Acute Myeloid Leukemia Therapy Improved by CRISPR Stem Cell Transplant

For highly aggressive types of blood cancer, stem cell transplantation is often the only potentially curative therapy. Yet, these cancers can often return even after a transplant. Notably, CAR T cell therapy has not been effective against all blood cancers, including acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS).

A recent Phase I/II multicenter clinical trial, led by researchers at Washington University School of Medicine in St. Louis, shows that a stem cell transplant, that removes CD33 from donor cells using CRISPR, can help prevent cancer recurrence.

The work was published in Nature Medicine and titled, “CRISPR−Cas9 CD33-deleted allogeneic hematopoietic cell transplantation with gemtuzumab ozogamicin maintenance in AML: a Phase I/II trial.” The study was conducted at Siteman Cancer Center, based at Barnes-Jewish Hospital and WashU Medicine, and 14 other sites in the U.S. and Canada. 30 adult patients with AML or MDS at high risk of relapse received the stem cell transplant.

Myeloid cancers, such as AML and MDS, are difficult to treat with CAR T cells because the same proteins targets are present on both cancer cells and healthy myeloid cells, leading to toxicity risks.

“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 John DiPersio, MD, PhD, professor of medicine at WashU Medicine and corresponding author of the study. “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.”

As proof of concept, patients also received a maintenance therapy that targets CD33, after completion of the stem cell transplant. While not a CD33-targeted CAR T cell, the maintenance therapy, called gemtuzumab ozogamicin, is an engineered antibody that targets CD33 and carries an anti-cancer drug. Gemtuzumab ozogamicin is approved by the Food and Drug Administration (FDA) to treat CD33-positive AML and is in clinical trials for CD33-positive MDS. While it helps prevent relapse, the drug’s use is limited because it can cause liver toxicity and damage to blood cells, including dangerously low counts of white blood cells, red blood cells, and platelets.

All patients from the trial achieved engraftment of their transplanted stem cells by day 28. Some patients met this goal sooner with platelet production returning by day 16 on average. These timeframes are comparable to those of standard transplanted stem cells.

Average survival was just over 14 months. Nineteen patients received at least one cycle of the antibody maintenance therapy as part of a dose-escalation protocol. The authors found that patients maintained blood cell counts across all doses, suggesting that the gene-edited stem cell transplant protected patients from low blood cell counts typically seen following a standard stem cell transplant.

DiPersio and colleagues published a single case study detailing a patient with high-risk AML who received a CD33-deleted stem cell transplant. Upon relapse after the transplant, the patient received a CD33-targeted CAR T cell therapy, which used T cells from the same donor who provided the stem cell transplant.

The treatment resulted in complete remission and the patient remains cancer free over one year after receiving the CAR T cell therapy. Normal blood cell production returned with all blood cells lacking CD33, providing evidence that the genetically engineered donor cells had established themselves in the bone marrow.

DiPersio said the results of the study lay the groundwork for developing paired CD33-deleted stem cell transplant and CD33-targeted immunotherapy interventions that avoid destruction of healthy donor cells in the course of cancer treatment.

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