Discovery of Exceptionally Potent, Selective, and Efficacious PROTAC Degraders of CBP and p300 Proteins

Stereochemical inversion at a 1,4-cyclohexyl PROTAC linker fine-tunes conformation and binding affinity

Proteolysis targeting chimeras (PROTACs) are heterobifunctional small-molecule degraders made of a linker connecting a target-binding moiety to a ubiquitin E3 ligase-binding moiety. The linker unit is known to influence the physicochemical and pharmacokinetic properties of PROTACs, as well as the properties of ternary complexes, in turn impacting on their degradation activity in cells and in vivo. Our LRRK2 PROTAC XL01126, bearing a trans-cyclohexyl group in the linker, is a better and more cooperative degrader than its corresponding cis- analogue despite its much weaker binary binding affinities. Here, we investigate how this subtle stereocenter alteration in the linker affects the ligand binding affinity to the E3 ligase VHL. We designed a series of molecular matched pairs, truncating from the full PROTACs down to the VHL ligand, and find that across the series the trans-cyclohexyl compounds showed consistently weaker VHL-binding affinity compared to the cis- counterparts. High-resolution co-crystal structures revealed that the trans linker exhibits a rigid stick-out conformation, while the cis linker collapses into a folded-back conformation featuring a network of intramolecular contacts and long-range interactions with VHL. These observations are noteworthy as they reveal how a single stereochemical inversion within a PROTAC linker impacts conformational rigidity and binding mode, in turn fine-tuning differentiated propensity to binary and ternary complex formation, and ultimately cellular degradation activity.

Application and challenges of nitrogen heterocycles in PROTAC linker

The absence of effective active pockets makes traditional molecularly targeted drug strategies ineffective against 80 % of human disease-related proteins. The PROTAC technology effectively makes up for the deficiency of traditional molecular targeted drugs, which produces drug activity by degrading rather than inhibiting the target protein. The degradation of PROTAC is not only affected by POI ligand and E3 ligand, but by the selection of suitable linker which can play an important role in the efficiency and selectivity of the degradation. In the early exploring stage of the PROTAC, flexible chains were priorly applied as the linker of PROTAC. Although PROTAC with flexible chains as linkers sometimes perform well in vitro bioactivity evaluations, the introduction of lipophilic flexible chains reduces the hydrophilicity of these molecules, resulting in generally poor pharmacokinetic characteristics and pharmacological activities in vivo. In addition, recent reports have also shown that some PROTAC with flexible chains have some risks to causing hemolysis in vivo. Therefore, PROTAC with flexible chains show less druggability and large difficulty to entering the clinical trial stage. On the other hand, the application of nitrogen heterocycles in the design of PROTAC linkers has been widely reported in recent years. More and more reports have shown that the introduction of nitrogen heterocycles in the linker not only can effectively improves the metabolism of PROTAC in vivo, but also can enhance the degradation efficiency and selectivity of PROTAC. These PROTAC with nitrogen heterocycle linkers have attracted much attention of pharmaceutical chemists. The introduction of nitrogen heterocycles in the linker deserves priority consideration in the primary design of the PROTAC based on various druggabilities including pharmacokinetic characteristics and pharmacological activity. In this work, we summarized the optimization process and progress of nitrogen heterocyclic rings as the PROTAC linker in recent years. However, there were still limited understanding of how to discover, design and optimize PROTAC. For example, the selection of the types of nitrogen heterocycles and the optimization sites of this linker are challenges for researchers, choosing between four to six-membered nitrogen heterocycles, selecting from saturated to unsaturated ones, and even optimizing the length and extension angle of the linker. There is a truly need for theoretical explanation and elucidation of the PROTAC to guide the developing of more effective and valuable PROTAC.

Discovery of a Promising CBP/p300 Degrader XYD129 for the Treatment of Acute Myeloid Leukemia

The epigenetic target CREB (cyclic-AMP responsive element binding protein) binding protein (CBP) and its homologue p300 were promising therapeutic targets for the treatment of acute myeloid leukemia (AML). Herein, we report the design, synthesis, and evaluation of a class of CBP/p300 PROTAC degraders based on our previously reported highly potent and selective CBP/p300 inhibitor 5. Among the compounds synthesized, 11c (XYD129) demonstrated high potency and formed a ternary complex between CBP/p300 and CRBN (AlphaScreen). The compound effectively degraded CBP/p300 proteins and exhibited greater inhibition of growth in acute leukemia cell lines compared to its parent compound 5. Furthermore, 11c demonstrated significant inhibition of tumor growth in a MOLM-16 xenograft model (TGI = 60%) at tolerated dose schedules. Our findings suggest that 11c is a promising lead compound for the treatment of AML.

Pharmacological targeting of the cancer epigenome

Epigenetic dysregulation is increasingly appreciated as a hallmark of cancer, including disease initiation, maintenance and therapy resistance. As a result, there have been advances in the development and evaluation of epigenetic therapies for cancer, revealing substantial promise but also challenges. Three epigenetic inhibitor classes are approved in the USA, and many more are currently undergoing clinical investigation. In this Review, we discuss recent developments for each epigenetic drug class and their implications for therapy, as well as highlight new insights into the role of epigenetics in cancer.

Paralogue-selective degradation of the lysine acetyltransferase EP300

The transcriptional coactivators EP300 and CREBBP are critical regulators of gene expression that share high sequence identity but exhibit non-redundant functions in basal and pathological contexts. Here, we report the development of a bifunctional small molecule, MC-1, capable of selectively degrading EP300 over CREBBP. Using a potent aminopyridine-based inhibitor of the EP300/CREBBP catalytic domain in combination with a VHL ligand, we demonstrate that MC-1 preferentially degrades EP300 in a proteasome-dependent manner. Mechanistic studies reveal that selective degradation cannot be predicted solely by target engagement or ternary complex formation, suggesting additional factors govern paralogue-specific degradation. MC-1 inhibits cell proliferation in a subset of cancer cell lines and provides a new tool to investigate the non-catalytic functions of EP300 and CREBBP. Our findings expand the repertoire of EP300/CREBBP-targeting chemical probes and offer insights into the determinants of selective degradation of highly homologous proteins.

Design, synthesis and biological evaluation of new RNF126-based p300/CBP degraders

Histone acetyltransferase CREB-binding protein (CBP) and its homologous protein p300 are key transcriptional activators that can activate oncogene transcription, which present promising targets for cancer therapy. Here, we designed and synthesized a series of p300/CBP targeted low molecular weight PROTACs by assembling the covalent ligand of RNF126 E3 ubiquitin ligase and the bromodomain ligand of the p300/CBP. The optimal molecule A8 could effectively degrade p300 and CBP through the ubiquitin-proteasome system in time- and concentration-dependent manners, with half-maximal degradation (DC50) concentrations of 208.35/454.35 nM and 82.24/79.45 nM for p300/CBP in MV4-11 and Molm13 cell lines after 72 h of treatment. And the degradation of p300/CBP by A8 is dependent on the ubiquitin-proteasome pathway and its simultaneous interactions with the target proteins and RNF126. A8 exhibits good antiproliferative activity in a series of p300/CBP-dependent cancer cells. It could transcriptionally inhibit the expression of c-Myc, induce cell cycle arrest in the G0/G1 phase and apoptosis in MV4-11 cells. This study thus provided us a new chemotype for the development of drug-like PROTACs targeting p300/CBP, which is expected to be applied in cancer therapy.

Discovery of Highly Potent and Efficient CBP/p300 Degraders with Strong In Vivo Antitumor Activity

The transcriptional coactivator cAMP response element binding protein (CREB)-binding protein (CBP) and its homologue p300 have emerged as attractive therapeutic targets for human cancers such as acute myeloid leukemia (AML). Herein, we report the design, synthesis, and biological evaluation of a series of cereblon (CRBN)-recruiting CBP/p300 proteolysis targeting chimeras (PROTACs) based on the inhibitor CCS1477. The representative compounds 14g (XYD190) and 14h (XYD198) potently inhibited the growth of AML cells with low nanomolar IC50 values and effectively degraded CBP and p300 proteins in a concentration- and time-dependent manner. Mechanistic studies confirmed that 14g and 14h can selectively bind to CBP/p300 bromodomains and induce CBP and p300 degradation in bromodomain family proteins in a CRBN- and proteasome-dependent manner. 14g and 14h displayed remarkable antitumor efficacy in the MV4;11 xenograft model (TGI = 88% and 93%, respectively). Our findings demonstrated that 14g and 14h are useful lead compounds and deserve further optimization and activity evaluation for the treatment of human cancers.

CBP/p300 Degrader: A Promising Therapeutic Strategy for Treatment of Prostate Cancer and Beyond

Transcriptional coactivators CBP/p300 have emerged as potential targets for cancer therapeutics. This Viewpoint discusses recent results that demonstrate an exceptionally potent and orally efficacious CBP/p300 degrader for the treatment of prostate cancer. This degrader stands out as a promising new candidate for cancer therapy and deserves further research.

Discovery of CBPD-409 as a Highly Potent, Selective, and Orally Efficacious CBP/p300 PROTAC Degrader for the Treatment of Advanced Prostate Cancer

CBP/p300 are critical transcriptional coactivators of the androgen receptor (AR) and are promising cancer therapeutic targets. Herein, we report the discovery of highly potent, selective, and orally bioavailable CBP/p300 degraders using the PROTAC technology with CBPD-409 being the most promising compound. CBPD-409 induces robust CBP/p300 degradation with DC50 0.2-0.4 nM and displays strong antiproliferative effects with IC50 1.2-2.0 nM in the VCaP, LNCaP, and 22Rv1 AR+ prostate cancer cell lines. It has a favorable pharmacokinetic profile and achieves 50% of oral bioavailability in mice. A single oral administration of CBPD-409 at 1 mg/kg achieves >95% depletion of CBP/p300 proteins in the VCaP tumor tissue. CBPD-409 exhibits strong tumor growth inhibition and is much more potent and efficacious than two CBP/p300 inhibitors CCS1477 and GNE-049 and the AR antagonist Enzalutamide. CBPD-409 is a promising CBP/p300 degrader for further extensive evaluations for the treatment of advanced prostate cancer and other types of human cancers.

Discovery of CBPD-268 as an Exceptionally Potent and Orally Efficacious CBP/p300 PROTAC Degrader Capable of Achieving Tumor Regression

CBP/p300 proteins are key epigenetic regulators and promising targets for the treatment of castration-resistant prostate cancer and other types of human cancers. Herein, we report the discovery and characterization of CBPD-268 as an exceptionally potent, effective, and orally efficacious PROTAC degrader of CBP/p300 proteins. CBPD-268 induces CBP/p300 degradation in three androgen receptor-positive prostate cancer cell lines, with DC50 ≤ 0.03 nM and Dmax > 95%, leading to potent cell growth inhibition. It has an excellent oral bioavailability in mice and rats. Oral administration of CBPD-268 at 0.3-3 mg/kg resulted in profound and persistent CBP/p300 depletion in tumor tissues and achieved strong antitumor activity in the VCaP and 22Rv1 xenograft tumor models in mice, including tumor regression in the VCaP tumor model. CBPD-268 was well tolerated in mice and rats and displayed a therapeutic index of >10. Taking these results together, CBPD-268 is a highly promising CBP/p300 degrader as a potential new cancer therapy.

Exploration of bromodomain ligand-linker conjugation sites for efficient CBP/p300 heterobifunctional degrader activity

Synthesis of proteolysis targeting chimeras (PROTACs) involves conjugation of an E3 ligase binding ligand to a ligand targeting a protein of interest via a rigid or flexible chemical linker. The choice of linker conjugation site on these ligands can be informed by structural analysis of ligand-target binding modes, the feasibility of synthetic procedures to access specific sites, and computational modeling of predicted ternary complex formations. Small molecules that target bromodomains - epigenetic readers of lysine acetylation - typically offer several potential options for linker conjugation sites. Here we describe how varying the linker attachment site (exit vector) on a CBP/p300 bromodomain ligand along with linker length affects PROTAC degradation activity and ternary complex formation. Using kinetic live cell assays of endogenous CBP and p300 protein abundance and bead-based proximity assays for ternary complexes, we describe the structure-activity relationships of a diverse library of CBP/p300 degraders (dCBPs).

Discovery and Characterization of Active CBP/EP300 Degraders Targeting the HAT Domain

Proteolysis Targeting Chimeras (PROTACs) are bifunctional molecules that simultaneously bind an E3 ligase and a protein of interest, inducing degradation of the latter via the ubiquitin-proteasome system. Here we present the development of degraders targeting CREB-binding protein (CBP) and E1A-associated protein (EP300)-two homologous multidomain enzymes crucial for enhancer-mediated transcription. Our PROTAC campaign focused on CPI-1612, a reported inhibitor of the histone acetyltransferase (HAT) domain of these two proteins. A novel asymmetric synthesis of this ligand was devised, while PROTAC-SAR was explored by measuring degradation, target engagement, and ternary complex formation in cellulo. Our study demonstrates that engagement of Cereblon (CRBN) and a sufficiently long linker between the E3 and CBP/EP300 binders (≥21 atoms) are required for PROTAC-mediated degradation using CPI-1612 resulting in a new active PROTAC dCE-1. Lessons learned from this campaign, particularly the importance of cell-based assays to understand the reasons underlying PROTAC performance, are likely applicable to other targets to assist the development of degraders.

Histone lysine acetyltransferase inhibitors: an emerging class of drugs for cancer therapy

Lysine acetyltransferases (KATs) are a family of epigenetic enzymes involved in the regulation of gene expression; they represent a promising class of emerging drug targets. The frequent molecular dysregulation of these enzymes, as well as their mechanistic links to biological functions that are crucial to cancer, have led to exploration around the development of small-molecule inhibitors against KATs. Despite early challenges, recent advances have led to the development of potent and selective enzymatic and bromodomain (BRD) KAT inhibitors. In this review we discuss the discovery and development of new KAT inhibitors and their application as oncology therapeutics. Additionally, new chemically induced proximity approaches are presented, offering opportunities for unique target selectivity profiles and tissue-specific targeting of KATs. Emerging clinical data for CREB binding protein (CREBBP)/EP300 BRD inhibitors and KAT6 catalytic inhibitors indicate the promise of this target class in cancer therapeutics.

Discovery of Novel PROTAC Degraders of p300/CBP as Potential Therapeutics for Hepatocellular Carcinoma

Adenoviral E1A binding protein 300 kDa (p300) and its closely related paralog CREB binding protein (CBP) are promising therapeutic targets for human cancer. Here, we report the first discovery of novel potent small-molecule PROTAC degraders of p300/CBP against hepatocellular carcinoma (HCC), one of the most common solid tumors. Based upon the clinical p300/CBP bromodomain inhibitor CCS1477, a conformational restriction strategy was used to optimize the linker to generate a series of PROTACs, culminating in the identification of QC-182. This compound effectively induces p300/CBP degradation in the SK-HEP-1 HCC cells in a dose-, time-, and ubiquitin-proteasome system-dependent manner. QC-182 significantly downregulates p300/CBP-associated transcriptome in HCC cells, leading to more potent cell growth inhibition compared to the parental inhibitors and the reported degrader dCBP-1. Notably, QC-182 potently depletes p300/CBP proteins in mouse SK-HEP-1 xenograft tumor tissue. QC-182 is a promising lead compound toward the development of p300/CBP-targeted HCC therapy.

Transcriptional co-activators: emerging roles in signaling pathways and potential therapeutic targets for diseases

Specific cell states in metazoans are established by the symphony of gene expression programs that necessitate intricate synergic interactions between transcription factors and the co-activators. Deregulation of these regulatory molecules is associated with cell state transitions, which in turn is accountable for diverse maladies, including developmental disorders, metabolic disorders, and most significantly, cancer. A decade back most transcription factors, the key enablers of disease development, were historically viewed as 'undruggable'; however, in the intervening years, a wealth of literature validated that they can be targeted indirectly through transcriptional co-activators, their confederates in various physiological and molecular processes. These co-activators, along with transcription factors, have the ability to initiate and modulate transcription of diverse genes necessary for normal physiological functions, whereby, deregulation of such interactions may foster tissue-specific disease phenotype. Hence, it is essential to analyze how these co-activators modulate specific multilateral processes in coordination with other factors. The proposed review attempts to elaborate an in-depth account of the transcription co-activators, their involvement in transcription regulation, and context-specific contributions to pathophysiological conditions. This review also addresses an issue that has not been dealt with in a comprehensive manner and hopes to direct attention towards future research that will encompass patient-friendly therapeutic strategies, where drugs targeting co-activators will have enhanced benefits and reduced side effects. Additional insights into currently available therapeutic interventions and the associated constraints will eventually reveal multitudes of advanced therapeutic targets aiming for disease amelioration and good patient prognosis.