The Company Behind the CRISPR Leukemia Breakthrough Shut Itself Down a Year Before the Paper Ran

The patient had run out of good options. High-risk acute myeloid leukemia, the kind that comes back, and it did come back, even after a stem-cell transplant from a donor whose cells had been stripped of a single surface protein by CRISPR. So the doctors went again. They took the same donor’s cells, reengineered them a second time into CD33-targeted CAR-T cells, and infused them. The leukemia disappeared. More than a year later, according to a case report published last October in JCO Precision Oncology and described by WashU Medicine, it still had not returned.

That result got a press release this month, and the admiring coverage that trails a press release. What none of it dwelled on: the company that designed the therapy laid off 95 percent of its staff in May 2025, closed its clinical and manufacturing operations, and left the program behind more than a year before the paper describing it ran in Nature Medicine.

The therapy is called tremtelectogene empogeditemcel, mercifully shortened to trem-cel, and the idea behind it belongs to Miriam Y. Kim. She first described the concept as a postdoctoral researcher at the University of Pennsylvania, then carried it into John F. DiPersio’s laboratory at Washington University in St. Louis. The problem she set out to solve is old and brutal. In AML, the cancer cells wear a protein called CD33. So do healthy blood cells. Any drug aimed at CD33 hits both, which is why CD33-targeted therapy after a transplant has always risked destroying the blood system the transplant just rebuilt.

Kim’s fix was to delete CD33 from the donor stem cells with CRISPR-Cas9 before transplanting them. Do that, and the patient’s new blood system goes invisible to CD33 drugs while the leukemia stays lit up as a target, opening a window to hunt residual cancer without friendly fire. The reason it looked safe to try: people born without CD33 have no apparent health problems, so the protein appears expendable.

The phase 1/2 trial, published in Nature Medicine in May 2026, tested whether the edited graft behaves like a normal one. It enrolled 30 adults with AML or myelodysplastic syndrome at high risk of relapse, across 15 sites in the United States and Canada. All 30 engrafted by day 28. Platelets recovered by day 16 on average. Nineteen of the 30 went on to at least one cycle of maintenance with gemtuzumab ozogamicin, the CD33 drug, and their blood counts held. On the narrow question it was built to answer, whether a CRISPR-edited transplant can take hold and tolerate CD33 maintenance, the answer was a clean yes.

What the trial proved is engraftment and tolerability. What it did not prove is that anyone lived longer. This was open-label, with no control arm, measured against the historical record of standard transplantation, which is how DiPersio framed it: the CD33-deleted transplant, he said, “looks very similar to the outcomes of standard stem cell transplantation.” Average survival came in at just over 14 months, and 7 of the 30 patients died, four when the cancer returned and three from the transplant itself, the familiar litany of kidney failure, liver toxicity, and sepsis. That is a proof of mechanism in a small, gravely ill population, which is what a phase 1/2 trial is for, and nothing more.

The maintenance drug carries its own history, and it is not flattering. Gemtuzumab ozogamicin, sold as Mylotarg by Pfizer, won accelerated FDA approval in 2000, then was pulled from the market in 2010 after a required confirmatory trial found no survival benefit and a higher rate of fatal toxicity in the patients who got it. It came back in 2017 at a lower, fractionated dose. So the elegant new transplant is engineered to pair with a drug the agency once decided was doing more harm than good. That does not make trem-cel a bad idea. It does make “breakthrough” feel a size too large for the garment.

Then there is the sponsor. Trem-cel was funded, manufactured, and largely staffed by Vor Biopharma, the Cambridge biotech that built it, and several of the paper’s authors were Vor employees when the work was done. On May 8, 2025, Vor announced it was winding down, cutting roughly 95 percent of its workforce to about eight people, shutting its clinical and manufacturing operations, and putting itself up for strategic review. The company blamed “currently available clinical data from its key clinical programs” and a “challenging fundraising environment.” It had been expected to report updated results in the first half of that year. Instead it closed. The transplants engrafted. The company did not survive them.

By June, Vor had licensed an autoimmune drug from the Chinese biotech RemeGen, installed a new chief executive, and reintroduced itself as an autoimmune-disease company. Trem-cel was not part of the new story.

This is the maneuver the press releases are built to smooth over. A publicly traded company can produce a promising result, publish it in one of the field’s best journals, collect the admiring write-ups, and at the same time decide the asset is not worth what it would cost to finish. The trial asked whether the editing works. The market asked whether anyone would pay to carry it through phase 3, manufacture it at scale, and walk it past the FDA. Vor, pointing at its own clinical data, answered no.

None of that erases what Kim and DiPersio built. The editing held, the graft took, and the approach may yet be picked up by someone with deeper pockets, because good ideas in oncology rarely stay orphaned for long. But the reader who meets the headline this month should know what they are looking at: a technology whose inventor’s own company studied the numbers, the runway, and the odds, and spent its last dollars on something else. Kim’s window is still open. The company that opened it is already gone.

Sources

1. Nature Medicine – DiPersio et al., CRISPR-Cas9 CD33-deleted allogeneic HCT with gemtuzumab ozogamicin maintenance in AML, phase 1/2 trial (2026)
2. WashU Medicine / Siteman Cancer Center – Gene-edited stem cell transplant shows promise for aggressive blood cancers
3. BioPharma Dive – Vor Bio to wind down, lay off 95% of staff (2025-05)
4. BioPharma Dive – Vor, with new CEO, pivots to autoimmune disease via RemeGen license (2025-06)
5. FDA / Federal Register – Withdrawal of approval of Mylotarg (gemtuzumab ozogamicin) (2010/2011)
6. Knobbe Martens – FDA re-approves Mylotarg seven years after withdrawal (2017)