Antitumor Activity In Preclinical Models Research Articles

A NOVEL IMMUNOSTIMULATORY PD-L1/OX40 TETRAVALENT BISPECIFIC ANTIBODY FOR CANCER IMMUNOTHERAPY

Abstract Single agent immune checkpoint therapy has shown substantial and durable clinical activity in many tumor types; however, only a fraction of the patients could benefit from this approach. To improve beyond the anti-PD-1/PD-L1 treatment options, bispecific antibodies (BsAb) that combines PD-L1 blockade and conditional co-stimulatory receptor activation simultaneously in one molecule have been developed and demonstrated superior anti-tumor activity in pre-clinical models. However, many of these PD-L1 based BsAb faced challenge in clinical development due to insufficient activity or unexpected toxicity. Here, we demonstrated that OX40 might be a more suitable partner for PD-L1 based BsAb design than other agonistic targets (CD27 and 4-1BB, etc.) currently in clinical studies. A novel Fc silenced tetravalent PD-L1/OX40 (EMB-09) BsAb targeting optimal OX40 binding epitope has been developed based on EpimAb’s proprietary FIT-Ig® technology. Results showed that EMB-09 maintained the parental mAb binding characteristic and retained the functional properties of each parental mAb including OX40 agonistic as well as PD-L1/PD1 inhibitory pathways. In addition, EMB-09 induced OX40 activation only in the context of PD-L1 engagement. Concurrent PD-L1/PD-1 blockade and OX40 co-stimulation by EMB-09 led to synergistic activation of T cell in vitro and exerted superior anti-tumor activity in mouse tumor models compared to anti-PD-L1 mAb. The underlining mechanism was extensively analyzed, which indicated an increased CD8+ tumor-infiltrating T-cells (TIL) as well as enhanced CD8 TIL activation status upon EMB-09 treatment. Additionally, EMB-09 was well tolerated in cynomolgus monkeys at high dose levels with a favorable safety and PK profile in a GLP-TOX study. In conclusion, as a PD-L1/OX40 BsAb with a novel biology mechanism, EMB-09 demonstrated a markedly improved anti-tumor activity compared to anti-PD-L1 mAb. The first-in-human clinal study of EMB-09 has been initiated (NCT05263180).

Abstract PR006: PMR-116, a novel inhibitor of ribosome biogenesis with antitumor activity in preclinical models of prostate cancer

Abstract Advanced prostate cancer is characterised by mutations and amplifications of genes involved in regulating protein synthesis. PTEN-loss stimulates activity of the mTOR pathway, while amplification of MYC leads to increased ribosome biogenesis and elevated mRNA translation rate. Our previous work has demonstrated the efficacy of co-targeting ribosome biogenesis, via inhibition of RNA Pol I activity, and 4E-BP1 phosphorylation to suppress prostate cancer growth in vivo in GEMM of PCa and in patient-derived xenografts(1-3). In a collaboration with Pimera Inc., we investigated the efficacy of their new lead RNA Pol I inhibitor PMR-116 in models of prostate cancer. PMR-116 is well tolerated in vivo in mice and can be given at 300mg/kg weekly. Using the Hi-MYC mouse model of PCa we show that dosing 6-month-old mice once weekly for 4 weeks can decrease the incidence of invasive lesions by up to 85% compared to vehicle control while reverting glands to patterns of low grade intraepithelial neoplasia. PMR-116 rapidly inhibits proliferation in the Hi-MYC model with a 50% decrease in Ki67 observed 12 hours after oral administration. Conversely, PMR-116 showed minimal anti-tumour efficacy in the PTEN-null model of PCa suggesting that elevated MYC signalling may be required for optimal response. To further validate our promising GEMM results in more clinically relevant human-derived models, we used patient-derived xenografts lines we established from multidrug-resistant, metastatic PCa(4). PMR-116 treatment decreased tumour volume in all PDX tested including complete response in a line in which tumour volume decreased by ~90% compared to baseline. We believe this new RNA Pol I inhibitor shows promising results in a wide range of preclinical models of androgen receptor dependent and independent PCa and may exert higher efficacy in tumours expressing high levels of MYC. PMR-116 is currently in Phase I dose escalation trial in patient with solid tumours (ACTRN12620001146987).

A novel orally bioavailable small molecular ENPP1 inhibitor (OC-1) for cancer immunotherapy.

e15136 Background: Ectonucleotide pyrophosphatase/phosphodiesterase I (ENPP1) is a type II transmembrane glycoprotein with nucleotide pyrophosphatase and phosphodiesterase enzymatic activities. Inhibiting ENPP1 activity has emerged as an attractive approach in cancer immunotherapy, based on two distinct mechanisms. One mechanism is the stabilization of cyclic GMP–AMP (2’3’-cGAMP), which binds to stimulator of IFN genes (STING) and functions as an endogenous second messenger to activate the innate immune system; the other is the lowering of adenosine concentrations in the tumor microenvironment. ENPP1 inhibitors have shown promising monotherapy activity and synergy with various targeted therapies in preclinical models. Methods: OC-1 was characterized, for activity and selectivity, in the following in vitro assays: a luminescence-based enzymatic assay; cell proliferation assays; selectivity assays against 15 phosphodiesterases (PDE) based on inhibition of enzymatic activity; and a cytokine release assay in human THP cells and mouse splenocytes looking at changes in IFN-β/CXCL10. Oral bioavailability of OC-1 in mice and rats was studied following oral gavage. Plasma exposures were quantified using standard LC/MS/MS methods. In vivo tumor growth inhibition (TGI) of OC-1 was evaluated in syngeneic mouse tumor models (CT26 and MC38), as monotherapy and in combination with an anti-PD1 antibody. OC-1 was dosed orally in all in-vivo models. Results: We have discovered a novel small molecule inhibitor of ENPP1, OC-1, with a Ki value less than 10 nM in an ENPP1 enzymatic assay. Increase in gene expression of IFN-β and CXCL10/IP10 was observed in OC-1-treated THP1 cells. No inhibition in cell proliferation in various cancer lines was seen up to the highest concentration tested (30 µM). OC-1 was also selective against 15 PDEs with IC50 > 30 µM. The mean oral bioavailability of OC-1 was 72% and 63% in mice and rats, respectively. OC-1 showed single agent activity (20 – 40% TGI) and combination activity of approximately 75% TGI when it was combined with an anti-PD-1 antibody in in vivo efficacy studies with no impact on body weight. Conclusions: In summary, OC-1 is an orally bioavailable ENPP1 inhibitor with promising anti-tumor activity in preclinical models. Its physicochemical properties make it a suitable candidate for clinical investigation.

First-in-human study of ABBV-011, a seizure-related homolog protein 6 (SEZ6)–targeting antibody-drug conjugate, in patients with small cell lung cancer.

3002 Background: Small cell lung cancer (SCLC) has a dismal prognosis and new therapies are urgently needed. SEZ6 is a transmembrane protein expressed in SCLC tumors that may be used as a therapeutic target. ABBV-011 is an antibody-drug conjugate (ADC) targeting SEZ6 with a calicheamicin payload, which has shown antitumor activity in preclinical models of SCLC. Preliminary results from the monotherapy dose-escalation and -expansion cohorts of the first-in-human ABBV-011 study are presented. Methods: Phase 1, open-label, multicenter study (NCT03639194) of ABBV-011 alone or in combination with budigalimab, a programmed cell death 1 inhibitor. Primary objectives were to assess the safety and tolerability and to determine the maximum tolerated dose (MTD) and/or recommended phase 2 dose of ABBV-011. Adults (≥18 years) with relapsed/refractory SCLC (1–3 lines of prior therapy) were enrolled. Dose escalation was guided by Bayesian continual reassessment method. ABBV-011 was administered intravenously at doses from 0.3 to 2.0 mg/kg once every 3 weeks. Dose expansion was conducted in SEZ6-selected patients. Results: At data cutoff on August 22, 2022, 99 patients were treated with ABBV-011 monotherapy. Median age was 63 years (range, 41–79), 50% of patients were male, and 68% had received ≥2 prior therapies. ABBV-011 ADC pharmacokinetics were approximately dose-proportional with an elimination half-life of 4.6 days across the dose range of 0.3–2.0 mg/kg. In dose escalation (n=26), 1 patient had a dose-limiting toxicity of grade (G) 3 fatigue at 2.0 mg/kg. We report safety and efficacy results for 40 patients in the dose-expansion 1.0-mg/kg ABBV-011 cohort. Median duration of treatment was 12 weeks (range, 1.9–63.3). Treatment-emergent adverse events (TEAEs) occurred in 39 (98%) patients, the most frequent being fatigue (48%), nausea (45%), anorexia (38%), thrombocytopenia (38%), and vomiting (35%). G3 TEAEs occurred in 18 (45%) patients, the most frequent being fatigue, thrombocytopenia, and neutropenia (10% each); 1 G4 TEAE of dyspnea was reported. Seven patients died due to malignant neoplasm/disease progression (n=6) or respiratory distress (n=1); none were related to ABBV-011. Hepatotoxicity was observed, including G≥2 TEAEs of hyperbilirubinemia (18%), increased gamma-glutamyltransferase (8%), ascites (5%), veno-occlusive liver disease (3%), and portal hypertension (3%). Confirmed objective response rate was 25% (10 partial responses [PR]), with median duration of response of 4.2 months (95% CI: 2.6, 6.7). Clinical benefit rate (CBR) was 65% (10 PR and 16 stable disease) and CBR lasting >12 weeks was 43%. The median progression-free survival was 3.5 months. Conclusions: The MTD was not reached and ABBV-011 was well tolerated at 1.0 mg/kg with promising antitumor activity observed. Further evaluation of ABBV-011 is ongoing. Clinical trial information: NCT03639194 .

First-in-human, phase I study of KC1036, a multiple kinase inhibitor, as a single agent in patients with advanced solid tumors.

e15000 Background: KC1036 is a novel and selective multi-kinase inhibitor that targets cell proliferation, angiogenesis, metastasis and drug resistance by inhibiting receptor tyrosine kinases (AXLs and VEGFRs) involved in the tumor microenvironment. KC1036 has demonstrated anti-tumor activity in preclinical models of solid tumors, including non-small cell lung cancer (NSCLC). Methods: This phase I, first-in-human study of KC1036 (NCT04387916) enrolled adult patients (pts) with advanced or metastatic solid tumors. This study was comprised of dose escalation (Part A: With the accelerated titration of the 10 mg QD, in which only one patient was enrolled, a standard 3+3 design was applied for 20, 40, 60, and 80 mg QD) followed by dose expansion (Part B) and recommended phase II dose (RP2D) expansion (Part C). The primary safety endpoints were to determine dose limiting toxicities (DLTs) and RP2D. Secondary endpoints included pharmacokinetics (PK) and preliminary efficacy evaluated per RECIST v1.1 criterion. Results: As of November 30, 2022, a total of 41 pts had received at least 1 dose of KC1036 orally with 13 pts in Part A (10 mg [n = 1], 20 mg [n = 3], 40 mg [n = 3], 60 mg [n = 4], 80 mg [n = 3]) and 28 pts in Part B (60 mg [n = 21], 80 mg [n = 7]). In Part A, no DLTs were observed, and the maximum tolerated dose (MTD) has not been reached. Consequently, 60 mg and 80 mg were recommended for Part B. Pts most commonly had NSCLC (31.7%, 13/41), esophageal cancer (14.6%, 6/41), colorectal cancer (12.1%, 5/41). All pts were heavily pre-treated with 65% (27/41) receiving three or more lines of therapy, and average targeted tumor size was 58.7 mm with the largest 162 mm. Of the 41 pts, the median duration of KC1036 exposure was 105.0 days (range, 51.0-168.0). Any-grade TRAEs occurred in 39 pts (95.1%), with 16 pts (39.0%) reported with ≥ Grade 3. The most commonly reported ≥ Grade 3 TRAEs at 60 mg was hypertension (16.7%, 4/24), and that at 80 mg was diarrhea (30.0%, 3/10). PK analysis showed that KC1036 exhibited with dose-proportional increases in exposure over the range of 10 to 40 mg, but increased less proportionally within the range of 40-80 mg. Of 36 pts who had at least one efficacy evaluation, 5 (13.9%) had a best response of partial response (PR) and 24 (66.7%) had stable disease (SD). Thus, ORR was 13.9% (95% CI: 4.7, 29.5) and DCR was 80.6% (95% CI: 64.0, 91.2). Among the 9 evaluable pts with lung adenocarcinoma (6 on 60 mg and 3 on 80 mg), 2 pts had PR, 5 had SD and 2 had progressive disease (PD) as best response. Sustainable PR for 42 weeks was observed in 1 patient with lung adenocarcinoma on 60 mg. Conclusions: KC1036 demonstrated a manageable safety profile and preliminary antitumor activity in pts with advanced solid tumors. Taking safety, efficacy and clinical PK into consideration, 60 mg QD was recommended as RP2D for Part C, and this part was ongoing. Further studies in pts with lung adenocarcinoma would be considered promising. Clinical trial information: NCT04387916 .

Tumor-intrinsic sensitivity to the pro-apoptotic effects of IFN-γ is a major determinant of CD4+ CAR T-cell antitumor activity

CD4+ T cells and CD4+ chimeric antigen receptor (CAR) T cells display highly variable antitumor activity in preclinical models and in patients; however, the mechanisms dictating how and when CD4+ T cells promote tumor regression are incompletely understood. With the help of functional intravital imaging, we report that interferon (IFN)-γ production but not perforin-mediated cytotoxicity was the dominant mechanism for tumor elimination by anti-CD19 CD4+ CAR T cells. Mechanistically, mouse or human CD4+ CAR T-cell-derived IFN-γ diffused extensively to act on tumor cells at distance selectively killing tumors sensitive to cytokine-induced apoptosis, including antigen-negative variants. In anti-CD19 CAR T-cell-treated patients exhibiting elevated CAR CD4:CD8 ratios, strong induction of serum IFN-γ was associated with increased survival. We propose that the sensitivity of tumor cells to the pro-apoptotic activity of IFN-γ is a major determinant of CD4+ CAR T-cell efficacy and may be considered to guide the use of CD4+ T cells during immunotherapy.

Abstract CT272: Pharmacodynamic and immunophenotyping analyses of ATR inhibitor M1774 in a Phase I study in patients with solid tumors (DDRiver Solid Tumors 301)

Abstract Background: Ataxia telangiectasia and Rad3-related (ATR) protein kinase plays a critical role in the DNA damage response. M1774, a potent, selective, orally administered ATR inhibitor with antitumor activity in preclinical models, was evaluated in Part A1 of an open-label, single-arm study (NCT04170153) for safety, tolerability, maximum tolerated dose, pharmacokinetics (PK) and pharmacodynamics (PD). M1774 monotherapy in patients with advanced solid tumors was well-tolerated and the totality of evidence, including quantitative model-based analyses, suggested the recommended dose for expansion (RDE) as 180 mg QD 2 weeks on/1 week off.1 Here, we report findings of the M1774 PD and immunophenotyping analyses. Methods: M1774 PD was explored by assessing phosphorylation by ATR of CHK1 (p-CHK1) in tumor and of H2AX (γ-H2AX) in serial blood samples, stimulated ex vivo with the radiomimetic 4-Nitroquinoline N-oxide or Dimethyl sulfoxide as control. A flow cytometry quantitative assay was used to measure γ-H2AX in the CD45+ lymphocytes fraction. The effect of M1774 on the immunophenotype was explored by flow cytometry. Blood samples were collected at baseline, 3 and 24 hours after first M1774 administration on day 1 of cycle 1 for the γ-H2AX analysis, and on Days 1 and 15 of Cycles 1 and 2 before treatment for immunophenotyping. Results: Preclinical tumor tissue-blood bridging PD analyses in a mouse model demonstrated that the inhibition of γ-H2AX in lymphocytes highly correlated with inhibition of p-CHK1 in tumor. Clinical data of γ-H2AX levels and immunophenotyping were generated for the blood samples collected from the 55 participants of Part A1 of the study. Exploratory PK-PD analysis using γ-H2AX levels 3 h post-dose on day 1 across the doses predicted target inhibition >80% for doses ≥130 mg, suggesting target engagement. The levels of ɣ-H2AX at 24 h after first dose intake were variable and mean levels rebounded to baseline value. M1774 treatment did not cause any significant and consistent change in the levels of all explored immune cell subsets at the tested dose levels, including myeloid-derived suppressor cells, T and B lymphocytes, monocytes, and natural killer cells. Conclusions: PD analyses showed that M1774 efficiently inhibited ATR at the RDE without impacting the immunophenotype. 1TA Yap, et al. Ann Oncol. 2022; 33(suppl_7): S197-S224. Citation Format: Ruth Plummer, Anthony W. Tolcher, Timothy A. Yap, Giuseppe Sessa, Jatinder K. Mukker, Annick Seithel-Keuth, Christine Hicking, Zoltan Szucs, Ioannis Gounaris, Giuseppe Locatelli, Johann S. de Bono. Pharmacodynamic and immunophenotyping analyses of ATR inhibitor M1774 in a Phase I study in patients with solid tumors (DDRiver Solid Tumors 301) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr CT272.

Abstract 6320: CUSP06/AMT-707, a new CDH6-targeting antibody-drug conjugate, demonstrates potent antitumor activity in preclinical models

Abstract Cadherin-6 (CDH6), also known as K-cadherin, is a type II classic cadherin molecule that plays an important role in the embryonic development of the kidney, but has very limited expression in adult tissues. It is overexpressed in several human malignancies, primarily in ovarian, renal, but also (to a lesser degree) gastric, thyroid, cholangiocarcinoma and other tumor types. The characteristics of limited expression in normal tissues, high expression in tumor tissues, and rapid internalization upon antibody binding makes CDH6 a well-suited Antibody-drug Conjugate (ADC) target. We developed a novel CDH6-targeting ADC, CUSP06, consisting of a proprietary humanized antibody selective for CDH6, a protease cleavable linker, and an exatecan payload, with a drug-to-antibody ratio (DAR) of 8. CUSP06 selectively bound to cell surface CDH6 and was efficiently internalized into CDH6 positive ovarian and renal cancer cells. CUSP06 exhibited strong antiproliferative activity against several CDH6 positive cancer cell lines in vitro. Furthermore, compared to a DXd-based ADC, a exatecan-based ADC demonstrated improved bystander effect. Treatment with CUSP06 resulted in tumor regression in several CDH6 positive cell line derived xenograft (CDX) models, including PA-1, OVCAR3, and 786-O. In addition, CUSP06 demonstrated potent antitumor activity with tumor regression observed in two CDH6 low and high expressing patient derived xenograft (PDX) models. The preclinical activity of CUSP06 against CDH6-expressing tumors provide compelling support for the clinical development of CUSP06 in CDH6-expressing human cancers. CUSP06 is currently in IND-enabling studies. CUSP06 showed an expected toxicity profile consistent with it's exatecan payload in the ongoing pilot toxicology studies. We plan to initiate a Phase 1 first-in-human clinical trial in the 2nd half of 2023. Citation Format: Wei Lu, Jing Shi, Shu-Hui Liu, Nicole Covino, Xun Meng, Eric D. Slosberg. CUSP06/AMT-707, a new CDH6-targeting antibody-drug conjugate, demonstrates potent antitumor activity in preclinical models [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 6320.

Abstract 6185: Debio 0123 is a selective WEE1 inhibitor that effectively penetrates the brain and demonstrates anti-tumor activity in preclinical models of glioblastoma

Abstract Background: Glioblastoma (GBM) is one of the most aggressive and hard-to-treat cancers with a 5-year survival rate of 5.8%, in part due to the presence of the blood brain barrier (BBB) that prevents most therapeutics from reaching the tumors at efficacious concentrations. Debio 0123 is a WEE1 inhibitor that has previously shown significant preclinical efficacy in combination with DNA damaging agents or radiotherapy and is currently being investigated in phase 1 clinical studies either as a monotherapy or in combination with carboplatin. Herein we present data that demonstrates Debio 0123 effectively penetrates in to the brain and subsequently leads to anti-tumor activity in GBM models both in vitro and in vivo as a monotherapy or in combination with standard-of-care therapies (SOC). Methods: Radio- and chemo-sensitizing effects of Debio 0123 were evaluated in GBM cell lines in vitro using standard clonogenic assays or cytotoxicity assays. In vivo, distribution and bioanalysis studies were performed to determine the amount of Debio 0123 present in the different parts of the brain and intracranially implanted tumors. Efficacy of Debio 0123 as monotherapy or in combination with the SOC, temozolomide (TMZ), was further assessed in vivo in U87MG-luc intracranial or subcutaneous xenograft models. Results: In vitro, Debio 0123 in combination with radiation enhanced cell death in GBM cell lines and reduced TMZ IC50 in primary GBM cell lines. In vivo studies demonstrated substantial Debio 0123 brain penetration in mice, rats and monkeys, with mean brain-to-plasma concentration ratios of ~0.6 and 1.52 and 4, respectively. Furthermore, a mean tumor-to-plasma ratio of 0.62 was determined in orthotopically implanted brain tumors in mice. In efficacy studies, Debio 0123 administered at 30 or 60mg/kg every day for 28 days displayed significant anti-tumor activity in a GBM xenograft model U87-MG implanted in immunodeficient mice either subcutaneously, with up to 57.5% tumor growth inhibition, or orthotopically in the brain, with up to 73.7% tumor growth inhibition. Finally, efficacy of Debio 0123 in combination with TMZ was also assessed in intracranially transplanted U87-MG tumors. Here, TMZ was highly active, however, the combination of Debio 0123 and TMZ resulted in significantly enhanced anti-tumor activity leading to sustained complete regressions in 75% of animals for up to 100 days and was well tolerated throughout the 28-day treatment. Conclusion: These results demonstrate that following oral administration, Debio 0123 effectively crosses the BBB, has significant efficacy as a monotherapy and improves response to standard-of-care chemo or radiotherapy in preclinical models. Collectively, these data highlight Debio 0123 as a promising candidate for further clinical development in GBM patients that may provide improved response to SOC therapy. Citation Format: Luke Piggott, Noemie Luong, Frederic Massiere, Annett Kunze, Christophe Chardonnens, Anne Vaslin. Debio 0123 is a selective WEE1 inhibitor that effectively penetrates the brain and demonstrates anti-tumor activity in preclinical models of glioblastoma. [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 6185.

Abstract 6158: BGB-24714, a novel oral IAP antagonist, displayed significant anti-tumor activities in preclinical models as a monotherapy and in combination with paclitaxel

Abstract Background: Evasion of apoptosis is identified as one of the essential hallmarks of cancer and upregulation of inhibitor of apoptosis proteins (IAPs) is one of the mechanisms by which tumor cells evade apoptosis. An oral SMAC mimetic and antagonist of cellular IAP1 (cIAP1) and X-linked IAP (XIAP), BGB-24714, is currently investigated in a phase 1a/1b oncology trial in patients with advanced or metastatic solid tumors (NCT05381909). Here, we evaluated the anti-tumor activity of BGB-24714 as a single agent or in combination with paclitaxel in preclinical models. Results: BGB-24714 effectively inhibited cIAP1 by inducing its degradation in MDA-MB-231 cells, with an EC50 of 2.5 nM. BGB-24714 also potently antagonized the inhibitory interaction of XIAP with caspase-9 and induced caspase-9 autoactivation in MDA-MB-231 cells, with an EC50 of 23 nM. In a total of 25 breast cancer cell lines treated with TNFα, BGB-24714 potently inhibited the in vitro proliferation of 5 breast cancer cells with EC50 < 100 nM. In pharmacodynamics studies, single dose administration of BGB-24714 significantly induced degradation of cIAP1 and antagonism of the XIAP: Smac interaction in the MDA-MB-231 xenograft model in a dose dependent manner. Using the same model, BGB-24714 exhibited dose-dependent anti-tumor activities as a single agent. The tumor growth inhibition rates were 30%, 52% and 73% in low to high dosage treatment groups. Furthermore, BGB-24714 at medium dosage level demonstrated synergized anti-tumor activity in HCC1806 xenograft model when used in combination with paclitaxel. In intermittent dosing study, BGB-24714 with the intermittent dosing schedule demonstrated significant but slightly less effective anti-tumor activity than the continuous dosing schedule. In summary, BGB-24714, as a novel oral IAP antagonist, showing significant anti-tumor activities in preclinical models, which is promising and warrants the testing of the compound in human. Citation Format: Jie Li, Nan Hu, Yilu Zhang, Xiaolong Yang, Minjuan Deng, Wenfeng Gong, Longbo Yin, Yong Liu, Yajuan Gao, Wei Wei, Xing Wang, Xinyi Liang, Yanwen Ma, Xuxing Sang, Chang Liu, Jingyuan Wang, Qianqian Lu, Fengtao Song, Xi Yuan, Yibing Wang, Jing Li, Wei Jin, Xuesong Liu, Xiaomin Song. BGB-24714, a novel oral IAP antagonist, displayed significant anti-tumor activities in preclinical models as a monotherapy and in combination with paclitaxel [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 6158.

Abstract 6346: Discovery of HX011, a novel OX40 mAb, for potential cancer immunotherapy

Abstract OX40, tumor necrosis factor receptor superfamily member 4, is an immune co-stimulatory receptor, together with its ligand OX40L, forming OX40-OX40L trimer-trimer complex between the surface of T-cells (OX40) and APCs/NKs (OX40L). OX40 expresses constitutively on Treg as well as on conventional Teff, particularly within TME. OX40/OX40L interactions directly triggers conventional T cell activation via NF-kB1 pathway and inhibits bcl-x and survivin to prevent apoptosis; and it also downregulates FoxP3/CTLA4, thus Treg function. OX40 agonistic antibodies have also been shown to activate tumor immunity and increase anti-tumor activity in preclinical models. Here we report that we identified/constructed a novel OX40-agonistic humanized IgG1 monoclonal antibody (mAb), HX011, with high affinity through hybridoma technology. HX011 was subsequently confirmed of the similar binding properties and functions as the original mouse mAb clone, 9C12: 1) binding to human OX40 (EC50 ~0.012 nM; 2) similar function binding in the reporter assay (EC50 ~6.6 nM); but 3) has little blockade of the OX40L binding to OX40. In another word, HX011 is similar to BMS-986178 for the first two assays, but contrastingly different for the third assay. Non-competing with OX40L may actually maximize T-cell activation, a potential advantage over a competing antibody. In vivo pharmacology modeling of HX011 in the humanized syngeneic CRC model, MC38, where syngeneic C57BL/6 mouse mice were knocked in with huOX40 (“HuGEMM-OX40”), demonstrated strong anti-tumor activity (TGI ~100% at 1 mg/kg, twice weekly). HX011 is being further investigated as a potential new immunotherapy for the treatment of cancers. Citation Format: Hang Ke, Faming Zhang, Jialin Li, Feiyu Peng, Cen Chen, Henry Qixiang Li. Discovery of HX011, a novel OX40 mAb, for potential cancer immunotherapy [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 6346.

Abstract 6157: USP7 is a promising thersapeutic target for neuroblastoma

Abstract Background: The ubiquitin-proteasome system (UPS) plays an essential role in post-translational modification of proteins to maintain proteostasis. Ubiquitination of target protein is coordinated by different enzymes, which add and link ubiquitin to each other in specific ways that determine the fate of the protein for to be degraded or to be included into protein complex to induce signaling pathways. Ubiquitination is further regulated by deubiquitinases, which remove ubiquitin and rescue target protein from being degraded or diverted from it signaling role. UPS dysregulation has been linked to many human diseases, including cancer. Ubiquitin-specific protease 7 (USP7) is a deubiquitinase that plays a critical role in immune response, tumor suppression and DNA repair. USP7 overexpression has been associated with tumor aggressiveness in a variety of tumors, including in neuroblastoma (NB). Therefore, USP7 is a potential therapeutic target for NB. Some new specific USP7 inhibitors are highly potent and selective and have demonstrated significant antitumor activity in preclinical models of adult cancer. We hypothesize that inhibition of USP7 alone or in combination with epigenetic drugs like HDAC inhibitors will be more effective against NB tumor growth. To evaluate the efficacy of USP7 inhibition with specific USP7 inhibitors alone or with combination with epigenetic drugs like HDAC inhibitors (Saha) or chemotherapy against NB tumor growth, NB cell lines were treated with increasing concentrations of USP7 inhibitors alone or in combination with chemotherapy or with HDAC inhibitors. Cell proliferation was measured using continuous live cell imaging and cell viability using AlamarBlue assay. Apoptosis was measured by caspase cleavage using Caspase 3/7 apoptosis assay as well as by PARP cleavage by Western Blotting (WB). Changes in various proteins were measured by WB and co-immunoprecipitation were performed to analyze the ubiquitin status of specific protein targets regulated by USP7. Pharmacologic inhibition of USP7 resulted in significant decreases in both cell proliferation and cell viability in vitro only in p53 wild-type NB cell lines. There was no correlation seen between MYCN amplification status and treatment response. USP7 inhibition induced apoptosis as well as necroptosis with dose-dependent increase. Treatment with USP7 inhibitors led to decrease p53 ubiquitination leading increased of p53 protein expression in all sensitive cell lines tested and decreased MYCN protein level in MYCN-amplified cell lines. EZH2, which is a predominant target of USP7 is significantly downregulated with increasing dose of USP7 inhibitors. In addition, the combination of USP7 inhibitors with chemotherapy commonly used in NB or with combination with HDAC inhibitors showed enhanced efficacy. Our data suggests that USP7 inhibition may be a promising therapeutic strategy for children with high-risk and relapsed neuroblastoma. Citation Format: Christophe Le Clorennec, Karen Lee, Peter Zage. USP7 is a promising thersapeutic target for neuroblastoma [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 6157.

Abstract 3452: TNG908, a brain-penetrant MTA-cooperative PRMT5 inhibitor, is efficacious in preclinical glioblastoma models

Abstract Glioblastoma multiforme (GBM) is one of the most aggressive primary malignant brain tumors in adults. The median overall survival (OS) of GBM patients is poor (1-2 years) on standard of care therapies, which include surgery, radiotherapy, and the alkylating agent temozolomide. Therefore, developing new treatments for GBM is an urgent unmet medical need. TNG908 is a clinical stage MTA-cooperative PRMT5 inhibitor that inhibits PRMT5 activity selectively in MTAP-null cells leveraging a synthetic lethal interaction. Approximately 40% of GBM tumors are homozygous null for MTAP [1, 2]. In preclinical in vitro studies, the antiproliferation activity of TNG908 was 15 times more potent in MTAP-null cancer cell lines than in MTAP WT cells. TNG908 has high passive permeability and is not a substrate for P-glycoprotein nor Breast Cancer Resistant Protein efflux transporters, attributes which are favorable for crossing the blood-brain barrier. TNG908 demonstrated in vivo brain penetration in multiple preclinical studies, including non-human primates and mice, suggesting that TNG908 may fulfill the significant unmet need in the treatment of GBM. Pharmacodynamic activity of TNG908 to inhibit PRMT5 was demonstrated by decreased SDMA-modified protein levels in a dose-dependent manner in a GBM xenograft model. TNG908 demonstrated dose-dependent antitumor activity in multiple hyper- and hypomethylated GBM subcutaneous models, including cell lines and patient-derived xenograft models. Despite reduced Kpuu in rodents compared to primates, oral administration of TNG908 drove near tumor stasis and increased median survival by 3-fold in a highly aggressive murine GBM orthotopic model. In summary, TNG908 is a potent, brain-penetrant small molecule PRMT5 inhibitor that is selective for MTAP-null tumor cells with good drug-like properties and strong antitumor activity in preclinical models of GBM. As such, TNG908 may provide a novel treatment strategy for MTAP-deleted GBM patients. *MZ and AT contributed equally to the work. Reference: 1.Cerami et al., Cell, 2013. 2.Gao et al., Sci Signal, 2013 Citation Format: Minjie Zhang, Alice Tsai, Kevin M. Cottrell, Brian B. Haines, Erik Wilker, Heather DiBenedetto, Ron Weitzman, Alan Huang, Charles B. Davis, John P. Maxwell, Kimberly J. Briggs. TNG908, a brain-penetrant MTA-cooperative PRMT5 inhibitor, is efficacious in preclinical glioblastoma models [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 3452.

Evaluation of Fruquintinib in the Continuum of Care of Patients with Colorectal Cancer

The management of patients with metastatic colorectal cancer (mCRC) has the continuum of care as the treatment paradigm. To date, trifluridine/tipiracil, a biochemically modulated fluoropyrimidine, and regorafenib, a multi-kinase inhibitor, remain the main options for the majority of patients who progressed to standard doublet- or triplet-based chemotherapies, although a tailored approach could be indicated in certain circumstances. Being highly selective for vascular endothelial growth factor receptor (VEGFR)-1, -2 and -3, fruquintinib demonstrated a strong anti-tumor activity in preclinical models and received approval from China's National Medical Products Administration (NMPA) in 2018 for the treatment of patients with chemo-refractory mCRC. The approval was based on the results of the phase III FRESCO trial. Then, in order to overcome geographic differences in clinical practice, the FRESCO-2 trial was conducted in the US, Europe, Japan, and Australia. In a heavily pretreated patient population, the study met its primary endpoint, demonstrating an advantage of fruquintinib over a placebo in overall survival (OS). Here, we review the clinical development of fruquintinib and its perspectives in gastrointestinal cancers. Then, we discuss the introduction of fruquintinib in the continuum of care of CRC paying special attention to unmet needs, including the identification of cross-resistant and potentially susceptible populations, evaluation of radiological response, and identification of novel biomarkers of clinical benefit.

Abstract P6-10-07: CDK2 inhibition with BLU-222 in combination with ribociclib demonstrates robust antitumor activity in pre-clinical models of CDK4/6 inhibitor-naïve and -resistant HR+/HER2- breast cancer

Abstract Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors are standard of care in combination with endocrine therapy in advanced hormone receptor-positive (HR+), human epidermal growth factor receptor 2 negative (HER2-) breast cancer. Despite improved progression-free and overall survival, almost all patients develop CDK4/6 inhibitor resistance and experience disease progression on treatment. Aberrant activation of CDK2/cyclin E is a key resistance mechanism by which tumors can evade CDK4/6 blockade. Therefore, patients with HR+/HER2- breast cancer could benefit from treatment with a selective CDK2 inhibitor in combination with CDK4/6 inhibitors both in the resistant and first-line (1L) settings. BLU-222 is a novel, potent, and selective small-molecule inhibitor of CDK2 with favorable pharmacokinetic properties when administered orally, currently in early-stage clinical development. In pre-clinical studies using the MCF-7 xenograft model of HR+ CDK4/6-responsive breast cancer, treatment with BLU-222 combined with the CDK4/6 inhibitor ribociclib led to pronounced and durable tumor regression superior to ribociclib alone. In a derived palbociclib resistant MCF-7 xenograft model, ribociclib had no anti-tumor activity while BLU-222 led to a strong and durable anti-tumor response (83% tumor growth inhibition [TGI]) that was further improved when given in combination with ribociclib (110% TGI). To further explore the mechanism of aberrant CDK2 activation in CDK4/6 resistant, HR+ breast cancer, we engineered isogenic T47D cell lines to overexpress cyclin E1 (CCNE1) with or without p16, an endogenous inhibitor of CDK4/6 activity. In in vitro proliferation assays, co-expression of CCNE1 and p16 sensitized T47D cells to BLU-222 by approximately 10-fold compared to the parental control (110 nM vs 1078 nM, respectively). CDK4/6 inhibition with ribociclib had no anti-proliferative effect in the CCNE1 overexpressing cell lines regardless of p16 expression status. In vivo, treatment of the empty vector control T47D xenografts with ribociclib led to tumor stasis, while ribociclib in combination with BLU-222 led to tumor regression. T47D xenografts overexpressing both CCNE1 and p16 were resistant to ribociclib however CDK2 inhibition with BLU-222 single-agent treatment led to tumor regression. Finally, the activity of BLU-222 was evaluated in a patient-derived xenograft (PDX) model of CDK4/6 inhibitor-resistant HR+/HER2- breast cancer where the patient had progressed on 1L palbociclib/fulvestrant and 2L abemaciclib/fulvestrant therapy. In this PDX model, BLU-222 in combination with ribociclib led to tumor stasis, even in the absence of fulvestrant. In conclusion, these data support CDK2/cyclin E activation as a key vulnerability in CDK4/6 resistant, HR+/HER2- breast cancer and provide a rationale for the study of BLU-222 in patients with disease progression following CDK4/6 inhibitor treatment. Additionally, the improved durability of response when BLU-222 is combined with CDK4/6 inhibitors in the CDK4/6-naïve setting supports the combination of these agents as 1L treatment. BLU-222 is currently under investigation in VELA (NCT05252416), a phase 1/2, first-in-human trial for patients with cyclin E aberrant cancers and HR+/HER2- breast cancer. Citation Format: Victoria Brown, Nealia House, Phil Ramsden, Karen Ho, Hsin-Jung Wu, Erik Wilker, Jian Guo, Maxine Chen, Douglas Wilson, Neil Bifulco, Steve Wenglowsky, Yoon Jong Choi, Kerrie Faia. CDK2 inhibition with BLU-222 in combination with ribociclib demonstrates robust antitumor activity in pre-clinical models of CDK4/6 inhibitor-naïve and -resistant HR+/HER2- breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P6-10-07.

Abstract P1-11-03: VRN101099, Brain Permeable HER2 Kinase Inhibitor, Shows the Anti-tumor Activity in Preclinical Models of HER2-positive Cancers

Abstract The brain is a common target organ for breast cancer metastasis, and the risk of brain metastasis is usually high for patients with HER2-positive breast cancer. While T-DXd showed positive results in HER2-positive breast cancer brain metastasis (BCBM), its activity and blood-brain barrier (BBB) permeability in active BCBM need to be further validated (TUXEDO-1 included 15 patients with active brain metastasis). Tucatinib plus Trastuzumab and Capecitabine triplet showed efficacy in patients with brain metastasis, but Tucatinib alone has poor BBB penetration. There is still an unmet medical need for active HER2-positive BCBM. VRN101099 is a highly selective kinase inhibitor of HER2 (Eurofins scanMAX Kinase Profiling, S-score (35) of 0.01). VRN101099 has single- to double-digit nanomolar (IC50) cellular potency in HER2-dependent cancer cells, and Ba/F3 cells expressing HER2 wild type or mutations with selectivity over wildtype EGFR. VRN101099 inhibited proliferation of BT474 with 3.6 nM IC50 but HaCaT with 829.2 nM IC50. VRN101099 binds HER2 kinase by forming a covalent bond to the Cys805 and its irreversible inhibition resulted in a longer target resident time than Tucatinib, confirmed by in vitro washout experiments. Robust in vivo activity of VRN101099 was observed in HER2-positive BT474 xenograft models. Moreover, once daily oral administration of VRN101099 significantly regressed intracranial BT474 tumor, demonstrating greater efficacy than twice daily oral administration of Tucatinib. These results were well explained by the superior brain to plasma exposure of VRN101099 to Tucatinib. Also, VRN101099 showed high exposure in the target organ, the fat pad, which potentiated better clinical translation. These anti-tumor efficacies are correlated with pharmacodynamic responses, as confirmed by decreased HER2, AKT, and ERK phosphorylation. In summary, VRN101099 is a brain penetrant, orally bioavailable, irreversible, and highly selective inhibitor of HER2 with therapeutic potential in HER2-positive BCBM. These data support the clinical development of VRN101099 in HER2-driven cancers. Citation Format: Yikyung Ko, Jihye yoo, Hong-ryul Jung, Hyerim Lim, YeongDeok Lee, Se Hyuk Kim, Dong Guk Shin, Serin Cho, Myung hoe Heo, Haelee Kim, Ha Yeon Cho, Ah Reum Han, Eunhwa Ko, Hwan Geun Choi, Deakwon Kim, Sunghwan Kim. VRN101099, Brain Permeable HER2 Kinase Inhibitor, Shows the Anti-tumor Activity in Preclinical Models of HER2-positive Cancers [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P1-11-03.

Trials in progress: TAM kinase inhibitor PF-07265807 and sasanlimab plus axitinib in patients with advanced or metastatic renal cell carcinoma—A phase 1, open-label, pharmacokinetic, safety and tolerability study.

TPS743 Background: Tumor-associated macrophage kinases (TAMK) are expressed on tumor cells during malignant transformation. Inhibition of AXL and MERTK, members of the TAMK family, may lower the immune activation threshold and promote antitumor immunity. PF-07265807 (ARRY-067) is a small-molecule inhibitor of MERTK and AXL showing antitumor activity in preclinical models as monotherapy or combined with anti-programmed cell death protein 1 (anti-PD-1) antibodies (ab). Sasanlimab is an anti-PD-1 ab with acceptable safety profile being tested (300 mg SC Q4W, Phase 3) in non-muscle invasive bladder cancer. Axitinib is a VEGF inhibitor approved (single agent and combined with anti-PD-1 inhibitors) for advanced renal cell carcinoma (RCC). This first-in-human Phase 1, open-label, multi-center study (NCT04458259) will evaluate the safety, tolerability, pharmacokinetics, and preliminary antitumor activity of PF-07265807 in patients (pts) with advanced or metastatic solid tumors as a single agent and in combination with sasanlimab with or without axitinib. We describe here the dose escalation (Part 3) and dose expansion (Part 4) for the triplet combination of PF-07265807 + sasanlimab + axitinib. Methods: Eligible participants for the triplet cohorts are adult pts with confirmed unresectable advanced or metastatic clear cell RCC, with estimated creatinine clearance ≥30 mL/min and urinary protein <2+ or ≥2+ but 24-h urine protein:creatinine ratio <2 g/24 h. Pts for Part 4 need to have intermediate and poor risk RCC and no prior systemic therapy for metastatic disease. Other key eligibility criteria: measurable disease by RECIST 1.1 or non-measurable disease, ECOG PS 0–2, adequate bone marrow and liver function, and resolved acute effects of prior therapy. Each cycle is 21 days. Part 3 will determine the maximum tolerated dose/recommended phase 2 dose (MTD/RP2D) of PF-07265807 in triplet therapy using a Bayesian logistic regression model and the escalation with overdose control principle. After the MTD/RP2D is identified, Part 4 will further evaluate the safety and preliminary efficacy of PF-07265807 combined with sasanlimab and axitinib. Treatment with study drug will continue until disease progression, consent withdrawal, or unacceptable toxicity, whichever occurs first. Primary endpoints of Part 3 are incidence of dose-limiting toxicities, adverse events (AE), and laboratory (lab) abnormalities. For Part 4, the primary endpoints are objective response rate and complete response rate; secondary endpoints include disease control rate; time-to-event endpoints; AEs; lab abnormalities; concentrations of PF-07265807 and its metabolite, sasanlimab, and axitinib; and incidence and titer of anti-sasanlimab anti-drug antibodies response. The study started in late 2020 and is recruiting. Clinical trial information: NCT04458259 .

143D, a novel selective KRASG12C inhibitor exhibits potent antitumor activity in preclinical models.

The KRASG12C mutant has emerged as an important therapeutic target in recent years. Covalent inhibitors have shown promising antitumor activity against KRASG12C-mutant cancers in the clinic. In this study, a structure-based and focused chemical library analysis was performed, which led to the identification of 143D as a novel, highly potent and selective KRASG12C inhibitor. The antitumor efficacy of 143D in vitro and in vivo was comparable with that of AMG510 and of MRTX849, two well-characterized KRASG12C inhibitors. At low nanomolar concentrations, 143D showed biochemical and cellular potency for inhibiting the effects of the KRASG12C mutation. 143D selectively inhibited cell proliferation and induced G1-phase cell cycle arrest and apoptosis by downregulating KRASG12C-dependent signal transduction. Compared with MRTX849, 143D exhibited a longer half-life and higher maximum concentration (Cmax) and area under the curve (AUC) values in mouse models, as determined by tissue distribution assays. Additionally, 143D crossed the blood‒brain barrier. Treatment with 143D led to the sustained inhibition of KRAS signaling and tumor regression in KRASG12C-mutant tumors. Moreover, 143D combined with EGFR/MEK/ERK signaling inhibitors showed enhanced antitumor activity both in vitro and in vivo. Taken together, our findings indicate that 143D may be a promising drug candidate with favorable pharmaceutical properties for the treatment of cancers harboring the KRASG12C mutation.

Discovery of HCD3514 as a potent EGFR inhibitor against C797S mutation in vitro and in vivo.

Osimertinib (AZD9291), a third-generation epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKI), has significantly improved the survival of non-small cell lung cancer (NSCLC) patients with EGFRT790M mutation, the major mechanism of acquired resistance to first-generation EGFR TKI. However, resistance to AZD9291 arises eventually and EGFRC797S mutation was reported to be a major resistance mechanism. Thus, it is highly valuable to develop novel EGFR fourth-generation inhibitors targeting C797S mutation to override the acquired resistance. In this study, we identified HCD3514 as a novel EGFR fourth-generation inhibitors targeting C797S triple mutation. It strongly inhibited EGFRL858R/T790M/C797S and EGFR19del/T790M/C797S mutations with IC50 values of 1.0 and 2.0 nM, respectively. HCD3514 dose-dependently inhibited the activation of EGFR in both engineered BaF3 cells and tumor cells harboring EGFRC797S triple mutant and thus effectively suppressed the proliferation of the cells. Moreover, HCD3514 caused a dose-dependent increase of apoptosis in C797S triple mutant cells accompanied by increased levels of cleaved caspase-3 and cleaved PARP. Furthermore, HCD3514 induced tumor growth inhibition in EGFR19del/T790M/C797S xenograft model as a single oral agent by decreasing the activation of EGFR. In addition to EGFRC797S triple mutations, HCD3514 also potently and selectively inhibited EGFRT790M double mutations (L858R/T790M and 19del/T790M). Collectively, HCD3514 is a highly selective and potent EGFR inhibitor against EGFRC797S triple mutations as well as EGFRT790M double mutations and is confirmed potently anti-tumor activity in preclinical models.

Activating a Non-Genetic MYC-Regulating Pathway Sensitizes Acute Myeloid Leukemia to Bromodomain Inhibition

MYC (c-Myc) plays a central role in the oncogenic progression and is commonly overexpressed or amplified in more than 50% of human cancers, including acute myeloid leukemia (AML), which is an aggressive disease with over 70% of patients who cannot survive more than five years. Elevated MYC expression is associated with therapeutic resistance and predicts a poorer outcome in AML patients. While numerous lines of evidence have validated MYC as a therapeutic target of AML and many other cancer types, MYC protein itself has been a challenge for therapeutic targeting. Recently, MYC gene transcription has been linked to the bromodomain and extra-terminal domain (BET) family proteins. BET inhibitors (BETi) inhibit MYC expression and have shown promising antitumor activities in preclinical models and are being evaluated in clinical trials. Despite the therapeutic promise, BETi-associated drug resistance and dose-limiting side effects hinder the clinical applications. Therefore, there is a pressing need to understand the mechanisms controlling drug response and resistance to BET inhibitors. To understand the mechanisms that regulate BETi sensitivity, we leveraged a functional CRISPR screen focusing on about 1,700 human transcription factors. We discovered the aryl hydrocarbon receptor (AHR) as a novel regulator of BETi sensitivity in AML. Our results have further identified that a novel AHR-ELMSAN1 axis is critically involved in regulating MYC and BETi sensitivity. Specifically, we show that constitutive or ligand activation of AHR sensitizes leukemia cells to BETi. AHR signaling represses MYC and synergizes with BETi for maximum inhibition of MYC transcription. Ectopic expression of MYC could rescue the anti-leukemia effects induced by combined AHR activation and BET blockade. Mechanistically, our unbiased bioinformatic analysis unexpectedly identified that AHR functions through activating a non-canonical target, ELMSAN1, a less-studied epigenetic factor known as a member of the MiDAC histone deacetylase complex. Depletion of ELMSAN1 leads to upregulation of MYC and blunts AHR signaling-mediated MYC repression and BETi sensitization. Importantly, we demonstrate that AHR agonist and BETi synergistically inhibit MYC and leukemia cell growth in CD34+ primary AML patient samples in vitro and patient-derived xenografts in vivo. In summary, our data suggest that AHR-ELMSAN1 is a novel pathway for MYC repression, activation of which could sensitize AML to BET-targeting agents. As neither AHR nor ELMSAN1 is found mutated in AML, our research may open a new door to new MYC targeting paradigms by activating novel non-genetic MYC repressors.