Perturbation of the c-Myc-Max protein-protein interaction via synthetic α-helix mimetics.

The rational design of inhibitors of the bHLH-ZIP oncoprotein c-Myc is hampered by a lack of structure in its monomeric state. We describe herein the design of novel, low-molecular-weight, synthetic α-helix mimetics that recognize helical c-Myc in its transcriptionally active coiled-coil structure in association with its obligate bHLH-ZIP partner Max. These compounds perturb the heterodimer's binding to its canonical E-box DNA sequence without causing protein-protein dissociation, heralding a new mechanistic class of "direct" c-Myc inhibitors. In addition to electrophoretic mobility shift assays, this model was corroborated by further biophysical methods, including NMR spectroscopy and surface plasmon resonance. Several compounds demonstrated a 2-fold or greater selectivity for c-Myc-Max heterodimers over Max-Max homodimers with IC50 values as low as 5.6 μM. Finally, these compounds inhibited the proliferation of c-Myc-expressing cell lines in a concentration-dependent manner that correlated with the loss of expression of a c-Myc-dependent reporter plasmid despite the fact that c-Myc-Max heterodimers remained intact.