U.S. drugmaker Bristol-Myers Squibb Co on Wednesday said it will pay $2.4 billion to acquire Medarex Inc , a biotechnology company that has been helping it develop a promising treatment for melanoma since 2005.

Medarex's expertise in making antibody-based drugs could help Bristol-Myers as it strives to regain its stature as one of the world's leading players in the oncology market, and to develop treatments for immunologic conditions such as arthritis, lupus and psoriasis.

The agreed offer of $16 a share represents a 90 percent premium to Medarex's closing share price on Wednesday of $8.40 per share on Nasdaq.

Bristol already owns a 2 percent stake in Medarex, through its four-year-old partnership with its neighbor in Princeton, New Jersey.

Medarex has developed so-called transgenic mice with human immune systems that are able to generate fully human antibodies that can be used as drugs.

The premium is not out of whack when you look at the scarcity value of this technology and the appetite for biotech companies right now, a source familiar with the deal told Reuters. It's a small deal in size but a large deal in terms of the potential scope they gain with this science.

Bristol-Myers and Medarex are developing one of the mouse-generated antibodies, called ipilimumab, as a treatment for patients in late stages of melanoma -- the most deadly form of skin cancer. There are now no highly effective treatments for it.

In three mid-stage clinical trials, 30 to 42 percent of patients with metastatic melanoma treated with ipilimumab were still alive after two years, which Bristol-Myers said established a survival benefit.

The companies are now conducting a larger late-stage trial, requested by U.S. regulators, designed to show an unequivocal survival benefit.

The partners are also conducting a mid-stage trial of ipilimumab among lung cancer patients and a late-stage study of it against advanced prostate cancer.

Medarex's technology has been used to develop several recently approved medicines, for which the company receives royalties.

They include Johnson & Johnson's Simponi (golimumab), approved in the United States for rheumatoid arthritis, and J&J's Stelara (ustekinumab), recently cleared in Europe to treat psoriasis. A Medarex antibody is also the basis of Ilaris, a Novartis AG treatment for children with an inflammatory disease caused by rare genetic mutations.

Medarex is one of the grand-daddies of the antibody research field, and it has finally come of age, said Steve Brozak, an analyst at WBB Securities.

Bristol-Myers spokesman Brian Henry said Medarex is testing 10 other drugs in clinical trials, some with other large drugmakers.

This deal will give us (full) rights to ipilimumab and broadly position Bristol-Myers for long-term leadership in biologics, Henry said, referring to complicated biotech drugs that are typically given by injection.

This deal clearly expands our opportunities in oncology and immunology, Henry said.

Medarex and a company called Abgenix were the first to develop transgenic mouse systems.

Abgenix was acquired by Amgen in December 2005 for $2.2 billion in cash plus the assumption of debt. Abgenix gave Amgen full ownership of panitumumab, now known as Vectibix, an approved treatment for colorectal cancer.

It also removed the necessity of Amgen to pay a royalty to Abgenix on future sales of denosumab, a bone drug that is now considered Amgen's big hope for the future.

Other companies that specialize in monoclonal antibodies include Regeneron Pharmaceuticals Inc , Seattle Genetics , which has multiple collaborations, and Immunogen Inc .

Bristol-Myers said its purchase of Medarex has been unanimously approved by the boards of both companies and is expected to close in late August.

(Reporting by Ransdell Pierson, Jessica Hall and Toni Clarke; Editing by Gary Hill and Carol Bishopric)

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