We chose mouse tumor macrophages (J774A.1) for our cell studies because these macrophages take up particles and behave as TAMs. Furthermore, J774A.1 macrophages can be activated to produce ROS by exposure to lipopolysaccharides (LPS), to trigger the degradation of our nanoparticles at a defined point in time. Apart from photoacoustic contrast, our particles exhibit some degree of autofluorescence, allowing us to monitor their degradation using confocal laser scanning microscopy (CLSM). The macrophages are incubated with P1 particles for 3 h and then induced with LPS to trigger ROS production. We follow the degradation over the course of 2 days using CLSM (see Figure and Figure ). We observe that the nanoparticles are taken up well by the macrophages and are broken down inside the cells within a day (see Figure ). However, some particles are only taken up at a later stage of the experiment and we also observe residual red fluorescence of free particles after 2 days (see Figure ). To rule out the potential effect of photobleaching, we monitor the particles with medium and subject the particles to the same laser irradiation sequence, but without cells for 2 days. The fluorescence of the particles does not change significantly, indicating that the reduction in fluorescence signal during cell culture is due to degradation and not an effect of photobleaching (see Figure ). To investigate whether the degradation of particles takes place inside the cells, we analyze the cell lysate for the small molecular products. After degradation in cell culture over 2 days, the media is removed from the cells, the cells are lysed and the supernatant with the degradation products is subjected to LC-MS (see Figure ). Here, we detect the prodrug monomer (Lenvatinib + imidazole) and Lenvatinib with one and a half imidazole rings (Lenvatinib + 1.5 imidazole)