Mechanisms of imipridones in targeting mitochondrial metabolism in cancer cells

Abstract

ONC201 is the first member of the imipridone family of anticancer drugs to enter the clinic for the treatment of diverse solid and hematologic cancers. A subset of pediatric and adult patients with highly aggressive brain tumors has shown remarkable clinical responses to ONC201, and recently, the more potent derivative ONC206 entered clinical trials as a single agent for the treatment of central nervous system (CNS) cancers. Despite the emerging clinical interest in the utility of imipridones, their exact molecular mechanisms are not fully described. In fact, the existing literature points to multiple pathways (e.g. tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) signaling, dopamine receptor antagonism, and mitochondrial metabolism) as putative drug targets. We have performed a comprehensive literature review and highlighted mitochondrial metabolism as the major target of imipridones. In support of this, we performed a meta-analysis of an ONC201 screen across 539 human cancer cell lines and showed that the mitochondrial caseinolytic protease proteolytic subunit (ClpP) is the most significant predictive biomarker of response to treatment. Herein, we summarize the main findings on the anticancer mechanisms of this potent class of drugs, provide clarity on their role, and identify clinically relevant predictive biomarkers of response.

Keywords: ClpP; ONC201; ONC206; ONC212; imipridone.

Fig. 1

Anticancer mechanism of imipridones. Data from the existing literature indicates that imipridones exert their anticancer effects primarily by binding to and potently activating the mitochondrial Clp protease proteolytic subunit ClpP, causing ClpP to lose its dependence on the chaperone protein ClpX (Step 1). Hyper-active Clp protease then depletes its target substrates including components of the respiratory complex chain, most strongly complex I and II proteins (Step 2). In turn, OXPHOS is impaired and cellular ATP is depleted (Step 2). Mitochondrial structural damage and distress occurs, concomitant with the state of energy deprivation leading to integrated stress response (ISR) activation (Step 3). The ISR is relayed to the nucleus through an undefined mechanism, involving the typical (phospho-eIF2α-dependent) or atypical (phospho-eIF2α-independent) pathway. ISR activation causes global translational attenuation, including reduced levels of cyclin D1 leading to cell cycle arrest (Step 4A). In conditions of prolonged stress, ATF4 and CHOP are upregulated, and together these transcription factors increase the expression of their target genes including GADD34, which promotes further protein synthesis and stress (thus further activating the ISR); the TRAIL receptor DR5, which can promote TRAIL-mediated extrinsic cell death; and pro-apoptotic Bcl-2 family proteins, which promote the intrinsic, mitochondrial cell death program (Step 4B). Created with BioRender.com.

Fig. 2

Comprehensive cell viability screen of human cancer cell lines identified ClpP as the primary predictor of ONC201 response. Correlation between cell viability at 2.5 μM ONC201 and gene expression levels (log2 TPM) across 539 human cancer lines from the Cancer Cell Line Encyclopedia (A). Scatter plots and boxplots show ONC201 sensitivity compared to gene expression levels for CLPP (B) and DRD2 (C). Box plots show Spearman’s ρ between ONC201 sensitivity and gene expression levels for 16 individual cancer types. Calculations are based on 11 (rhabdoid) to 106 (lung) cancer cell lines (D). All gliomas among brain cancer cell lines are IDH-wild type. Primary drug screening data for human cancer cell lines was derived from the PRISM Repurposing Screen (19Q3) and gene expression data from the Cancer Dependency Map (20Q3). Spearman’s ρ and -log₁₀P values are shown. Boxplots show the first quartile, median value, third quartile, and minimum/maximum (whiskers) of the data. Blue lines show linear regression results between ONC201 sensitivity and gene expression levels. Classification of gene expression levels into low/high is based on median. DMSO, dimethylsulfoxide.