In the perpetual battle over U.S. prescription drug prices, pharmacy-benefit managers may have found a potent new weapon: biosimilars. Pharma investors look ill-prepared.

Biosimilars are analogous to generic versions of traditional small-molecule pharmaceuticals. A biosimilar isn’t an exact copy of the treatment it mimics and requires considerably more resources and expertise to develop than a generic drug. One reason for that is, unlike generics, biosimilars need to go through clinical trials to win approval from the Food...