Preempting a planned IPO by Peloton Therapeutics Inc., Merck & Co. Inc. has fielded a $2.2 billion buyout offer for the privately held drug developer, which has raised nearly $300 million to date. The proposal, including $1.05 billion in cash up front, would bring Merck at least two hypoxia inducible factor-2 alpha (HIF-2 alpha) antagonists, including Peloton's lead drug, PT-2977, a potential oral treatment for renal cell carcinoma (RCC).