Photodynamic performance of zinc phthalocyanine in HeLa cells: A comparison between DPCC liposomes and BSA as delivery systems

- BSA allows specific uptake of ZnPc, which retains its fluorescence lifetime and PDT activity.
- DPPC transport ZnPc in a less specific manner and impacts its fluorescence lifetime and PDT activity.
- When transported by BSA, ZnPc induces cell apoptosis, while DPPC leads to necrosis.

Comparable intracellular concentrations (≈ 30 pmol/106 cells) of bovine serum albumin-ZnPc (BSA) adduct outperformed dipalmitoyl-phosphatidyl-choline (DPPC) liposomes containing ZnPc at photodynamic-killing of human cervical cancer cells (HeLa) after only 15 min of irradiation using red light (λ > 620 nm, 30 W/cm2). This result could not be simply explained in terms of dye aggregation within the carrier, since in the liposomes the dye was considerably less aggregated than in bovine serum albumin, formulation that was capable to induce cell apoptosis upon red light exposure. Thus, using specific organelle staining, our cumulative data points towards intrinsic differences in intra-cellular localization depending on the cargo vehicle used, being ZnPc:BSA preferentially located in the near vicinity of the nucleus and in the Golgi structures, while the liposomal formulation ZnPc:DPPC was preferentially located in cellular membrane and cytoplasm. In addition to those differences, using real-time advanced fluorescence lifetime imaging of HeLa cells loaded with the photosensitizer contained in the different vehicles, we have found that only for the ZnPc:BSA formulation, there was no significant changes in the fluorescence lifetime of the photosensitizer inside the cells. This contrasts with the in situ ≈ two-fold reduction of the fluorescence lifetime measured for the liposomal ZnPc formulation. Those observations point towards the superiority of the protein to preserve dye aggregation, and its photochemical activity, post-cell uptake, demonstrating the pivotal role of the delivery vehicle at determining the ultimate fate of a photosensitizer.