A Johnson & Johnson banner is displayed on the front of the New York Stock Exchange (NYSE) in New York City, in New York City, U.S., December 5, 2023. REUTERS/Brendan McDermid

(Reuters) -Johnson & Johnson is in discussions to buy Protagonist Therapeutics, a source familiar with the matter told Reuters.

Shares of Protagonist surged more than 30% in afternoon trading. The company had a market capitalization of $4.2 billion as of Thursday's close.

The two companies are working on the development of an oral treatment, icotrokinra, for immune diseases, including plaque psoriasis and ulcerative colitis, with J&J holding the exclusive rights to commercialize the product.

Leerink Partners analysts believe icotrokinra could be "one of the most impactful immunology drug launches of this decade," and estimate its peak global sales could reach $9.5 billion.

The potential deal comes at a time J&J is looking to bolster its drug pipeline. Its top-selling immune-disease drug Stelara has been losing market share to lower-cost biosimilars, and the company has warned of accelerating sales declines.

Protagonist is also developing rusfertide, a late-stage blood cancer drug candidate, with Takeda Pharmaceutical.

J&J did not immediately respond to Reuters' request for comment and Protagonist said it does not provide comment on speculations or rumors.

The Wall Street Journal reported the potential deal news earlier in the day.

(Reporting by Sabrina Valle in New York and Kamal Choudhury in Bengaluru; Editing by Anil D'Silva, Tasim Zahid and Shinjini Ganguli)