Celgene Corp. (CELG) - Get Report  shares traded sharply lower Thursday after one of Bristol-Myers Squibb Co.'s  (BMY) - Get Report key investors said it $74 billion takeover bid for the cancer drug specialist was too risky and expensive.

Wellington Management, which holds an 8% stake in Bristol-Myers, added its concern for the proposed deal alongside activist investor Starboard Value LP, which is attempting to add supporters to the Bristol Myers board in order to prevent the deal from going through. Bristol Myers shareholders are set to vote on the takeover on April 12.

"Since announcing the Celgene transaction on January 3, our Board and management team have had numerous conversations and meetings with our stockholders across our ownership base, including Wellington," Bristol Myers said in an emailed statement to TheStreet. "We believe that we are acquiring Celgene at an attractive price, and that this transaction presents an important and unique opportunity to create sustainable value."

"We look forward to holding our special meeting of stockholders on April 12, 2019 and delivering the enhanced value this combination creates," Bristol Myers said.

Celgene shares were marked 6.4% lower in early Thursday trading and changing hands at $85.16 each. Bristol Myers shares were pegged 1.14% higher at $51.54 per share.

"While Wellington agrees that Bristol-Myers should be active in business development that secures differentiated science and broadens the future revenue base, Wellington does not believe that the Celgene transaction is an attractive path towards accomplishing this goal," the group said late Wednesday. 

Bristol-Myers offered Celgene investors one Bristol-Myers share and $50 in cash for each Celgene holding, as well as a special rights issue that will pay off if the merged group meets certain business targets, when it unveiled the deal on January 3. The deal values Celgene at $102.43 each, the companies said, a 53.7% premium to the previous session's closing price.

The combined group -- which will be 69% owned by Bristol-Myers -- would have a portfolio with nine drugs that generate more than $1 billion in sales, the companies said.

"While Wellington's decision is likely to prompt other funds who remain unconvinced by the upside of the acquisition to identify themselves in the coming weeks, ultimately we still expect the majority shareholder to support the deal at the upcoming April 12 vote," Barclays analysts, which maintained an overweight rating with a $102 price target on Celgene and an equal weight rating and a $53 target for Bristol Myers, argued Thursday.

"Beyond the lack of realistic, potential alternatives that could collectively provide a similar level of upside, we continue to see strong rationale for the acquisition," the bank said.