancer therapies significantly safer and far more effective. The innovative approach centers on removing a specific surface protein from donor cells, effectively creating a cellular shield that allows subsequent targeted therapies to hunt down and destroy cancer cells while leaving healthy, transplanted tissue completely unharmed.
The clinical trial was conducted at the Siteman Cancer Center—based at Barnes-Jewish Hospital and WashU Medicine—alongside 14 other specialized medical centers across the United States and Canada. The findings from this multi-center effort have been published in the prestigious journal Nature Medicine, marking a potentially vital step forward in the management of hard-to-treat hematologic malignancies.
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 could help surmount a formidable obstacle that has historically limited the success of chimeric antigen receptor, or CAR-T, cell therapy in certain types of blood cancer.
While CAR-T cell therapy has achieved astonishing success against several aggressive blood cancers, its clinical impact has been considerably less impressive when applied to diseases such as acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). The underlying dilemma, as DiPersio explained, lies in a frustrating biological overlap: many of the characteristic proteins found abundantly on the surface of AML and MDS cancer cells also happen to appear on normal, healthy myeloid cells, including the vital donor stem cells introduced during a transplant.
If physicians program CAR-T cells to target and destroy one of these shared proteins, the engineered immune cells cannot easily distinguish between friend and foe. Consequently, they may indiscriminately annihilate the patient’s healthy blood stem cells alongside the malignant cancer cells. This collateral damage can spark a severe and potentially life-threatening inflammatory response throughout the body. Furthermore, it significantly compromises the efficacy of the cancer treatment itself, as vast numbers of valuable CAR-T cells become sidetracked and exhausted by attacking healthy cellular targets instead of concentrating their destructive force on the tumor.
The conceptual foundation for avoiding this biological trap was originally conceived by Dr. Miriam Y. Kim, now an assistant professor of medicine at WashU Medicine. Kim began investigating this protective strategy as a postdoctoral researcher at the University of Pennsylvania, later continuing the work within the DiPersio laboratory before transitioning to an independent investigator role within the WashU Medicine Division of Oncology. Alongside her research endeavors, Kim treats patients and serves as a research member at Siteman Cancer Center.
Removing CD33 From Healthy Stem Cells
In the recent clinical trial, patients diagnosed with high-risk AML and MDS received donor stem cells that had been precisely engineered using advanced gene-editing techniques to eliminate a specific surface protein known as CD33. The overarching objective of this genetic modification was to generate a population of healthy blood cells that would become entirely invisible and invulnerable to subsequent medical treatments specifically designed to home in on and destroy cells carrying 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," said DiPersio, who also holds the directorship 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."
Expanding upon this promise, DiPersio and his collaborative team previously documented a single, compelling case involving a high-risk AML patient who received a CD33-deleted stem cell transplant. When the patient’s cancer eventually relapsed, clinicians treated the recurrence using CD33-targeted CAR-T cells manufactured using T cells provided by the exact same donor who supplied the original stem cells.
Following this specialized intervention, the patient—who was fighting one of the most aggressive clinical presentations of AML—achieved a complete remission and remained entirely cancer-free for more than one year post-treatment. Crucially, the patient’s normal blood cell production fully recovered, and genetic testing confirmed that all circulating blood cells lacked CD33. This definitive finding demonstrated that the genetically engineered donor cells had successfully established themselves and thrived within the bone marrow microenvironment. DiPersio served as the senior author of that preceding study, which was published in October 2025 in JCO Precision Oncology.
Shielding Healthy Blood Cells
Cell surface proteins like CD33 represent exceptionally attractive targets for this kind of sophisticated therapeutic shielding strategy because the protein is expressed almost exclusively on blood-forming cells and is notably absent from other bodily tissues. Moreover, clinical evidence strongly suggests that CD33 is not biologically essential for the normal functioning of blood stem cells; individuals who are naturally born without the protein do not appear to suffer from related health complications.
The overarching therapeutic theory posits that following a successful transplantation procedure using CD33-deleted stem cells, any remaining cells in the body that still carry CD33 should theoretically belong to the cancer. Physicians could then deploy a CAR-T cell therapy or an alternative CD33-directed immunotherapy with maximum aggression, confident that the treatment would eradicate the malignant cells while leaving the healthy, donor-derived blood and immune system completely unharmed.
To evaluate this strategy clinically, the phase 1/2 multicenter trial enrolled 30 adult patients suffering from AML or MDS who faced an elevated risk of disease relapse. Before the transplantation procedure took place, the donor stem cells were treated with CRISPR gene-editing technology to cleanly excise the gene responsible for producing CD33.
The resulting specialized cellular product, known scientifically as tremtelectogene empogeditemcel or simply trem-cel, was developed by Vor Biopharma, the corporate entity that also provided financial support for the clinical study.
Testing a CD33-Targeted Cancer Treatment
To rigorously test whether these gene-edited stem cells could successfully withstand a subsequent therapeutic assault directed against CD33, the trial participants were administered a specific maintenance treatment following their transplant procedure.
The pharmacological agent utilized, a drug called gemtuzumab ozogamicin, functions differently from a CAR-T therapy. Rather than living immune cells, it is an engineered antibody designed to recognize CD33 and deliver a potent anti-cancer toxin directly inside any cell bearing the protein.
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 clinical utility has historically been reined in by severe toxic side effects. The drug can induce significant liver damage alongside profound suppression of healthy blood cell production, frequently plunging patients into dangerously low counts of white blood cells, red blood cells, and platelets.
Gene-Edited Cells Successfully Engrafted
Despite the complexities of the protocol, clinical outcomes from the trial demonstrated encouraging viability. All 30 participating patients successfully achieved engraftment by day 28 of the study, confirming that the transplanted, gene-edited stem cells had successfully migrated to the bone marrow and initiated the production of new blood cells. Several patients reached this crucial physiological milestone even earlier, with platelet production returning by day 16 on average.
Significantly, these recovery timeframes closely mirrored the benchmarks typically observed in standard, unedited stem cell transplantation procedures.
Across the patient cohort, the average overall survival rate registered at just over 14 months. Nineteen of the trial participants received at least one cycle of gemtuzumab ozogamicin as part of a structured dose-escalation protocol, which allowed the research team to successfully identify a recommended and safe dosage range.
Across the various administered doses, patients managed to maintain stable blood cell counts. This observation strongly suggests that the gene-edited transplant successfully insulated them from the precipitous and often hazardous drops in blood cells that typically accompany this specific maintenance therapy when administered following conventional stem cell transplants.
Side Effects Remained Similar to Standard Transplants
The adverse events and side effects documented throughout the course of the trial were broadly consistent with those routinely encountered during standard stem cell transplantation procedures. Documented complications included anemia, depressed platelet counts, persistent fevers, various infections, and graft-versus-host disease—a challenging phenomenon in which the newly introduced donor immune cells mount an attack against the recipient’s healthy tissues.
Tragically, seven patients died during the course of the study. Of those fatalities, four were directly attributed to the relentless progression of the underlying cancer, while three were linked to complications arising from the transplant process itself, including kidney failure, liver toxicity, and systemic sepsis.
In reflecting on the implications of the work, DiPersio emphasized that the data provide a robust clinical foundation for future therapeutic regimens. By pairing CD33-deleted stem cell transplantation with targeted immunotherapies, researchers hope to give clinicians the tools needed to combat aggressive blood cancers with unprecedented force, eliminating the traditional compromise between destroying malignant cells and preserving the vital blood system required for patient survival.
