Pheophorbide a Is a Specific Probe for ABCG2 Function and Inhibition

Pheophorbide a (PhA), a chlorophyll catabolite, was shown to be an ABCG2 substrate based on Abcg2−/− knockout mouse studies (J. W. Jonker et al. , Proc. Natl. Acad. Sci. USA, 99: 15649–15654, 2002). We developed a functional assay for ABCG2 using PhA and the ABCG2 inhibitor fumitremorgin C. In selected cell lines expressing high levels of P-glycoprotein, multidrug resistance-associated protein 1, or ABCG2, PhA transport was observed only in cells expressing ABCG2. Fumitremorgin C-inhibitable PhA transport was found to correlate with cell surface ABCG2 expression as measured by the anti-ABCG2 antibody 5D3. We found that 100 μm of the cyclin-dependent kinase inhibitor UCN-01 or 1 μm of the P-glycoprotein inhibitor tariquidar inhibited ABCG2-mediated PhA transport. In 4-day cytotoxicity assays, ABCG2-mediated resistance to SN-38 and topotecan was abrogated in ABCG2-transfected HEK-293 cells treated with 1 μm tariquidar, and ABCG2-transfected cells were 6–7-fold resistant to UCN-01. PhA is an ABCG2-specific substrate with potential value in measuring ABCG2 function and expression in clinical samples.