AstraZeneca agreed to buy a 55-percent stake in Acerta Pharma BV for $4 billion that will give the UK drugmaker a potential blockbuster medicine for blood cancers as well as diseases in which the body attacks itself. AstraZeneca will pay $2.5 billion upfront, and another $1.5 billion when closely held Acerta's experimental drug acalabrutinib is approved by a regulator or at the end of 2018, whichever comes first, the London-based drugmaker said in a statement Thursday.