Pharmacokinetics of single-dose rectal zonisamide administration in normal dogs.

BACKGROUND: Few medications are available for parental administration to animals with seizures. Rectal administration of medications is often used if the animal cannot be administered oral medications. HYPOTHESIS/OBJECTIVES: To determine the pharmacokinetic differences in zonisamide when administered rectally in either of 2 vehicles and p.o. to dogs. ANIMALS: Eight healthy research dogs. METHODS: Randomized cross-over design. Zonisamide, 10 mg/kg, was administered rectally in polyethylene glycol (PEG-R), rectally in water (H2O-R), and as an oral capsule. Plasma zonisamide concentrations were measured until 72 hours after administration. Zonisamide was quantitated by HPLC and plasma concentration versus time curve data was analyzed by using noncompartmental modeling. RESULTS: Mean maximum plasma zonisamide concentrations (µg/mL) were significantly higher after oral administration (11.56 ± 4.04) compared to H2O-R (5.00 ± 1.83) (P = .004). Disappearance half-life (hours) and mean time to maximum concentration (hours) were not significantly different between methods of administration. Mean relative bioavailability of PEG-R (85 ± 69%) was significantly higher than that of H2O-R (53 ± 37%) (P = .039). Dogs tolerated all dosing forms with no evidence of adverse effects. CONCLUSIONS AND CLINICAL IMPORTANCE: The vehicle in which zonisamide is dissolved influences rectal bioavailability, with PEG preferred to H2O-R. Because of the prolonged time to maximum concentration, rectal administration of zonisamide should not be used to treat status epilepticus in dogs. A dose higher than what was used in this study might be necessary, if currently recommended minimum therapeutic concentrations (10 µg/mL) are to be achieved with a single-dose administration.

Diagnosis and surgical resection of a choroid plexus cyst in a dog.

A three-year-old neutered male toy fox terrier presented for a Chiari-like malformation. No neurological deficits were found on examination, although diffuse cervical, thoracolumbar and head pain were present. A mass within the fourth ventricle was apparent on magnetic resonance imaging (MRI) of the brain. The lesion was hyperintense to brain parenchyma on T2-weighted images, hypointense on T1-weighted images and there was strong, homogeneous contrast enhancement. The cystic mass was removed through a suboccipital craniectomy. Histopathology was consistent with a choroid plexus cyst. The dog recovered well from the procedure and was clinically normal three months after surgery. To the authors' knowledge this is the first description of the appearance of a choroid plexus cyst on MRI in a dog and of its surgical removal. Although they are an uncommon finding, choroid plexus cysts should be considered as a differential diagnosis for mass lesions within the fourth ventricle.