Lara K. Maxwell

Pharmacokinetics of valacyclovir in the adult horse

Recent outbreaks of equine herpes virus type-1 infections have stimulated renewed interest in the use of effective antiherpetic drugs in horses. The purpose of this study was to investigate the pharmacokinetics of valacyclovir (VCV), the prodrug of acyclovir (ACV), in horses. Six adult horses were used in a randomized cross-over design. Treatments consisted of 10 mg/kg ACV infused intravenously, 5 g (7.7-11.7 mg/kg) VCV delivered intragastrically (IG) and 15 g (22.7-34.1 mg/kg) VCV administered IG. Serum samples were obtained at predetermined times for acyclovir assay using high-performance liquid chromatography. Following the administration of 5 g VCV, the mean observed maximum serum ACV concentration (C(max)) was 1.45 +/- 0.38 (SD) microg/mL, at 0.74 +/- 0.43 h. At a dose of 15 g VCV, the mean C(max) was 5.26 +/- 2.82 microg/mL, at 1 +/- 0.27 h. The mean bioavailability of ACV from oral VCV was 60 +/- 12% after 5 g of VCV and 48 +/- 12% after 15 g VCV, and did not differ significantly between dose rates (P > 0.05). Superposition suggested that a loading dose of 27 mg/kg VCV every 8 h for 2 days, followed by a maintenance dose of 18 mg/kg every 12 h, will maintain effective serum ACV concentrations.

Effect of fentanyl on visceral and somatic nociception in conscious horses.

BACKGROUND Transdermal fentanyl is used clinically in horses based on pharmacokinetic data and antinociceptive effects documented in other species. HYPOTHESIS Fentanyl IV administration increases both visceral and somatic nociceptive threshold in conscious horses. ANIMALS Six clinically normal horses, each fitted with a permanent gastric cannula. METHODS Visceral nociception was evaluated with 2 methods of threshold detection--olorectal distention and duodenal distention. Somatic nociception was assessed by measurement of thermal threshold. Fentanyl was administered as an increasing stepwise infusion followed by a continuous-rate infusion for a total of 2 hours. There were 4 doses of fentanyl and 1 dose each of saline and xylazine administered to each horse. Serum fentanyl concentrations were measured and the resulting data were used to determine pharmacokinetic parameters for each horse. All data were analyzed by means of a 3-factor analysis of variance followed by either a simple t test or a Bonferroni t test for multiple comparisons. RESULTS Fentanyl administration did not result in significant changes in duodenal or colorectal distention threshold. Thermal threshold showed an increased trend at the 15-minute time point for the highest fentanyl group only, with a corresponding mean serum fentanyl concentration of 7.82 +/- 2.10 ng/mL. Two horses in this group became agitated and tachycardic during the first 15 minutes of the infusion. CONCLUSIONS AND CLINICAL IMPORTANCE Fentanyl did not produce a significant antinociceptive effect at the doses used, 2 of which resulted in serum concentrations above the nociceptive threshold in other species.

Pharmacokinetics of lidocaine following the application of 5% lidocaine patches to cats.

Lidocaine patches have been used to provide local analgesia in dogs and cats. We conducted this study to assess the systemic and local absorption of lidocaine from topical patches in cats. Eight 2-year-old cats received either intravenous lidocaine at 2 mg/kg or one 700 mg lidocaine patch placed on the lateral thorax for 72 h, in a cross-over randomized repeated measures design. Plasma was collected at specific times and the skin was biopsied at the time of patch removal for the quantitative analysis of lidocaine and its major metabolite, monoethylglycinexylidide (MEGX), by gas chromatography with mass spectrometry. Percent absorption time plots for systemic lidocaine appearance were constructed using the Loo-Riegelman method. Approximately, constant rate absorption was observed from 12-72 h after patch application at a mean +/- SD rate of 109 +/- 49 microg/kg/h, resulting in steady-state lidocaine plasma concentrations of 0.083 +/- 0.032 microg/mL and MEGX concentrations of 0.012 +/- 0.009 microg/mL. Overall bioavailability of transdermal lidocaine was 6.3 +/- 2.7%, and only 56 +/- 29% of the total lidocaine dose delivered by the patch reached systemic circulation. Skin lidocaine concentrations were much higher than plasma concentrations, at 211 +/- 113 microg/g in the thoracic skin beneath the patch and 2.2 +/- 0.6 microg/g in the contralateral thoracic skin without the patch. As both lidocaine and MEGX were recovered from contralateral skin, it is likely that lidocaine accumulated in the skin from low systemic concentrations of circulating lidocaine over the 72-h period of patch application. Plasma lidocaine concentrations remained well below systemically toxic concentrations, and no obvious clinical side effects were observed in any of the cats. The low systemic absorption rate coupled with high local lidocaine concentrations on the skin support the safe use of lidocaine patches in cats.

Plasma concentrations of lidocaine in dogs following lidocaine patch application.

Transdermal absorption of lidocaine was determined by measuring plasma lidocaine concentrations following skin application of 5% lidocaine patches. Two lidocaine patches were placed on the ventral abdominal midline of seven dogs for 72 hours. Lidocaine was detectable in plasma 12 hours after patch application, and it reached steady-state concentrations between 24 and 48 hours. Plasma lidocaine levels decreased dramatically at 60 hours post-application. Low plasma lidocaine concentrations remained for 6 hours after patch removal. No clinically significant side effects were noted.

Pharmacokinetics of ganciclovir and valganciclovir in the adult horse

Equine herpes myeloencephalopathy, resulting from equine herpes virus type 1 (EHV-1) infection, is associated with substantial morbidity and mortality in the horse. As compared to other antiviral drugs, such as acyclovir, ganciclovir has enhanced potency against EHV-1. This study investigated the pharmacokinetics of ganciclovir and its oral prodrug, valganciclovir, in six adult horses in a randomized cross-over design. Ganciclovir sodium was administered intravenously as a slow bolus at a dose of 2.5 mg/kg, and valganciclovir was administered orally at a dose of 1800 mg per horse. Intravenously administered ganciclovir disposition was best described by a three-compartment model with a prolonged terminal half-life of 72 ± 9 h. Following the oral administration of valganciclovir, the mean observed maximum serum ganciclovir concentration was 0.58 ± 0.37 μg/mL, and bioavailability of ganciclovir from oral valganciclovir was 41 ± 20%. Superposition predicted that oral dosing of 1800-mg valganciclovir two times daily would fail to produce and maintain effective plasma concentrations of ganciclovir. However, superposition suggested that i.v. administration of ganciclovir at 2.5 mg/kg every 8 h for 24 h followed by maintenance dosing of 2.5 mg/kg every 12 h would maintain effective ganciclovir serum concentrations in most horses throughout the dosing interval.

Effect of body weight on the pharmacokinetics of flunixin meglumine in miniature horses and quarter horses.

In most species, large variations in body size necessitate dose adjustments based on an allometric function of body weight. Despite the substantial disparity in body size between miniature horses and light-breed horses, there are no studies investigating appropriate dosing of any veterinary drug in miniature horses. The purpose of this study was to determine whether miniature horses should receive a different dosage of flunixin meglumine than that used typically in light-breed horses. A standard dose of flunixin meglumine was administered intravenously to eight horses of each breed, and three-compartmental analysis was used to compare pharmacokinetic parameters between breed groups. The total body clearance of flunixin was 0.97 ± 0.30 mL/min/kg in miniature horses and 1.04 ± 0.27 mL/min/kg in quarter horses. There were no significant differences between miniature horses and quarter horses in total body clearance, the terminal elimination rate, area under the plasma concentration versus time curve, apparent volume of distribution at steady-state or the volume of the central compartment for flunixin (P > 0.05). Therefore, flunixin meglumine may be administered to miniature horses at the same dosage as is used in light-breed horses.

Simultaneous quantification of vinblastine and desacetylvinblastine concentrations in canine plasma and urine samples using LC-APCI-MS/MS.

A highly sensitive and specific liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry (LC/APCI-MS/MS) method has been developed and validated for simultaneous quantification of vinblastine and its metabolite, desacetylvinblastine, in canine plasma and urine samples. Plasma and urine samples were processed by a solid phase extraction procedure. The optimal chromatographic behavior of these analytes was achieved on pentafluorophenyl (PFP) propyl analytical column (5μm, 50×2.1mm) under isocratic elution of 0.75mL/min with a mobile phase of 5mM ammonium acetate and methanol. The samples were analyzed in positive ion, multiple reaction monitoring mode. The calibration curves were linear over 0.125-2ng/mL (lower calibration curve); 2-100ng/mL (higher calibration curve) and 0.125-5ng/mL for vinblastine and desacetylvinblastine in plasma, and over 1-2000ng/mL and 0.5-100ng/mL for vinblastine and desacetylvinblastine in urine samples, respectively. The limits of quantitation of vinblastine and desacetylvinblastine were 0.125ng/mL in both matrices. The intra and interday accuracy was above 89% and precision below 8.6% for both analytes in both matrices. The developed method was successfully applied to ongoing in vivo vinblastine pharmacokinetic studies in dogs.

Plasma d-dimer concentrations during experimental EHV-1 infection of horses

BACKGROUND Central nervous system blood vessel thrombosis is a part of the pathogenesis of equid herpesvirus-associated myeloencephalopathy (EHM). D-dimers (DD) are stable breakdown products of cross-linked fibrin, and increased DD-plasma concentrations could reflect the degree of systemic coagulation during EHV-1 infection. HYPOTHESIS We hypothesized that blood DD concentrations will be increased during periods of EHV-1 fever and viremia, reflecting an activated coagulation cascade with fibrinolysis. ANIMALS Twenty-eight equids were infected with EHV-1 in 3 experimental infection studies. Three (uninfected) horses were included in a separate study to evaluate methodology for DD concentration measurements. METHODS Clinical data and quantitative viremia were evaluated, and DD concentrations were measured in blood samples on the day before the infection and during days 1-12 postchallenge. Uninfected horses were sampled every 3 hours for 48 hours. Logistic and linear regression was used to investigate the potential association between the fever and viremia with the presence or absence of DD concentrations in peripheral blood. RESULTS DD concentrations were increased for 1-8 days in the majority of infected animals. Both viremia (odds ratio [OR] 6.3; 95% confidence interval [CI] 3.4-11.8; P = .0013) and fever (OR 4.9; CI 2.3-10.1; P = .001) were strongly associated with the likelihood of detecting DD in peripheral blood. CONCLUSIONS AND CLINICAL IMPORTANCE EHV-1 viremia is associated with increases in DD concentration in horses and ponies. This indicates that EHV-1 viremia can lead to an activation of coagulation and fibrinolysis.

Efficacy of the early administration of valacyclovir hydrochloride for the treatment of neuropathogenic equine herpesvirus type-1 infection in horses

OBJECTIVE To determine whether prophylactic administration of valacyclovir hydrochloride versus initiation of treatment at the onset of fever would differentially protect horses from viral replication and clinical disease attributable to equine herpesvirus type-1 (EHV-1) infection. ANIMALS 18 aged mares. PROCEDURES Horses were randomly assigned to receive an oral placebo (control), treatment at detection of fever, or prophylactic treatment (initiated 1 day prior to viral challenge) and then inoculated intranasally with a neuropathogenic strain of EHV-1. Placebo or valacyclovir was administered orally for 7 or 14 days after EHV-1 inoculation or detection of fever (3 horses/group). Effects of treatment on viral replication and clinical disease were evaluated. Plasma acyclovir concentrations and viremia were assessed to determine inhibitory concentrations of valacyclovir. RESULTS Valacyclovir administration decreased shedding of virus and viremia, compared with findings for control horses. Rectal temperatures and clinical disease scores in horses that received valacyclovir prophylactically for 2 weeks were lower than those in control horses. The severity of but not the risk for ataxia was decreased by valacyclovir administration. Viremia was decreased when steady-state trough plasma acyclovir concentrations were > 0.8 μg/mL, supporting the time-dependent activity of acyclovir. CONCLUSIONS AND CLINICAL RELEVANCE Valacyclovir treatment significantly decreased viral replication and signs of disease in EHV-1-infected horses; effects were greatest when treatment was initiated before viral inoculation, but treatment was also effective when initiated as late as 2 days after inoculation. During an outbreak of equine herpesvirus myeloencephalopathy, antiviral treatment may be initiated in horses at various stages of infection, including horses that have not yet developed signs of viral disease.

A single viral gene determines lethal cross-species neurovirulence of baboon herpesvirus HVP2

Alpha-herpesviruses can produce more severe infections in non-natural host species than in their natural host. Isolates of the baboon alpha-herpesvirus Papiine herpesvirus 2 (HVP2) are either very neurovirulent in mice (subtype nv) or non-virulent (subtype ap), but no such difference is evident in the natural baboon host. Comparative genome sequencing was used to identify subtype-specific sequence differences (SSDs) between HVP2nv and HVP2ap isolates. Some genes were identified that despite exhibiting sequence variation among isolates did not have any SSDs, while other genes had comparatively high levels of SSDs. Construction of genomic recombinants between HVP2nv and HVP2ap isolates mapped the mouse neurovirulence determinant to within three genes. Construction of gene-specific recombinants demonstrated that the UL39 ORF is responsible for determining the lethal neurovirulence phenotype of HVP2 in mice. These results demonstrate that differences in a single viral gene can determine the severity of herpesvirus infection in a non-natural host species.