Topiramate

Overview

Topiramate is an anticonvulsant that may be useful for the treatment of refractory seizure disorders in dogs, particularly partial seizures. It may also be beneficial in cats, although clinical information in this species is limited.

Topiramate has a very short elimination half-life in dogs (approximately 2–4 hours). Therapeutic activity may persist because of its high affinity for receptors in the brain, but the short half-life may still make frequent dosing necessary and can limit the drug’s usefulness for long-term maintenance. Generic formulations are available, although treatment cost may still be a consideration.

Mechanism of Action (MOA): Although its exact anticonvulsant mechanism is unknown, topiramate blocks repetitive neuronal action potentials, inhibits voltage-dependent sodium and calcium channels, enhances GABAA-mediated inhibitory activity, and antagonizes kainate/AMPA receptors without affecting NMDA receptors. It is also a weak inhibitor of carbonic anhydrase isoenzymes CA-II and CA-IV; this effect is not believed to contribute significantly to its antiepileptic activity, but it may influence the drug’s adverse effect profile.

Indications

Topiramate is used primarily as an adjunctive anticonvulsant for refractory seizure disorders in dogs. Its use in cats is less well established because of limited clinical information.

• Refractory seizure disorders (dogs): Adjunctive treatment, particularly for partial seizures.
• Refractory seizure disorders (cats): May be beneficial, although clinical experience is limited.
• Feline idiopathic ulcerative dermatitis (cats): Reported in a single case.
• Neuropathic pain (dogs): Reported as an adjunctive analgesic; alternative medications should be considered first.
• Feline hyperesthesia syndrome (cats): An anticonvulsant that may also be used to manage self-mutilation (repetitive biting, chewing, licking, or scratching that causes self-injury).

Dosage (Reference)

Dog

In dogs, topiramate is used as an adjunctive anticonvulsant for refractory seizure disorders. Therapy should be initiated at the lower end of the dosage range and adjusted gradually according to clinical response.

Cat

Clinical dosage information in cats is limited. Published information includes anecdotal anticonvulsant dosing and a single case report for feline idiopathic ulcerative dermatitis.

Warnings & Precautions

Topiramate should be used cautiously where hepatic or renal function is impaired, and veterinary experience with the drug remains limited. Appropriate precautions should also be taken when handling this medication due to its hazardous drug classification.

• Contraindicated in patients with known hypersensitivity to topiramate.
• Use cautiously in patients with impaired hepatic or renal function.
• Hazardous drug: Topiramate is classified by the National Institute for Occupational Safety and Health (NIOSH) as a hazardous drug. Appropriate personal protective equipment (PPE) should be used to minimize exposure during handling.
• Pregnancy & lactation: Teratogenic effects have been reported in mice and rats at dosages equivalent to those used in humans, and topiramate enters maternal milk. Because safety has not been established in veterinary patients, use only when the benefits outweigh the potential risks.

Drug Interactions

The following drug interactions have been reported or are considered theoretical. Concurrent use is not necessarily contraindicated unless otherwise stated, but additional monitoring may be required.

• Amitriptyline: Topiramate may increase amitriptyline concentrations.
• Carbonic anhydrase inhibitors (e.g., acetazolamide, dichlorphenamide): Concurrent use may increase the risk of renal calculus formation; topiramate itself acts as a mild carbonic anhydrase inhibitor.
• CNS depressants (e.g., acepromazine, cannabidiol, gabapentin, trazodone): May exacerbate the CNS adverse effects of topiramate.
• Furosemide: Concurrent use may increase the risk of hypokalemia.
• Hydrochlorothiazide: May increase topiramate exposure and the risk of hypokalemia.
• Methenamine: Topiramate may alter urine pH and decrease the efficacy of methenamine.
• Opioids (e.g., buprenorphine, hydrocodone, tramadol): May exacerbate the CNS adverse effects of topiramate.
• Phenobarbital: Concurrent use may decrease topiramate serum concentrations; in dogs, phenobarbital co-administration was associated with a 5.6-fold greater topiramate clearance and an approximately 4-fold shorter elimination half-life.
• Posaconazole: Concurrent use may increase topiramate plasma concentrations and the risk of toxicity.

Side Effects & Overdose

Side Effects

Because topiramate has not been widely used in veterinary patients, its adverse effect profile has not been fully established. Most reported adverse effects involve the gastrointestinal and central nervous systems.

• Dogs: Gastrointestinal distress, sedation, ataxia, inappetence, weight loss, and irritability. These signs often lessen or resolve over time.
• Cats: Sedation and inappetence have been reported.
• Renal tubular acidosis: Reported in a cat; resolved after discontinuation of topiramate and supportive care.
• Rare adverse effects (humans): Acute myopia with secondary angle-closure glaucoma has been reported.
• Renal effects: May cause hyperchloremic metabolic acidosis, increased urine pH, reduced urinary citrate excretion, and an increased risk of calcium phosphate renal calculi; in people the incidence of kidney stones is 2 to 4 times that of the general population.
• Hematologic effects: Mild subclinical anemia developed in four of eight healthy cats given the extended-release formulation once daily for 30 days; no clinically apparent adverse effects were seen, but the finding calls the safety of long-term administration into question.

Overdose

Clinical signs following topiramate overdose primarily involve the central nervous system. Treatment is largely supportive.

• Clinical signs (dogs and cats): Lethargy, ataxia, and vomiting (cats).
• Signs reported in people: Overdose in humans has caused convulsions, drowsiness or lethargy, slurred speech, blurred and double vision, impaired mentation, stupor, ataxia, metabolic acidosis, hypotension, agitation, and abdominal pain.
• Management: Consider gastrointestinal decontamination if ingestion was recent.
• Supportive care: Provide supportive therapy as needed. Hemodialysis enhances elimination of topiramate.
• Poison consultation: Consultation with a veterinary poison center is recommended for known or suspected overdoses.

Key Notes

Practical clinical points that help optimize the safe and effective use of topiramate in dogs and cats:

• Do not discontinue abruptly: Sudden withdrawal may result in recurrence of seizures.
• Administration: May be given with or without food. If gastrointestinal upset occurs when given on an empty stomach, administer with food or a small treat.
• Dose consistency: Administer doses at the same time each day and avoid missed doses to help maintain seizure control.
• Seizure diary: Maintaining a seizure diary can help evaluate treatment effectiveness and guide therapy adjustments.
• Pharmacokinetics: Oral absorption in dogs is rapid but bioavailability varies between 30 and 60%. In people the drug is not extensively metabolized and about 70% is excreted unchanged in the urine.
• Dose adjustment on co-therapy: When topiramate is added to a dog already receiving an enzyme-inducing anticonvulsant such as phenobarbital, a higher topiramate dose is likely to be needed to reach the same plasma concentration.
• Blood levels: Plasma concentrations are not routinely monitored; where they are measured, the human therapeutic range is 2 to 25 mg/L. No specific laboratory-test interference has been reported.
• Extended-release formulation: In healthy cats, 10 mg/kg once daily maintained target plasma concentrations, but the effect on seizure control was not studied and safety with long-term use is not established.
• Dispensing: Store tablets in tight containers at room temperature and sprinkle capsules below 25°C; protect both from moisture.