The US health regulator has approved a therapy developed by Johnson & Johnson and its China-focused partner Legend Biotech Corp to treat a type of white blood cell cancer, the US healthcare company said on Monday.

The Food and Drug Administration's decision paves the way for Legend's first approved product in the United States, at a time when the regulator has stepped up its scrutiny of drug trials conducted in China. The Legend-J&J therapy was tested initially in China, and then in the United States and Japan.

"This is the first of many cell therapies we plan to bring to patients as we continue advancing our pipeline across disease states," said Ying Huang, Legend's chief executive officer and chief financial officer.

The treatment, Carvykti/Cilta-cel, belongs to a class of drugs known as CAR-T therapies, or chimeric antigen receptor T-cell therapies.

CAR-T drugs work by harvesting a patient's own disease-fighting T-cells, genetically engineering them to target specific proteins on cancer cells, and replacing them to seek out and attack cancer.

Legend and J&J will sell the drug in Greater China at 70-30 split in profit, and in all other countries in a 50-50 split in profit.

A decision on the CAR-T therapy was initially expected to come by the end of November after it was given priority review six months earlier, but this was extended by three months to allow sufficient time to review information submitted by J&J following an FDA request.

Legend, which initiated the development, signed an agreement with Janssen Biotech Inc, an arm of J&J, in 2017, to jointly develop and commercialize the drug, with Legend getting $350 million in upfront payment.

Besides the United States, Carvykti/Cilta-cel is also being reviewed by health authorities in Japan and Europe. A Marketing Authorisation Application (MAA) for the therapy in China is yet to be filed.