Discovery of ARD-2051 as a Potent and Orally Efficacious Proteolysis Targeting Chimera (PROTAC) Degrader of Androgen Receptor for the Treatment of Advanced Prostate Cancer

Design, synthesis, and biological evaluation of novel BTK-targeting proteolysis targeting chimeras (PROTACs) with enhanced pharmacokinetic properties

Bruton's tyrosine kinase (BTK) has been an attractive target in the B-cell malignancies. Significant progress has been achieved in developing effective BTK-targeting small-molecule inhibitors and proteolysis targeting chimeras (PROTACs). Based on noncovalent inhibitor ARQ-531, we previously developed two potent BTK PROTACs 6e and SC-3e, which exhibited poor pharmacokinetic property. Herein, we present our extensive structure-activity relationship (SAR) studies focused on BTK binder, linker and cereblon (CRBN) ligand of SC-3e, resulting in two novel BTK PROTACs FDU28 (compound 25) and FDU73 (compound 27). Compounds 25 and 27 selectively induced rapid and robust degradation of wild type (WT) and C481S mutant BTK in a concentration-, time- and ubiquitin-proteasome system (UPS)-dependent manner without affecting CRBN neo-substrates. Furthermore, compound 27 displayed excellent cell antiproliferative activities, metabolic stability in mouse liver microsomes and improved bioavailability in mice. Overall, 27 is a highly effective and selective BTK degrader that is suitable for in vivo efficacy investigations.

PSMA-Guided PROTAC Degraders for Tumor-Specific Protein Degradation in Prostate Cancer

PROTACs that degrade target proteins to treat diseases represent a highly promising strategy in drug design. However, the degradation of target proteins in nondisease tissues may lead to systemic toxicity. Herein, capitalizing on the characteristic overexpression of PSMA in prostate cancer, we devised PSMA-guided PROTACs-specific targeting to prostate cancer. By conjugation of AR degraders and BET degraders separately with PSMA ligands via cleavable linkers, two classes of PSMA-guided PROTACs were obtained. In vitro experiments demonstrated that PSMA-guided PROTACs selectively degraded target proteins in PSMA-overexpressing prostate cancer cells without affecting target proteins in other cells. In vivo studies revealed that compared to conventional PROTACs, PSMA-guided PROTACs enhanced drug exposure in prostate cancer tissues and prolonged half-life and consequently achieved stronger and more sustained therapeutic effects. This work provides a novel avenue for disease tissue-specific PROTAC research, holding significant implications for targeted therapy in prostate cancer.

Identification of a Potent and Selective CDK9 Degrader as a Targeted Therapeutic Option for the Treatment of Small-Cell Lung Cancer

Small-cell lung cancer (SCLC) represents a significant public health challenge due to its increasing incidence and high mortality. Most SCLC patients are diagnosed at advanced stages, and there are limited effective targeted therapies available. In this study, a potent and selective CDK9 degrader, C3, was developed through PROTAC modification of the CDK9 inhibitor, AT-7519. C3 effectively induced apoptosis in various SCLC cell lines at low nanomolar concentrations and demonstrated favorable in vivo tolerance and adequate oral bioavailability. Notably, PROTAC C3 significantly reduced the proliferation of primary tumor samples from patients in mini-PDX models. Our findings indicate that the targeted degradation of CDK9 could become a viable strategy for treating SCLC, highlighting its potential therapeutic value. Additionally, this research offers a general structural optimization and evaluation strategy to improve the degradative selectivity, metabolic stability, and oral availability of PROTAC molecules.

Discovery of a highly potent, N-terminal domain-targeting degrader of AR-FL/AR-V7 for the treatment of prostate cancer

The clinical development of PROTACs targeting the androgen receptor (AR) for degradation has made significant progress. However, effective treatments for metastatic prostate cancers containing the androgen receptor splice variant 7 (AR-V7), a constitutively active mutant without the ligand-binding domain (LBD), are still lacking. Here, we reported the identification of a highly potent, noncovalent PROTAC targeting the N-terminal domain (NTD) of AR, NP18, which is developed from the covalent AR-NTD antagonist EPI-002, and effectively degrades both AR-FL and AR-V7 in 22Rv1 cells (DC50: 18 and 26 nM respectively). Mechanistically, NP18 interacts with the N-terminal domain (NTD) of both full-length AR (AR-FL) and splice variant 7 (AR-V7), leading to their selective and proteasomal degradation. Importantly, NP18 exhibited remarkably superior antitumor activity in both 22Rv1 xenograft and patient-derived xenograft (PDX) models than EPI-002. Taken together, these findings highlight NP18 as a promising candidate to counteract AR splice variant-driven resistance.

Next-generation cancer therapeutics: PROTACs and the role of heterocyclic warheads in targeting resistance

One of the major obstacles to sustained cancer treatment effectiveness is the development of medication resistance. Current therapies that block proteins associated with cancer progression often lose their efficacy due to acquired drug resistance, which is frequently driven by mutated or overexpressed protein targets. Proteolysis-targeting chimeras (PROTACs) offer an alternative therapeutic strategy by hijacking the cell's ubiquitin-proteasome system to degrade disease-causing proteins, presenting several potential advantages. Over the past few years, PROTACs have been developed to target various cancer-related proteins, offering new treatment options for patients with previously untreatable malignancies and serving as a foundation for next-generation therapeutics. One of the notable benefits of PROTACs is their ability to overcome certain resistance mechanisms that limit the effectiveness of conventional targeted therapies, as shown in several recent studies. Additionally, research teams are investigating how PROTACs can selectively degrade mutant proteins responsible for resistance to first-line cancer therapies. In the pursuit of novel and effective treatments, this review highlights recent advancements in the development of PROTACs aimed at overcoming cancer resistance. When it comes to drug design, heterocyclic scaffolds often serve as a foundational framework, offering opportunities for modification and optimization of novel molecules. Researchers are similarly exploring various heterocyclic derivatives as "warheads" in the design of PROTACs has been instrumental in pushing the boundaries of targeted protein degradation. As warheads, these heterocyclic compounds are responsible for recognizing and binding to the target protein, which ultimately leads to its degradation via the ubiquitin-proteasome system. This study aims to provide a comprehensive overview of cutting-edge strategies in PROTAC design, offering detailed insights into key concepts and methodologies for creating effective PROTACs. Special emphasis is placed on structure-based rational design, the development of novel warheads, and their critical in influencing biological activity.

PROTACs as Therapeutic Modalities for Drug Discovery in Castration-Resistant Prostate Cancer

Castration-resistant prostate cancer (CRPC) presents significant challenges in clinical management due to its resistance to conventional androgen receptor (AR)-targeting therapies. The advent of proteolysis targeting chimeras (PROTACs) has revolutionized cancer therapy by enabling the targeted degradation of key molecular players implicated in CRPC progression. In this review we discuss the developments of PROTACs for CRPC treatment, focusing on AR and other CRPC-associated regulators. We provide an overview of the strategic trends in AR PROTAC development from the aspect of targeting site selection and preclinical antitumor evaluation, as well as updates on AR degraders in clinical applications. Additionally, we briefly address the current status of selective AR degrader development. Furthermore, we review new developments in PROTACs as potential CRPC treatment paradigms, highlighting those targeting chromatin modulators BRD4, EZH2, and SWI/SNF; transcription regulator SMAD3; and kinases CDK9 and PIM1. Given the molecular targets shared between CRPC and neuroendocrine prostate cancer (NEPC), we also discuss the potential of PROTACs in addressing NEPC.

You Win Some, You Lose Some: Modifying the Molecular Periphery of Nitrofuran-Tagged Diazaspirooctane Reshapes Its Antibacterial Activity Profile

The use of the concept of privileged structures significantly accelerates the search for new leads and their optimization. 6-(methylsulfonyl)-8-(4-methyl-4H-1,2,4-triazol-3-yl)-2-(5-nitro-2-furoyl)-2,6-diazaspiro[3.4]octane 1 has been identified as a lead, with MICs of 0.0124-0.0441 μg/mL against MTb multiresistant strains. Several series of structural analogues have been synthesized, including variations in the periphery and simplifications of their scaffolds. All synthesized compounds were tested against the MTb H37Rv strain and ESKAPE panel of pathogens using serial broth dilutions. However, an attempt to optimize structure of 1 did not lead to the development of more active compounds which can work against MTb, but to substances with high activity against S. aureus. Induced-fit docking and MM-GBSA calculations determined a change in the likely biotarget from deazaflavin-dependent nitroreductase to azoreductases. The privileged nature of the scaffold was demonstrated by the detection of a different type of activity.

Discovery of a Novel Non-invasive AR PROTAC Degrader for the Topical Treatment of Androgenetic Alopecia

Elevated expression levels and enhanced activity of androgen receptor (AR) proteins are key factors in the development of androgenetic alopecia (AGA). AR proteolysis-targeting chimera (PROTAC) degraders have shown therapeutic potential, but their poor skin permeability requires invasive delivery methods. In this study, we conducted a structure feature analysis to investigate the effects of different linkers and E3 ligands of AR PROTACs on skin retention properties and degradation potency. Among these, compound C6 was discovered with excellent skin retention properties and nanomolar level AR degradation. By degrading AR, C6 regulated the expression levels of downstream paracrine factors associated with AGA. Additionally, after non-invasive topical application, C6 demonstrated excellent skin accumulation and achieved hair regeneration in an AGA mouse model. Overall, the development of non-invasive C6 offers a promising new strategy for AGA treatment and highlights the potential for using PROTACs in treating other skin diseases.

Discovery of ERD-12310A as an Exceptionally Potent and Orally Efficacious PROTAC Degrader of Estrogen Receptor α (ERα)

Inhibition of estrogen receptor alpha (ERα) signaling is an established therapeutic approach for the treatment of ER-positive (ER+) breast cancers, but new therapeutic strategies are urgently needed to overcome clinical resistance. In the present study, we describe the discovery and extensive evaluation of ERD-12310A as an exceptionally potent and orally efficacious PROTAC degrader of ERα. ERD-12310A achieved a DC50 value of 47 pM and is 10 times more potent than ARV-471. ERD-12310A displayed an improved pharmacokinetic profile in mice and rats over ARV-471. ERD-12310A attained tumor regression in the ER+, estrogen-dependent MCF-7 breast cancer xenograft model with wild-type ER and is more potent than ARV-471. Importantly, ERD-12310A achieved strong tumor growth inhibition in MCF-7 xenograft tumors harboring the clinically relevant ESR1Y537S mutation, which confers resistance to traditional antiestrogens. Our data position ERD-12310A as a promising candidate for further development as a potential therapy for ER+ breast cancer.

Characteristic roadmap of linker governs the rational design of PROTACs

Proteolysis targeting chimera (PROTAC) technology represents a groundbreaking development in drug discovery, leveraging the ubiquitin‒proteasome system to specifically degrade proteins responsible for the disease. PROTAC is characterized by its unique heterobifunctional structure, which comprises two functional domains connected by a linker. The linker plays a pivotal role in determining PROTAC's biodegradative efficacy. Advanced and rationally designed functional linkers for PROTAC are under development. Nonetheless, the correlation between linker characteristics and PROTAC efficacy remains under-investigated. Consequently, this study will present a multidisciplinary analysis of PROTAC linkers and their impact on efficacy, thereby guiding the rational design of linkers. We will primarily discuss the structural types and characteristics of PROTAC linkers, and the optimization strategies used for their rational design. Furthermore, we will discuss how factors like linker length, group type, flexibility, and linkage site affect the biodegradation efficiency of PROTACs. We believe that this work will contribute towards the advancement of rational linker design in the PROTAC research area.

Structural and Physicochemical Features of Oral PROTACs

Achieving oral bioavailability with Proteolysis Targeting Chimeras (PROTACs) is a key challenge. Here, we report the in vivo pharmacokinetic properties in mouse, rat, and dog of four clinical oral PROTACs and compare with an internally derived data set. We use NMR to determine 3D molecular conformations and structural preorganization free in solution, and we introduce the new experimental descriptors, solvent-exposed H-bond donors (eHBD), and acceptors (eHBA). We derive an upper limit of eHBD ≤ 2 for oral PROTACs in apolar environments and show a greater tolerance for other properties (eHBA, polarity, lipophilicity, and molecular weight) than for Rule-of-5 compliant oral drugs. Within a set of structurally related PROTACs, we show that examples with eHBD > 2 have much lower oral bioavailability than those that have eHBD ≤ 2. We summarize our findings as an experimental "Rule-of-oral-PROTACs" in order to assist medicinal chemists to achieve oral bioavailability in this challenging space.

Discovery of a Series of Orally Bioavailable Androgen Receptor Degraders for the Treatment of Prostate Cancer

Androgen receptor (AR) signaling plays a key role in the progression of prostate cancer. This study describes the discovery and optimization of a novel series of AR PROTAC degraders that recruit the Cereblon (CRBN) E3 ligase. Having identified a series of AR ligands based on 4-(4-phenyl-1-piperidyl)-2-(trifluoromethyl)benzonitrile, our PROTAC optimization strategy focused on linker connectivity and CRBN ligand SAR to deliver potent degradation of AR in LNCaP cells. This work culminated in compounds 11 and 16 which demonstrated good rodent oral bioavailability. Subsequent SAR around the AR binding region brought in an additional desirable feature, degradation of the important treatment resistance mutation L702H. Compound 22 (AZ'3137) possessed an attractive profile showing degradation of AR and L702H mutant AR with good oral bioavailability across species. The compound also inhibited AR signaling in vitro and tumor growth in vivo in a mouse prostate cancer xenograft model.

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.

Orally Bioavailable Proteolysis-Targeting Chimeras: An Innovative Approach in the Golden Era of Discovering Small-Molecule Cancer Drugs

Proteolysis-targeting chimeras (PROTACs) are an emerging therapeutic modality that show promise to open a target space not accessible to conventional small molecules via a degradation-based mechanism. PROTAC degraders, due to their bifunctional nature, which is categorized as 'beyond the Rule of Five', have gained attention as a distinctive therapeutic approach for oral administration in clinical settings. However, the development of PROTACs with adequate oral bioavailability remains a significant hurdle, largely due to their large size and less than ideal physical and chemical properties. This review encapsulates the latest advancements in orally delivered PROTACs that have entered clinical evaluation as well as developments highlighted in recent scholarly articles. The insights and methodologies elaborated upon in this review could be instrumental in supporting the discovery and refinement of novel PROTAC degraders aimed at the treatment of various human cancers.

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.

Structure-Activity Relationship (SAR) Studies of Novel Monovalent AR/AR-V7 Dual Degraders with Potent Efficacy against Advanced Prostate Cancer

Androgen receptor (AR) has been extensively established as a potential therapeutic target for nearly all stages of prostate cancer (PCa). However, acquired resistance to AR-targeted drugs inevitably develops and severely limits their clinical efficacy. Particularly, there currently exists no efficient treatment for patients expressing the constitutively active AR splice variants, such as AR-V7. Herein, we report the structure-activity relationship studies of 55 N-heterocycle-substituted hydantoins, which identified the structural motifs required for AR/AR-V7 degradation. Among them, the most potent compound 27c exhibited selective AR/AR-V7 degradation over other hormone receptors and excellent antiproliferative activities in LNCaP and 22RV1 cells. RNA sequence analysis confirmed that 27c effectively suppressed transcriptional activity of the AR signaling pathway. Importantly, 27c demonstrated potent antitumor efficacy in an enzalutamide-resistant 22RV1 xenograft model. These results highlight the potential of 27c as a promising dual AR/AR-V7 degrader for overcoming drug resistance in advanced PCa expressing AR splice variants.

Design and Synthesis of Dual-Target Inhibitors Targeting Androgen Receptors and Glucocorticoid Receptors to Overcome Antiandrogen Resistance in Castration-Resistant Prostate Cancer

Androgen receptor (AR) antagonists play important roles in the treatment of castration-resistant prostate cancer (CRPC). The glucocorticoid receptor (GR) upregulation leads to drug resistance for clinically used antiandrogens. Therefore, blocking AR/GR signaling simultaneously has become an efficient strategy to overcome the drug resistance of CRPC. Our previous work indicated that Z19 could inhibit the activity of both AR and GR. Herein, we optimized the structure of Z19 and identified GA32 as a potent AR/GR dual inhibitor. GA32 efficiently reduced the mRNA and protein levels of AR/GR downstream genes. GA32 efficiently inhibited the proliferation of enzalutamide resistance CRPC both in vitro and in vivo. GA32 could directly bind to AR and GR, and the predicted binding modes for GA32 with AR/GR suggested that GA32 binds to the AR or GR hormone binding pocket. This work provides a potential lead compound with dual AR/GR inhibitory activity to conquer the drug resistance of CRPC.

Proteolysis Targeting Chimera Degraders of the METTL3-14 m6A-RNA Methyltransferase

Methylation of adenine N6 (m6A) is the most frequent RNA modification. On mRNA, it is catalyzed by the METTL3-14 heterodimer complex, which plays a key role in acute myeloid leukemia (AML) and other types of blood cancers and solid tumors. Here, we disclose the first proteolysis targeting chimeras (PROTACs) for an epitranscriptomics protein. For designing the PROTACs, we made use of the crystal structure of the complex of METTL3-14 with a potent and selective small-molecule inhibitor (called UZH2). The optimization of the linker started from a desfluoro precursor of UZH2 whose synthesis is more efficient than that of UZH2. The first nine PROTAC molecules featured PEG- or alkyl-based linkers, but only the latter showed cell penetration. With this information in hand, we synthesized 26 PROTACs based on UZH2 and alkyl linkers of different lengths and rigidity. The formation of the ternary complex was validated by a FRET-based biochemical assay and an in vitro ubiquitination assay. The PROTACs 14, 20, 22, 24, and 30, featuring different linker types and lengths, showed 50% or higher degradation of METTL3 and/or METTL14 measured by Western blot in MOLM-13 cells. They also showed substantial degradation on three other AML cell lines and prostate cancer cell line PC3.

Journey of Von Hippel-Lindau (VHL) E3 ligase in PROTACs design: From VHL ligands to VHL-based degraders

The scientific community has shown considerable interest in proteolysis-targeting chimeras (PROTACs) in the last decade, indicating their remarkable potential as a means of achieving targeted protein degradation (TPD). Not only are PROTACs seen as valuable tools in molecular biology but their emergence as a modality for drug discovery has also garnered significant attention. PROTACs bind to E3 ligases and target proteins through respective ligands connected via a linker to induce proteasome-mediated protein degradation. The discovery of small molecule ligands for E3 ligases has led to the prevalent use of various E3 ligases in PROTAC design. Furthermore, the incorporation of different types of linkers has proven beneficial in enhancing the efficacy of PROTACs. By far more than 3300 PROTACs have been reported in the literature. Notably, Von Hippel-Lindau (VHL)-based PROTACs have surfaced as a propitious strategy for targeting proteins, even encompassing those that were previously considered non-druggable. VHL is extensively utilized as an E3 ligase in the advancement of PROTACs owing to its widespread expression in various tissues and well-documented binders. Here, we review the discovery of VHL ligands, the types of linkers employed to develop VHL-based PROTACs, and their subsequent modulation to design advanced non-conventional degraders to target various disease-causing proteins. Furthermore, we provide an overview of other E3 ligases recruited in the field of PROTAC technology.

Insight into Recent Advances in Degrading Androgen Receptor for Castration-Resistant Prostate Cancer

Induced protein degradation has emerged as an innovative drug discovery approach, complementary to the classical method of suppressing protein function. The androgen receptor signaling pathway has been identified as the primary driving force in the development and progression of lethal castration-resistant prostate cancer. Since androgen receptor degraders function differently from androgen receptor antagonists, they hold the promise to overcome the drug resistance challenges faced by current therapeutics. Proteolysis-targeting chimeras (PROTACs), monomeric degraders, hydrophobic tagging, molecular glues, and autophagic degradation have demonstrated their capability in downregulating intracellular androgen receptor concentrations. The potential of these androgen receptor degraders to treat castration-resistant prostate cancer is substantiated by the advancement of six PROTACs and two monomeric androgen receptor degraders into phase I or II clinical trials. Although the chemical structures, in vitro and in vivo data, and degradation mechanisms of androgen receptor degraders have been reviewed, it is crucial to stay updated on recent advances in this field as novel androgen receptor degraders and new strategies continue to emerge. This review thus provides insight into recent advancements in this paradigm, offering an overview of the progress made since 2020.

Current advances and development strategies of orally bioavailable PROTACs

Proteolysis-targeting chimeras (PROTACs) have been an area of intensive research with the potential to extend drug space not target to traditional molecules. In the last half decade, we have witnessed several PROTACs initiated phase I/II/III clinical trials, which inspired us a lot. However, the structure of PROTACs beyond "rule of 5" resulted in developing PROTACs with acceptable oral pharmacokinetic (PK) properties remain one of the biggest bottleneck tasks. Many reports have demonstrated that it is possible to access orally bioavailable PROTACs through rational ligand and linker modifications. In this review, we systematically reviewed and highlighted the most recent advances in orally bioavailable PROTACs development, especially focused on the medicinal chemistry campaign of discovery process and in vivo oral PK properties. Moreover, the constructive strategies for developing oral PROTACs were proposed comprehensively. Collectively, we believe that the strategies summarized here may provide references for further development of oral PROTACs.

Discovery of ARD-1676 as a Highly Potent and Orally Efficacious AR PROTAC Degrader with a Broad Activity against AR Mutants for the Treatment of AR + Human Prostate Cancer

We report herein the discovery and extensive characterization of ARD-1676, a highly potent and orally efficacious PROTAC degrader of the androgen receptor (AR). ARD-1676 was designed using a new class of AR ligands and a novel cereblon ligand. It has DC50 values of 0.1 and 1.1 nM in AR+ VCaP and LNCaP cell lines, respectively, and IC50 values of 11.5 and 2.8 nM in VCaP and LNCaP cell lines, respectively. ARD-1676 effectively induces degradation of a broad panel of clinically relevant AR mutants. ARD-1676 has an oral bioavailability of 67, 44, 31, and 99% in mice, rats, dogs, and monkeys, respectively. Oral administration of ARD-1676 effectively reduces the level of AR protein in the VCaP tumor tissue in mice and inhibits tumor growth in the VCaP mouse xenograft tumor model without any sign of toxicity. ARD-1676 is a highly promising development candidate for the treatment of AR+ human prostate cancer.

Discovery of as a Potent and Orally Efficacious Estrogen Receptor PROTAC Degrader with Strong Antitumor Activity

Estrogen receptor α (ERα) is a prime target for the treatment of ER-positive (ER+) breast cancer. Despite the development of several effective therapies targeting ERα signaling, clinical resistance remains a major challenge. In this study, we report the discovery of a new class of potent and orally bioavailable ERα degraders using the PROTAC technology, with ERD-3111 being the most promising compound. ERD-3111 exhibits potent in vitro degradation activity against ERα and demonstrates high oral bioavailability in mice, rats, and dogs. Oral administration of ERD-3111 effectively reduces the levels of wild-type and mutated ERα proteins in tumor tissues. ERD-3111 achieves tumor regression or complete tumor growth inhibition in the parental MCF-7 xenograft model with wild-type ER and two clinically relevant ESR1 mutated models in mice. ERD-3111 is a promising ERα degrader for further extensive evaluations for the treatment of ER+ breast cancer.