A stapled peptide called MAX-7 dragged the cancer protein MYC to the cell's recycling bin and destroyed it there, shrinking tumors in mice with triple-negative breast cancer (TNBC). The work, published July 14 in the Journal of Medicinal Chemistry ↗, is notable less for the target than for the route it took to reach it.
MYC is one of the most wanted proteins in cancer biology and one of the least druggable. It is a master switch that tells cells to grow and divide, and it is cranked up in a large share of human tumors. But it has no deep pocket for a small-molecule drug to grip, and it lives inside the nucleus rather than on the cell surface where antibodies work. For forty years it has sat on the list of targets everyone wants and almost nobody can hit.
What a staple buys
MAX-7 is a stapled peptide, a short chain of amino acids locked into a rigid corkscrew shape by a hydrocarbon brace that bridges two turns of the helix. In this case the brace spans from one residue to the residue seven positions along, an arrangement chemists write as (i, i+7). The staple does real work. Compared with the same sequence left unstapled, the authors report that MAX-7 holds its helical shape better, resists being chopped up by the body's enzymes, and crosses into cells more readily. Those three gains are the whole reason stapled peptides exist as a drug class: an ordinary peptide aimed at an intracellular target is usually too floppy, too fragile, and too stuck outside the cell to matter.
The lysosome, not the proteasome
The mechanism is the part worth arguing about. Most of the targeted-degradation field, the PROTACs and molecular glues that have drawn billions in investment, works by tagging a protein for the proteasome, the cell's protein shredder. MAX-7 does something different. It sends MYC to the lysosome through a pathway called chaperone-mediated autophagy (CMA), the route cells use to selectively haul worn-out proteins into an acidic compartment for disposal. The authors show the depletion is lysosome-dependent and that it bypasses the ubiquitin-proteasome system entirely. PROTACs, molecular glues, and CMA-based degraders like MAX-7 all aim at the same goal, destroying a protein rather than blocking it, but only the last route runs through the lysosome.
That distinction is not cosmetic. A degrader that works through a second, independent disposal route is a degrader that could still work in tumors that have learned to resist proteasome-based drugs, and it opens a design space that most of the field has left alone. In the breast cancer cells, MAX-7 cut proliferation, migration, and invasion and pushed the cells into apoptosis. In a 4T1 mouse model, a standard aggressive breast cancer line, the peptide produced what the paper calls significant tumor regression with a favorable safety profile. The abstract does not attach numbers to those claims, so the size of the effect will have to wait for the full data.
This is the second stapled peptide against a supposedly undruggable cancer driver to cross our desk in as many days. Last week a different group used one to switch a tumor suppressor back on and knock down two drivers at once ↗ in colorectal cancer. That one restored a pathway; MAX-7 destroys a protein. Both sit on peptidemodel's anticancer shelf ↗, and both point at the same shift: the stapled-peptide format is being pushed at exactly the intracellular targets that defeated small molecules. MAX-7 is a lead, not a drug. It has cleared cells and one mouse model, which is where most promising peptides stall. The interesting claim is the door it walked through, and whether other undruggable proteins can be sent the same way.