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“Pexion” – the new antiepileptic drug
Since the release of Pexion, much has been spoken about this new antiepileptic drug
(AED). Many practitioners wonder if this drug is a ‘better drug’ to control epilepsy
and if this means it is advisable to start or change the AED treatment in epileptic
patients to Pexion. As one might expect, there is no clear answer to this enquiry and
every patient and every drug has several aspects to consider.
The anticonvulsant efficacy of Pexion (imepitoin) along with its safety profile in dogs
has lead to its development as a new AED for canine epilepsy. Following long-term
laboratory and clinical research, imepitoin became licensed to initiate treatment in
dogs with idiopathic epilepsy and was launched in April 20131. The major advantage
with the release of Pexion is that another licensed AED is now available to treat
canine epilepsy next to phenobarbital (also licensed to initiate therapy) and potassium
bromide (licensed as an adjunctive therapy). Although other AEDs are available and
very much have their place in treating epilepsy for the benefit of our patients, these
drugs are un-licensed and many have limited research data behind them.
Canine epilepsy
By definition, epilepsy is a chronic neurological disorder characterised by two or
more incidents of seizure activity. Epilepsy is the most common chronic neurological
disease in dogs. The prevalence of ‘idiopathic’ epilepsy in first opinion practice in the
UK is estimated to be around 0.6%, which equals about 56,000 dogs of the UK
population2. Many of these dogs receive long-term AED therapy and about 2/3 of
dogs with idiopathic epilepsy respond well to one or a combination of two AEDs,
whereas 1/3 of epileptic dogs remain poorly controlled. Dogs with poorly controlled
epilepsy have an increased risk of behaviour changes, a reduced quality of life and
premature death3, 4, 5. And here it is important to recognise, that epilepsy does not only
affect the quality of life of the dog but also that of the owner3.
Diagnosis, AED treatment and its expectations
The diagnosis of “idiopathic epilepsy” is made by exclusion in dogs aged 6 months to
6 years with recurrent seizure activity and a normal interictal period. Minimum
database is considered a full haematology and biochemistry profile. Initiation of AED
therapy is advised if a patient presents with status epilepticus, cluster seizures, severe
postictal periods, increasing frequency and severity of seizures, or two or more
isolated seizures in a six month period.
Unfortunately, there is no ideal drug to treat epilepsy. The general advice is to start a
dog on one AED, monitor its efficacy and increase the dose as necessary.
Phenobarbital and potassium bromide require assessment of serum levels and the dose
can be increased until medium to high serum level is reached (e.g. up to 35 μg/mL for
phenobarbital) if dogs don’t respond to lower doses. If sufficient seizure control is not
reached with appropriate serum level and dosing on monotherapy, a second AED can
be considered. Once optimum drug dose is chosen and more drugs are added, the
chances of seizure control seems to decline with each added drug. It further appears
that this phenomenon is only little related to the type of available AED used. This is
similar to people and a study in 1098 epileptic humans reported a higher probability
of seizure freedom in patients receiving 1 drug compared to 2 drug regimens, and 2
drugs compared to 3 drug regimens. Similar to veterinary experience, the study
©Dr. Annette Wessmann, November 2013
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concluded that the chance of seizure freedom declined with successive drug
regimens.6
With the release of Pexion we have a another choice in how to initiate therapy for
patients with idiopathic epilepsy and the below information may be useful in helping
when to decide if it might be the right choice for a patient presenting with seizures.
When to use Pexion?1
- Licensed for idiopathic epilepsy. It is not licensed for seizures caused by a
known aetiology e.g. brain tumour, meningoencephalitis or metabolic
disturbances.
- Licensed as monotherapy for the reduction of the frequency of generalised
seizures. It is not licensed for add-on therapy. However, study results suggest
improved seizure control similar to potassium bromide and no harmful clinical
interactions when used as an add-on therapy to phenobarbital. Information
about combining Pexion with potassium bromide therapy is not available.
- Licensed for dogs only. Not for cats. No reports are available about
experiences in feline epilepsy.
- Licensed for single generalised seizures. The efficacy of Pexion to treat dogs
with status epilepticus and cluster seizures has not been studied; therefore it
should not be used as a primary treatment in dogs with cluster seizures or
status epilepticus. Moreover, Pexion comes in an oral formulation only
(tablets) limiting its practical application for dogs in status epilepticus.
Effectiveness, side effects and quality of life aspect
Study results showed similar efficacy in terms of seizure control when comparing
Pexion and phenobarbital. Pexion reduced the average number of generalised seizures
from 2.3 to 1.1 per month after 20 weeks of treatment compared with a reduction of
2.4 seizures to 1.1 per month with phenobarbital. During the evaluation phase of 12
weeks, 47% (30 out of 64) of Pexion treated dogs were free from generalised seizures,
whilst 58% (51 out of 88) of phenobarbital treated dogs had no seizures.1
Mild and generally transient side effects occurred rarely and more often at the
beginning of the treatment, including polyphagia, hyperactivity, ataxia and others less
frequently. 1 In particular, some owners in the study commented on the beneficial
aspect of the anxiolytic properties of Pexion for their pet.
While anecdotally seizure control is often the main focus in canine idiopathic epilepsy
treatment, the quality of life aspect has gained increased attention and importance in
recent years. Quality of life is not only determined by the number of seizures but also
by the overall well being, side effects and the overall impact on life in an epileptic
dog. In people, the quality of life aspect is considered to be important in determining
the more suitable antiepileptic drug. In a questionnaire-based veterinary study
published in 2012, around half of owners perceived that their dogs and their own QoL
was negatively affected by the adverse effect profile of what was then the standard
AEDs used in veterinary medicine3. The safety profile of Pexion, with reported fewer
side effects, makes this drug a good alternative to other AEDs available.
©Dr. Annette Wessmann, November 2013
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Pexion: Dosage, metabolism, excretion, tolerance, risk1
Action
- Low affinity partial agonist of the benzodiazepine receptor. It
inhibits seizures by potentiating the effect of GABA on the
GABAA-receptor
- Imepitoin only works in the presence of GABA meaning that
it will exert its inhibitory effects in overactive neurons, thus
specifically targeting the seizure activity.
Dosage and
- Start with 10mg/kg twice daily.
administration
- Increase by 50-100% in weekly increments to a maximum of
30mg/kg twice daily if seizures are not adequately controlled.
Administration
- Oral medication (tablets)
- Can be administered with or without food. The timing of
administration in relation to feeding should be kept
consistent.
- Bioavailability is 92% on an empty stomach and slightly
reduced if given with food.
Steady state
- Half life of about 1.5 hours
- Steady state reached within 3 days yet starts working within a
few hours
Metabolism &
- Rapidly metabolised via oxidative metabolism in the liver
Elimination
- No change in liver values reported
- Majority excreted in the faeces rather than urine.
Monitoring
- Therapeutic levels are achieved with a dose of 10-30mg/kg
BID
- No drug level testing required, as there is no correlation
between efficacy and serum levels
- No evidence of liver enzyme induction; does not involve the
cytochrome P450 system.
Physical
- No dependence has been shown to develop with Pexion
dependence
Tolerance
- No loss of anticonvulsant efficacy (tolerance development)
development
with continuous treatment
Not to be used
- In dogs with severely impaired liver, kidney or heart function.
How to change from phenobarbital to Pexion
Many practitioners enquired how to change from phenobarbital to Pexion following
the release of Pexion. It should be noted that the decision of changing the AED for an
epileptic patient should not be taken lightly. Clinical judgement is important in the
decision of whether or not changing the seizure treatment is the appropriate choice for
the individual patient and here side effects and seizure control are the main influences
to consider. If a patient is adequately controlled and has no or only minor side effects,
a change in AED therapy is not recommended. The concern is that inappropriate
change in AED therapy may result in seizure re-occurrence in a previously wellcontrolled patient. In particular, if phenobarbital is stopped abruptly there is a risk of
severe withdrawal seizures upon termination of the treatment. If the choice is made,
the change should be performed carefully. It is usually done by reducing the AED by
25% each month and at the same time starting the patient on the new AED.7
©Dr. Annette Wessmann, November 2013
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Conclusion
With the release of Pexion, we clinicians are in the fortunate position to be able to
choose between two licensed veterinary products to initiate the treatment of dogs with
idiopathic epilepsy. Pexion, as well as phenobarbital, is a well researched AED and
the level of seizure control that is achievable is similar with either product. In the past,
treatment options were limited if patients did not respond well to the main AED
therapy and the release of Pexion has opened the window for an increased treatment
choice for the benefit of our patients. If an epileptic dog is not well controlled and
shows severe side effects, it is entirely appropriate to consider AED change with the
aim of improving seizure control and quality of life of our epileptic patients.
Important to note is, that a change should not be performed abruptly. Instead, care
should be taken and a protocol followed that reduces the AED by 25% each month at
the same time as starting the patient on the new AED. If a dog is adequately
controlled by current medication and side effects don’t, or don’t severely affect the
quality of life of the patient, then the current AED therapy should be continued.
References
1.) Pexion® Summary of Product Characteristics, 2013. Available at
http://www.ema.europa.eu/docs/en_GB/document_library/EPAR__Summary_for_the_public/veterinary/002543/WC500140843.pdf
and
European Public Assessment Report (EPAR): Pexion (imepitoin). London:
European
Medicines
Agency;
2013.
Available
at
http://www.ema.europa.eu/ema/index.jsp?curl=pages/medicines/veterinary/me
dicines/002543/vet_med_000268.jsp&mid=WC0b01ac058001fa1c
2.) Kearsley-Fleet L, O'Neill DG, Volk HA, Church DB, Brodbelt DC:
Prevalence and risk factors for canine epilepsy of unknown origin in the UK.
The Veterinary Record 2013, 172(13):338.
3.) Wessmann A, Volk H A, Parkin T, Ortega M and Anderson T J (2012). Living
with canine idiopathic epilepsy: a questionnaire-based evaluation of quality of
life, Proceedings of the 24th Symposium ESVN-ECVN, J Vet Intern Med
26(3) 823-852.
4.) Shihab N, Bowen J, Volk HA. Behavioral changes in dogs associated with the
development of idiopathic epilepsy. Epilepsy & behavior 2011, 21(2):160167.
5.) Berendt M, Gredal H, Ersbøll AK, Alving J. Premature death, risk factors, and
life patterns in dogs with epilepsy. Journal of Veterinary Internal Medicine
2007, 21(4):754-759.
6.) Brodie MJ, Barry SJ, Bamagous GA, Norrie JD, Kwan P “Patterns of
treatment response in newly diagnosed epilepsy.” Neurology. 2012,
78(20):1548.
7.) Penderis J, Volk H: Switching between medications for the management of
epilepsy in dogs. The Veterinary Record 2013, 173(13):323-324.
©Dr. Annette Wessmann, November 2013
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