Photodynamic therapy (PDT) involves the combination of a photosensitizer and light of a specific wavelength. Upon light activation in the presence of oxygen, photosensitizer molecules generate reactive oxygen species that cause cytotoxicity by inducing oxidative stress. Aminolevulinic acid (ALA) is a pro-drug used for the diagnosis and PDT treatment of various solid tumors based on the endogenous production of heme precursor protoporphyrin IX (PpIX). Although nearly all types of human cells express heme biosynthesis enzymes and produce PpIX, tumor cells are found to have more fluorescent PpIX production and accumulation than normal cells, allowing for the detection and treatment of solid tumors.
The preferential accumulation of ALA-mediated PpIX in tumors is crucial for the successful application of this modality for tumor imaging and treatment. Tumor cellular PpIX level is determined by the ALA uptake, ALA-PpIX biosynthesis, PpIX bioconversion and efflux. All these factors are subject to alterations in the expression and activity of heme biosynthesis enzymes and transporters in tumors and can be pharmacologically modified to enhance tumor PpIX levels. The objective of my research is to identify important determinants affecting tumor PpIX level and evaluate therapeutic approaches to enhance ALA-based tumor detection and therapy. This research focused on triple negative breast cancer cells (TNBC) that is currently lack of effective treatment options and aimed for evaluating clinically relevant agents for enhancing ALA-based modalities.
Our study showed that TNBC cells exhibited a reduced ALA-PpIX level and resistance to ALA-PDT due to high ABCG2 transporter activity. Use of an ABCG2 inhibitor significantly increased ALA-PpIX and sensitized TNBC cells to ALA-PDT. This finding led to the search for a clinically relevant ABCG2 inhibitor for improving the application of ALA for the TNBC. We found that lapatinib, an FDA-approved small molecule kinase inhibitor, significantly increased ALA-PpIX level and ALA-PDT response in a panel of TNBC cells whereas normal breast epithelial cells were not affected. Lapatinib was also able to increase ALA-PpIX fluorescence in TNBC tumors. These results indicate that lapatinib can be used in combination with ALA for fluorescence-guided resection and PDT of TNBC.
The other focus of my work was on the evaluation of ferrochelatase (FECH) inhibition for therapeutic enhancement of ALA. Although inhibiting FECH-catalyzed PpIX bioconversion is a rational approach for enhancing ALA-PpIX/PDT, inhibition of PpIX bioconversion by iron chelators yielded mixed results in both preclinical and clinical studies. Using FECH-knockdown tumor cell lines, we found that the effectiveness of chelators for enhancing ALA-PpIX/PDT was dependent on the FECH level, particularly the relative FECH level in tumor versus normal tissues. Tumor cells with FECH deficiency did not benefit from the combination of ALA and chelators. Our study suggests the necessity of selecting tumors with high FECH levels for this enhancement approach of ALA. Overall, my Ph.D. research on ALA and its enhancement for tumor detection and PDT indicates that tumor phenotypic and genotypic characteristics such as increased ABCG2 activity and reduced FECH activity significantly influence tumor ALA-PpIX levels, and ALA enhancement approaches should focus on addressing these particular tumor features.
|School:||University of the Sciences in Philadelphia|
|School Location:||United States -- Pennsylvania|
|Source:||DAI-B 79/12(E), Dissertation Abstracts International|
|Subjects:||Toxicology, Pharmacology, Pharmaceutical sciences|
|Keywords:||Abcg2, Aminolevulinic acid, Breast cancer, Ferrochelatase, Photodynamic therapy, Protoporphyrin ix|
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