Carboranes as potent phenyl mimetics. A comparative study on the reversal of ABCG2-mediated drug resistance by carboranylquinazolines and their organic isoters.

Multidrug resistance (MDR) is a major challenge in clinical cancer therapy. In particular, overexpression of certain ATP-binding cassette (ABC) transporter proteins, like the efflux transporter ABCG2 (or BCRP), has been associated with the development of resistance to applied chemotherapeutic agents in cancer therapies, and therefore targeted inhibition of BCRP-mediated transport might lead to reversal of MDR. Previously, we have described the introduction of closo-dicarbadodecaborane or carborane as an inorganic pharmacophore into a polymethoxylated 2-phenylquinazolin-4-amine backbone. Here, the scope was extended to the corresponding amide derivatives. As most of the amide derivatives suffered from poor solubility, only the amide derivative QCe and the two amine derivatives DMQCc and DMQCd were further investigated. Carboranes are often considered as sterically demanding phenyl mim etics or isosteres. Therefore, the phenyl and adamantyl analogues of the most promising carborane derivatives were also investigated. The studies showed that the previously described DMQCd, a penta-methoxylated N-carboranyl-2-phenylquinazolin-4-amine, was by far superior to its organic analogues in terms of cytotoxicity, inhibition of the human ABCG2 transporter, as well as the ability to reverse BCRP-mediated mitoxantrone resistance in MDCKII-hABCG2 and HT29 colon cancer cells. Thus, DMQCd is a promising candidate for further studies in combination therapy for ABCG2-overexpressing cancers.