Scalable synthesis of bryostatin 1 and analogs, adjuvant leads against latent HIV

Defining the Effects of PKC Modulator HIV Latency-Reversing Agents on Natural Killer Cells

Background

Latency reversing agents (LRAs) such as protein kinase C (PKC) modulators can reduce rebound-competent HIV reservoirs in small animal models. Furthermore, administration of natural killer (NK) cells following LRA treatment improves this reservoir reduction. It is currently unknown why the combination of a PKC modulator and NK cells is so potent and whether exposure to PKC modulators may augment NK cell function in some way.

Methods

Primary human NK cells were treated with PKC modulators (bryostatin-1, prostratin, or the designed, synthetic bryostatin-1 analog SUW133), and evaluated by examining expression of activation markers by flow cytometry, analyzing transcriptomic profiles by RNA sequencing, measuring cytotoxicity by co-culturing with K562 cells, assessing cytokine production by Luminex assay, and examining the ability of cytokines and secreted factors to independently reverse HIV latency by co-culturing with Jurkat-Latency (J-Lat) cells.

Results

PKC modulators increased expression of proteins involved in NK cell activation. Transcriptomic profiles from PKC-treated NK cells displayed signatures of cellular activation and enrichment of genes associated with the NFκB pathway. NK cell cytotoxicity was unaffected by prostratin but significantly decreased by bryostatin-1 and SUW133. Cytokines from PKC-stimulated NK cells did not induce latency reversal in J-Lat cell lines.

Conclusions

Although PKC modulators have some significant effects on NK cells, their contribution in "kick and kill" strategies is likely due to upregulating HIV expression in CD4+ T cells, not directly enhancing the effector functions of NK cells. This suggests that PKC modulators are primarily augmenting the "kick" rather than the "kill" arm of this HIV cure approach.

A Marine Natural Product, Harzianopyridone, as an Anti-ZIKV Agent by Targeting RNA-Dependent RNA Polymerase

The Zika virus (ZIKV) is a mosquito-borne virus that already poses a danger to worldwide human health. Patients infected with ZIKV generally have mild symptoms like a low-grade fever and joint pain. However, severe symptoms can also occur, such as Guillain-Barré syndrome, neuropathy, and myelitis. Pregnant women infected with ZIKV may also cause microcephaly in newborns. To date, we still lack conventional antiviral drugs to treat ZIKV infections. Marine natural products have novel structures and diverse biological activities. They have been discovered to have antibacterial, antiviral, anticancer, and other therapeutic effects. Therefore, marine products are important resources for compounds for innovative medicines. In this study, we identified a marine natural product, harzianopyridone (HAR), that could inhibit ZIKV replication with EC50 values from 0.46 to 2.63 µM while not showing obvious cytotoxicity in multiple cellular models (CC50 > 45 µM). Further, it also reduced the expression of viral proteins and protected cells from viral infection. More importantly, we found that HAR directly bound to the ZIKV RNA-dependent RNA polymerase (RdRp) and suppressed its polymerase activity. Collectively, our findings provide HAR as an option for the development of anti-ZIKV drugs.

Quantification of Binding of Small Molecules to Native Proteins Overexpressed in Living Cells

The affinity and selectivity of small molecules for proteins drive drug discovery and development. We report a fluorescent probe cellular binding assay (FPCBA) for determination of these values for native (untagged) proteins overexpressed in living cells. This method uses fluorophores such as Pacific Blue (PB) linked to cell-permeable protein ligands to generate probes that rapidly and reversibly equilibrate with intracellular targets, as established by kinetic assays of cellular uptake and efflux. To analyze binding to untagged proteins, an internal ribosomal entry site (IRES) vector was employed that allows a single mRNA to encode both the protein target and a separate orthogonal fluorescent protein (mVenus). This enabled cellular uptake of the probe to be correlated with protein expression by flow cytometry, allowing measurement of cellular dissociation constants (Kd) of the probe. This approach was validated by studies of the binding of allosteric activators to eight different Protein Kinase C (PKC) isozymes. Full-length PKCs expressed in transiently transfected HEK293T cells were used to measure cellular Kd values of a probe comprising PB linked to the natural product phorbol via a carbamate. These values were further used to determine competitive binding constants (cellular Ki values) of the nonfluorescent phorbol ester PDBu and the anticancer agent bryostatin 1 for each isozyme. For some PKC-small molecule pairs, these cellular Ki values matched known biochemical Ki values, but for others, altered selectivity was observed in cells. This approach can facilitate quantification of interactions of small molecules with physiologically relevant native proteins.

Biological evaluation of indolactams for in vitro bryostatin 1-like activity

Small molecule activators of protein kinase C (PKC) have traditionally been classified as either tumor promoters or suppressors. Although bryostatin 1 has well established anti-cancer activity, most natural products that target the PKC regulator domain exhibit tumor promotion properties. In this study, we examine a focused library of indolactam analogues in cell-based assays to establish the structural features of the scaffold that enhance bryostatin 1-like activity. These systematic biological assessments identified specific indole substitution patterns that impart diminished tumor promotion behavior in vitro for indolactam analogues, while still maintaining nanomolar potency for PKC.

Total Synthesis of the Proposed Structure of Neaumycin B

The total synthesis of the proposed structure of anti-glioblastoma natural product neaumycin B was achieved in 22 steps (longest linear sequence). The synthesis features HCl-mediated [6,6]-spiroketalization, a combination of Krische iridium-catalyzed crotylation, Marshall palladium-catalyzed propargylation, Fürstner nickel-catalyzed regio- and enantioselective vicinal monoprotected diol formation, Brown crotylation and asymmetric halide-aldehyde cycloaddition, so as to establish the challenging contiguous stereocenters.

Industrial total synthesis of natural medicines

Total synthesis offers a key approach to the production of natural medicines if sufficient quantities cannot be obtained due to low natural abundance or lack of efficient fermentation or semi-synthesis methods. This Viewpoint outlines the previous and current states of research as they apply to the total synthesis of natural medicines on an industrial scale and provides a holistic view on the potential for future developments in the field.

Effect of Solvent on the Rate of Ozonolysis: Development of a Homogeneous Flow Ozonolysis Protocol

Herein, we describe the effect of organic solvents on ozone solubility and the rate of ozonolysis reactions using rapid injection NMR spectroscopy. The tabulated solubility and kinetic data allowed for the design of a homogeneous ozonolysis flow reactor capable of delivering precise quantities of dissolved ozone to various olefin substrates.

Capturing primary ozonides for a syn-dihydroxylation of olefins

Ozonolysis is a widely used and practical synthetic technique for the deconstructive oxidation of olefins using ozone. While there are numerous ozonolysis reactions that give a myriad of products and functionalities, almost all of them involve scission at the olefin double bond. Using ozone as a constructive reagent rather than a deconstructive one would open new domains of chemical reactivity and amplify molecular complexity in synthetic methodology. Here we report the use of primary ozonides as preparative synthetic intermediates for a safe and green olefin syn-dihydroxylation reaction. Furthermore, we have demonstrated this method using a continuous flow reactor that virtually eliminates peroxide accumulation and extended these applications towards the synthesis of pharmaceutically relevant small molecules such as guaifenesin, the active ingredient in Mucinex, and a precursor to ponesimod, a drug to treat multiple sclerosis.

Bryostatin-1: a promising compound for neurological disorders

The central nervous system (CNS) is the most complex system in human body, and there is often a lack of effective treatment strategies for the disorders related with CNS. Natural compounds with multiple pharmacological activities may offer better options because they have broad cellular targets and potentially produce synergic and integrative effects. Bryostatin-1 is one of such promising compounds, a macrolide separated from marine invertebrates. Bryostatin-1 has been shown to produce various biological activities through binding with protein kinase C (PKC). In this review, we mainly summarize the pharmacological effects of bryostatin-1 in the treatment of multiple neurological diseases in preclinical studies and clinical trials. Bryostatin-1 is shown to have great therapeutic potential for Alzheimer's disease, multiple sclerosis, fragile X syndrome, stroke, traumatic brain injury, and depression. It exhibits significant rescuing effects on the deficits of spatial learning, cognitive function, memory and other neurological functions caused by diseases, producing good neuroprotective effects. The promising neuropharmacological activities of bryostatin-1 suggest that it is a potential candidate for the treatment of related neurological disorders although there are still some issues needed to be addressed before its application in clinic.

Secreted factors induced by PKC modulators do not indirectly cause HIV latency reversal

HIV can establish a long-lived latent infection in cells harboring integrated non-expressing proviruses. Latency reversing agents (LRAs), including protein kinase C (PKC) modulators, can induce expression of latent HIV, thereby reducing the latent reservoir in animal models. However, PKC modulators such as bryostatin-1 also cause cytokine upregulation in peripheral blood mononuclear cells (PBMCs), including cytokines that might independently reverse HIV latency. To determine whether cytokines induced by PKC modulators contribute to latency reversal, primary human PBMCs were treated with bryostatin-1 or the bryostatin analog SUW133, a superior LRA, and supernatant was collected. As anticipated, LRA-treated cell supernatant contained increased levels of cytokines compared to untreated cell supernatant. However, exposure of latently-infected cells with this supernatant did not result in latency reactivation. These results indicate that PKC modulators do not have significant indirect effects on HIV latency reversal in vitro and thus are targeted in their latency reversing ability.

Trimethylene Methane Dianion Equivalent for the Asymmetric Consecutive Allylation of Aldehydes: Applications to Prins-Driven Macrocyclizations for the Synthesis of Bryostatin 1 and Analogues

We report a one-step (one-flask) generation and reaction of a bifunctional allylating reagent, a trimethylene methane dianion equivalent, that provides a route for the asymmetric 2-(trimethylsilylmethyl) allylation of aldehydes. The product of the first aldehyde allylation process is then set to engage in a second separate aldehyde allylation, providing an improved Prins macrocyclization strategy both for the scalable synthesis of bryostatin 1 and for the total synthesis of a new potent bryostatin analogue.

Medium-Chain Lipid Conjugation Facilitates Cell-Permeability and Bioactivity

The majority of bioactive molecules act on membrane proteins or intracellular targets and therefore needs to partition into or cross biological membranes. Natural products often exhibit lipid modifications to facilitate critical molecule-membrane interactions, and in many cases their bioactivity is markedly reduced upon removal of a lipid group. However, despite its importance in nature, lipid-conjugation of small molecules is not commonly used in chemical biology and medicinal chemistry, and the effect of such conjugation has not been systematically studied. To understand the composition of lipids found in natural products, we carried out a chemoinformatic characterization of the "natural product lipidome". According to this analysis, lipidated natural products predominantly contain saturated medium-chain lipids (MCLs), which are significantly shorter than the long-chain lipids (LCLs) found in membranes and lipidated proteins. To study the usefulness of such modifications in probe design, we systematically explored the effect of lipid conjugation on five different small molecule chemotypes and find that permeability, cellular retention, subcellular localization, and bioactivity can be significantly modulated depending on the type of lipid tail used. We demonstrate that MCL conjugation can render molecules cell-permeable and modulate their bioactivity. With all explored chemotypes, MCL-conjugates consistently exhibited superior uptake or bioactivity compared to LCL-conjugates and either comparable or superior uptake or bioactivity to short-chain lipid (SCL)-conjugates. Together, our findings suggest that conjugation of small molecules with MCLs could be a powerful strategy for the design of probes and drugs.

Latency reversal plus natural killer cells diminish HIV reservoir in vivo

HIV is difficult to eradicate due to the persistence of a long-lived reservoir of latently infected cells. Previous studies have shown that natural killer cells are important to inhibiting HIV infection, but it is unclear whether the administration of natural killer cells can reduce rebound viremia when anti-retroviral therapy is discontinued. Here we show the administration of allogeneic human peripheral blood natural killer cells delays viral rebound following interruption of anti-retroviral therapy in humanized mice infected with HIV-1. Utilizing genetically barcoded virus technology, we show these natural killer cells efficiently reduced viral clones rebounding from latency. Moreover, a kick and kill strategy comprised of the protein kinase C modulator and latency reversing agent SUW133 and allogeneic human peripheral blood natural killer cells during anti-retroviral therapy eliminated the viral reservoir in a subset of mice. Therefore, combinations utilizing latency reversal agents with targeted cellular killing agents may be an effective approach to eradicating the viral reservoir.

Enantioselective Syntheses of Yohimbine Alkaloids: Proving Grounds for New Catalytic Transformations

The total synthesis of bioactive alkaloids is an enduring challenge and an indication of the state of the art of chemical synthesis. With the explosion of catalytic asymmetric methods over the past three decades, these compelling targets have been fertile proving grounds for enantioselective bond forming transformations. These activities are summarized herein both to highlight the power and versatility of these methods and to instill future inspiration for new syntheses of these privileged natural products.

Asymmetric Total Synthesis of Oxazolomycins B and C

Efforts towards the first total synthesis of (-)-oxazolomycin B and (+)-oxazolomycin C from the intermediate of our previous synthesis of (+)-neoxazolomycin are reported. The syntheses were achieved in a longest linear sequence of 25 steps from the amino acid serine in 3.6 and 2.7 % overall yields, respectively. The efficiency of our approach is derived from silyl triflate-mediated reductive oxazolidine ring-opening and Fürstner's Ru-catalyzed hydrosilylation and protodesilylation reactions. The obtained spectra and optical rotations were in good agreement with those of natural products, thus confirming the structures.

Marine Natural Products: A Potential Source of Anti-hepatocellular Carcinoma Drugs

Hepatocellular carcinoma (HCC) has high associated morbidity and mortality rates. Although chemical medication represents a primary HCC treatment strategy, low response rates and therapeutic resistance serve to reduce its efficacy. Hence, identifying novel effective drugs is urgently needed, and many researchers have sought to identify new anti-cancer drugs from marine organisms. The marine population is considered a "blue drug bank" of unique anti-cancer compounds with diverse groups of chemical structures. Here, we discuss marine-derived compounds, including PM060184 and bryostatin-1, with demonstrated anti-cancer activity in vitro or in vivo. Based on the marine source (sponges, algae, coral, bacteria, and fungi), we introduce pharmacological parameters, compound-induced cytotoxicity, effects on apoptosis and metastasis, and potential molecular mechanisms. Cumulatively, this review provides insights into anti-HCC research conducted to date in the field of marine natural products and marine-derived compounds, as well as the potential pharmacological mechanisms of these compounds and their status in drug development.

Application of marine natural products in drug research

New diseases are emerging as the environment changes, so drug manufacturers are always on the lookout for new resources to develop effective and safe drugs. In recent years, many bioactive substances have been produced in the marine environment, which represents an alternative resource for new drugs used to combat major diseases such as cancer or inflammation. Many marine-derived medicinal substances are in preclinical or early stage of clinical development, and some marine drugs have been put on the market, such as ET743 (Yondelis®). This review presents the sources, activities, mechanisms of action and syntheses of bioactive substances based on marine natural products in clinical trials and on the market, which is helpful to understand the progress of drug research by application of marine natural products.

Structural insights into C1-ligand interactions: Filling the gaps by in silico methods

Protein Kinase C isoenzymes (PKCs) are the key mediators of the phosphoinositide signaling pathway, which involves regulated hydrolysis of phosphatidylinositol (4,5)-bisphosphate to diacylglycerol (DAG) and inositol-1,4,5-trisphosphate. Dysregulation of PKCs is implicated in many human diseases making this class of enzymes an important therapeutic target. Specifically, the DAG-sensing cysteine-rich conserved homology-1 (C1) domains of PKCs have emerged as promising targets for pharmaceutical modulation. Despite significant progress, the rational design of the C1 modulators remains challenging due to difficulties associated with structure determination of the C1-ligand complexes. Given the dearth of experimental structural data, computationally derived models have been instrumental in providing atomistic insight into the interactions of the C1 domains with PKC agonists. In this review, we provide an overview of the in silico approaches for seven classes of C1 modulators and outline promising future directions.

Preclinical and Clinical Studies on Bryostatins, A Class of Marine-Derived Protein Kinase C Modulators: A Mini-Review

Bryostatins are complex macrolactones isolated from marine organisms Bryozoan Bugula neritina. They are potent modulators of protein kinase C isozymes (PKCα: ki = 1.3-188 nM), and are one of the most extensively investigated marine natural products in clinical trials. Although ~21 natural bryostatins have been isolated, however only bryostatin-1 (1) has received much interest among medicinal chemists and clinicians. The structure-activity relationship of bryostatins has been well established, with the identification of key pharmacophoric features important for PKC modulation. The low natural abundance and the long synthetic route have prompted medicinal chemists to come-up with simplified analogs. Bryostatin skeleton comprises three pyran rings connected to each other to form a macrocyclic lactone. The simplest analog 27 contains only one pyran, which is also able to modulate the PKCα activity; however, the cyclic framework appears to be essential for the desired level of potency. Another simplified analog 17 ("picolog") exhibited potent and in-vivo efficacy against lymphoma. Bryostatin-1 (1) has shown an acceptable intravenous pharmacokinetic profile in mice and displayed promising in-vivo efficacy in mice models of various cancers and Alzheimer's disease. Bryostatin-1 was investigated in numerous Phase I/II oncology clinical trials; it has shown minimal effect as a single agent, however, provided encouraging results in combination with other chemotherapy agents. FDA has granted orphan drug status to bryostatin-1 in combination with paclitaxel for esophageal cancer. Bryostatin-1 has also received orphan drug status for fragile X syndrome. Bryostatin-1 was also investigated in clinical studies for Alzheimer's disease and HIV infection. In a nutshell, the natural as well as synthetic bryostatins have generated a strong hope to emerge as treatment for cancer along with many other diseases.

Designed PKC-targeting bryostatin analogs modulate innate immunity and neuroinflammation

Neuroinflammation characterizes multiple neurologic diseases, including primary inflammatory conditions such as multiple sclerosis and classical neurodegenerative diseases. Aberrant activation of the innate immune system contributes to disease progression, but drugs modulating innate immunity, particularly within the central nervous system (CNS), are lacking. The CNS-penetrant natural product bryostatin-1 attenuates neuroinflammation by targeting innate myeloid cells. Supplies of natural bryostatin-1 are limited, but a recent scalable good manufacturing practice (GMP) synthesis has enabled access to it and its analogs (bryologs), the latter providing a path to more efficacious, better tolerated, and more accessible agents. Here, we show that multiple synthetically accessible bryologs replicate the anti-inflammatory effects of bryostatin-1 on innate immune cells in vitro, and a lead bryolog attenuates neuroinflammation in vivo, actions mechanistically dependent on protein kinase C (PKC) binding. Our findings identify bryologs as promising drug candidates for targeting innate immunity in neuroinflammation and create a platform for evaluation of synthetic PKC modulators in neuroinflammatory diseases.

Synthesis and biological activities of simplified aplysiatoxin analogs focused on the CH/π interaction

Debromoaplysiatoxin (DAT) is a potent protein kinase C (PKC) activator with tumor-promoting and pro-inflammatory activities. Irie and colleagues have found that 10-methyl-aplog-1 (1), a simplified analog of DAT, has strong anti-proliferative activity against several cancer cell lines with few adverse effects. Therefore, 1 is a potential lead compound for cancer therapy. We synthesized a new derivative 2 which has a naphthalene ring at the side chain terminal position instead of a benzene ring, to increase CH/π interactions with Pro-241 of the PKCδ-C1B domain. Based on the synthetic route of 1, 2 was convergently synthesized in 26 linear steps from 6-hydroxy-1-naphthoic acid with an overall yield of 0.18%. Although the anti-proliferative activity of 2 was more potent than that of 1, the binding potency of 2 to the PKCδ-C1B domain did not exceed that of 1. Molecular dynamics simulation indicated the capability of 2 to simultaneously form hydrogen bonds and CH/π interactions with the PKCδ-C1B domain. Focusing on the hydrogen bonds, their geometry in the binding modes involving the CH/π interactions seemed to be sub-optimal, which may explain the slightly lower affinity of 2 compared to 1. This study could be of help in optimizing such interactions and synthesizing a promising lead cancer compound.

Marine Anticancer Agents: An Overview with a Particular Focus on Their Chemical Classes

The marine environment is a rich source of biologically active molecules for the treatment of human diseases, especially cancer. The adaptation to unique environmental conditions led marine organisms to evolve different pathways than their terrestrial counterparts, thus producing unique chemicals with a broad diversity and complexity. So far, more than 36,000 compounds have been isolated from marine micro- and macro-organisms including but not limited to fungi, bacteria, microalgae, macroalgae, sponges, corals, mollusks and tunicates, with hundreds of new marine natural products (MNPs) being discovered every year. Marine-based pharmaceuticals have started to impact modern pharmacology and different anti-cancer drugs derived from marine compounds have been approved for clinical use, such as: cytarabine, vidarabine, nelarabine (prodrug of ara-G), fludarabine phosphate (pro-drug of ara-A), trabectedin, eribulin mesylate, brentuximab vedotin, polatuzumab vedotin, enfortumab vedotin, belantamab mafodotin, plitidepsin, and lurbinectedin. This review focuses on the bioactive molecules derived from the marine environment with anticancer activity, discussing their families, origin, structural features and therapeutic use.

Function-Oriented Synthesis: Design, Synthesis, and Evaluation of Highly Simplified Bryostatin Analogues

Using a function-oriented synthesis strategy, we designed, synthesized, and evaluated the simplest bryostatin 1 analogues reported to date, in which bryostatin's A- and B-rings are replaced by a glutarate linker. These analogues, one without and one with a C26-methyl group, exhibit remarkably different protein kinase C (PKC) isoform affinities. The former exhibited bryostatin-like binding to several PKC isoforms with K_i's < 5 nM, while the latter exhibited PKC affinities that were up to ∼180-fold less potent. The analogue with bryostatin-like PKC affinities also exhibited bryostatin-like PKC translocation kinetics in vitro, indicating rapid cell permeation and engagement of its PKC target. This study exemplifies the power of function-oriented synthesis in reducing structural complexity by activity-informed design, thus enhancing synthetic accessibility, while still maintaining function (biological activity), collectively providing new leads for addressing the growing list of therapeutic indications exhibited by PKC modulators.

Functional proteomic analysis reveals roles for PKCδ in regulation of cell survival and cell death: Implications for cancer pathogenesis and therapy

Protein Kinase C-δ (PKCδ), regulates a broad group of biological functions and disease processes, including well-defined roles in immune function, cell survival and apoptosis. PKCδ primarily regulates apoptosis in normal tissues and non-transformed cells, and genetic disruption of the PRKCD gene in mice is protective in many diseases and tissue damage models. However pro-survival/pro-proliferative functions have also been described in some transformed cells and in mouse models of cancer. Recent evidence suggests that the contribution of PKCδ to specific cancers may depend in part on the oncogenic context of the tumor, consistent with its paradoxical role in cell survival and cell death. Here we will discuss what is currently known about biological functions of PKCδ and potential paradigms for PKCδ function in cancer. To further understand mechanisms of regulation by PKCδ, and to gain insight into the plasticity of PKCδ signaling, we have used functional proteomics to identify pathways that are dependent on PKCδ. Understanding how these distinct functions of PKCδ are regulated will be critical for the logical design of therapeutics to target this pathway.

Unconventional Macrocyclizations in Natural Product Synthesis

Over the past several decades, macrocyclic compounds have emerged as increasingly significant therapeutic candidates in drug discovery. Their pharmacological activity hinges on their rotationally restricted three-dimensional orientation, resulting in a unique conformational preorganization and a high enthalpic gain as a consequence of high-affinity macrocycle-protein binding interactions. Synthetic access to macrocyclic drug candidates is therefore crucial. From a synthetic point of view, the efficiency of macrocyclization events commonly suffers from entropic penalties as well as undesired intermolecular couplings (oligomerization). Although over the past several decades ring-closing metathesis, macrolactonization, or macrolactamization have become strategies of choice, the toolbox of organic synthesis provides a great number of versatile transformations beyond the aforementioned. This Outlook focuses on a selection of examples employing what we term unconventional macrocyclizations toward the synthesis of natural products or analogues.

Experimental Systems for Measuring HIV Latency and Reactivation

The final obstacle to achieving a cure to HIV/AIDS is the presence of latent HIV reservoirs scattered throughout the body. Although antiretroviral therapy maintains plasma viral loads below the levels of detection, upon cessation of therapy, the latent reservoir immediately produces infectious progeny viruses. This results in elevated plasma viremia, which leads to clinical progression to AIDS. Thus, if a HIV cure is ever to become a reality, it will be necessary to target and eliminate the latent reservoir. To this end, tremendous effort has been dedicated to locate the viral reservoir, understand the mechanisms contributing to latency, find optimal methods to reactivate HIV, and specifically kill latently infected cells. Although we have not yet identified a therapeutic approach to completely eliminate HIV from patients, these efforts have provided many technological breakthroughs in understanding the underlying mechanisms that regulate HIV latency and reactivation in vitro. In this review, we summarize and compare experimental systems which are frequently used to study HIV latency. While none of these models are a perfect proxy for the complex systems at work in HIV+ patients, each aim to replicate HIV latency in vitro.

The quest for supernatural products: the impact of total synthesis in complex natural products medicinal chemistry

Covering: 2000 up to 2020This review presents select recent advances in the medicinal chemistry of complex natural products that are prepared by total synthesis. The underlying studies highlight enabling divergent synthetic strategies and methods that permit the systematic medicinal chemistry studies of key analogues bearing deep-seated structural changes not readily accessible by semisynthetic or biosynthetic means. Select and recent examples are detailed where the key structural changes are designed to improve defined properties or to overcome an intrinsic limitation of the natural product itself. In the examples presented, the synthetic efforts provided supernatural products, a term first introduced by our colleague Ryan Shenvi (Synlett, 2016, 27, 1145-1164), with properties superseding the parent natural product. The design principles and approaches for creating the supernatural products are highlighted with an emphasis on the properties addressed that include those that improve activity or potency, increase selectivity, enhance durability, broaden the spectrum of activity, improve chemical or metabolic stability, overcome limiting physical properties, add mechanisms of action, enhance PK properties, overcome drug resistance, and/or improve in vivo efficacy. Some such improvements may be regarded by some as iterative enhancements whereas others, we believe, truly live up to their characterization as supernatural products. Most such efforts are also accompanied by advances in synthetic organic chemistry, inspiring the development of new synthetic methodology and providing supernatural products with improved synthetic accessibility.

Development of New Innovative Synthetic Organic Chemistry Using Lone Pairs of Oxygen Atoms

Oxygen atoms have a lone pair of electrons, so they have high chelation ability, high nucleophilic ability, stabilizing ability of adjacent cations, and take a chelate or oxocarbenium ion structure with Lewis acids and metals. I took advantage of these properties to develop three new reactions, 1) asymmetric synthesis of chiral quaternary carbon centers, 2) asymmetric synthesis using acetal functions, and 3) organic chemistry using acetal-type reactive salt chemical species, and applied them to biologically active natural products synthesis. New reactions described here are all innovative and useful for natural products synthesis. In particular, the first asymmetric synthesis of fredericamycin A, and concise asymmetric synthesis of anthracycline antibiotics, scyphostatin, (+)-Sch 642305, (-)-stenine, clavolonine, (+)-rubrenolide, (+)-rubrynolide, (+)-centrolobine, and decytospolide A and B, etc., are noteworthy. Furthermore, since reactions using acetal-type reactive salt chemical species allow the coexistence of functional groups that normally cannot coexist, the reactions using reactive salts have potential to change the retrosynthesis planned based on conventional reactions.

Soft Heteroleptic N-Heterocyclic Carbene Palladium(II) Species for Efficient Catalytic Routes to Alkynones via Carbonylative Sonogashira Coupling

N,N'-Substituted di-isopropyl (NHC-1), benzyl-isopropyl (NHC-2), and adamantyl-isopropyl (NHC-3) benzimidazolium salts react with palladium(II) bromide in pyridine to afford the corresponding trans-dibromidopyridinepalladium(II) complexes Pd-C1, Pd-C2, and Pd-C3 in high yields. A distorted square planar geometry for Pd-C2 and Pd-C3 was confirmed by single-crystal X-ray diffraction. The palladium(II) complexes show a remarkably higher catalytic activity and selectivity, compared to the literature data, in carbonylative Sonogashira coupling reactions of aryl iodides and aryl diiodides with aryl alkynes, alkyl alkynes, and dialkynes. Excellent yields with as low as 0.03 mol % loading of the catalyst were obtained. In the series of benzimidazolium (NHC) precursors, the ¹H NMR signals of the α hydrogen show a consistent probing of the N-substituent donor strength. The density functional theory (DFT) quantum mechanical descriptors of the frontier orbitals were calculated. A linear correlation of the calculated absolute softness of the complexes versus the calculated percent buried volume (%V _bur) of their corresponding ligands was obtained. The catalytic activity experimental data are consistent with the hard soft acid base (HSAB)-predicted high affinity of the softest Pd-C3 complex for soft substrates, such as aryl iodides.

Stereoselective Synthesis of Trisubstituted (Z)-Alkenes from Ketones via the Julia-Kocienski Olefination Using 1-Methyl- and 1-tert-Butyl-1H-tetrazol-5-yl Alkyl Sulfones

1-Methyl-1H-tetrazol-5-yl (MT) alkyl sulfones 1b, 3b, and 7b react with various unsymmetrical ketones in the presence of LiHMDS in THF at low temperature to give trisubstituted (Z)-alkenes in good yields stereoselectively (Z/E = 91:9 to 99:1). For sterically less demanding ketones, olefination using t-Bu reagents 1d and 3d generated (Z)-alkenes with higher stereoselectivity (93:7-99:1).

Total synthesis of bryostatin 3

Bryostatins are a family of 21 complex marine natural products with a wide range of potent biological activities. Among all the 21 bryostatins, bryostatin 3 is structurally the most complex. Whereas nine total syntheses of bryostatins have been achieved to date, bryostatin 3 has only been targeted once and required the highest number of steps to synthesize (43 steps in the longest linear sequence and 88 total steps). Here, we report a concise total synthesis of bryostatin 3 using 22 steps in the longest linear sequence and 31 total steps through a highly convergent synthetic plan by the use of highly atom-economical and chemoselective transformations in which alkynes played a major role in reducing step count.

Bryostatin 1 Promotes Synaptogenesis and Reduces Dendritic Spine Density in Cortical Cultures through a PKC-Dependent Mechanism

The marine natural product bryostatin 1 has demonstrated procognitive and antidepressant effects in animals and has been entered into human clinical trials for treating Alzheimer's disease (AD). The ability of bryostatin 1 to enhance learning and memory has largely been attributed to its effects on the structure and function of hippocampal neurons. However, relatively little is known about how bryostatin 1 influences the morphology of cortical neurons, key cells that also support learning and memory processes and are negatively impacted in AD. Here, we use a combination of carefully designed chemical probes and pharmacological inhibitors to establish that bryostatin 1 increases cortical synaptogenesis while decreasing dendritic spine density in a protein kinase C (PKC)-dependent manner. The effects of bryostatin 1 on cortical neurons are distinct from those induced by neural plasticity-promoting psychoplastogens such as ketamine. Compounds capable of increasing synaptic density with concomitant loss of immature dendritic spines may represent a unique pharmacological strategy for enhancing memory by improving signal-to-noise ratio in the central nervous system.

Prodrugs of PKC modulators show enhanced HIV latency reversal and an expanded therapeutic window

AIDS is a pandemic disease caused by HIV that affects 37 million people worldwide. Current antiretroviral therapy slows disease progression but does not eliminate latently infected cells, which resupply active virus, thus necessitating lifelong treatment with associated compliance, cost, and chemoexposure issues. Latency-reversing agents (LRAs) activate these cells, allowing for their potential clearance, thus presenting a strategy to eradicate the infection. Protein kinase C (PKC) modulators-including prostratin, ingenol esters, bryostatin, and their analogs-are potent LRAs in various stages of development for several clinical indications. While LRAs are promising, a major challenge associated with their clinical use is sustaining therapeutically meaningful levels of the active agent while minimizing side effects. Here we describe a strategy to address this problem based on LRA prodrugs, designed for controllable release of the active LRA after a single injection. As intended, these prodrugs exhibit comparable or superior in vitro activity relative to the parent compounds. Selected compounds induced higher in vivo expression of CD69, an activation biomarker, and, by releasing free agent over time, significantly improved tolerability when compared to the parent LRAs. More generally, selected prodrugs of PKC modulators avoid the bolus toxicities of the parent drug and exhibit greater efficacy and expanded tolerability, thereby addressing a longstanding objective for many clinical applications.

Synthesis and evaluation of designed PKC modulators for enhanced cancer immunotherapy

Bryostatin 1 is a marine natural product under investigation for HIV/AIDS eradication, the treatment of neurological disorders, and enhanced CAR T/NK cell immunotherapy. Despite its promising activity, bryostatin 1 is neither evolved nor optimized for the treatment of human disease. Here we report the design, synthesis, and biological evaluation of several close-in analogs of bryostatin 1. Using a function-oriented synthesis approach, we synthesize a series of bryostatin analogs designed to maintain affinity for bryostatin’s target protein kinase C (PKC) while enabling exploration of their divergent biological functions. Our late-stage diversification strategy provides efficient access to a library of bryostatin analogs, which per our design retain affinity for PKC but exhibit variable PKC translocation kinetics. We further demonstrate that select analogs potently increase cell surface expression of CD22, a promising CAR T cell target for the treatment of leukemias, highlighting the clinical potential of bryostatin analogs for enhancing targeted immunotherapies.

Bryostatin 1 is a unique therapeutic lead, however its scarce natural sources have hampered its use in treatment of human disease. Here, the authors use a scalable synthesis of bryostatin 1 to make close-in analogs which potently induce increased cell surface expression holding potential for immunotherapy.

Synthesis of Karrikinolide Using the Aldol-Type Acetal Addition Reaction

A short step total synthesis of karrikinolide has been achieved. Both α and α' positions of O-acylated acetol were alkylated by the boron-mediated aldol-type acetal addition reaction. The one-pot sequence including the Arbuzov reaction, intramolecular Horner-Wadsworth-Emmons reaction, acidic hydrolysis of acetals, and pyran formation provided karrikinolide. This procedure was applicable to the gram-scale synthesis of karrikinolide.

Evaluation of Toxic Amyloid 42 Oligomers in Rat Primary Cerebral Cortex Cells and Human iPS-derived Neurons Treated with 10-Me-Aplog-1, a New PKC Activator

Amyloid β42 (Aβ42), a causative agent of Alzheimer’s disease (AD), is derived extracellularly from Aβ precursor protein (APP) following the latter’s cleavage by β-secretase, but not α-secretase. Protein kinase Cα (PKCα) activation is known to increase α-secretase activity, thereby suppressing Aβ production. Since Aβ42 oligomer formation causes potent neurotoxicity, APP modulation by PKC ligands is a promising strategy for AD treatment. Although bryostatin-1 (bryo-1) is a leading compound for this strategy, its limited natural availability and the difficulty of its total synthesis impedes further research. To address this limitation, Irie and colleagues have developed a new PKC activator with few side effects, 10-Me-Aplog-1, (1), which decreased Aβ42 in the conditioned medium of rat primary cerebral cortex cells. These results are associated with increased α-secretase but not PKCε-dependent Aβ-degrading enzyme. The amount of neuronal embryonic lethal abnormal vision (nELAV), a known β-secretase stabilizer, was reduced by treatment with 1. Notably, 1 prevented the formation of intracellular toxic oligomers. Furthermore, 1 suppressed toxic oligomerization within human iPS-derived neurons such as bryo-1. Given that 1 was not neurotoxic toward either cell line, these findings suggest that 1 is a potential drug lead for AD therapy.

New diagnostic method for Alzheimer’s disease based on the toxic conformation theory of amyloid β

Recent investigations suggest that soluble oligomeric amyloid β (Aβ) species may be involved in early onset of Alzheimer’s disease (AD). Using systematic proline replacement, solid-state NMR, and ESR, we identified a toxic turn at position 22 and 23 of Aβ42, the most potent neurotoxic Aβ species. Through radicalization, the toxic turn can induce formation of the C-terminal hydrophobic core to obtain putative Aβ42 dimers and trimers. Synthesized dimer and trimer models showed that the C-terminal hydrophobic core plays a critical role in the formation of high molecular weight oligomers with neurotoxicity. Accordingly, an anti-toxic turn antibody (24B3) that selectively recognizes a toxic dimer model of E22P-Aβ42 was developed. Sandwich enzyme-linked immunosorbent assay with 24B3 and 82E1 detected a significantly higher ratio of Aβ42 with a toxic turn to total Aβ42 in cerebrospinal fluid of AD patients compared with controls, suggesting that 24B3 could be useful for early onset of AD diagnosis.

Unlocking the Drug Potential of the Bryostatin Family: Recent Advances in Product Synthesis and Biomedical Applications

Bryostatins are a class of naturally occurring macrocyclic lactones with a unique fast developing portfolio of clinical applications, including treatment of AIDS, Alzheimer's disease, and cancer. This comprehensive account summarizes the recent progress (2014-present) in the development of bryostatins, including their total synthesis and biomedical applications. An emphasis is placed on the discussion of bryostatin 1, the most-studied analogue to date. This review highlights the synthetic and biological challenges of bryostatins and provides an outlook on their future development.

Natural Products as a Foundation for Drug Discovery

Many natural products have been used as drugs for the treatment of diverse indications. Although most U.S. pharmaceutical companies have reduced or eliminated their in-house natural-product research over the years, new approaches for compound screening and chemical synthesis are resurrecting interest in exploring the therapeutic value of natural products. The aim of this commentary is to review emerging strategies and techniques that have made natural products a viable strategic choice for inclusion in drug discovery programs. Published 2019. U.S. Government.

Munc13 Is a Molecular Target of Bryostatin 1

Bryostatin 1 is a natural macrolide shown to improve neuronal connections and enhance memory in mice. Its mechanism of action is largely attributed to the modulation of novel and conventional protein kinase Cs (PKCs) by binding to their regulatory C1 domains. Munc13-1 is a C1 domain-containing protein that shares common endogenous and exogenous activators with novel and conventional PKC subtypes. Given the essential role of Munc13-1 in the priming of synaptic vesicles and neuronal transmission overall, we explored the potential interaction between bryostatin 1 and Munc13-1. Our results indicate that in vitro bryostatin 1 binds to both the isolated C1 domain of Munc13-1 (K_i = 8.07 ± 0.90 nM) and the full-length Munc13-1 protein (K_i = 0.45 ± 0.04 nM). Furthermore, confocal microscopy and immunoblot analysis demonstrated that in intact HT22 cells bryostatin 1 mimics the actions of phorbol esters, a previously established class of Munc13-1 activators, and induces plasma membrane translocation of Munc13-1, a hallmark of its activation. Consistently, bryostatin 1 had no effect on the Munc13-1^H567K construct that is insensitive to phorbol esters. Effects of bryostatin 1 on the other Munc13 family members, ubMunc13-2 and bMunc13-2, resembled those of Munc13-1 for translocation. Lastly, we observed an increased level of expression of Munc13-1 following a 24 h incubation with bryostatin 1 in both HT22 and primary mouse hippocampal cells. This study characterizes Munc13-1 as a molecular target of bryostatin 1. Considering the crucial role of Munc13-1 in neuronal function, these findings provide strong support for the potential role of Munc13s in the actions of bryostatin 1.

Algae metabolites: from in vitro growth inhibitory effects to promising anticancer activity

Covering: 1957 to 2017 Algae constitute a heterogeneous group of eukaryotic photosynthetic organisms, mainly found in the marine environment. Algae produce numerous metabolites that help them cope with the harsh conditions of the marine environment. Because of their structural diversity and uniqueness, these molecules have recently gained a lot of interest for the identification of medicinally useful agents, including those with potential anticancer activities. In the current review, which is not a catalogue-based one, we first highlight the major biological events that lead to various types of cancer, including metastatic ones, to chemoresistance, thus to any types of current anticancer treatment relating to the use of chemotherapeutics. We then review algal metabolites for which scientific literature reports anticancer activity. Lastly, we focus on algal metabolites with promising anticancer activity based on their ability to target biological characteristics of cancer cells responsible for poor treatment outcomes. Thus, we highlight compounds that have, among others, one or more of the following characteristics: selectivity in reducing the proliferation of cancer cells over normal ones, potential for killing cancer cells through non-apoptotic signaling pathways, ability to circumvent MDR-related efflux pumps, and activity in vivo in relevant pre-clinical models.

Marine Spirotetronates: Biosynthetic Edifices That Inspire Drug Discovery

Spirotetronates are actinomyces-derived polyketides that possess complex structures and exhibit potent and unexplored bioactivities. Due to their anticancer and antimicrobial properties, they have potential as drug hits and deserve further study. In particular, abyssomicin C and tetrocarcin A have shown significant promise against antibiotic-resistant S. aureus and tuberculosis, as well as for the treatment of various lymphomas and solid tumors. Improved synthetic routes to these compounds, particularly the class II spirotetronates, are needed to access sufficient quantities for structure optimization and clinical applications.