Antibody-Mediated Delivery of Chimeric BRD4 Degraders. Part 2: Improvement of In Vitro Antiproliferation Activity and In Vivo Antitumor Efficacy

The importance of cellular degradation kinetics for understanding mechanisms in targeted protein degradation

Targeted protein degradation has exploded over the past several years due to preclinical and early clinical therapeutic success of numerous compounds, and the emergence of new degradation modalities, which has broadened the definition of what a degrader is. The most characterized and well-studied small molecule degraders are molecular glues and proteolysis targeting chimeras (PROTACs). These degraders induce a ternary complex between a target protein, degrader, and E3 ligase component, resulting in ubiquitination and subsequent degradation of the target protein via the ubiquitin proteasomal system (UPS). This event-driven process requires success at all steps through a complex cascade of events. As more systems, degraders, and targets are tested, it has become increasingly clear that achieving degradation is only the first critical milestone in a degrader development program. Rather highly efficacious degraders require a combination of multiple optimized parameters: rapid degradation, high potency, high maximal degradation (D_max), and sustained loss of target without re-dosing. Success to meet these more rigorous goals depends upon the ability to characterize and understand the dynamic cellular degradation profiles and relate them to the underlying mechanism for any given target treated with a specific concentration of degrader. From this starting point, optimization and fine tuning of multiple kinetic parameters such as how fast degradation occurs (the rate), how much of the target is degraded (the extent), and how long the target remains degraded (the duration) can be performed. In this review we explore the diversity of cellular kinetic degradation profiles which can arise after molecular glue and PROTAC treatment and the potential implications of these varying responses. As the overall degradation kinetics are a sum of individual mechanistic steps, each with their own kinetic contributions, we discuss the ways in which changes at any one of these steps could potentially influence the resultant kinetic degradation profiles. Looking forward, we address the importance in characterizing the kinetics of target protein loss in the early stages of degrader design and how this will enable more rapid discovery of therapeutic agents to elicit desired phenotypic outcomes.

Enriching Proteolysis Targeting Chimeras with a Second Modality: When Two Are Better Than One

Proteolysis targeting chimera (PROTAC)-mediated protein degradation has prompted a radical rethink and is at a crucial stage in driving a drug discovery transition. To fully harness the potential of this technology, a growing paradigm toward enriching PROTACs with other therapeutic modalities has been proposed. Could researchers successfully combine two modalities to yield multifunctional PROTACs with an expanded profile? In this Perspective, we try to answer this question. We discuss how this possibility encompasses different approaches, leading to multitarget PROTACs, light-controllable PROTACs, PROTAC conjugates, and macrocycle- and oligonucleotide-based PROTACs. This possibility promises to further enhance PROTAC efficacy and selectivity, minimize side effects, and hit undruggable targets. While PROTACs have reached the clinical investigation stage, additional steps must be taken toward the translational development of multifunctional PROTACs. A deeper and detailed understanding of the most critical challenges is required to fully exploit these opportunities and decisively enrich the PROTAC toolbox.

Proteolysis-targeting chimera (PROTAC) delivery system: advancing protein degraders towards clinical translation

Proteolysis Targeting Chimeras (PROTACs), an emerging therapeutic entity designed to degrade target proteins by hijacking the ubiquitin-proteasome system, have the potential to revolutionize the healthcare industry. The broad applicability of this protein degradation strategy has been verified with a few E3 ligases and a variety of distinct targets through the construction of modular chimeric structures. Despite recent efforts to promote the use of PROTACs for clinical applications, most PROTACs do not make it beyond the preclinical stage of drug development. There are several reasons that prevent PROTACs from reaching the market, and the inadequate delivery to the target site is one of the most challenging hurdles. With the increasing need for accelerating the translational process, combining the concepts of PROTACs and delivery systems has been explored to enhance the in vivo performance of PROTACs. These improved delivery strategies can eliminate unfavorable physicochemical properties of PROTACs, improve their targetability, and decrease their off-target side effects. The integration of powerful PROTACs and versatile delivery systems will inaugurate a burgeoning orientation for the field of targeted protein degradation. In this review, we will survey the latest progress in improving the in vivo degradation efficacy of PROTACs through delivery strategies, outline design principles for PROTAC-based delivery systems, discuss the current challenges with PROTACs, and outlook future opportunities in this field.

New Technologies Bloom Together for Bettering Cancer Drug Conjugates

Drug conjugates, including antibody-drug conjugates, are a step toward realizing Paul Ehrlich's idea from over 100 years ago of a "magic bullet" for cancer treatment. Through balancing selective targeting molecules with highly potent payloads, drug conjugates can target specific tumor microenvironments and kill tumor cells. A drug conjugate consists of three parts: a targeting agent, a linker, and a payload. In some conjugates, monoclonal antibodies act as the targeting agent, but new strategies for targeting include antibody derivatives, peptides, and even small molecules. Linkers are responsible for connecting the payload to the targeting agent. Payloads impact vital cellular processes to kill tumor cells. At present, there are 12 antibody-drug conjugates on the market for different types of cancers. Research on drug conjugates is increasing year by year to solve problems encountered in conjugate design, such as tumor heterogeneity, poor circulation, low drug loading, low tumor uptake, and heterogenous expression of target antigens. This review highlights some important preclinical research on drug conjugates in recent years. We focus on three significant areas: improvement of antibody-drug conjugates, identification of new conjugate targets, and development of new types of drug conjugates, including nanotechnology. We close by highlighting the critical barriers to clinical translation and the open questions going forward. SIGNIFICANCE STATEMENT: The development of anticancer drug conjugates is now focused in three broad areas: improvements to existing antibody drug conjugates, identification of new targets, and development of new conjugate forms. This article focuses on the exciting preclinical studies in these three areas and advances in the technology that improves preclinical development.

Homobivalent, Trivalent, and Covalent PROTACs: Emerging Strategies for Protein Degradation

Proteolysis-targeting chimeras (PROTACs) is a fast-growing technology providing many strengths over inhibition of protein activity directly and is attracting increasing interest in new drug discovery and development. However, efficiently identifying potent and drug-like degraders is still challenging in the development of PROTACs. Complementary to traditional PROTACs, several emerging types of PROTACs, such as homobivalent PROTACs based on two E3 ligases (e.g., CRBN, VHL, MDM2, TRIM24), chemical- or biological-based trivalent/multitargeted PROTACs, and covalent PROTACs, are rising for targeted protein degradation. These new types of PROTACs have several advantages over the traditional PROTACs including high selectivity, low toxicity, better therapeutic effects, and so on. In this perspective, we will summarize the latest development of representative PROTACs focusing on research mainly in past 10 years and discuss their advantages and disadvantages. Moreover, the outlook and perspectives on the associated challenges and future directions will be provided.

PROTACs: past, present and future

Proteolysis-targeting chimeras (PROTACs) are heterobifunctional molecules consisting of one ligand that binds to a protein of interest (POI) and another that can recruit an E3 ubiquitin ligase. The chemically-induced proximity between the POI and E3 ligase results in ubiquitination and subsequent degradation of the POI by the ubiquitin-proteasome system (UPS). The event-driven mechanism of action (MOA) of PROTACs offers several advantages compared to traditional occupancy-driven small molecule inhibitors, such as a catalytic nature, reduced dosing and dosing frequency, a more potent and longer-lasting effect, an added layer of selectivity to reduce potential toxicity, efficacy in the face of drug-resistance mechanisms, targeting nonenzymatic functions, and expanded target space. Here, we highlight important milestones and briefly discuss lessons learned about targeted protein degradation (TPD) in recent years and conjecture on the efforts still needed to expand the toolbox for PROTAC discovery to ultimately provide promising therapeutics.

PROTACs: great opportunities for academia and industry (an update from 2020 to 2021)

PROteolysis TArgeting Chimeras (PROTACs) technology is a new protein-degradation strategy that has emerged in recent years. It uses bifunctional small molecules to induce the ubiquitination and degradation of target proteins through the ubiquitin–proteasome system. PROTACs can not only be used as potential clinical treatments for diseases such as cancer, immune disorders, viral infections, and neurodegenerative diseases, but also provide unique chemical knockdown tools for biological research in a catalytic, reversible, and rapid manner. In 2019, our group published a review article “PROTACs: great opportunities for academia and industry” in the journal, summarizing the representative compounds of PROTACs reported before the end of 2019. In the past 2 years, the entire field of protein degradation has experienced rapid development, including not only a large increase in the number of research papers on protein-degradation technology but also a rapid increase in the number of small-molecule degraders that have entered the clinical and will enter the clinical stage. In addition to PROTAC and molecular glue technology, other new degradation technologies are also developing rapidly. In this article, we mainly summarize and review the representative PROTACs of related targets published in 2020–2021 to present to researchers the exciting developments in the field of protein degradation. The problems that need to be solved in this field will also be briefly introduced.

Target and tissue selectivity of PROTAC degraders

Targeted protein degradation (TPD) strategies have revolutionized how scientists tackle challenging protein targets deemed undruggable with traditional small molecule inhibitors. Many promising campaigns to inhibit proteins have failed due to factors surrounding inhibition selectivity and targeting of compounds to specific tissues and cell types. One of the major improvements that PROTAC (proteolysis targeting chimera) and molecular glue technology can exert is highly selective control of target inhibition. Multiple studies have shown that PROTACs can gain selectivity for their protein targets beyond that of their parent ligands via optimization of linker length and stabilization of ternary complexes. Due to the bifunctional nature of PROTACs, the tissue selective nature of E3 ligases can be exploited to uncover novel targeting mechanisms. In this review, we provide critical analysis of the recent progress towards making selective PROTAC molecules and new PROTAC technologies that will continue to push the boundaries of achieving selectivity. These efforts have wide implications in the future of treating disease as they will broaden the possible targets that can be addressed by small molecules, like undruggable proteins or broadly active targets that would benefit from degradation in specific tissue types.

Degrader-antibody conjugates

Degrader-antibody conjugates (DACs) are novel entities that combine a proteolysis targeting chimera (PROTAC) payload with a monoclonal antibody via some type of chemical linker. This review provides a current summary of the DAC field. Many general aspects associated with the creation and biological performance of traditional cytotoxic antibody-drug conjugates (ADCs) are initially presented. These characteristics are subsequently compared and contrasted with related parameters that impact DAC generation and biological activity. Several examples of DACs assembled from both the scientific and the patent literature are utilized to highlight differing strategies for DAC creation, and specific challenges associated with DAC construction are documented. Collectively, the assembled examples demonstrate that biologically-active DACs can be successfully prepared using a variety of PROTAC payloads which employ diverse E3 ligases to degrade multiple protein targets.

Smart PROTACs Enable Controllable Protein Degradation for Precision Cancer Therapy

Proteolysis-targeting chimeras (PROTACs) are heterobifunctional chemicals that degrade proteins at the post-translational level, which represent an emerging therapeutic modality to fight cancer and other diseases. Although several PROTACs have now entered clinical trials, potential off-tissue side effects have resulted from nonspecific accumulation at non-cancerous sites after systemic administration, and this remains a major challenge. To this end, in the past 3 years, activatable PROTACs whose activity can only be launched on demand have gained tremendous momentum. In this review, we provide an overview of these new smart activatable PROTACs, which exert protein degradation action only in response to internal or external stimuli. We categorize these activatable PROTACs according to their activation mechanism contributed by different stimuli, including reduction-activatable, hypoxia-activatable, and enzyme-activatable PROTACs and photo-caged or photo-switchable PROTACs. The use of stimuli-responsive chemical blocks in these activatable PROTACs allows local activation of the antitumor effects while reducing the incidence of off-site side effects for precision cancer therapy. The design principle and category of smart PROTACs are introduced along with an overview of their therapeutic prospects and challenges.

In Silico Modeling and Scoring of PROTAC-Mediated Ternary Complex Poses

Proteolysis targeting chimeras (PROTACs) are molecules that induce protein degradation via formation of ternary complexes between an E3 ubiquitin ligase and a target protein. The rational design of PROTACs requires accurate knowledge of the native configuration of the PROTAC-induced ternary complex. This study demonstrates that native and non-native ternary complex poses can be distinguished based on the pose occupancy time in MD, where native poses exhibit longer occupancy times at both room and higher temperatures. Candidate poses are generated by MD sampling and pre-ranked by classic MM/GBSA. A specific heating scheme is then applied to accelerate ternary pose departure, with the pose occupancy time and fraction being measured. This scoring identifies the native pose in all systems tested. Its success is partially attributed to the dynamic nature of pose departure analyses, which accounts for entropic effects typically neglected in the faster static scoring methods, while entropy plays a greater role in protein-protein than in protein-ligand systems.

Modeling the CRL4A ligase complex to predict target protein ubiquitination induced by cereblon-recruiting PROTACs

PROteolysis TArgeting Chimeras (PROTACs) are hetero-bifunctional small molecules that can simultaneously recruit target proteins and E3 ligases to form a ternary complex, promoting target protein ubiquitination and degradation via the Ubiquitin-Proteasome System (UPS). PROTACs have gained increasing attention in recent years due to certain advantages over traditional therapeutic modalities and enabling targeting of previously “undruggable” proteins. To better understand the mechanism of PROTAC-induced Target Protein Degradation (TPD), several computational approaches have recently been developed to study and predict ternary complex formation. However, mounting evidence suggests that ubiquitination can also be a rate-limiting step in PROTAC-induced TPD. Here, we propose a structure-based computational approach to predict target protein ubiquitination induced by cereblon (CRBN)-based PROTACs by leveraging available structural information of the CRL4A ligase complex (CRBN/DDB1/CUL4A/Rbx1/NEDD8/E2/Ub). We generated ternary complex ensembles with Rosetta, modeled multiple CRL4A ligase complex conformations, and predicted ubiquitination efficiency by separating the ternary ensemble into productive and unproductive complexes based on the proximity of the ubiquitin to accessible lysines on the target protein. We validated our CRL4A ligase complex models with published ternary complex structures and additionally employed our modeling workflow to predict ubiquitination efficiencies and sites of a series of cyclin-dependent kinases (CDKs) after treatment with TL12–186, a pan-kinase PROTAC. Our predictions are consistent with CDK ubiquitination and site-directed mutagenesis of specific CDK lysine residues as measured using a NanoBRET ubiquitination assay in HEK293 cells. This work structurally links PROTAC-induced ternary formation and ubiquitination, representing an important step toward prediction of target “degradability.”

Targeting the histone H3 lysine 79 methyltransferase DOT1L in MLL-rearranged leukemias

Disrupting the methylation of telomeric silencing 1-like (DOT1L)-mediated histone H3 lysine 79 has been implicated in MLL fusion-mediated leukemogenesis. Recently, DOT1L has become an attractive therapeutic target for MLL-rearranged leukemias. Rigorous studies have been performed, and much progress has been achieved. Moreover, one DOT1L inhibitor, EPZ-5676, has entered clinical trials, but its clinical activity is modest. Here, we review the recent advances and future trends of various therapeutic strategies against DOT1L for MLL-rearranged leukemias, including DOT1L enzymatic activity inhibitors, DOT1L degraders, protein-protein interaction (PPI) inhibitors, and combinatorial interventions. In addition, the limitations, challenges, and prospects of these therapeutic strategies are discussed. In summary, we present a general overview of DOT1L as a target in MLL-rearranged leukemias to provide valuable guidance for DOT1L-associated drug development in the future. Although a variety of DOT1L enzymatic inhibitors have been identified, most of them require further optimization. Recent advances in the development of small molecule degraders, including heterobifunctional degraders and molecular glues, provide valuable insights and references for DOT1L degraders. However, drug R&D strategies and platforms need to be developed and preclinical experiments need to be performed with the purpose of blocking DOT1L-associated PPIs. DOT1L epigenetic-based combination therapy is worth considering and exploring, but the therapy should be based on a thorough understanding of the regulatory mechanism of DOT1L epigenetic modifications.

PROTAC targeted protein degraders: the past is prologue

Targeted protein degradation (TPD) is an emerging therapeutic modality with the potential to tackle disease-causing proteins that have historically been highly challenging to target with conventional small molecules. In the 20 years since the concept of a proteolysis-targeting chimera (PROTAC) molecule harnessing the ubiquitin-proteasome system to degrade a target protein was reported, TPD has moved from academia to industry, where numerous companies have disclosed programmes in preclinical and early clinical development. With clinical proof-of-concept for PROTAC molecules against two well-established cancer targets provided in 2020, the field is poised to pursue targets that were previously considered 'undruggable'. In this Review, we summarize the first two decades of PROTAC discovery and assess the current landscape, with a focus on industry activity. We then discuss key areas for the future of TPD, including establishing the target classes for which TPD is most suitable, expanding the use of ubiquitin ligases to enable precision medicine and extending the modality beyond oncology.

Driving E3 Ligase Substrate Specificity for Targeted Protein Degradation: Lessons from Nature and the Laboratory

Methods to direct the degradation of protein targets with proximity-inducing molecules that coopt the cellular degradation machinery are advancing in leaps and bounds, and diverse modalities are emerging. The most used and well-studied approach is to hijack E3 ligases of the ubiquitin-proteasome system. E3 ligases use specific molecular recognition to determine which proteins in the cell are ubiquitinated and degraded. This review focuses on the structural determinants of E3 ligase recruitment of natural substrates and neo-substrates obtained through monovalent molecular glues and bivalent proteolysis-targeting chimeras. We use structures to illustrate the different types of substrate recognition and assess the basis for neo-protein-protein interactions in ternary complex structures. The emerging structural and mechanistic complexity is reflective of the diverse physiological roles of protein ubiquitination. This molecular insight is also guiding the application of structure-based design approaches to the development of new and existing degraders as chemical tools and therapeutics. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Non-small molecule PROTACs (NSM-PROTACs): Protein degradation kaleidoscope

The proteolysis targeting chimeras (PROTACs) technology has been rapidly developed since its birth in 2001, attracting rapidly growing attention of scientific institutes and pharmaceutical companies. At present, a variety of small molecule PROTACs have entered the clinical trial. However, as small molecule PROTACs flourish, non-small molecule PROTACs (NSM-PROTACs) such as peptide PROTACs, nucleic acid PROTACs and antibody PROTACs have also advanced considerably over recent years, exhibiting the unique characters beyond the small molecule PROTACs. Here, we briefly introduce the types of NSM-PROTACs, describe the advantages of NSM-PROTACs, and summarize the development of NSM-PROTACs so far in detail. We hope this article could not only provide useful insights into NSM-PROTACs, but also expand the research interest of NSM-PROTACs.

Recent Developments in PROTAC-Mediated Protein Degradation: From Bench to Clinic

Proteolysis-targeting chimeras (PROTACs), an emerging paradigm-shifting technology, hijacks the ubiquitin-proteasome system for targeted protein degradation. PROTACs induce ternary complexes between an E3 ligase and POI, and this induced proximity leads to polyUb chain formation on substrates and eventual proteasomal-mediated POI degradation. PROTACs have shown great therapeutic potential by degrading many disease-causing proteins, such as the androgen receptor and BRD4. The PROTAC technology has advanced significantly in the last two decades, with the repertoire of PROTAC targets increased tremendously. Herein, we describe recent developments of PROTAC technology, focusing on mechanistic and kinetic studies, pharmacokinetic study, spatiotemporal control of PROTACs, covalent PROTACs, resistance to PROTACs, and new E3 ligands.

Design, Synthesis, and Evaluation of Trivalent PROTACs Having a Functionalization Site with Controlled Orientation

Trivalent PROTACs having a functionalization site with controlled orientation were designed, synthesized, and evaluated. Based on the X-ray structure of BRD protein degrader MZ1 (1) in complex with human VHL and BRD4BD2, we expected that the 1,2-disubstituted ethyl group near the JQ-1 moiety in MZ1 (1) could be replaced by a planar benzene tether as a platform for further functionalization. To test this hypothesis, we first designed six divalent MZ1 derivatives, 2a-c and 3a-c, by combining three variations of substitution patterns on the benzene ring (1,2-, 1,3-, and 1,4-substitution) and two variations in the number of ethylene glycol units (2 or 1). We then tested the synthesized compounds for the BRD4 degradation activity of each. As expected, we found that 1,2D-EG2-MZ1 (2a), an MZ1 derivative with 1,2-disubstituted benzene possessing two ethylene glycol units, had an activity profile similar to that of MZ1 (1). Based on the structure of 2a, we then synthesized and evaluated four isomeric trivalent MZ1 derivatives, 15a-15d, having a tert-butyl ester unit on the benzene ring as a handle for further functionalization. Among the four isomers, 1,2,5T-EG2-MZ1 (15c) retained a level of BRD4 depletion activity similar to that of 2a without inducing a measurable Hook effect, and its BRD4 depletion kinetics was the same as that of MZ1 (1). Other isomers were also shown to retain BRD4 depletion activity. Thus, the trivalent PROTACs we synthesized here may serve as efficient platforms for further applications.

Strategies for designing proteolysis targeting chimaeras (PROTACs)

Proteolysis targeting chimaeras (PROTACs) is a cutting edge and rapidly growing technique for new drug discovery and development. Currently, the largest challenge in the molecular design and drug development of PROTACs is efficient identification of potent and drug-like degraders. This review aims to comprehensively summarize and analyse state-of-the-art methods and strategies in the design of PROTACs. We provide a detailed illustration of the general principles and tactics for designing potent PROTACs, highlight representative case studies, and discuss the advantages and limitations of these strategies. Particularly, structure-based rational PROTAC design and emerging new types of PROTACs (e.g., homo-PROTACs, multitargeting PROTACs, photo-control PROTACs and PROTAC-based conjugates) will be focused on.

Integrative Modeling of PROTAC-Mediated Ternary Complexes

Proteolysis-targeting chimeras (PROTACs), which selectively induce targeted protein degradation, represent an emerging drug discovery technology. Although numerous PROTACs have been reported, designing potent PROTACs still remains a great challenge, to some extent, due to insufficient structural data of Target-PROTAC-E3 ternary complexes. In this work, PROTAC-Model, an integrative computational method by combining the FRODOCK-based protocol and RosettaDock-based refinement, was developed to predict PROTAC-mediated ternary complex structures and tested on 14 cases. The quality of the models was evaluated using the criteria of the critical assessment of predicted interactions (CAPRI). Using the unbound structures, the FRODOCK-based protocol can generate the ternary complex structures with medium or high quality for 8 cases out of 14. With the refinement by RosettaDock, the cases with medium or high quality increase to 12. Compared with PRosettaC and the method developed by Drummond et al., PROTAC-Model shows better performance. In summary, PROTAC-Model should be useful for the rational design of PROTACs.

Drug discovery for epigenetics targets

Dysregulation of the epigenome is associated with the onset and progression of several diseases, including cancer, autoimmune, cardiovascular, and neurological disorders. Members from the three families of epigenetic proteins (readers, writers, and erasers) have been shown to be druggable using small-molecule inhibitors. Increasing knowledge of the role of epigenetics in disease and the reversibility of these modifications explain why pharmacological intervention is an attractive strategy for tackling epigenetic-based disease. In this review, we provide an overview of epigenetics drug targets, focus on approaches used for initial hit identification, and describe the subsequent role of structure-guided chemistry optimisation of initial hits to clinical candidates. We also highlight current challenges and future potential for epigenetics-based therapies.

Folate-Guided Protein Degradation by Immunomodulatory Imide Drug-Based Molecular Glues and Proteolysis Targeting Chimeras

Molecular glues and proteolysis targeting chimeras (PROTACs) are promising new therapeutic modalities. However, the lack of specificity for molecular glue- or PROTAC-mediated proteolysis in cancer cells versus normal cells raises potential toxicity concerns that will likely limit their clinical applications. Here, we developed a general strategy to deliver immunomodulatory imide drug (IMiD)-based molecular glues and PROTACs to folate receptor α (FOLR1)-positive cancer cells. Specifically, we designed a folate-caged pomalidomide prodrug, FA-S2-POMA, by incorporating a folate group as a caging and guiding element and validated its degradation effect on its neo-substrates in FOLR1-positive cancer cells in a FOLR1-dependent manner. We also developed a folate-caged pomalidomide-based anaplastic lymphoma kinase (ALK) PROTAC, FA-S2-MS4048, which effectively degraded ALK fusion proteins in cancer cells, again in a FOLR1-dependent manner. This novel approach provides a generalizable platform for the targeted delivery of IMiD-based molecular glues and PROTACs to FOLR1-expressing cancer cells with the potential to ameliorate toxicity.

Native Mass Spectrometry for the Study of PROTAC GNE-987-Containing Ternary Complexes

PROteolysis TArgeting Chimeras (PROTACs) promote the degradation, rather than inhibition, of a drug target as a mechanism for therapeutic treatment. Bifunctional PROTAC molecules allow simultaneous binding of both the target protein and an E3-Ubiquitin ligase, bringing the two proteins into close spatial proximity to allow ubiquitinylation and degradation of the target protein via the cell's endogenous protein degradation pathway. We utilized native mass spectrometry (MS) to study the ternary complexes promoted by the previously reported PROTAC GNE-987 between Brd4 bromodomains 1 and 2, and Von Hippel Lindeau E3-Ubiquitin Ligase. Native MS at high resolution allowed us to measure ternary complex formation as a function of PROTAC concentration to provide a measure of complex affinity and stability, whilst simultaneously measuring other intermediate protein species. Native MS provides a high-throughput, low sample consumption, direct screening method to measure ternary complexes for PROTAC development.

Light-Controllable PROTACs for Temporospatial Control of Protein Degradation

PROteolysis-TArgeting Chimeras (PROTACs) is an emerging and promising approach to target intracellular proteins for ubiquitination-mediated degradation, including those so-called undruggable protein targets, such as transcriptional factors and scaffold proteins. To date, plenty of PROTACs have been developed to degrade various disease-relevant proteins, such as estrogen receptor (ER), androgen receptor (AR), RTK, and CDKs. However, the on-target off-tissue and off-target effect is one of the major limitation that prevents the usage of PROTACs in clinic. To this end, we and several other groups have recently developed light-controllable PROTACs, as the representative for the third generation controllable PROTACs, by using either photo-caging or photo-switch approaches. In this review, we summarize the emerging light-controllable PROTACs and the prospective for other potential ways to achieve temporospatial control of PROTACs.

Site-Specific Antibody Conjugation to Engineered Double Cysteine Residues

Site-specific antibody conjugations generate homogeneous antibody-drug conjugates with high therapeutic index. However, there are limited examples for producing the site-specific conjugates with a drug-to-antibody ratio (DAR) greater than two, especially using engineered cysteines. Based on available Fc structures, we designed and introduced free cysteine residues into various antibody CH2 and CH3 regions to explore and expand this technology. The mutants were generated using site-directed mutagenesis with good yield and properties. Conjugation efficiency and selectivity were screened using PEGylation. The top single cysteine mutants were then selected and combined as double cysteine mutants for expression and further investigation. Thirty-six out of thirty-eight double cysteine mutants display comparable expression with low aggregation similar to the wild-type antibody. PEGylation screening identified seventeen double cysteine mutants with good conjugatability and high selectivity. PEGylation was demonstrated to be a valuable and efficient approach for quickly screening mutants for high selectivity as well as conjugation efficiency. Our work demonstrated the feasibility of generating antibody conjugates with a DAR greater than 3.4 and high site-selectivity using THIOMAB^TM method. The top single or double cysteine mutants identified can potentially be applied to site-specific antibody conjugation of cytotoxin or other therapeutic agents as a next generation conjugation strategy.

Aptamer-PROTAC Conjugates (APCs) for Tumor-Specific Targeting in Breast Cancer

Development of proteolysis targeting chimeras (PROTACs) is emerging as a promising strategy for targeted protein degradation. However, the drug development using the heterobifunctional PROTAC molecules is generally limited by poor membrane permeability, low in vivo efficacy and indiscriminate distribution. Herein an aptamer-PROTAC conjugation approach was developed as a novel strategy to improve the tumor-specific targeting ability and in vivo antitumor potency of conventional PROTACs. As proof of concept, the first aptamer-PROTAC conjugate (APC) was designed by conjugating a BET-targeting PROTAC to the nucleic acid aptamer AS1411 (AS) via a cleavable linker. Compared with the unmodified BET PROTAC, the designed molecule (APR) showed improved tumor targeting ability in a MCF-7 xenograft model, leading to enhanced in vivo BET degradation and antitumor potency and decreased toxicity. Thus, the APC strategy may pave the way for the design of tumor-specific targeting PROTACs and have broad applications in the development of PROTAC-based drugs.

Cancer Selective Target Degradation by Folate-Caged PROTACs

PROTACs (proteolysis targeting chimeras) are an emerging class of promising therapeutic modalities that degrade intracellular protein targets by hijacking the cellular ubiquitin-proteasome system. However, potential toxicity of PROTACs in normal cells due to the off-tissue on-target degradation effect limits their clinical applications. Precise control of a PROTAC's on-target degradation activity in a tissue-selective manner could minimize potential toxicity/side-effects. To this end, we developed a cancer cell selective delivery strategy for PROTACs by conjugating a folate group to a ligand of the VHL E3 ubiquitin ligase, to achieve targeted degradation of proteins of interest (POIs) in cancer cells versus noncancerous normal cells. We show that our folate-PROTACs, including BRD PROTAC (folate-ARV-771), MEK PROTAC (folate-MS432), and ALK PROTAC (folate-MS99), are capable of degrading BRDs, MEKs, and ALK, respectively, in a folate receptor-dependent manner in cancer cells. This design provides a generalizable platform for PROTACs to achieve selective degradation of POIs in cancer cells.

Antibody-Mediated Delivery of Chimeric BRD4 Degraders. Part 1: Exploration of Antibody Linker, Payload Loading, and Payload Molecular Properties

The biological and medicinal impacts of proteolysis-targeting chimeras (PROTACs) and related chimeric molecules that effect intracellular degradation of target proteins via ubiquitin ligase-mediated ubiquitination continue to grow. However, these chimeric entities are relatively large compounds that often possess molecular characteristics, which may compromise oral bioavailability, solubility, and/or in vivo pharmacokinetic properties. We therefore explored the conjugation of such molecules to monoclonal antibodies using technologies originally developed for cytotoxic payloads so as to provide alternate delivery options for these novel agents. In this report, we describe the first phase of our systematic development of antibody-drug conjugates (ADCs) derived from bromodomain-containing protein 4 (BRD4)-targeting chimeric degrader entities. We demonstrate the antigen-dependent delivery of the degrader payloads to PC3-S1 prostate cancer cells along with related impacts on MYC transcription and intracellular BRD4 levels. These experiments culminate with the identification of one degrader conjugate, which exhibits antigen-dependent antiproliferation effects in LNCaP prostate cancer cells.