Astellas Pharma’s bid to acquire CV Therapeutics is getting even more hostile with the news that it is suing the US firms over its ‘poison pill’ plan.

Last week, CVT again rejected a $1.1 billion unsolicited offer from the Japanese firm, having already turned down the same $16 per share offer made privately in November. Astellas has responded by launching its hostile bid, which represents a 41% premium to CVT’s closing price of $11.35 on January 26, the last day of trading before the offer was made public. It expires on March 27.

Astellas said that it still prefers to reach a negotiated agreement, the “refusal to engage with us regarding our proposal has left us with no alternative but to take our offer directly” to CVT’s stockholders. The Tokyo-based group added that the offer represents “immediate cash value that exceeds what the company could reasonably expect to deliver on it own, particularly given current uncertain market conditions and execution risks inherent in CVT’s standalone strategy”.

Then things got nastier and Astellas filed a lawsuit in a Delaware Court to prevent CVT from applying its recently-amended stockholders rights plan in a way that would prevent shareholders from tendering their shares into the offer. Astellas is also trying to stop CVT from claiming that a 2000 agreement between the two firms has been violated by the bid.

A month ago, CVT extended the expiration date of its shareholder rights plan from February 1, 2009 to February 1 next year. The move will act as a ‘poison pill’, forcing any bidder to negotiate with management and it allows existing shareholders to buy stock cheaply to fend off a hostile takeover.

CVT responded by advising stockholders to “take no action at this time” in response to Astellas’ offer, saying that it will advise them of its formal position regarding the bid within the next fortnight.