Indoloazepinone-Constrained Oligomers as Cell-Penetrating and Blood-Brain-Barrier-Permeating Compounds

Non-cationic and amphipathic indoloazepinone-constrained (Aia) oligomers have been synthesized as new vectors for intracellular delivery. The conformational preferences of the [l-Aia-Xxx](n) oligomers were investigated by circular dichroism (CD) and NMR spectroscopy. Whereas Boc-[l-Aia-Gly](2,4)-OBn oligomers 12 and 13 and Boc-[l-Aia-(3)-h-l-Ala](2,4)-OBn oligomers 16 and 17 were totally or partially disordered, Boc-[l-Aia-l-Ala](2)-OBn (14) induced a typical turn stabilized by C-5- and C-7-membered H-bond pseudo-cycles and aromatic interactions. Boc-[l-Aia-l-Ala](4)-OBn (15) exhibited a unique structure with remarkable T-shaped -stacking interactions involving the indole rings of the four l-Aia residues forming a dense hydrophobic cluster. All of the proposed FITC-6-Ahx-[l-Aia-Xxx](4)-NH2 oligomers 19-23, with the exception of FITC-6-Ahx-[l-Aia-Gly](4)-NH2 (18), were internalized by MDA-MB-231 cells with higher efficiency than the positive references penetratin and Arg(8). In parallel, the compounds of this series were successfully explored in an in vitro blood-brain barrier (BBB) permeation assay. Although no passive diffusion permeability was observed for any of the tested Ac-[l-Aia-Xxx](4)-NH2 oligomers in the PAMPA model, Ac-[l-Aia-l-Arg](4)-NH2 (26) showed significant permeation in the in vitro cell-based human model of the BBB, suggesting an active mechanism of cell penetration.