Takeda Pharmaceutical of Japan has announced plans to purchase US biotech firm Millennium Pharmaceuticals in a deal valued at nearly $9 billion.

The deal is the largest transaction involving a Japanese company so far this year and brings the Osaka-based Takeda a new therapeutic area — cancer treatments.

Millennium’s leading product is Velcade, a blood cancer treatment expected to generate revenues of as much as $345 million this year, according to analysts. Millennium also has seven compounds in clinical trials. Those compounds are designed to treat cancer, Crohn’s disease and multiple sclerosis.

The Millenium deal is the latest of a handful pulled off by Takeda in recent months.

In February, Takeda brokered a deal to purchase Amgen’s Japan unit and gain marketing rights to 13 Amgen drugs for Japan and Asia. In March, Takeda bought out partner Abbott Laboratories in its TAP Pharmaceuticals joint venture.

Takeda is Japan’s largest drugmaker in terms of revenue but is staring at looming patent expiration on its biggest-selling products – the proton pump inhibitor Prevacid which expires in 2009, and diabetes treatment Actos, which will see expiry in 2011.

Takeda has arranged to purchase Cambridge, MA-based Millennium for $25 a share, or a total of $8.8 billion. The price represents a sweet 53% premium to Wednesday’s $16.35 closing price for Millennium’s stock, the Wall Street Journal reported. The deal is conditional upon a majority of shareholders accepting the terms.