A forward chemical screen using zebrafish embryos with novel 2-substituted 2H-chromene derivatives

Recent Advances in Drug Discovery Toxicology

Major advances in scientific discovery and insights that stem from the development and use of new techniques and models can bring remarkable progress to conventional toxicology. Although animal testing is still considered as the "gold standard" in traditional toxicity testing, there is a necessity for shift from animal testing to alternative methods regarding the drug safety testing owing to the emerging state-of-art techniques and the proposal of 3Rs (replace, reduce, and refine) towards animal welfare. This review describes some recent research methods in drug discovery toxicology, including in vitro cell and organ-on-a-chip, imaging systems, model organisms (C. elegans, Danio rerio, and Drosophila melanogaster), and toxicogenomics in modern toxicology testing.

Development of a New Methodology for Dearomative Borylation of Coumarins and Chromenes and Its Applications to Synthesize Boron-Containing Retinoids

Dearomative borylation of coumarins and chromenes via conjugate addition represents a relatively unexplored and challenging task. To address this issue, herein, we report a new and general copper (I) catalyzed dearomative borylation process to synthesize boron-containing oxacycles. In this report, the borylation of coumarins, chromones, and chromenes comprising functional groups, such as esters, nitriles, carbonyls, and amides, has been achieved. In addition, the method generates different classes of potential boron-based retinoids, including the ones with oxadiazole and anthocyanin motifs. The borylated oxacycles can serve as suitable intermediates to generate a library of compounds.

Synthesis of a boron-containing amidoxime reagent and its application to synthesize functionalized oxadiazole and quinazolinone derivatives

Herein, we report the design, synthesis and application of a borylated amidoxime reagent for the direct synthesis of functionalized oxadiazole and quinazolinone derivatives. This reagent exhibits broad synthetic utility to obtain a variety of biologically relevant drug-like molecules. It can be easily prepared at large scale from relatively inexpensive reagents, and can undergo facile transformations to obtain target compounds. The developed amidoxime reagent was synthesized from 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzonitrile and hydroxyl amine hydrochloride using N,N-diisopropylethylamine as a base in ethanol under reflux conditions. Overall advantages include a metal-free route to boronated oxadiazoles, quinazolinone derivatives, and restriction of the multistep sequences. Importantly, the boron-rich pharmacophore derived compounds were obtained through an efficient and inexpensive strategy.

Interference Potential of Tannins and Chlorophylls in Zebrafish Phenotypic-Based Assays

Natural products are prolific producers of diverse chemical scaffolds, which have yielded several clinically useful drugs. However, the complex features of natural products present challenges for identifying bioactive molecules using high-throughput screens. For most assays, measured endpoints are either colorimetric or luminescence based. Thus, the presence of the major metabolites, tannins, and chlorophylls, in natural products could potentially interfere with these measurements to give either false-positive or false-negative hits. In this context, zebrafish phenotypic assays provide an alternative approach to bioprospect naturally occurring bioactive compounds. Whether tannins and/or chlorophylls interfere in zebrafish phenotypic assays, is unclear. In this study, we evaluated the interference potential of tannins and chlorophylls against efficacy of known small-molecule inhibitors that are known to cause phenotypic abnormalities in developing zebrafish embryos. First, we fractionated tannin-enriched fraction (TEF) and chlorophyll-enriched fraction (CEF) from Camellia sinensis and cotreated them with PD0325901 [mitogen-activated protein kinase-kinase (MEK) inhibitor] and sunitinib malate (SM; anti-[lymph]angiogenic drug). While TEF and CEF did not interfere with phenotypic or molecular endpoints of PD0325901, TEF at 100 μg/mL partially masked the antiangiogenic effect of SM. On the other hand, CEF (100 μg/mL) was toxic when treated up to 6 dpf. Furthermore, CEF at 100 μg/mL potentially enhanced the activity of γ-secretase inhibitors, resulting in toxicity of treated embryos. Our study provides evidence that the presence of tannin and/or chlorophyll in natural products do interfere with zebrafish phenotype assays used for identifying potential hits. However, this may be target/assay dependent and thus requiring additional optimization steps to assess interference potential of tannins and chlorophylls before performing any screening assay.

Small molecules targeting LapB protein prevent Listeria attachment to catfish muscle

Listeria monocytogenes is a Gram-positive foodborne pathogen and the causative agent of listeriosis. L. monocytogenes lapB gene encodes a cell wall surface anchor protein, and mutation of this gene causes Listeria attenuation in mice. In this work, the potential role of Listeria LapB protein in catfish fillet attachment was investigated. To achieve this, boron-based small molecules designed to interfere with the active site of the L. monocytogenes LapB protein were developed, and their ability to prevent L. monocytogenes attachment to fish fillet was tested. Results indicated that seven out of nine different small molecules were effective in reducing the Listeria attachment to catfish fillets. Of these, three small molecules (SM3, SM5, and SM7) were highly effective in blocking Listeria attachment to catfish fillets. This study suggests an alternative strategy for reduction of L. monocytogenes contamination in fresh and frozen fish products.

Chemical screening in zebrafish for novel biological and therapeutic discovery

Zebrafish chemical screening allows for an in vivo assessment of small molecule modulation of biological processes. Compound toxicities, chemical alterations by metabolism, pharmacokinetic and pharmacodynamic properties, and modulation of cell niches can be studied with this method. Furthermore, zebrafish screening is straightforward and cost effective. Zebrafish provide an invaluable platform for novel therapeutic discovery through chemical screening.

15 years of zebrafish chemical screening

In 2000, the first chemical screen using living zebrafish in a multi-well plate was reported. Since then, more than 60 additional screens have been published describing whole-organism drug and pathway discovery projects in zebrafish. To investigate the scope of the work reported in the last 14 years and to identify trends in the field, we analyzed the discovery strategies of 64 primary research articles from the literature. We found that zebrafish screens have expanded beyond the use of developmental phenotypes to include behavioral, cardiac, metabolic, proliferative and regenerative endpoints. Additionally, many creative strategies have been used to uncover the mechanisms of action of new small molecules including chemical phenocopy, genetic phenocopy, mutant rescue, and spatial localization strategies.

Metalloradical approach to 2H-chromenes

Cobalt(III)-carbene radicals, generated through metalloradical activation of salicyl N-tosylhydrazones by cobalt(II) complexes of porphyrins, readily undergo radical addition to terminal alkynes to produce salicyl-vinyl radical intermediates. Subsequent hydrogen atom transfer (HAT) from the hydroxy group of the salicyl moiety to the vinyl radical leads to the formation of 2H-chromenes. The Co(II)-catalyzed process can tolerate various substitution patterns and produces the corresponding 2H-chromene products in good isolated yields. EPR spectroscopy and radical-trapping experiments with TEMPO are in agreement with the proposed radical mechanism. DFT calculations reveal the formation of the salicyl-vinyl radical intermediate by a metalloradical-mediated process. Unexpectedly, subsequent HAT from the hydroxy moiety to the vinyl radical leads to formation of an o-quinone methide intermediate, which dissociates spontaneously from the cobalt center and easily undergoes an endocyclic, sigmatropic ring-closing reaction to form the final 2H-chromene product.

Gold(I)-catalyzed hydroarylation reaction of aryl (3-iodoprop-2-yn-1-yl) ethers: synthesis of 3-iodo-2H-chromene derivatives

An efficient entry to the preparation of elusive 4-unsubstituted-3-iodo-2H-chromenes has been accomplished as result of a catalytic cyclization. Thus, upon exposition of [(3-iodoprop-2-yn-1-yl)oxy]arenes to IPrAuNTf₂ (3 mol %), in 1,4-dioxane at 100 °C, the desired heterocyclic motif is readily assembled. This process nicely tolerates a variety of functional groups and, interestingly, it is compatible with the presence of strong electron-withdrawing groups attached to the arene. The overall transformation can be termed as a new example of a migratory cycloisomerization and, formally, it involves well-blended 1,2-iodine shift and hydroarylation steps.

Multicomponent assembly of 2-amino-4-substituted 4H-chromenes using cyclic Michael donors

An efficient one-pot synthesis has been disclosed to access a series of 2-amino-4-substituted 4H-chromenes from salicylaldehyde, malononitrile, and a few cyclic nucleophiles using indium trichloride as a catalyst. This methodology affords high yields of products in a shorter reaction time.

Design and synthesis of 3,5-disubstituted boron-containing 1,2,4-oxadiazoles as potential combretastatin A-4 (CA-4) analogs

We have designed and synthesized a small library of 3,5-disubstituted-1,2,4-oxadiazole containing combretastatin A-4 (CA-4) analogs. Our objective is to increase the efficacy of the CA-4 as an anti-tubulin and antimitotic agent by substituting the cis-alkene bond with one of its bioisosteres, the 1,2,4-oxadiazole ring. We also modified the substituents attached to both of the phenyl rings (ring A and B in Fig. 1) of CA-4 for the purpose of diversifying our analogs based on SAR. These compounds were synthesized via a coupling reaction between an amidoxime and a carboxylic acid in DMF solvent, with HOBt as a base, and utilizing EDCI as a coupling reagent. Using this protocol, we synthesized a small library of 10 compounds with moderate to good yields. A detailed biological study is currently undergoing in our laboratory to evaluate the activity of these compounds.

Zebrafish: as an integrative model for twenty-first century toxicity testing

The zebrafish embryo is a useful small model for investigating vertebrate development because of its transparency, low cost, transgenic and morpholino capabilities, conservation of cell signaling, and concordance with mammalian developmental phenotypes. From these advantages, the zebrafish embryo has been considered as an alternative model for traditional in vivo developmental toxicity screening. The use of this organism in conjunction with traditional in vivo developmental toxicity testing has the potential to reduce cost and increase throughput of testing the chemical universe, prioritize chemicals for targeted toxicity testing, generate predictive models of developmental toxicants, and elucidate mechanisms and adverse outcome pathways for abnormal development. This review gives an overview of the zebrafish embryo for pre dictive toxicology and 21st century toxicity testing. Developmental eye defects were selected as an example to evaluate data from the U.S. Environmental Protection Agency's ToxCast program comparing responses in zebrafish embryos with those from pregnant rats and rabbits for a subset of 24 environmental chemicals across >600 in vitro assay targets. Cross-species comparisons implied a common basis for biological pathways associated with neuronal defects, extracellular matrix remodeling, and mitotic arrest.

Novel retinoic acid receptor alpha agonists for treatment of kidney disease

Development of pharmacologic agents that protect podocytes from injury is a critical strategy for the treatment of kidney glomerular diseases. Retinoic acid reduces proteinuria and glomerulosclerosis in multiple animal models of kidney diseases. However, clinical studies are limited because of significant side effects of retinoic acid. Animal studies suggest that all trans retinoic acid (ATRA) attenuates proteinuria by protecting podocytes from injury. The physiological actions of ATRA are mediated by binding to all three isoforms of the nuclear retinoic acid receptors (RARs): RARα, RARβ, and RARγ. We have previously shown that ATRA exerts its renal protective effects mainly through the agonism of RARα. Here, we designed and synthesized a novel boron-containing derivative of the RARα-specific agonist Am580. This new derivative, BD4, binds to RARα receptor specifically and is predicted to have less toxicity based on its structure. We confirmed experimentally that BD4 binds to RARα with a higher affinity and exhibits less cellular toxicity than Am580 and ATRA. BD4 induces the expression of podocyte differentiation markers (synaptopodin, nephrin, and WT-1) in cultured podocytes. Finally, we confirmed that BD4 reduces proteinuria and improves kidney injury in HIV-1 transgenic mice, a model for HIV-associated nephropathy (HIVAN). Mice treated with BD4 did not develop any obvious toxicity or side effect. Our data suggest that BD4 is a novel RARα agonist, which could be used as a potential therapy for patients with kidney disease such as HIVAN.

Design, synthesis and biological study of pinacolyl boronate-substituted stilbenes as novel lipogenic inhibitors

A pilot library of novel 4,4,5,5-tetramethyl-2-(4-substitutedstyrylphenyl)-1,3,2 dioxaborolane derivatives has been synthesized. 4-(4,4,5,5-Tetramethyl-1,3,2-dioxaboratophenyl)-methyl triphenylphosphonium bromide 3 was treated with various aldehydes in the presence of 3 equiv of (t)BuONa in DMF, and stirred at room temperature for 4-6h to yield the corresponding boron-containing stilbene derivatives in 71-94% yields. Several of them, including BF102 and BF175, have the lipogenesis inhibitory effect by suppressing lipogenic gene expression in mammalian hepatocytes. Further, BF102 also inhibits cholesterol biosynthesis by suppressing HMG-CoA reductase gene expression in hepatocytes. Interestingly, our preliminary in vivo data suggests that BF102 has no significant toxicity in mice at the highest possible dose we can administered. Thus, BF102 is a potential lead for the next generation of lipid-lowering drugs.

Chemical screening in zebrafish for novel biological and therapeutic discovery

Zebrafish chemical screening allows for an in vivo assessment of small molecule modulation of biological processes. Compound toxicities, chemical alterations by metabolism, pharmacokinetic and pharmacodynamic properties, and modulation of cell niches can be studied with this method. Furthermore, zebrafish screening is straightforward and cost-effective. Zebrafish provide an invaluable platform for novel therapeutic discovery through chemical screening.

Designing zebrafish chemical screens

The zebrafish is proving to be highly amenable to in vivo small molecule screening. With a growing number of screens successfully completed, a rich interface is being created between disciplines that have historically used zebrafish (e.g., embryology and genetics) and disciplines focused on small molecules (e.g., chemistry and pharmacology). Navigating this interface requires consideration of the unique demands of conducting high-throughput screening in vivo. In this chapter, we discuss design elements of successful zebrafish screens, established screening methods, and approaches for mechanism of action studies following discovery of novel small molecules. These methods are enabling the zebrafish to have an increasingly positive impact on biomedical research and drug development.

Synthesis of 2-perfluoroalkyl 4H- and 2H-chromenylphosphonates mediated by amines and phosphines

An efficient synthesis of 2-perfluoroalkyl 4H-chromen-3-ylphosphonates 4a-i (R(F) = CF(3)) and 5a-i (R(F) = C(2)F(5)) has been accomplished via regioselective cycloaddition of 2-hydroxybenzaldehydes to diethyl 3,3,3-trifluoropropyn-1-yl- and diethyl 3,3,4,4,4-pentafluorobutyn-1-ylphosphonate, using trialkyl amines or phosphines as mediators. 2H-Chromen-3-ylphosphonates 6a-i were regioselectively obtained in the presence of triphenylphosphine. A convenient method for the isomerization of 4H-chromen-3-ylphosphonates into 2H-chromen-3-ylphosphonates 6a-i (R(F) = CF(3)) and 7a-i (R(F) = C(2)F(5)) was established.

Small molecule screening in zebrafish: an in vivo approach to identifying new chemical tools and drug leads

In the past two decades, zebrafish genetic screens have identified a wealth of mutations that have been essential to the understanding of development and disease biology. More recently, chemical screens in zebrafish have identified small molecules that can modulate specific developmental and behavioural processes. Zebrafish are a unique vertebrate system in which to study chemical genetic systems, identify drug leads, and explore new applications for known drugs. Here, we discuss some of the advantages of using zebrafish in chemical biology, and describe some important and creative examples of small molecule screening, drug discovery and target identification.

Synthesis of function-oriented 2-phenyl-2H-chromene derivatives using L-pipecolinic acid and substituted guanidine organocatalysts

Organocatalytic domino oxa-Michael/aldol reactions between salicylaldehyde with electron deficient olefins are presented. We screened guanidine, 1,1,3,3-tetramethylguanidine (TMG) and L-pipecolinic acid as organocatalysts for this transformation. 3-Substituted 2-phenyl-2H-chromene derivatives are synthesized with high yields and with poor enantioselectivity (5-17% ee) using L-pipecolinic acid while TMG works well with cinnamaldehyde without using co-catalyst. These 3-substituted-2-phenyl-2H-chromene derivatives are further derivatized to synthesize triazole and biotin-containing chromene derivatives, to facilitate purification of protein targets.

Design and synthesis of novel pinacolylboronate containing combretastatin 'antimitotic agent' analogues

We developed a procedure to synthesize pinacolyl boronate containing stilbene derivatives and used this procedure to synthesize boron-containing combretastatin analogues. The key step involves the Wittig reaction of the ylide 4-(4,4,5,5-tetramethyl-1,3,2-dioxaboratophenyl)-methyl triphenylphosphonium bromide 11 with 3,4,5-trimethoxy benzaldehyde in the presence of (t)BuONa in DMF, providing 88% yield. We are now in a position to evaluate the biological activity of these derivatives as modulators of TGF-beta signaling pathways.