The novel isoxazoline ectoparasiticide, sarolaner, was identified during a lead optimization program for an orally-active compound with efficacy against fleas and ticks on dogs. The aim of the discovery program was to identify a novel isoxazoline specifically for use in companion animals, beginning with de novo synthesis in the Zoetis research laboratories. The sarolaner molecule has unique structural features important for its potency and pharmacokinetic (PK) properties, including spiroazetidine and sulfone moieties. The flea and tick activity resides in the chirally pure S-enantiomer, which was purified to alleviate potential off-target effects from the inactive enantiomer. The mechanism of action was established in electrophysiology assays using CHO-K1 cell lines stably expressing cat flea (Ctenocephalides felis) RDL (resistance-to-dieldrin) genes for assessment of GABA-gated chloride channel (GABACls) pharmacology. As expected, sarolaner inhibited GABA-elicited currents at both susceptible (CfRDL-A285) and resistant (CfRDL-S285) flea GABACls with similar potency. Initial whole organism screening was conducted in vitro using a blood feeding assay against C. felis. Compounds which demonstrated robust activity in the flea feed assay were subsequently tested in an in vitro ingestion assay against the soft tick, Ornithodoros turicata. Efficacious compounds which were confirmed safe in rodents at doses up to 30mg/kg were progressed to safety, PK and efficacy studies in dogs. In vitro sarolaner demonstrated an LC80 of 0.3μg/mL against C. felis and an LC100 of 0.003μg/mL against O. turicata. In a head-to-head comparative in vitro assay with both afoxolaner and fluralaner, sarolaner demonstrated superior flea and tick potency. In exploratory safety studies in dogs, sarolaner demonstrated safety in dogs≥8 weeks of age upon repeated monthly dosing at up to 20mg/kg. Sarolaner was rapidly and well absorbed following oral dosing. Time to maximum plasma concentration occurred within the first day post-dose. Bioavailability for sarolaner was calculated at >85% and the compound was highly protein bound (>99.9%). The half-life for sarolaner was calculated at 11-12 days. Sarolaner plasma concentrations indicated dose proportionality over the range 1.25-5mg/kg, and these same doses provided robust efficacy (>99%) for ≥35days against both fleas (C. felis) and multiple species of ticks (Rhipicephalus sanguineus, Ixodes ricinus and Dermacentor reticulatus) after oral administration to dogs. As a result of these exploratory investigations, sarolaner was progressed for development as an oral monthly dose for treatment and control of fleas and ticks on dogs.

ABSTRACT:Neurological side effects consistent with ivermectin toxicity have been observed in dogs when high doses of the common heartworm prevention agent ivermectin are coadministered with spinosad, an oral flea prevention agent. Based on numerous reports implicating the role of the ATP-binding cassette drug transporter P-glycoprotein (P-gp) in ivermectin efflux in dogs, an in vivo study was conducted to determine whether ivermectin toxicity results from a pharmacokinetic interaction with spinosad. Beagle dogs were randomized to three groups treated orally in parallel: Treatment group 1 (T01) received ivermectin (60 g/kg), treatment group 2 (T02) received spinosad (30 mg/kg), and treatment group 3 (T03) received both ivermectin and spinosad. Whereas spinosad pharmacokinetics were unchanged in the presence of ivermectin, ivermectin plasma pharmacokinetics revealed a statistically significant increase in the area under the curve (3.6-fold over the control) when ivermectin was coadministered with spinosad. The majority of the interaction is proposed to result from inhibition of intestinal and/or hepatic P-gp-mediated secretory pathways of ivermectin. Furthermore, in vitro Transwell experiments with a human multidrug resistance 1-transfected Madin-Darby canine kidney II cell line showed polarized efflux at concentrations <2 M, indicating that spinosad is a high-affinity substrate of P-gp. In addition, spinosad was a strong inhibitor of the P-gp transport of digoxin, calcein acetoxymethyl ester (IC 50 ‫؍‬ 3.2 M), and ivermectin (IC 50 ‫؍‬ 2.3 M). The findings suggest that spinosad, acting as a P-gp inhibitor, increases the risk of ivermectin neurotoxicity by inhibiting secretion of ivermectin to increase systemic drug levels and by inhibiting P-gp at the blood-brain barrier.

Three laboratory studies were conducted to determine the appropriate dose of sarolaner, a novel isoxazoline, for the treatment and month-long control of infestations of fleas and ticks on dogs. In the first study, dogs were treated orally with sarolaner suspension formulations at 1.25, 2.5 or 5.0mg/kg, and infested with Dermacentor reticulatus, Rhipicephalus sanguineus ticks and with Ctenocephalides felis felis (cat flea) prior to treatment and then weekly for up to 8 weeks. Fleas and ticks were counted 48h after treatment and after each subsequent infestation at 24h for fleas and 48h for ticks. The lowest dose of sarolaner (1.25mg/kg) provided 100% efficacy against fleas from treatment through Day 35 and 98.4% at Day 56. This dose of sarolaner resulted in 99.7-100% control of both species of ticks through Day 28. In Study 2, dogs were dosed orally with placebo or sarolaner suspension formulations at 0.625, 1.25 or 2.5mg/kg and infested with Ixodes scapularis prior to treatment and weekly for 6 weeks, Amblyomma americanum (pretreatment and Day 26), Dermacentor variabilis (Day 33) and A. maculatum (Day 41). Ixodes scapularis was the most susceptible; the lowest dose (0.625mg/kg) providing>95% efficacy through Day 43. Efficacy against D. variabilis on Day 35 was>95% at 1.25 and 2.5mg/kg, whereas the 0.625mg/kg dose gave only 61.4% efficacy. Amblyomma spp. were the least susceptible ticks; efficacy of the 1.25mg/kg dose at Day 28 for A. americanum was markedly lower (88.5%) than achieved for D. reticulatus (100%) at Day 28 and also lower than for D. variabilis at Day 35 (96.2%). In Study 3, dogs were dosed orally with placebo or sarolaner in the proposed commercial tablet (Simparica™) at 1.0, 2.0 or 4.0mg/kg, and infested with A. maculatum, one of the ticks determined to be dose limiting, prior to treatment and then weekly for 5 weeks. All doses gave 100% control of the existing infestation. The two highest dosages resulted in >93% control of subsequent challenges for 5 weeks. There was no significant improvement in efficacy provided by the 4.0 mg/kg dose over the 2.0mg/kg dose (P>0.05) at any time point. The 2.0mg/kg dose was superior to the 1.0mg/kg on Day 14 (P=0.0086) and as efficacy for 1.0mg/kg declined below 90% at Day 28, a single 1mg/kg dose would not provide a full month of tick control. Thus, 2.0mg/kg was selected as the sarolaner dose rate to provide flea and tick control for at least one month following a single oral treatment.

The metabolism of the pyrethroids deltamethrin (DLM), cis-permethrin (CPM) and trans-permethrin (TPM) was studied in human expressed cytochrome P450 (CYP) and carboxylesterase (CES) enzymes. DLM, CPM and TPM were metabolised by human CYP2B6 and CYP2C19, with the highest apparent intrinsic clearance (CL) values for pyrethroid metabolism being observed with CYP2C19. Other CYP enzymes contributing to the metabolism of one or more of the three pyrethroids were CYP1A2, CYP2C8, CYP2C9*1, CYP2D6*1, CYP3A4 and CYP3A5. None of the pyrethroids were metabolised by CYP2A6, CYP2E1, CYP3A7 or CYP4A11. DLM, CPM and TPM were metabolised by both human CES1 and CES2 enzymes. Apparent CL values for pyrethroid metabolism by CYP and CES enzymes were scaled to per gram of adult human liver using abundance values for microsomal CYP enzymes and for CES enzymes in liver microsomes and cytosol. TPM had the highest and CPM the lowest apparent CL values for total metabolism (CYP and CES enzymes) per gram of adult human liver. Due to their higher abundance, all three pyrethroids were extensively metabolised by CES enzymes in adult human liver, with CYP enzymes only accounting for 2%, 10% and 1% of total metabolism for DLM, CPM and TPM, respectively.