Activist Starboard Value accused Pfizer of threatening litigation against two renegade former executives that initially were supportive of the investor's turnaround campaign at the pharmaceutical giant.

Pfizer Inc. company officials threatened legal action against two former top executives who had been working with Starboard Value to push for changes at the drugmaker, the activist investor alleged Thursday in a letter to the company’s board.

Former Pfizer CEO Ian Read and ex-CFO Frank D'Amelio said they will not be involved with activist investor Starboard Value, which reportedly wants the U.S. drug giant to make changes to turn its performance around.