By Hendra Lukman 
For patients battling some of the most aggressive forms of blood cancer, a stem cell transplant has long represented the most potent chance for a cure. However, the specter of cancer recurrence after this intensive procedure has often left clinicians with a starkly limited array of follow-up treatment options. Now, a groundbreaking clinical trial spearheaded by researchers at the Washington University School of Medicine in St. Louis is illuminating a path forward, suggesting that genetically modifying donor stem cells before transplantation can pave the way for safer and potentially more effective post-transplant therapies. This innovative strategy involves the precise removal of a specific protein from donor cells, enabling targeted therapies to attack residual cancer cells while preserving the integrity of the transplanted healthy cells.
The pivotal study, conducted across the Siteman Cancer Center at Barnes-Jewish Hospital and WashU Medicine, alongside 14 other leading medical institutions in the United States and Canada, has yielded results published in the esteemed journal Nature Medicine. This research marks a significant stride in overcoming a formidable hurdle that has previously hampered the efficacy of advanced immunotherapies, particularly CAR-T cell therapy, in treating certain challenging blood cancers.
The CAR-T Conundrum: A Shared Target Creates a Dilemma
Dr. John F. DiPersio, MD, PhD, the Virginia E. & Sam J. Golman Professor of Medicine at WashU Medicine and the corresponding author of the study, highlighted the critical challenge that this gene-editing approach aims to address. CAR-T cell therapy, a revolutionary treatment that engineers a patient’s own immune T cells to recognize and destroy cancer cells, has demonstrated remarkable success against certain blood cancers. However, its effectiveness has been notably constrained when applied to diseases like acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS).
The fundamental issue, as explained by Dr. DiPersio, lies in the shared expression of certain proteins on both cancerous AML and MDS cells and healthy myeloid cells, including the crucial donor stem cells used in transplantation. When CAR-T cells are programmed to target such a protein, they risk indiscriminately attacking and destroying these vital healthy stem cells alongside the malignant ones. This collateral damage can trigger a severe and potentially life-threatening inflammatory response, known as cytokine release syndrome. Furthermore, it can dilute the therapeutic impact of the CAR-T therapy itself, as a significant portion of the engineered cells expend their energy on attacking benign targets rather than concentrating their efforts on eradicating the cancer.
The conceptual foundation for circumventing this critical limitation was initially laid by Dr. Miriam Y. Kim, MD, now an assistant professor of medicine at WashU Medicine. Dr. Kim initiated this line of inquiry as a postdoctoral researcher at the University of Pennsylvania, continuing her work in Dr. DiPersio’s lab at WashU Medicine before establishing herself as an independent investigator within the Division of Oncology. Her clinical practice and research endeavors are based at Siteman Cancer Center.
Precision Engineering: Removing CD33 from Donor Stem Cells
The clinical trial meticulously enrolled patients diagnosed with AML and MDS who received donor stem cells that had undergone genetic modification to precisely excise the CD33 protein. The overarching objective was to engineer healthy blood cells that would be rendered impervious to therapies specifically designed to target CD33, thereby creating a safe haven for these essential cells.
"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 Dr. DiPersio, who also leads 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."
In a compelling demonstration of the strategy’s potential, Dr. DiPersio and his colleagues reported on a single patient with high-risk AML whose cancer recurred after receiving a CD33-deleted stem cell transplant. In this critical juncture, the patient was successfully treated with CD33-targeted CAR-T cells derived from the same donor who had provided the original stem cells. Remarkably, this patient, who had one of the most aggressive forms of AML, achieved a complete remission and has remained cancer-free for over a year following the CAR-T therapy. Crucially, normal blood cell production was restored, and all of the patient’s blood cells were confirmed to be CD33-negative, a finding that definitively indicated the successful engraftment and sustained presence of the genetically engineered donor cells within the bone marrow. This remarkable case was detailed in a study published in October 2025 in JCO Precision Oncology, with Dr. DiPersio serving as the senior author.
Shielding Healthy Blood Cells: The Rationale Behind CD33 Deletion
The choice of CD33 as the target for this innovative approach is strategically significant. The protein is predominantly expressed on blood-forming cells, with minimal presence in other tissues. Furthermore, scientific evidence suggests that CD33 is not essential for the normal functioning of blood stem cells; individuals born without this protein do not appear to suffer any related health consequences.
The underlying hypothesis is that following a successful transplant with CD33-deleted stem cells, any cells that continue to express CD33 are likely to be residual cancer cells. Consequently, a CD33-targeted CAR-T therapy or another form of immunotherapy directed at CD33 could then selectively eliminate these malignant cells without compromising the health and function of the newly established, CD33-negative donor-derived blood cells.
The Clinical Trial: A Multicenter Endeavor
The phase 1/2 multicenter trial enrolled 30 adult patients with AML or MDS who were identified as having a high risk of disease relapse. Prior to transplantation, the donor stem cells were precisely modified using CRISPR gene-editing technology to remove the CD33 protein. The resulting genetically engineered product, known as tremtelectogene empogeditemcel (trem-cel), was developed by Vor Biopharma, which also provided funding for the study.
Testing the Defense: A CD33-Targeted Maintenance Therapy
To rigorously assess the ability of the gene-edited stem cells to withstand a direct assault on CD33, patients in the trial received a post-transplant maintenance therapy. This therapy involved the administration of gemtuzumab ozogamicin, an antibody-drug conjugate that targets CD33. Unlike CAR-T cells, gemtuzumab ozogamicin is an engineered antibody that binds to CD33 and delivers a potent anti-cancer drug directly to cells expressing the protein.
Gemtuzumab ozogamicin is already approved by the U.S. Food and Drug Administration (FDA) for use in CD33-positive AML and is being investigated in clinical trials for CD33-positive MDS. While this treatment can play a role in preventing relapse, its application has been historically limited by significant side effects, including liver toxicity and damage to healthy blood cells, leading to potentially dangerous reductions in white blood cell, red blood cell, and platelet counts.
Engraftment Success: Gene-Edited Cells Take Root
A critical benchmark for the success of any stem cell transplant is engraftment, the process by which the transplanted stem cells migrate to the bone marrow and begin producing new blood cells. In this trial, all 30 patients achieved engraftment by day 28 post-transplantation, a timeline consistent with standard stem cell transplantation. Some patients achieved this vital milestone even sooner. Notably, platelet production returned by an average of day 16. These recovery metrics are highly encouraging, suggesting that the genetic modification did not impede the fundamental engraftment capabilities of the donor stem cells.
The average survival observed in the trial was just over 14 months. Nineteen of the participants received at least one cycle of gemtuzumab ozogamicin as part of a carefully designed dose-escalation protocol. This protocol was instrumental in identifying a recommended safe and effective dose for the maintenance therapy. Across the various dose levels administered, patients were able to maintain their blood cell counts. This finding is particularly significant, as it strongly suggests that the gene-edited transplant effectively shielded the patients from the severe hematological nadirs that are often encountered when gemtuzumab ozogamicin is administered following a conventional stem cell transplant.
Side Effect Profile: Similarities and Distinctions
The spectrum of side effects experienced by patients in this trial was broadly comparable to those typically associated with standard stem cell transplantation. These included expected complications such as anemia, low platelet counts, fever, infections, and graft-versus-host disease (GVHD), a serious complication where the donor immune cells attack the recipient’s healthy tissues.
During the study period, seven patients succumbed to their illness. Four of these deaths were attributed to the progression of their underlying cancer, while the remaining three were linked to transplant-related complications, including kidney failure, liver toxicity, and sepsis.
Future Directions: A Foundation for Enhanced Therapies
Dr. DiPersio emphasized that the findings from this trial provide a robust foundation for the development of future treatment paradigms. The vision is to seamlessly integrate CD33-deleted stem cell transplantation with CD33-targeted immunotherapies. This dual approach aims to empower clinicians to mount a more aggressive assault on cancer cells while simultaneously safeguarding the critical donor-derived healthy cells that are indispensable for reconstituting the patient’s blood system. This strategy holds the promise of significantly improving outcomes for patients with some of the most challenging and life-threatening hematological malignancies.
The research was supported by Vor Biopharma. Several co-authors of the study were employees of the company at the time the research was conducted. The successful demonstration of engraftment and the ability of gene-edited cells to withstand targeted therapy represent a pivotal step forward, offering a tangible glimmer of hope for patients facing dire prognoses. The implications of this work extend beyond AML and MDS, potentially paving the way for similar gene-editing strategies in other cancers where targeted therapies are hampered by the expression of the target antigen on essential healthy cells.
