9‑Deazapurines as Broad-Spectrum Inhibitors of the ABC Transport Proteins P‑Glycoprotein, Multidrug Resistance-Associated Protein 1, and Breast Cancer Resistance Protein

P-Glycoprotein (P-gp, ABCB1), multidrug resistance-associated protein 1 (MRP1, ABCC1), and breast cancer resistance protein (BCRP, ABCG2) are the three major ABC transport proteins conferring resistance to many structurally diverse anticancer agents, leading to the phenomenon called multidrug resistance (MDR). Much effort has been put into the development of clinically useful compounds to reverse MDR. Broad-spectrum inhibitors of ABC transport proteins can be of great use in cancers that simultaneously coexpress two or three transporters. In this work, we continued our effort to generate new, potent, nontoxic, and multiply effective inhibitors of the three major ABC transporters. The best compound was active in a very low micromolar concentration range against all three transporters and restored sensitivity toward daunorubicin (P-gp and MRP1) and SN-38 (BCRP) in A2780/ADR (P-gp), H69AR (MRP1), and MDCK II BCRP (BCRP) cells. Additionally, the compound is a noncompetitive inhibitor of daunorubicin (MRP1), calcein AM (P-gp), and pheophorbide A (BCRP) transport.