Perspectives in animal health: old targets and new opportunities

Antiparasitic properties of leaf extracts derived from selected Nicotiana species and Nicotiana tabacum varieties

Within the traditional pharmacopeia, tobacco (Nicotiana spp.) is often cited as an efficient pesticide. This activity is generally attributed to nicotine, but tobacco plants contain other alkaloids that could potentially contribute to this effect. In this study, we tested methanolic extracts of N. glutinosa, N. glauca, N. debneyi, and N. tabacum (putrescine N-methyltransferase line, burley TN90 and Stella, Virginia ITB 683 and K326), selected according to alkaloid content. Their antiparasitic activity was evaluated in bioassays against adult fleas (Ctenocephalides felis), blowfly (Lucilia cuprina) larvae, nematodes (Caenorhabditis elegans), and ticks (Rhipicephalus sanguineus larvae and adults, Ixodes ricinus nymphs). None of the extracts killed fleas and blowfly larvae effectively at the concentrations tested. Only N. tabacum K326 and N. glutinosa exhibited moderate anthelmintic activity. All extracts significantly repelled R. sanguineus ticks, but not I. ricinus, and the nicotine-rich extracts rapidly knocked down all tick species and stages at high concentrations. The link between nicotine and tick knockdown was confirmed by successfully testing the pure alkaloid at concentrations found in the tobacco extracts. In contrast, repellent activity could not be correlated to the individually tested alkaloids (nicotine, nornicotine, anabasine, anatabine), although anatabine and nornicotine were active in the tick bioassay at high concentrations.

Pharmacokinetics and metabolism of eprinomectin in cats when administered in a novel topical combination of fipronil, (S)-methoprene, eprinomectin and praziquantel

Four studies were conducted to determine the pharmacokinetic characteristics and in vitro metabolism of eprinomectin, a semi-synthetic avermectin, in cats. Pharmacokinetic parameters including bioavailability of eprinomectin were determined in a parallel study design comprised of one group of eight cats which were treated once topically at 0.12 mL/kg bodyweight with BROADLINE(®), a novel combination product (fipronil 8.3% (w/v), (S)-methoprene 10% (w/v), eprinomectin 0.4% (w/v) and praziquantel 8.3% (w/v)), delivering a dose of 0.5mg eprinomectin per kg body weight, and a group of six cats which received 0.4% (w/v) eprinomectin at 0.4 mg/kg bodyweight once by intravenous injection. For cats treated by topical application, the average eprinomectin (B1a component) maximum plasma concentration (Cmax) was 20 ng/mL. The maximum concentrations were reached 24h after dosing in the majority of the animals (six of eight cats). The average terminal half-life was 114 h due to slow absorption ('flip-flop' kinetics). Following intravenous administration the average Cmax was 503 ng/mL at 5 min post-dose, and the mean elimination half-life was 23 h. Eprinomectin was widely distributed with a mean volume of distribution of 2,390 mL/kg, and the clearance rate was 81 mL/h/kg. Mean areas under the plasma concentration versus time curves extrapolated to infinity were 2,100 ngh/mL and 5,160 ngh/mL for the topical and intravenous doses, respectively. Topical eprinomectin was absorbed with an average absolute bioavailability of 31%. In a second parallel design study, the dose proportionality of eprinomectin after single topical administration of BROADLINE(®) was studied. Four groups of eight cats each were treated once topically with 0.5, 1, 2 or 5 times the minimum recommended dose of the combination, 0.12 mL/kg bodyweight. Based on comparison of areas under the plasma concentration versus time curves from the time of dosing to the last time point at which eprinomectin B1a was quantified, and Cmax, dose proportionality was established. In addition, the metabolic pathway of eprinomectin using cat liver microsomes, and plasma protein binding using cat, rat, and dog plasma were studied in vitro. Results of the analyses of eprinomectin B1a described here showed that it is metabolically stable and highly protein bound (>99%), and thus likely to be, as with other species, excreted mainly as unchanged parent drug in the feces of cats.

Functional characterisation of a nicotinic acetylcholine receptor α subunit from the brown dog tick, Rhipicephalus sanguineus

Ticks and tick-borne diseases have a major impact on human and animal health worldwide. Current control strategies rely heavily on the use of chemical acaricides, most of which target the CNS and with increasing resistance, new drugs are urgently needed. Nicotinic acetylcholine receptors (nAChRs) are targets of highly successful insecticides. We isolated a full-length nAChR α subunit from a normalised cDNA library from the synganglion (brain) of the brown dog tick, Rhipicephalus sanguineus. Phylogenetic analysis has shown this R. sanguineus nAChR to be most similar to the insect α1 nAChR group and has been named Rsanα1. Rsanα1 is distributed in multiple tick tissues and is present across all life-stages. When expressed in Xenopus laevis oocytes Rsanα1 failed to function as a homomer, with and without the addition of either Caenorhabditis elegans resistance-to-cholinesterase (RIC)-3 or X. laevis RIC-3. When co-expressed with chicken β2 nAChR, Rsanα1 evoked concentration-dependent, inward currents in response to acetylcholine (ACh) and showed sensitivity to nicotine (100 μM) and choline (100 μM). Rsanα1/β2 was insensitive to both imidacloprid (100 μM) and spinosad (100 μM). The unreliable expression of Rsanα1 in vitro suggests that additional subunits or chaperone proteins may be required for more robust expression. This study enhances our understanding of nAChRs in arachnids and may provide a basis for further studies on the interaction of compounds with the tick nAChR as part of a discovery process for novel acaricides.

Overview of the animal health drug development and registration process: an industry perspective

Products for animal health commercialization follow a structured progression from initial concept through to regulatory approval. Typically, products are developed for use in either food animals or companion animals. These can be for the intention of disease intervention, productivity enhancement or improvement in a quality of life capacity. The animal health industry is a regulated industry, meaning that a government agency is responsible for oversight of products, both pre- and post-approval. There are three primary US government agencies that ensure quality, safety and effectiveness for the approval of new products and post-marketing compliance.

The effect of oral anthelmintics on the survivorship and re-feeding frequency of anthropophilic mosquito disease vectors

In the Tropics, there is substantial temporal and spatial overlap of diseases propagated by anthropophilic mosquito vectors (such as malaria and dengue) and human helminth diseases (such as onchocerciasis and lymphatic filariasis) that are treated though mass drug administrations (MDA). This overlap will result in mosquito vectors imbibing significant quantities of these drugs when they blood feed on humans. Since many anthelmintic drugs have broad anti-invertebrate effects, the possibility of combined helminth control and mosquito-borne disease control through MDA is apparent. It has been previously shown that ivermectin can reduce mosquito survivorship when administered in a blood meal, but more detailed examinations are needed if MDA is to ever be developed into a tool for malaria or dengue control. We examined concentrations of drugs that follow human pharmacokinetics after MDA and that matched with mosquito feeding times, for effects against the anthropophilic mosquito vectors Anopheles gambiae s.s. and Aedes aegypti. Ivermectin was the only human-approved MDA drug we tested that affected mosquito survivorship, and only An. gambiae s.s. were affected at concentrations respecting human pharmacokinetics at indicated doses. Ivermectin also delayed An. gambiae s.s. re-feeding frequency and defecation rates, and two successive ivermectin-spiked blood meals following human pharmacokinetic concentrations compounded mortality effects compared to controls. These findings suggest that ivermectin MDA in Africa may be used to decrease malaria transmission if MDAs were administered more frequently. Such a strategy would broaden the current scope of polyparasitism control already afforded by MDAs, and which is needed in many African villages simultaneously burdened by many parasitic diseases.

Comparative pharmacokinetics of some injectable preparations containing ivermectin in dogs

Little is known about the kinetics of ivermectin formulations following subcutaneous administration in dogs. The vehicle components used in production may change the pharmacokinetics of the drug. The present study was aimed at the comparison of the pharmacokinetics of seven injectable ivermectin formulation of different brand names (A-G). The animals were allocated to seven groups, each comprising seven dogs. The dogs were administered ivermectin at a dose of 200 microg/kg bw by subcutaneous route and blood samples were collected from all groups up to 288h post-injection. Plasma ivermectin analyses were performed using a HPLC with a fluorescence detector. Compared to Group 1(A), it was determined that statistically significant differences existed in Groups 2(B), 3(C), 4(D), 5(E), and 7(G) for C(max) values; and in Groups 3(C), 4(D), 6(F), 7(G) for AUC(0-->288) and AUC(0-->infinity) values. These values were highest in Group 1(A) and lowest in Group 7(F). The results obtained in the present study demonstrated that, in cases which require subacute administration, optimal exposure is achieved with the preparation A. However, it must be noted that this evaluation was based on pharmacokinetic parameters and not antiparasitic efficacy.

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.

Modern agrochemical research: a missed opportunity for drug discovery?

The word "agrochemical" has often taken on a pejorative character in the public mind. Some of the negative tone might have coloured the perception of the industry by pharma, together with views on the chemical nature of agrochemicals that seem to be based on older pesticides that date back to the 1950s and 1960s. In this review, we try to address some of these concerns, draw out the similarities between agrochemical and pharmaceutical research and highlight opportunities for drug discovery that are offered by pesticide-related compounds, particularly with regard to herbicides and compounds with leadlike physical properties.

Synthesis and anthelmintic activity of 7-substituted 3,4a-dimethyl-4a,5a,8a,8b-tetrahydro-6H-pyrrolo[3′,4′:4,5]furo[3,2-b]pyridine-6,8(7H)-diones

The racemic 7-substituted 3,4a-dimethyl-4a,5a,8a,8b-tetrahydro-6H-pyrrolo[3',4':4,5]furo[3,2-b]pyridine-6,8(7H)-diones represent novel tricyclic compounds with strong in vivo efficacy against the parasitic nematode Haemonchus contortus Rudolphi in sheep. Here we report on the synthesis of tricyclic endo-2,3-dihydro[3,2-b]pyridine-type cycloadducts and describe the separation of the racemic 3,4a-dimethyl-7-ethyl-4a,5a,8a,8b-tetrahydro-6H-pyrrolo[3',4':4,5]furo[3,2]pyridine-6,8(7H)-dione into the enantiomers by HPLC. The absolute configuration of the most anthelmintically active (4aS,5aS,8aS,8bR)-enantiomer was determined by single crystal X-ray analysis using its stable (4aS,5aS,8aS,8bR)-enantiomer-CuCl2 (2:1)-complex.

Chimeric cyclodepsipeptides as mimetics for the anthelmintic PF1022A

In the anthelmintic cyclooctadepsipeptide PF1022A (1) didepsipeptide units have been exchanged for the beta-turn mimetics (D)-Pro-(L)-Pro and BTD (7) in order to elucidate the functional role of the depsipeptide backbone. Compounds 12 and 14 are the first PF1022A analogues in which a substantial part of the PF1022A backbone has been replaced with an improvement of anthelmintical activity. Preliminary structure-activity relationships suggest a symmetric conformation to be the biological active one.

Determination of expression of cyclooxygenase-1 and -2 isozymes in canine tissues and their differential sensitivity to nonsteroidal anti-inflammatory drugs

OBJECTIVE

To evaluate cyclooxygenase isozyme distribution in tissues from dogs and determine the differential sensitivity of canine cyclooxygenase (COX)-1 and -2 isozymes to nonsteroidal anti-inflammatory drugs (NSAIDs).

SAMPLE POPULATION

Canine tissue samples (stomach, duodenum, ileum, jejunum, colon, spleen, cerebral cortex, lung, ovary, kidney, and liver) were obtained from 2 dogs for northern and western blot analyses, and blood for whole blood COX assays was obtained from 15 dogs.

PROCEDURE

11 NSAIDs were evaluated to determine their COX-2 selectivity in whole blood assays. The concentrations of the drug needed to inhibit 50% of enzyme activity (IC50) were then calculated for comparison. Expression and tissue distribution of COX isozymes were determined by northern and western blot analysis.

RESULTS

Aspirin, diclofenac, indomethacin, ketoprofen, meclofenamic acid, and piroxicam had little selectivity toward COX isozymes, whereas NS398, carprofen, tolfenamic acid, nimesulide, and etodolac had more than 5 times greater preference for inhibiting COX-2 than COX-1. All canine tissues examined, including those from the gastrointestinal tract, coexpressed COX-1 and -2 mRNA, although protein expression was observed only for COX-1.

CONCLUSIONS AND CLINICAL RELEVANCE

Canine COX-2 was selectively inhibited by etodolac, nimesulide, and NS398; tolfenamic acid and carprofen also appeared to be preferential COX-2 inhibitors in dogs. The roles of COX-1 as a constitutive housekeeping enzyme and COX-2 as a proinflammatory inducible enzyme (as determined in humans) appear to apply to dogs; therefore, COX-2-selective inhibitors should prove useful in reducing the adverse effects associated with nonselective NSAIDs.

Development of antiparasitic drugs in the 21st century

Prospects for discovering new antiparasitic drugs for veterinary medicine in the coming century will be determined by economic, social and scientific factors. Consolidation in the pharmaceutical industry in general, and the animal health industry in particular, changes the business conditions in which drug discovery for veterinary medicine occurs. Social pressures on traditional animal agriculture and companion animal ownership have shifted the interest of animal companies primarily to pet medicine. Antiparasitic drug discovery is more than ever targeted to the most lucrative market segments, but the excellence of available drugs, and the apparent lack of resistance in important parasites, reduces industrial motivation to invest in parasitology. Veterinary parasitologists in academia will still have the chance to interact with their industrial counterparts in the traditional ways of supporting drug discovery and development. Nonetheless, there are many new opportunities to expand the research horizons of veterinary parasitology to strengthen the case for retaining a significant presence in the animal health industry.

Restricted conformation analogues of an anthelmintic cyclodepsipeptide

Six analogues of the anthelmintic cyclodepsipeptide PF1022A were prepared, each containing a small ring fused to the macrocycle to restrict the number of conformations the larger ring can adopt. It was anticipated that such conformational changes could lead to enhanced biological activity and selectivity. The analogues form two series of three members each. In one series, a carbon-based molecular bridge joins the methyl of a leucine residue with the methyl of its closest lactic acid residue to form five-, six-, and seven-membered lactam rings. In the second series, a leucine residue is replaced with five-, six-, and seven-membered nitrogen heterocycles. Decreasing the size of the small ring in the lactam series increasingly distorts the macrocycle and consistently decreases activity relative to PF1022A. In the leucine series, a similar trend is observed. Molecular modeling of PF1022A along with the analogues described herein suggests that the ability to exist in a highly symmetrical conformational state is a necessary condition for biological activity.

Synthesis of nodulisporic acid 2' '-oxazoles and 2' '-thiazoles

[reaction--see text] The semisynthetic conversion of nodulisporic acid A (1) into a set of three heterocyclic side chain derivatives provided compounds, highlighted by 6, with an improved spectrum of ectoparasiticidal activity and pharmacokinetic profile relative to the natural product.