By Atsuko Fukase And Eric Pfanner 

TOKYO--Otsuka Pharmaceutical Co. agreed to buy Avanir Pharmaceuticals Inc. of the U.S. for $3.5 billion, the latest large deal by a Japanese drug maker looking to expand overseas.

The deal gives Otsuka, maker of the schizophrenia and depression drug Abilify, access to more treatments for central-nervous-system diseases. California-based Avanir sells Nuedexta, a treatment for a condition known as pseudobulbar affect that causes sudden and involuntary outbursts in patients with neurological diseases such as amyotrophic lateral sclerosis.

Otsuka Pharmaceutical, part of Otsuka Holdings Co., said it would pay $17 a share in cash for Nasdaq-traded Avanir. The deal is the largest purchase by Otsuka Holdings, which listed its shares in an initial public offering in Tokyo in 2010, giving it more resources for acquisitions. Like other Japanese pharmaceutical companies, Otsuka has been looking abroad as growth slows in the domestic market, the second-largest after the U.S.

Otsuka Holdings President Tatsuo Higuchi said at a news conference that Avanir's strengths in neurologic diseases would complement Otsuka's experience in the area of mental illnesses. "We'll try to maximize the integration effect," he said.

Avanir Chief Executive Keith Katkin said in a written statement, "Together, our organizations will be able to more rapidly develop and commercialize needed medications." Avanir, which was founded in 1988 as Lidak Pharmaceuticals, is conducting trials of medicines for depression, migraine headaches and other neurological diseases.

Some pharmaceutical companies have been reducing their involvement in those areas because of the large number of generic drugs available and the long period required for clinical trials and regulatory approval. But Otsuka is trying to build on its existing business and find new products to make up for Abilify, whose patent is set to expire in 2015 according to the company.

Otsuka has been bulking up in recent years and bought U.S. cancer-drug maker Astex Pharmaceuticals Inc. last year for about $886 million.

"From our experience, we value creativity which can be generated from cultural diversity and different ways of thinking," Mr. Higuchi said. "I learned it from the late Chairman Otsuka," he added, a reference to longtime executive Akihiko Otsuka, who died last Friday at the age of 77. Mr. Otsuka, a member of the founding family, was involved in the development of the company's food and beverage arm, helping to create the Pocari Sweat sports drink and Calorie Mate energy bar.

Mr. Higuchi said the yen's recent fall didn't affect Otsuka's decision-making on overseas acquisitions. "We'll make a move when it should be done," Mr. Higuchi said, adding that Otsuka would use funds it holds in dollars for part of the purchase.

The acquisition of Avanir is the latest among sizable overseas deals by Japanese pharmaceutical companies, which have included Takeda Pharmaceutical Co.'s purchases of Millennium Pharmaceuticals and Nycomed, Astellas Pharma Inc.'s acquisition of OSI Pharmaceuticals and Daiichi Sankyo Co.'s deal for a controlling stake in Ranbaxy Laboratories Ltd.

The deals have given the Japanese companies broader access to overseas markets, but some have run into trouble. Daiichi sold its Ranbaxy stake this year, six years after the acquisition, following a series of regulatory problems. Takeda has faced questions from shareholders over its integration of Nycomed and Millennium. The company recently brought in a French executive as president and has said it would seek to break down internal silos.

Anjie Zheng contributed to this article.

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