Immunotherapy can help the immune system attack cancer cells, but it often works poorly for liver metastases, where immune responses are naturally suppressed. This study proposes a new way to deliver immune-stimulating drugs directly into liver tumors via the tumor’s blood supply, using drug carriers that slowly release the drug into the tumor over time (immunoembolization). We developed and tested several different drug carriers, including microspheres and emulsions, and showed that the immunotherapy drug is released into the tumor over days to weeks, resulting in liver tumor shrinkage in pigs. Extended-release immunoembolization has the potential to maximize immune activation in liver metastases (in situ vaccination) while reducing autoimmune side effects.