US pharmaceutical maker Pfizer and biopharmaceutical company Global Blood Therapeutics (GBT) have entered a definitive agreement under which Pfizer will acquire GBT in an all-cash deal valued at $5.4 billion, they said on Monday. Pfizer said the acquisition will drive growth by enhancing its work in rare hematology while reinforcing the company’s commitment to sickle cell disease research and treatment. Under the terms of the transaction, Pfizer will acquire all the outstanding shares of California-based GBT for $68.50 per share in cash, for a total enterprise value of approximately $5.4 billi…