Among glial tumours of the central nervous system (CNS), one of the most devastating is the category of H3K27-altered midline gliomas because of their uniform and rapid mortality, high neurological symptom burden and predilection for paediatric, adolescent and young adult populations.¹ Historically, these tumours were divided into two categories: diffuse interstitial pontine glioma (DIPG), seen most frequently in the paediatric and adolescent populations, with rare manifestations outside of the brainstem and spinal cord, and diffuse midline gliomas (DMG), occurring primarily in the thalamus and more rarely in the midbrain of older adolescents and young adults. At the time, the only known characteristics shared by these tumours were their midline location and their dismally short median overall survival (mOS) of approximately 11 months from diagnosis.

In 2012, the histone-based H3K27M mutation was first described in DIPG,² characterized by an H3F3A gene mutation, as well as immunohistochemical evidence of both the methionine alteration and the loss of tri-methylation of histone H3 on immunohistochemical analysis, after a concerted effort to pursue tissue-based diagnostic assessments was made in these difficult-to-resect tumours.³ Because of a similar midline location in adults and similarly poor clinical outcomes, this effort was duplicated in the primarily thalamic DMGs, finding an identical histone alteration and underscoring the importance of genotype-driven classification as both a diagnostic and prognostic marker.³ The 2016 WHO classification of CNS tumours acknowledged these advancements and unified these diagnoses into a single molecular diagnostic category of ‘diffuse midline glioma, H3K27M-mutant’ to highlight the unifying genetic features and clinical outcomes of the tumour⁴. The 2021 WHO diagnostic criteria further modified this to ‘diffuse midline glioma, H3K27-altered’ to be more inclusive, as other mechanisms of histone gene mutation were discovered.⁵

Despite improvements in diagnostic precision, there still remain few treatment options. H3K27-altered midline gliomas cannot be maximally resected due to the tumour location, and though chemoradiation strategies are employed, these tumours are very radiation- and chemoresistant.^6,7 The dismal prognosis, despite these interventions, highlights the urgent need for novel therapies, and the molecular insights over the past decade have reignited the hope of developing pharmaceuticals that are attuned to this devastating group of tumours, of which the discovery of the new drug class of imipridones and their novel targets is the subject of this review.³

Methods

PubMed was used by all contributing authors to determine relevant literature for this review and to determine relevance to the topics based on review of abstracts. For the mechanism of action and future directions, the following search terms were used: TIC10 OR imipridone OR ONC201 OR dordaviprone AND mechanism NOT review NOT clinical trial, and abstracts were subsequently reviewed by the authors for relevance to cancer and for the further elimination of articles that were not research works (editorials, comments, errata) as well as non-English works. For the clinical trial section, the following search terms were used: ONC201 clinical trial NOT review with identical exclusion criteria. Duplicates between the searches were harmonized. Only trials studying imipridones as monotherapy were included. Reviews were only included for discussing relevant background topics.

Imipridone discovery and preclinical determination of mechanism of action

The discovery: From screening to re-discovery as a potential tumour necrosis factor-related apoptosis-inducing ligand receptor ligand

The first imipridone was discovered in 1973 using a phenotypic screening approach to identify small compounds with anti-cancer potential.⁸ One of the drug candidates, determined to be chemically inert and subsequently shelved, featured a heterocyclic ring structure, which would later be the foundation of the new drug class, the imipridones, named for their imide and pyridone chemical moieties.⁹ Re-discovery in an unbiased drug screen decades later found it to have activity against a colorectal cancer line as a potential ligand of the tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) receptor, subsequently named TRAIL-inducing compound 10 (TIC10).¹⁰ The TRAIL pathway is a complex network of intracellular signalling, on the one hand, pro-apoptotic by initiating the extrinsic cell death cascade culminating in the activation of caspases, and on the other, modulating the cell survival signalling cascades associated with nuclear factor-kappa B (NF-κB), phosphoinositide-3-kinase (PI3K)/AKT and mitogen-activated protein kinase (MAPK). TRAIL receptors comprise several functional and decoy subunits that combine into homotrimers, of which death receptor 5 (DR5) is a major contributor. Early preclinical work with TIC10, subsequently re-named ONC201 once acquired for further study by Oncoceutics (Oncoceutics compound 201), focused on its capacity to stimulate the extrinsic cell death pathway and apoptosis, specifically its ability to initiate caspase cleavage/activation and enhance DR5 expression, as well as on rational combinations to inhibit parallel MAPK and PI3K/AKT pathways.¹¹

ONC201 sensitized cancer cells to mTOR (mechanistic target of rapomycin) inhibition, a downstream effector of PI3K/AKT, leading to increased TRAIL receptor DR5 expression, as well as activation of caspase-8.¹² These effects were at least partly reversible when the TRAIL receptor was sequestered, caspase-8 activation was inhibited, or when mTOR was degraded.¹⁰ Experimental and computational approaches confirmed that ONC201 synergizes with MAPK and AKT inhibition.^13,14

However, the mechanistic primacy of TRAIL receptor activation by imipridones was quickly called into doubt. Activation of non-TRAIL-related pathways was observed, including those related to mitochondrial apoptosis, such as mitochondrial respiration machinery failure, oxidative phosphorylation-related gene suppression and structural damage, as well as poly-ADP-ribose polymerase (PARP) cleavage.^12,13,15 More bioavailable and potent analogues of ONC201 made it even clearer that imipridones were poor activators of TRAIL, with highly variable activation of caspases and DR5.¹⁶ TRAIL receptor activation has therefore fallen out of favour as anything more than a downstream or overlapping effect of other mechanisms.

The reputation: Antagonism of the dopamine receptor D2 pathway

Dopamine is a catecholamine neurotransmitter whose function ranges from voluntary movement to reward behaviour and other neurological functions.¹⁷ Dopamine-related signalling occurs through two groups of G-protein-coupled receptors: the D1-like receptors, comprised of dopamine receptor D1 and dopamine receptor D5 (DRD5), which activate adenylyl cyclase, and the D2-like receptors, comprised of dopamine receptor D2 (DRD2), dopamine receptor D3 and dopamine receptor D4, which inhibit adenylyl cyclase. Though beyond the scope of this review, a link between dopamine and cancer has been posited for decades, stemming from observations of an inverse linkage between Parkinson’s disease (a dopamine-deficiency disorder) and cancer, as well as complex and conflicting relationships between cancer and the use of dopaminergic antipsychotics.¹⁸ This was further clarified in the molecular era when reviews of The Cancer Genome Atlas (TCGA) revealed broad DRD2 overexpression across a variety of tumour types, including the aggressive brain tumor glioblastoma (GBM), that correlated with poor prognosis.¹⁹

An unbiased drug screen, using a Bayesian design for drug-target prediction, initially discovered the link between DRD2 and ONC201.²⁰ Integrating data about drug structure, function and receptor-interaction biology, the interaction was predicted in an unbiased approach by comparing with novel drug-receptor binding partners to established data about drugs with known targets and established interactions.²⁰ This was confirmed experimentally, whereby GBM cell sensitivity to ONC201 is inversely correlated with DRD5 expression, a negative regulator of DRD2 activity.¹⁹ Radioligand binding studies subsequently demonstrated that ONC201 antagonizes DRD2 via allosteric changes at concentrations needed for anti-proliferative effects and in a dopamine-independent manner.^21,22

The downstream consequences are several-fold. First, ONC201 disrupts DRD2 receptor engagement with scaffolding proteins, such as beta-arrestin, which inhibits pro-proliferative MAPK and PI3K/AKT signalling pathways, as well as a metabolic reprogramming that suppresses glycolysis and oxidative phosphorylation.^21,23,24 Second, because DRD2 normally suppresses adenylyl cyclase levels, ONC201 raises cyclic adenosine monophosphate (cAMP) levels and leads to increases in pro-apoptotic, cAMP-responsive factors, including activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP).²¹ Finally, DRD2 has been shown to activate MET to promote GBM stem-like cell growth.²⁵ Interestingly, ONC201 may have additional effects on the tumour microenvironment, in which GBM-co-cultured monocyte-derived macrophages make an inflammatory switch to a pro-inflammatory state in the presence of ONC201, but whether this is a direct effect of ONC201 or an interaction with GBM cells is not fully elucidated.²⁶

Though DRD2 antagonism is an established mechanism of action for imipridones, it again does not appear to be the dominant mechanism, in large part due to observations of loss of mitochondrial integrity and function, which has no established link to DRD2 signalling. Furthermore, pharmacological inhibition and genetic knockout of DRD2 did not fully abrogate ONC201, recapitulating its known cytotoxic effects.²⁷ While imipridones are still associated with DRD2 antagonism, a third, more prominent mechanism has emerged.

The contender: Agonism of the caseinolytic mitochondrial matrix peptidase proteolytic subunit (Caseinolytic Protease P, ClpP)

The most recent and most significant mechanism of action is agonism of Caseinolytic Protease P (ClpP). ClpP is part of the two-subunit mitochondrial matrix protease (ClpXP), wherein the Clp subunit X (ClpX) functions as a cap that recognizes and translocates serine-phosphorylated substrates into the ClpP receptacle to initiate proteolysis and protein degradation.²⁸ Its primary role is thought to be the degradation of faulty mitochondrial respiratory chain proteins.

A drug screening panel of ONC201 analogues discovered highly potent inhibition of cell proliferation via induction of the mitochondrial integrated stress response (ISR) proteins ATF4 and CHOP.²⁹ Mass spectroscopic and small interfering ribonucleic acid (siRNA) knockout studies, as well as ClpP knockout studies, confirmed the interaction between ONC201 and ClpP, with ONC201 binding ClpP and causing an allosteric transition of the protease to a consistently extended, activated conformation.^29–31 Crystallographic studies further elucidated the breadth of conformational changes, in which ONC201 binds ClpP external to the catalytic site and induces widening of the ClpP entry pore, structurally alters the catalytic triad within the active site to enhance its activity, and increases the dynamism of the N-terminal residues that disrupt the regulatory function of the ClpX ‘cap’.³² The ClpP protease is therefore thought to increase degradation of mitochondrial proteins either by negating the ClpX gatekeeper function, which then permits degradation of non-serine-phosphorylated substrates by a conformationally altered catalytic domain, or by outright dissociation or degradation of the ClpX subunit in the context of an otherwise widened ClpP entry pore.^28,33

The most direct effect of ClpP agonism and hyperactivation by ONC201 is damage to the mitochondria. Observed effects include mitochondrial swelling, reduction in the mitochondrial membrane potential and increases in reactive oxygen species (ROS) production, with eventual cristae whirl formation and disintegration leading to total collapse of the mitochondria and vacuolation.^33–35 This results in activation of the ISR and upregulation of its downstream effectors, including phosphor-eukaryotic initiating factor 2-alpha (eIF2-alpha), DR5 and, notably, ATF4 and CHOP, followed by activation of intrinsic apoptosis via cleaved caspase-3 and -7, PARP and X-linked Inhibitor of Apoptosis (XIAP).^33,36,37 Other consequences of mitochondrial disruption include inhibition of oxidative phosphorylation and a switch towards glycolytic adenosine triphosphate (ATP) production, with consequent alterations in glutamate transport dynamics, with cancer cells most dependent on oxidative phosphorylation at baseline demonstrating increased sensitivity to imipridones.^26,38 In a similar vein, upregulation of genes associated with mitochondrial metabolism (steroid biosynthesis, oxidative phosphorylation and the tricyclic acid (TCA) cycle) predicts radiographic responses to ONC201 in patients.³⁹ Imipridones also cause transcriptional changes, with lowered expression of mitochondrial inner membrane and matrix genes, and epigenetic changes, including upregulation of genes involved in chromatin accessibility and downregulation of cell cycle regulator and neuroglial differentiation genes, with others observing a phenotypic shift from an oligodendrocyte-like progenitor cell to a more mature and differentiated astrocytic-like cell population.^33,39,40 Finally, IDH mutations and H3K27M mutations are mutually exclusive, with ONC201 increasing glutamate and L-2-hydroxyglutarate (L-2HG) concentrations in H3K27M-altered glioma responders, again highlighting imipridone mechanisms related to mitochondrial metabolism.³⁹

Mechanistic summary: A confluence of pathway activation

The mechanism of imipridone anti-cancer activity is multifaceted, and it is difficult to assign primacy to any one pathway. The evidence to date suggests that imipridone does not operate by any single mechanism but instead has distinctive pathways with overlapping effects. For example, reduction of ROS products by pre-treatment with N-acetylcysteine decreased the efficacy of imipridones, while pre-treatment with the DRD2 agonist dopamine had no effect on activation of the ISR, confirming ClpP agonism as a separate, DRD2-independent mechanism of action.^22,35 However, both ClpP agonism and DRD2 antagonism lead to increased expression of ATF4 and CHOP, as well as initiating cell cycle arrest and other cellular alterations that can lead to reduced DRD2 expression, thus showing overlapping effects.⁴¹ Similarly, ClpP agonism has been demonstrated to initiate extrinsic cell death pathways independent of ClpP dysregulation, and even the original TRAIL-receptor ligand hypothesis has been implicated as a separate mechanism from mitochondrial disruption, though past observations of mTOR inhibitor synergy with imipridones may be due to the effects of mTOR inhibition on increasing cellular dependency on mitochondrial respiration rather than to PI3K/AKT dysregulation.^13,41–43

Expansion of clinical trial work leading to FDA approval

The first-in-human trial of ONC201 (A First-in-Human Phase I Single-Agent Open-Label Dose-Escalation Study of Every Three-Week Dosing of Oral ONC201 in Patients With Advanced Solid Tumors; ClinicalTrials.gov identifier: NCT02250781) began in early 2015 and established the recommended phase II dose of 625 mg weekly.³⁸ This original trial recruited patients with refractory solid tumours but did not include any patients with primary CNS tumours.⁴⁴ A subsequent phase II trial began in early 2016 and tested weekly ONC201 as monotherapy in 17 patients with recurrent isocitrate dehydrogenase (IDH)-wild-type GBM. This study failed to meet its primary endpoint of 6-month progression-free survival (PFS6) (11.8%); however, a single patient with what was at the time classified as a secondary GBM with an H3.3K27M mutation had a sustained partial response.⁴⁵ A follow-up phase II trial in recurrent IDH-wild-type GBM similarly failed to meet its primary endpoint, with PFS6 of 5% and a median PFS of 1.8 months, but once again, three patients with H3K27M-mutant tumours were incidentally enrolled in this study, one of whom had a complete response of multiple lesions.⁴⁶ On the basis of these two unexpected responses, ONC201 was selected for further testing in patients with H3K27M mutant tumours, now known as DMG (H3K27-altered DMG).

Multiple single-arm phase II trials, as well as expanded access programmes (EAPs), subsequently opened evaluating ONC201 in adult and paediatric patients with H3K27-altered DMG. Results from 18 patients enrolled in the EAP through April 2019 demonstrated clinical benefit, with 5/18 patients continuing treatment without progression for a median of 53 weeks (range, 41–81.9 weeks). Of these, one adult patient had a complete response sustained for over 17 months at the time of publication, and another had a partial response sustained for over 12 months.⁴⁷ Further analysis of patient outcomes from the phase I paediatric trial ONC014 and the EAP ONC018 for paediatric and adult patients, as part of a broader assessment of mechanism of response, also suggested a benefit in H3K27-altered DMGs. The combined mOS was 21.7 months in those enrolled post-radiation prior to progression and 9.3 months for those enrolled at disease recurrence. Despite numeric differences in survival between the two studies individually, these were not statistically significant (Table 1).^39,48,49

Because of the rarity and malignancy of the disease and the small size of each study, patient data from five studies – three clinical trials ONC006 (Oral ONC201 in Adult Recurrent Glioblastoma; ClinicalTrials.gov identifier: NCT02525692), ONC013 (A Phase II, Open-label Study of ONC201 in Adults With Recurrent High-grade Glioma; ClinicalTrials.gov identifier: NCT03295396), ONC014 (ONC201 in Newly Diagnosed Diffuse Intrinsic Pontine Glioma and Recurrent/Refractory Pediatric H3 K27M Gliomas; ClinicalTrials.gov identifier: NCT03416530), and two EAPs (ONC018 [Expanded Access to ONC201 for Patients With H3 K27M-mutant and/or Midline High Grade Gliomas; ClinicalTrials.gov identifier: NCT03134131] and ONC016 [Expanded Access Use of ONC201 in a Patient With Diffuse Intrinsic Pontine Gliomas; ClinicalTrials.gov identifier: NCT05392374]) – were pooled into a combined analysis to provide meaningful safety and efficacy data for the US Food and Drug Administration (FDA) review.^{45,46,50–52} The FDA was involved in guiding the prespecified eligibility criteria used to identify patients eligible for analysis, and patients were subsequently evaluated by blinded independent central review, with overall response rate (ORR), based on Response Assessment in Neuro-Oncology High-grade Glioma (RANO-HGG) criteria, serving as the primary endpoint.

Patients eligible for the integrated analysis were at least 2 years old with recurrent or progressive H3K27M-mutant glioma and measurable disease per RANO-HGG. All patients had a Karnofsky performance status of at least 60, received radiation at least 90 days prior to the first dose of ONC201 and were on a stable or decreasing dose of steroids for at least 3 days prior to the baseline scan. Patients with leptomeningeal involvement, primary spinal tumours and DIPG were excluded.

Fifty out of 374 patients met eligibility, with negative or unknown H3K27M status (n=92) and absence of progressive or measurable disease (n=89) being the most common reasons for exclusion. The majority were adults (64%), with a median age of 30 years (range, 8–70). One patient received ONC201 every 3 weeks, but all others were administered weekly. ORR was 20% by RANO-HGG (95% confidence interval [CI], 10.0–33.7), and the duration of response was 11.2 months (3.8–NR [not reached]). Signs of clinical benefit, including corticosteroid response (time to response [TTR], 3.7 months; range, 1.9–5.6 months) and improvement in performance status (TTR, 3.5 months; range, 1.9–22.4 months), occurred prior to objective response (median TTR, 8.3 months; range, 1.9–15.9 months). Treatment–related adverse events (TRAEs) were common but typically mild, with fatigue, nausea and headache reported in over 30% of patients. Ten patients experienced a grade 3 TRAE, with fatigue being the only one to occur in more than two patients.⁵⁰

After reviewing these data, the FDA approved dordaviprone on 6 August 2025.⁵³ To more definitively assess benefit in newly diagnosed patients with H3K27-altered DMG, the phase III ACTION study, randomizing patients with thalamic H3K27-altered DMG to ONC201 versus placebo post-radiation, is ongoing.⁵⁴ This international, multi-site trial continues to accrue outside of the USA post-approval, and results are expected in the latter half of 2026.

Combining data from small studies is not a preferred standard for FDA approval. The inclusion of only 50 out of 372 total patients (13.4%), based primarily on molecular, anatomical and clinical parameters, highlights the push–pull of scientific jurisprudence and selection bias risks. The researchers, in partnership with the FDA, sensibly sought to make as homogeneous a population as possible and reduce bias with blinded independent review. Nonetheless, small numbers and carefully stratified patients limit the ability to account for significant heterogeneity in terms of study designs and their respective eligibility criteria (i.e. stringency/flexibility in phase II versus phase I versus EAP settings, none of which were randomized), as well as age groups (i.e. paediatric versus young adults, where disease biology is different). Furthermore, exclusion of other known midline locations and leptomeningeal disease (LMD) limits the generalizability. While the FDA decision is, on the one hand, justifiable to approve a well-tolerated medication for an orphan disease that is rapidly and universally fatal and in dire need of treatment options, it is also prudent to recognize the limitations and appreciate the importance of follow-up research in more rigorously establishing the efficacy and generalizability of dordaviprone.

Future directions of imipridone therapies

While a major advancement to the field of neuro-oncology and a welcome reprieve for patients, dordaviprone therapy in its current iteration is not a panacea. The efficacy of dordaviprone is both time- and anatomically limited, with thalamic locations deriving more benefit than pontine and spinal ones. Whether this is due to differences in tumour biology or pharmacology remains to be seen, but future directions are focusing on building upon imipridone monotherapy by either optimizing the compound and/or fashioning rational combination therapies.

Rational combination therapy with the current generation of imipridones

Much of solid tumour oncology care with systemic agents is delivered in combination with other regimens to target multiple vulnerabilities and/or pathways simultaneously and minimize the development of resistance, but this is a difficult prospect in CNS tumours, where survival is short and quality of life can be compromised by agents with otherwise low efficacy. However, because dordaviprone is well-tolerated by patients and demonstrates minimal off-target toxicity, combination regimens may be more feasible.⁵⁰ The lowest barrier for evaluation is adding dordaviprone to the current standard of care, radiation with temozolomide, indicated for high-grade gliomas, which has shown promise in GBM models.⁵⁵ Co-targeting the mitogen-activated protein kinase (MAPK) and/or the phosphatidylinositol-3-kinase (PI3K)/AKT pathway is also being investigated, including adding a MEK inhibitor to dordaviprone as a dual therapy or in combination with AKT inhibition as a triple therapy.^14,56 Others have considered combinations with epigenetic modifiers, given the outsized effects in histone-altered gliomas, including Enhancer of Zeste 1 (EZH1) and Histone Deacetylase (HDAC) inhibitors, and in vitro studies suggesting co-administration with epigenetic modifiers may have effects on the immune microenvironment, such as increasing M1-weighted macrophage and CD3+ and CD8+ T-cell populations.^57,58 Finally, inhibition of glycolysis and/or glucose restriction might also exacerbate the ISR in combination with imipridone therapy.³⁸ Finally, although imipridones are known to activate TRAIL without themselves being TRAIL ligands, the addition of TLY012 (a chemically modified TRAIL receptor agonist) to imipridone therapy in vitro and in patient-derived xenograft (PDX) models appears to amplify caspase cleavage and cell death.⁵⁹

Optimizing next-generation imipridones and ClpP inhibitors

Imipridone derivatives ONC206 and ONC212 were synthesized in 2017 to improve upon dordaviprone.²³ ONC206 is a structural analogue of dordaviprone with ten-fold greater potency and stronger inhibition of cellular migration, while showing similar mechanistic effects, such as increasing ClpP and decreasing anti-apoptotic markers.^33,60 Other analogues are also in development, including IMP075, which shifts around the chemical moieties of ONC021 to achieve higher ClpP agonist potency, and XT6, which promotes formation of the human ClpP (hClpP) tetradecamer and further boosts ClpP-associated mitochondrial protein degradation.^61,62 Consistently, there has been an appreciation that maintaining the core chemical structure of the imipridone is sufficient to maintain properties, while modifications can alter the potency.^63,64

Other groups have modified the current imipridone structure while adding new functionality, and those in development include compounds modified by fluoridation, ferrocene-containing compounds, additions of alkyne- or triazole-linked warheads and [1,8]naphthyridinone scaffolding.^16,65–67

Finally, more recent efforts have moved away entirely from imipridones to fashion inhibitors directly by screening for ClpP interaction and agonism and to create wholly new compounds. One example is compound 9, tested in breast cancer models, that disrupts mitochondrial electron transport chain activity more potently. Another are TR compounds, which are highly selective small-molecule activators of ClpP which, when tested in colorectal cancer (CRC) cell lines, arrested cell cycle activity at nanomolar concentrations and potently downregulated mitochondrial function, leading to mitophagy and ferroptosis.^68,69

Conclusions

Imipridones started from a humble beginning, rescued from the dusty shelves of history by both demonstrating preclinical and clinical efficacy in a disease that previously had no available therapies and by leading to the discovery and characterization of under-appreciated or outright new mechanisms of anti-cancer therapy via DRD2 antagonism and ClpP agonism, respectively. The story is not over for imipridones, including their derivatives, modifications and followers, and the promise of effective anti-cancer therapies through both new and better-tolerated means remains tantalizingly close on the horizon.