Investors welcomed the sudden departure of Pfizer Inc CEO Jeffrey Kindler by pushing up the stock in the hopes that a new leader will more act more aggressively to revive the giant pharmaceutical company.

While questions remain about the abruptness of Kindler's retirement, announced on Sunday evening, Pfizer shares rose 1.1 percent on Monday as investors bet that new CEO Ian Read will push through sales, cost cuts, share buybacks or higher dividends for the world's largest drugmaker.

Pfizer also may seek more acquisitions as it confronts a daunting milestone in November 2011 -- the U.S. patent expiration of the top-selling Lipitor cholesterol drug, analysts said.

People are hopeful there will be some more aggressive restructuring that takes place within Pfizer, said Edward Jones analyst Linda Bannister. We're up against the patent expiration of Lipitor...It's right around the corner.

Kindler, 55, said in a statement issued by Pfizer that the job had been extremely demanding and that he wanted to recharge my batteries.

Since Kindler became CEO in July 2006, Pfizer's shares have fallen roughly 27 percent compared with a 10 percent decline for the NYSE Arca Pharmaceutical index of large U.S. and European drugmakers.

When a company's shares underperform, shareholders express their frustrations to the board, and the board expresses its frustrations by making management changes, said Les Funtleyder, portfolio manager of the Miller Tabak Healthcare Transformation Fund, which does not hold Pfizer shares.

Goldman Sachs analyst Jami Rubin applauded the change, saying she has long argued that Pfizer should be more aggressive in achieving greater efficiencies in both its $28.5 billion operating expenses base as well as its massive balance sheet and portfolio of various businesses, some of which should be divested.

We are delighted to see the board taking action as Pfizer's share price continues to underperform amid a flurry of questions about strategic direction, Rubin said in research note.

Rubin said Pfizer should remove its 2012 forecast, which she said was set unrealistically high and has been a source of anxiety.

Pfizer's 2012 forecast includes the first full year of impact from losing exclusive U.S. rights to Lipitor, which had sales of $11.4 billion last year, and has implied somewhat stable results despite the Lipitor loss.

Kindler's departure comes more than a year after the drugmaker completed the signature move of his tenure -- the $67 billion acquisition of rival Wyeth.

That acquisition, which brought Pfizer more access to biotech drugs and vaccines as well as cost cuts, was intended to help Pfizer maneuver through the decline of Lipitor.

But the deal has failed to spur stock gains. Pfizer shares have fallen 5.2 percent since the company bought Wyeth in October 2009. By contrast, shares of Merck & Co have jumped 15 percent since it clinched its big purchase of Schering-Plough Corp, in November 2009.

JP Morgan analyst Chris Schott said the CEO change could lead to more aggressive actions at Pfizer, including share repurchases or dividend increases, as well as more business development or divestitures.

While a CEO transition in the midst of a major merger integration will likely create added uncertainty with the story, the key question, in our view, remains on the changes this transition will bring to the Pfizer story, Schott said in a research note.

Kindler drove Pfizer to become a leaner organization, and the Wyeth deal helped diversify the company's revenue base and decrease its dependence on Lipitor, said Deutsche Bank analyst Barbara Ryan.

Despite these accomplishments, Jeff was not able to develop a sufficient level of support from the investment community, Ryan said in a research note. She said his departure likely resulted from a mutual assessment on the part of the board and Kindler.

Pfizer's appointment of Read, who joined the company in 1978, was a logical one, Ryan said, noting that he is well-regarded within the organization.

Rubin of Goldman Sachs called Read a seasoned executive with over 20 years of experience running many different regions and businesses in pharma.

Some investors may be concerned that the new CEO may not shake up the company to the degree that some would like to see, but he has the ability to focus the strategy and get the ship going in the right direction, Rubin said.

But Read can only do so much restructuring, and the company will need some promising experimental drugs to succeed, Edward Jones' Bannister said, citing drugs in development for pain, rheumatoid arthritis, blood clots and Alzheimer's disease.

In order for the stock to really start to move on a sustainable basis, it's going to have to come from positive results from their pipeline, Bannister said.

Pfizer's shares were up 33 cents at $17.06 in morning trading on the New York Stock Exchange.

(Reporting by Lewis Krauskopf and Ransdell Pierson. Editing by Maureen Bavdek and Robert MacMillan)

Pfizer Switch