Today, Kadcyla (ado-trastuzumab emtansine, or T-DM1) received a long-awaited approval from the Food and Drug Administration (FDA) to treat patients with metastatic, HER2-positive breast cancer who have previously been treated with Herceptin (trastuzumab) or taxanes, a class of drugs that includes paclitaxel and docetaxel.Kadcyla belongs to a new class of drugs called antibody-drug conjugates and works by fusing the powerful chemotherapy drug emtansine to the antibody Herceptin, which targets HER2 receptors on cancer cells.The approval was based on results from the phase 3 EMILIA trial, which enrolled 991 patients who had previously been treated with Herceptin and a taxane-based chemotherapy. Patients were randomly assigned to either receive Kadcyla, or the current standard--a combination of Xeloda (capecitabine) and lapatinib. Those in the Kadcyla arm had a median overall survival of 30.9 months compared with 25.1 months for those in the combination arm. Patients receiving Kadcyla also had improved median progression-free survival: 9.6 months compared with 6.4. Furthermore, patients on Kadcyla experienced fewer severe side effects (grade 3 or higher) than those on Xeloda and lapatinib (43.1 percent compared with 59.2 percent).Common side effects included low platelet count, nausea, fatigue and joint or muscle pain. This drug also carries a boxed warning (the highest warning the FDA can give) advising doctors and patients that in some instances, this drug can cause liver toxicity, heart toxicity and death. It can also lead to severe birth defects.The drug is expected to be available within two weeks, and the estimated cost will be $9,800 a month, or $94,000 for a course of treatment. Genentech, the drug's manufacturer, is planning to begin patient assistance programs to help patients that might not be able to afford Kadcyla.For more information on Kadcyla, visit kadcyla.com or call 877-436-3683 . And for more information on treating HER2 breast cancer, read our web-exclusive article here.