The red box: a selective G-quadruplex binder that downregulates c-MYC expression in human cells.

G-quadruplex (G4) structures within oncogene promoters represent promising therapeutic targets, particularly in the context of transcriptional regulation. Here, we report that the red box, a fully organic macrocyclic cationic receptor, exhibits high affinity and selectivity for G4 structures. Biophysical analyses reveal that the red box binds the c-MYC G4 with nanomolar affinity. Notably, the red box displays greater selectivity for c-MYC over duplex DNA compared to the well-characterized porphyrin TMPyP4, and its macrocyclic architecture is essential for this activity, as its acyclic analogue displays negligible G4 stabilization. In cellular assays, treatment with non-cytotoxic concentrations of the red box selectively downregulated c-MYC expression in HEK293T cells, with minimal effects on c-KIT and hTERT. Unlike previously reported G4-selective ligands based on Pd(ii)/Pt(ii) metallacycles, the red box achieves nanomolar affinity and selectivity through a fully metal-free, hydrazone-based scaffold, addressing key limitations related to cost, biodegradability, and long-term biocompatibility. Furthermore, c-MYC downregulation was consistently observed across multiple cell lines, including tumour-derived cellular models, underscoring the translational relevance of this new class of organic G4 binders.