The present invention relates to the duocarmycin-containing antibody-drug conjugate (ADC) trastuzumab vc-seco-DUBA (SYD985) for use in the treatment of trastuzumab emtansine (T-DM1) refractory HER2 IHC 3+ or HER2 IHC 2+/FISH positive cancer patients, particularly T-DM1 refractory breast cancer patients.