Abstract The E1A-associated protein p300 (EP300) functions as a key regulator of oncogenic transcriptional programs, positioning it as an attractive therapeutic target in cancer. However, the high sequence homology between p300 and its paralog CREB-binding protein (CBP) has limited the development of selective inhibitors, often resulting in dose-limiting toxicities. In this study, we report the discovery of a highly potent and selective degrader of p300. Distinct from dual p300/CBP degraders, this compound exhibits enhanced formation and stability of the ternary complex with p300, drives stronger ubiquitination and proteasomal recruitment, and targets a unique lysine residue on p300 for degradation. Hematological malignancies including multiple myeloma, non-Hodgkin’s lymphoma, and acute myeloid leukemia were particularly sensitive to p300-selective degradation, which elicited a cytotoxic response in cancer cells and demonstrated robust antitumor activity in xenograft models. Together, these findings establish selective p300 degradation as a promising therapeutic approach for hematologic cancers and a novel strategy to disrupt oncogenic transcriptional dependencies. All Authors were or are employees of AbbVie. The design, study conduct, and financial support for this research were provided by AbbVie. AbbVie participated in the interpretation of data, review, and approval of the publication. No honoraria or payments were made for authorship. Citation Format: Marwa Asem, Yan Zhai, Xiaohong Song, Milad Rouhimoghadam, Sreenivas Punna, Fritz G Buchanan, Daniel T Cohen, Ryan McClure, Stephanie Sandoval, Anlu Chen, Shaun McLoughlin, Colin Woodford, Peter Kovar, Vlasios Manaves, Alla V Korepanova, Justin M Reitsma, Andrea Shergalis, Judith A Ronau, Yifei Kong, Yu Shen, Jurgen Dinges, . Discovery of a paralog selective p300 protein degrader with potent anti-cancer activity in hematological malignancies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 5803.

The E1A-associated protein p300 (EP300) is a key regulator of oncogenic transcription factors, making it a promising target for cancer therapy. However, its high sequence similarity to its paralog, CREB-binding protein (CBP), has hindered the development of selective inhibitors, leading to dose-limiting toxicities. Here, we describe the discovery of a highly potent and selective p300 degrader. Unlike dual p300/CBP degraders, this compound forms a more stable ternary complex with p300, driving enhanced proteasomal recruitment and ubiquitination. Notably, our data uncover a previously unrecognized mechanism of paralog selectivity mediated by regioselective ubiquitination of a unique lysine residue on p300. Hematological malignancies, including multiple myeloma, non-Hodgkin lymphoma, and acute myeloid leukemia, exhibit marked sensitivity to selective p300 degradation, resulting in cell lethality and robust antitumor activity in xenograft models. These findings establish selective p300 degradation as a mechanistically distinct and promising therapeutic strategy in hematological malignancies.

TAM receptor tyrosine kinases have emerged as promising therapeutic targets for cancer treatment due to their roles in both tumor intrinsic survival mechanisms and suppression of antitumor immunity within the tumor microenvironment. Inhibiting MerTK and Axl selectively is believed to hinder cancer cell survival, reverse the protumor myeloid phenotype, and suppress efferocytosis, thereby eliciting an antitumor immune response. In this study, we present the discovery of A-910, a highly potent and selective dual MerTK/Axl inhibitor, achieved through a structure-based medicinal chemistry campaign. The lead compound exhibits favorable oral bioavailability, exceptional kinome selectivity, and significantly improved in vivo target engagement. These findings support the use of A-910 as an orally bioavailable in vivo tool compound for investigating the immunotherapy potential of dual MerTK/Axl inhibition.

Inhibition of the receptor tyrosine kinase MerTK by small molecules has the potential to augment the immune response to tumors. Potent, selective inhibitors with high levels of in vivo target engagement are needed to fully evaluate the potential use of MerTK inhibitors as cancer therapeutics. We report the discovery and optimization of a series of pyrazinamide-based type 1.5 MerTK inhibitors bearing an azetidine-benzoxazole substituent. Compound 31 potently engages the target in vivo and demonstrates single agent activity in the immune-driven MC-38 murine syngeneic tumor model.

Amino Acid Sequence of AM-1 VH Region

Table S1: Mouse Strains Used for Xenograft Studies; Table S2: ABT-414 Growth Inhibition of Xenograft Tumors; Figure S1: Efficacy of ABT-414, ABT-806-vcMMAE, and unconjugated MMAE co-dosed with ABT-806 in U87MGde2-7 Tumor-Bearing Mice.

Supplementary file containing all supplementary figures and tables

Activation of apoptosis in malignant cells is an established strategy for controlling cancer and is potentially curative. To assess the impact of concurrently inducing the extrinsic and intrinsic apoptosis-signaling pathways in acute myeloid leukemia (AML), we evaluated activity of the TRAIL receptor agonistic fusion protein eftozanermin alfa (eftoza; ABBV-621) in combination with the B-cell lymphoma protein-2 selective inhibitor venetoclax in pre-clinical models and human patients. Simultaneously stimulating intrinsic and extrinsic apoptosis-signaling pathways with venetoclax and eftoza, respectively, enhanced their activities in AML cell lines and patient-derived ex vivo/in vivo models. Eftoza activity alone or plus venetoclax required death receptor 4/5 (DR4/DR5) expression on the plasma membrane but was independent of TP53 or FLT3-ITD status. The safety/tolerability of eftoza as monotherapy and in combination with venetoclax was demonstrated in patients with relapsed/refractory AML in a phase 1 clinical trial. Treatment-related adverse events were reported in 2 of 4 (50%) patients treated with eftoza monotherapy and 18 of 23 (78%) treated with eftoza plus venetoclax. An overall response rate of 30% (7/23; 4 complete responses [CRs], 2 CRs with incomplete hematologic recovery, and 1 morphologic leukemia-free state) was reported in patients who received treatment with eftoza plus venetoclax and 67% (4/6) in patients with myoblasts positive for DR4/DR5 expression; no tumor responses were observed with eftoza monotherapy. These data indicate that combination therapy with eftoza plus venetoclax to simultaneously activate the extrinsic and intrinsic apoptosis-signaling pathways may improve clinical benefit compared with venetoclax monotherapy in relapsed/refractory AML with an acceptable toxicity profile. This trial was registered at www.clinicaltrials.gov as #NCT03082209.

Abstract TRAIL can activate cell surface death receptors, resulting in potent tumor cell death via induction of the extrinsic apoptosis pathway. Eftozanermin alfa (ABBV-621) is a second generation TRAIL receptor agonist engineered as an IgG1-Fc mutant backbone linked to two sets of trimeric native single-chain TRAIL receptor binding domain monomers. This hexavalent agonistic fusion protein binds to the death-inducing DR4 and DR5 receptors with nanomolar affinity to drive on-target biological activity with enhanced caspase-8 aggregation and death-inducing signaling complex formation independent of FcγR-mediated cross-linking, and without clinical signs or pathologic evidence of toxicity in nonrodent species. ABBV-621 induced cell death in approximately 36% (45/126) of solid cancer cell lines in vitro at subnanomolar concentrations. An in vivo patient-derived xenograft (PDX) screen of ABBV-621 activity across 15 different tumor indications resulted in an overall response (OR) of 29% (47/162). Although DR4 (TNFSFR10A) and/or DR5 (TNFSFR10B) expression levels did not predict the level of response to ABBV-621 activity in vivo, KRAS mutations were associated with elevated TNFSFR10A and TNFSFR10B and were enriched in ABBV-621–responsive colorectal carcinoma PDX models. To build upon the OR of ABBV-621 monotherapy in colorectal cancer (45%; 10/22) and pancreatic cancer (35%; 7/20), we subsequently demonstrated that inherent resistance to ABBV-621 treatment could be overcome in combination with chemotherapeutics or with selective inhibitors of BCL-XL. In summary, these data provide a preclinical rationale for the ongoing phase 1 clinical trial (NCT03082209) evaluating the activity of ABBV-621 in patients with cancer. Significance: This study describes the activity of a hexavalent TRAIL-receptor agonistic fusion protein in preclinical models of solid tumors that mechanistically distinguishes this molecular entity from other TRAIL-based therapeutics.

Cancer cells are highly dependent on NAD+/NADH produced via the nicotinamide salvage pathway. The rate-limiting enzyme in this pathway is the nicotinamide phosphoribosyltransferase (NAMPT), which we have targeted with novel NAMPT inhibitors. NAMPT inhibition elicits depletion of total cellular NAD+ levels and ultimately cytotoxicity via depletion of cellular ATP levels. 18F-fluorodeoxyglucose- positron emission tomography (FDG-PET) is a translational imaging tool to assess glucose utilization in tumors and normal tissue. We used FDG-PET to understand the timing of ATP depletion in vivo and better understand the pharmacology of NAMPT inhibitors. Because of the intimate relationship between cellular ATP levels and cell viability, we developed an in-depth understanding of our NAMPT inhibitor pharmacology and the relationship with changes in tumor FDG uptake. Taken together, we show that FDG-PET could be used as a biomarker in clinical studies to understand dose and provide proof of mechanism for NAMPT inhibitors. SIGNIFICANCE STATEMENT: Our imaging data suggest that tumor 18F-fluorodeoxyglucose uptake can provide insight into the ATP status inside the tumor after nicotinamide phosphoribosyltransferase (NAMPT) therapy, with a novel NAMPT inhibitor. Such an approach could be used clinically as a pharmacodynamic biomarker to help understand the implications of dose, schedule, rescue strategy, or other clinical biomarkers.

In continuation of our previous research towards the discovery of potent, selective and drug-like Wee1 inhibitors, 2 novel series of biaryl heterocycles were designed, synthesized and evaluated. The new biaryl cores were designed to enable structure - activity exploration of substituents at C-8 or N-8 which were used for tuning compound properties and to improve compound profiles. The lead molecule 33 demonstrated a desirable pharmacokinetic profile and potentiated the anti-proliferative activity of irinotecan in vivo when dosed orally in the human breast MX-1 xenograft model.

Abstract ProbodyTM therapeutics are antibody prodrugs designed to remain largely inactive until proteolytically activated in the tumor microenvironment (TME), potentially enabling the safer targeting of antigens that are highly expressed in both tumor and normal tissue. CD71 (transferrin receptor) is an example of an ideal Probody Drug Conjugate (PDC) target, not only because it efficiently internalizes and can deliver a cytotoxic payload intracellularly, but also because it is expressed at high levels both in many different tumor types as well as in dividing normal cells. We have previously demonstrated that while an anti-CD71 antibody drug conjugate (ADC) is highly toxic, a CD71-targeting PDC is both efficacious in mouse tumor models and well tolerated in nonhuman primates. Two key components of a Probody therapeutic prodomain that reduce its binding to normal tissue and allow for its tumor-specific activation are 1) a mask that reduces the ability of the antibody binding site to interact with target antigen and 2) a protease-activatable substrate that is cleaved in the TME, resulting in removal of the mask. Here, we demonstrate how modulating mask strength and substrate cleavability can optimize efficacy and safety of a CD71-targeting PDC in preclinical models. Through this process, we have selected a lead molecule, CX-2029, for further development. CX-2029 is a CD71-targeting PDC conjugated to vcMMAE with a Drug to Probody Ratio (DPR) of 2, achieved by purification. At dose levels consistent with those expected in humans, a more strongly masked PDC, CX-2018, was less efficacious in a mouse xenograft tumor model compared to PDC CX-2016, which has a weaker mask, demonstrating that mask strength affects antitumor activity. Further, PDC CX-2019, which has the same mask but a less cleavable substrate than CX-2016, was similarly efficacious in a mouse xenograft tumor model, demonstrating that both substrates are sufficiently cleaved in the TME to activate the PDCs. However, CX-2019 was better tolerated in NHP at 6 mg/kg than CX-2016, suggesting that the less cleavable substrate in CX-2019 leads to a better therapeutic index. Using a LC/MS/MS method, we showed lower levels of circulating activated CX-2019 compared with circulating activated CX-2016, which is consistent with CX-2019’s improved tolerability. Lead CX-2029 contains the same mask and substrate as CX-2019 but differs in having a DPR of 2 versus ~3 for CX-2019. Up to 6 mg/kg of CX-2029 as a single dose produced complete regressions and durable responses in mouse xenograft tumor models encompassing multiple indications, and was tolerated in monkeys at doses of up to 12 mg/kg. These data demonstrate that, in preclinical models, tuning of mask strength and substrate cleavability can optimize the efficacy and tolerability of Probody Therapeutics and have the potential to enable the safe and effective targeting of highly expressed tumor antigens like CD-71. CX-2029 is currently under development, with an IND filing expected in 2018. PROBODY is a trademark of CytomX Therapeutics, Inc. Citation Format: Shweta Singh, Laura Serwer, Niharika Chauhan, Amy DuPage, Michael Krimm, Ken Wong, Yuanhui Huang, Andrew Jang, Eric Ureno, Adam Miller, Sarah Patrick, Shanti Duvur, Fritz Buchanan, Matthew M. Ravn, Rob Leanna, Ilaria Badagnani, Tracy Henriques, Shouchun Liu, Claus Krebber, Sridhar Viswanathan, Jennifer Richardson, Susan Morgan-Lappe, Michael Kavanaugh. Optimizing a CD71-targeting Probody drug conjugate (PDC) for activity in multiple solid tumor and lymphoma models and for tolerability in nonhuman primates [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B116.

In the originally published version of this Letter, the authors Arthur F. Kluge, Michael A. Patane and Ce Wang were inadvertently omitted from the author list. Their affiliations are: I-to-D, Inc., PO Box 6177, Lincoln, Massachusetts 01773, USA (A.F.K.); Mitobridge, Inc. 1030 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA (M.A.P.); and China Novartis Institutes for BioMedical Research, No. 4218 Jinke Road, Zhangjiang Hi-Tech Park, Pudong District, Shanghai 201203, China (C.W.). These authors contributed to the interpretation of results and design of compounds. In addition, author ‘Edward A. Kesicki’ was misspelled as ‘Ed Kesicki’. These errors have been corrected online.

Abstract The dynamic and reversible acetylation of proteins catalyzed by histone acetyltransferases (HATs) and histone deacetylases (HDACs) is a major epigenetic regulatory mechanism of gene transcription associated with multiple diseases. While HDAC inhibitors are approved to treat certain cancers, progress on the development of drug-like HAT inhibitors has lagged. The HAT paralogs p300 and CBP (p300/CBP) are key transcriptional co-activators essential for a multitude of cellular processes and also implicated in human pathological conditions, including cancer. Current p300/CBP HAT domain inhibitors including natural products, bi-substrate analogs (Lys-CoA) and the widely utilized C646 lack potency or selectivity. Here, we describe A-485, a potent, selective and drug-like p300/CBP catalytic inhibitor. We show the first high resolution (1.95Å) co-crystal structure of a small molecule bound to the catalytic active site of p300 and demonstrate that A-485 is acetyl-CoA competitive. A-485 selectively inhibited proliferation across lineage-specific tumor types, including several hematological malignancies and androgen receptor-positive prostate cancer. A-485 inhibited the androgen receptor transcriptional program in both androgen sensitive and castrate resistant prostate cancer and inhibited tumor growth in a castration resistant xenograft model. These results demonstrate the feasibility of selectively targeting the catalytic activity of histone acetyltransferases. Citation Format: Kenneth D. Bromberg, Loren M. Lasko, Clarissa G. Jakob, Wei Qiu, Debra Montgomery, Enrico L. Digiammarino, Todd M. Hansen, Roberto M. Risi, Robin R. Frey, Vlasios Manaves, Bailin Shaw, Mikkel Algire, Paul Hessler, Lloyd T. Lam, Tamar Uziel, Emily Favire, Debra Ferguson, Fritz G. Buchanan, Ruth L. Martin, Maricel Torrent, Saul H. Rosenberg, Michael R. Michaelides, Albert Lai. Discovery of a potent catalytic p300/CBP inhibitor that targets lineage-specific tumors [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr LB-A23.

AbstractDepatuxizumab mafodotin (depatux-m, ABT-414) is a tumor-selective antibody drug conjugate (ADC) comprised of the anti-EGFR antibody ABT-806 and the monomethyl auristatin F (MMAF) warhead. Depatux-m has demonstrated promising clinical activity in glioblastoma multiforme (GBM) patients and is currently being evaluated in clinical trials in first-line and recurrent GBM disease settings. Depatux-m responses have been restricted to patients with amplified EGFR, highlighting the need for therapies with activity against tumors with nonamplified EGFR overexpression. In addition, depatux-m dosing has been limited by corneal side effects common to MMAF conjugates. We hypothesized that a monomethyl auristatin E (MMAE) ADC utilizing an EGFR-targeting antibody with increased affinity may have broader utility against tumors with more modest EGFR overexpression while mitigating the risk of corneal side effects. We describe here preclinical characterization of ABBV-221, an EGFR-targeting ADC comprised of an affinity-matured ABT-806 conjugated to MMAE. ABBV-221 binds to a similar EGFR epitope as depatux-m and retains tumor selectivity with increased binding to EGFR-positive tumor cells and greater in vitro potency. ABBV-221 displays increased tumor uptake and antitumor activity against wild-type EGFR-positive xenografts with a greatly reduced incidence of corneal side effects relative to depatux-m. ABBV-221 has similar activity as depatux-m against an EGFR-amplified GBM patient derived xenograft (PDX) model and is highly effective alone and in combination with standard-of-care temozolomide in an EGFRvIII-positive GBM xenograft model. Based on these results, ABBV-221 has advanced to a phase I clinical trial in patients with advanced solid tumors associated with elevated levels of EGFR. Mol Cancer Ther; 17(4); 795–805. ©2018 AACR.

Nat. Chem. Biol. 13, 389–395 (2017); published online 30 January 2017; corrected after print 14 June 2017 In the version of this article initially published, the keys for the graphs in Figure 5b–e incorrectly stated GDK126 instead of GSK126. The error has been corrected in the HTML and PDF versions of the article.

potent and selective catalytic inhibitor of p300/CBP histone acetyltransferases suppresses tumour proliferation across multiple cell lineages, illustrating the therapeutic potential of drug-like small molecules that target histone acetyltransferases.