Discovery and mode of action of afoxolaner, a new isoxazoline parasiticide for dogs.

Afoxolaner is an isoxazoline compound characterized by a good safety profile and extended effectiveness against fleas and ticks on dogs following a single oral administration. In vitro membrane feeding assay data and in vivo pharmacokinetic studies in dogs established an afoxolaner blood concentration of 0.1-0.2 µg/ml to be effective against both fleas (Ctenocephalides felis) and ticks (Dermacentor variabilis). Pharmacokinetic profiles in dogs following a 2.5mg/kg oral dosage demonstrated uniform and predictable afoxolaner plasma concentrations above threshold levels required for efficacy for more than one month. Dose ranging and a 5-month multi-dose experimental study in dogs, established that the 2.5mg/kg oral dosage was highly effective against fleas and ticks, and produced predictable and reproducible pharmacokinetics following repeated dosing. Mode of action studies showed that afoxolaner blocked native and expressed insect GABA-gated chloride channels with nanomolar potency. Afoxolaner has comparable potency between wild type channels and channels possessing the A302S (resistance-to-dieldrin) mutation. Lack of cyclodiene cross-resistance for afoxolaner was confirmed in comparative Drosophila toxicity studies, and it is concluded that afoxolaner blocked GABA-gated chloride channels via a site distinct from the cyclodienes.

Efficacy of a capsule conjugate vaccine against inhalational anthrax in rabbits and monkeys.

Bacillus anthracis, the causative agent of anthrax, is recognized as one of the most serious bioterrorism threats. The current human vaccines are based on the protective antigen component of the anthrax toxins. Concern about possible vaccine resistant strains and reliance on a single antigen has prompted the search for additional immunogens. Bacterial capsules, as surface-expressed virulence factors, are well-established components of several licensed vaccines. In a previous study we showed that an anthrax vaccine consisting of the B. anthracis poly-γ-D-glutamic acid capsule covalently conjugated to the outer membrane protein complex of Neisseria meningitidis serotype B protected mice against parenteral B. anthracis challenge. Here we tested this vaccine in rabbits and monkeys against an aerosol spore challenge. The vaccine induced anti-capsule antibody responses in both species, measured by ELISA and a macrophage opsono-adherence assay. While rabbits were not protected against a high aerosol challenge dose, significant protection was observed in monkeys receiving the capsule conjugate vaccine. The results confirm that the capsule is a protective immunogen against anthrax, being the first non-toxin antigen shown to be efficacious in monkeys and suggest that addition of capsule may broaden and enhance the protection afforded by protective antigen-based vaccines.

Discovery of the development candidate N-tert-butyl nodulisporamide: a safe and efficacious once monthly oral agent for the control of fleas and ticks on companion animals.

Nodulisporic acid A (1) is a structurally complex fungal metabolite that exhibits systemic efficacy against fleas via modulation of an invertebrate specific glutamate-gated ion channel. In order to identify a nodulisporamide suitable for monthly oral dosing in dogs, a library of 335 nodulisporamides was examined in an artificial flea feeding system for intrinsic systemic potency as well as in a mouse/bedbug assay for systemic efficacy and safety. A cohort of 66 nodulisporamides were selected for evaluation in a dog/flea model; pharmacokinetic analysis correlated plasma levels with flea efficacy. These efforts resulted in the identification of the development candidate N-tert-butyl nodulisporamide (3) as a potent and efficacious once monthly oral agent for the control of fleas and ticks on dogs and cats which was directly compared to the topical agents fipronil and imidacloprid, with favorable results obtained. Multidose studies over 3 months confirmed the in vivo ectoparasiticidal efficacy and established that 3 lacked overt mammalian toxicity. Tissue distribution studies in mice using [(14)C]-labeled 3 indicate that adipose beds serve as ligand depots, contributing to the long terminal half-lives of these compounds.

Comparative disposition and metabolism of paraherquamide in sheep, gerbils, and dogs.

The disposition and metabolism of paraherquamide (PHQ), a potent and broad-spectrum anthelminthic, were examined in sheep, dogs, and gerbils. The metabolism of PHQ in these species was extensive and marked by significant species differences both in vitro and in vivo. In sheep and gerbils, PHQ metabolism occurs mainly at the pyrrolidine moiety, generating several metabolites that, for the most part, retained nematodicidal activity in vitro. In dogs, the dioxepene group was also extensively metabolized, ultimately resulting in formation of a catechol and loss of pharmacological activity. After oral administration of [3H]PHQ to intact sheep, gerbils, and dogs, the majority of the administered radioactivity was recovered in feces. Intact PHQ accounted for 0% (dogs) to approximately 30% (sheep and gerbils) of drug-related material in feces. A detailed investigation of the composition of the intestinal content of sheep indicated that a significant amount of the dose was still present in the rumen 24 h after dose and that PHQ underwent significant dehydration in the cecum. The oral pharmacokinetic parameters of PHQ in sheep and dogs suggest that its absorption is rapid in both species but that its apparent elimination rate is significantly higher in the dog (t(1/2) approximately 1.5 h) than it is in sheep (t(1/2) approximately 8.5 h). The short elimination half-life and the absence of PHQ or other active components in the dog gastrointestinal tract provide a potential explanation of the lack of efficacy of PHQ in this species.

Isolation and insecticidal/anthelmintic activity of xanthonol, a novel bis-xanthone, from a non-sporulating fungal species.

Xanthonol, a novel dimeric xanthone, was isolated from a fermentation broth of a non-sporulating fungal species using Sephadex LH20 followed by HPLC and the structure elucidated by spectral analysis. Xanthonol exhibited insecticidal and anthelmintic activities against larvae of Lucilia sericata, Aedes aegypti, and Haemonchus contortus with LD90 of 33, 8, and 50 microg/ml, respectively.

Platensimycin is a selective FabF inhibitor with potent antibiotic properties.

Bacterial infection remains a serious threat to human lives because of emerging resistance to existing antibiotics. Although the scientific community has avidly pursued the discovery of new antibiotics that interact with new targets, these efforts have met with limited success since the early 1960s. Here we report the discovery of platensimycin, a previously unknown class of antibiotics produced by Streptomyces platensis. Platensimycin demonstrates strong, broad-spectrum Gram-positive antibacterial activity by selectively inhibiting cellular lipid biosynthesis. We show that this anti-bacterial effect is exerted through the selective targeting of beta-ketoacyl-(acyl-carrier-protein (ACP)) synthase I/II (FabF/B) in the synthetic pathway of fatty acids. Direct binding assays show that platensimycin interacts specifically with the acyl-enzyme intermediate of the target protein, and X-ray crystallographic studies reveal that a specific conformational change that occurs on acylation must take place before the inhibitor can bind. Treatment with platensimycin eradicates Staphylococcus aureus infection in mice. Because of its unique mode of action, platensimycin shows no cross-resistance to other key antibiotic-resistant strains tested, including methicillin-resistant S. aureus, vancomycin-intermediate S. aureus and vancomycin-resistant enterococci. Platensimycin is the most potent inhibitor reported for the FabF/B condensing enzymes, and is the only inhibitor of these targets that shows broad-spectrum activity, in vivo efficacy and no observed toxicity.

p38 MAP kinase inhibitors: metabolically stabilized piperidine-substituted quinolinones and naphthyridinones.

Quinolinones and naphthyridinones with C7 N-t-butyl piperidine substituents were found to be potent p38 MAP kinase inhibitors. These compounds significantly suppress TNF-alpha release in both cellular and LPS-stimulated whole blood assays. They also displayed excellent PK profiles across three animal species. Quinolinone at 10 mpk showed comparable oral efficacy to that of dexamethasone at 1 mpk in a murine collagen-induced arthritis model.

Immunogenicity and protective efficacy of Bacillus anthracis poly-gamma-D-glutamic acid capsule covalently coupled to a protein carrier using a novel triazine-based conjugation strategy.

The capsular polypeptide of Bacillus anthracis is composed of a unique polyglutamic acid polymer in which D-glutamate monomers are joined by gamma-peptidyl bonds. The capsule is poorly immunogenic, and efforts at exploiting the polymer for vaccine development have focused on increasing its inherent immunogenicity through chemical coupling to immune-stimulating protein carriers. The usual strategy has employed carbodiimide-based condensing reagents for activation of free alpha-carboxyl groups, despite reports that this chemistry may lead to chain scission. We have purified the high molecular mass capsule to >95% homogeneity and have demonstrated that the polymer contains >99% poly-gamma-D-glutamic acid. The predominant structure of the polymer as assessed by circular dichroism and multiangle laser light scattering was unordered at near-neutral pH. We investigated the effects of various activation chemistries, and we demonstrated that carbodiimide treatment under aqueous conditions results in significant cleavage of the gamma-peptidyl bond, whereas scission is significantly reduced in nonaqueous polar solvents, although undesired side chain modification was still observed. An activation chemistry was developed using the triazine-based reagent 4-(4,6-dimethoxy (1,3,5)triazin-2-yl)-4-methylmorpholinium chloride, which allowed for controlled and reproducible derivatization of alpha-carbonyls. In a two-pot reaction scheme, activated capsule was derivatized with a sulfhydryl-reactive heterobifunctional moiety and was subsequently coupled to thiolated carrier protein. This conjugate elicited very high capsule-specific immune titers in mice. More importantly, mice immunized with conjugated capsule exhibited good protection against lethal challenge from a virulent B. anthracis strain in two models of infection. We also showed, for the first time, that treatment of capsule with carbodiimide significantly reduced recognition by capsule-specific antisera concurrent with the reagent-induced reduction of polymer mass. The data suggested that for vaccine development, maintenance of the high mass of the polymer may be important.

Nodulisporic acids D-F: structure, biological activities, and biogenetic relationships.

Nodulisporic acids D, E, and F are the newest members of a family of nontremorogenic indole-diterpenoids that are potent, orally bioavailable, antiflea agents derived from a fungus belonging to the genus Nodulisporium. The four members of the D series are each devoid of an isoprene residue that is present at C-26 in the three nodulisporic acids described originally (the A series). Nodulisporic acid E (11a) has a simpler structure, which lacks not only the isoprene residue at C-26 but also two that form the A/B rings. Nodulisporic acid F is the simplest of all nodulisporic acids and is devoid of all three isoprene residues of the indole unit; as such, it represents the earliest biosynthetic intermediate in this series. A biogenetic grid based on mutation studies is proposed that encompasses all the known nodulisporic acids. Structure-activity relationships of the known natural nodulisporic acids have been elucidated. Within a series the most active compound possesses a dienoic acid chain, and overall, the end product of the biogenetic grid, i.e., nodulisporic acid A, exhibits the most potent antiflea activity. Additionally, the stereochemistries of C-3' ' and C-4' ' of nodulisporic acid D(2) and therefore of nodulisporic acids A(2), B(2), and C(2) have been assigned.

Rediocides B-E, potent insecticides from Trigonostemon reidioides.

Four new congeners, rediocides B-E (2-5), of the previously reported rediocide A (1) were isolated from a methanol extract of the roots of the plant Trigonostemon reidioides. The structures of these minor analogues were elucidated by comparison of their NMR and mass spectral data with those of rediocide A and confirmed by extensive 2D NMR spectral analysis. They all possess potent activity against fleas (Ctenocephalides felis) in an artificial membrane feeding system and exhibited LD(90) values ranging from 0.25 to 0.5 ppm.

Nodulisporic acids C, C1, and C2: a series of D-ring-opened nodulisporic acids from the fungus Nodulisporium sp.

Nodulisporic acids C (4a), C1 (5a), and C2 (6a), a series of D-ring-opened nodulisporic acids, were isolated from fermentations of a mutant culture of Nodulisporium sp. MF5954. Nodulisporic acid C, the most potent of the three, showed good activity against fleas with an LD90 of 10 micro g/mL. These compounds tend to be unstable in the free acid form and were isolated as stable sodium salts.

Hybrid-designed inhibitors of p38 MAP kinase utilizing N-arylpyridazinones.

Imidazo[1,2-a]pyridyl N-arylpyridazinones were hybridized from the classic pyridinylimidazoles and the more recent dual hydrogen bond acceptors, resulting in a new structural class of selective p38 MAP kinase inhibitors.

Nodulisporic acid side-chain modifications: access to the 2", 3", 4", and 6" registers.

Efficient routes to access the 2", 3", 4", and 6" registers of the nodulisporic acid (NsA) side chain are disclosed. A mild one-carbon, Ph(2)CdoublebondNCH(2)CtriplebondN mediated homologation of NsA's 3"-aldehyde permitted access to the 4"-register. Curtius reaction of NsA's 3"-acid yielded the corresponding 2"-aldehyde 4 from which the unnatural Delta(2",3")-olefin isomer 2b was obtained. In addition, Arndt-Eistert reactions of the parent NsA permitted a one-carbon homologation to the 6" register. These efforts identified new analogues with significant flea activity and illustrated the biological significance of unsaturation at the 1",2" register.

Amphidial structure of ivermectin-resistant and susceptible laboratory and field strains of Haemonchus contortus.

The development of anthelmintic resistance by nematode parasites is a growing problem for veterinarians, pet owners, and producers. The intensive use of the macrocyclic lactones for the treatment of a variety of parasitic diseases has hastened the development of resistance to this family of parasiticides. As a result, resistance to ivermectin, moxidectin, nemadectin, and doramectin by Haemonchus contortus has been documented throughout the world. Sensory neurons located in the cephalic end of nematodes are in close contact with the external environment. Through these neurons, important chemical and thermal cues are gathered by the parasite. Examination of serial electron micrographs of ivermectin-susceptible and ivermectin-resistant H. contortus allows for comparison of neuronal structure, arrangement of neurons within the amphidial channel, and distance of the tip of the dendritic processes to the amphidial pore. The latter of these characteristics provides a useful means by which to compare the association between the neurons and the external environment of the worm. Comparison of parental laboratory strains of ivermectin-susceptible strains of H. contortus with related selected, ivermectin-resistant strains and with a wild-type ivermectin-susceptible field strain of H. contortus from Louisiana reveal that the ivermectin-resistant worms examined have markedly shorter sensory cilia than their ivermectin-susceptible parental counterparts. Additionally, the amphidial neurons of ivermectin-resistant worms are characterized by generalized degeneration and loss of detail, whereas other neurons outside of the channels, such as the labial and cephalic neurons, are normal in structure. These findings raise a number of questions regarding the relationship between amphidial structure and ivermectin resistance as well as the role of amphids as a means of entry for ivermectin. While shortened amphidial sensilla are associated with ivermectin resistance, it remains unclear if such a structural modification facilitates survival of nematodes exposed to macrocyclic lactones.

Nodulisporic acid B, B1, and B2: a series of 1'-deoxy-nodulisporic acids from Nodulisporium sp.

During the re-isolation of the lead compound nodulisporic acid A (1a) and targeted chemical screening for related compounds, we discovered a series of 1'-deoxy congeners named herein nodulisporic acids B (1b), B1 (2b), and B2 (3b). In comparison with nodulisporic acid A, these compounds were less active and were chemically unstable resulting into formation of delta23 dehydro derivatives. Therefore, these compounds were stabilized and isolated as sodium salts and methyl ester. Nodulisporic acid B is 100-fold less active than nodulisporic acid A against fleas. The isolation, structure elucidation, and biological activities of these compounds are described.

Nodulisporic acid: its chemistry and biology.

The discovery of the natural product nodulisporic acid A (NsA A) and its potent, systemic insecticidal activity at Merck Research Laboratories in 1992 stimulated intense scientific scrutiny. Interest in this new class of indole diterpenes led to extensive delineation of its properties, both chemical and biological. Synthetic modification of NsA A served to identify its pharmacologically permissive and non-permissive regions, produced semisynthetic derivatives with enhanced adulticidal flea efficacy (both in vitro and in vivo), and provided useful tools to support biological studies. Early observations in rodent models showed a wide therapeutic index for NsA A and detailed mechanism of action investigations demonstrated that it selectively modulated an invertebrate specific glutamate-gated ion channel for which no mammalian homolog exists. Consistent with these mechanistic conclusions, dogs treated orally with either NsA A or closely related amide analogs (15 mg/kg dosages) showed no apparent toxicity, and measurement of systemic flea efficacy in these animals demonstrated that prolonged antiparasitic activity was attained, up to 14 days subsequent to treatment with a single p.o. dose for fleas or up to 4 weeks for ticks. The extended flea efficacy was correlated to both the in vivo pharmacokinetic profile of a given analog and its intrinsic in vitro potency against fleas. In addition, studies directed towards the total synthesis of NsA A have been reported.

Side-chain homologation of nodulisporic acid: synthesis of potent new dienyl derivatives.

A series of new, diene-modified nodulisporic acid analogues (2) bearing diverse functionality at the 3"- and 4"-sites was efficiently prepared from the 3"-aldehyde 3. Biological evaluation of these synthetic nodulisporic acid analogues for systemic flea efficacy identified potent compounds and further clarified the structural requirements for ectoparasite activity.