H3 K27M-mutant Glioma Patients Research Articles

Abstract 4914: Role of ClpP in the anti-cancer effects of imipridone ONC201 and ONC206

Abstract ONC201 is the first bitopic dopamine receptor D2 (DRD2) antagonist and allosteric mitochondrial protease ClpP agonist. Downstream of target engagement, ONC201 activates the integrated stress response pathway resulting in selective tumor cell death. ONC201 is well tolerated and induces durable tumor regressions in H3 K27M-mutant glioma patients. ONC206, a derivative of ONC201 currently in Phase I trials for adult and pediatric brain tumors, is also a bitopic DRD2 antagonist and ClpP agonist with nanomolar potency. We further evaluated in vitro efficacy and mechanism of action specific to ClpP for ONC206 and investigated acquired resistance to ONC201 and ONC206. FITC-casein and peptide degradation assays confirmed that ONC206 acts as an agonist of human ClpP with enhanced potency relative to ONC201. Broad efficacy in the nanomolar range for ONC206 (GI50 <78-889nM, 72h) was observed in 1,088 Genomics of Drug Sensitivity in Cancer (GDSC) cancer cell lines with increased sensitivity in cell lines exhibiting high ClpP mRNA expression. Among solid tumors, nervous system-related cell lines were particularly sensitive. Gene Expression Profiling Interactive Analysis (GEPIA) database analysis showed ClpP mRNA was overexpressed in glioblastoma cells relative to normal cells. GDSC observations were confirmed in glioblastoma (GBM) (T98G, A172, U87MG), H3.3 K27M mutant diffuse intrinsic pontine glioma (DIPG) (SF8628, SF7761) and astrocytoma (H4, U118MG) cell lines. ONC206 was more potent than ONC201 in cell viability assays for all glioma lines tested. SF8628 cells with a CRISPR-mediated ClpP knockout were shown to demonstrate resistance to both ONC201 and ONC206. Polyclonal resistant cell lines were generated through prolonged passaging in increasing concentrations of ONC201 or ONC206 (SF8628 and T98G cells). Cells with acquired resistance to ONC201 (ONC201Res) or ONC206 (ONC206Res) showed cross resistance to both imipridones. Time-stability of resistance was confirmed by removing drug for 2 to 4 weeks and retesting. RNAseq analysis revealed upregulation of the integrated stress response with imipridone treatment in T98G parental cells but not resistant clones. Whole genome sequencing revealed ClpP mutations in both ONC201Res and ONC206Res cells. Our results confirm the role of ClpP in the mechanism of action of ONC201 and ONC206 in tumor cells. Citation Format: Andrew Lee, Cristina Maranto, Scott Foster, Sara Morrow, Joshua E. Allen, Randall Lanier, Phiroze Sethna, Varun V. Prabhu. Role of ClpP in the anti-cancer effects of imipridone ONC201 and ONC206. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4914.

EXTH-104. ROLE OF CLPP AND MITOCHONDRIAL METABOLISM IN THE ANTI-CANCER EFFECTS OF IMIPRIDONE ONC201 AND ONC206 IN GLIOBLASTOMA AND DIPG

Abstract ONC201 is the first bitopic dopamine receptor D2 (DRD2) antagonist and allosteric mitochondrial protease ClpP agonist. ONC201 is well tolerated and induces durable tumor regressions in H3 K27M-mutant glioma patients. ONC206, a derivative of ONC201 currently in Phase I trials, is also a bitopic DRD2 antagonist and ClpP agonist with enhanced nanomolar potency relative to ONC201. We evaluated in vitro efficacy, mechanism(s) of action and acquired resistance for ONC201 and ONC206 in glioblastoma (GBM) (T98G, A172, U87MG), H3.3 K27M mutant DIPG (SF8628, SF7761) and astrocytoma (H4, U118MG) cell lines. ONC206 was more potent than ONC201 in cell viability assays for all line. A mutation in ClpP has previously been associated with resistance to imipridones. Accordingly, SF8628 cells with a CRISPR-mediated ClpP knockout were resistant to both ONC201 and ONC206. Polyclonal resistant cell lines were generated through prolonged passaging in increasing concentrations of ONC201 (SF8628 and T98G cells) or ONC206 (T98G cells). Cells with acquired resistance to ONC201 (ONC201Res) or ONC206 (ONC206Res) showed cross resistance to both imipridones. Time-stability of resistance was confirmed by removing drug for 2 to 4 weeks and retesting. Whole genome sequencing revealed ClpP mutations in both ONC201Res and ONC206Res T98G cells. RNAseq analysis revealed upregulation of the integrated stress response, apoptosis pathways, and amino acid metabolism with imipridone treatment in T98G parental cells but not resistant clones. Depleting glucose in the media enhanced cell viability inhibition by imipridones, indicating mitochondrial metabolism may contribute to their mechanism of action. Our results identify ClpP and mitochondrial metabolism as key aspects for the mechanism of action of ONC201 and ON206 in DIPG and GBM cell lines.

CTNI-36. SAFETY OF ONC201 ADMINISTERED TWO CONSECUTIVE DAYS PER WEEK IN PEDIATRIC H3 K27M-MUTANT GLIOMA PATIENTS

Abstract ONC201, an anti-cancer DRD2 antagonist and ClpP agonist, is in Phase II trials for adult H3 K27M-mutant diffuse midline gliomas. The recommended phase 2 dose (RP2D) of 625mg ONC201 once a week has been established as a biologically active dose that is well tolerated in adult and pediatric populations. Radiographic regressions with single agent ONC201 have been reported in recurrent H3 K27M-mutant glioma patients. In another study, twice/week dosing was explored in adult patients and deemed to be safe (no DLTs observed). This warranted exploration of twice/week dosing in pediatric patients and will be discussed in this presentation. This multi-arm, dose-escalation and dose-expansion trial (ONC014; NCT03416530) determined the pediatric RP2D of ONC201 administered once per week and twice per week on two consecutive days. ONC201 was orally administered and scaled by body weight. Dose escalation was performed by a 3 + 3 design beginning with two 125mg capsules less than the adult RP2D equivalent. Twelve children (8 females; 4 males) with H3 K27M-mutant gliomas (pons: 8; thalamus: 2; spinal cord: 2) aged 4-19 years have been treated post-radiation: 3 at dose level -1; 3 at dose level 1; 6 as part of the dose expansion cohort on dose level 2. Median KPS was 90 (range 70-100). One treatment cycle was 21 days (6 doses), which also defined the DLT window. Patients were on-treatment for a median length of 4 cycles (range: 2-11). Twice weekly dosing of ONC201 was tolerated well, as observed with weekly dosing, with no instance of DLT. A total of 4 SAEs were reported, none of which were related to the study drug. The most common AEs (regardless of relatedness) included headache, facial nerve disorder, abducens nerve disorder, nausea, fatigue and ataxia. Additional safety data, PK, and clinical outcomes from this arm will be reported.

Abstract 1370: ADME and toxicology profiles of first-in-class DRD2/ClpP-targeted imipridone ONC201

Abstract Imipridone ONC201 is a first-in-class DRD2 antagonist and ClpP agonist that is well tolerated and induces durable tumor regressions in H3 K27M-mutant glioma patients with once weekly dosing. We evaluated the toxicology, absorption, distribution, metabolism and excretion of ONC201 in animals. Repeat-dose 28-day studies with weekly oral ONC201 in rats and dogs revealed NOAELs (75 and 60 mg/kg, respectively) that represent higher dose levels compared to the 625 mg clinical dose. Genotoxicity (AMES, in vitro and in vivo micronucleus assays) assays demonstrated that ONC201 is not mutagenic at exposures above the RP2D and 2µM average Cmax in patients. The 3T3 neural red uptake assay showed ONC201 is not a phototoxin (photo-irritant factor 1.1). In embryofetal development studies, ONC201 did not lead to maternal effects in rats or rabbits and caused fetal malformations in 3/61 rabbit fetuses with daily dosing (from gestation day 7-17 or 19) at 62.5 and 25 mg/kg, respectively. A quantitative whole-body autoradiography study conducted in rats with one dose of [14C]-ONC201 showed rapidly tissue distribution peaking at 1h in most tissues and exhibiting a plasma terminal half-life of 5.6h. The endocrine, metabolic/excretory, ocular and GI tract tissues contained the highest distribution of [14C]ONC201. [14C]-ONC201 was distributed evenly across CNS substructures, including the midline region. Consistent with the uniform distribution, ONC201 displayed high permeability (23-31×10-6 cm/s, 7-700 μM) and was not a substrate of efflux transporters in Caco-2 cells (efflux ratio 0.46-0.79). ONC201 exerted inhibitory potential on MDR1- and BCRP-mediated transport at 200μM. Clinical trials with ONC201 restrict use of concomitant medications that induce or inhibit CYP enzymes, but several supportive medications can affect these enzymes. In vitro studies revealed that ONC201 is not an inducer of 7 major human CYP enzymes but does inhibit and is a substrate of CYP 2B6, 2C8, 2C9, 2C19, 2D6 and 3A4 (IC50 20-90µM). Analysis of dexamethasone, a known inducer of CYP3A4, did not show a correlation between baseline dose level and ONC201 plasma concentrations in H3 K27M-mutant glioma patients (n=22). Metabolic profiling in human hepatocytes revealed only one metabolite, ONC207, that was >10% in abundance. ONC207 did not achieve an IC50 in cancer cell viability and DRD2 antagonism assays. Assaying by LC-MS/MS of plasma samples obtained from ONC201-treated H3 K27M-mutant glioma patients (n=20) confirmed the presence of ONC207 as the major metabolite. A mass balance study conducted in rats with one dose of [14C]-ONC201 showed that the main route of excretion was biliary (61.18%) via the feces (69.78%), with urine (24.99%) also contributing to overall recovery. Together, these data inform the administration of ONC201 to advanced cancer patients who often require a range of comedications and comorbidities. Citation Format: Sara Morrow, Rohinton Tarapore, Ailan Guo, Magdalena Nej, Yunlan Fang, Gina Theerman, Leah Lake, Martin Stogniew, Varun V. Prabhu, Joshua E. Allen. ADME and toxicology profiles of first-in-class DRD2/ClpP-targeted imipridone ONC201 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1370.

Abstract 393: Predictive biomarker evaluation and molecular differentiation for imipridones ONC201 and ONC206

Abstract Imipridone ONC201 is a first-in-class dopamine receptor D2 (DRD2) antagonist and mitochondrial protease ClpP agonist that is well tolerated and induces durable tumor regressions in H3 K27M-mutant glioma patients. ONC206, a chemical derivative of ONC201 currently in Phase I trials for central nervous system tumors, is also a DRD2 and ClpP dual targeting imipridone with differentiated target engagement and nanomolar potency. We explored molecular differentiation and predictive biomarker studies for ONC201 and ONC206 using gene expression profiling, development of resistance clones and RNA-seq/proteomics in large cancer cell line panels including the 1000-cell line GDSC panel. Gene expression profiling revealed ONC206 and ONC201 induce partially overlapping signatures in U87 glioblastoma cells. Similarly, T98G glioblastoma cells with acquired resistance to ONC201 or ONC206 revealed partial cross resistance and combinatorial efficacy, supporting distinct functional effects. Next, we evaluated a curated 35-gene panel corroborated from prior literature on ONC201 including DRD2- (DRD2, DRD5, TH, EGFR), ClpP- (ClpP, NDUFS7, NDUFA12, SDHA, SDHB, POLD2, POLDIP2), epigenetic- (EZH2, EED, EZHIP, KDM1A, KDM3A, KDM4A, KDM6A, KDM6B), hypoxia- (HIF1, HIF2, HIF3, VHL, VEGFA, HES1, TGM2, CELF2, P4HA2, CEBPB) and downstream signaling-related (ATF3, ATF4, MYC, MYCN, GNA11, GNA15) genes as predictive biomarkers for ONC201/ONC206 efficacy (IC50, IC90, AUC and mRNA fold change) in the GDSC panel using CCLE RNA-seq data. Optimized thresholds for predictive power revealed that the top molecular correlatives prioritized for ONC201 (ClpP, TGM2, NDUFS7, EZH2, EGFR, POLD2, HIF2A, CEBPB, HES1) and ONC206 (ClpP, TGM2, NDUFS7, EZH2, EGFR, HIF1, VHL, ATF4, MYC, KDM6B) in GDSC partially overlap. Biomarker combinations with the prioritized markers improved predictive power compared to single markers. Dual (both markers must be co-expressed) or independent combinations involving EGFR, NDUFS7, EZH2 and ClpP were most predictive for ONC206 while those involving ClpP, EGFR and EZH2 were most predictive for ONC201. Results from GDSC for ONC201 were further confirmed using the PRISM dataset with RNA-seq and proteomics that revealed similar results. Hypoxic tumor cell cultures confirmed GDSC findings that elevated tumor cell hypoxia can impart ONC201 or ONC206 resistance. Thus, ONC206 is a distinct agent that may be uniquely poised to address tumors that are not addressed by or have developed acquired resistance to ONC201. Our results indicate that for accurate prediction of ONC201 and ONC206 clinical benefit, a curated combinatorial biomarker approach for each tumor type, using diverse detection approaches such as protein expression, genomics, and transcriptomics, may be used in ongoing clinical trials. Citation Format: Sara Morrow, Kirti Nath, Yiqun Zhang, Mathew J. Garnett, Ultan McDermott, Cyril H. Benes, Joel Basken, Wafik S. El-Deiry, Joshua E. Allen, Varun V. Prabhu. Predictive biomarker evaluation and molecular differentiation for imipridones ONC201 and ONC206 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 393.

DIPG-52. PHASE I CLINICAL TRIAL OF ONC201 IN PEDIATRIC H3 K27M-MUTANT GLIOMA OR NEWLY DIAGNOSED DIPG

H3 K27M-mutant gliomas often manifest as midline gliomas, have a dismal prognosis, and have no effective treatments. ONC201 efficacy has been shown in high-grade glioma preclinical models and durable responses with single agent ONC201 have been reported in adults with recurrent H3 K27M-mutant gliomas. These observations led to a Phase I pediatric clinical trial of ONC201 dosed by body weight. This multi-center, open-label, 3 + 3 dose-escalation and dose-expansion clinical trial (NCT03416530) for H3 K27M-mutant glioma or non-biopsied DIPG has 6 arms: arms A and E determine the RP2D in pediatric post-radiation (recurrent or not-recurrent) H3 K27M-mutant glioma patients with ONC201 administered as an oral capsule as well as a liquid formulation, respectively. Both arms have completed accrual. The study is currently enrolling newly diagnosed DIPG patients to determine the RP2D for ONC201 in combination with radiation (arm B). Dedicated assessment of intratumoral ONC201 concentrations in midline gliomas patients (arm C) and the effects of ONC201 in H3K27M DNA levels in circulating CSF (arm D) are currently enrolling patients. ONC201 as a single agent in patients with progressive H3K27M mutant tumors following irradiation (excluding DIPG/spinal cord tumors) was recently opened (arm F). Once the RP2D is confirmed, there is a dose-expansion cohort to confirm the safety, radiographic efficacy and survival with ONC201. The primary endpoints of arms A, B, and E have been established with the RP2D of 625mg scaled by body weight as a capsule or liquid formulation administered alone or in combination with radiation without incidence of dose-limiting toxicity.

HGG-18. CLINICAL EFFICACY OF ONC201 IN THALAMIC H3 K27M-MUTANT GLIOMA

ONC201, a bitopic DRD2 antagonist and allosteric ClpP agonist, has shown encouraging efficacy in H3 K27M-mutant glioma. Given that the thalamus has the highest extra-striatal expression of DRD2, we performed an integrated preclinical and clinical analysis of ONC201 in thalamic H3 K27M-mutant glioma. ONC201 was effective in mouse intra-uterine electroporation (IUE)-generated H3 K27M-mutant gliomas, with an in vitro IC50 of 500 nM and 50% prolongation of median survival in vivo (p=0.02, n=14). We analyzed thalamic H3 K27M-mutant glioma patients treated with ONC201 on active clinical trials as of 5/22/19 enrollment (n=19 recurrent and 10 post-radiation, non-recurrent; 5–70 years old). As of 12/18/2019, PFS6 and OS12 are 26.3% and 36.8%, respectively, in the recurrent group. For non-recurrent patients, with median follow up of 21.9 months (8.6–26.6) from diagnosis, median PFS or OS have not been reached. This surpasses historical OS of 13.5 months. Best response by RANO includes 1 CR, 3 PR, 4 SD, 8 PD for recurrent patients and 2 PR, 4 SD, 1 PD for non-recurrent patients (4 on-trial patients experienced regressions that are yet unconfirmed responses). Median duration of response for recurrent patients is 14.0 months (2.0–33.1). Furthermore, H3 K27M cell-free tumor DNA in plasma and CSF correlated with MRI response. In summary, single agent ONC201 administered at recurrence, or adjuvantly following radiation, demonstrates promising clinical efficacy in thalamic H3 K27M-mutant glioma patients who currently have no effective treatments following radiation. Investigations are ongoing to assess whether micro-environmental DRD2 expression explains the early exceptional responses in thalamic H3 K27M-mutant glioma.

EXTH-74. IND-ENABLING CHARACTERIZATION OF DUAL DRD2- AND ClpP-TARGETING AGENT ONC206 AS THE NEXT IMIPRIDONE FOR CLINICAL NEURO-ONCOLOGY

Abstract ONC201 is the first bitopic antagonist of dopamine receptor D2 (DRD2) and allosteric mitochondrial protease ClpP agonist, that is well tolerated and induces durable tumor regressions in H3 K27M-mutant glioma patients. ONC206, a derivative of ONC201, is also a bitopic DRD2 antagonist that exhibits enhanced non-competitive effects, nanomolar potency, and disruption of DRD2 homodimers. In these studies, a FITC-casein degradation assay revealed that ONC206 also acts as an agonist of human ClpP and has a 3-fold improved potency. GEPIA database analysis showed ClpP mRNA was overexpressed in glioblastoma cells relative to normal cells. Broad nanomolar efficacy of ONC206 (GI50 < 78-889nM, 72h) was observed in >1,000 GDSC cancer cell lines with the highest sensitivity in cell lines exhibiting high ClpP and/or DRD2+/DRD5- RNA expression signatures. Among solid tumors, nervous system related cell lines were particularly sensitive. ONC206 reduced the viability of normal human fibroblasts at higher doses (GI50 > 5µM), suggesting a wide therapeutic window. Antitumor efficacy without body weight loss was observed with 50 mg/kg weekly oral ONC206 in a subcutaneous xenograft model of DRD2-overexpressing, dopamine-secreting tumor cells. Oral ONC206 at 50mg/kg exhibited a ~12 µM plasma Cmax and ~6 hours terminal half-life in Sprague-Dawley rats. Additionally, 5–10 fold higher ONC206 concentrations were observed in adrenal gland, bile duct, brain and bone marrow relative to plasma. GLP toxicology studies with weekly oral ONC206 in Sprague-Dawley rats and beagle dogs revealed no dose-limiting toxicities. Mild and reversible body weight changes were observed at the highest evaluated dose in both species. The no-observed-adverse-effect level was ≥ 16.7 mg/kg in dogs and ≥ 50 mg/kg in rats that exceed efficacious doses. A 50 mg starting dose of ONC206 was selected for the first-in-human open label, dose escalation, and food effect Phase I study in biomarker-enriched adult recurrent primary CNS tumors.

CTNI-15. CLINICAL EFFICACY OF ONC201 IN NEWLY DIAGNOSED DIPG AND IN PREVIOUSLY IRRADIATED PEDIATRIC H3 K27M-MUTANT GLIOMAS

Abstract ONC201, an anti-cancer DRD2 antagonist and ClpP agonist, is in Phase II trials for adult H3 K27M-mutant diffuse midline gliomas. In adults, the recommended phase 2 dose (RP2D) of 625mg ONC201 once a week has been established as a biologically active dose that is well tolerated. Radiographic regressions with single agent ONC201 have been reported in adult recurrent H3 K27M-mutant glioma patients. This multi-arm, dose-escalation and dose-expansion trial determined the pediatric RP2D of ONC201 administered as an oral capsule (Arm A) or liquid formulation (Arm E) in post-radiation H3 K27M-mutant glioma (Arm A) or in newly diagnosed DIPG (Arm B) patients. Molecular assessments include intratumoral ONC201 concentrations (Arm C) and CSF H3 K27M DNA levels (Arm D). Enrollment as of April 30, 2020 is complete in Arm A (22) and Arm E (26) and continues in Arm B (18/24), Arm C (5/12), and Arm D (22/24). The RP2D of weekly 625mg ONC201 scaled by body weight was confirmed when administered as a capsule or a liquid formulation as a single agent or in combination with radiation without dose-limiting toxicity. The most frequent adverse events regardless of attribution to the drug were predominantly low grade: ONC201 capsule alone was headache (54.5%), nausea (36.4%), and fatigue (36.4%); ONC201 liquid formulation was vomiting (31.8%), headache (22.7%), VIth nerve disorder (22.7%); ONC201 capsules in combination with radiation (Arm B) was headache (47.1%), vomiting (52.9%), nausea (41.2%). Pharmacokinetic analysis in plasma of Arm A patients revealed T1/2: 8.4h; Tmax: 2.1h; Cmax: 2.3ug/mL; AUC0-tlast: 16.4ug/mL, with similar exposure across body weights. In conclusion, when scaled by body weight the ONC201 capsule or liquid formulation alone or in combination with radiation were associated with safety and pharmacokinetic profiles in pediatric H3 K27M-mutant diffuse midline glioma patients that are similar to the experience in adults.

DDRE-33. MOLECULAR DIFFERENTIATION OF IMIPRIDONES ONC201 AND ONC206

Abstract ONC201 is the first bitopic antagonist of dopamine receptor D2 (DRD2) and agonist of ClpP in oncology. In clinical trials, the small molecule has induced durable tumor regressions and clinical benefit in H3 K27M-mutant glioma patients while being well tolerated. ONC206 is a chemical derivative of ONC201 with nanomolar anti-cancer potency. In this study, we describe receptor pharmacology, gene expression profiling, acquired resistance and biodistribution studies that suggest ONC206 exhibits distinct therapeutic properties relative to ONC201. ONC206 exhibited a nanomolar Ki for DRD2 with complete specificity across human GPCRs and complete DRD2 antagonism. Schild analyses of ONC206 in cAMP and β-Arrestin recruitment assays revealed hallmarks of non-competitive DRD2 antagonism, unlike antipsychotics but similar to ONC201. Shotgun mutagenesis across DRD2 identified 7 residues critical for ONC206-mediated antagonism at orthosteric and allosteric sites. Six residues were critical for ONC201 and ONC206, however the impact varied between the two compounds and one allosteric residue was exclusive to ONC206. Structural mapping revealed that some ONC206-critical allosteric residue interactions are located at the interface of TM-IV and –V that mediates the DRD2 homodimer interface. Gene expression profiling revealed ONC206 and ONC201 induce distinct signatures in U87 glioblastoma cells, further supporting distinct functional effects. Similarly, T98G glioblastoma cells with acquired resistance to ONC201 or ONC206 reveal partial cross resistance. Finally, rat biodistribution studies revealed nanomolar CSF concentrations that exceed therapeutic thresholds, unlike ONC201. In summary, ONC206 exhibits increased non-competitive DRD2 antagonism, nanomolar potency, distinct biodistribution, differentiated gene expression and disruption of DRD2 dimers relative to ONC201. Thus, ONC206 may be uniquely poised to address tumors that are not addressed by ONC201 or have developed acquired resistance.

BIOM-47. LONGITUDINAL MONITORING OF PLASMA H3K27M IN DIFFUSE MIDLINE GLIOMA PATIENTS TREATED WITH ONC201

Abstract Diffuse midline glioma, H3 K27M-mutant (DMG) is a 2016 WHO Grade IV glioma that has no established treatment beyond first-line radiation. ONC201 is an investigational small molecule that has been shown to be clinically active in recurrent DMG clinical trials. While biopsies of DMG are sometimes feasible, many patients defer secondary to complication risk. MR scans have many limitations in monitoring DMG progression, including distinguishing pseudoprogression and pseudoresponse and measuring diffuse lesions that often do not contrast enhance. Digital droplet PCR (ddPCR) is capable of sensitively detecting and quantifying the allelic frequency of circulating-tumor DNA (ctDNA) fragments against a backdrop of non-tumor DNA. Using sequence-specific probes for H3F3A (H3.3 K27M) and HIST1H3B (H3.1 K27M) ddPCR detects very low frequency variants and provides an assessment of mutational burden. A pilot cohort of 5 patients treated with ONC201 who had a range of outcomes were assessed with serial ctDNA analyses. Two patients with immediately progressive disease had a concordant H3 K27M ctDNA increase that precedes radiographic detection by 4 weeks. Two patients with >50% tumor regressions while on ONC201 had concordant H3 K27M ctDNA burden at the onset of response and subsequent radiographic progression was preceded by increases in ctDNA 8–16 weeks prior. One patient who had prolonged stable disease had decreased H3 K27M ctDNA burden over time. Upon radiographic progression, the addition of bevacizumab with ONC201 caused a radiographic pseudoresponse, however H3 K27M ctDNA remained stable. These pilot results suggest H3 K27M ctDNA may be a sensitive and accurate biomarker of disease burden. Longitudinal evaluation of H3 K27M ctDNA in a cohort of 34 recurrent contrast-enhancing H3 K27M-mutant glioma patients while on ONC201 will be reported. Primary tumor locations range across the thalamus, cerebellum, basal ganglia, temporal lobe, and midbrain; median age is 31 years old (range 20–70).

EXTH-70. CYP450 AND METABOLIC PROFILING OF ANTI-CANCER IMIPRIDONE ONC201

Abstract Imipridone ONC201 is an investigational agent in phase II clinical trials for high grade gliomas as a single agent once weekly. Systemic and tumor exposure to ONC201 exceeds therapeutic thresholds consistently; however, exposure is variable despite fasted conditions and does not strongly covary with body weight or body surface area. Clinical trials with ONC201 have restricted the use of concomitant medication that induce or inhibit cytochrome P450 (CYP) enzymes; however, several supportive medications for glioma patients such as corticosteroids and anti-seizure medications can affect these enzymes. Studies in pooled human liver microsomes revealed that ONC201 is not an inducer of seven major drug-metabolizing human CYP enzymes: CYP 1A2, 2B6, 2C8, 2C9, 2C19, 2D6, and 3A4. However, ONC201 inhibits multiple CYPs IC50 values ranging from 34.9 to 428.6 μM and is substrate of CYP3A4 and, to lesser extent, other CYPs. Given potential metabolism of ONC201 by these enzymes, we conducted metabolic profiling in human hepatocytes that revealed at least 13 metabolites; however, only one was >10% abundance. The major metabolite, termed ONC207, that lacks a benzyl moiety of ONC201 was assayed by LC-MS-MS in plasma samples obtained from 16 ONC201-treated H3 K27M-mutant glioma patients. Peak plasma concentrations of ONC207 relative to ONC201 was 22% (range: 6%-48%) and its abundance was not correlated with ONC201, suggesting that variable exposure to ONC201 may not be associated with variable metabolism. ONC207 does not inhibit dopamine receptor D2 or cancer cell viability in contrast to the parent compound. In summary, ONC201 is an inhibitor and substrate, but not an inducer, of multiple CYP enzymes. The major metabolite of ONC207 appears to be biologically inactive and is unlikely to explain variable ONC201 exposure or its prolonged biological activity that outlives its systemic presence.

Abstract 5688: IND-enabling characterization of ONC206 as the next bitopic antagonist for oncology

Abstract ONC201 is the first clinical bitopic antagonist of dopamine receptor D2 (DRD2), that is well tolerated and induces durable tumor regressions in H3 K27M-mutant glioma patients. ONC206, a derivative of ONC201 that shares the imipridone core structure, is also a bitopic DRD2 antagonist that exhibits enhanced non-competitive effects, high specificity, nanomolar potency, and disruption of DRD2 homodimers. ONC206 exhibited a Ki of ~320nM for DRD2 with complete specificity across human GPCRs and complete DRD2 antagonism. Schild analyses of ONC206 in cAMP and β-Arrestin recruitment assays revealed hallmarks of non-competitive DRD2 antagonism, unlike antipsychotics but similar to ONC201. Shotgun mutagenesis across DRD2 identified 7 residues critical for ONC206-mediated antagonism at orthosteric and allosteric sites. Six residues were critical for ONC201 and ONC206, however the impact varied between the two compounds and one allosteric residue was exclusive to ONC206 located at the region that mediates the DRD2 homodimer interface. Gene expression profiling revealed ONC206 and ONC201 (upon 200nM treatment, 72 h) induce distinct signatures in U87 glioblastoma cells, further supporting distinct functional effects. Cell lines resistant to ONC201 and ONC206 are being generated to profile acquired-resistance mechanisms. Broad nanomolar efficacy of ONC206 (GI50 <78-889nM, 72h) was observed in >1,000 GDSC cancer cell lines with the highest sensitivity in cell lines exhibiting a DRD2+/DRD5- RNA expression signature. ONC206 reduced the viability of normal human fibroblasts at higher doses (GI50 > 5µM), suggesting a wide therapeutic window. Antitumor efficacy without body weight loss was observed with 50 mg/kg weekly oral ONC206 in a dopamine-secreting HuCCT1 cholangiocarcinoma subcutaneous xenograft model. Oral ONC206 at 50mg/kg exhibited a ~12 µM plasma Cmax and ~6 hours terminal half-life in Sprague-Dawley rats. Additionally, 5-10 fold higher ONC206 concentrations were observed in adrenal gland, bile duct, brain and bone marrow relative to plasma. Nanomolar concentrations were also observed in the CSF above DRD2 antagonism thresholds, unlike ONC201. GLP toxicology studies with weekly oral ONC206 in Sprague-Dawley rats and beagle dogs revealed no dose-limiting toxicities. Mild and reversible body weight changes were observed at the highest evaluated dose in both species. The no observed adverse effect level was ≥ 16.7 mg/kg in dogs and ≥ 50 mg/kg in rats that exceed efficacious doses. A 50 mg starting dose of ONC206 was selected for the first-in-human clinical trial in biomarker-enriched adult recurrent CNS tumors. In summary, ONC206 is poised for clinical introduction as the next imipridone bitopic DRD2 antagonist for oncology that exhibits differentiated target engagement, signaling, and biodistribution profiles. Citation Format: Varun Vijay Prabhu, Sara Morrow, Caroline A. Cuoco, Abed R. Kawakibi, Jinkyu Jung, Neel Madhukar, Matthew J. Garnett, Ultan McDermott, Cyril H. Benes, Robert Wechsler-Reya, Lakshmi Anantharaman, Neil Charter, Joseph B. Rucker, Benjamin J. Doranz, Joel Basken, Olivier Elemento, R. Benjamin Free, David R. Sibley, Martin Stogniew, Wolfgang Oster, Mark R. Gilbert, Sharon DeMorrow. IND-enabling characterization of ONC206 as the next bitopic antagonist for oncology [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5688.

Abstract 4808: ONC201 decreases protein chaperone ClpX to unleash mitochondrial protease ClpP activity, intregrated stress response and tumor cell death

Abstract Brain tumors are one of the most lethal cancers in children and young adults. We previously reported that first-in-class small-molecule imipridone ONC201 can antagonize dopamine receptor D2 (DRD2), inactivate ERK/AKT, induce the integrated stress response (ISR), upregulate pro-apoptotic TRAIL receptor DR5, deplete cancer stem cells, and induce growth arrest or cell death in tumor cells. ONC201 crosses the blood-brain barrier and has induced durable tumor regressions in adult and pediatric H3K27M-mutant glioma patients. We hypothesized that ONC201 may synergize with radiotherapy and temozolomide in brain tumors. Glioblastoma (GBM: SNB19, T98G and U251), diffuse intrinsic pontine glioma (DIPG: SU-DIPG-IV and SU-DIPG-36) and atypical teratoid rhabdoid tumor (ATRT: BT-12, BT-16) cell lines were tested using cell viability or colony formation assays with ONC201 up to 20 μM alone or in combination with radiotherapy up to 8 Gy and temozolomide up to 100 μM. We observed synergy between ONC201 and radiation and temozolomide by multiple assays with the best combination index of 0.31 under the treatment of 0.625 μM ONC201, 25 μM TMZ and 4 Gy irradiation in cell viability assay. We observed induction of PKA substrate phosphorylation as a marker of DRD2 antagonism, induction of ATF4 as a marker of ISR activation, and multiple markers of cell death in treated brain tumor cells. Mitochondrial dysfunction induced by ONC201 was greatly reflected in reduction of maximal cell respiration in brain tumor cell lines. ClpXP is a proteolytic machine in mitochondria comprising of the protein chaperone ClpX and protease ClpP, a recently described binding target of ONC201. We have observed that ONC201 induced cytotoxicity depends on ClpP and knockdown of ClpP strongly protects multiple human cancer cell lines from ONC201-mediated (but not other drugs, e.g. temozolomide) cytotoxicity. The ClpX protein level decreased dramatically upon ONC201 treatment. We are exploring the potential interplay between ClpP, dopamine receptors, the ISR and TRAIL signaling pathways after single agent or combinatorial treatments. Our data suggests that ONC201 may be combined synergistically with temozolomide or radiation to address gliomas, along with potential pharmacodynamic biomarkers. The results implicate ClpX and ClpP in downstream cell death induction following ONC201 treatment of brain tumor cells. Citation Format: Lanlan Zhou, Howard P. Safran, Michael J. Glantz, Wafik S. El-Deiry. ONC201 decreases protein chaperone ClpX to unleash mitochondrial protease ClpP activity, intregrated stress response and tumor cell death [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4808.

ONC201 in previously irradiated pediatric H3 K27M-mutant glioma or newly diagnosed DIPG.

3619 Background: ONC201 is a first-in-class DRD2 antagonist and ClpP agonist that has demonstrated promising activity in high-grade glioma preclinical models and radiographic regressions with single agent ONC201 in recurrent H3 K27M-mutant glioma patients . The recommended phase 2 dose (RP2D) of 625mg ONC201 orally once a week has been established in adult patients as well tolerated and biologically active. ONC201 efficacy has been shown in high-grade glioma preclinical models and radiographic regressions with single agent ONC201 have been reported in adult recurrent H3 K27M-mutant glioma patients. We report results from the first Phase I pediatric clinical trial of ONC201. Methods: This open-label, multi-center trial for pediatric H3 K27M-mutant glioma or non-biopsied DIPG employed a 3+3 dose-escalation and dose-expansion design with 6 arms. Arms A and E, which have completed accrual, determined the RP2D of ONC201 using oral capsule and liquid formulations in post-radiation pediatric H3 K27M-mutant glioma patients ONC201, respectively. Arm B aims to determine the RP2D for ONC201 in combination with radiotherapy in patients with newly diagnosed DIPG. Arms C and D aim to measure intratumoral ONC201 concentrations in midline glioma patients and the impact of ONC201 on H3 K27M DNA levels in CSF, respectively. Arm F was recently opened to study ONC201 as a single agent in patients with progressive H3 K27M-mutant tumors (excluding DIPG and spinal cord tumors) following radiotherapy. After determining the RP2D, a dose-expansion cohort will evaluate the safety, radiographic response, and activity of ONC201. Results: An RP2D of weekly 625mg ONC201 scaled by body weight as a capsule or in liquid formulation was established in the primary endpoints of arms A, B and E alone or in combination with radiation, without incidence of dose-limiting toxicity (DLT). Pharmacokinetic profiles were similar to those observed in adults (T1/2: 8.4h; Tmax: 2.1h; Cmax: 2.3ug/mL; AUC0-tlast: 16.4ug/mL), with similar exposure across body weights. Conclusions: ONC201 was well tolerated without DLTs at the same adult RP2D scaled by body weight as monotherapy or in combination with radiotherapy in pediatric H3 K27M-mutant glioma patients. Further investigation of ONC201 to treat H3 K27M-mutant glioma and DIPG is warranted. Clinical trial information: NCT03416530 .

Single-agent ONC201 in recurrent H3 K27M-mutant diffuse midline glioma.

3615 Background: Recurrent H3 K27M-mutant diffuse midline glioma is a lethal brain tumor that predominantly affects children and young adults and has no effective therapy. ONC201 is a first-in-class orally administered, anti-cancer small molecule that selectively antagonizes the dopamine receptors DRD2/DRD3 and agonizes ClpP, a mitochondrial protease. Prior studies have indicated dysregulated dopamine receptor expression and enhanced ONC201 sensitivity among H3 K27M-mutant gliomas. Methods: Adults with midline H3 K27M-mutant glioma patients were enrolled to a dedicated Phase II clinical trial (NCT03295396), a multi-arm Phase II clinical trial (NCT02525692), and expanded access protocols under the Sponsor’s IND. Results were pooled among patients treated with ONC201 monotherapy through any of these trial with H3 K27M confirmed glioma, progressive and measurable disease by RANO, > 90 days from completion of prior radiation, no evidence of leptomeningeal dissemination, midline location other than primarily pons or spinal cord, and baseline KPS > 60. Using an enrollment cutoff of February 15, 2019 and data cutoff of July 31, 2019, there were 20 patients (NCT03295396, 12; NCT02525692, 7; expanded access, 1). Dosage was 625 mg weekly in 19 and once every 3 weeks in 1. Results: No DLTs or treatment discontinuations due to toxicity occurred. Midline gliomas can exhibit minimal contrast enhancement or exhibit a mixture of contrast-enhancing and non-contrast enhancing regions in the tumor. As a result, blinded independent central review (BICR) of tumor response by MRI was assessed by RANO-HGG and RANO-LGG for each patient to capture contrast-enhancing lesions by T1 post-contrast and non-contrast-enhancing assessments by T2/FLAIR, respectively, in the object response rate. The best response by RANO-HGG or RANO-LGG is 30% (95% CI, 11.9-54.3%). Duration of response by RANO-HGG is median 52.7 weeks (range 15.9-138.3). One patient with stable disease as of this data cutoff has continued on treatment beyond 12 months and recently underwent an investigator-reported PR by RANO-HGG that is pending confirmation. Conclusions: Single agent ONC201 is well tolerated and clinically active in recurrent H3 K27M-mutant diffuse midline glioma patients. Clinical trial information: NCT03295396, NCT02525692 .

Clinical efficacy of ONC201 in thalamic H3 K27M-mutant glioma.

3617 Background: Diffuse midline gliomas, H3 K27M-mutant are associated with a poor prognosis compared to H3 wild-type gliomas and have no effective therapy following first-line radiation. ONC201 is a bitopic DRD2 antagonist and allosteric ClpP agonist that has shown encouraging single agent efficacy in recurrent H3 K27M-mutant gliomas located in various midline structures of the brain. In addition to tumor and immune cells, the pharmacodynamics of ONC201 extend to stromal cells that can mediate a bystander antitumor response in preclinical models. Given this observation and that the thalamus has the highest extrastriatal expression of DRD2, we report the clinical experience of ONC201 in a subgroup of H3 K27M-mutant glioma patients with primary tumors located in the thalamus. Methods: We analyzed 29 thalamic H3 K27M-mutant glioma patients treated with ONC201 in clinical trials enrolled as of 5/22/19. Nineteen enrolled with recurrent disease whereas 10 enrolled following radiation prior to recurrence. Twelve patients enrolled on NCT03295396, 10 NCT03416530, 4 NCT02525692, and 3 expanded access. Median age was 22 years old (range: 5-70) and baseline KPS was 80 (range: 60-90). Median time from radiation to start of ONC201 was 1.8 months (range: 0.2-8.7) for non-recurrent patients and 7.2 months (range: 1.4-102.0) for recurrent patients. Results: As of 12/18/2019, PFS6 and OS12 measured relative to initiation of ONC201 are 26.3% and 36.8%, respectively, in the recurrent group. For patients initiating ONC201 post-radiation prior to recurrence, median PFS or OS have not been reached with a median follow up of 21.9 months (8.6-26.6) from diagnosis, which surpass historical OS of 13.5 months. Best response for evaluable recurrent patients by RANO: 1 CR, 3 PR, 4 SD, 8 PD, 3 not reported; for non-recurrent patients: 2 PR, 4 SD, 1 PD, 3 not reported. Median duration of response for recurrent patients is 14.0 months (2.0-33.1). ONC201 was well tolerated and no dose-limiting toxicities or treatment discontinuations due to toxicity occurred. Furthermore, H3 K27M cell-free tumor DNA in plasma and CSF correlated with MRI response. Conclusions: In summary, single agent ONC201 administered at recurrence or following radiation, demonstrates promising clinical efficacy in thalamic H3 K27M-mutant glioma patients. Investigations are ongoing to assess whether micro-environmental DRD2 expression correlates with responses of thalamic H3 K27M-mutant glioma to ONC201. Clinical trial information: NCT03295396, NCT03416530, NCT02525692 .

Clinical experience of ONC201 in patients with recurrent H3 K27M-mutant spinal cord glioma.

2563 Background: High-grade gliomas of the spinal cord are a rare and understudied entity, representing < 5% of all spinal cord tumors. Reported median survival times range from 10-16 months. Up to 53% of tumors harbor the H3 K27M mutation, which is associated with an unfavorable prognosis. Postsurgical treatment often includes radiation ± temozolomide, although the role of chemotherapy has not been conclusively established. At recurrence, there are no effective therapies and most clinical studies exclude patients with spinal cord tumors. We report our clinical experience with ONC201, a small molecule DRD2 antagonist and caseinolytic protease P agonist, in patients with recurrent H3 K27M-mutant diffuse gliomas of the spinal cord (scDG). Methods: Adults and children with recurrent H3 K27M-mutant scDG received ONC201 in two Phase II clinical trials enrolling adult recurrent H3 K27M-mutant glioma patients (NCT02525692; NCT03295396) and in one Phase I clinical trial enrolling pediatric patients (NCT03416530). Adult patients received ONC201 at the RP2D dose of 625 mg weekly and pediatric patients received the RP2D of 625 mg weekly, scaled by body weight. All patients began ONC201 as a single agent until disease progression. Five patients continued ONC201 combined with bevacizumab beyond progression. Results: As of January 15, 2020, 12 evaluable patients (adult n = 8, pediatric n = 4) received ONC201. The median age was 20.9 (range: 7-72) years. The median follow-up time for the single agent ONC201 group was 5.4 (range 1.3-9.7) months while that of the combination group is 7.4 (range 6.2-25.1) months. The median number of ONC201 doses was 10 (range: 5-39) for the ONC201 single agent group and 34 (range: 21-100) for the combination group. Five of 7 patients remain alive in the ONC201 single agent group while 3 of 5 patients remain alive in the combination group. Three patients in the ONC201 single agent group and 2 patients in the combination group continue on treatment. There were no drug-related toxicities requiring dose reduction or discontinuation. Conclusions: Treatment with ONC201 alone or combined with bevacizumab is well tolerated in patients with recurrent H3 K27M-mutant scDG and a subset of patients experiences prolonged survival that exceeds historical outcomes. Clinical trial information: NCT02525692; NCT03295396; NCT03416530 .

DDIS-20. IMIPRIDONE STRUCTURE ACTIVITY RELATIONSHIP UNCOVERS ONC206 AS THE NEXT BITOPIC DRD2 ANTAGONIST FOR ONCOLOGY WITH DIFFERENTIATED RECEPTOR PHARMACOLOGY

Abstract ONC201 is the first bitopic antagonist of dopamine receptor D2 (DRD2) in oncology clinical trials, which have shown that the small molecule is well tolerated and induces durable tumor regressions in H3 K27M-mutant glioma patients. ONC201 possesses a unique tri-heterocyclic core structure that prompted a medicinal chemistry exploration following its successful clinical translation. Chemical derivatization of the ONC201 pharmacophore from an angular to a linear isomer ablated DRD2 antagonist activity and anti-cancer activity, indicating the core structure integrity is critical. A series of analogs that share the same core structure, called imipridones, were profiled for modulation of β-arrestin recruitment to GPCRs and anti-cancer efficacy. The profiled imipridones exhibited a heterogeneous spectrum of GPCR agonist/antagonist activity that was exclusive to Class A GPCRs. The addition of electron withdrawing groups to one of two peripheral benzyl rings enhanced the potency of GPCR engagement and anti-cancer effects, while derivatization of the other benzyl ring was inactivating. Among the GPCR hits identified, maximal variance in imipridone GPCR antagonism was identified for DRD2/DRD3. ONC206 emerged as the most selective and potent antagonist for DRD2/DRD3 with a Ki of ~320nM for DRD2 with complete specificity across human GPCRs and complete DRD2 antagonism. Schild analyses of ONC206 in cAMP and β-Arrestin recruitment assays revealed hallmarks of non-competitive DRD2 antagonism, unlike antipsychotics but similar to ONC201. Shotgun mutagenesis across DRD2 identified 7 residues critical for ONC206-mediated antagonism at orthosteric and allosteric sites. Six residues were critical for ONC201 and ONC206, however the impact varied between the two compounds and one allosteric residue was exclusive to ONC206. Structural mapping revealed that some ONC206-critical allosteric residue interactions are located at the interface of TM-IV and –V that mediates the DRD2 homodimer interface. Thus, ONC206 is a bitopic DRD2 antagonist that may be poised to address oncogenic DRD2 monomers or dimers.

ACTR-34. SINGLE AGENT ONC201 IN PREVIOUSLY-TREATED, PROGRESSIVE ADULT H3 K27M-MUTANT GLIOMA

Abstract H3 K27M-mutant gliomas often manifest as midline gliomas, have a dismal prognosis, and have no established or effective treatments at recurrence. ONC201 is the first clinical bitopic DRD2 antagonist/ClpP agonist and is under evaluation in Phase II trials for gliomas and other cancers. We previously reported in vitro studies suggesting dysregulated dopamine receptor expression and enhanced ONC201 sensitivity among H3 K27M-mutant gliomas. Following these observations, adults with midline H3 K27M-mutant glioma patients were enrolled to a dedicated Phase II clinical trial (NCT03295396), a multi-arm Phase II trial (NCT0252569), and expanded access protocols under the Sponsor’s IND. An integrated radiographic analysis with an objective response rate primary endpoint in patients who received ONC201 monotherapy with confirmed H3 K27M-mutant glioma (not primarily in the pons or spinal cord and without leptomeningeal spread) that was progressive and measurable disease by RANO criteria, >90 days from completion of prior radiation, and had KPS >60. As of December 15, 2018, 15 patients have received single agent ONC201 who meet these criteria (n=9 NCT03295396; n=5 NCT0252569; n=1 expanded access). ONC201 was orally administered at 625 mg weekly, except for one patient dosed once every 3 weeks. As midline gliomas can exhibit a mixture of contrast-enhancing and non-contrast-enhancing disease, objective response was assessed by blinded independent central review using RANO-HGG and RANO-LGG criteria for each patient. Best response to date by RANO-HGG criteria is at least 27%: 1 CR, 3 PR, 7 SD, and 4 PD; by RANO-LGG is at least 36%: 1 CR, 1 PR, 3 minor response (MR), 4 SD, 5 PD, 1 unevaluable. By RANO-HGG, median onset of response is 2.6 months (range 1.3–3.4); median duration of response has not been reached with a median follow-up of 7.7 months (range 1.8–29.8). Updated radiographic response, pharmacodynamics, safety, and other clinical outcomes will be reported.