5-ALA stimulates the production of protoporphyrin IX in tumor cells, and is currently evaluated in clinical trials for post-operative and interstitial PDT. Both compound 15 and 5-ALA showed full water solubility in the high micromolar range, and we did not detect the formation of any aggregates with diameter size >10 nm. We also compared the in vitro toxicity of different concentrations of compound 15 and 5-ALA in U87 glioblastoma cells and observed that compound 15 was able to induce significant levels of cell death at similar doses as 5-ALA and with complete lack of toxicity in HCN-2 cells (Figure ). Taken together, these results highlight the biocompatibility and potential utility of compound 15 for light-controlled ablation of cancer cells with comparable phototoxicity to PDT agents under clinical evaluation. ). The number of cancer cells (Figure ). Finally, we examined whether the in vivo treatment with compound 15 caused side effects in zebrafish. First, we compared the morphology of treated and untreated zebrafish, with no evident abnormalities between the two groups (Figure ). Next, we examined their behavior and vital signs after the PDT treatment. Larvae from control and treated groups showed similar swimming behavior, as shown by the tracks of individual larva (n≥10 per group, Figure ) with no significant differences in distance covered or velocity (Figure , Videos S1 and S2). Heartbeat monitoring in both groups also revealed that treatment with compound 15 and light was safe and did not cause any evident deleterious side effects (Videos S3 and S4). In summary, our data indicate that photosensitive metabolites represent a new viable chemical strategy to harness the aberrant early metabolic features of aggressive cancer cells and to open avenues for minimally-invasive therapeutic strategies to safely eliminate microtumors.