The present invention relates to therapeutic ADCs comprising SN-38 attached to an anti-Trop-2 antibody or antigen-binding antibody fragment. The ADC may be administered at a dosage of between 4 mg/kg and 18 mg/kg, preferably 4, 6, 8, 9, 10, 12, 16 or 18 mg/kg, most preferably 8 to 10 mg/kg. When administered at specified dosages and schedules, the ADC can reduce solid tumors in size, reduce or eliminate metastases and is effective to treat cancers resistant to standard therapies, such as radiation therapy, chemotherapy or immunotherapy. Preferably, the ADC is administered in combination with one or more other therapeutic agents, such as a PARP inhibitor, a microtubule inhibitor, a Bruton kinase inhibitor or a PI3K inhibitor. Most preferably, the ADC is of use for treating a Trop-2 expressing cancer, such as metastatic urothelial cancer.