Patients with triple-negative breast cancer are predominantly treated by chemotherapy, but the responsiveness is low due to the capacity of TNBC cells to activate DNA double-strand break (DSB) repair pathways.1 Recently, the interaction between proteins IGFBP-3 and NONO was discovered to modulate DNA DSB repair via non-homologous end joining in TNBC cells.2 Inhibiting this novel protein-protein interaction is anticipated to disrupt DNA DSB repair in TNBC cells and thus enhance sensitivity to front-line chemotherapy.
Here we introduce the development of the first IGFBP-3/NONO inhibitors by peptide stapling, where two side-chains of a native IGFBP-3 peptide are covalently linked to form macrocyclic compounds. Two stapled peptides were synthesised via an on-resin intramolecular C2 arylation of tryptophan with either 3 or 4-iodophenylalanine at i/i + 4 spacing through a Pd(II)-catalysed C(sp2)-H functionalisation.3 Another three stapled peptides were made via solution-phase dicysteine stapling with dichloroacetone,4 dibromoxylene5 and perfluorobenzene6 at i/i + 4 spacing. All candidates showed improved alpha-helicity, proteolytic stability and binding affinity as characterised by circular dichroism, chymotrypsin degradation assay and fluorescence polarisation (FP) assay, along with some degree of cellular uptake by confocal fluorescence microscopy. This work demonstrates the feasibility of a stapled peptide strategy against the target protein NONO, paving the way for future drug development.