PEAKS activation by PROTAC EPIC-0726 potentiates temozolomide in glioblastoma via K63/K48 ubiquitination-dependent ERK/AKT suppression and p21 stabilization

BACKGROUND: Despite advances in small-molecule inhibitors (SMIs), the clinical outcomes for glioblastoma (GBM) remain bleak. Recently, polymerase I and transcript release factor (PTRF/Cavin1) has emerged as a promising therapeutic target, with its inhibitor EPIC-1042 demonstrating preclinical antitumor activity. However, the therapeutic limitations of SMIs necessitate alternative strategies to achieve enduring target suppression. METHODS: EPIC-0726, a proteolysis-targeting chimera (PROTAC) degrader of PTRF, was developed through computer-aided drug design (CADD). Target engagement and degradation specificity were validated by Western blot. Quantitative proteomics identified downstream effectors, while mechanistic insights were elucidated through co-immunoprecipitation, immunofluorescence, and ubiquitination profiling. Orthotopic GBM models were used to assess therapeutic efficacy and temozolomide (TMZ) sensitization. RESULTS: EPIC-0726 induced dose-dependent PTRF degradation via the ubiquitin-proteasome system (UPS), requiring ternary complex formation. Proteomic analysis revealed RBX1, a core component of E3 ligase complexes, as a key downstream target. PTRF degradation by EPIC-0726 destabilized RBX1, concurrently suppressing K63-linked ubiquitination-mediated ERK1/2/AKT activation and stabilizing p21 via impaired K48-dependent proteasomal degradation. In vivo, EPIC-0726 monotherapy inhibited GBM growth and synergized with TMZ, with effects more potent than that of EPIC-1042. CONCLUSION: This study establishes PROTAC-mediated PTRF degradation as a mechanistically distinct manner to activate proteolysis strategy-enhanced TMZ efficacy by ERK1/2/AKT kinase suppression and p21 stabilization (PEAKS) through the PTRF-RBX1 regulatory axis. The superior efficacy of EPIC-0726 over EPIC-1042, particularly in overcoming TMZ resistance, provides a paradigm-shifting therapeutic approach for GBM. Our findings warrant the clinical translation of EPIC-0726 as both a monotherapy and a backbone for combination regimens.