The phototoxic effect of zinc phthalocyanine on melanoma cells grown as a monolayer and three-dimensional multicellular spheroids

Melanoma is a dreadful skin malignancy caused by genetic mutations in melanocytes. The inherent unresponsiveness of melanoma cells to conventional therapies leads to uncontrollable tumour growth and alarming fatalities rates. Photodynamic therapy (PDT) is a novel therapeutic option for the eradication of malignant tumours and, compared to traditional therapies, is minimally invasive and exhibits increased efficacy. Currently, most PDT experiments are still conducted on two-dimensional (2-D) monoculture, which may not sufficiently mimic the physiological conditions and the three-dimensional (3-D) architecture of native tumours. Therefore, 3-D cell cultures serve as excellent models to replicate tumour tissue in terms of structural and functional properties. Commercially available A375 melanoma cells were cultivated as monolayers and 3-D tumour spheroids for this study. A375 cells were treated with zinc phthalocyanine tetrasulfonic acid (ZnPcS4) at varying doses (0.125-20 µM) and photoactivation was achieved using a 673 nm diode laser at a fluency of 10 J/cm². Photoactivated ZnPcS4 resulted in a dose-dependent reduction in cell proliferation, and increased cytotoxicity as determined by adenosine 5′-triphosphate (ATP), and lactate dehydrogenase (LDH) assay respectively. Morphological changes also confirmed the phototoxic effect of ZnPcS4, while cell death pathways were detected via annexin V-FITC-PI. The half-maximal inhibitory concentration (IC50) of ZnPcS4-mediated PDT on 3-D tumour spheroids was higher than that of monolayers. In conclusion, 3-D cell cultures are unresponsive to PDT compared to traditional monolayer cell cultures and can therefore provide more realistic data and reduce significant discrepancies between in vitro and in vivo studies for better prediction in clinical responses.