How much does trastuzumab emtansine actually help? The numbers explained

Trastuzumab emtansine, also called T-DM1 and sold as Kadcyla, has clear evidence of benefit in two situations. The most important is early HER2-positive breast cancer where some invasive cancer is still found at surgery after chemotherapy and HER2-targeted treatment given beforehand. This is called residual disease. In the large KATHERINE trial, people in this situation were given either fourteen cycles of trastuzumab emtansine or the standard trastuzumab alone. Trastuzumab emtansine roughly halved the risk of the cancer coming back or death over the first few years. Put simply, at three years about eighty-eight in every hundred people on trastuzumab emtansine were free of invasive cancer, compared with about seventy-seven in every hundred on trastuzumab. Longer follow-up, published about seven years after the trial began reporting, showed the gap held, and that trastuzumab emtansine also helped people live longer, with about five more people in every hundred alive. The second situation is HER2-positive breast cancer that has spread, where it was shown to delay growth and extend life compared with an older combination, although newer medicines such as trastuzumab deruxtecan are now often used earlier. These are averages from trials. They cannot predict what will happen to one person, but they give a fair sense of the size of the benefit your team is weighing against side effects.

Strongest evidence: residual disease after surgery

For early HER2-positive cancer with cancer left at surgery, it reduced recurrence by about half.

Longer life, not just longer control

Long-term results showed more people alive with trastuzumab emtansine than with trastuzumab.

A place in advanced disease too

It remains an option for spread HER2-positive cancer, usually after other HER2-targeted medicines.

Trial results are the best guide available, but they need careful reading.

Averages hide wide variation

Some people in the trial had a very high risk and some lower. Your personal benefit may be larger or smaller than the average.

Most people on trastuzumab also did well

Many people on trastuzumab alone stayed free of cancer. Trastuzumab emtansine adds to an already helpful treatment rather than making the difference between nothing and everything.

Side effects are part of the trade

Low platelets, liver changes, tiredness and tingling were more common, and some people stopped early.

What this page cannot tell you

It cannot calculate your own risk or say whether the benefit outweighs the side effects for you. That needs your oncologist, who knows your pathology report and health.

In KATHERINE, the benefit was seen across the main groups studied. Subgroup results are less reliable than the main result, but they offer some reassurance.

Hormone receptor status

People with both hormone receptor-positive and hormone receptor-negative cancers benefited.

Amount of residual cancer

Those with small amounts of residual cancer and those with more both appeared to gain, though the absolute gain is naturally larger when the starting risk is higher.

Earlier pertuzumab

Some people had pertuzumab before surgery, and they seemed to benefit as well, although numbers in that group were smaller.