By Gnaneshwar Rajan and Gursimran Mehar

(Reuters) -The U.S. health regulator said on Monday companies will be required to add a serious warning on the prescribing information for cancer therapies known as CAR-T, made by Gilead Sciences (NASDAQ:), Johnson & Johnson (NYSE:), and others.

The other cancer therapies include Bristol Myers (NYSE:) Squibb's Breyanzi and its partnered therapy, Abecma, with 2seventy bio, J&J unit Janssen and Legend Biotech's Carvykti, Novartis AG (SIX:)'s Kymriah, and Gilead unit Kite's Tecartus and Yescarta.

Novartis said it will update the prescribing information for its CAR-T cell therapy Kymriah to include instances of T-cell malignancies occurring after treatment with Kymriah.

The Swiss drugmaker said, to date, it has not identified a causal relationship between Kymriah and secondary T-cell malignancies and remains confident in the therapy's profile.

Novartis shares were trading 1.5% lower premarket on Tuesday, at 92.91 Swiss francs. The shares traded at a record high of around 94.50 francs on Monday, according to LSEG data.

Last November, the U.S Food and Drug Administration said it had received reports of patients developing a type of T-cell blood cancer after being treated with CAR-T therapies.

The FDA said in its letters to the companies on Monday that since the approval, it had identified adverse events and clinical trial reports describing T-cell malignancies.

The health regulator said it considered the risks of T-cell malignancy, which refers to a group of blood disorders including lymphoma and leukemia, resulting in hospitalization and death, to be applicable to all therapies in the category.

Bristol Myers also confirmed the FDA's letters, and said it was "evaluating next steps on the labels for Abecma and Breyanzi".

"To date, BMS has not observed any CAR-positive T-cell malignancy cases, and therefore, we have not found a causal relationship between our products and secondary T-cell malignancies."

The CAR-T treatment generally involves extracting disease-fighting white blood cells known as T-cells from a patient, re-engineering them to attack cancer and infusing them back into the body.

Since 2017, six CAR-T cell therapies have been approved by the FDA.

Gilead, J&J, 2seventy and Legend Biotech did not immediately respond to requests for comments.