title: ABCG2-mediated transport of photosensitizers: potential impact on photodynamic therapy.
abstract: 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.
authors.affiliation: Cancer Therapeutics Branch, Center for Cancer Research, National Institutes of Health, Bethesda, Maryland 20892, USA. robeyr@mail.nih.gov
authors.forename: Robert W
authors.initials: RW
authors.lastname: Robey
authors.forename: Kenneth
authors.initials: K
authors.lastname: Steadman
authors.forename: Orsolya
authors.initials: O
authors.lastname: Polgar
authors.forename: Susan E
authors.initials: SE
authors.lastname: Bates
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000378:metabolism; D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters / Q000037:antagonists & inhibitors / Q000502:physiology*; D002282:Adenocarcinoma, Bronchiolo-Alveolar / Q000188:drug therapy / Q000378:metabolism* / Q000473:pathology; D000622:Aminolevulinic Acid / Q000378:metabolism; D001692:Biological Transport; D049109:Cell Proliferation / Q000187:drug effects; D002734:Chlorophyll / Q000031:analogs & derivatives / Q000378:metabolism; D002735:Chlorophyllides; D005434:Flow Cytometry; D005453:Fluorescence; D006801:Humans; D007211:Indoles / Q000494:pharmacology; D007668:Kidney / Q000378:metabolism; D008175:Lung Neoplasms / Q000188:drug therapy / Q000378:metabolism* / Q000473:pathology; D027425:ATP-Binding Cassette, Sub-Family C Proteins / Q000378:metabolism; D009363:Neoplasm Proteins / Q000037:antagonists & inhibitors / Q000502:physiology*; D010778:Photochemotherapy; D017319:Photosensitizing Agents / Q000378:metabolism*; D011166:Porphyrins / Q000378:metabolism / Q000627:therapeutic use; D011524:Protoporphyrins / Q000627:therapeutic use; D014407:Tumor Cells, Cultured
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters; D000622:Aminolevulinic Acid; D002734:Chlorophyll; D002735:Chlorophyllides; D007211:Indoles; D027425:ATP-Binding Cassette, Sub-Family C Proteins; D009363:Neoplasm Proteins; D017319:Photosensitizing Agents; D011166:Porphyrins; D011524:Protoporphyrins; C116614:ABCG2 protein, human; C437342:tetra(3-hydroxyphenyl)porphine; C040298:pyropheophorbide a; C062985:phytochlorin; C028025:protoporphyrin IX; C010522:tryptoquivaline; C421109:multidrug resistance-associated protein 1