Pfizer said it still aims to complete the acquisition, valued at $68 billion when the merger was announced in January, by the end of the third quarter or in the fourth quarter.
Pfizer's biggest focus is small molecule drugs, meaning traditional oral medicines made from synthetic chemicals. Wyeth's strong suit is biotech -- injectable or intravenous drugs that are made in living cells and are adept at blocking or activating specific proteins linked to diseases. Wyeth is also a global leader in vaccines.
To maximize new opportunities in biopharmaceutical research, Pfizer will form two distinct research organizations after the merger, Pfizer said in a release.
Martin Mackay will give up his title as head of global research and development and instead lead a newly created PharmaTherapeutics Group focusing on discovery of small molecule drugs.
A separate BioTherapeutics Research Group, focusing on vaccines and biotech medicines, will be led by Mikael Dolsten, Wyeth's current head of research, Pfizer said.
Mackay and Dolsten will serve as presidents of their respective groups and report to Pfizer Chief Executive Jeff Kindler.
Ian Read, now president of global pharmaceutical operations at Pfizer, is slated to become group president of Pfizer BioPharmaceutical Businesses.
Other senior Wyeth researchers are also being reserved top posts at Pfizer.
Emilio Emini, a respected industry scientist who heads Wyeth vaccines and formerly ran Merck & Co's
Menelas Pangalos, Wyeth's current head of discovery who has spearheaded efforts to develop treatments for Alzheimer's disease, is slated to become Pfizer's chief scientific officer of neuroscience research.
Geno Germano, currently president of Wyeth's U.S. and Pharmaceuticals business units, will become president of Pfizer's Specialty Care and Vaccines business.
Cavan Redmond, now head of Wyeth's consumer healthcare business, will become group president of Pfizer Diversified Businesses.
Pfizer said it will have nine global healthcare businesses after the merger, including nutritionals and consumer units to be acquired from Wyeth.
Pfizer shares were down 1 percent at $13.58 on the New York Stock Exchange on Tuesday morning, in line with declines for the drug sector.
(Reporting by Ransdell Pierson; Editing by Steve Orlofsky and Matthew Lewis)