Aureobasidins: Structure-activity relationships for the inhibition of the human MDR1 P-glycoprotein ABC-transporter

Cyclic depsipeptide cyclo-[D-Hmp(1)-L-MeVal(2)-L-Phe(3)-L-MePhe(4)-L-Pro(5) -L-aIle(6)-L-meVal(7)-L-Leu(8)-L-beta HOMeVal(9)], the antifungal antibioti c aureobasidin A (AbA), was reported to interfere with ATP-binding cassette (ABC) transporters in yeast and mammalian cells, particularly the MDRI P-g lycoprotein (Pgp), a transmembrane phospholipid flippase or "hydrophobic va cuum cleaner" that mediates multidrug resistance (MDR) of cancer cells. In a standardized assay that measures Pgp function by the Pgp-mediated efflux of the calcein-AM Pgp substrate and uses human lymphoblastoid MDR-CEM (VBL1 00) cells as highly resistant Pgp-expressing cells and the cyclic undecapep tide cyclosporin A (CsA) as a reference MDR-reversing agent (IC50 of 3.4 mu M), AbA was found to be a more active Pgp inhibitor (IC50 of 2.3 mu M). Cu t of seven natural analogues and 18 chemical derivatives of AbA, several we re shown to display even more potent Pgp-inhibitory activity. The Pgp-inhib itory activity was increased about 2-fold by some minor modifications such as those found in the naturally occurring aureobasidins AbB ([D-Hiv(1)]-AbA ), AbC ([Val(6)]-AbA), and AbD [gamma HOMeVal(9)]-AbA). The replacement of the [Phe(3)-MePhe(4)-Pro(5)] tripeptide by an 8-aminocaprylic acid or the N -7-desmethylation of MeVal(7) led to only a 3.3-fold decreased capacity to inhibit Pgp function, suggesting that the Pgp inhibitory potential of aureo basidins, though favored by the establishment of an antiparallel beta-sheet between the [D-Hmp(1)-L-MeVal(2)-L-Phe(3)] and [L-aIle(6)-L-MeVal(7)-L-Leu (8)-] tripeptides, does not critically depend on the occurrence of the [L-P he(3)-L-MePhe(4)-L-Pro(5)-L-aIle(6)] type II' beta-turn secondary structure . In contrast, the most potent Pgp inhibitors were found among AbA analogue s with [beta HOMeVal(9)] residue alterations, with some data suggesting a n egative impact of the [L-Leu(8)-L-beta HOMeVal(9)-D-Hmp(1)] gamma-turn seco ndary structure on Pgp inhibitory potential. The [2,3-dehydroMeVal(9)]-AbA was the most potent Pgp inhibitory aureobasidin, being 13-fold more potent than AbA and 19-fold more potent (on a molar basis) than CsA. Finally, ther e was no correlation between the SAR for the human MDR1 Pgp inhibition and the SAR for Saccharomyces cerevisiae antifungal activity, which is mediated by an inositol phosphoceramide synthase activity.