Described herein are compositions and methods of use of radionuclide-antibody conjugates (for RAIT) and drug-antibody conjugates (ADC). The combination of RAIT and ADC was more efficacious than either RAIT alone, ADC alone, or the sum of effects of RAIT and ADC. The unexpected synergy resulted in decreased tumor growth rate and increased survival, with a high incidence of tumor-free survival in Capan-1 human pancreatic cancer xenografts in nude mice.