AstraZeneca is to pay $4bn to buy a 55% stake in Acerta Pharma

British pharmaceuticals group AstraZeneca said Thursday it had agreed to buy a 55 percent stake in Acerta Pharma, which is developing a medicine for blood cancers.

AstraZeneca said it will pay $2.5 billion upfront, then a further $1.5 billion once Dutch- and US-based Acerta’s experimental drug acalabrutinib obtains regulatory approval from US authorities or else by the end of 2018.

Acalabrutinib has shown promise in treating leukemia and also auto-immune diseases such as lupus.

AstraZeneca, which rejected a takeover bid from US giant Pfizer last year, is seeking to rebuild its portfolio of exclusive medicines amid intense competition from generic drugmakers.

Depending on whether it obtains shareholders' approval, AstraZeneca will have the option to buy the remaining 45 percent of Acerta for a reported $3 billion assuming acalabrutinib wins clearance.

The British group is pushing several new innovative treatments for respiratory diseases as well as diabetes and cancers.