Differential sensitivity in cell lines to photodynamic therapy in combination with ABCG2 inhibition

BackgroundABCG2 is an ATP-binding cassette transporter protein which has a role in the regulation of endogenous protoporphyrin IX (PpIX) levels.ObjectiveTo understand the influence of ABCG2 on porphyrin-based photodynamic therapy (PDT) and fluorescence diagnosis (FD), we examined the role of endogenous ABCG2 in four human cell lines from the epidermis (HaCaT keratinocytes), oesophagus (OE19 adenocarcinoma), brain (SH-SY5Y neuroblastoma) and bladder (HT1197 carcinoma).MethodsCells were incubated with ALA or MAL in the presence or absence of the ABCG2 activity inhibitor Ko-143. Porphyrin accumulation was detected by spectrofluorimetric analysis and high performance liquid chromatography (HPLC) with porphyrin localisation observed by confocal laser scanning microscopy. PDT efficacy was assessed 24 h post irradiation (1.5 J/cm2 red light) by the neutral red (NR) assay.ResultsWe show cell-specific differences when Ko-143 was co-incubated with ALA or, in particular with, MAL. Enhanced PDT-induced cell kill was shown in HaCaT, OE19 and HT1197 cells, but not SH-SY5Y cells and could be explained by porphyrin accumulation and expression of ABCG2. We have also found that despite high levels of intracellular PpIX, the OE19 cells were protected from phototoxic cell death by PpIX compartmentalisation. This could be reversed by Ko-143.ConclusionThe results from this study show a possible cause of reduced sensitivity to ALA/MAL-PDT, with a potential solution to overcome this effect in certain tissue types. The potential to improve PDT with Ko-143 remains promising