Josephine Sheedy

BMI1 as a novel target for drug discovery in cancer

Growing evidence has demonstrated that clonogenic cancer stem (initiating) cells are responsible for tumor regrowth and disease relapse. Bmi‐1 plays a critical role in the self‐renewal of adult stem cells. The Bmi‐1 protein is elevated in many types of cancers, and experimental reduction of Bmi‐1 protein levels by small interfering RNA (siRNA) causes apoptosis and/or senescence in tumor cells in vitro and increases susceptibility to cytotoxic agents. The Bmi‐1 protein has no known enzymatic activity, but serves as the key regulatory component of the PRC1 complex (polycomb repressive complex‐1). This complex influences chromatin structure and regulates transcriptional activity of a number of important loci including the Ink4a locus which encodes the tumor suppressor proteins p16Ink4a and p14Arf. In this prospective study, we will discuss the implication of BMI1 in cancers, the biology of BMI1, and the regulatory control of BMI1 expression. The target validation and the future prospects of targeting BMI1 in cancer therapy are also discussed. J. Cell. Biochem. 112: 2729–2741, 2011.

Structure–Activity Relationship (SAR) Optimization of 6-(Indol-2-yl)pyridine-3-sulfonamides: Identification of Potent, Selective, and Orally Bioavailable Small Molecules Targeting Hepatitis C (HCV) NS4B

A novel, potent, and orally bioavailable inhibitor of hepatitis C RNA replication targeting NS4B, compound 4t (PTC725), has been identified through chemical optimization of the 6-(indol-2-yl)pyridine-3-sulfonamide 2 to improve DMPK and safety properties. The focus of the SAR investigations has been to identify the optimal combination of substituents at the indole N-1, C-5, and C-6 positions and the sulfonamide group to limit the potential for in vivo oxidative metabolism and to achieve an acceptable pharmacokinetic profile. Compound 4t has excellent potency against the HCV 1b replicon, with an EC50 = 2 nM and a selectivity index of >5000 with respect to cellular GAPDH. Compound 4t has an overall favorable pharmacokinetic profile with oral bioavailability values of 62%, 78%, and 18% in rats, dogs, and monkeys, respectively, as well as favorable tissue distribution properties with a liver to plasma exposure ratio of 25 in rats.

Discovery and Optimization of Small Molecule Splicing Modifiers of Survival Motor Neuron 2 as a Treatment for Spinal Muscular Atrophy

The underlying cause of spinal muscular atrophy (SMA) is a deficiency of the survival motor neuron (SMN) protein. Starting from hits identified in a high-throughput screening campaign and through structure-activity relationship investigations, we have developed small molecules that potently shift the alternative splicing of the SMN2 exon 7, resulting in increased production of the full-length SMN mRNA and protein. Three novel chemical series, represented by compounds 9, 14, and 20, have been optimized to increase the level of SMN protein by >50% in SMA patient-derived fibroblasts at concentrations of <160 nM. Daily administration of these compounds to severe SMA Δ7 mice results in an increased production of SMN protein in disease-relevant tissues and a significant increase in median survival time in a dose-dependent manner. Our work supports the development of an orally administered small molecule for the treatment of patients with SMA.

Pharmacokinetics, pharmacodynamics, and efficacy of a small-molecule SMN2 splicing modifier in mouse models of spinal muscular atrophy

Spinal muscular atrophy (SMA) is caused by the loss or mutation of both copies of the survival motor neuron 1 (SMN1) gene. The related SMN2 gene is retained, but due to alternative splicing of exon 7, produces insufficient levels of the SMN protein. Here, we systematically characterize the pharmacokinetic and pharmacodynamics properties of the SMN splicing modifier SMN-C1. SMN-C1 is a low-molecular weight compound that promotes the inclusion of exon 7 and increases production of SMN protein in human cells and in two transgenic mouse models of SMA. Furthermore, increases in SMN protein levels in peripheral blood mononuclear cells and skin correlate with those in the central nervous system (CNS), indicating that a change of these levels in blood or skin can be used as a non-invasive surrogate to monitor increases of SMN protein levels in the CNS. Consistent with restored SMN function, SMN-C1 treatment increases the levels of spliceosomal and U7 small-nuclear RNAs and corrects RNA processing defects induced by SMN deficiency in the spinal cord of SMNΔ7 SMA mice. A 100% or greater increase in SMN protein in the CNS of SMNΔ7 SMA mice robustly improves the phenotype. Importantly, a ∼50% increase in SMN leads to long-term survival, but the SMA phenotype is only partially corrected, indicating that certain SMA disease manifestations may respond to treatment at lower doses. Overall, we provide important insights for the translation of pre-clinical data to the clinic and further therapeutic development of this series of molecules for SMA treatment.

The nucleoside analog clitocine is a potent and efficacious readthrough agent

Nonsense mutations resulting in a premature stop codon in an open reading frame occur in critical tumor suppressor genes in a large number of the most common forms of cancers and are known to cause or contribute to the progression of disease. Low molecular weight compounds that induce readthrough of nonsense mutations offer a new means of treating patients with genetic disorders or cancers resulting from nonsense mutations. We have identified the nucleoside analog clitocine as a potent and efficacious suppressor of nonsense mutations. We determined that incorporation of clitocine into RNA during transcription is a prerequisite for its readthrough activity; the presence of clitocine in the third position of a premature stop codon directly induces readthrough. We demonstrate that clitocine can induce the production of p53 protein in cells harboring p53 nonsense-mutated alleles. In these cells, clitocine restored production of full-length and functional p53 as evidenced by induced transcriptional activation of downstream p53 target genes, progression of cells into apoptosis, and impeded growth of nonsense-containing human ovarian cancer tumors in xenograft tumor models. Thus, clitocine induces readthrough of nonsense mutations by a previously undescribed mechanism and represents a novel therapeutic modality to treat cancers and genetic diseases caused by nonsense mutations.

4-Hydroxy-2-pyridones: Discovery and evaluation of a novel class of antibacterial agents targeting DNA synthesis

The continued emergence of bacteria resistant to current standard of care antibiotics presents a rapidly growing threat to public health. New chemical entities (NCEs) to treat these serious infections are desperately needed. Herein we report the discovery, synthesis, SAR and in vivo efficacy of a novel series of 4-hydroxy-2-pyridones exhibiting activity against Gram-negative pathogens. Compound 1c, derived from the N-debenzylation of 1b, preferentially inhibits bacterial DNA synthesis as determined by standard macromolecular synthesis assays. The structural features of the 4-hydroxy-2-pyridone scaffold required for antibacterial activity were explored and compound 6q, identified through further optimization of the series, had an MIC90 value of 8 μg/mL against a panel of highly resistant strains of E. coli. In a murine septicemia model, compound 6q exhibited a PD50 of 8 mg/kg in mice infected with a lethal dose of E. coli. This novel series of 4-hydroxy-2-pyridones serves as an excellent starting point for the identification of NCEs treating Gram-negative infections.

Discovery of Novel Small Molecule Inhibitors of VEGF Expression in Tumor Cells Using a Cell-Based High Throughput Screening Platform

Current anti-VEGF (Vascular Endothelial Growth Factor A) therapies to treat various cancers indiscriminately block VEGF function in the patient resulting in the global loss of VEGF signaling which has been linked to dose-limiting toxicities as well as treatment failures due to acquired resistance. Accumulating evidence suggests that this resistance is at least partially due to increased production of compensatory tumor angiogenic factors/cytokines. VEGF protein production is differentially controlled depending on whether cells are in the normal “homeostatic” state or in a stressed state, such as hypoxia, by post-transcriptional regulation imparted by elements in the 5’ and 3’ untranslated regions (UTR) of the VEGF mRNA. Using the Gene Expression Modulation by Small molecules (GEMS™) phenotypic assay system, we performed a high throughput screen to identify low molecular weight compounds that target the VEGF mRNA UTR-mediated regulation of stress-induced VEGF production in tumor cells. We identified a number of compounds that potently and selectively reduce endogenous VEGF production under hypoxia in HeLa cells. Medicinal chemistry efforts improved the potency and pharmaceutical properties of one series of compounds resulting in the discovery of PTC-510 which inhibits hypoxia-induced VEGF expression in HeLa cells at low nanomolar concentration. In mouse xenograft studies, oral administration of PTC-510 results in marked reduction of intratumor VEGF production and single agent control of tumor growth without any evident toxicity. Here, we show that selective suppression of stress-induced VEGF production within tumor cells effectively controls tumor growth. Therefore, this approach may minimize the liabilities of current global anti-VEGF therapies.

Abstract 5517: PTC596-induced Bmi1 hyper-phosphorylation via Cdk1/2 activation resulting in tumor stem cell depletion

The Polycomb group (PcG) transcription repressor BMI1 is highly expressed in human cancers and is required for the clonogenic self-renewal and tumorigenesis of human cancer cells including those in hematological cancer and neuroblastoma. PTC596 is efficacious in vivo across a range of xenograft tumor models, including models of glioblastoma, fibrosarcoma and leukemia as well as orthotopic models of GBM. With EC50 values of 30-200 nM in a variety of tumor cell lines, PTC596 selectively reduces the level of functional BMI1 protein resulting in the depletion of the tumor stem cell fraction. PTC596 induces the hyper-phosphorylation of BMI1 leading to its degradation and the reduction of polycomb repressive complex 1 (PRC1) activity. Mechanistic studies suggest that PTC596 inhibits APC/CCDC20 activity resulting in the persistent activation of CDK1 and CDK2 which mediate the hyperphosphorylation of BMI1. Studies are ongoing to elucidate the mechanism of PTC596 inhibition of APC/CCDC20 and its preferential depletion of the tumor stem cell fraction. Citation Format: Min Jung Kim, Liangxian Cao, Josephine Sheedy, Nicole Risher, Melissa Dumble, Chang-Sun Lee, Nadiya Sydorenko, Ramil Baiazitov, Wu Du, Young-Choon Moon, Marla L. Weetall, Joseph Colacino, Thomas W. Davis. PTC596-induced Bmi1 hyper-phosphorylation via Cdk1/2 activation resulting in tumor stem cell depletion. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5517. doi:10.1158/1538-7445.AM2014-5517

The minor gentamicin complex component, X2, is a potent premature stop codon readthrough molecule with therapeutic potential

Nonsense mutations, resulting in a premature stop codon in the open reading frame of mRNAs are responsible for thousands of inherited diseases. Readthrough of premature stop codons by small molecule drugs has emerged as a promising therapeutic approach to treat disorders resulting from premature termination of translation. The aminoglycoside antibiotics are a class of molecule known to promote readthrough at premature termination codons. Gentamicin consists of a mixture of major and minor aminoglycoside components. Here, we investigated the readthrough activities of the individual components and show that each of the four major gentamicin complex components representing 92–99% of the complex each had similar potency and activity to that of the complex itself. In contrast, a minor component (gentamicin X2) was found to be the most potent and active readthrough component in the gentamicin complex. The known oto- and nephrotoxicity associated with aminoglycosides preclude long-term use as readthrough agents. Thus, we evaluated the components of the gentamicin complex as well as the so-called “designer” aminoglycoside, NB124, for in vitro and in vivo safety. In cells, we observed that gentamicin X2 had a safety/readthrough ratio (cytotoxicity/readthrough potency) superior to that of gentamicin, G418 or NB124. In rodents, we observed that gentamicin X2 showed a safety profile that was superior to G418 overall including reduced nephrotoxicity. These results support further investigation of gentamicin X2 as a therapeutic readthrough agent.

BMI1 is a therapeutic target in recurrent medulloblastoma

Medulloblastoma (MB) is the most frequent malignant pediatric brain tumor, representing 20% of newly diagnosed childhood central nervous system malignancies. Although advances in multimodal therapy yielded a 5-year survivorship of 80%, MB still accounts for the leading cause of childhood cancer mortality. In this work, we describe the epigenetic regulator BMI1 as a novel therapeutic target for the treatment of recurrent human Group 3 MB, a childhood brain tumor for which there is virtually no treatment option beyond palliation. Current clinical trials for recurrent MB patients based on genomic profiles of primary, treatment-naive tumors will provide limited clinical benefit since recurrent metastatic MBs are highly genetically divergent from their primary tumor. Using a small molecule inhibitor against BMI1, PTC-028, we were able to demonstrate complete ablation of self-renewal of MB stem cells in vitro. When administered to mice xenografted with patient tumors, we observed significant reduction in tumor burden in both local and metastatic compartments and subsequent increased survival, without neurotoxicity. Strikingly, serial in vivo re-transplantation assays demonstrated a marked reduction in tumor initiation ability of recurrent MB cells upon re-transplantation of PTC-028-treated cells into secondary recipient mouse brains. As Group 3 MB is often metastatic and uniformly fatal at recurrence, with no current or planned trials of targeted therapy, an efficacious targeted agent would be rapidly transitioned to clinical trials.