Pfizer Inc. (NYSE:PFE) announced that its chief executive Jeffrey Kindler has retired from the company and he will be replaced by Ian Read, currently head of its global biopharmaceutical operations.

The changeover comes at a time when the world's biggest drug maker, whose shares have fallen since 2006, is preparing to lose patent protection for its blockbuster Lipitor cholesterol drug. The patent on Lipitor, which accounted for $11.4 billion of Pfizer’s $50 billion sales last year, expires in November next year.

The sudden management change also comes a week after Merck & Co said its President Ken Frazier would succeed Richard Clark as its chief executive.

During Kindler's tenure as the chief executive for four and a half years, Pfizer's stock has fallen and to address those concerns, Kindler cut costs and resorted to major acquisitions, most notably $68 billion buy of Wyeth and $3.6 billion purchase of King Pharmaceuticals.

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, Kindler said of his decision to retire.

Kindler said he has concluded the time is right to turn the leadership over to Ian Read with the company about to complete a full year of operating Pfizer and Wyeth together.

Read, who joined Pfizer in 1978, has led Pfizer's worldwide biopharmaceutical businesses since 2006. The operations now comprises five global business units – Primary Care, Specialty Care, Oncology, Established Products and Emerging Markets – and accounts for around 85 percent of Pfizer's annual revenue.

Constance Horner, Pfizer's lead independent director said Kindler moved aggressively to drive change at Pfizer, including putting new, more focused and agile business units in place and due to his efforts the company is now a stronger, more diversified and more focused company.

Horner said Read has redefined the company's go to market approach with the creation of global business units and has demonstrated the need to drive change and innovation, and move quickly to adapt to competitive pressures.

Last month, Pfizer reported a 70 percent plunge in third quarter profit, as charges offset a 39 percent growth in revenue. The company and Bristol-Myers Squibb Co. also announced they have halted a late-stage study of their experimental blood-thinner apixaban because of safety concerns.

Shares of Pfizer closed Friday's trading at $16.72. The shares have been trading in the 52-week range of $14 to $20.36.