A new leukemia drug being hailed by doctors as a breakthrough could prove among the most expensive therapies ever on the market: For a single treatment, the price is expected to reach hundreds of thousands of dollars.

“It is a revolutionary treatment,” said Dr. Prakash Satwani, a pediatric hematologist at NewYork-Presbyterian/Columbia University Medical Center, noting that the therapy can help some leukemia patients with no other options. But, he added, “the price will be phenomenal.”

“From what we’re hearing, this will be a quantum leap more expensive than other cancer drugs,” said Leonard Saltz, chief of gastrointestinal oncology at Memorial Sloan Kettering Cancer Center in New York.

Switzerland-based Novartis hasn’t announced a price for the medicine, but British health authorities have said the drug could cost $649,000 for a one-time treatment that would have significant benefits.

The cancer therapy was unanimously approved by a Food and Drug Administration advisory committee in July, and its approval seems all but certain.

The treatment, CTL019, belongs to a new class of medications called CAR T-cell therapies, which involve harvesting patients’ immune cells and genetically altering them to kill cancer. It’s been tested in patients whose leukemia has relapsed in spite of chemotherapy or a bone-marrow transplant.

The prognosis for these patients is normally bleak. But in a clinical trial, 83% of those treated with CAR T-cell therapy — which has been described as a “living drug” (one using patients’ cells) — have gone into remission.

CAR T cells have been successful only in a limited number of cancers, however, and are being suggested for use as a last resort when all else has failed. As a result, only a few hundred patients a year would be eligible for them, at least initially, said Dr. J. Leonard Lichtenfeld, deputy chief medical officer for the American Cancer Society.

The FDA is scheduled to decide on approval by Oct. 3. The agency is also considering a CAR T-cell therapy from Kite Pharma.

A third company, Juno Therapeutics, halted the development of one of its CAR T-cell therapies after five patients died from complications of treatment.

Rather than wait for Novartis to announce a price, an advocacy group called Patients for Affordable Drugs has launched a pre-emptive strike, asking to meet with company officials to discuss a “fair” price for the therapy. The Novartis drug has the potential to be one of the most expensive drugs ever sold, said David Mitchell, the patients group’s president, who has been treated for multiple myeloma, a blood cancer, since 2010. (The Laura and John Arnold Foundation, which provides some funding for Kaiser Health News, supports Patients for Affordable Drugs.)