(Adds analyst comment, background; updates shares)
By Bill Berkrot
May 16 (Reuters) - Pfizer Inc, which is in the process of trying to acquire British rival AstraZeneca for more than $100 billion, on Friday said it planned to file for U.S. approval for palbociclib, its experimental drug for advanced breast cancer, in the third quarter.
The largest U.S. drugmaker said it made the decision to submit its application to the Food and Drug Administration following discussions with the regulatory agency about the results of a midstage clinical trial.
Morningstar analyst Damien Conover said he believed Pfizer may be looking to strengthen its position in negotiations with AstraZeneca by showcasing its own cancer drugs, although he had considered an FDA filing based on the Phase II data a strong possibility.
The strength of AstraZeneca's portfolio of experimental cancer medicines has been cited as one of the reasons Pfizer wants to acquire the company, and has been a centerpiece of the case Astra has made in seeking a significantly higher offer.
AstraZeneca has rejected a $106 billion approach from its larger U.S. rival. Pfizer is widely expected to raise its cash and stock bid for Astra ahead of a May 26 deadline under UK law.
Palbociclib had received the FDA's new breakthrough designation given to help speed up the approval process for potentially important new medicines.
Pfizer presented data from the trial at a cancer meeting last month. Patients in the study who took the experimental Pfizer medicine plus the hormone drug letrozole went 20.2 months on average before their cancer began to worsen, compared with 10.2 months for those that received only letrozole.
Palbociclib is considered to be one of the most important medicines in Pfizer's developmental pipeline. A rival drug is also being developed by Eli Lilly and Co, which several analysts have said appears promising.
Morningstar's Conover forecast peak sales of more than $2 billion for palbociclib and gives the drug a 70 percent chance of approval based on the current data.
He also noted that potential rival drugs from Lilly and Swiss drugmaker Novartis lag behind Pfizer in the development process.
"Pfizer will have the first mover advantage, which is pretty significant, especially in cancer," Conover said.
Pfizer shares were up 0.5 percent at $29.19 in afternoon trading on the New York Stock Exchange. (Reporting by Bill Berkrot, editing by G Crosse)