Cyclic Peptide PROTACs Knock Down HIF-1α — Until Hypoxia Outpaces Them
- Custom bifunctional amino acids — carrying a VHL-recruiting ligand and a variable linker — are incorporated directly during solid-phase peptide synthesis, an alternative to conventional PEG-based linkers for building cyclic peptide degraders.
- A SICLOPPS-derived cyclic peptide HIF-1 inhibitor serves as the warhead, yielding cyclic peptide PROTACs (CP-PROTACs) that engage VHL in a productive ternary complex.
- The degraders lower HIF-1α levels and hypoxia-response signaling across multiple cancer cell lines.
- Degradation is oxygen-state dependent — robust under moderate hypoxia but largely lost under more severe hypoxia, even though the degrader still engages and marks HIF-1α there.
- The activity loss reflects a kinetic mismatch, not a binding failure: HIF-1α resynthesis outpaces degradation, pointing to a general ceiling on PROTAC efficacy against rapidly replenished targets.
Smart move folding the VHL ligand and linker into a bifunctional amino acid for direct SPPS incorporation — that sidesteps the PEGylation/conjugation bottleneck that makes cyclic peptide degraders such a pain to iterate on, and it turns linker SAR into a synthesis-cycle variable instead of a separate coupling step. The SICLOPPS-derived warhead is a nice proof that a cyclic peptide can grip a transcription-factor surface a small molecule can't easily touch, so on chemotype comparison this favors the peptide side. But the real flag for my own work is the hypoxia dropout: ternary complex still forms, HIF-1α still gets marked, degradation just can't keep up. That's not a binding problem I'd fix by optimizing the VHL warhead — it's a turnover-rate problem, and it says something uncomfortable about any PROTAC campaign against a target with fast resynthesis kinetics.
Cyclic Peptide PROTACs Restore VHL-Mediated HIF-1α Degradation in Hypoxia ↗