Purpose: The ATP-binding cassette protein ABCG2 (breast cancer resistance protein) effluxes some of the photosensitizers used in photodynamic therapy (PDT) and, thus, may confer resistance to this treatment modality. Tyrosine kinase inhibitors (TKI) can block the function of ABCG2. Therefore, we tested the effects of the TKI imatinib mesylate (Gleevec) on photosensitizer accumulation and in vitro and in vivo PDTefficacy. Experimental Design: Energy-dependent photosensitizer efflux and imatinib mesylate's effects on intracellular accumulation of clinically used second-and first-generation photosensitizers were studied by flow cytometry in murine and human cells with and without ABCG2 expression. Effects of ABCG2 inhibition on PDT were examined in vitro using cell viability assays and in vivo measuring photosensitizer accumulation and time to regrowth in a RIF-1tumor model. Results: Energy-dependent efflux of 2-(1-hexyloxethyl)-2-devinyl pyropheophorbide-a (HPPH, Photochlor), endogenous protoporphyrin IX (PpIX) synthesized from 5-aminolevulenic acid, and the benzoporphyrin derivative monoacid ring A (BPD-MA, Verteporfin) was shown in ABCG2+ cell lines, but the first-generation multimeric photosensitizer porfimer sodium (Photofrin) and a novel derivative of HPPH conjugated to galactose were minimally transported. Imatinib mesylate increased accumulation of HPPH, PpIX, and BPD-MA from 1.3-to 6-fold in ABCG2+ cells, but not in ABCG2À cells, and enhanced PDTefficacy both in vitro and in vivo. Conclusions: Second-generation clinical photosensitizers are transported out of cells by ABCG2, and this effect can be abrogated by coadministration of imatinib mesylate. By increasing intracellular photosensitizer levels in ABCG2+ tumors, imatinib mesylate or other ABCG2 transport inhibitors may enhance efficacy and selectivity of clinical PDT.Photodynamic therapy (PDT) is used for the treatment of many cancers. Photosensitizers are taken up by tumor cells and then activated by light (1), generating reactive oxygen species that cause cell death by necrosis or apoptosis (2). Expression of ATP-binding cassette (ABC) transport proteins renders tumor cells resistant to chemotherapy drugs that are substrates of these proteins (3), and the effect of these transporters on intracellular photosensitizer accumulation has been examined as a potential cause of resistance to PDT. The ABC family transport protein that has been most thoroughly investigated is ABCB1, or P-glycoprotein, but photosensitizers were found not to be substrates for this pump (4 -8), nor were they substrates for ABCC1, or multidrug resistance-associated protein-1 (8). In contrast, another ABC family transport protein, ABCG2 or breast cancer resistance protein, has been found to transport some photosensitizers and to decrease intracellular photosensitizer accumulation (8). Jonker et al. (9) showed that ABCG2 knock-out mice were photosensitive because of increased protoporphyrin IX (PpIX) levels. Robey et al. found that pheophorbide a is a specific substrate for...

The ATP-dependent transporter ABCG2 exports certain photosensitizers (PS) from cells, implying that the enhanced expression of ABCG2 by cancer cells may confer resistance to photodynamic therapy (PDT) mediated by those PS. In 35 patient-derived primary cultures of lung epithelial and stromal cells, PS with different subcellular localization and affinity for ABCG2 displayed cell-type specific retention both independent and dependent on ABCG2. In the majority of cases, the ABCG2 substrate 2-[1-hexyloxyethyl]-2-devinyl pyropheophorbide-a (HPPH) was lost from fibroblastic cells more rapidly than from their epithelial counterparts, even in the absence of detectable ABCG2 expression, facilitating selective eradication by PDT of epithelial over fibroblastic cells in tumor/stroma co-cultures. Pairwise comparison of normal and transformed epithelial cells also identified tumor cells with elevated or reduced retention of HPPH, depending on ABCG2. Enhanced ABCG2 expression led to the selective PDT survival of tumor cells in tumor/stroma co-cultures. This survival pattern was reversible through HPPH derivatives that are not ABCG2 substrates or the ABCG2 inhibitor imatinib mesylate. PS retention, not differences in subcellular distribution or cell signaling responses, was determining cell type selective death by PDT. These data suggest that up-front knowledge of tumor characteristics, specifically ABCG2 status, could be helpful in individualized PDT treatment design.