Potentiation of chlorin e6 photodynamic activity in vitro with peptide-based intracellular vehicles

Photodynamic therapy (PDT) is a targeted treatment modality where photosens itizers accumulate into cells and are selectively activated by light leadin g to the production of toxic species and cell death. Focusing the action of photosensitizers to a unique intracellular target may enhance their cytoto xicity. In this study, we demonstrate that the routing of the porphyrin-bas ed photosensitizer chlorin e(6), to the nucleus of cells can significantly alter its toxicity profile. The cellular localization of chlorin e(6) was a chieved by coupling the chromophore during solid-phase synthesis to a nucle us-directed linear peptide (Ce6-peptide) or a branched peptide (Ce6-loligom er) composed of eight identical arms displaying the sequence of the Ce6-pep tide. These constructs incorporated signals guiding their cytoplasmic uptak e and nuclear localization. Ce6-peptide and Ce6-loligomer displayed an enha nced photodynamic activity compared to unconjugated chlorin es, lowering th e observed CD50 values for CHO and RIF-1 cells by 1 or more orders of magni tude. The intracellular accumulation of Ce6-peptide and Ce6-loligomer was a ssessed by electron and confocal microscopy as well as by flow cytometry. C onstructs were internalized by cells within an hour and by 6 h, the release of active oxygen species could be observed within the nucleus of cells pre treated with Ce6-loligomer. These results highlight the utility of designin g peptides as vehicles for regulating the intracellular distribution of pho tosensitizers such as chlorin e(6) in order to maximize their efficacy in P DT.