Reprogramming RNA processing: an emerging therapeutic landscape.

The production of a mature mRNA requires coordination of multiple processing steps, which ultimately control its content, localization, and stability. These steps include some of the largest macromolecular machines in the cell, which were, until recently, considered undruggable due to their biological complexity. Building from an expanded understanding of the underlying mechanisms that drive these processes, a new wave of therapeutics is seeking to target RNA processing. With a focus on impacting gene regulation at the RNA level, such modalities offer potential for sequence-specific resolution in drug design. Here, we review our current understanding of RNA-processing events and their role in gene regulation, with a focus on the therapeutic opportunities that have emerged within this landscape.

Discovery of Aminopyrazole Derivatives as Potent Inhibitors of Wild-Type and Gatekeeper Mutant FGFR2 and 3.

Fibroblast growth factor receptors (FGFR) 2 and 3 have been established as drivers of numerous types of cancer with multiple drugs approved or entering late stage clinical trials. A limitation of current inhibitors is vulnerability to gatekeeper resistance mutations. Using a combination of targeted high-throughput screening and structure-based drug design, we have developed a series of aminopyrazole based FGFR inhibitors that covalently target a cysteine residue on the P-loop of the kinase. The inhibitors show excellent activity against the wild-type and gatekeeper mutant versions of the enzymes. Further optimization using SAR analysis and structure-based drug design led to analogues with improved potency and drug metabolism and pharmacokinetics properties.

Discovery of 2,6-Dimethylpiperazines as Allosteric Inhibitors of CPS1.

Carbamoyl phosphate synthetase 1 (CPS1) is a potential synthetic lethal target in LKB1-deficient nonsmall cell lung cancer, where its overexpression supports the production of pyrimidine synthesis. In other cancer types, CPS1 overexpression and activity may prevent the accumulation of toxic levels of intratumoral ammonia to support tumor growth. Herein we report the discovery of a novel series of potent and selective small-molecule inhibitors of CPS1. Piperazine 2 was initially identified as a promising CPS1 inhibitor through a high-throughput screening effort. Subsequent structure-activity relationship optimization and structure-based drug design led to the discovery of piperazine H3B-616 (25), a potent allosteric inhibitor of CPS1 (IC50 = 66 nM).

Estrogen Receptor Covalent Antagonists: The Best Is Yet to Come.

The development of tamoxifen and subsequent estrogen receptor alpha (ERα) antagonists represents a tremendous therapeutic breakthrough in the treatment of breast cancer. Despite the ability of ERα antagonists to increase survival rates, resistance to these therapies is an all-too-common occurrence. The majority of resistant tumors, including those with hotspot mutations in the ligand-binding domain of ERα, remain dependent on ERα signaling, indicating that either a more potent or novel class of antagonist could have clinical benefit. With this thought in mind, we developed a novel ERα antagonist that exhibits enhanced potency due to its ability to covalently target a unique cysteine in ER. This review describes the design of this antagonist, H3B-5942, and discusses opportunities for future improvements, which could reduce the risk of escape mutations to this therapeutic modality.

Discovery of Selective Estrogen Receptor Covalent Antagonists for the Treatment of ERαWT and ERαMUT Breast Cancer.

Mutations in estrogen receptor alpha (ERα) that confer resistance to existing classes of endocrine therapies are detected in up to 30% of patients who have relapsed during endocrine treatments. Because a significant proportion of therapy-resistant breast cancer metastases continue to be dependent on ERα signaling, there remains a critical need to develop the next generation of ERα antagonists that can overcome aberrant ERα activity. Through our drug-discovery efforts, we identified H3B-5942, which covalently inactivates both wild-type and mutant ERα by targeting Cys530 and enforcing a unique antagonist conformation. H3B-5942 belongs to a class of ERα antagonists referred to as selective estrogen receptor covalent antagonists (SERCA). In vitro comparisons of H3B-5942 with standard-of-care (SoC) and experimental agents confirmed increased antagonist activity across a panel of ERαWT and ERαMUT cell lines. In vivo, H3B-5942 demonstrated significant single-agent antitumor activity in xenograft models representing ERαWT and ERαY537S breast cancer that was superior to fulvestrant. Lastly, H3B-5942 potency can be further improved in combination with CDK4/6 or mTOR inhibitors in both ERαWT and ERαMUT cell lines and/or tumor models. In summary, H3B-5942 belongs to a class of orally available ERα covalent antagonists with an improved profile over SoCs.Significance: Nearly 30% of endocrine therapy-resistant breast cancer metastases harbor constitutively activating mutations in ERα. SERCA H3B-5942 engages C530 of both ERαWT and ERαMUT, promotes a unique antagonist conformation, and demonstrates improved in vitro and in vivo activity over SoC agents. Importantly, single-agent efficacy can be further enhanced by combining with CDK4/6 or mTOR inhibitors. Cancer Discov; 8(9); 1176-93. ©2018 AACR.This article is highlighted in the In This Issue feature, p. 1047.

The cryo-EM structure of the SF3b spliceosome complex bound to a splicing modulator reveals a pre-mRNA substrate competitive mechanism of action.

Somatic mutations in spliceosome proteins lead to dysregulated RNA splicing and are observed in a variety of cancers. These genetic aberrations may offer a potential intervention point for targeted therapeutics. SF3B1, part of the U2 small nuclear RNP (snRNP), is targeted by splicing modulators, including E7107, the first to enter clinical trials, and, more recently, H3B-8800. Modulating splicing represents a first-in-class opportunity in drug discovery, and elucidating the structural basis for the mode of action opens up new possibilities for structure-based drug design. Here, we present the cryogenic electron microscopy (cryo-EM) structure of the SF3b subcomplex (SF3B1, SF3B3, PHF5A, and SF3B5) bound to E7107 at 3.95 Å. This structure shows that E7107 binds in the branch point adenosine-binding pocket, forming close contacts with key residues that confer resistance upon mutation: SF3B1R1074H and PHF5AY36C The structure suggests a model in which splicing modulators interfere with branch point adenosine recognition and supports a substrate competitive mechanism of action (MOA). Using several related chemical probes, we validate the pose of the compound and support their substrate competitive MOA by comparing their activity against both strong and weak pre-mRNA substrates. Finally, we present functional data and structure-activity relationship (SAR) on the PHF5AR38C mutation that sensitizes cells to some chemical probes but not others. Developing small molecule splicing modulators represents a promising therapeutic approach for a variety of diseases, and this work provides a significant step in enabling structure-based drug design for these elaborate natural products. Importantly, this work also demonstrates that the utilization of cryo-EM in drug discovery is coming of age.

H3B-6527 Is a Potent and Selective Inhibitor of FGFR4 in FGF19-Driven Hepatocellular Carcinoma.

Activation of the fibroblast growth factor receptor FGFR4 by FGF19 drives hepatocellular carcinoma (HCC), a disease with few, if any, effective treatment options. While a number of pan-FGFR inhibitors are being clinically evaluated, their application to FGF19-driven HCC may be limited by dose-limiting toxicities mediated by FGFR1-3 receptors. To evade the potential limitations of pan-FGFR inhibitors, we generated H3B-6527, a highly selective covalent FGFR4 inhibitor, through structure-guided drug design. Studies in a panel of 40 HCC cell lines and 30 HCC PDX models showed that FGF19 expression is a predictive biomarker for H3B-6527 response. Moreover, coadministration of the CDK4/6 inhibitor palbociclib in combination with H3B-6527 could effectively trigger tumor regression in a xenograft model of HCC. Overall, our results offer preclinical proof of concept for H3B-6527 as a candidate therapeutic agent for HCC cases that exhibit increased expression of FGF19. Cancer Res; 77(24); 6999-7013. ©2017 AACR.

Identification of nicotinamide phosphoribosyltransferase (NAMPT) inhibitors with no evidence of CYP3A4 time-dependent inhibition and improved aqueous solubility.

Herein we report the optimization efforts to ameliorate the potent CYP3A4 time-dependent inhibition (TDI) and low aqueous solubility exhibited by a previously identified lead compound from our NAMPT inhibitor program (1, GNE-617). Metabolite identification studies pinpointed the imidazopyridine moiety present in 1 as the likely source of the TDI signal, and replacement with other bicyclic systems was found to reduce or eliminate the TDI finding. A strategy of reducing the number of aromatic rings and/or lowering cLogD7.4 was then employed to significantly improve aqueous solubility. These efforts culminated in the discovery of 42, a compound with no evidence of TDI, improved aqueous solubility, and robust efficacy in tumor xenograft studies.

Discovery of potent and efficacious cyanoguanidine-containing nicotinamide phosphoribosyltransferase (Nampt) inhibitors.

A co-crystal structure of amide-containing compound (4) in complex with the nicotinamide phosphoribosyltransferase (Nampt) protein and molecular modeling were utilized to design and discover a potent novel cyanoguanidine-containing inhibitor bearing a sulfone moiety (5, Nampt Biochemical IC50=2.5nM, A2780 cell proliferation IC50=9.7nM). Further SAR exploration identified several additional cyanoguanidine-containing compounds with high potency and good microsomal stability. Among these, compound 15 was selected for in vivo profiling and demonstrated good oral exposure in mice. It also exhibited excellent in vivo antitumor efficacy when dosed orally in an A2780 ovarian tumor xenograft model. The co-crystal structure of this compound in complex with the NAMPT protein was also determined.

Identification of amides derived from 1H-pyrazolo[3,4-b]pyridine-5-carboxylic acid as potent inhibitors of human nicotinamide phosphoribosyltransferase (NAMPT).

Potent, 1H-pyrazolo[3,4-b]pyridine-containing inhibitors of the human nicotinamide phosphoribosyltransferase (NAMPT) enzyme were identified using structure-based design techniques. Many of these compounds exhibited nanomolar antiproliferation activities against human tumor lines in in vitro cell culture experiments, and a representative example (compound 26) demonstrated encouraging in vivo efficacy in a mouse xenograft tumor model derived from the A2780 cell line. This molecule also exhibited reduced rat retinal exposures relative to a previously studied imidazo-pyridine-containing NAMPT inhibitor. Somewhat surprisingly, compound 26 was only weakly active in vitro against mouse and monkey tumor cell lines even though it was a potent inhibitor of NAMPT enzymes derived from these species. The compound also exhibited only minimal effects on in vivo NAD levels in mice, and these changes were considerably less profound than those produced by an imidazo-pyridine-containing NAMPT inhibitor. The crystal structures of compound 26 and the corresponding PRPP-derived ribose adduct in complex with NAMPT were also obtained.

Structure-based discovery of novel amide-containing nicotinamide phosphoribosyltransferase (nampt) inhibitors.

Crystal structures of several urea- and thiourea-derived compounds in complex with the nicotinamide phosphoribosyltransferase (Nampt) protein were utilized to design a potent amide-containing inhibitor bearing an aza-indole moiety (7, Nampt BC IC50 = 9.0 nM, A2780 cell proliferation IC50 = 10 nM). The Nampt-7 cocrystal structure was subsequently obtained and enabled the design of additional amide-containing inhibitors which incorporated various other fused 6,5-heterocyclic moieties and biaryl sulfone or sulfonamide motifs. Additional modifications of these molecules afforded many potent biaryl sulfone-containing Nampt inhibitors which also exhibited favorable in vitro ADME properties (microsomal and hepatocyte stability, MDCK permeability, plasma protein binding). An optimized compound (58) was a potent inhibitor of multiple cancer cell lines (IC50 <10 nM vs U251, HT1080, PC3, MiaPaCa2, and HCT116 lines), displayed acceptable mouse PK properties (F = 41%, CL = 52.4 mL/min/kg), and exhibited robust efficacy in a U251 mouse xenograft model.

Discovery of potent and efficacious urea-containing nicotinamide phosphoribosyltransferase (NAMPT) inhibitors with reduced CYP2C9 inhibition properties.

Potent, reversible inhibition of the cytochrome P450 CYP2C9 isoform was observed in a series of urea-containing nicotinamide phosphoribosyltransferase (NAMPT) inhibitors. This unwanted property was successfully removed from the described inhibitors through a combination of structure-based design and medicinal chemistry activities. An optimized compound which did not inhibit CYP2C9 exhibited potent anti-NAMPT activity (17; BC NAMPT IC50=3 nM; A2780 antiproliferative IC50=70 nM), good mouse PK properties, and was efficacious in an A2780 mouse xenograft model. The crystal structure of this compound in complex with the NAMPT protein is also described.

Structure-based identification of ureas as novel nicotinamide phosphoribosyltransferase (Nampt) inhibitors.

Nicotinamide phosphoribosyltransferase (Nampt) is a promising anticancer target. Virtual screening identified a thiourea analogue, compound 5, as a novel highly potent Nampt inhibitor. Guided by the cocrystal structure of 5, SAR exploration revealed that the corresponding urea compound 7 exhibited similar potency with an improved solubility profile. These studies also indicated that a 3-pyridyl group was the preferred substituent at one inhibitor terminus and also identified a urea moiety as the optimal linker to the remainder of the inhibitor structure. Further SAR optimization of the other inhibitor terminus ultimately yielded compound 50 as a urea-containing Nampt inhibitor which exhibited excellent biochemical and cellular potency (enzyme IC50 = 0.007 µM; A2780 IC50 = 0.032 µM). Compound 50 also showed excellent in vivo antitumor efficacy when dosed orally in an A2780 ovarian tumor xenograft model (TGI of 97% was observed on day 17).

The characterization of 3,4-dimethylmethcathinone (3,4-DMMC).

Analogs and derivatives of traditional illicit drugs are ever increasing in variety and creativity. Staying abreast of the new developments is a constant challenge for every forensic laboratory. Recently, a seizure from Australian Customs Service presented our laboratory with the designer cathinone 3,4-dimethylmethcathinone (3,4-DMMC). Gas chromatography/mass spectrometry (GC/MS), liquid chromatography/mass spectrometry (LC/MS), nuclear magnetic resonance (NMR) spectroscopy, infrared (IR) spectroscopy, and ultraviolet (UV) spectrophotometry were employed to analyze the spectroscopic characteristics of this cathinone. As an analog, 3,4-DMMC exhibits similar if not identical IR and UV profiles to mephedrone (4-MMC) and methcathinone; however, the retention time from GC is unique as expected, and the electron impact fragmentation pattern is consistent with the fragmentation pattern of other cathinones. The chemical shifts of the carbons and hydrogens were assigned by both one- and two-dimensional NMR techniques, while the molecular weight was confirmed by LC/MS.

Discovery of a potent and orally bioavailable benzolactam-derived inhibitor of Polo-like kinase 1 (MLN0905).

This article describes the discovery of a series of potent inhibitors of Polo-like kinase 1 (PLK1). Optimization of this benzolactam-derived chemical series produced an orally bioavailable inhibitor of PLK1 (12c, MLN0905). In vivo pharmacokinetic-pharmacodynamic experiments demonstrated prolonged mitotic arrest after oral administration of 12c to tumor bearing nude mice. A subsequent efficacy study in nude mice achieved tumor growth inhibition or regression in a human colon tumor (HT29) xenograft model.

Anomeric oxygen to carbon rearrangements of alkynyl tributylstannane derivatives of furanyl (gamma)- and pyranyl (delta)-lactols.

Tetrahydropyran and tetrahydrofuran containing natural products, drugs and agrochemicals often possess carbon-carbon bonds adjacent to the heteroatom. Consequently, new methods for the construction of anomeric carbon-carbon bonds are of considerable importance. We have devised a new strategy to access these systems that requires the treatment of O-glycoside alkynyl tributylstannane derivatives of furanyl and pyranyl lactols with Lewis acid to effect oxygen to carbon rearrangements. This leads to the formation of the corresponding carbon linked alkynol products that can be further manipulated to produce key structural motifs and building blocks for the assembly of complex molecules.