The chief executive officer of Pfizer Inc,, the world's largest drugmaker, retired in an unexpected move to recharge his batteries after completing the mega merger with rival Wyeth.

Jeffrey Kindler, 55, is being replaced by the global head of pharmaceuticals, Ian Read, who is 57, Pfizer said.

In a statement issued by the company late on Sunday, Kindler said: The combination of meeting the requirements of our many stakeholders around the world and the 24/7 nature of my responsibilities, has made this period extremely demanding on me personally.

Read, who joined Pfizer in 1978, has led Pfizer's worldwide pharmaceuticals business since 2006. The business includes primary care, specialty care, oncology, established products and emerging markets and accounts for about 85 percent of Pfizer's annual revenue. He has been responsible for more than 40,000 employees.

Kindler, previously the company's general counsel with little drug industry experience, was a surprise choice when he was named to the drugmaker's top job in 2006.

He orchestrated Pfizer's mega merger with rival Wyeth, but he retires before the drugmaker confronts a significant challenge -- the U.S. patent expiration of the blockbuster cholesterol fighter Lipitor.

I am excited at the opportunity to recharge my batteries, spend some rare time with my family, and prepare for the next challenge in my career, Kindler said in the statement.

Pfizer said its board will elect a non-executive chairman from its current membership at its next regularly scheduled meeting that will take place within the next two weeks.

Pfizer's leadership change comes only days after U.S. rival Merck & Co announced its own CEO change. However, Merck's promotion of president Ken Frazier to succeed Richard Clark as CEO was well-telegraphed and widely expected.

Kindler's resignation comes after the surprise departure of Pfizer research executive Martin Mackay in May.

Mackay had been co-head of research with Mikael Dolsten, former head of research for Wyeth, after Pfizer acquired Wyeth for $67 billion in October 2009.

But Mackay unexpectedly jumped ship just seven months later, to head research at rival AstraZeneca Plc, leaving Dolsten in command of research for the world's largest drugmaker.

Following Mackay's unexpected departure, Kindler's resignation could undermine a recent rebound in faith among analysts in the company's prospects.

(Editing by Anshuman Daga)

Pfizer Merck & Co.