By Svea Herbst-Bayliss and Michael Erman

Oct 10 (Reuters) - The corporate clash between Pfizer and Starboard Value heated up on Thursday when the activist investment firm pressed the U.S. drugmaker to investigate actions by its board, after two former executives backed out of its campaign against the company.

Jeffrey Smith, who runs Starboard, pushed Pfizer's board on Thursday to set up a special committee to probe potential "coercive conduct" that may have prompted the two executives who had planned on working with the hedge fund to change their minds.

Starboard built a roughly $1-billion position in the company, which has a $167-billion market capitalization, and said former chief executive Ian Read and former chief financial officer Frank D'Amelio were concerned about the company's path and offered their assistance.

Starboard alleged the former executives reversed course because people within Pfizer or their representatives contacted Read and D'Amelio and "purportedly threatened to commence costly litigation against them."

In a letter to the Pfizer board, Smith wrote he has been told of recent events that put Read and D'Amelio under pressure. If they did not publicly support Pfizer CEO Albert Bourla they could risk having previous compensation clawed back and the cancellation of unvested performance stock units, the letter said.

Hours earlier, Guggenheim Securities, Pfizer's longtime banker, said Read and D'Amelio "decided not to be involved in the efforts of Starboard regarding Pfizer" adding that they fully support Bourla.

Pfizer did not immediately respond to a request for comment.

The dueling statements signal a widening gulf between the two sides before they have had their first meeting, which is scheduled for next Wednesday.

Lawyers and bankers said hedge funds often rely on help from other large investors during their campaigns but that pulling in former executives so publicly and so early was unusual.

Smith said in his letter he hopes to have "constructive engagement" with Pfizer.

Starboard has not provided details of what it wants Pfizer to do, noting only that the company's share price has dropped dramatically since 2019 when Bourla took over as CEO as Read's hand-picked successor. Over the last 52 weeks, the stock price has dropped 11% but it has climbed 4% in the last five days to trade at $29.60.

The hedge fund also pointed to Pfizer spending $70 billion on mergers, which has been criticized by some Wall Street analysts as problematic.

Pfizer is best known for its COVID-19 vaccine and before that, erectile-dysfunction drug Viagra and cholesterol-lowering drug Lipitor.

With lower demand for the vaccine after the pandemic, Pfizer's stock price has suffered. The company's stock is currently trading at about half its pandemic-era high.

The company has also contended with disappointing data for an experimental obesity drug, the weak launch of its respiratory syncytial virus vaccine, and pulling its sickle-cell-disease treatment Oxbryta due to deaths in clinical trials.

Starboard has previously waged campaigns against healthcare companies, including unsuccessfully trying to scuttle Bristol-Myers Squibb's bid to buy Celgene in 2019.

(Reporting by Svea Herbst-Bayliss and Michael Erman in New York with additional reporting by Bhanvi Satija in Bengaluru and 정품 시알리스 ; Editing by Anil D'Silva and Rod Nickel)