(Bloomberg) -- US regulators are closely looking at cell therapies used to treat blood cancers, focusing on concerns that the treatments may cause additional malignancies as side effects. 

The Food and Drug Administration is studying the impact of a type of therapy known as CAR-T, in which immune cells are genetically modified in a laboratory to bind to cancer cells and kill them, according to a statement Tuesday. The high-priced therapies are used in hard-to-treat lymphomas and other blood cancers.

The FDA said it’s investigating reports from clinical trials and post-marketing studies that certain CAR-T therapies may cause malignancies including lymphoma. While the benefits still outweigh potential risks, the regulator said it’s looking at whether there’s a need for regulatory action. 

Several CAR-T therapies are being scrutinized by the FDA, including Abecma from Bristol Myers Squibb Co. and Carvykti from Johnson & Johnson. Shares of Cabaletta Bio Inc., which is developing a CAR-T, fell as much as 49% as of 3 p.m. in New York.

The focus of the probe may be Novartis AG’s Kymriah, RBC Capital Markets analysts led by Brian Abrahams said in a note to clients. The regulator’s adverse events database points to the therapy as being associated with far more frequent cases of malignancies than other, similar treatments, the note said. 

“A distinct pattern emerges in which Novartis’s Kymriah appears to have by far the highest rates,” the RBC analysts said. Novartis’ American depositary receipts fell as much as 0.9%. 

More than 10,000 patients have been treated with Kymriah since it was cleared for US use in 2017 and Novartis has not identified a causal relationship between the treatment and secondary cancers, a spokesperson said in an email.

A Bristol spokesperson said the company has treated more than 4,700 patients during and after clinical trials with both Abecma and Breyanzi, another cell therapy named by the FDA, and has not observed any malignancies connected to its CAR-T treatments. 

--With assistance from Gerry Smith.

(Updates with comment from Bristol Myers Squibb in final paragraph.)

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