Gene therapy trial shows durable promise for rare blood disorders

Patients with two severe inherited blood disorders remained free of symptoms three years after receiving a single gene therapy infusion, according to long-term trial results published Wednesday — the strongest evidence yet that one-time genetic treatments can deliver lasting cures for diseases that once demanded lifelong care.

Of the 31 patients enrolled in the international trial, 28 have required no blood transfusions since treatment, the study reported. Before therapy, participants had needed transfusions every three to five weeks, on average, since early childhood.

The therapy works by harvesting a patient’s own blood stem cells, correcting the faulty gene in the laboratory, and returning the cells after a conditioning regimen clears space in the bone marrow. The corrected cells then produce healthy blood for what researchers hope is a lifetime.

“For the first time, we are talking about these conditions in the past tense for a majority of treated patients,” said the trial’s lead investigator, presenting the data at a hematology congress. “Three years is not forever. But biologically, these grafts look permanent.”

For the first time, we are talking about these conditions in the past tense for a majority of treated patients.

Independent specialists urged measured optimism, noting that the conditioning chemotherapy carries real risks and that three patients did not achieve transfusion independence. Longer follow-up, they said, remains essential.

The larger obstacle may be access. The therapy’s list price is expected to exceed $2 million per patient, and the disorders it treats are most prevalent in regions with the least capacity to deliver complex cell therapies.

Trial sponsors said they are developing a simplified, in-body delivery method that would remove the need for stem cell harvesting and chemotherapy — an approach that, if successful, could cut costs dramatically. Early human studies are expected to begin next year.

For the trial’s participants, the statistics are personal. One, a 24-year-old who received her infusion in 2023, told the congress she had recently completed her first marathon. “I used to plan my life around a hospital chair,” she said. “Now I plan it around a race calendar.”