Marlene Drag

MDR1-deficient genotype in Collie dogs hypersensitive to the P-glycoprotein substrate ivermectin.

Multidrug resistance (MDR) phenotypes in cancer cells are associated with overexpression of the drug carrier P-glycoprotein. The antiparasitic drug ivermectin, one of its substrates, abnormally accumulates in the brain of transgenic mice lacking the P-glycoprotein, resulting in neurotoxicity. Similarly, an enhanced sensitivity to ivermectin has been reported in certain dogs of the Collie breed. To explore the basis of this phenotype, we analyzed the canine P-glycoprotein-encoding MDR1 gene, and we report the first characterization of the cDNA for wild-type (Beagle) P-glycoprotein. The corresponding transcripts from ivermectin-sensitive Collies revealed a homozygous 4-bp exonic deletion. We established, by genetic testings, that the MDR1 frame shift is predictable. Accordingly, no P-glycoprotein was detected in the homozygote-deficient dogs. In conclusion, we characterized a unique case of naturally occurring gene invalidation. This provides a putative novel model that remains to be exploited in the field of human therapeutics and that might significantly affect tissue distribution and drug bioavailability studies.

The intravenous and oral pharmacokinetics of afoxolaner used as a monthly chewable antiparasitic for dogs.

The pharmacokinetics of afoxolaner in dogs was evaluated following either intravenous or after oral administration of NEXGARD(®), a soft chewable formulation. Afoxolaner is a member of one of the newest classes of antiparasitic agents, known as antiparasitic isoxazolines. The soft chewable formulation underwent rapid dissolution, and afoxolaner was absorbed quickly following oral administration of the minimum effective dose of 2.5mg/kg, with maximum plasma concentrations (Cmax) of 1,655 ± 332 ng/mL observed 2-6h (Tmax) after treatment. The terminal plasma half-life was 15.5 ± 7.8 days, and oral bioavailability was 73.9%. Plasma concentration-versus-time curves fit a 2-compartment model and increased proportionally with dose over the oral dose range of 1.0-4.0mg/kg, and over the oral dose range from 1.0 to 40 mg/kg. Following an intravenous dose of 1mg/kg, the volume of distribution (Vd) was 2.68 ± 0.55 L/kg, and the systemic clearance was 4.95 ± 1.20 mL/h/kg. Afoxolaner plasma protein binding was >99.9% in dogs. One major metabolite, formed following hydroxylation of afoxolaner, was identified in dog plasma, urine and bile. When afoxolaner is administered orally, there is a strong correlation between afoxolaner plasma concentration and efficacy with EC90 values of 23 ng/mL for Ctenocephalides felis and ≥ 100 ng/mL for Rhipicephalus sanguineus sensu lato and Dermacentor variabilis. The pharmacokinetic properties of afoxolaner are suited for a monthly administration product because the fast absorption and long terminal half-life support a rapid onset of action while ensuring month-long efficacy.

Assessment of the efficacy of orally administered afoxolaner against Rhipicephalus sanguineus sensu lato.

Two studies were conducted to confirm that a single oral dose of the novel insecticide/acaricide afoxolaner is efficacious against existing infestations of Rhipicephalus sanguineus sensu lato in dogs and can control re-infestation for up to 35 days. Each study utilized 16 purpose bred adult dogs using a controlled randomized block design. One or two days prior to treatment, all dogs were infested with 50 unfed adult ticks. On Day 0 one group was treated with an oral chewable formulation of afoxolaner at a dose as close as possible to the minimum dose of 2.5mg/kg. Weekly re-infestations with 50 adult unfed ticks were repeated for five weeks. Forty-eight hours after treatment and after each re-infestation, the number of remaining live ticks on each dog was counted. Treatment with afoxolaner resulted in efficacies of 98.8-100% within 48 h on existing tick infestations, while the efficacy against new tick infestations was >95.7% over five weeks.

Safety evaluation of orally administered afoxolaner in 8-week-old dogs

The safety profile of afoxolaner, a new isoxazoline molecule, was evaluated following the regulatory requirements when administered six times orally in a soft chewable formulation at a dose of at least 1×, 3× or 5× the maximum exposure dose (6.3mg/kg) in 8-week-old Beagle dogs. Thirty-two healthy puppies (16 males and 16 females) were enrolled and allocated randomly to one of four treatment groups. Treatments were administered at three, one-month dose intervals (Days 0, 28 and 56) followed by three, 2-week dose intervals (Days 84, 98 and 112). The study ended at Day 126. The groups were: Group 1: non-treated control; Group 2: afoxolaner chews administered at a dosage of at least 6.3mg/kg (1×); Group 3: afoxolaner chews administered at a dosage of at least 18.9 mg/kg (3×); and Group 4: afoxolaner chews administered at a dosage of at least 31.5mg/kg (5×). All dogs were examined for general health twice a day beginning on at least Day-14. Physical examinations, and blood collections for clinical pathology analysis and afoxolaner plasma concentrations, were performed throughout the study. On Day 126, 2 weeks following the last treatment, all dogs were humanely euthanized prior to the conduction of a full necropsy with tissue collection. No afoxolaner-related changes were observed in growth, physical variables, clinical pathology variables, or tissues examined histologically. No clinically or statistically significant health abnormalities related to the administration of afoxolaner were observed. Vomiting and diarrhea were observed sporadically across all groups including the controls. The kinetics of afoxolaner plasma concentrations was linear following 6 doses of 6.3, 18.9 and 31.5mg/kg and dose proportionality was demonstrated. There were no statistical differences (p<0.05) between samples taken on Days 55 and 83 when compared to Day 27. Based upon the results of this study, afoxolaner was shown to be safe when administered repeatedly in a soft chewable formulation at up to 5× the maximum exposure dose in dogs as young as 8 weeks of age.

Assessment of the onset of action of afoxolaner against existing adult flea (Ctenocephalides felis) infestations on dogs.

The speed of kill of afoxolaner against experimental infestations by Ctenocephalides felis was evaluated after oral administration of afoxolaner in a soft chew (NEXGARD(®)) at a dose to achieve 2.5mg/kg bodyweight. Forty beagles were allocated to two treatment groups. Dogs in Treatment Group 1 were untreated controls. Dogs in Treatment Group 2 were treated on Day-0 with afoxolaner, according to their pre-treatment bodyweight. All dogs were infested with approximately 100 C. felis on Day-1. Live fleas were counted upon removal at 5 time points after treatment (i.e., 2, 4, 8, 12 and 24h after treatment). For each time point, counts were performed on 4 dogs from each of the treated and the untreated groups. Early curative flea killing efficacy was evaluated with respect to the untreated control group. The afoxolaner treated group had significantly fewer fleas than the untreated control group at 8, 12, and 24h (p<0.001). The percent efficacies of orally administered afoxolaner were 15.0%, 87.8%, 99.5%, 100.0%, and 100.0% at 2, 4, 8, 12, and 24h, respectively. In this study, afoxolaner began killing fleas by 2h after treatment with increasing efficacy at subsequent time points and had >99.5% efficacy at 8, 12, and 24h after treatment demonstrating an early onset of action.

Evaluation of the efficacy of afoxolaner against Haemaphysalis longicornis on dogs

A controlled study to assess the acaricidal efficacy of afoxolaner in dogs after a single oral administration was conducted against Haemaphysalis longicornis ticks. The study was characterized by a negative controlled randomized block design and included sixteen beagle dogs of both sexes. Starting two days before treatment, each dog was infested weekly with 50 ticks over 4 weeks. The number of live ticks was determined 48 h after treatment and then 48 h after each infestation. The mean dose of afoxolaner received by dogs was 3.0mg/kg (range: 2.5-3.1mg/kg). Afoxolaner rapidly eliminated pre-existing tick infestations (100% ticks killed within 48 h of treatment) and controlled weekly re-infestations (91.9% prophylactic efficacy at Day 30).

Evaluation of the effect of simulated rainfall on the efficacy of Ivomec Pour-On against Cooperia spp. infection in cattle.

Abstract Four controlled studies, one each in Australia, Germany, the United Kingdom, and the United States, involving 30 calves each were conducted to evaluate the effect of simulated rainfall on the efficacy of Ivomec Pour-On against infections of Cooperia spp. At 3 weeks before treatment the calves were infected orally with third-stage larvae of Cooperia spp. In each study a recent, locally derived field isolate was used. The calves were allocated by restricted randomization based on body weight within sex to one of the following treatments: unmedicated control with no rain, Ivomec Pour-On with no rain, Ivomec Pour-On with rain starting at 40 min before treatment, Ivomec Pour-On with rain starting at 10 min after treatment, and Ivomec Pour-On with rain starting at 60 min after treatment. Ivomec Pour-On was applied topically at a dose rate of 1 ml/10 kg body weight (500 μg ivermectin/kg body weight). The simulated rainfall was equivalent to a heavy shower of approximately 12.5 mm of water during a 30-min period. The calves were necropsied for worm counting at 14 or 15 days after treatment. An evaluation of the pooled data showed that as compared with the untreated controls, the Ivomec Pour-On-treated calves with no rain had significantly (P < 0.01) fewer C. oncophora (>99%), C. punctata (>99%), C. surnabada (>98%), and combined Cooperia spp. (>99%). The reduction in Cooperia numbers noted for calves exposed to simulated rainfall was >96% for all Cooperia species, regardless of when the rainfall started relative to the application of Ivomec Pour-On. There was no significant (P > 0.1) difference between the Ivomec Pour-On-treated calves with no rain and the pooled groups with simulated rainfall or between the group with rain before treatment and the pooled groups with rain after treatment. Ivomec Pour-On was highly effective against established infections of Cooperia spp. when applied to wet animals or to animals becoming wet shortly after treatment.