Targeting fusion oncoproteins in childhood cancers: Challenges and future opportunities for developing therapeutics.

Fusion oncoproteins are associated with childhood cancers and have proven challenging to target, aside from those that include kinases. As part of its efforts for targeting childhood cancers, the National Cancer Institute recently conducted a series on 'Novel Chemical Approaches for Targeting Fusion Oncoproteins'. Key learnings on leading platforms and technologies which can be utilized to advance the development of molecular therapeutics that target fusion oncoproteins in childhood cancers are described. Recent breakthroughs in medicinal chemistry and chemical biology provide new ground and creative strategies to exploit for the development of targeted agents for improving outcomes against these recalcitrant cancers.

New approaches for challenging therapeutic targets.

Despite successes with new drug approvals over the past two decades through conventional drug development approaches, many human diseases remain intractable to current therapeutic interventions. Possible barriers may be that the complexity of the target, and disease biology, are impervious to such conventional drug development approaches. The US National Institutes of Health hosted a workshop with the goal of identifying challenges and opportunities with alternative modalities for developing treatments across diseases associated with historically undruggable targets. This report highlights key issues discussed during the workshop that, if addressed, could expand the pool of therapeutic approaches for treating various diseases.

Assessing interobserver variability and accuracy in the histological diagnosis and classification of cutaneous neurofibromass.

BACKGROUND: Cutaneous neurofibromas (cNFs) are the most common tumors in people with neurofibromatosis type 1 (NF1) and are associated with reduced quality of life. There is currently no widely accepted standardized language for describing cNFs clinically or histopathologically. The objective of this study was to evaluate interobserver agreement across pathologists in describing and reporting of neurofibromas involving the skin. METHODS: Twenty-eight (H&E)-stained slides of cNF were scanned using an Aperio XT scanner. The digital images were reviewed by 6 pathologists, who entered free text of up to a 200 word description for each case into a REDcap database. Responses were analyzed for the most commonly used terms based on frequency, as well as agreement (reported as concordance) between reviewers. RESULTS: A set of the terms most commonly used by pathologists for the histological classification of cNF along with areas of agreement and disagreement have been identified. The study shows that there was strong agreement across reviewers that not all neurofibromas involving the skin are cutaneous neurofibromas and regarding the presence or absence of atypical features and heterologous elements. Areas of less concordance were identified and include cNF subtypes, definition of extension and pattern of growth, as well as the distinction of a cNF from a plexiform without an intraneural component involving skin. CONCLUSIONS: This work is the first step towards development of a robust classification system and devising "gold standard" histopathologic diagnostic criteria for cutaneous neurofibromas.

Funding community collaboration to develop effective therapies for neurofibromatosis type 1 tumors.

The time from identifying a drug target to a new drug approval is often measured in decades and can take even longer for therapies to treat rare diseases. In fact, 95% of rare diseases do not have a specific therapy approved at all. Coordinated efforts to augment the drug development pipeline along with long-term and comprehensive support that enable scientific breakthroughs for rare diseases are possible, but it requires integration across multiple stakeholders. This article analyzes the coordinated funding efforts of four federal and philanthropic organizations to advance drug development for neurofibromatosis type 1-associated tumors and discusses how these organizations have been collaborating and evolved practices to optimize funding and research support.

Creating a comprehensive research strategy for cutaneous neurofibromas.

OBJECTIVE: Outside of procedural-based methods, there are currently no established medical treatments for cutaneous neurofibroma (cNF), which afflict up to 99% of patients with NF1. Further, adult patients often report cNF are the greatest burden of living with NF1. The Neurofibromatosis Therapeutic Acceleration Program (NTAP) launched a think tank to address core questions to facilitate development of effective therapeutics for cNF in people with NF1. METHODS: Experts (with and without explicit experience with NF1 or cNF) from multiple scientific and medical disciplines, representing the ranks of academia, industry, and government agencies, were invited to become a member of a team addressing a specific subset of questions pertinent to cNF. Teams met monthly to review published and unpublished materials, and created summaries about the material known and unknown that may influence therapeutic development for cNF. Teams prioritized questions and organized supporting data, which was presented to the entire body of experts by each team at a research summit. RESULTS: Four themes were identified as being relevant to creating a comprehensive research strategy for cNF: (1) establishing definitions of cNF, (2) determining the biology of cNF with respect to tumor initiation, progression, and maintenance, (3) outlining the factors that guide therapies development, and (4) defining core considerations for clinical trials design and optimization for cNF. CONCLUSION: Considerations and key questions for each of the thematic areas were identified and provided basis for a request for applications launched by NTAP focused on cNF and are described in the accompanying articles of this supplement.

Cutaneous neurofibromas: Current clinical and pathologic issues.

OBJECTIVE: To present the current terminology and natural history of neurofibromatosis 1 (NF1) cutaneous neurofibromas (cNF). METHODS: NF1 experts from various research and clinical backgrounds reviewed the terms currently in use for cNF as well as the clinical, histologic, and radiographic features of these tumors using published and unpublished data. RESULTS: Neurofibromas develop within nerves, soft tissue, and skin. The primary distinction between cNF and other neurofibromas is that cNF are limited to the skin whereas other neurofibromas may involve the skin, but are not limited to the skin. There are important cellular, molecular, histologic, and clinical features of cNF. Each of these factors is discussed in consideration of a clinicopathologic framework for cNF. CONCLUSION: The development of effective therapies for cNF requires formulation of diagnostic criteria that encompass the clinical and histologic features of these tumors. However, there are several areas of overlap between cNF and other neurofibromas that make distinctions between cutaneous and other neurofibromas more difficult, requiring careful deliberation with input across the multiple disciplines that encounter these tumors and ultimately, prospective validation. The ultimate goal of this work is to facilitate accurate diagnosis and meaningful therapeutics for cNF.

Considerations for development of therapies for cutaneous neurofibroma.

OBJECTIVE: The only therapies currently available for cutaneous neurofibromas (cNF) are procedural. The goals of the Therapies Development Working Group were to (1) summarize currently available treatment options for cNF, (2) define key considerations for drug discovery and development generally, and specifically for cNF, and (3) outline recommendations for the successful development of medical therapies for cNF. METHODS: The subgroup reviewed published and unpublished data on procedural, drug/device, and medical treatment approaches utilized for cNFs via literature search. The team defined disease- and patient-specific factors to consider for therapies development in a series of consensus meetings. RESULTS: The team identified 5 approaches entailing procedural and drug/device methods currently under study. There have been 4 clinical studies exploring various interventional therapies, from which outcomes were highly variable. The team identified 4 key factors to prioritize during the development of products for the treatment for cNF: safety, anatomic distribution of cNF, numbers of tumors to be treated, and route of administration. CONCLUSIONS: The number, size, and distribution of cNF is highly variable among patients with NF1 and it is possible that different phenotypes will require different drug development paths. The nonfatal nature of the disease and relatively limited patient numbers suggest that for any product to have a higher likelihood of acceptance, it will have to (1) demonstrate an effect that is clinically meaningful, (2) have a safety profile conducive to long-term dosing, and (3) have a low manufacturing cost.

The biology of cutaneous neurofibromas: Consensus recommendations for setting research priorities.

OBJECTIVE: A group of experts in dermatology, genetics, neuroscience, and regenerative medicine collaborated to summarize current knowledge on the defined factors contributing to cutaneous neurofibroma (cNF) development and to provide consensus recommendations for future research priorities to gain an improved understanding of the biology of cNF. METHODS: The group members reviewed published and unpublished data on cNF and related diseases via literature search, defined a set of key topic areas deemed critical in cNF pathogenesis, and developed recommendations in a series of consensus meetings. RESULTS: Five specific topic areas were identified as being relevant to providing an enhanced understanding of the biology of cNF: (1) defining the human cells of origin; (2) understanding the role of the microenvironment, focusing on neurons, mast cells, and fibroblasts; (3) defining the genetic and molecular differences between the cNFs, focusing on size and number; (4) understanding if sex hormones are critical for cNF development or progression; and (5) identifying challenges in establishing in vitro and in vivo models representing human cNF. CONCLUSIONS: The complexity of cNF biology stems from its heterogeneity at multiple levels including genetic, spatial involvement, temporal development, and cellular composition. We propose a unified working model for cNF that builds a framework to address the key questions about cNF that, when answered, will provide the necessary understanding of cNF biology to allow meaningful development of therapies.

Clinical trial design for cutaneous neurofibromas.

OBJECTIVE: Several clinical trials targeting cutaneous neurofibromas (cNF) have been conducted; however, none has resulted in meaningful changes to care. The Clinical Trial Design and Development subgroup's goals were to (1) define key considerations in the design of clinical trials for cNF, (2) summarize existing data in relation to these considerations, and (3) provide consensus recommendations about key elements of trial design to accelerate the clinical development of therapies for cNF. METHODS: The subgroup, with experts from genetics, dermatology, neurology, oncology, and basic science, spanning academia, government research, and regulatory programs, and industry, reviewed published and unpublished data on clinical trials for cNF and other diseases in the skin. Discussions of these data resulted in formulation of a list of priority issues to address in order to develop efficient and effective clinical trials for cNF. RESULTS: The subgroup identified 2 natural history studies of cNF, 4 priority outcome measures, and 6 patient-reported outcome tools for potential use in efficacy trials of cNF. Time to initiate intervention, patient eligibility, mechanism of action, route of administration, safety monitoring, and regulatory agency interactions were identified as key factors to consider when designing clinical trials for cNF. CONCLUSIONS: Alignment on endpoints and methods for the measurement and quantification of cNF represent a priority for therapeutic development for cNF. Advances in technological methods and outcome tools utilized in other skin diseases may be applicable to cNF studies. Patient age is an important factor guiding trial design and clinical development path.

Inhibition of the histone lysine methyltransferase EZH2 for the treatment of cancer.

In recent years, there has been an increased effort in the development of therapies which target an epigenetic mode of action. Among thes e efforts include progress in the development of inhibitors of EZH2 (Enhancer of Zeste Homolog 2), a key epigenetic target with strong disease implications to cancer. Over the last 3+ years, multiple reports describing small molecule inhibitors of EZH2 have been described, including those for chemical probes and drug candidates which have entered the clinic as first-in-class agents. Recent progress in this emerging area is presented in this review.

Recent progress in the discovery of epigenetic inhibitors for the treatment of cancer.

Epigenetics investigates heritable changes in gene transcription that do not involve a change in DNA sequence, and an increased understanding in the role of epigenetic misregulation as a key contributor to cancer has triggered the development of epigenetic targeted cancer therapies. Among these include efforts around a class of enzymes known as histone methyltransferases (HMTs). The level of interest in the development of HMT inhibitors as a class of anticancer agents has significantly grown beyond academic settings, and in the last 5 years whole research groups from biotech and big pharma have been dedicated to this area. There are now multiple reports describing small-molecule HMT inhibitors, including chemical probes and drug candidates entering the clinic as first-in-class agents. Recent progress in this emerging area is the topic of this chapter.

A687V EZH2 is a driver of histone H3 lysine 27 (H3K27) hypertrimethylation.

The EZH2 methyltransferase silences gene expression through methylation of histone H3 on lysine 27 (H3K27). Recently, EZH2 mutations have been reported at Y641, A677, and A687 in non-Hodgkin lymphoma. Although the Y641F/N/S/H/C and A677G mutations exhibit clearly increased activity with substrates dimethylated at lysine 27 (H3K27me2), the A687V mutant has been shown to prefer a monomethylated lysine 27 (H3K27me1) with little gain of activity toward H3K27me2. Herein, we demonstrate that despite this unique substrate preference, A687V EZH2 still drives increased H3K27me3 when transiently expressed in cells. However, unlike the previously described mutants that dramatically deplete global H3K27me2 levels, A687V EZH2 retains normal levels of H3K27me2. Sequencing of B-cell-derived cancer cell lines identified an acute lymphoblastic leukemia cell line harboring this mutation. Similar to exogenous expression of A687V EZH2, this cell line exhibited elevated H3K27me3 while possessing H3K27me2 levels higher than Y641- or A677-mutant lines. Treatment of A687V EZH2-mutant cells with GSK126, a selective EZH2 inhibitor, was associated with a global decrease in H3K27me3, robust gene activation, caspase activation, and decreased proliferation. Structural modeling of the A687V EZH2 active site suggests that the increased catalytic activity with H3K27me1 may be due to a weakened interaction with an active site water molecule that must be displaced for dimethylation to occur. These findings suggest that A687V EZH2 likely increases global H3K27me3 indirectly through increased catalytic activity with H3K27me1 and cells harboring this mutation are highly dependent on EZH2 activity for their survival.

Long residence time inhibition of EZH2 in activated polycomb repressive complex 2.

EZH2/PRC2 catalyzes transcriptionally repressive methylation at lysine 27 of histone H3 and has been associated with numerous cancer types. Point mutations in EZH2 at Tyr641 and Ala677 identified in non-Hodgkin lymphomas alter substrate specificity and result in increased trimethylation at histone H3K27. Interestingly, EZH2/PRC2 is activated by binding H3K27me3 marks on histones, and this activation is proposed as a mechanism for self-propagation of gene silencing. Recent work has identified GSK126 as a potent, selective, SAM-competitive inhibitor of EZH2 capable of globally decreasing H3K27 trimethylation in cells. Here we show that activation of PRC2 by an H3 peptide trimethylated at K27 is primarily an effect on the rate-limiting step (kcat) with no effect on substrate binding (Km). Additionally, GSK126 is shown to have a significantly longer residence time of inhibition on the activated form of EZH2/PRC2 as compared to unactivated EZH2/PRC2. Overall inhibition constant (Ki*) values for GSK126 were determined to be as low as 93 pM and appear to be driven by slow dissociation of inhibitor from the activated enzyme. The data suggest that activation of EZH2 allows the enzyme to adopt a conformation that possesses greater affinity for GSK126. The long residence time of GSK126 may be beneficial in vivo and may result in durable target inhibition after drug systemic clearance.

Recent progress in the discovery of small-molecule inhibitors of the HMT EZH2 for the treatment of cancer.

The histone lysine methyltransferase EZH2 is the catalytic component of the multi-protein PRC2 complex and methylates lysine 27 on histone H3. EZH2 overexpression is implicated in tumorigenesis and correlates with poor prognosis in several tumor types. Inhibition of aberrant EZH2 activity might attenuate tumorigenesis resulting from misregulated gene transcription derived from aberrant EZH2 activity. In the last year, the first reports of small molecules demonstrating potent and selective inhibition of EZH2 have been published by multiple groups. Herein, we review recent progress reported in the discovery of small molecule inhibitors of EZH2.

EZH2 is required for germinal center formation and somatic EZH2 mutations promote lymphoid transformation.

The EZH2 histone methyltransferase is highly expressed in germinal center (GC) B cells and targeted by somatic mutations in B cell lymphomas. Here, we find that EZH2 deletion or pharmacologic inhibition suppresses GC formation and functions. EZH2 represses proliferation checkpoint genes and helps establish bivalent chromatin domains at key regulatory loci to transiently suppress GC B cell differentiation. Somatic mutations reinforce these physiological effects through enhanced silencing of EZH2 targets. Conditional expression of mutant EZH2 in mice induces GC hyperplasia and accelerated lymphomagenesis in cooperation with BCL2. GC B cell (GCB)-type diffuse large B cell lymphomas (DLBCLs) are mostly addicted to EZH2 but not the more differentiated activated B cell (ABC)-type DLBCLs, thus clarifying the therapeutic scope of EZH2 targeting.

EZH2 inhibition as a therapeutic strategy for lymphoma with EZH2-activating mutations.

In eukaryotes, post-translational modification of histones is critical for regulation of chromatin structure and gene expression. EZH2 is the catalytic subunit of the polycomb repressive complex 2 (PRC2) and is involved in repressing gene expression through methylation of histone H3 on lysine 27 (H3K27). EZH2 overexpression is implicated in tumorigenesis and correlates with poor prognosis in several tumour types. Additionally, somatic heterozygous mutations of Y641 and A677 residues within the catalytic SET domain of EZH2 occur in diffuse large B-cell lymphoma (DLBCL) and follicular lymphoma. The Y641 residue is the most frequently mutated residue, with up to 22% of germinal centre B-cell DLBCL and follicular lymphoma harbouring mutations at this site. These lymphomas have increased H3K27 tri-methylation (H3K27me3) owing to altered substrate preferences of the mutant enzymes. However, it is unknown whether specific, direct inhibition of EZH2 methyltransferase activity will be effective in treating EZH2 mutant lymphomas. Here we demonstrate that GSK126, a potent, highly selective, S-adenosyl-methionine-competitive, small-molecule inhibitor of EZH2 methyltransferase activity, decreases global H3K27me3 levels and reactivates silenced PRC2 target genes. GSK126 effectively inhibits the proliferation of EZH2 mutant DLBCL cell lines and markedly inhibits the growth of EZH2 mutant DLBCL xenografts in mice. Together, these data demonstrate that pharmacological inhibition of EZH2 activity may provide a promising treatment for EZH2 mutant lymphoma.

Development and validation of reagents and assays for EZH2 peptide and nucleosome high-throughput screens.

Histone methyltransferases (HMT) catalyze the methylation of histone tail lysines, resulting in changes in gene transcription. Misregulation of these enzymes has been associated with various forms of cancer, making this target class a potential new area for the development of novel chemotherapeutics. EZH2 is the catalytic component of the polycomb group repressive complex (PRC2), which selectively methylates histone H3 lysine 27 (H3K27). EZH2 is overexpressed in prostate, breast, bladder, brain, and other tumor types and is recognized as a molecular marker for cancer progression and aggressiveness. Several new reagents and assays were developed to aid in the identification of EZH2 inhibitors, and these were used to execute two high-throughput screening campaigns. Activity assays using either an H3K27 peptide or nucleosomes as substrates for methylation are described. The strategy to screen EZH2 with either a surrogate peptide or a natural substrate led to the identification of the same tractable series. Compounds from this series are reversible, are [(3)H]-S-adenosyl-L-methionine competitive, and display biochemical inhibition of H3K27 methylation.

Identification of Potent, Selective, Cell-Active Inhibitors of the Histone Lysine Methyltransferase EZH2.

The histone H3-lysine 27 (H3K27) methyltransferase EZH2 plays a critical role in regulating gene expression, and its aberrant activity is linked to the onset and progression of cancer. As part of a drug discovery program targeting EZH2, we have identified highly potent, selective, SAM-competitive, and cell-active EZH2 inhibitors, including GSK926 (3) and GSK343 (6). These compounds are small molecule chemical tools that would be useful to further explore the biology of EZH2.

Asymmetric synthesis of the carbapenam core from serine.

The stereospecific synthesis of the functionalized carbapenam core 16 from the serine-derived trisubstituted pyrrolidine 9 is reported. The synthetic strategy relies on synthesizing an appropriately functionalized pyrrolidine, followed by an intramolecular azetidone formation utilizing a modified Mukiyama reagent. The efficient one-pot conversion of the benzenesulfonamide-protected pyrrolidine 9 to the Cbz-protected pyrrolidine 10 is also reported.

Hydroxylated pyrrolidines. Enantiospecific synthesis of all-cis 2,3,4,5-substituted pyrrolidine derivatives from serine.

We report the enantiospecific synthesis of the sterically congested all-cis 2,3,4,5-substituted pyrrolidines 4, 5, and 6, from either D- or L-serine. Hemiaminal intermediate 13 is converted to the fully substituted pyrrolidine 15 by way of a tandem Wittig-Michael reaction. The endo stereochemistry of the C-3 methyl group of compound 15 is set by stereoselective reduction of the double bond in 11, driven by a preference for hydrogenation from the rear side of the molecule. The all-cis configuration of these fully substituted pyrrolidines has been established by X-ray analysis of compound 6. Removal of the benzenesulfonyl group from the highly substituted and functionalized intermediate 15 is successfully accomplished by sodium naphthalenide reduction.