Pfizer appointed a former chief executive officer at Vanguard, the drug giant's biggest investor, to its board on Tuesday as it faces pressure from activist hedge fund Starboard Value.

Activist investor Starboard Value claims the pharma giant’s former CEO and CFO were “threatened” into backing current management.

Starboard’s stake comes amid a time of struggle at the drugmaker, which has begun to cut costs aggressively as demand for its Covid-19 medicines recedes.

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.

In a letter to Pfizer’s board, the activist investor claimed former CEO Ian Read and CFO Frank D’Amelio were pressured to abandon a Starboard-led attempt to change the company’s direction.