The acidic microenvironment of solid tumors is a potential source of selectivity in the anti-cancer activity of ionophores, which requires delicate control of their biophysical properties. In this context, we have systematically studied fluorine substitutions in the aromatic side chains of HCl-binding pseudopeptidic cages. Interconnected factors like chloride binding, protonation, lipophilicity, and conformation and diffusiveness of the cages can impact their ability to transport HCl through the aqueous-lipid interphase, as demonstrated by robust experimental (X-ray, nuclear magnetic resonance [NMR], fluorescence) and theoretical results. The fine-tuning of these properties allows the modulation of their pH-dependent cytotoxicity against cancer cells, from essentially non-cytotoxic at pH 7.5 (like the extracellular surroundings of healthy tissues) to highly toxic in slightly acidic microenvironments (like those around solid tumors). Thus, a distal fluorine substitution produces a big impact on the physicochemical and biological properties of the cages, improving their selectivity as potential therapeutic ionophores.
