An experimental cancer therapy being developed by Novartis AG eliminated an aggressive form of blood cancer in 82 percent of children and young adults treated with modified immune cells in a mid-stage trial, the company said on Saturday.

Interim results from the multi-center trial for 50 patients with acute lymphoblastic leukemia whose cancer returned or did not respond to other treatment, showed that 41 were disease-free three months after treatment with the drug, called CTL019.

The trial results were presented at a meeting of the American Society of Hematology in San Diego.

Novartis estimated that 60 percent of those responders were relapse-free after six months. Complete remission was defined as including "remission with incomplete blood count recovery."

CTL019 is part of an experimental class of drugs that are made by genetically altering a patient's T-cells, a type of white blood cell, in the lab to help the immune system find and kill cancer cells. The modified cells, called chimeric antigen receptor T-cells, or CAR-T, are infused into the patient.

Novartis said it plans to file early next year for US Food and Drug Administration approval of CTL019.

The company said nearly half of trial patients experienced severe cytokine release syndrome, a dangerous buildup of toxic debris known as CRS, and 15 percent experienced serious neurological problems including delirium. Researchers said the side effects were treated, and no patients died due to CRS.

CRS, a known side effect of CAR-T therapy, led to the decision last week to halt a trial of JCAR015, a rival drug being developed by Juno Therapeutics Inc.

"We have learned that the patients who come in with more leukemia in their body have a much higher risk of getting sick," said Dr Stephan Grupp, the Novartis trial's lead investigator and director of the cancer immunotherapy program at the Children's Hospital of Philadelphia.

He said the trial did not show a big difference between patients whose leukemia was refractory, meaning they had stopped responding to treatment, and patients who had relapsed, or might have responded temporarily to chemotherapy. "We do see a difference in toxicity," the researcher said. "Refractory patients with high disease burden can get sicker temporarily on the way to remission."