In photodynamic therapy (PDT), a tumor-selective photosensitizer is administered followed by activation of the photosensitizer by exposure to a light source of a given wavelength. This, in turn, generates reactive oxygen species that induce cellular apoptosis and necrosis in tumor tissue. Based on our earlier finding that the photosensitizer pheophorbide a is an ABCG2 substrate, we explored the ability of ABCG2 to transport photosensitizers with a structure similar to that of pheophorbide a. ABCG2-overexpressing NCI-H1650 MX50 bronchoalveolar carcinoma cells were found to have reduced intracellular accumulation of pyropheophorbide a methyl ester and chlorin e6 compared to parental cells as measured by flow cytometry. The ABCG2 inhibitor fumitremorgin C was found to abrogate ABCG2-mediated transport. Intracellular fluorescence of hematoporphyrin IX, meso-tetra(3-hydroxyphenyl)porphyrin, and meso-tetra(3-hydroxyphenyl)chlorin was not substantially affected by ABCG2. ABCG2-overexpressing cells also displayed decreased intracellular fluorescence of protoporphyrin IX generated by exogenous application of 5-aminolevulinic acid. Mutations at amino acid 482 in the ABCG2 protein known to affect substrate specificity were not found to impact transport of the photosensitizers. In cytotoxicity assays, ABCG2-transfected HEK-293 cells were 11-fold, 30-fold, 4-fold, and >7-fold resistant to PDT with pheophorbide a, pyropheophorbide a methyl ester, chlorin e6, and 5-aminolevulinic acid, respectively. ABCG2-transfected cells were not resistant to PDT with meso-tetra(3-hydroxyphenyl) chlorin. Neither multidrug resistance-associated protein 1 expression nor P-glycoprotein expression appreciably decreased the intracellular fluorescence of any of the photosensitizers examined as determined by flow cytometry. The results presented here implicate ABCG2 as a possible cause for cellular resistance to photodynamic therapy.
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