AstraZeneca has signed a deal which will see Daiichi Sankyo for the co-promotion and supply of its proton pump inhibitor Nexium in Japan.

Under the terms of the agreement, the Anglo-Swedish drugmaker and Daiichi Sankyo will co-promote the product after it is approved for use in Japan. AstraZeneca first submitted a regulatory application for Nexium (esomeprazole) in the country in February this year.

It will then manufacture and develop the product and Daiichi Sankyo will be responsible for its distribution. The Tokyo-based firm is making an initial payment of $100 million to AstraZeneca and will pay further undisclosed sums on approval and when certain sales targets are achieved.

Nexium was the jewel in AstraZeneca's crown for a number of years and is still a big-seller. Approved in more than 120 countries for the treatment of gastroesophageal reflux disease, its third-quarter sales edged up 2% to $1.24 billion.

AstraZeneca's biggest seller nowadays is Crestor (rosuvastatin) and earlier this week it filed a lawsuit in Delaware, USA to stop Watson Pharmaceuticals launching a generic version of the cholesterol blockbuster.

In the summer, the drugs major had won a court case reaffirmed the validity of its patent for Crestor but Watson subsequently  filed a subsequent New Drug Application seeking approval to market rosuvastatin zinc tablets, a new salt version of Crestor.

AstraZeneca is arguing that Watson's NDA is based on clinical trials of Crestor to prove safety and efficacy and is looking to block the generic company's product prior to patent expiration on March 30, 2013 or "until final resolution of the matter before the court".

For the third quarter, Crestor had US sales of $626 million, up 20%.