Indoleamine 2,3-Dioxygenase Inhibitors Isolated from the Sponge Xestospongia vansoesti: Structure Elucidation, Analogue Synthesis, and Biological Activity

Halenaquinol Blocks Staphylococcal Protein A Anchoring on Cell Wall Surface by Inhibiting Sortase A in Staphylococcus aureus

Sortase A (SrtA) is a cysteine transpeptidase that binds to the periplasmic membrane and plays a crucial role in attaching surface proteins, including staphylococcal protein A (SpA), to the peptidoglycan cell wall. Six pentacyclic polyketides (1-6) were isolated from the marine sponge Xestospongia sp., and their structures were elucidated using spectroscopic techniques and by comparing them to previously reported data. Among them, halenaquinol (2) was found to be the most potent SrtA inhibitor, with an IC50 of 13.94 μM (4.66 μg/mL). Semi-quantitative reverse transcription PCR data suggest that halenaquinol does not inhibit the transcription of srtA and spA, while Western blot analysis and immunofluorescence microscopy images suggest that it blocks the cell wall surface anchoring of SpA by inhibiting the activity of SrtA. The onset and magnitude of the inhibition of SpA anchoring on the cell wall surface in S. aureus that has been treated with halenaquinol at a value 8× that of the IC50 of SrtA are comparable to those for an srtA-deletion mutant. These findings contribute to the understanding of the mechanism by which marine-derived pentacyclic polyketides inhibit SrtA, highlighting their potential as anti-infective agents targeting S. aureus virulence.

Silver-catalyzed cyclization of α-imino-oxy acids to fused tetralone derivatives

A silver-catalyzed intramolecular radical relay cyclization of α-imino-oxy acids under mild conditions has been described. This reaction offers facile access to a diverse range of fused tetralone derivatives with exquisite stereoselectivity in moderate to good yields (40-98%). Experimental studies show that the reaction undergoes a decarboxylation and acetone fragmentation/1,5-hydrogen atom transfer (HAT)/cyclization process.

Indoleamine 2, 3-dioxygenase 1 inhibitory compounds from natural sources

L-tryptophan metabolism is involved in the regulation of many important physiological processes, such as, immune response, inflammation, and neuronal function. Indoleamine 2, 3-dioxygenase 1 (IDO1) is a key enzyme that catalyzes the first rate-limiting step of tryptophan conversion to kynurenine. Thus, inhibiting IDO1 may have therapeutic benefits for various diseases, such as, cancer, autoimmune disease, and depression. In the search for potent IDO1 inhibitors, natural quinones were the first reported IDO1 inhibitors with potent inhibitory activity. Subsequently, natural compounds with diverse structures have been found to have anti-IDO1 inhibitory activity. In this review, we provide a summary of these natural IDO1 inhibitors, which are classified as quinones, polyphenols, alkaloids and others. The overview of in vitro IDO1 inhibitory activity of natural compounds will help medicinal chemists to understand the mode of action and medical benefits of them. The scaffolds of these natural compounds can also be used for further optimization of potent IDO1 inhibitors.

Metal-/catalyst-free one-pot three-component thioamination of 1,4-naphthoquinone in a sustainable solvent

A metal-/catalyst-free protocol for the direct thioamination of 1,4-naphthoquinone with thiophenols and amines using air as an oxidant in a green solvent has been developed. This environmentally friendly strategy was...

Indoleamine 2,3-dioxygenase 1 (IDO1) inhibitors and PROTAC-based degraders for cancer therapy

Indoleamine 2,3-dioxygenase 1 (IDO1), a known immunosuppressive enzyme that catalyzes the rate-limiting step in the oxidation of tryptophan (Trp) to kynurenine (Kyn), has received increasing attention as an attractive immunotherapeutic target for cancer therapy. Up to now, eleven small-molecule IDO1 inhibitors have entered clinical trials for the treatment of cancers. In addition, proteolysis targeting chimera (PROTAC) based degraders also provide prospects for cancer therapy. Herein we present a comprehensive overview of the medicinal chemistry strategies and potential therapeutic applications of IDO1 inhibitors in nonclinical trials and IDO1-PROTAC degraders.

Asymmetric total synthesis of (+)-xestoquinone and (+)-adociaquinones A and B

The asymmetric total synthesis of (+)-xestoquinone and (+)-adociaquinones A and B was achieved in 6-7 steps using an easily accessible meso-cyclohexadienone derivative. The [6,6]-bicyclic decalin B-C ring and the all-carbon quaternary stereocenter at C-6 were prepared via a desymmetric intramolecular Michael reaction with up to 97% ee. The naphthalene diol D-E ring was constructed through a sequence of Ti(Oi-Pr)4-promoted photoenolization/Diels-Alder, dehydration, and aromatization reactions. This asymmetric strategy provides a scalable route to prepare target molecules and their derivatives for further biological studies.

Diverse chemical space of indoleamine-2,3-dioxygenase 1 (Ido1) inhibitors

Indoleamine-2,3-dioxygenase 1 (IDO1) catalyses the first and rate limiting step of kynurenine pathway accounting for the major contributor of L-Tryptophan degradation. The Kynurenine metabolites are identified as essential cofactors, antagonists, neurotoxins, immunomodulators, antioxidants as well as carcinogens. The catalytic active site of IDO1 enzyme consists of hydrophobic Pocket-A positioned in the distal heme site and remains connected to a second hydrophobic Pocket-B towards the entrance of the active site. IDO1 enzyme also relates directly to the modulation of the innate and adaptive immune system. Various studies proved that the over expression of IDO1 enzyme play a predominant role in the escape of immunity during cancer progression. Recently, there has been considerable interest in evaluating the potential of IDO1 inhibitors to mobilize the body's immune system against solid tumours. In the last two decades, enormous attempts to advance new IDO1 inhibitors are on-going both in pharmaceutical industries and in academia which resulted in the discovery of a diverse range of selective and potent IDO1 inhibitors. The IDO1 inhibitors have therapeutic utility in various diseases and in the near future, it may have utility in the treatment of COVID-19. Despite various reviews on IDO1 inhibitors in last five years, none of the reviews provide a complete overview of diverse chemical space including naturally occurring and synthetic IDO1 inhibitors with detailed structure activity relationship studies. The present work provides a complete overview on the IDO1 inhibitors known in the literature so far along with the Structure-Activity Relationship (SAR) in each class of compounds.

Discovery of Novel Indoleamine 2,3-Dioxygenase 1 (IDO1) and Histone Deacetylase 1 (HDAC1) Dual Inhibitors Derived from the Natural Product Saprorthoquinone

The discovery of IDO1 and HDAC1 dual inhibitors may provide a novel strategy for cancer treatment by taking advantages of both immunotherapeutic and epigenetic drugs. In this paper, saprorthoquinone (1) and 13 of its analogues from Salvia prionitis Hance were investigated for their SAR against IDO1, the results demonstrated the ortho-quinone was a key pharmacophore. Then a series of IDO1 and HDAC dual inhibitors connected by appropriate linkers were designed, synthesized, and evaluated from the hit compound saprorthoquinone (1). Among them, compound 33d showed balanced activity against both IDO1 (IC₅₀ = 0.73 μM) and HDAC1 (IC₅₀ = 0.46 μM). Importantly, the structure of 33d suggested that an ortho-quinone pharmacophore and a N-(2-aminophenyl) amide pharmacophore were necessary for the IDO inhibition and HDAC inhibition respectively. Meanwhile, these two pharmacophore groups should be combined by a pentane linker. Moreover, the binding modes of 33d to the enzyme active site showed that the hydrogen bond with Leu234 of IDO1 appeared to confer increased potency to this class of inhibitors, which may explain the higher activity of 33d. This study provides a new strategy for future IDO1/HDAC dual inhibitors with synergistic antitumor activity started from lead compound 33d.

Biologically active marine natural products and their molecular targets discovered using a chemical genetics approach

Covering: 2000 to 2019The discovery of new natural products that have some combination of unprecedented chemical structures, biological activities of therapeutic interest for urgent medical needs, and new molecular targets provides the fuel that sustains the vitality of natural products chemistry research. Unfortunately, finding these important new compounds is neither routine or trivial and a major challenge is finding effective discovery paradigms. This review presents examples that illustrate the effectiveness of a chemical genetics approach to marine natural product (MNP) discovery that intertwines compound discovery, molecular target identification, and phenotypic response/biological activity. The examples include MNPs that have complex unprecedented structures, new or understudied molecular targets, and potent biological activities of therapeutic interest. A variety of methods to identify molecular targets are also featured.

Marine Pharmacology in 2014-2015: Marine Compounds with Antibacterial, Antidiabetic, Antifungal, Anti-Inflammatory, Antiprotozoal, Antituberculosis, Antiviral, and Anthelmintic Activities; Affecting the Immune and Nervous Systems, and Other Miscellaneous Mechanisms of Action

The systematic review of the marine pharmacology literature from 2014 to 2015 was completed in a manner consistent with the 1998-2013 reviews of this series. Research in marine pharmacology during 2014-2015, which was reported by investigators in 43 countries, described novel findings on the preclinical pharmacology of 301 marine compounds. These observations included antibacterial, antifungal, antiprotozoal, antituberculosis, antiviral, and anthelmintic pharmacological activities for 133 marine natural products, 85 marine compounds with antidiabetic, and anti-inflammatory activities, as well as those that affected the immune and nervous system, and 83 marine compounds that displayed miscellaneous mechanisms of action, and may probably contribute to novel pharmacological classes upon further research. Thus, in 2014-2015, the preclinical marine natural product pharmacology pipeline provided novel pharmacology as well as new lead compounds for the clinical marine pharmaceutical pipeline, and thus continued to contribute to ongoing global research for alternative therapeutic approaches to many disease categories.

Recent advances in the discovery of indoleamine 2,3-dioxygenase 1 (IDO1) inhibitors

Indoleamine 2,3-dioxygenase 1 (IDO1), an important immunoregulatory enzyme ubiquitously expressed in various tissues and cells, plays a key role in tryptophan metabolism via the kynurenine pathway and has emerged as an attractive therapeutic target for the treatment of cancer and other diseases, such as Alzheimer's disease and arthritis. IDO1 has diverse biological roles in immune suppression and tumor progression by tryptophan catabolism. In addition, IDO1-mediated immune tolerance assists tumor cells in escaping the immune surveillance. Recently, extensive and enormous investigations have been made in the discovery of IDO1 inhibitors in both academia and pharmaceutical companies. In this review, IDO1 inhibitors are grouped as tryptophan derivatives, inhibitors with an imidazole, 1,2,3-triazole or tetrazole scaffold, inhibitors with quinone or iminoquinone, N-hydroxyamidines and other derivatives, and their enzymatic inhibitory activity, selectivity and other biological activities are also introduced and summarized.

Recent advances in small molecule based cancer immunotherapy

Immunotherapy has been increasingly utilized for the treatment of cancer. Currently available cancer immunotherapies mainly involve the use of antibodies, which have advantages in terms of pharmacodynamics such as efficacy and specificity, however, they exhibit disadvantages in regard to the pharmacokinetics including but not limited to poor tissue and tumor penetration, very long half-life, and the lack of oral bioavailability. Also they are immunogenic and may cause undesired side effects. In addition, they are difficult and expensive to produce. In contrast to therapeutic antibodies, small molecule immuno-oncology agents generally have favorable pharmacokinetics, for example, better oral bioavailability, higher tissue and tumor penetration, reasonable half-lives etc. Furthermore, some small molecules are highly selective and efficacious with benign toxicity profiles. Therefore, small molecule immuno-oncology agents have the potential to overcome the drawbacks of therapeutic antibodies, and they can complement existing therapeutic antibodies and may also be used in combination with antibodies to achieve synergistic effects. In this article, we summarize the current advances in the field of small molecule approaches in tumor immunology which include the small molecules in clinical trials and preclinical studies, and the reported crystal structures of small molecules and their target proteins as well as the binding interactions between small molecules and the targets. The tumorigenesis mechanism of different targets (the programmed cell death 1/programmed cell death ligand 1(PD1/PD-L1), retinoic acid-related orphan receptor-gamma t (RORγt), Chemokine receptor, Stimulator of Interferon Genes (Sting), Indoleamine 2,3-dioxygenase (IDO), toll-like receptors (TLR) etc.) are also elucidated.

Total Synthesis of (-)-Xestosaprol N and O

The first total synthesis of (-)-xestosaprol N and O is described. This synthetic work features a convergent strategy: (1) a Pd-catalyzed arylation followed by cyclization to build a naphthalene fragment (ring C, D); (2) utilization of (-)-quinic acid to construct the chiral hydroxyl group at C-2; (3) a substrate controlled intramolecular Heck reaction to construct a quaternary carbon center (ring B); (4) introduction of a hypotaurine moiety at a late stage to furnish the E ring.

Orbicularisine: A Spiro-Indolothiazine Isolated from Gills of the Tropical Bivalve Codakia orbicularis

A novel spiro-indolofuranone fused to a thiazine skeleton, orbicularisine (1), was isolated from gills of the mollusk Codakia orbicularis. The isolation and structure elucidation using spectroscopic evidence including mass and NMR spectroscopy are described. The final structure of 1 was supported by key HMBC correlation.

Marine natural products

This review covers the literature published in 2014 for marine natural products (MNPs), with 1116 citations (753 for the period January to December 2014) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1378 in 456 papers for 2014), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

IDO as a drug target for cancer immunotherapy: recent developments in IDO inhibitors discovery

This review highlights the recent advances in research related to the role of IDO in immune escape in cancer and novel small-molecule IDO inhibitors with an emphasis on their chemical structures and modes of action.