Novartis said on Tuesday it paused eight clinical trials of an experimental cell therapy, called rap-cel, targeting autoimmune and neurological disorders in late August, after three patients died.
The halt was implemented on August 24 after Novartis was notified of three cases of a severe, life-threatening immune reaction.
Complications from the reaction, in which the body's defence system overreacts and attacks its own organs, led to the deaths, the Swiss drugmaker said.
US drugmaker Bristol Myers also said it has implemented a voluntary pause on studies for a rival CAR T-cell treatment, zola-cel, "out of an abundance of caution" as it reviews safety data.
Novartis said it was conducting a comprehensive review of the deaths and is working with independent safety boards to look into what happened as well as to find ways to catch dangerous side effects earlier.
Novartis said its two trials in patients with lymphoma and leukemia were not affected by the halt.
The drugmaker said that this type of severe immune reaction is a known, potentially life-threatening risk of CAR T therapy.
Novartis added that it is actively sharing information with health authorities and will continue to monitor patients who have already received the treatment.
The trials affected include those for inflammatory diseases such as lupus, rheumatoid arthritis, and vasculitis and nerve- and muscle-weakening disorders including multiple sclerosis and myasthenia gravis.
Rap-cel is a type of CAR T-cell therapy, a relatively new type of treatment in which a patient's immune cells are removed, modified to recognise and destroy specific target cells, and reinfused.
The complex process has seen success with blood cancers, and drugmakers have more recently been testing these therapies for autoimmune and other conditions.
Shares of the Swiss drugmaker were up 4 per cent on Tuesday as investors focused on the late-stage success of its oral multiple sclerosis drug, remibrutinib.
The Wall Street Journal first reported the trial halt.
A spokesperson for Bristol Myers said its pause was put in place after patients experienced temporary, reversible inflammation.
The company added that the drug's overall safety profile remains in line with other CAR T therapies.
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