Self-aggregation of verteporfin in glioblastoma multiforme cells: a static and time-resolved fluorescence study

Monomeric state of photosensitizer molecules can induce biological responses in a variety of neoplastic tissues in photodynamic therapy, but the self-aggregate forms are mostly inactive. Although verteporfin has showed a history of success as a photosensitizer, its self-aggregation inside cancer cells has not been fully described. This study aimed to elucidate the self-aggregation of verteporfin in glioblastoma multiforme cells at various concentrations, using steady-state/time-resolved fluorescence and fluorescence lifetime imaging microscopy based on our previous approach (Calori and Tedesco, 2020). According to the findings, low internalization mainly led to the monomeric state of verteporfin inside cells, with a fluorescence lifetime of ~6.0 ns. J-type fluorescent self-aggregates of verteporfin were found in intermediate conditions, in coexistence with monomers, presenting a lower fluorescence lifetime of ~2.5 ns. At high internalization of verteporfin, the amount of self-aggregates increased and the monomer's lifetime decreased, showing the dependence of the aggregation of verteporfin on concentration. The findings suggest that the amount of monomers of verteporfin inside cells reaches a saturation limit. This limitation could be a critical factor in photodynamic therapy that uses poor intracellular soluble photosensitizers.