Miami (AFP) - US regulators on Wednesday approved the first gene therapy against cancer -- a treatment that uses a patient's own immune cells to fight leukemia -- opening a new era in the fight against one of the world's top killers.

The treatment is made by Novartis and is called Kymriah (tisagenlecleucel).

This type of anti-cancer immunotherapy, known as a CAR-T cell therapy, was known by CTL019 until now.

"This marks the first-ever CAR-T cell therapy to be approved anywhere in the world," Novartis CEO Joseph Jimenez told reporters on a conference call.

"It uses a new approach that is wholly personalized by using a patient's own T-cells."

Kymriah was approved by the US Food and Drug Administration for children and young adult patients up to age 25 with a form of acute lymphoblastic leukemia (ALL).

To qualify for treatment, patients must have B-cell precursor ALL that is refractory, or the patient has relapsed at least twice.

The FDA described the approval as "a historic action" and a "new approach to the treatment of cancer and other serious and life-threatening diseases," said a statement.

- How it works -

The treatment is not a pill or a form of chemotherapy, which can weaken the body's natural defenses.

Instead, it harnesses a patient's own immune cells, called T-cells and white blood cells, and trains them to recognize and fight cancer.

The patient's immune cells are removed with a special blood filtration process, sent to a lab, and genetically encoded to be able to hunt down cancer cells.

These re-engineered T-cells are then transfused back into the patient, where they can begin attacking leukemia.

Studies have shown that 83 percent of patients responded to the treatment, achieving remission within three months, Novartis said.

An application with the European Medicines Agency is expected to be filed by the end of the year.

- $475,000 a dose -

The price of Kymriah -- which is delivered to a patient just once -- is $475,000, said Bruno Strigini, CEO of Oncology at Novartis.

Patients who do not respond to the treatment within the first month would not be expected to pay, he told reporters.

The more common treatment for leukemia -- bone marrow transplants -- can cost between $540,000 and $800,000 the first year in the United States, Strigini said.

Meanwhile, outside analyses have set a cost-effective price for Kymriah between $600,000 and $750,000, he added.

"Recognizing our responsibility we set the price below that level," said Strigini.

Most patients who fit the criteria for treatment would likely be covered by insurance, since they are under 25 and would either be on their parents' insurance or covered by government-sponsored Medicaid, a Novartis spokesman said.

The price tag is high -- as are costs throughout the cancer industry -- a trend that is "unsustainable," said Craig Devoe, acting chief of medical oncology and hematology at Northwell Health Cancer Institute in New York.

Asked if he thought Novartis's pricing was reasonable, he told AFP: "It's not."

Devoe also said that while the positive results are exciting, many questions remain about how long these novel cancer treatments -- developed mainly in the past decade or so -- will last.

"Not just responding but staying in response long term is what patients want," he said.

"We really don't know how long these cells are going to stay active."

- Breakthrough therapy -

The treatment was pioneered by Carl June at the University of Pennsylvania.

Its most high-profile patient is Emily Whitehead, now 12.

Six years ago, she was the first child to receive what was widely considered a risky treatment.

She has been cancer-free ever since.

In 2014, US regulators designated CTL019 as a "breakthrough therapy" and put the experimental immunotherapy agent on the fast track to market approval.

It was the first cancer immunotherapy to receive the breakthrough designation. More are expected to follow in the coming years as the field of immunotherapy grows.

Last month, an advisory panel to the FDA unanimously urged approval of the cutting-edge cancer therapy.

About two dozen centers in the United States are expected to be able to treat patients seeking Kymriah, with as many as 35 up and running by the end of the year.

ALL is the most common form of childhood leukemia, with about 3,000 cases diagnosed in the US each year.

Of those, 600 US patients would likely meet the criteria of being under 25 and having relapsed or refractory ALL, according to Novartis.