Federal regulators approved a highly anticipated breast cancer drug Friday, two and a half years after they sparked patient protests by refusing to grant it a speedy review.

The drug, which will be sold under the brand name Kadcyla, is considered a breakthrough treatment because it deploys a potent toxin that combines with an existing drug, Herceptin, to kill breast cancer cells. It could initially benefit as many as 15,000 patients who have a particularly aggressive form of the disease.