Provided are drug delivery compositions and devices useful for the treatment and/or prevention of cancer and metastatic tumors. For example, a drug delivery device is provided that comprises a biodegradable scaffold carrying one or more anti-cancer therapeutic agents that activate the innate immune system (e.g., STING agonists) and/or the adaptive immune system (e.g., anti-PD-1 antibodies). The compositions and devices may include a cytokine (e.g., IL-15 superagonist). The drug delivery device can be implanted in the void volume of a resected tumor to prevent tumor regrowth and tumor metastasis. Also provided are methods of making the drug delivery compositions and devices as well as kits containing materials to provide the compositions and devices.