Performing Department
Small Animal Clinical Sciences
Non Technical Summary
Seizures are the most common neurologic disorder in dogs, with idiopathic epilepsy being the most common cause of epileptic seizure activity. The goal for treatment of seizures is to reduce the severity and frequency of seizures, which left untreated, can lead to significant, chronic morbidity and mortality. Beyond the physical ramifications of seizures, repeated seizures are a source of emotional stress and financial strain on the owner. Currently, there are options for treatment of seizures; however, not all patients respond to the most commonly used drugs and many of these drugs carry undesirable side effects. Zonisamide is an anti-convulsant withseveral likely mechanisms of action. Side effects associated with zonisamide administration are typically minor, including mild sedation, ataxia, and vomiting, all of which are usually temporary.A mild increase in alkaline phosphatase activity is possible with chronic use. Apparenthepatic necrosis has also been reported in 2 dogs with zonisamide use, and is likely attributable to sulfonamide hypersensitivity. This appears to be a rare reaction.Additionally, renal tubular acidosis has been reported in 1 dog for which the mechanism is unknown.Zonisamide is most commonly used as adjunctive maintenance therapy for dogs with refractory idiopathic epilepsy. This use is supported by several studies that demonstrated a significant decrease in seizure activity and frequency in dogs that had zonisamide added-on to their current first-line seizure therapy. In a significant number of cases it was possible to decrease the dosage of the first-line seizure medication, thus reducing side effects to the patient and cost to the client. Anecdotally, oral zonisamide has been used as the first-line or sole therapy in idiopathic epilepsy with success. The use of zonisamide monotherapy is supported by astudyshowing a 60% response rate (≥ 50% reduction in seizure activity) in dogs treated with zonisamide alone.Currently, zonisamide is only commercially available in a powder capsule for oral use. Previous pharmacokinetic studies have custom-made zonisamide into an injectable solution; however, it is unknown if injectable zonisamide will ever be commercially available. Since it is not available in injectable form, zonisamide has been administered rectally to dogs that are too sedate from drug side effects or neurologic disease to safely swallow medication. This practice is extrapolated from supportive published data using rodent models; however, there are no published pharmacologic data supporting the use of rectal administration of zonisamide in dogs. The current rectal zonisamide dosage used (30 mg/kg) is inferred from studies using rats and the appropriate canine dosing for rectal administration is unknown.The previously-mentioned study in rodents showed that rectally administered zonisamide formulated in both water and polyethylene glycol (PEG) suppository reached rapid therapeutic levels in the blood. Although both formulations performed well, the PEG formulation slightly out-performed the aqueous formulation.Although this data support the use of rectal zonisamide in dogs, rodent models are often inadequate for mimickingmetabolism for other species. We propose to measure the pharmacokinetics of a single dose of rectally administered zonisamide, to identify an appropriate canine rectal dosage, and to determine the ideal drug-delivery substrate (water v. PEG). This information can then be compared with previous pharmacokinetic studies on oral administration of zonisamide to determine whether rectally administered zonisamide is an appropriate substitute for oral zonisamide in patients unable to take oral medications.We hypothesize that: The pharmacokinetics of rectal zonisamide will make it a suitable alternative to oral zonisamide for use in patients who are unable to take oral medications.Aims of this proposal are:1. To measure the pharmacokinetics of rectally administered zonisamide2. To measure the pharmacokinetic effect of 2 doses (20mg/kg and 30mg/kg) of rectal zonisamide in the dog3. To compare the pharmacokinetics of zonisamide water formulation with PEG formulationAmerican College of Veterinary Internal Medicine, $7,853, 04/01/2014 to 03/31/2014
Animal Health Component
100%
Research Effort Categories
Basic
(N/A)
Applied
100%
Developmental
(N/A)
Goals / Objectives
1.Measure the pharmacokinetics of rectally administered zonisamide in the dog2.Measure the pharmacokinetic effect of 2 doses (20mg/kg and 30mg/kg) of rectal zonisamide in the dog3.Compare the pharmacokinetics of zonisamide water formulation with PEG formulation in the dog
Project Methods
Experimental subjects We plan to use 8 healthy, purpose-bred research hound dogs obtained from current protocols within the University of Tennessee - College of Veterinary Medicine teaching/research animal pools. The number of dogs selected is based on previously performed pharmacokinetic studies and commonly accepted minimum number necessary to collect meaningful pharmacokinetic data.Experimental designThe proposed study is a prospective, single-blind, cross-over design. Normal research dogs will be housed within the College of Veterinary Medicine. The principle investigator will perform a complete physical exam, within 14 days prior to administering zonisamide, to detect conditions that may preclude administering zonisamide. In addition, a complete blood count (CBC) and serum chemistry panel with electrolytes will be performed. Blood samples for CBC and serum chemistry panels will be obtained by jugular, cephalic or lateral saphenous venipuncture using a 20ga, 1 inch needle and a vacutainer or 3cc syringe.Food will be withheld 12 hours prior to rectal administration of zonisamide. Dogs will have free access to water at all times.Zonisamide rectal solution will be generated by mixing the powder contained in commercially available capsules with either water or PEG to a concentration of 100mg/mL. Doses administered in the study will include 20mg/kg and 30mg/kg. These doses were chosen based on anecdotal usage of rectal zonisamide as well as previous studies demonstrating rectal administration of zonisamide in rats and dogs.11,12 The rectal zonisamide solution will be administered using a red rubber catheter inserted approximately 2cm into the rectum.Eight dogs will be randomly divided into 4 treatment groups (n = 2/group) and the study completed over four treatment phases. All dogs will receive all 4 treatments over the course of the study according to the following schedule:Treatment Arm 1:Group 1: 20mg/kg zonisamide in water to a concentration of 100mg/mLGroup 2: 30mg/kg zonisamide in water to a concentration of 100mg/mLGroup 3: 20mg/kg zonisamide in PEG to a concentration of 100mg/mLGroup 4: 30mg/kg zonisamide in PEG to a concentration of 100mg/mLTreatment Arm 2:Group 1: 30mg/kg zonisamide in water to a concentration of 100mg/mLGroup 2: 20mg/kg zonisamide in water to a concentration of 100mg/mLGroup 3: 30mg/kg zonisamide in PEG to a concentration of 100mg/mLGroup 4: 20mg/kg zonisamide in PEG to a concentration of 100mg/mLTreatment Arm 3:Group 1: 20mg/kg zonisamide in PEG to a concentration of 100mg/mLGroup 2: 30mg/kg zonisamide in PEG to a concentration of 100mg/mLGroup 3: 20mg/kg zonisamide in water to a concentration of 100mg/mLGroup 4: 30mg/kg zonisamide in water to a concentration of 100mg/mLTreatment Arm 4:Group 1: 30mg/kg zonisamide in PEG to a concentration of 100mg/mLGroup 2: 20mg/kg zonisamide in PEG to a concentration of 100mg/mLGroup 3: 30mg/kg zonisamide in water to a concentration of 100mg/mLGroup 4: 20mg/kg zonisamide in water to a concentration of 100mg/mLAll blood samples will be taken using jugular, cephalic or lateral saphenous venipuncture with a 20ga, 1 inch needle and vacutainer or 6cc syringe. A baseline (0 hour) blood sample will be obtained. Zonisamide will then be administered per rectum using a 100mg/mL solution via a red rubber catheter to each treatment group using the dosing schedule as described above. Blood samples will be obtained at 1, 4, 8, 12, 18, 24 and 48 hours post-rectal administration. Blood samples will be collected in lithium heparin collection tubes, immediately centrifuged for 15 minutes and plasma decanted into separate cryopreservation tubes. The plasma will be stored frozen at -20 degrees Celsius until assayed. This method of sample handling is in accordance with previously described methods for measurement of plasma zonisamide concentrations.9,10,13 Timing for collection of blood samples is based on known pharmacokinetic properties of zonisamide as well as previous pharmacokinetic study designs.9,10,13All dogs will have a 7-day washout period between each of the four treatment phases during which they will be returned to their regular housing. After the washout periods, the groups will be rotated and the procedures described above will be repeated until all dogs have received all four treatment types. The study will then end and the dogs will return to their previous housing areas. Analytical proceduresSerum zonisamide concentrations at each time point will be measured using a previously validated immunoassay procedure. This analysis will be performed at Auburn University College of Veterinary Medicine Clinical Pharmacology Laboratory under the direction of Dr. Dawn Boothe. While previous pharmacokinetic studies on zonisamide have employed high pressure liquid chromatography (HPLC) for determining serum concentrations, immunoassay is now the preferred method of analysis at Auburn and has advantages over HPLC in simplicity, speed and lack of special equipment needed while providing good correlation with HPLC measurements14.Pharmacokinetic analysis will be performed by Auburn University College of Veterinary Medicine Clinical Pharmacology Laboratory. Measurements of plasma concentration - time curve and values including area under the curve (AUC), bioavailability (F), maximum concentration (Cmax), time to maximum concentration (tmax), clearance (Cl), elimination half-life (t1/2) and volume of distribution (Vd) will be performed.Statistical analysisStatistical analysis will be performed under the guidance of Ann Reed at the University of Tennessee. All parameters will be reported as mean +/- standard deviation. One Way Analysis of Variance (ANOVA) will be used to determine if significant differences occur between the pharmacokinetic parameters (i.e. bioavailability, Cmax and tmax) of two levels of zonisamide formulations and two levels of zonisamide dosage. Assumptions will be assessed and a description of analysis will be provided with a P-value of 0.05 used to determine significance.