We all know cats are not small dogs, so how does levetiracetam (leh-vuh-tr-A-suh-tam) differ between species?
Metabolism
The mechanism of action (modification of the SV2A receptor) is the same for cats and dogs. This mechanism of action (MOA) is unique to levetiracetam and different that the MOA for phenobarbital.
Formulations
There are two formulations available 1) standard release (SRL) and 2) extended release (XRL). The dosage of 20 mg/kg PO q8h for the SRL formulation, comes from pharmacokinetic analysis of this drug in a cohort of healthy cats. A therapeutic range has not yet been developed for cats therefore if seizure control is poor, the dosage is often increased until signs of toxicity are noted and then reduced to the highest effective dose with minimal side effects. When doing that, the prescriber is using the individual animal as a guide for toxicity rather than an established therapeutic range. Reported side effects include hypersalivation (mild, transient), inappetence and mild lethargy. There are very few efficacy studies for cats, however in 2008, a single study reported a greater than 50% reduction in seizures in 7 of 10 cats when levetiracetam was added to phenobarbital. Liquid formulations are readily available through compounding pharmacies and can be used interchangeably with the 250 mg size tablets. Use caution when prescribing the liquid formulation to ensure it does not have xylitol as an added ingredient.
Extended release levetiracetam is available in 500 mg and 750 mg size tablets. Historically, this has limited its use in cats. In 2017, I decided it was high time we changed that thinking so we evaluated the pharmacokinetics of a single dose of 500 mg XRL in healthy cats and found that it was well tolerated with minimal side effects. Furthermore, we identified that a reduce dosing interval from q8hr (SRL) to q24h when using XRL was appropriate for cats! The serum levetiracetam concentrations were really high therefore we subsequently evaluated the use of levetiracetam over 10 days to monitor for drug accumulation. Thankfully, none was identified! No efficacy studies have been performed using 500 mg PO XRL q24h in cats, to date, however I do recommend this dosage for cats, when levetiracetam is needed and q8h dosing isn't an option.
The story doesn't end there! Medicating cats is such a terrible thing to do to a cat (and horrifying for some owners) that I then explored the idea of transdermal levetiracetam (TD).The dosage of 20 mg/kg transdermal q8h resulted in serum concentrations similar to those of the oral route with minimal side effects. We have not evaluated TD levetiracetam long-term so efficacy remains unknown. Do I use TD levetiracetam? Yes. I ensure that the clients know that this is cutting edge research and therefore long-term efficacy studies have not been performed; purely that it is well tolerated.
That's all for now! Please reach out with any suggested topics and stay tuned for a super fun neurology CE event coming this summer. Shhhh...it's still in the planning stages!
Have a great week!