New data on a breast cancer treatment from AstraZeneca and the Japanese drug maker Daiichi Sankyo brought a standing ovation from cancer doctors attending their annual meeting in Chicago on Sunday. And with good reason.

The drug, Enhertu, enabled women with advanced breast cancer to live six months longer than others treated with conventional chemotherapy. Oncologists call the results “practice-changing” for metastatic breast cancer.

And the good news extends well beyond breast cancer. The evidence suggests the drug could one day be used to treat many other types of tumours, opening broad opportunities for Enhertu and other drugs like it...