A fluorescence study examining how 14-valerate side chain substitution modulates anthracycline binding to small unilamellar phospholipid vesicles

Abstract

The intrinsic fluorescence properties of the anthracycline antitumor antibiotics were studied in an effort to understand how 14-valerate side chain substitution modulates drug associations with small unilamellar phospholipid vesicles (SUVs) under near physiological conditions. Drug location and dynamics in fluid-phase dimyristoylphosphatidylcholine (DMPC) bilayers were evaluated for several analogs; accessibilities of bound fluorophores to membrane-impermeable iodide were evaluated in quenching experiments, while the diffusive motions of these agents were studied using lifetime-resolved anisotropy plots. The bulky and hydrophobic valerate substituent was found to further hinder the rotations experienced by a bound drug molecule, with apparent limiting anisotropy ( α ∞) values showing increases of 13–82% upon valerate group substitution. In addition, the bimolecular quenching rate constants (unit, 10 9 M −1 · s −1) for membrane-bound adriamycin (1.4), N-trifluoroacetyladriamycin (0.4), and their corresponding valerate-substituted analogs ( k q values of 1.1 and 0.5, respectively) reveal that the side chain is a weak modulator of fluorophore penetration into the bilayer, with stronger modulation being achieved through amino group substitution. Similar results were obtained for drugs bound to negatively-charged dimyristoylphosphatidylglycerol (DMPG) bilayers. Finally, comparison of the equilibrium binding affinities of the various congeners for electroneutral DMPC versus negatively-charged DMPG bilayers demonstrate that positively-charged parent anthracyclines display high levels of selective binding to negatively-charged phospholipids, unlike valerate-substituted analogs which display no such selectivity. The modulation of anthracycline-membrane interactions achieved through valerate substitution offers potential explanations, at least in part, for some of the novel biological properties of valerate-containing anthracyclines.