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A new class of specific breast cancer resistance protein (BCRP) inhibitors was identified, showing no inhibition of the ATP binding cassette (ABC) transporters P-gp and MRP1. Some of these modulators inhibit BCRP with high potency; they are only slightly less potent than Ko143 and could serve as promising lead structures for the design of novel effective BCRP inhibitors. These inhibitors are structurally related to tariquidar (XR9576) and belong to a library of multidrug-resistance modulators synthesized by our research group. The absence of the tetrahydroisoquinoline substructure appears to play a crucial role for specificity; we found that the presence of this substructure is not essential for interaction with BCRP. To determine the type of interaction between pheophorbide A and compounds with and without the tetrahydroisoquinoline substructure, various substrate pheophorbide A concentrations were used in enzyme kinetics assays. The resulting data show that these compounds share a noncompetitive-type interaction with pheophorbide A. Experiments with imatinib and pheophorbide A revealed a mixed-type interaction. The combination of imatinib and compounds with and without the tetrahydroisoquinoline substructure resulted in a positive cooperative effect, indicating that imatinib engages a binding site distinct from that of the new compounds on one side and distinct from that of pheophorbide A on the other side as well. The results of this study suggest that the category of BCRP-specific inhibitors, which includes only fumitremorgin C, Ko143 and analogues, and novobiocin needs to be extended by this new class of inhibitors, which possess three key characteristics: specificity, potency, and low toxicity.

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Multidrug resistance observed in cancer chemotherapy is commonly attributed to overexpression of efflux transporter proteins. These proteins act as ATP-dependent drug efflux pumps, actively extruding chemotherapeutic agents from cells and causing a decrease in intracellular drug accumulation. Besides the well-recognized role of P-glycoprotein (P-gp, ABCB1), the breast cancer resistance protein (BCRP, ABCG2) is becoming increasingly accepted as playing an important role in multidrug resistance. In contrast to P-glycoprotein, only a few inhibitors of ABCG2 are known. According to the literature, tyrosine kinase inhibitors (TKIs) can be considered to be broad-spectrum inhibitors, interacting with ABCB1, ABCC1 and ABCG2. Here, we investigated seven different TKIs, gefitinib, erlotinib, AG1478, PD158780, PD153035, nilotinib and imatinib, for their potential to restore ABCG2 sensitivity to cells. Furthermore, we analyzed the alteration of ABCG2 expression caused by TKIs and demonstrated that EGFR inhibitors such as gefitinib and PD158780 reduced both total and surface expression of ABCG2 in EGRF-positive MDCK BCRP cells by interaction with the PI3K/Akt signaling pathway. The reduced ABCG2 content led to an increased effect of XR9577, a well-known ABCG2 modulator, lowering the concentration required for half maximal inhibition. On the other hand, BCR-ABL inhibitors had no influence on ABCG2 expression and modulator activity. Interestingly, a combination of an EGFR inhibitor with the PI3K/Akt inhibitor LY294002 led to a significant reduction of ABCG2 expression at low concentrations of the drugs. Based on our results, we assume that EGFR exerts a post-transcriptional enhancing effect on ABCG2 expression via the PI3K/Akt signaling pathway, which can be attenuated by EGFR inhibitors. Blocking the key signaling pathway regulating ABCG2 expression with EGFR inhibitors, combined with the inhibition of ABCG2 with potent modulators might be a promising approach to circumvent MDR in cancer cells.

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