Bristol-Myers Squibb Co. agreed to buy cancer drug-maker Mirati Therapeutics Inc. for $4.8 billion on Sunday, a price that disappointed investors after reported interest from rival Sanofi.

Pharma companies remain fixated on cancer drugs — the sector had $196 billion in global sales last year, according to IQVIA — even as investors become more optimistic about innovative products like those used to treat obesity. However, Mirati's purchase price illustrates a difficult operating environment for biotechnology companies, according to Mizuho analyst Jared Holz.

"A take-under really not what the biotech sector needed," Holz said in the note. A bid above Bristol's is unlikely as "the asset was probably in play and explored by others."