Wellington said the acquisition was asking Bristol shareholders to accept too many risks while there were other ways to create value for shareholders. The investment company is also concerned that the completion of the acquisition will be more difficult than expected by the management of the company.
In addition, Wellington said the terms of the agreement provided for "Bristol shares for Celgene shareholders at a price well below the implied value of the assets".
Bristol-Myers said that since the announcement of the Celgene transaction on Jan. 3, its board of directors and management team have had numerous meetings and conversations with shareholders, including Wellington.
"We believe we are buying Celgene at an attractive price and that this transaction represents a unique and important opportunity to create sustainable value," Bristol-Myers said in a statement.
Bristol said he looked forward to his shareholder meeting in April to provide "the increased value that this combination creates".
The two companies began talking about the deal in September 2018, when Bristol contacted Celgene.
Celgene has also worked on experimental gene therapy, which is a highly competitive and potentially profitable area of the biotechnology industry. By buying Celgene, Bristol has access to anti-cancer drugs, a region Bristol is struggling to compete with Merck.
WATCH: Starboard Appoints 5 Directors to BMY's Board of Directors