By Hendra Lukman 
For patients battling some of the most aggressive forms of blood cancer, a stem cell transplant represents a critical, often the sole, curative treatment option. However, the specter of cancer recurrence post-transplant can leave clinicians with a dwindling arsenal of therapeutic choices. Now, groundbreaking research emerging from the Washington University School of Medicine in St. Louis is illuminating a path forward, suggesting that genetically engineering donor stem cells before transplantation can pave the way for safer and potentially more potent follow-up cancer treatments. This innovative strategy involves the precise removal of a specific protein from the donor cells, thereby enabling targeted therapies to selectively attack residual cancer cells while preserving the integrity of the life-saving transplanted cells.
The pivotal clinical trial, a collaborative effort involving researchers at the Siteman Cancer Center—a joint enterprise of Barnes-Jewish Hospital and WashU Medicine—along with 14 other leading medical institutions across the United States and Canada, has yielded results published in the esteemed journal Nature Medicine. These findings represent a significant leap in the fight against recalcitrant hematologic malignancies.
Overcoming a Crucial Hurdle in CAR-T Therapy
At the heart of this advancement lies a solution to a persistent challenge that has historically hampered the efficacy of chimeric antigen receptor T-cell (CAR-T) therapy, particularly in certain aggressive blood cancers. As explained by Dr. John F. DiPersio, the Virginia E. & Sam J. Golman Professor of Medicine at WashU Medicine and the study’s corresponding author, the gene-editing approach addresses a fundamental limitation of CAR-T therapy.
CAR-T therapy, a revolutionary form of immunotherapy where a patient’s own T-cells are genetically modified to recognize and attack cancer cells, has demonstrated remarkable success against some blood cancers. However, its effectiveness has been notably constrained when applied to diseases such as acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). The crux of the problem, according to Dr. DiPersio, stems from the fact that many proteins found on the surface of AML and MDS cancer cells are also present on healthy myeloid cells. This includes the very donor stem cells that are crucial for transplantation. Consequently, if CAR-T cells are engineered to target such a shared protein, they risk indiscriminately destroying healthy blood stem cells alongside the malignant ones.
This collateral damage can precipitate a severe and potentially life-threatening inflammatory response. Furthermore, it can dilute the effectiveness of the cancer treatment itself, as a significant proportion of the engineered CAR-T cells expend their energy attacking benign targets rather than concentrating their efforts on eradicating the cancerous population.
The conceptual framework for circumventing this critical issue was initially articulated by Dr. Miriam Y. Kim, now an assistant professor of medicine at WashU Medicine. Dr. Kim’s foundational research in this area began during her postdoctoral fellowship at the University of Pennsylvania, continued within Dr. DiPersio’s laboratory at WashU Medicine, and culminated in her establishment as an independent investigator within the WashU Medicine Division of Oncology. She also actively treats patients at Siteman Cancer Center, where she is a research member.
Precision Engineering: Eliminating CD33 from Healthy Stem Cells
The clinical trial meticulously examined the outcomes of patients diagnosed with AML and MDS who received donor stem cells that had undergone precise genetic modification to remove a protein known as CD33. The overarching objective was to cultivate a population of healthy blood cells that would be rendered impervious to therapies specifically designed to target CD33.
"We are profoundly encouraged by the results of this study, which indicate that a stem cell transplant with CD33-deleted cells yields outcomes that closely mirror those of standard stem cell transplantation," stated Dr. DiPersio, who also holds the directorship of WashU Medicine’s Center for Gene and Cellular Immunotherapy. "Looking ahead, our hope is to integrate this approach with CD33-targeted immunotherapies, such as CAR-T cells, thereby enhancing treatment paradigms for patients afflicted with these particularly aggressive hematologic malignancies."
In a compelling illustration of the therapeutic potential, Dr. DiPersio and his research colleagues have also documented a singular case involving a patient with high-risk AML. This individual underwent a CD33-deleted stem cell transplant. When the cancer subsequently reappeared, the patient was treated with CD33-targeted CAR-T cells, which were derived from the T-cells of the very same donor who had provided the initial stem cells. This patient, who had one of the most aggressive subtypes 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 observed to be CD33-negative. This critical observation confirmed that the genetically engineered donor cells had successfully engrafted and established themselves within the patient’s bone marrow. Dr. DiPersio served as the senior author of this case study, which was published in October 2025 in JCO Precision Oncology.
The Strategy of Shielding Healthy Blood Cells
CD33 emerges as an exceptionally attractive target for this innovative strategy due to its specific expression. The protein is predominantly found on blood-forming cells and is largely absent from other tissues in the body. Furthermore, scientific evidence suggests that CD33 is not indispensable for the normal functioning of blood stem cells; individuals born without this protein do not appear to experience any associated health complications.
The underlying principle is that following a successful transplant utilizing CD33-deleted stem cells, any remaining cells expressing CD33 are likely to be cancerous. Subsequently, a CAR-T therapy or another form of immunotherapy designed to target CD33 could then be deployed to precisely eliminate these cancer cells, leaving the healthy donor-derived blood cells unharmed.
The phase 1/2 multicenter trial involved the enrollment of 30 adult patients diagnosed with AML or MDS who were identified as having a high risk of disease relapse. Prior to transplantation, the donor stem cells underwent modification using CRISPR gene-editing technology to excise the CD33 protein. The resulting product, a CD33-deleted stem cell formulation, is known by the investigational name tremtelectogene empogeditemcel (trem-cel). This novel cell therapy was developed by Vor Biopharma, the entity that provided funding for the study.
Rigorous Testing of a CD33-Targeted Cancer Treatment
To rigorously assess the capacity of the gene-edited stem cells to withstand a therapeutic intervention directed at CD33, patients in the trial also received a maintenance treatment following their transplantation. This maintenance therapy involved the administration of gemtuzumab ozogamicin. While not a CAR-T therapy, gemtuzumab ozogamicin is an engineered antibody that specifically recognizes CD33 and delivers an anti-cancer drug directly to cells bearing this protein.
Gemtuzumab ozogamicin has already received approval from the Food and Drug Administration (FDA) for the treatment of CD33-positive AML and is currently under investigation in clinical trials for CD33-positive MDS. Although this drug can be effective in preventing disease recurrence, its clinical utility has been historically limited by significant side effects. These can include liver toxicity and damage to healthy blood cells, leading to dangerously low levels of white blood cells, red blood cells, and platelets.
Evidence of Successful Engraftment of Gene-Edited Cells
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 to produce new blood cells. In this trial, all 30 participating patients achieved successful engraftment by day 28 post-transplant. Some patients reached this crucial milestone even earlier, with platelet production typically returning by day 16. These recovery timelines were found to be comparable to those observed in patients receiving standard stem cell transplantation, underscoring the safety and feasibility of the gene-editing process.
The average survival observed in the trial was just over 14 months. A significant portion of the cohort, 19 patients, received at least one cycle of gemtuzumab ozogamicin as part of a carefully designed dose-escalation protocol. This phased approach allowed researchers to accurately identify a recommended therapeutic dose for the maintenance therapy. Notably, across the various dose levels, patients were able to maintain their blood cell counts. This finding is particularly significant as it suggests that the gene-edited transplant effectively shielded them from the severe drops in blood cell counts that are frequently encountered when gemtuzumab ozogamicin is administered following a conventional stem cell transplant.
Side Effect Profile Remains Comparable to Standard Transplants
Importantly, the adverse events reported during the course of the treatment were broadly consistent with those typically associated with standard stem cell transplantation. These included common 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.
Dr. DiPersio emphasized that these findings lay a robust foundation for the development of future therapeutic strategies. These strategies aim to combine CD33-deleted stem cell transplantation with CD33-targeted immunotherapies. The ultimate goal is to empower clinicians to mount a more aggressive assault on cancer cells while simultaneously safeguarding the vital donor cells necessary for reconstituting the patient’s blood system.
This groundbreaking research was made possible through the financial support of Vor Biopharma. It is important to note that several co-authors of the study were employees of Vor Biopharma at the time the research was conducted, highlighting the collaborative nature of scientific advancement in this field.
Background and Context: The Enduring Challenge of Relapsed Cancers
Stem cell transplantation, also known as bone marrow transplantation, has long been a cornerstone in the management of hematologic malignancies that have proven resistant to conventional chemotherapy or have relapsed after initial treatment. The procedure involves replacing a patient’s diseased or damaged bone marrow with healthy stem cells, either from a matched donor (allogeneic transplant) or, in some cases, from the patient themselves (autologous transplant). In the context of aggressive blood cancers like AML and MDS, allogeneic transplantation is often the preferred approach due to its potential for a graft-versus-leukemia effect, where the donor immune cells actively target and eliminate any remaining cancer cells.
However, the success of allogeneic transplantation is not guaranteed. Even with meticulous donor matching and supportive care, relapse remains a significant concern, accounting for a substantial proportion of treatment failures. This relapse can occur because residual cancer cells may have survived the conditioning regimen prior to transplant, or they may have acquired resistance mechanisms. The limited options for treating relapsed disease after transplant underscore the urgent need for novel therapeutic strategies.
Timeline of Advancement: From Concept to Clinical Trial
The conceptualization of removing targetable antigens from donor cells to enable subsequent targeted therapies can be traced back several years. Dr. Miriam Kim’s initial research laid the groundwork for understanding how to engineer cells to resist specific therapeutic attacks. The development of advanced gene-editing technologies, such as CRISPR-Cas9, in the early to mid-2010s provided the precise tools necessary to implement these concepts.
The preclinical development and optimization of the CD33-deleted stem cell product (trem-cel) likely occurred over several years, involving rigorous laboratory testing and animal model studies to ensure safety and efficacy. The initiation of the phase 1/2 clinical trial marked a crucial step, transitioning these promising preclinical findings into human subjects. The trial, as reported, enrolled 30 adult patients, a sample size typical for early-phase studies designed to assess safety, tolerability, and preliminary efficacy. The publication of results in Nature Medicine signifies a major milestone, bringing this innovative approach closer to potential clinical adoption. The subsequent reporting of the single-patient case in JCO Precision Oncology further illustrates the real-world application and positive outcomes achievable with this technology.
Broader Implications for Cancer Treatment
The implications of this research extend beyond AML and MDS. The principle of genetically modifying donor cells to resist targeted therapies could be applied to other blood cancers and potentially even solid tumors where similar challenges exist. If a particular protein is found on both cancer cells and essential donor cells, and if that protein is not critical for the function of healthy cells, then removing it from donor cells before transplantation opens up a new avenue for post-transplant therapy.
This approach has the potential to revolutionize the treatment of refractory and relapsed blood cancers by:
- Enhancing Efficacy of Immunotherapies: It allows for the full utilization of potent immunotherapies like CAR-T cells, which might otherwise be too toxic to administer in the context of a standard transplant.
- Reducing Treatment-Related Toxicity: By sparing healthy donor cells, the risk of severe side effects, such as myelosuppression and debilitating inflammatory responses, could be significantly mitigated.
- Expanding Treatment Options: It provides a much-needed lifeline for patients who have exhausted conventional treatment pathways and face limited choices.
While the results are highly promising, continued research and larger clinical trials will be necessary to fully validate the long-term safety and efficacy of this approach. Nevertheless, this gene-editing strategy represents a significant paradigm shift, offering a sophisticated and targeted method to combat aggressive blood cancers and improve outcomes for patients who need it most.
