Genomic analysis of smoothened inhibitor resistance in basal cell carcinoma

Crystal structure of a multi-domain human smoothened receptor in complex with a super stabilizing ligand

The Smoothened receptor (SMO) belongs to the Class Frizzled of the G protein-coupled receptor (GPCR) superfamily, constituting a key component of the Hedgehog signalling pathway. Here we report the crystal structure of the multi-domain human SMO, bound and stabilized by a designed tool ligand TC114, using an X-ray free-electron laser source at 2.9 Å. The structure reveals a precise arrangement of three distinct domains: a seven-transmembrane helices domain (TMD), a hinge domain (HD) and an intact extracellular cysteine-rich domain (CRD). This architecture enables allosteric interactions between the domains that are important for ligand recognition and receptor activation. By combining the structural data, molecular dynamics simulation, and hydrogen-deuterium-exchange analysis, we demonstrate that transmembrane helix VI, extracellular loop 3 and the HD play a central role in transmitting the signal employing a unique GPCR activation mechanism, distinct from other multi-domain GPCRs.

Smoothened receptors (SMO) play a key role in the Hedgehog signalling pathway. Here the authors present the structure of a multi-domain human SMO with a rationally designed stabilizing ligand bound in the transmembrane domain of the receptor, and propose a model for SMO activation.

Basal cell carcinoma: PD-L1/PD-1 checkpoint expression and tumor regression after PD-1 blockade

Monoclonal antibodies that block immune regulatory proteins such as programmed death-1 (PD-1) have demonstrated remarkable efficacy in controlling the growth of multiple tumor types. Unresectable or metastatic basal cell carcinoma, however, has largely gone untested. Because PD-Ligand-1 (PD-L1) expression in other tumor types has been associated with response to anti-PD-1, we investigated the expression of PD-L1 and its association with PD-1 expression in the basal cell carcinoma tumor microenvironment. Among 40 basal cell carcinoma specimens, 9/40 (22%) demonstrated PD-L1 expression on tumor cells, and 33/40 (82%) demonstrated PD-L1 expression on tumor-infiltrating lymphocytes and associated macrophages. PD-L1 was observed in close geographic association to PD-1+ tumor infiltrating lymphocytes. Additionally, we present, here, the first report of an objective anti-tumor response to pembrolizumab (anti-PD-1) in a patient with metastatic PD-L1 (+) basal cell carcinoma, whose disease had previously progressed through hedgehog pathway-directed therapy. The patient remains in a partial response 14 months after initiation of therapy. Taken together, our findings provide a rationale for testing anti-PD-1 therapy in patients with advanced basal cell carcinoma, either as initial treatment or after acquired resistance to hedgehog pathway inhibition.

Role of Vismodegib in the Management of Advanced Periocular Basal Cell Carcinoma

BACKGROUND

Vismodegib is the first selective hedgehog pathway inhibitor approved to treat locally advanced and metastatic basal cell carcinoma (BCC). Limited information is available concerning its role in managing advanced BCC around the eye.

METHODS

The medical literature was searched for cases of nonsyndromic periocular BCC treated with vismodegib. Clinical information was abstracted and analyzed. In addition, a review of the pharmacology of vismodegib, including general effectiveness and safety, was conducted.

RESULTS

Thirty study patients with nonsyndromic periocular BCC treated with vismodegib were found in the literature. Vismodegib was used in 3 ways: medical therapy, adjuvant therapy prior to surgery or radiotherapy, and treatment of positive surgical margins. Complete regression was reported in 9 study patients (30%), with follow-up visits after therapy averaging fewer than 5 months. Four study participants developed squamous cell carcinoma while receiving treatment.

CONCLUSIONS

Too few cases exist to draw any conclusions on the role that vismodegib might play in the management of periocular BCC. In addition, long-term follow-up data are not yet available. Although the objective response rate of advanced BCC is impressive in study patients receiving vismodegib, well-controlled clinical studies are needed to determine whether vismodegib has any impact on survival or quality of life.

Hedgehog Signaling: From Basic Biology to Cancer Therapy

The Hedgehog (HH) signaling pathway was discovered originally as a key pathway in embryonic patterning and development. Since its discovery, it has become increasingly clear that the HH pathway also plays important roles in a multitude of cancers. Therefore, HH signaling has emerged as a therapeutic target of interest for cancer therapy. In this review, we provide a brief overview of HH signaling and the key molecular players involved and offer an up-to-date summary of our current knowledge of endogenous and exogenous small molecules that modulate HH signaling. We discuss experiences and lessons learned from the decades-long efforts toward the development of cancer therapies targeting the HH pathway. Challenges to develop next-generation cancer therapies are highlighted.

Silibinin Treatment Inhibits the Growth of Hedgehog Inhibitor-Resistant Basal Cell Carcinoma Cells via Targeting EGFR-MAPK-Akt and Hedgehog Signaling

Basal cell carcinoma (BCC) is the most common skin malignancy. Deregulated hedgehog signaling plays a central role in BCC development; therefore, hedgehog inhibitors have been approved to treat locally advanced or metastatic BCC. However, the development of resistance to hedgehog inhibitors is the major challenge in effective treatment of this disease. Herein, we evaluated the efficacy of a natural agent silibinin to overcome resistance with hedgehog inhibitors (Sant-1 and GDC-0449) in BCC cells. Silibinin (25-100 μm) treatment for 48 h strongly inhibited growth and induced death in ASZ001, Sant-1-resistant (ASZ001-Sant-1) and GDC-0449-resistant (ASZ001-GDC-0449) BCC cells. Furthermore, colony-forming ability of ASZ001, ASZ001-Sant-1 and ASZ001-GDC-0449 cells was completely inhibited by silibinin treatment. Molecular analysis showed that silibinin treatment decreased the level of phosphorylated EGFR (Tyrosine 1173) and total EGFR in ASZ001-Sant-1 cells, key signaling molecules responsible for BCC resistance toward hedgehog inhibitors. Further, silibinin treatment decreased the phosphorylated Akt (Serine 473), phosphorylated ERK1/2 (Threonine 202/Tyrosine 204), cyclin D1 and Gli-1 level but increased the SUFU expression in ASZ001-Sant-1-resistant cells. Silibinin treatment of ASZ001-Sant-1-resistant cells also decreased bcl-2 but increased cleaved caspase 3 and PARP cleavage, suggesting induction of apoptosis. Together, these results support silibinin use to target hedgehog inhibitor-resistant BCC cells.

Acute myeloid leukemia - strategies and challenges for targeting oncogenic Hedgehog/GLI signaling

Treatment of acute myeloid leukemia (AML), an aggressive and heterogeneous hematological malignancy, remains a challenge. Despite advances in our understanding of the complex genetics and biology of AML pathophysiology, these findings have been translated to the clinic with only limited success, and poor outcomes persist for the majority of patients. Thus, novel treatment strategies are clearly needed for achieving deeper and prolonged remissions and for avoiding the development of resistance. Due to its profound role in (cancer) stem cell biology and differentiation, the Hedgehog (HH)/Glioma-associated Oncogene Homolog (GLI) signaling pathway may be an attractive novel therapeutic target in AML. In this review, we aim to provide a critical and concise overview of the currently known potential and challenges of HH/GLI targeting. We describe the biological role of the HH/GLI pathway in AML pathophysiology. We specifically focus on ways of targeting non-canonical HH/GLI signaling in AML, particularly in combination with standard treatment regimens, which may overcome some hurdles observed with approved HH pathway inhibitors in solid tumors.

Reprogramming Medulloblastoma-Propagating Cells by a Combined Antagonism of Sonic Hedgehog and CXCR4

The CXCR4 chemokine and Sonic Hedgehog (SHH) morphogen pathways are well-validated therapeutic targets in cancer, including medulloblastoma. However, single-agent treatments with SHH or CXCR4 antagonists have not proven efficacious in clinical trials to date. Here, we discovered that dual inhibition of the SHH and CXCR4 pathways in a murine model of SHH-subtype medulloblastoma exerts potent antitumor effects. This therapeutic synergy resulted in the suppression of tumor-propagating cell function and correlated with increased histone H3 lysine 27 trimethylation within the promoters of stem cell genes, resulting in their decreased expression. These results demonstrate that CXCR4 contributes to the epigenetic regulation of a tumor-propagating cell phenotype. Moreover, they provide a mechanistic rationale to evaluate the combination of SHH and CXCR4 inhibitors in clinical trials for the treatment of medulloblastoma, as well as other cancers driven by SHH that coexpress high levels of CXCR4. Cancer Res; 77(6); 1416-26. ©2016 AACR.

Nek2A/SuFu feedback loop regulates Gli-mediated Hedgehog signaling pathway

Suppressor of Fused (SuFu), one of the most conserved components of the Hedgehog (Hh) signaling, binds Gli transcription factors and impedes activation of target gene expression in mammalian cells. Despite the central importance of SuFu in the Hh pathway, little is known about SuFu regulation. In a previous study, we identified NIMA-related expressed kinase 2A (Nek2A) as a SuFu-interacting protein. Here, we show that Nek2A stabilizes SuFu through impairing ubiquitin/proteasome degradation of SuFu. In addition, Nek2A negatively regulates target genes of Hh signaling as well as Gli2 transcriptional activity. In turn, inhibition of Hh signaling by GANT61 diminishes mRNA and protein levels of Nek2A, and Hh agonist promotes transcription of NEK2A gene. Chromatin immunoprecipitation assays revealed that Gli1 and Gli2 directly bind to the promoter regions of NEK2A gene and induced its transcription. Thus, we uncovered one of the mechanisms by which Nek2A acts as a modulator of the Hh signaling pathway in the context of a novel negative-feedback loop, which may offer new insights into Gli-mediated Hh signaling regulation in development and human diseases.

GLI1, a master regulator of the hallmark of pancreatic cancer

Hedgehog signaling is highly conserved across species and governs proper embryonic development. Germline gene mutations that reduce this signaling activity cause a variety of developmental abnormalities such as holoprosencephaly, while those that enhance Hedgehog signaling activity induce a tumor-predisposition condition Nevoid basal cell carcinoma syndrome. Furthermore, dysregulated activation of Hedgehog signaling has been recognized in various sporadic malignancies, including pancreatic adenocarcinoma. Pancreatic adenocarcinoma develops through a multistep carcinogenesis starting with oncogenic mutation of the KRAS gene. During this process, precancerous or cancer cells secrete Hedgehog ligand proteins to promote characteristic desmoplastic stroma around the cells, which in turn activates the expression of the downstream transcription factor GLI1 inside the cells. The quantitative and spatiotemporal dysregulation of GLI1 subsequently leads to the expression of transcriptional target genes of GLI1 that govern the hallmark of malignant properties. Here, after a brief introductory outline, a perspective is offered of Hedgehog signaling with a special focus on the role of GLI1 in pancreatic carcinogenesis.

A molecular dynamics approach to identify an oxysterol-based hedgehog pathway inhibitor

BACKGROUND

Multiple oxysterols (OHCs) have demonstrated the ability to act as agonists or antagonists of the hedgehog (Hh) signaling pathway, a developmental signaling pathway that has been implicated as a potential therapeutic target in a variety of human diseases. These OHCs are known to modulate Hh signaling through direct binding interactions with the N-terminal cysteine rich domain (CRD) of Smoothened, a key regulator of Hh signal transduction.

METHODS

Homology modeling, molecular dynamics simulations, and MM/GBSA energy calculations were utilized to explore binding interactions between the OHC scaffold and the human Smoothened CRD. Follow-up cellular assays explored the in vitro activity of potential Hh pathway modulators.

RESULTS

Structural features that govern key molecular interactions between the Smoothened CRD and the OHC scaffold were identified. Orientation of the iso-octyl side chain as well as the overall entropy of the OHC-CRD complex are important for determining activity against the Hh pathway. OHC 9, which was previously thought to be inactive because it was not an Hh agonist, was identified as an inhibitor of Hh signal transmission.

CONCLUSIONS

Calculated MM/GBSA binding energies for OHCs in complex with the CRD of Smoothened correlate well with in vitro Hh modulatory activity. Compounds with high affinity stabilize Smoothened and are antagonists, whereas compounds with reduced affinity allow a conformational change in Smoothened that results in pathway activation.

GENERAL SIGNIFICANCE

Computational modeling and molecular dynamics simulations can be used to predict whether a small molecule that binds the Smoothened CRD will be an agonist or antagonist of the pathway.

Design, Synthesis, and Pharmacological Evaluation of 2-(2,5-Dimethyl-5,6,7,8-tetrahydroquinolin-8-yl)-N-aryl Propanamides as Novel Smoothened (Smo) Antagonists

A series of novel Smo antagonists were developed either by directly incorporating the basic skeleton of the natural product artemisinin or by first breaking artemisinin into structurally simpler and stable intermediates and then reconstructing into diversified heterocyclic derivatives, equipped with a Smo-targeting bullet. 2-(2,5-Dimethyl-5,6,7,8-tetrahydroquinolin-8-yl)-N-arylpropanamide 65 was identified as the most potent, with an IC₅₀ value of 9.53 nM against the Hh signaling pathway. Complementary mechanism studies confirmed that 65 inhibits Hh signaling pathway by targeting Smo and shares the same binding site as that of the tool drug cyclopamine. Meanwhile, 65 has a good plasma exposure and an acceptable oral bioavailability. Dose-dependent antiproliferative effects were observed in ptch+/-;p53-/- medulloblastoma cells, and significant tumor growth inhibitions were achieved for 65 in the ptch+/-;p53-/- medulloblastoma allograft model.

Metastatic basal cell carcinoma with amplification of PD-L1: exceptional response to anti-PD1 therapy

Metastatic basal cell carcinomas are rare malignancies harbouring Hedgehog pathway alterations targetable by SMO antagonists (vismodegib/sonidegib). We describe, for the first time, the molecular genetics and response of a patient with Hedgehog inhibitor-resistant metastatic basal cell carcinoma who achieved rapid tumour regression (ongoing near complete remission at 4 months) with nivolumab (anti-PD1 antibody). He had multiple hallmarks of anti-PD1 responsiveness including high mutational burden (>50 mutations per megabase; 19 functional alterations in tissue next-generation sequencing (NGS; 315 genes)) as well as PDL1/PDL2/JAK2 amplification (as determined by both tissue NGS and by analysis of plasma-derived cell-free DNA). The latter was performed using technology originally developed for the genome-wide detection of sub-chromosomal copy-number alterations (CNAs) in noninvasive prenatal testing and showed numerous CNAs including amplification of the 9p24.3-9p22.2 region containing PD-L1, PD-L2 and JAK2. Of interest, PD-L1, PD-L2 and JAK2 amplification is a characteristic of Hodgkin lymphoma, which is exquisitely sensitive to nivolumab. In conclusion, selected SMO antagonist-resistant metastatic basal cell carcinomas may respond to nivolumab based on underlying molecular genetic mechanisms that include PD-L1 amplification and high tumour mutational burden.

Solasonine, A Natural Glycoalkaloid Compound, Inhibits Gli-Mediated Transcriptional Activity

The major obstacle limiting the efficacy of current Smoothened (Smo) inhibitors is the primary and acquired resistance mainly caused by Smo mutations and Gli amplification. In this context, developing Hh inhibitors targeting Gli, the final effector of this signaling pathway, may combat the resistance. In this study we found that solasonine, a natural glycoalkaloid compound, significantly inhibited the hedgehog (Hh) pathway activity. Meanwhile, solasonine may obviously inhibit the alkaline phosphatase (ALP) activity in C3H10T1/2 cells, concomitantly with reductions of the mRNA expression of Gli1 and Ptch1. However, we found that solasonine exhibited no effect on the transcriptional factors activities provoked by TNF-α and PGE2, thus suggesting its selectivity against Hh pathway activity. Furthermore, we identified that solasonine inhibited the Hh pathway activity by acting on its transcriptional factor Gli using a series of complementary data. We also observed that solasonine obviously inhibited the Gli-luciferase activity provoked by ectopic expression of Smo mutants which may cause the resistance to the current Smo inhibitors. Our study suggests that solasonine may significantly inhibit the Hh pathway activity by acting on Gli, therefore indicating the possibility to use solasonine as a lead compound to develop anticancer drugs for combating the resistance of current Smo inhibitors.

Inhibition of Hedgehog-dependent tumors and cancer stem cells by a newly identified naturally occurring chemotype

Hedgehog (Hh) inhibitors have emerged as valid tools in the treatment of a wide range of cancers. Indeed, aberrant activation of the Hh pathway occurring either by ligand-dependent or -independent mechanisms is a key driver in tumorigenesis. The smoothened (Smo) receptor is one of the main upstream transducers of the Hh signaling and is a validated target for the development of anticancer compounds, as underlined by the FDA-approved Smo antagonist Vismodegib (GDC-0449/Erivedge) for the treatment of basal cell carcinoma. However, Smo mutations that confer constitutive activity and drug resistance have emerged during treatment with Vismodegib. For this reason, the development of new effective Hh inhibitors represents a major challenge for cancer therapy. Natural products have always represented a unique source of lead structures in drug discovery, and in recent years have been used to modulate the Hh pathway at multiple levels. Here, starting from an in house library of natural compounds and their derivatives, we discovered novel chemotypes of Hh inhibitors by mean of virtual screening against the crystallographic structure of Smo. Hh functional based assay identified the chalcone derivative 12 as the most effective Hh inhibitor within the test set. The chalcone 12 binds the Smo receptor and promotes the displacement of Bodipy-Cyclopamine in both Smo WT and drug-resistant Smo mutant. Our molecule stands as a promising Smo antagonist able to specifically impair the growth of Hh-dependent tumor cells in vitro and in vivo and medulloblastoma stem-like cells and potentially overcome the associated drug resistance.

Effects of mutations in Wnt/β-catenin, hedgehog, Notch and PI3K pathways on GSK-3 activity-Diverse effects on cell growth, metabolism and cancer

Glycogen synthase kinase-3 (GSK-3) is a serine/threonine kinase that participates in an array of critical cellular processes. GSK-3 was first characterized as an enzyme that phosphorylated and inactivated glycogen synthase. However, subsequent studies have revealed that this moon-lighting protein is involved in numerous signaling pathways that regulate not only metabolism but also have roles in: apoptosis, cell cycle progression, cell renewal, differentiation, embryogenesis, migration, regulation of gene transcription, stem cell biology and survival. In this review, we will discuss the roles that GSK-3 plays in various diseases as well as how this pivotal kinase interacts with multiple signaling pathways such as: PI3K/PTEN/Akt/mTOR, Ras/Raf/MEK/ERK, Wnt/beta-catenin, hedgehog, Notch and TP53. Mutations that occur in these and other pathways can alter the effects that natural GSK-3 activity has on regulating these signaling circuits that can lead to cancer as well as other diseases. The novel roles that microRNAs play in regulation of the effects of GSK-3 will also be evaluated. Targeting GSK-3 and these other pathways may improve therapy and overcome therapeutic resistance.

Sonidegib for the treatment of advanced basal cell carcinoma: a comprehensive review of sonidegib and the BOLT trial with 12-month update

The Hedgehog inhibitors are promising alternative for patients with advanced basal cell carcinoma that are not amenable to radiotherapy or surgery. Sonidegib, also known as LDE225, is an orally available SMO antagonist that was recently approved by the US FDA for the treatment of patients with locally advanced basal cell carcinoma. This article will provide an overview of the pharmacology and pharmacokinetics of sonidegib and in-depth analysis of the BOLT trial with additional data from the 12-month update. The present challenges associated with Hedgehog inhibitors will also be discussed.

Structural basis of Smoothened regulation by its extracellular domains

Developmental signals of the Hedgehog (Hh) and Wnt families are transduced across the membrane by Frizzledclass G-protein-coupled receptors (GPCRs) composed of both a heptahelical transmembrane domain (TMD) and an extracellular cysteine-rich domain (CRD). How the large extracellular domains of GPCRs regulate signalling by the TMD is unknown. We present crystal structures of the Hh signal transducer and oncoprotein Smoothened, a GPCR that contains two distinct ligand-binding sites: one in its TMD and one in the CRD. The CRD is stacked a top the TMD, separated by an intervening wedge-like linker domain. Structure-guided mutations show that the interface between the CRD, linker domain and TMD stabilizes the inactive state of Smoothened. Unexpectedly, we find a cholesterol molecule bound to Smoothened in the CRD binding site. Mutations predicted to prevent cholesterol binding impair the ability of Smoothened to transmit native Hh signals. Binding of a clinically used antagonist, vismodegib, to the TMD induces a conformational change that is propagated to the CRD, resulting in loss of cholesterol from the CRD-linker domain-TMD interface. Our results clarify the structural mechanism by which the activity of a GPCR is controlled by ligand-regulated interactions between its extracellular and transmembrane domains.

Resistance of Nonmelanoma Skin Cancer to Nonsurgical Treatments. Part II: Photodynamic Therapy, Vismodegib, Cetuximab, Intralesional Methotrexate, and Radiotherapy

A wide range of treatments is now available for nonmelanoma skin cancer, including 5-fluorouracil, ingenol mebutate, imiquimod, diclofenac, photodynamic therapy, methotrexate, cetuximab, vismodegib, and radiotherapy. All are associated with high clinical and histologic response rates. However, some tumors do not respond due to resistance, which may be primary or acquired. Study of the resistance processes is a broad area of research that aims to increase our understanding of the nature of each tumor and the biologic features that make it resistant, as well as to facilitate the design of new therapies directed against these tumors. In this second article, having covered the topical treatments of nonmelanoma skin cancer, we review resistance to other nonsurgical treatments, such as monoclonal antibodies against basal and squamous cell carcinomas, intralesional chemotherapy, photodynamic therapy, and radiotherapy.

AKT1 Activation is Obligatory for Spontaneous BCC Tumor Growth in a Murine Model that Mimics Some Features of Basal Cell Nevus Syndrome

Patients with basal cell nevus syndrome (BCNS), also known as Gorlin syndrome, develop numerous basal cell carcinomas (BCC) due to germline mutations in the tumor suppressor PTCH1 and aberrant activation of Hedgehog (Hh) signaling. Therapies targeted at components of the Hh pathway, including the smoothened (SMO) inhibitor vismodegib, can ablate these tumors clinically, but tumors recur upon drug discontinuation. Using SKH1-Ptch1^+/- as a model that closely mimics the spontaneous and accelerated growth pattern of BCCs in patients with BCNS, we show that AKT1, a serine/threonine protein kinase, is intrinsically activated in keratinocytes derived from the skin of newborn Ptch1^+/- mice in the absence of carcinogenic stimuli. Introducing Akt1 haplodeficiency in Ptch1^+/- mice (Akt1^+/- Ptch1^+/-) significantly abrogated BCC growth. Similarly, pharmacological inhibition of AKT with perifosine, an alkyl phospholipid AKT inhibitor, diminished the growth of spontaneous and UV-induced BCCs. Our data demonstrate an obligatory role for AKT1 in BCC growth, and targeting AKT may help reduce BCC tumor burden in BCNS patients. Cancer Prev Res; 9(10); 794-802. ©2016 AACR.

SCF (Fbxl17) ubiquitylation of Sufu regulates Hedgehog signaling and medulloblastoma development

Skp1-Cul1-F-box protein (SCF) ubiquitin ligases direct cell survival decisions by controlling protein ubiquitylation and degradation. Sufu (Suppressor of fused) is a central regulator of Hh (Hedgehog) signaling and acts as a tumor suppressor by maintaining the Gli (Glioma-associated oncogene homolog) transcription factors inactive. Although Sufu has a pivotal role in Hh signaling, the players involved in controlling Sufu levels and their role in tumor growth are unknown. Here, we show that Fbxl17 (F-box and leucine-rich repeat protein 17) targets Sufu for proteolysis in the nucleus. The ubiquitylation of Sufu, mediated by Fbxl17, allows the release of Gli1 from Sufu for proper Hh signal transduction. Depletion of Fbxl17 leads to defective Hh signaling associated with an impaired cancer cell proliferation and medulloblastoma tumor growth. Furthermore, we identify a mutation in Sufu, occurring in medulloblastoma of patients with Gorlin syndrome, which increases Sufu turnover through Fbxl17-mediated polyubiquitylation and leads to a sustained Hh signaling activation. In summary, our findings reveal Fbxl17 as a novel regulator of Hh pathway and highlight the perturbation of the Fbxl17-Sufu axis in the pathogenesis of medulloblastoma.

Hedgehog pathway inhibition in advanced basal cell carcinoma: latest evidence and clinical usefulness

Treatment of locally advanced basal cell carcinomas (laBCCs) with large, aggressive, destructive, and disfiguring tumors, or metastatic disease is challenging. Dysregulation of the Hedgehog (Hh) signaling pathway has been identified in the vast majority of basal cell carcinomas (BCCs). There are two United States Food and Drug Administration (US FDA)-approved Hh pathway inhibitors (HPIs) that exhibit antitumor activity in advanced BCC with an acceptable safety profile. Common adverse effects include muscle spasms, dysgeusia, alopecia, fatigue, nausea and weight loss.

Synthetic Small Molecule Inhibitors of Hh Signaling As Anti-Cancer Chemotherapeutics

The hedgehog (Hh) pathway is a developmental signaling pathway that is essential to the proper embryonic development of many vertebrate systems. Dysregulation of Hh signaling has been implicated as a causative factor in the development and progression of several forms of human cancer. As such, the development of small molecule inhibitors of Hh signaling as potential anti-cancer chemotherapeutics has been a major area of research interest in both academics and industry over the past ten years. Through these efforts, synthetic small molecules that target multiple components of the Hh pathway have been identified and advanced to preclinical or clinical development. The goal of this review is to provide an update on the current status of several synthetic small molecule Hh pathway inhibitors and explore the potential of several recently disclosed inhibitory scaffolds.

A Recurrent Mosaic Mutation in SMO, Encoding the Hedgehog Signal Transducer Smoothened, Is the Major Cause of Curry-Jones Syndrome

Curry-Jones syndrome (CJS) is a multisystem disorder characterized by patchy skin lesions, polysyndactyly, diverse cerebral malformations, unicoronal craniosynostosis, iris colobomas, microphthalmia, and intestinal malrotation with myofibromas or hamartomas. Cerebellar medulloblastoma has been described in a single affected individual; in another, biopsy of skin lesions showed features of trichoblastoma. The combination of asymmetric clinical features, patchy skin manifestations, and neoplastic association previously led to the suggestion that this could be a mosaic condition, possibly involving hedgehog (Hh) signaling. Here, we show that CJS is caused by recurrent somatic mosaicism for a nonsynonymous variant in SMO (c.1234C>T [p.Leu412Phe]), encoding smoothened (SMO), a G-protein-coupled receptor that transduces Hh signaling. We identified eight mutation-positive individuals (two of whom had not been reported previously) with highly similar phenotypes and demonstrated varying amounts of the mutant allele in different tissues. We present detailed findings from brain MRI in three mutation-positive individuals. Somatic SMO mutations that result in constitutive activation have been described in several tumors, including medulloblastoma, ameloblastoma, and basal cell carcinoma. Strikingly, the most common of these mutations is the identical nonsynonymous variant encoding p.Leu412Phe. Furthermore, this substitution has been shown to activate SMO in the absence of Hh signaling, providing an explanation for tumor development in CJS. This raises therapeutic possibilities for using recently generated Hh-pathway inhibitors. In summary, our work uncovers the major genetic cause of CJS and illustrates strategies for gene discovery in the context of low-level tissue-specific somatic mosaicism.

Development of thieno- and benzopyrimidinone inhibitors of the Hedgehog signaling pathway reveals PDE4-dependent and PDE4-independent mechanisms of action

From a high content in vivo screen for modulators of developmental patterning in embryonic zebrafish, we previously identified eggmanone (EGM1, 3) as a Hedgehog (Hh) signaling inhibitor functioning downstream of Smoothened. Phenotypic optimization studies for in vitro probe development utilizing a Gli transcription-linked stable luciferase reporter cell line identified EGM1 analogs with improved potency and aqueous solubility. Mechanistic profiling of optimized analogs indicated two distinct scaffold clusters: PDE4 inhibitors able to inhibit downstream of Sufu, and PDE4-independent Hh inhibitors functioning between Smo and Sufu. Each class represents valuable in vitro probes for elucidating the complex mechanisms of Hh regulation.

Repurposing the Clinically Efficacious Antifungal Agent Itraconazole as an Anticancer Chemotherapeutic

Itraconazole (ITZ) is an FDA-approved member of the triazole class of antifungal agents. Two recent drug repurposing screens identified ITZ as a promising anticancer chemotherapeutic that inhibits both the angiogenesis and hedgehog (Hh) signaling pathways. We have synthesized and evaluated first- and second-generation ITZ analogues for their anti-Hh and antiangiogenic activities to probe more fully the structural requirements for these anticancer properties. Our overall results suggest that the triazole functionality is required for ITZ-mediated inhibition of angiogenesis but that it is not essential for inhibition of Hh signaling. The synthesis and evaluation of stereochemically defined des-triazole ITZ analogues also provides key information as to the optimal configuration around the dioxolane ring of the ITZ scaffold. Finally, the results from our studies suggest that two distinct cellular mechanisms of action govern the anticancer properties of the ITZ scaffold.

Genomic analysis identifies new drivers and progression pathways in skin basal cell carcinoma

Basal cell carcinoma (BCC) of the skin is the most common malignant neoplasm in humans. BCC is primarily driven by the Sonic Hedgehog (Hh) pathway. However, its phenotypic variation remains unexplained. Our genetic profiling of 293 BCCs found the highest mutation rate in cancer (65 mutations/Mb). Eighty-five percent of the BCCs harbored mutations in Hh pathway genes (PTCH1, 73% or SMO, 20% (P = 6.6 × 10(-8)) and SUFU, 8%) and in TP53 (61%). However, 85% of the BCCs also harbored additional driver mutations in other cancer-related genes. We observed recurrent mutations in MYCN (30%), PPP6C (15%), STK19 (10%), LATS1 (8%), ERBB2 (4%), PIK3CA (2%), and NRAS, KRAS or HRAS (2%), and loss-of-function and deleterious missense mutations were present in PTPN14 (23%), RB1 (8%) and FBXW7 (5%). Consistent with the mutational profiles, N-Myc and Hippo-YAP pathway target genes were upregulated. Functional analysis of the mutations in MYCN, PTPN14 and LATS1 suggested their potential relevance in BCC tumorigenesis.

Evaluation of WO2014207069 A1: Multitarget Hedgehog pathway inhibitors and uses thereof

In recent years, the involvement of the Hedgehog (Hh) signaling pathway in various human diseases and dysfunctions has been clearly demonstrated. Smoothened (Smo), one of the upstream signal transducers, has been the most druggable target of the Hh pathway. However, the emergence of resistance to Smo inhibitors and the identification of Smo-independent activation of the Hh pathway led to the need to find new chemical entities able to interfere with downstream components, such as Gli. For this purpose, two different computational approaches have been applied to a small-sized library of natural compounds. As a result, an isoflavone derivative that showed ability to inhibit both Smo and Gli1 has been identified; namely, Glabrescione B. A new synthetic approach has been planned for this compound and its derivatives. Biological evaluation demonstrated the mechanism of action and showed a promising preclinical profile.

Targeting the Sonic Hedgehog Signaling Pathway: Review of Smoothened and GLI Inhibitors

The sonic hedgehog (Shh) signaling pathway is a major regulator of cell differentiation, cell proliferation, and tissue polarity. Aberrant activation of the Shh pathway has been shown in a variety of human cancers, including, basal cell carcinoma, malignant gliomas, medulloblastoma, leukemias, and cancers of the breast, lung, pancreas, and prostate. Tumorigenesis, tumor progression and therapeutic response have all been shown to be impacted by the Shh signaling pathway. Downstream effectors of the Shh pathway include smoothened (SMO) and glioma-associated oncogene homolog (GLI) family of zinc finger transcription factors. Both are regarded as important targets for cancer therapeutics. While most efforts have been devoted towards pharmacologically targeting SMO, developing GLI-targeted approach has its merit because of the fact that GLI proteins can be activated by both Shh ligand-dependent and -independent mechanisms. To date, two SMO inhibitors (LDE225/Sonidegib and GDC-0449/Vismodegib) have received FDA approval for treating basal cell carcinoma while many clinical trials are being conducted to evaluate the efficacy of this exciting class of targeted therapy in a variety of cancers. In this review, we provide an overview of the biology of the Shh pathway and then detail the current landscape of the Shh-SMO-GLI pathway inhibitors including those in preclinical studies and clinical trials.

DYRK1B as therapeutic target in Hedgehog/GLI-dependent cancer cells with Smoothened inhibitor resistance

A wide range of human malignancies displays aberrant activation of Hedgehog (HH)/GLI signaling, including cancers of the skin, brain, gastrointestinal tract and hematopoietic system. Targeting oncogenic HH/GLI signaling with small molecule inhibitors of the essential pathway effector Smoothened (SMO) has shown remarkable therapeutic effects in patients with advanced and metastatic basal cell carcinoma. However, acquired and de novo resistance to SMO inhibitors poses severe limitations to the use of SMO antagonists and urgently calls for the identification of novel targets and compounds.Here we report on the identification of the Dual-Specificity-Tyrosine-Phosphorylation-Regulated Kinase 1B (DYRK1B) as critical positive regulator of HH/GLI signaling downstream of SMO. Genetic and chemical inhibition of DYRK1B in human and mouse cancer cells resulted in marked repression of HH signaling and GLI1 expression, respectively. Importantly, DYRK1B inhibition profoundly impaired GLI1 expression in both SMO-inhibitor sensitive and resistant settings. We further introduce a novel small molecule DYRK1B inhibitor, DYRKi, with suitable pharmacologic properties to impair SMO-dependent and SMO-independent oncogenic GLI activity. The results support the use of DYRK1B antagonists for the treatment of HH/GLI-associated cancers where SMO inhibitors fail to demonstrate therapeutic efficacy.

Smoothened regulation in response to Hedgehog stimulation

The Hedgehog (Hh) signaling pathway play critical roles in embryonic development and adult tissue homeostasis. A critical step in Hh signal transduction is how Hh receptor Patched (Ptc) inhibits the atypical G protein-coupled receptor Smoothened (Smo) in the absence of Hh and how this inhibition is release by Hh stimulation. It is unlikely that Ptc inhibits Smo by direct interaction. Here we discuss how Hh regulates the phosphorylation and ubiquitination of Smo, leading to cell surface and ciliary accumulation of Smo in Drosophila and vertebrate cells, respectively. In addition, we discuss how PI(4)P phospholipid acts in between Ptc and Smo to regulate Smo phosphorylation and activation in response to Hh stimulation.

Posaconazole, a Second-Generation Triazole Antifungal Drug, Inhibits the Hedgehog Signaling Pathway and Progression of Basal Cell Carcinoma

Deregulation of Hedgehog (Hh) pathway signaling has been associated with the pathogenesis of various malignancies, including basal cell carcinomas (BCC). Inhibitors of the Hh pathway currently available or under clinical investigation all bind and antagonize Smoothened (SMO), inducing a marked but transient clinical response. Tumor regrowth and therapy failure were attributed to mutations in the binding site of these small-molecule SMO antagonists. The antifungal itraconazole was demonstrated to be a potent SMO antagonist with a distinct mechanism of action from that of current SMO inhibitors. However, itraconazole represents a suboptimal therapeutic option due to its numerous drug-drug interactions. Here, we show that posaconazole, a second-generation triazole antifungal with minimal drug-drug interactions and a favorable side-effect profile, is also a potent inhibitor of the Hh pathway that functions at the level of SMO. We demonstrate that posaconazole inhibits the Hh pathway by a mechanism distinct from that of cyclopamine and other cyclopamine-competitive SMO antagonists but, similar to itraconazole, has robust activity against drug-resistant SMO mutants and inhibits the growth of Hh-dependent BCC in vivo Our results suggest that posaconazole, alone or in combination with other Hh pathway antagonists, may be readily tested in clinical studies for the treatment of Hh-dependent cancers. Mol Cancer Ther; 15(5); 866-76. ©2016 AACR.

Recent developments in brain tumor predisposing syndromes

The etiologies of brain tumors are in the most cases unknown, but improvements in genetics and DNA screening have helped to identify a wide range of brain tumor predisposition disorders. In this review we are discussing some of the most common predisposition disorders, namely: neurofibromatosis type 1 and 2, schwannomatosis, rhabdoid tumor predisposition disorder, nevoid basal cell carcinoma syndrome (Gorlin), tuberous sclerosis complex, von Hippel-Lindau, Li-Fraumeni and Turcot syndromes. Recent findings from the GLIOGENE collaboration and the newly identified glioma causing gene POT1, will also be discussed. Genetics. We will describe these disorders from a genetic and clinical standpoint, focusing on the difference in clinical symptoms depending on the underlying gene or germline mutation. Central nervous system (CNS) tumors. Most of these disorders predispose the carriers to a wide range of symptoms. Herein, we will focus particularly on tumors affecting the CNS and discuss improvements of targeted therapy for the particular disorders.

An Investigator-Initiated Open-Label Trial of Sonidegib in Advanced Basal Cell Carcinoma Patients Resistant to Vismodegib

PURPOSE

To assess the tumor response to the smoothened (SMO) inhibitor, sonidegib (LDE225), in patients with an advanced basal cell carcinoma (BCC) resistant to treatment with vismodegib (GDC0449).

EXPERIMENTAL DESIGN

Nine patients with an advanced BCC that was previously resistant to treatment with vismodegib were given sonidegib in this investigational, open-label study. Tumor response was determined using the response evaluation criteria in solid tumors. SMO mutations were identified using biopsy samples from the target BCC location.

RESULTS

The median duration of treatment with sonidegib was 6 weeks (range, 3-58 weeks). Five patients experienced progressive disease with sonidegib. Three patients experienced stable disease and discontinued sonidegib either due to adverse events (n = 1) or due to election for surgery (n = 2). The response of one patient was not evaluable. SMO mutations with in vitro data suggesting resistance to Hh pathway inhibition were identified in 5 patients, and none of these patients experienced responses while on sonidegib.

CONCLUSIONS

Patients with advanced BCCs that were previously resistant to treatment with vismodegib similarly demonstrated treatment resistance with sonidegib. Patients who have developed treatment resistance to an SMO inhibitor may continue to experience tumor progression in response to other SMO inhibitors.

Advanced basal cell carcinoma, the hedgehog pathway, and treatment options - role of smoothened inhibitors

Cutaneous basal cell carcinoma (BCC) is the most common human cancer and its incidence is rising worldwide. Ultraviolet radiation exposure, including tanning bed use, as well as host factors play a role in its development. The majority of cases are treated and cured with local therapies including surgery. Yet, the health care costs of diagnosis and treatment of BCCs in the US is substantial. In the United States, the cost of nonmelanoma skin cancer care in the Medicare population is estimated to be US$426 million per year. While rare, locally advanced BCCs that can no longer be controlled with surgery and/or radiation, and metastatic BCCs do occur and can be associated with significant morbidity and mortality. Vismodegib (GDC-0449), a smoothened inhibitor targeted at the hedgehog pathway, is the first US Food and Drug Association (FDA)-approved agent in the treatment of locally advanced, unresectable, and metastatic BCCs. This class of agents appears to be changing the survival rates in advanced BCC patients, but appropriate patient selection and monitoring are important. Multidisciplinary assessments are essential for the optimal care and management of these patients. For some patients with locally advanced BCC, treatment with a hedgehog inhibitor may eliminate the need for an excessively disfiguring or morbid surgery.

Targeting the Hedgehog Pathway in Pediatric Medulloblastoma

Medulloblastoma (MB), a primitive neuroectomal tumor of the cerebellum, is the most common malignant pediatric brain tumor. The cause of MB is largely unknown, but aberrant activation of Hedgehog (Hh) pathway is responsible for ~30% of MB. Despite aggressive treatment with surgical resection, radiation and chemotherapy, 70%–80% of pediatric medulloblastoma cases can be controlled, but most treated patients suffer devastating side effects. Therefore, developing a new effective treatment strategy is urgently needed. Hh signaling controls transcription of target genes by regulating activities of the three Glioma-associated oncogene (Gli1-3) transcription factors. In this review, we will focus on current clinical treatment options of MB and discuss mechanisms of drug resistance. In addition, we will describe current known molecular pathways which crosstalk with the Hedgehog pathway both in the context of medulloblastoma and non-medulloblastoma cancer development. Finally, we will introduce post-translational modifications that modulate Gli1 activity and summarize the positive and negative regulations of the Hh/Gli1 pathway. Towards developing novel combination therapies for medulloblastoma treatment, current information on interacting pathways and direct regulation of Hh signaling should prove critical.

A Group of ent-Kaurane Diterpenoids Inhibit Hedgehog Signaling and Induce Cilia Elongation

The Hedgehog (Hh) signaling pathway plays important roles in the tumorigenesis of multiple cancers and is a key target for drug discovery. In a screen of natural products extracted from Chinese herbs, we identified eight ent-Kaurane diterpenoids and two triterpene dilactones as novel Hh pathway antagonists. Epistatic analyses suggest that these compounds likely act at the level or downstream of Smoothened (Smo) and upstream of Suppressor of Fused (Sufu). The ent-Kauranoid-treated cells showed elongated cilia, suppressed Smo trafficking to cilia, and mitotic defects, while the triterpene dilactones had no effect on the cilia and ciliary Smo. These ent-Kaurane diterpenoids provide new prototypes of Hh inhibitors, and are valuable probes for deciphering the mechanisms of Smo ciliary transport and ciliogenesis.

Smoothened Regulation: A Tale of Two Signals

The G protein-coupled receptor (GPCR) Smoothened (Smo) is the signal transducer of the developmentally and therapeutically relevant Hedgehog (Hh) pathway. Although recent structural analyses have advanced our understanding of Smo biology, several questions remain. Chief among them are the identity of its natural ligand, the regulatory processes controlling its activation, and the mechanisms by which it signals to downstream effectors. In this review, we discuss recent discoveries from multiple model systems that have set the stage for solving these mysteries. We focus on the roles of distinct Smo functional domains, post-translational modifications, and trafficking, and conclude by discussing their contributions to signal output.

Digging a hole under Hedgehog: downstream inhibition as an emerging anticancer strategy

Hedgehog signaling is a key regulator of development and stem cell fate and its aberrant activation is a leading cause of a number of tumors. Activating germline or somatic mutations of genes encoding Hh pathway components are found in Basal Cell Carcinoma (BCC) and Medulloblastoma (MB). Ligand-dependent Hedgehog hyperactivation, due to autocrine or paracrine mechanisms, is also observed in a large number of malignancies of the breast, colon, skin, bladder, pancreas and other tissues. The key tumorigenic role of Hedgehog has prompted effort aimed at identifying inhibitors of this signaling. To date, only the antagonists of the membrane transducer Smo have been approved for therapy or are under clinical trials in patients with BCC and MB linked to Ptch or Smo mutations. Despite the good initial response, patients treated with Smo antagonists have eventually developed resistance due to the occurrence of compensating mechanisms. Furthermore, Smo antagonists are not effective in tumors where the Hedgehog hyperactivation is due to mutations of pathway components downstream of Smo, or in case of non-canonical, Smo-independent activation of the Gli transcription factors. For all these reasons, the research of Hh inhibitors acting downstream of Smo is becoming an area of intensive investigation. In this review we illustrate the progresses made in the identification of effective Hedgehog inhibitors and their application in cancer, with a special emphasis on the newly identified downstream inhibitors. We describe in detail the Gli inhibitors and illustrate their mode of action and applications in experimental and/or clinical settings.

Biochemical characterization of smoothened receptor antagonists by binding kinetics against drug-resistant mutant

Hedgehog (Hh) signaling critical for development, differentiation, and cell growth is involved in several cancers, including medulloblastoma and basal cell carcinoma. Although antagonism of the smoothened receptor (SMO), which mediates Hh signaling, is an attractive therapeutic target, a drug-resistant mutation in SMO (SMO-D473H) was identified in a clinical trial of the approved drug vismodegib. TAK-441 potently inhibits SMO-D473H, unlike vismodegib and another SMO antagonist, cyclopamine, whereas the differences in binding modes between these antagonists remain unknown. Here we report the biochemical characterization of TAK-441, vismodegib, and cyclopamine by binding kinetics. The association (kon) and dissociation (koff) rates were determined by kinetic binding studies using [(3)H]TAK-441, and dissociation was confirmed by label-free affinity selection-mass spectrometry (AS-MS). In the [(3)H]TAK-441 competition assay, TAK-441 but not vismodegib and cyclopamine showed time-dependent inhibition. Quantitative kinetic binding analysis revealed that koff of TAK-441 was >10-fold smaller than those of vismodegib and cyclopamine. To further assess the binding mode of antagonists, kinetic binding analysis was performed against SMO-D473H. The D473H mutation affected koff of TAK-441 but not kon. In contrast, only kon was changed by the D473H mutation in the case of vismodegib and cyclopamine. These results suggest that the difference in antagonist efficacy against D473H is associated with the binding mode of antagonists. These findings provide a new insight into the drug action of SMO antagonists and help develop potential therapeutics for drug-resistant mutants.

Vismodegib resistance in basal cell carcinoma: not a smooth fit

In this issue of Cancer Cell, two complementary papers by Atwood and colleagues and Sharpe and colleagues show that basal cell carcinomas resistant to the Smoothened (SMO) inhibitor vismodegib frequently harbor SMO mutations that limit drug binding, with mutations at some sites also increasing basal SMO activity.