The US Food and Drug Administration has approved Roche's much-touted antibody drug conjugate Kadcyla for patients with HER2-positive, late-stage breast cancer.

The drug, which was formerly known as T-DM1, combines the Swiss major's blockbuster Herceptin (trastuzumab) with partner ImmunoGen's chemotherapy DM1 (emtansine). It is intended for patients who were previously treated with Herceptin and taxanes.

Kadcyla is being approved with a boxed warning noting that it can cause liver and heart toxicity and death. The drug can also cause severe life-threatening birth defects "and pregnancy status should be verified prior to starting Kadcyla treatment," the FDA said.

Noting that it is the fourth approved drug that targets the HER2 protein - after Herceptin, GlaxoSmithKline's Tykerb/Tyverb (lapatinib) and Roche's own Perjeta (pertuzumab), approved last year -  Richard Pazdur, director of the Office of Hematology and Oncology Products in the FDA’s Center for Drug Evaluation and Research, noted that "Kadcyla delivers the drug to the cancer site to shrink the tumour, slow disease progression and prolong survival".

Roche's chief medical officer, Hal Barron, said Kadcyla represents "a completely new way to treat HER2-positive metastatic breast cancer", noting that in the Phase III trial called EMILIA, it helped patients live nearly six months longer". He added that “we currently have more than 25 ADCs in our pipeline and hope this promising approach will help us deliver more medicines to fight other cancers in the future".

Cowen & Co analyst Simos Simeonidis said the biggest news, given that approval was expected, was that the label allows for use in front-line HER-2 positive metastatic breast cancer. As for the black box warning, he said "this is neither a surprise, not an issue for the drug", as Herceptin, Tykerb and Perjeta, all come with similar warnings.

Kadcyla will cost $9,800 per month, which Mr Simeonidis said is higher than expected but "we don't expect to see significant payer pushback on pricing at launch, given the drug's efficacy and safety, and especially since Roche plans to offer patient assistance programmes".

The approval is also clearly good news for ImmunoGen as it triggers a $10.5 million milestone payment. The company will also earn royalties.