Antibody-drug-conjugates (ADC) are provided having a structure of : wherein L is PR3 ligand. The ADC has n drug moieties bound to the Trastuzumab antibody such that the ADC has a drug-to-antibody ratio (DAR) between 2 and 4 and the drug moieties are bound to the Trastuzumab antibody through cysteine (S) or lystine (Lys) residues. The disclosed ADCs are particularly useful in treating breast cancer.