Japan's Astellas Pharma has filed a lawsuit in Tokyo against Pfizer over rights to market blockbuster cholesterol drug Lipitor in Japan.

Astellas currently sells the drug under a marketing agreement that it claims gives it the right to sell Lipitor (atorvastatin) in Japan until July 2016, although Pfizer maintains the agreement will in fact come to an end in 2011, when the substance patent on atorvastatin in Japan expires.

Formulation and polymorph patents could keep the product free from generic competition in Japan until 2016, and Astellas insists it should be able to keep selling the drug until that time.

At stake is five years’ revenue from the world’s top-selling pharmaceutical, which had sales of more than $12 billion in 2005 and could knock on the door of $13 billion this year.

Astellas licensed Lipitor from Warner-Lambert, before it merged with Pfizer, and launched the product onto the Japanese market in 2000.

Meanwhile, Astellas also reported first half profit was down nearly 10% at 60.7 billion yen ($52m), because of a $300 million charge related to purchase of two developmental drugs to treat anaemia.

Astellas paid $300 million to US company FibroGen to license rights to the two drugs - FG-2216 and FG-4592 – and will pay FibroGen as much as $465 million during the course of development.

Sales at the drugmaker rose 5% to 448 billion yen, with revenues from Lipitor up 9% to 48 billion yen. The star performer in the company’s domestic business was blood pressure drug Micardis (telmisartan), up 44% to 24 billion yen, while overseas flagship immunosuppressant Prograf (tacrolimus) rose 14% to $719 million.