Optimized Pyridazinone Nutrient Channel Inhibitors Are Potent and Specific Antimalarial Leads.

Human and animal malaria parasites increase their host erythrocyte permeability to a broad range of solutes as mediated by parasite-associated ion channels. Molecular and pharmacological studies have implicated an essential role in parasite nutrient acquisition, but inhibitors suitable for development of antimalarial drugs are missing. Here, we generated a potent and specific drug lead using Plasmodium falciparum, a virulent human pathogen, and derivatives of MBX-2366, a nanomolar affinity pyridazinone inhibitor from a high-throughput screen. As this screening hit lacks the bioavailability and stability needed for in vivo efficacy, we synthesized 315 derivatives to optimize drug-like properties, establish target specificity, and retain potent activity against the parasite-induced permeability. Using a robust, iterative pipeline, we generated MBX-4055, a derivative active against divergent human parasite strains. MBX-4055 has improved oral absorption with acceptable in vivo tolerability and pharmacokinetics. It also has no activity against a battery of 35 human channels and receptors and is refractory to acquired resistance during extended in vitro selection. Single-molecule and single-cell patch-clamp indicate direct action on the plasmodial surface anion channel, a channel linked to parasite-encoded RhopH proteins. These studies identify pyridazinones as novel and tractable antimalarial scaffolds with a defined mechanism of action. SIGNIFICANCE STATEMENT: Because antimalarial drugs are prone to evolving resistance in the virulent human P. falciparum pathogen, new therapies are needed. This study has now developed a novel drug-like series of pyridazinones that target an unexploited parasite anion channel on the host cell surface, display excellent in vitro and in vivo ADME properties, are refractory to acquired resistance, and demonstrate a well defined mechanism of action.

Structure-activity relationships of a novel pyranopyridine series of Gram-negative bacterial efflux pump inhibitors.

Recently we described a novel pyranopyridine inhibitor (MBX2319) of RND-type efflux pumps of the Enterobacteriaceae. MBX2319 (3,3-dimethyl-5-cyano-8-morpholino-6-(phenethylthio)-3,4-dihydro-1H-pyrano[3,4-c]pyridine) is structurally distinct from other known Gram-negative efflux pump inhibitors (EPIs), such as 1-(1-naphthylmethyl)-piperazine (NMP), phenylalanylarginine-ß-naphthylamide (PAßN), D13-9001, and the pyridopyrimidine derivatives. Here, we report the synthesis and biological evaluation of 60 new analogs of MBX2319 that were designed to probe the structure activity relationships (SARs) of the pyranopyridine scaffold. The results of these studies produced a molecular activity map of the scaffold, which identifies regions that are critical to efflux inhibitory activities and those that can be modified to improve potency, metabolic stability and solubility. Several compounds, such as 22d-f, 22i and 22k, are significantly more effective than MBX2319 at potentiating the antibacterial activity of levofloxacin and piperacillin against Escherichia coli.

Human cytomegalovirus UL97 kinase is involved in the mechanism of action of methylenecyclopropane analogs with 6-ether and -thioether substitutions.

Methylenecyclopropane nucleoside (MCPN) analogs are being investigated for treatment of human cytomegalovirus (HCMV) infection because of favorable preclinical data and limited ganciclovir cross-resistance. Monohydroxymethyl MCPNs bearing ether and thioether functionalities at the purine 6 position have antiviral activity against herpes simplex virus (HSV) and varicella-zoster virus (VZV) in addition to HCMV. The role of the HCMV UL97 kinase in the mechanism of action of these derivatives was examined. When tested against a kinase-inactive UL97 K355M virus, a moderate 5- to 7-fold increase in 50% effective concentration (EC50) was observed, in comparison to a 13- to 25-fold increase for either cyclopropavir or ganciclovir. Serial propagation of HCMV under two of these compounds selected for three novel UL97 mutations encoding amino acid substitutions D456N, C480R,and Y617del. When transferred to baseline laboratory HCMV strains, these mutations individually conferred resistance to all of the tested MCPNs, ganciclovir, and maribavir. However, the engineered strains also demonstrated severe growth defects and abnormal cytopathic effects similar to the kinase-inactive mutant. Expressed and purified UL97 kinase showed in vitro phosphorylation of the newly tested MCPNs. Thus, HCMV UL97 kinase is involved in the antiviral action of these MCPNs, but the in vitro selection of UL97-defective viruses suggests that their activity against more typical ganciclovir-resistant growth-competent UL97 mutants may be relatively preserved.

Small molecule inhibitors of anthrax lethal factor toxin.

This manuscript describes the preparation of new small molecule inhibitors of Bacillus anthracis lethal factor. Our starting point was the symmetrical, bis-quinolinyl compound 1 (NSC 12155). Optimization of one half of this molecule led to new LF inhibitors that were desymmetrized to afford more drug-like compounds.

Human herpesvirus 6 U69 kinase phosphorylates the methylenecyclopropane nucleosides cyclopropavir, MBX 2168, and MBX 1616 to their monophosphates.

Dihydroxymethyl and monohydroxymethyl methylenecyclopropane nucleosides are effective inhibitors of both variants of human herpesvirus 6 (HHV-6). We investigated involvement of HHV-6 U69 protein kinase in their mechanism of action. Phosphorylation of the dihydroxymethyl analogue cyclopropavir and monohydroxymethyl nucleosides with either a 6-ether moiety (MBX 2168) or a 6-thioether moiety (MBX 1616) with purified U69 was examined. All three compounds were substrates of this viral kinase and had similar Michaelis-Menten kinetic parameters.

Investigating N-arylpyrimidinamine (NAPA) compounds as early-stage inhibitors against human cytomegalovirus.

Human cytomegalovirus (CMV) is a ubiquitous ß-herpesvirus that establishes latent asymptomatic infections in healthy individuals but can cause serious infections in immunocompromised people, resulting in increased risk of morbidity and mortality. The current FDA-approved CMV drugs target late stages of the CMV life-cycle. While these drugs are effective in most cases, they have serious drawbacks, including poor oral bioavailability, dose-limiting toxicity, and a low barrier to resistance. Given the clinical relevance of CMV-associated diseases, novel therapies are needed. Thus, a novel class of compounds that inhibits the early stages of the CMV life-cycle was identified and found to block infection of different strains in physiologically relevant cell types. This class of compounds, N-arylpyrimidinamine (NAPA), demonstrated potent anti-CMV activity against ganciclovir-sensitive and -resistant strains in in vitro replication assays, a selectivity index >30, and favorable in vitro ADME properties. Mechanism of action studies demonstrated that NAPA compounds inhibit an early step of virus infection. NAPA compounds are specific inhibitors of cytomegaloviruses and exhibited limited anti-viral activity against other herpesviruses. Collectively, we have identified a novel class of CMV inhibitor that effectively limits viral infection and proliferation.

Identification of a small-molecule entry inhibitor for filoviruses.

Ebola virus (EBOV) causes severe hemorrhagic fever, for which therapeutic options are not available. Preventing the entry of EBOV into host cells is an attractive antiviral strategy, which has been validated for HIV by the FDA approval of the anti-HIV drug enfuvirtide. To identify inhibitors of EBOV entry, the EBOV envelope glycoprotein (EBOV-GP) gene was used to generate pseudotype viruses for screening of chemical libraries. A benzodiazepine derivative (compound 7) was identified from a high-throughput screen (HTS) of small-molecule compound libraries utilizing the pseudotype virus. Compound 7 was validated as an inhibitor of infectious EBOV and Marburg virus (MARV) in cell-based assays, with 50% inhibitory concentrations (IC(50)s) of 10 µM and 12 µM, respectively. Time-of-addition and binding studies suggested that compound 7 binds to EBOV-GP at an early stage during EBOV infection. Preliminary Schrödinger SiteMap calculations, using a published EBOV-GP crystal structure in its prefusion conformation, suggested a hydrophobic pocket at or near the GP1 and GP2 interface as a suitable site for compound 7 binding. This prediction was supported by mutational analysis implying that residues Asn69, Leu70, Leu184, Ile185, Leu186, Lys190, and Lys191 are critical for the binding of compound 7 and its analogs with EBOV-GP. We hypothesize that compound 7 binds to this hydrophobic pocket and as a consequence inhibits EBOV infection of cells, but the details of the mechanism remain to be determined. In summary, we have identified a novel series of benzodiazepine compounds that are suitable for optimization as potential inhibitors of filoviral infection.

Time-dependent botulinum neurotoxin serotype A metalloprotease inhibitors.

Botulinum neurotoxins (BoNTs) are the most lethal of biological substances, and are categorized as class A biothreat agents by the Centers for Disease Control and Prevention. There are currently no drugs to treat the deadly flaccid paralysis resulting from BoNT intoxication. Among the seven BoNT serotypes, the development of therapeutics to counter BoNT/A is a priority (due to its long half-life in the neuronal cytosol and its ease of production). In this regard, the BoNT/A enzyme light chain (LC) component, a zinc metalloprotease responsible for the intracellular cleavage of synaptosomal-associated protein of 25 kDa, is a desirable target for developing post-BoNT/A intoxication rescue therapeutics. In an earlier study, we reported the high throughput screening of a library containing 70,000 compounds, and uncovered a novel class of benzimidazole acrylonitrile-based BoNT/A LC inhibitors. Herein, we present both structure-activity relationships and a proposed mechanism of action for this novel inhibitor chemotype.

Filociclovir Is an Active Antiviral Agent against Ocular Adenovirus Isolates In Vitro and in the Ad5/NZW Rabbit Ocular Model.

Presently, there is no FDA- or EMA-approved antiviral for the treatment of human adenovirus (HAdV) ocular infections. This study determined the antiviral activity of filociclovir (FCV) against ocular HAdV isolates in vitro and in the Ad5/NZW rabbit ocular model. The 50% effective concentrations (EC50) of FCV and cidofovir (CDV) were determined for several ocular HAdV types using standard plaque reduction assays. Rabbits were topically inoculated in both eyes with HAdV5. On day 1, the rabbits were divided into four topical treatment groups: (1) 0.5% FCV 4x/day × 10 d; (2) 0.1% FCV 4x/day × 10 d; (3) 0.5% CDV 2x/day × 7 d; (4) vehicle 4x/day × 10 d. Eyes were cultured for virus on days 0, 1, 3, 4, 5, 7, 9, 11, and 14. The resulting viral eye titers were determined using standard plaque assays. The mean in vitro EC50 for FCV against tested HAdV types ranged from 0.50 to 4.68 µM, whereas those treated with CDV ranged from 0.49 to 30.3 µM. In vivo, compared to vehicle, 0.5% FCV, 0.1% FCV, and 0.5% CDV produced lower eye titers, fewer numbers of positive eye cultures, and shorter durations of eye infection. FCV demonstrated anti-adenovirus activity in vitro and in vivo.

trans-Translation inhibitors bind to a novel site on the ribosome and clear Neisseria gonorrhoeae in vivo.

Bacterial ribosome rescue pathways that remove ribosomes stalled on mRNAs during translation have been proposed as novel antibiotic targets because they are essential in bacteria and are not conserved in humans. We previously reported the discovery of a family of acylaminooxadiazoles that selectively inhibit trans-translation, the main ribosome rescue pathway in bacteria. Here, we report optimization of the pharmacokinetic and antibiotic properties of the acylaminooxadiazoles, producing MBX-4132, which clears multiple-drug resistant Neisseria gonorrhoeae infection in mice after a single oral dose. Single particle cryogenic-EM studies of non-stop ribosomes show that acylaminooxadiazoles bind to a unique site near the peptidyl-transfer center and significantly alter the conformation of ribosomal protein bL27, suggesting a novel mechanism for specific inhibition of trans-translation by these molecules. These results show that trans-translation is a viable therapeutic target and reveal a new conformation within the bacterial ribosome that may be critical for ribosome rescue pathways.

Design, Synthesis, and Evaluation of Novel Prodrugs of Transition State Inhibitors of Norovirus 3CL Protease.

Ester and carbamate prodrugs of aldehyde bisulfite adduct inhibitors were synthesized in order to improve their pharmacokinetic and pharmacodynamic properties. The inhibitory activity of the compounds against norovirus 3C-like protease in enzyme and cell-based assays was determined. The ester and carbamate prodrugs displayed equivalent potency to those of the precursor aldehyde bisulfite adducts and precursor aldehydes. Furthermore, the rate of ester cleavage was found to be dependent on alkyl chain length. The generated prodrugs exhibited low cytotoxicity and satisfactory liver microsomes stability and plasma protein binding. The methodology described herein has wide applicability and can be extended to the bisulfite adducts of common warheads employed in the design of transition state inhibitors of serine and cysteine proteases of medical relevance.

Molecular Mechanism Underlying the Action of Influenza A Virus Fusion Inhibitor MBX2546.

Influenza A virus envelop protein hemagglutinin (HA) plays important roles in viral entry. We previously have reported that MBX2546, a novel influenza A virus inhibitor, binds to HA and inhibits HA-mediated membrane fusion. In this report, we show that (i) both binding and stabilization of HA by MBX2546 are required for the inhibition of viral infection and (ii) the binding of HA by MBX2546 represses the low-pH-induced conformational change in the HA, which is a prerequisite for membrane fusion. Mutations in MBX2546-resistant influenza A/PR/8/34 (H1N1) viruses are mapped in the HA stem region near the amino terminus of HA2. Finally, we have modeled the binding site of MBX2546 using molecular dynamics and find that the resulting structure is in good agreement with our results. Together, these studies underscore the importance of the HA stem loop region as a potential target for therapeutic intervention.

TAOK3 phosphorylates the methylenecyclopropane nucleoside MBX 2168 to its monophosphate.

Monohydroxymethyl methylenecyclopropane nucleosides (MCPNs) with ether or thioether substituents at the 6-position show promise as broad-spectrum herpes virus inhibitors. Their proposed mechanism of action involves sequential phosphorylation to a triphosphate, which can then inhibit viral DNA polymerase. The inhibition of herpes simplex virus (HSV) by these compounds is not dependent on the viral thymidine kinase (TK), which is known to phosphorylate acyclovir (ACV), a standard treatment for HSV infections. Previous studies on the mechanism of action of these compounds against human cytomegalovirus (HCMV) implicated a host kinase in addition to HCMV UL97 kinase in performing the initial phosphorylation. After first eliminating other candidate HSV-1 encoded kinases (UL13 and US3) as well as potential host nucleoside kinases, using activity-based fractionation, we have now identified the host serine-threonine protein kinase TAOK3 as the kinase responsible for transforming the representative monohydroxymethyl MCPN analog MBX 2168 to its monophosphate.

Novel Benzimidazole Inhibitors of Botulinum Neurotoxin/A Display Enzyme and Cell-Based Potency.

Botulinum Neurotoxins (BoNTs) are used therapeutically and in cosmetics, providing potential for bioterrorist activity, thus driving the search for small-molecule BoNT inhibitors. This report describes a 70,000-compound screen for inhibition of BoNT/A using a FRET assay to detect proteolysis of a peptide substrate. Hits were confirmed, followed by evaluation to determine compound specificity. Inhibitors fell into three main chemical classes, and on the basis of potency and specificity of inhibition, the activities of two chemotypes were examined further. Compounds exhibited specificity for BoNT/A LC inhibition with respect to other metalloproteases and displayed activity in a neuronal assay for botulinum intoxication.

Analysis of Botulinum Neurotoxin Serotype A Metalloprotease Inhibitors: Analogs of a Chemotype for Therapeutic Development in the Context of a Three-Zone Pharmacophore.

Botulinum neurotoxins (BoNTs), and in particular serotype A, are the most poisonous of known biological substances, and are responsible for the flaccid paralysis of the disease state botulism. Because of the extreme toxicity of these enzymes, BoNTs are considered highest priority biothreat agents. To counter BoNT serotype A (BoNT/A) poisoning, the discovery and development of small molecule, drug-like inhibitors as post-intoxication therapeutic agents has been/is being pursued. Specifically, we are focusing on inhibitors of the BoNT/A light chain (LC) (ie, a metalloprotease) subunit, since such compounds can enter neurons and provide post-intoxication protection of the enzyme target substrate. To aid/facilitate this drug development effort, a pharmacophore for inhibition of the BoNT/A LC subunit was previously developed, and is continually being refined via the incorporation of novel and diverse inhibitor chemotypes. Here, we describe several analogs of a promising therapeutic chemotype in the context of the pharmacophore for BoNT/A LC inhibition. Specifically, we describe: 1) the pharmacophoric 'fits' of the analogs and how these 'fits' rationalize the in vitro inhibitory potencies of the analogs and 2) pharmacophore refinement via the inclusion of new components from the most potent of the presented analogs.

Synthesis and biological evaluation of botulinum neurotoxin a protease inhibitors.

NSC 240898 was previously identified as a botulinum neurotoxin A light chain (BoNT/A LC) endopeptidase inhibitor by screening the National Cancer Institute Open Repository diversity set. Two types of analogues have been synthesized and shown to inhibit BoNT/A LC in a FRET-based enzyme assay, with confirmation in an HPLC-based assay. These two series of compounds have also been evaluated for inhibition of anthrax lethal factor (LF), an unrelated metalloprotease, to examine enzyme specificity of the BoNT/A LC inhibition. The most potent inhibitor against BoNT/A LC in these two series is compound 12 (IC(50) = 2.5 microM, FRET assay), which is 4.4-fold more potent than the lead structure and 11.2-fold more selective for BoNT/A LC versus the anthrax LF metalloproteinase. Structure-activity relationship studies have revealed structural features important to potency and enzyme specificity.