Abstract PS4-07-19: An agonist of mitochondrial CLPP, ONC206, overcomes mitochondrial dependencies and abates chemoresistance in triple negative breast cancer

Abstract Approximately 50% of triple-negative breast cancer (TNBC) patients treated with neoadjuvant chemotherapy such as platinums (e.g., carboplatin, CRB), taxanes (e.g., docetaxel, DTX), and anthracyclines administered alone, in combination, sequentially, or with or without anti-PD1 agents, retain residual cancer burden (RCB). RCB is strongly associated with rapid local and metastatic recurrence and mortality in TNBC. Thus, we conducted preclinical studies to address the urgent need to overcome chemoresistance. Mitochondrial function is known to promote chemoresistance and metastasis in TNBC and other cancers. We previously demonstrated that inhibiting mitochondrial oxidative phosphorylation (OXPHOS) or mitochondrial fusion could overcome chemoresistance in TNBC. In a recent preclinical trial using 50 orthotopic patient-derived xenograft (PDX) models of TNBC, multi-omic profiling revealed that mitochondrial pathways, particularly mitochondrial protein translation and OXPHOS, were significantly associated with resistance to DTX, CRB, or their combination. These findings are corroborated in human TNBC specimens (NCT02547987). Our recent work suggests that chemotherapy can induce the mitochondrial unfolded protein response (mtUPR), a retrograde signaling pathway that rewires nuclear transcription in response to mitochondrial proteotoxic stress. Caseinolytic peptidase P (CLPP), a mitochondrial matrix protease, plays a critical role in this response by targeting key proteins involved in mitochondrial respiration and homeostasis. In cancers, CLPP is essential for maintaining mitochondrial protein quality control. Both its inhibition, which causes protein accumulation, and its hyperactivation, which causes excessive proteolysis, can lead to mitochondrial dysfunction and cell death. Notably, high CLPP mRNA levels correlate with poor TNBC survival, while non-tumor cells are relatively insensitive to CLPP perturbation, suggesting a therapeutic window for targeting mtUPR in TNBC. ONC206, an analog of the first-in-class imipridone/dordaviprone/ONC201 with improved potency, is currently undergoing phase I clinical trials for CNS tumors (NCT04732065 and NCT04541082). We found that ONC206 exhibited strong single-agent activity and significantly enhanced chemosensitivity in select orthotopic PDX models, PDX-derived organoids (PDXOs), and human TNBC cell lines. Two PDX models, BCM-2665 (ONC206 responder) and BCM-0132 (ONC206 non-responder), were evaluated in combination chemotherapy trials. In BCM-2665, ONC206 induced complete regressions and suppressed long-term tumor regrowth when combined with CRB. One mouse showed no detectable tumor at euthanasia on day 225. No such effect was observed with DTX. In contrast, in BCM-0132, which did not respond to ONC206 alone, the addition of DTX but not CRB extended tumor regression. One mouse showed complete tumor clearance at day 190. ONC206 was tolerated in mouse trials even when combined with chemotherapy. To elucidate the mechanism of ONC206 activity, we knocked out CLPP, which completely abolished its efficacy. ONC206 treatment significantly reduced OXPHOS and levels of CLPP proteolytic substrates while elevating mtUPR-related proteins (PERK, GCN2, and phospho-eIF2α) in a CLPP-dependent manner. Moreover, ONC206 reversed chemotherapy-induced OXPHOS, suggesting that its combination with chemotherapy may further amplify mtUPR signaling and suppress OXPHOS to enhance therapeutic efficacy. Collectively, our findings provide evidence that ONC206 disrupts mitochondrial dependencies in TNBC by driving excessive CLPP-mediated proteolysis, impairing OXPHOS, and activating mtUPR. This represents a novel and potentially promising therapeutic strategy to enhance chemotherapy response in TNBC. Citation Format: L. M. Baek, A. S. Greer, L. Audra, J. T. Lei, J. C. Yang, L. E. Dobrolecki, S. F. Faucher, N. D. Griffith, C. M. Sallas, B. Lim, A. L. Welm, V. V. Prabhu, M. T. Lewis, G. V. Echeverria. An agonist of mitochondrial CLPP, ONC206, overcomes mitochondrial dependencies and abates chemoresistance in triple negative breast cancer abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-07-19.