Swiss biotech company Actelion is buying CoTherix ( CTRX) for $420 million, giving it control of the only approved inhaled treatment for pulmonary arterial hypertension in the U.S.

The deal was unanimously approved by both companies' boards of directors. Actelion will make a cash tender offer of $13.50 for each CoTherix share, representing a 21% premium to the stock's closing price Friday.

Subject to regulatory clearance and other customary conditions, the deal is expected to close early in the first quarter. Holders of about 32% of CoTherix's stock have agreed with Actelion to tender their shares.

In 2007, Actelion expects sales of the inhaled PAH drug Ventavis to contribute more than $100 million to its product revenue. PAH is a chronic, life-threatening disease characterized by high blood pressure in the arteries between the heart and lungs, causing limited exercise capacity and a reduced life expectancy.

CoTherix shares were up 19.3% to $13.36 Monday.

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