Confluence dependent resistance to photo-activated hypericin in HeLa cells

Hypericin is a natural photo-active pigment produced by plants of the genus Hypericum. The compound exhibits a potent photocytotoxic activity in vitro and in vivo. Using HeLa cells we further investigated whether the photocyt otoxic in vitro effect of hypericin is influenced by the cell density. It w as demonstrated that hypericin-induced photocytotoxicity in HeLa cells depe nds significantly on the amount of cells in culture, as low cell density cu ltures were more responsive to photodynamic therapy (PDT) than confluent or hyperconfluent cell cultures. This confluence dependent resistance (CDR) c an be explained in terms of a decrease in hypericin cellular uptake. The ph enomenon is not caused by the depletion of hypericin from the medium by hig h-density cell cultures since the extra-cellular availability of the drug i s not altered by the increase in cell density and does not appear to be a l imiting factor. Importantly, since confluent or hyperconfluent cell culture s can better mimic the high cell density of the solid tumour, CDR should be taken into consideration whenever resistance of solid tumours to PDT with hypericin is observed.