Here, we present a novel prodrug particle approach for the encapsulation and release of the anticancer drug Lenvatinib, a tyrosine kinase inhibitor (TKI). The prodrug-particles contain extremely high amounts of prodrug up to 90 wt%, while the beneficial properties of the conjugated polyimidazole nanoparticles are retained. We show that the particles exhibit contrast for photoacoustic imaging in tissue and that they are degraded through ROS by activated macrophages in cell culture. Effective release is demonstrated by mass spectrometry. The covalent incorporation of TKI prodrugs into the polymer backbone presents a new approach for theranostic nanoparticles.