ABCG2-mediated transport of photosensitizers: potential impact on photodynamic therapy

Cancer Biol Ther. 2005 Feb;4(2):187-94. Epub 2005 Feb 8.


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.

MeSH terms

  • ATP Binding Cassette Transporter, Subfamily B, Member 1 / metabolism
  • ATP Binding Cassette Transporter, Subfamily G, Member 2
  • ATP-Binding Cassette Transporters / antagonists & inhibitors
  • ATP-Binding Cassette Transporters / physiology*
  • Adenocarcinoma, Bronchiolo-Alveolar / drug therapy
  • Adenocarcinoma, Bronchiolo-Alveolar / metabolism*
  • Adenocarcinoma, Bronchiolo-Alveolar / pathology
  • Aminolevulinic Acid / metabolism
  • Biological Transport
  • Cell Proliferation / drug effects
  • Chlorophyll / analogs & derivatives
  • Chlorophyll / metabolism
  • Chlorophyllides
  • Flow Cytometry
  • Fluorescence
  • Humans
  • Indoles / pharmacology
  • Kidney / metabolism
  • Lung Neoplasms / drug therapy
  • Lung Neoplasms / metabolism*
  • Lung Neoplasms / pathology
  • Multidrug Resistance-Associated Proteins / metabolism
  • Neoplasm Proteins / antagonists & inhibitors
  • Neoplasm Proteins / physiology*
  • Photochemotherapy*
  • Photosensitizing Agents / metabolism*
  • Porphyrins / metabolism
  • Porphyrins / therapeutic use
  • Protoporphyrins / therapeutic use
  • Tumor Cells, Cultured


  • ABCG2 protein, human
  • ATP Binding Cassette Transporter, Subfamily B, Member 1
  • ATP Binding Cassette Transporter, Subfamily G, Member 2
  • ATP-Binding Cassette Transporters
  • Chlorophyllides
  • Indoles
  • Multidrug Resistance-Associated Proteins
  • Neoplasm Proteins
  • Photosensitizing Agents
  • Porphyrins
  • Protoporphyrins
  • tetra(3-hydroxyphenyl)porphine
  • Chlorophyll
  • pyropheophorbide a
  • phytochlorin
  • Aminolevulinic Acid
  • protoporphyrin IX
  • tryptoquivaline
  • multidrug resistance-associated protein 1