AstraZeneca (AZN.L) said on Thursday it had agreed to buy 55 percent stake of privately held biotech firm Acerta Pharma for $4 billion to give it access to a new kind of drug for fighting blood cancers. AstraZeneca will pay $2.5 billion upfront, with a further $1.5 billion paid either on receipt of the first regulatory approval for the medicine acalabrutinib or at the end of 2018, depending on which comes first. AstraZeneca believes the new medicine, which is now in final-stage testing, could sell more than $5 billion a year.