MedImmune is off the market.

Following months of prodding by its prominent shareholders, Gaithersburg-based MedImmune Inc. announced Monday that it has entered into a definitive agreement to be acquired by British pharmaceutical firm AstraZeneca PLC. Under the terms of the deal, AstraZeneca will purchase MedImmune for about $15.6 billion, or $58 per share. Two weeks ago, the local drug and vaccine company released information that its board of directors had hired Goldman Sachs, a global investment banker, to help the business find its best possible suitor.