Components:
Method of action:
Treatment option:
Medically reviewed by Militian Inessa Mesropovna, PharmD. Last updated on 10.04.2022
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Monotherapy in adults, adolescents and children over 6 years of age with partial seizures with or without secondary generalised seizures, and primary generalised tonic-clonic seizures.
Adjunctive therapy in children aged 2 years and above, adolescents and adults with partial onset seizures with or without secondary generalization or primary generalized tonic-clonic seizures and for the treatment of seizures associated with Lennox-Gastaut syndrome.
Topiramate is indicated in adults for the prophylaxis of migraine headache after careful evaluation of possible alternative treatment options. Topiramate is not intended for acute treatment.
Posology
It is recommended that therapy be initiated at a low dose followed by titration to an effective dose. Dose and titration rate should be guided by clinical response.
It is not necessary to monitor topiramate plasma concentrations to optimize therapy with Topiramate-Canon. On rare occasions, the addition of topiramate to phenytoin may require an adjustment of the dose of phenytoin to achieve optimal clinical outcome. Addition or withdrawal of phenytoin and carbamazepine to adjunctive therapy with Topiramate-Canon may require adjustment of the dose of Topiramate-Canon.
In patients with or without a history of seizures or epilepsy, antiepileptic drugs (AEDs) including topiramate should be gradually withdrawn to minimize the potential for seizures or increased seizure frequency. In clinical trials, daily dosages were decreased in weekly intervals by 50-100 mg in adults with epilepsy and by 25-50 mg in adults receiving topiramate at doses up to 100 mg/day for migraine prophylaxis. In paediatric clinical trials, topiramate was gradually withdrawn over a 2-8 week period.
Monotherapy epilepsy
General
When concomitantAEDs are withdrawn to achieve monotherapy with topiramate, consideration should be given to the effects this may have on seizure control. Unless safety concerns require an abrupt withdrawal of the concomitant AED, a gradual discontinuation at the rate of approximately one-third of the concomitant AED dose every 2 weeks is recommended.
When enzyme inducing medicinal products are withdrawn, topiramate levels will increase. A decrease in Topiramate-Canon (topiramate) dosage may be required if clinically indicated.
Adults
Dose and titration should be guided by clinical response. Titration should begin at 25 mg nightly for 1 week. The dosage should then be increased at 1- or 2-week intervals by increments of 25 or 50 mg/day, administered in two divided doses. If the patient is unable to tolerate the titration regimen, smaller increments or longer intervals between increments can be used.
The recommended initial target dose for topiramate monotherapy in adults is 100 mg/day to 200 mg/day in 2 divided doses. The maximum recommended daily dose is 500 mg/day in 2 divided doses. Some patients with refractory forms of epilepsy have tolerated topiramate monotherapy at doses of 1,000 mg/day. These dosing recommendations apply to all adults including the elderly in the absence of underlying renal disease.
Paediatric population (children over 6 years of age)
Dose and titration rate in children should be guided by clinical outcome. Treatment of children over 6 years of age should begin at 0.5 to 1 mg/kg nightly for the first week. The dosage should then be increased at 1 or 2 week intervals by increments of 0.5 to 1 mg/kg/day, administered in two divided doses. If the child is unable to tolerate the titration regimen, smaller increments or longer intervals between dose increments can be used.
The recommended initial target dose range for topiramate monotherapy in children over 6 years of age is 100 mg/day depending on clinical response, (this is about 2.0mg/kg/day in children 6-16 years).
Adjunctive therapy epilepsy (partial onset seizures with or without secondary generalization, primary generalized tonic-clonic seizures, or seizures associated with Lennox-Gastaut syndrome)
Adults
Therapy should begin at 25-50 mg nightly for one week. Use of lower initial doses has been reported, but has not been studied systematically. Subsequently, at weekly or bi-weekly intervals, the dose should be increased by 25-50 mg/day and taken in two divided doses. Some patients may achieve efficacy with once-a-day dosing.
In clinical trials as adjunctive therapy, 200 mg was the lowest effective dose. The usual daily dose is 200-400 mg in two divided doses.
These dosing recommendations apply to all adults, including the elderly, in the absence of underlying renal disease.
Paediatric population (children aged 2 years and above)
The recommended total daily dose of Topiramate-Canon (topiramate) as adjunctive therapy is approximately 5 to 9 mg/kg/day in two divided doses. Titration should begin at 25 mg (or less, based on a range of 1 to 3 mg/kg/day) nightly for the first week. The dosage should then be increased at 1- or 2-week intervals by increments of 1 to 3 mg/kg/day (administered in two divided doses), to achieve optimal clinical response.
Daily doses up to 30 mg/kg/day have been studied and were generally well tolerated.
Migraine
Adults
The recommended total daily dose of topiramate for prophylaxis of migraine headache is 100 mg/day administered in two divided doses. Titration should begin at 25 mg nightly for 1 week. The dosage should then be increased in increments of 25 mg/day administered at 1-week intervals. If the patient is unable to tolerate the titration regimen, longer intervals between dose adjustments can be used.
Some patients may experience a benefit at a total daily dose of 50 mg/day. Patients have received a total daily dose up to 200 mg/day. This dose may be benefit in some patients, nevertheless, caution is advised due to an increase incidence of side effects.
Paediatric population
Topiramate-Canon (topiramate) is not recommended for treatment or prevention of migraine in children due to insufficient data on safety and efficacy.
General dosing recommendations for Topiramate-Canon in special patient populations
Renal impairment
In patients with impaired renal function (CLCR ≤ 70 mL/min) topiramate should be administered with caution as the plasma and renal clearance of topiramate are decreased. Subjects with known renal impairment may require a longer time to reach steady-state at each dose. Half of the usual starting and maintenance dose is recommended.
In patients with end-stage renal failure, since topiramate is removed from plasma by haemodialysis, a supplemental dose of Topiramate-Canon equal to approximately one-half the daily dose should be administered on haemodialysis days. The supplemental dose should be administered in divided doses at the beginning and completion of the haemodialysis procedure. The supplemental dose may differ based on the characteristics of the dialysis equipment being used.
Hepatic impairment
In patients with moderate to severe hepatic impairment topiramate should be administered with caution as the clearance of topiramate is decreased.
Elderly
No dose adjustment is required in the elderly population providing renal function is intact.
Method of administration
Topiramate-Canon is available in film-coated tablets and a hard capsule formulation, for oral administration. It is recommended that film-coated tablets not be broken. The hard capsule formulation is provided for those patients who cannot swallow tablets, e.g. paediatric and the elderly.
Topiramate-Canon can be taken without regard to meals.
Migraine prophylaxis in pregnancy and in women of childbearing potential if not using a highly effective method of contraception.
In situations where rapid withdrawal of topiramate is medically required, appropriate monitoring is recommended.
As with other AEDs, some patients may experience an increase in seizure frequency or the onset of new types of seizures with topiramate. These phenomena may be the consequence of an overdose, a decrease in plasma concentrations of concomitantly used AEDs, progress of the disease, or a paradoxical effect.
Adequate hydration while using topiramate is very important. Hydration can reduce the risk of nephrolithiasis (see below). Proper hydration prior to and during activities such as exercise or exposure to warm temperatures may reduce the risk of heat-related adverse reactions.
Women of childbearing potential
Topiramate may cause fetal harm and fetal growth restriction (small for gestational age and low birth weight) when administered to a pregnant woman. The North American Antiepileptic Drug pregnancy registry data for topiramate monotherapy showed an approximate 3-fold higher prevalence of major congenital malformations (4.3%), compared with a reference group not taking AEDs (1.4%). In addition, data from other studies indicate that, compared with monotherapy, there is an increased risk of teratogenic effects associated with the use of AEDs in combination therapy.
Before the initiation of treatment with topiramate in a woman of childbearing potential, pregnancy testing should be performed and a highly effective contraceptive method advised. The patient should be fully informed of the risks related to the use of topiramate during pregnancy.
Oligohydrosis
Oligohydrosis (decreased sweating) has been reported in association with the use of topiramate. Decreased sweating and hyperthermia (rise in body temperature) may occur especially in young children exposed to high ambient temperature.
Mood disturbances/depression
An increased incidence of mood disturbances and depression has been observed during topiramate treatment.
Suicide/suicide ideation
Suicidal ideation and behaviour have been reported in patients treated with anti-epileptic agents in several indications. A meta-analysis of randomised placebo-controlled trials of AEDs has shown a small increased risk of suicidal ideation and behaviour. The mechanism of this risk is not known and the available data do not exclude the possibility of an increased risk for topiramate.
In double blind clinical trials, suicide related events (SREs) (suicidal ideation, suicide attempts and suicide) occurred at a frequency of 0.5% in topiramate treated patients (46 out of 8,652 patients treated) and at a nearly 3-fold higher incidence than those treated with placebo (0.2%; 8 out of 4,045 patients treated).
Patients therefore should be monitored for signs of suicidal ideation and behaviour and appropriate treatment should be considered. Patients (and caregivers of patients) should be advised to seek medical advice should signs of suicidal ideation or behaviour emerge.
Nephrolithiasis
Some patients, especially those with a predisposition to nephrolithiasis, may be at increased risk for renal stone formation and associated signs and symptoms such as renal colic, renal pain or flank pain.
Risk factors for nephrolithiasis include prior stone formation, a family history of nephrolithiasis and hypercalciuria. None of these risk factors can reliably predict stone formation during topiramate treatment. In addition, patients taking other medicinal products associated with nephrolithiasis may be at increased risk.
Decreased renal function
In patients with impaired renal function (CLCR ≤ 70 mL/min) topiramate should be administered with caution as the plasma and renal clearance of topiramate are decreased.
Decreased hepatic function
In hepatically-impaired patients, topiramate should be administered with caution as the clearance of topiramate may be decreased.
Acute myopia and secondary angle closure glaucoma
A syndrome consisting of acute myopia associated with secondary angle closure glaucoma has been reported in patients receiving topiramate. Symptoms include acute onset of decreased visual acuity and/or ocular pain. Ophthalmologic findings can include myopia, anterior chamber shallowing, ocular hyperaemia (redness) and increased intraocular pressure. Mydriasis may or may not be present. This syndrome may be associated with supraciliary effusion resulting in anterior displacement of the lens and iris, with secondary angle closure glaucoma. Symptoms typically occur within 1 month of initiating topiramate therapy. In contrast to primary narrow angle glaucoma, which is rare under 40 years of age, secondary angle closure glaucoma associated with topiramate has been reported in paediatric patients as well as adults. Treatment includes discontinuation of topiramate, as rapidly as possible in the judgment of the treating physician, and appropriate measures to reduce intraocular pressure. These measures generally result in a decrease in intraocular pressure.
Elevated intraocular pressure of any aetiology, if left untreated, can lead to serious sequelae including permanent vision loss.
A determination should be made whether patients with history of eye disorders should be treated with topiramate.
Visual field defects
Visual field defects have been reported in patients receiving topiramate independent of elevated intraocular pressure. In clinical trials, most of these events were reversible after topiramate discontinuation. If visual field defects occur at any time during topiramate treatment, consideration should be given to discontinuing the drug.
Metabolic acidosis
Hyperchloremic, non-anion gap, metabolic acidosis (i.e. decreased serum bicarbonate below the normal reference range in the absence of respiratory alkalosis) is associated with topiramate treatment. This decrease in serum bicarbonate is due to the inhibitory effect of topiramate on renal carbonic anhydrase. Generally, the decrease in bicarbonate occurs early in treatment although it can occur at any time during treatment. These decreases are usually mild to moderate (average decrease of 4 mmol/l at doses of 100 mg/day or above in adults and at approximately 6 mg/kg/day in paediatric patients). Rarely, patients have experienced decreases to values below 10 mmol/l. Conditions or therapies that predispose to acidosis (such as renal disease, severe respiratory disorders, status epilepticus, diarrhoea, surgery, ketogenic diet, or certain medicinal products) may be additive to the bicarbonate lowering effects of topiramate.
Chronic metabolic acidosis increases the risk of renal stone formation and may potentially lead to osteopenia.
Chronic metabolic acidosis in paediatric patients can reduce growth rates. The effect of topiramate on bone-related sequelae has not been systematically investigated in paediatric or adult populations.
Depending on underlying conditions, appropriate evaluation including serum bicarbonate levels is recommended with topiramate therapy. If signs or symptoms are present (e.g. Kussmaul's deep breathing, dyspnoea, anorexia, nausea, vomiting, excessive tiredness, tachycardia or arrhythmia), indicative of metabolic acidosis, measurement of serum bicarbonate is recommended. If metabolic acidosis develops and persists, consideration should be given to reducing the dose or discontinuing topiramate (using dose tapering).
Topiramate should be used with caution in patients with conditions or treatments that represent a risk factor for the appearance of metabolic acidosis.
Impairment of cognitive function
Cognitive impairment in epilepsy is multifactorial and may be due to the underlying aetiology, due to the epilepsy or due to the anti-epileptic treatment. There have been reports in the literature of impairment of cognitive function in adults on topiramate therapy which required reduction in dosage or discontinuation of treatment. However, studies regarding cognitive outcomes in children treated with topiramate are insufficient and its effect in this regard still needs to be elucidated.
Hyperammonemia and encephalopathy
Hyperammonemia with or without encephalopathy has been reported with topiramate treatment. The risk for hyperammonemia with topiramate appears dose-related. Hyperammonemia has been reported more frequently when topiramate is used concomitantly with valproic acid.
In patients who develop unexplained lethargy or changes in mental status associated with topiramate monotherapy or adjunctive therapy, it is recommended to consider hyperammonemic encephalopathy and measuring ammonia levels.
Nutritional supplementation
Some patients may experience weight loss whilst on treatment with topiramate. It is recommended that patients on topiramate treatment should be monitored for weight loss. A dietary supplement or increased food intake may be considered if the patient is losing weight while on topiramate.
Lactose intolerance
Topiramate-Canon tablets contain lactose. Patients with rare hereditary problems of galactose intolerance, Lapp lactase deficiency or glucose-galactose malabsorption should not take this medication.
Topiramate-Canon has minor or moderate influence on the ability to drive and use machines. Topiramate acts on the central nervous system and may produce drowsiness, dizziness or other related symptoms. It may also cause visual disturbances and/or blurred vision. These adverse reactions could potentially be dangerous in patients driving a vehicle or operating machinery, particularly until such time as the individual patient's experience with the medicinal products established.
The safety of topiramate was evaluated from a clinical trial database consisting of 4,111 patients (3,182 on topiramate and 929 on placebo) who participated in 20 double-blind trials and 2,847 patients who participated in 34 open-label trials, respectively, for topiramate as adjunctive treatment of primary generalized tonic-clonic seizures, partial onset seizures, seizures associated with Lennox-Gastaut syndrome, monotherapy for newly or recently diagnosed epilepsy or migraine prophylaxis. The majority of adverse reactions were mild to moderate in severity. Adverse reactions identified in clinical trials, and during post-marketing experience (as indicated by “*â€) are listed by their incidence in clinical trials in Table 1. Assigned frequencies are as follows:
| Very common | >1/10 |
| Common | >1/100 to <1/10 |
| Uncommon | >1/1,000 to <1/100 |
| Rare | >1/10,000 to <1/1,000 |
| Not known | cannot be estimated from the available data |
The most common adverse reactions (those with an incidence of >5% and greater than that observed in placebo in at least 1 indication in double-blind controlled studies with topiramate) include: anorexia, decreased appetite, bradyphrenia, depression, expressive language disorder, insomnia, coordination abnormal, disturbance in attention, dizziness, dysarthria, dysgeusia, hypoesthesia, lethargy, memory impairment, nystagmus, paresthesia, somnolence, tremor, diplopia, vision blurred, diarrhoea, nausea, fatigue, irritability, and weight decreased.
| Table 1: Topiramate Adverse Reactions | |||||
| System Organ Class | Very common | Common | Uncommon | Rare | Not known |
| Infections and infestations | Nasopharyngitis* | ||||
| Blood and lymphatic system disorders | Anaemia | Leucopenia, thrombocytopenia lymphadenopathy, eosinophilia | Neutropenia* | ||
| Immune system disorders | Hypersensitivity | Allergic oedema* | |||
| Metabolism and nutrition disorders | Anorexia, decreased appetite | Metabolic acidosis, hypokalaemia, increased appetite, polydipsia | Acidosis hyperchloraemic, hyperammonemia*, hyperammonemic encephalopathy* | ||
| Psychiatric disorders | Depression | Bradyphrenia, insomnia, expressive language disorder, anxiety, confusional state, disorientation, aggression, mood altered, agitation, mood swings, depressed mood, anger, abnormal behaviour | Suicidal ideation, suicide attempt, hallucination, psychotic disorder, hallucination auditory, hallucination visual, apathy, lack of spontaneous speech, sleep disorder, affect lability, libido decreased, restlessness, crying, dysphemia, euphoric mood, paranoia, perseveration, panic attack, tearfulness, reading disorder, initial insomnia, flat affect, thinking abnormal, loss of libido, listless, middle insomnia, distractibility, early morning awakening, panic reaction, elevated mood | Mania, panic disorder, feeling of despair*, hypomania | |
| Nervous system disorders | Paraesthesia, somnolence Dizziness | Disturbance in attention, memory impairment, amnesia, cognitive disorder, mental impairment, psychomotor skills impaired, convulsion, coordination abnormal, tremor, lethargy, hypoaesthesia, nystagmus, dysgeusia, balance disorder, dysarthria, intention tremor, sedation , | Depressed level of consciousness, grand mal convulsion, visual field defect, complex partial seizures, speech disorder, psychomotor hyperactivity, syncope, sensory disturbance, drooling, hypersomnia, aphasia, repetitive speech, hypokinesia, dyskinesia, dizziness postural, poor quality sleep, burning sensation, sensory loss, parosmia, cerebellar syndrome, dysaesthesia, hypogeusia, stupor, clumsiness, aura, ageusia, dysgraphia, dysphasia, neuropathy peripheral, presyncope, dystonia, formication | Apraxia, circadian rhythm sleep disorder, hyperaesthesia, hyposmia, anosmia, essential tremor, akinesia, unresponsive to stimuli | |
| Eye disorders | Vision blurred, diplopia, visual disturbance | Visual acuity reduced, scotoma, myopia*, abnormal sensation in eye*, dry eye, photophobia, blepharospasm, lacrimation increased, photopsia, mydriasis, presbyopia | Blindness unilateral, blindness transient, glaucoma, accommodation disorder, altered visual depth perception, scintillating scotoma, eyelid oedema*, night blindness, amblyopia | Angle closure glaucoma*, Maculopathy*, eye movement disorder* , conjunctival oedema* | |
| Ear and labyrinth disorders | Vertigo, tinnitus, ear pain | Deafness, deafness unilateral, deafness neurosensory, ear discomfort, hearing impaired | |||
| Cardiac disorders | Bradycardia, sinus bradycardia, palpitations | ||||
| Vascular disorders | Hypotension, orthostatic hypotension, flushing, hot flush | Raynaud's phenomenon | |||
| Respiratory, thoracic and mediastinal disorders | Dyspnoea , epistaxis, nasal congestion, rhinorrhoea, cough* | Dyspnoea exertional, Paranasal sinus hypersecretion, dysphonia | |||
| Gastrointestinal disorders | Nausea, diarrhoea | Vomiting, constipation, abdominal pain upper, dyspepsia, abdominal pain, dry mouth, stomach discomfort, paraesthesia oral, gastritis, abdominal discomfort | Pancreatitis, flatulence, gastrooesophageal reflux disease, abdominal pain lower, hypoaesthesia oral, gingival bleeding, abdominal distension, epigastric discomfort, abdominal tenderness, salivary hypersecretion, oral pain, breath odour, glossodynia | ||
| Hepatobiliary disorders | Hepatitis, Hepatic failure | ||||
| Skin and subcutaneous tissue disorders | Alopecia, rash, pruritus | Anhidrosis, hypoaesthesia facial, urticaria, erythema, pruritus generalised, rash macular, skin discolouration, dermatitis allergic, swelling face | Stevens-Johnson syndrome* erythema multiforme*, skin odour abnormal, periorbital oedema*, urticaria localised | Toxic epidermal necrolysis* | |
| Musculoskeletal and connective tissue disorders | Arthralgia, muscle spasms, myalgia, muscle twitching, muscular weakness, musculoskeletal chest pain | Joint swelling*, musculoskeletal stiffness, flank pain, muscle fatigue | Limb discomfort* | ||
| Renal and urinary disorders | Nephrolithiasis, pollakiuria, dysuria | Calculus urinary, urinary incontinence, haematuria, incontinence, micturition urgency, renal colic, renal pain | Calculus ureteric, renal tubular acidosis* | ||
| Reproductive system and breast disorders | Erectile dysfunction, sexual dysfunction | ||||
| General disorders and administration site conditions | Fatigue | Pyrexia, asthenia, irritability, gait disturbance, feeling abnormal, malaise | Hyperthermia, thirst, influenza like illness*, sluggishness, peripheral coldness, feeling drunk, feeling jittery | Face oedema, calcinosis | |
| Investigations | Weight decreased | Weight increased* | Crystal urine present, tandem gait test abnormal, white blood cell count decreased, Increase in liver enzymes | Blood bicarbonate decreased | |
| Social circumstances | Learning disability | ||||
| * identified as an adverse reaction from postmarketing spontaneous reports. Its frequency was calculated based on the incidence in clinical trials, or was calculated if the event did not occur in clinical trials. | |||||
Congenital malformations and fetal growth restrictions.
Paediatric population
Adverse reactions reported more frequently (>2-fold) in children than in adults in double-blind controlled studies include:
- Decreased appetite
- Increased appetite
- Hyperchloraemic acidosis
- Hypokalaemia
- Abnormal behaviour
- Aggression
- Apathy
- Initial insomnia
- Suicidal ideation
- Disturbance in attention
- Lethargy
- Circadian rhythm sleep disorder
- Poor quality sleep
- Lacrimation increased
- Sinus bradycardia
- Feeling abnormal
- Gait disturbance.
Adverse reactions that were reported in children but not in adults in double-blind controlled studies include:
- Eosinophilia
- Psychomotor hyperactivity
- Vertigo
- Vomiting
- Hyperthermia
- Pyrexia
- Learning disability.
Reporting of suspected adverse reactions
Reporting suspected adverse reactions after authorisation of the medicinal product is important. It allows continued monitoring of the benefit/risk balance of the medicinal product. Healthcare professionals are asked to report any suspected adverse reactions via the Yellow Card Scheme at: www.mhra.gov.uk/yellowcard.
Signs and symptoms
Overdoses of topiramate have been reported. Signs and symptoms included convulsions, drowsiness, speech disturbances, blurred vision, diplopia, impaired mentation, lethargy, abnormal coordination, stupor, hypotension, abdominal pain, agitation, dizziness and depression. The clinical consequences were not severe in most cases, but deaths have been reported after overdoses with multiple medicinal products including topiramate.
Topiramate overdose can result in severe metabolic acidosis.
Treatment
In acute topiramate overdose, if the ingestion is recent, the stomach should be emptied immediately by lavage or by induction of emesis. Activated charcoal has been shown to adsorb topiramate in vitro. Treatment should be appropriately supportive and the patient should be well hydrated. Haemodialysis has been shown to be an effective means of removing topiramate from the body.
Pharmacotherapeutic group: antiepileptics, other antiepileptics, antimigraine preparations, ATC code: N03AX11
Topiramate is classified as a sulfamate-substituted monosaccharide. The precise mechanism by which topiramate exerts its antiseizure and migraine prophylaxis effects are unknown. Electrophysiological and biochemical studies on cultured neurons have identified three properties that may contribute to the antiepileptic efficacy of topiramate.
Action potentials elicited repetitively by a sustained depolarization of the neurons were blocked by topiramate in a time-dependent manner, suggestive of a state-dependent sodium channel blocking action. Topiramate increased the frequency at which γ-aminobutyrate (GABA) activated GABAA receptors, and enhanced the ability of GABA to induce a flux of chloride ions into neurons, suggesting that topiramate potentiates the activity of this inhibitory neurotransmitter.
This effect was not blocked by flumazenil, a benzodiazepine antagonist, nor did topiramate increase the duration of the channel open time, differentiating topiramate from barbiturates that modulate GABAA receptors.
Because the antiepileptic profile of topiramate differs markedly from that of the benzodiazepines, it may modulate a benzodiazepine-insensitive subtype of GABAA receptor. Topiramate antagonized the ability of kainate to activate the kainate/AMPA (α -amino-3-hydroxy-5-methylisoxazole-4-propionic acid) subtype of excitatory amino acid (glutamate) receptor, but had no apparent effect on the activity of N-methyl-D-aspartate (NMDA) at the NMDA receptor subtype. These effects of topiramate were concentration-dependent over a range of 1 µM to 200 µM, with minimum activity observed at 1 µM to 10 µM.
In addition, topiramate inhibits some isoenzymes of carbonic anhydrase. This pharmacologic effect is much weaker than that of acetazolamide, a known carbonic anhydrase inhibitor, and is not thought to be a major component of topiramate's antiepileptic activity.
In animal studies, topiramate exhibits anticonvulsant activity in rat and mouse maximal electroshock seizure (MES) tests and is effective in rodent models of epilepsy, which include tonic and absence-like seizures in the spontaneous epileptic rat (SER) and tonic and clonic seizures induced in rats by kindling of the amygdala or by global ischemia. Topiramate is only weakly effective in blocking clonic seizures induced by the GABAA receptor antagonist, pentylenetetrazole.
Studies in mice receiving concomitant administration of topiramate and carbamazepine or phenobarbital showed synergistic anticonvulsant activity, while combination with phenytoin showed additive anticonvulsant activity. In well-controlled add-on trials, no correlation has been demonstrated between trough plasma concentrations of topiramate and its clinical efficacy. No evidence of tolerance has been demonstrated in man.
Absence seizures
Two small one arm studies were carried out with children aged 4-11 years old (CAPSS-326 and TOPAMAT-ABS-001). One included 5 children and the other included 12 children before it was terminated early due to lack of therapeutic response. The doses used in these studies were up to approximately 12 mg/kg in study TOPAMAT-ABS-001 and a maximum of the lesser of 9 mg/kg/day or 400 mg/day in study CAPSS-326. These studies do not provide sufficient evidence to reach conclusion regarding efficacy or safety in the paediatric population.
The film-coated tablet and hard capsule formulations are bioequivalent.
The pharmacokinetic profile of topiramate compared to other AEDs shows a long plasma half-life, linear pharmacokinetics, predominantly renal clearance, absence of significant protein binding, and lack of clinically relevant active metabolites.
Topiramate is not a potent inducer of drug metabolizing enzymes, can be administered without regard to meals, and routine monitoring of plasma topiramate concentrations is not necessary. In clinical studies, there was no consistent relationship between plasma concentrations and efficacy or adverse events.
Absorption
Topiramate is rapidly and well absorbed. Following oral administration of 100 mg topiramate to healthy subjects, a mean peak plasma concentration (Cmax) of 1.5 µg/ml was achieved within 2 to 3 hours (Tmax).
Based on the recovery of radioactivity from the urine the mean extent of absorption of a 100 mg oral dose of 14C-topiramate was at least 81%. There was no clinically significant effect of food on the bioavailability of topiramate.
Distribution
Generally, 13 to 17% of topiramate is bound to plasma protein. A low capacity binding site for topiramate in/on erythrocytes that is saturable above plasma concentrations of 4 µg/ml has been observed. The volume of distribution varied inversely with the dose. The mean apparent volume of distribution was 0.80 to 0.55 l/kg for a single dose range of 100 to 1200 mg. An effect of gender on the volume of distribution was detected, with values for females circa 50% of those for males. This was attributed to the higher percent body fat in female patients and is of no clinical consequence.
Biotransformation
Topiramate is not extensively metabolized (~20%) in healthy volunteers. It is metabolized up to 50% in patients receiving concomitant antiepileptic therapy with known inducers of drug metabolizing enzymes. Six metabolites, formed through hydroxylation, hydrolysis and glucuronidation, have been isolated, characterized and identified from plasma, urine and faeces of humans. Each metabolite represents less than 3% of the total radioactivity excreted following administration of 14C-topiramate. Two metabolites, which retained most of the structure of topiramate, were tested and found to have little or no anticonvulsant activity.
Elimination
In humans, the major route of elimination of unchanged topiramate and its metabolites is via the kidney (at least 81% of the dose). Approximately 66% of a dose of 14C-topiramate was excreted unchanged in the urine within four days. Following twice a day dosing with 50 mg and 100 mg of topiramate the mean renal clearance was approximately 18 ml/min and 17 ml/min, respectively. There is evidence of renal tubular reabsorption of topiramate. This is supported by studies in rats where topiramate was co-administered with probenecid, and a significant increase in renal clearance of topiramate was observed. Overall, plasma clearance is approximately 20 to 30 ml/min in humans following oral administration.
Linearity/non-linearity
Topiramate exhibits low intersubject variability in plasma concentrations and, therefore, has predictable pharmacokinetics. The pharmacokinetics of topiramate are linear with plasma clearance remaining constant and area under the plasma concentration curve increasing in a dose-proportional manner over a 100 to 400 mg single oral dose range in healthy subjects. Patients with normal renal function may take 4 to 8 days to reach steady-state plasma concentrations. The mean Cmax following multiple, twice a day oral doses of 100 mg to healthy subjects was 6.76 µg/ml. Following administration of multiple doses of 50 mg and 100 mg of topiramate twice a day, the mean plasma elimination half-life was approximately 21 hours.
Use with other AEDs
Concomitant multiple-dose administration of topiramate, 100 to 400 mg twice a day, with phenytoin or carbamazepine shows dose proportional increases in plasma concentrations of topiramate.
Renal impairment
The plasma and renal clearance of topiramate are decreased in patients with moderate and severe impaired renal function (CLCR ≤ 70 ml/min). As a result, higher steady-state topiramate plasma concentrations are expected for a given dose in renal-impaired patients as compared to those with normal renal function. In addition, patients with renal impairment will require a longer time to reach steady-state at each dose. In patients with moderate and severe renal impairment, half of the usual starting and maintenance dose is recommended.
Topiramate is effectively removed from plasma by haemodialysis. A prolonged period of hemodialysis may cause topiramate concentration to fall below levels that are required to maintain an anti-seizure effect. To avoid rapid drops in topiramate plasma concentration during hemodialysis, a supplemental dose of topiramate may be required. The actual adjustment should take into account 1) the duration of dialysis period, 2) the clearance rate of the dialysis system being used, and 3) the effective renal clearance of topiramate in the patient being dialyzed.
Hepatic impairment
Plasma clearance of topiramate decreased a mean of 26% in patients with moderate to severe hepatic impairment. Therefore, topiramate should be administered with caution in patients with hepatic impairment.
Elderly population
Plasma clearance of topiramate is unchanged in elderly subjects in the absence of underlying renal disease.
Paediatric population (pharmacokinetics, up to 12 years of age)
The pharmacokinetics of topiramate in children, as in adults receiving add-on therapy, are linear, with clearance independent of dose and steady-state plasma concentrations increasing in proportion to dose. Children, however, have a higher clearance and a shorter elimination half-life. Consequently, the plasma concentrations of topiramate for the same mg/kg dose may be lower in children compared to adults. As in adults, hepatic enzyme inducing AEDs decrease the steady-state plasma concentrations.
In nonclinical studies of fertility, despite maternal and paternal toxicity as low as 8 mg/kg/day, no effects on fertility were observed, in male or female rats with doses up to 100 mg/kg/day.
In preclinical studies, topiramate has been shown to have teratogenic effects in the species studied (mice, rats and rabbits). In mice, fetal weights and skeletal ossification were reduced at 500 mg/kg/day in conjunction with maternal toxicity. Overall numbers of fetal malformations in mice were increased for all drug-treated groups (20, 100 and 500 mg/kg/day).
In rats, dosage-related maternal and embryo/fetal toxicity (reduced fetal weights and/or skeletal ossification) were observed down to 20 mg/kg/day with teratogenic effects (limb and digit defects) at 400 mg/kg/day and above. In rabbits, dosage-related maternal toxicity was noted down to 10 mg/kg/day with embryo/fetal toxicity (increased lethality) down to 35 mg/kg/day, and teratogenic effects (rib and vertebral malformations) at 120 mg/kg/day.
The teratogenic effects seen in rats and rabbits were similar to those seen with carbonic anhydrase inhibitors, which have not been associated with malformations in humans. Effects on growth were also indicated by lower weights at birth and during lactation for pups from female rats treated with 20 or 100 mg/kg/day during gestation and lactation. In rats, topiramate crosses the placental barrier.
In juvenile rats, daily oral administration of topiramate at doses up to 300 mg/kg/day during the period of development corresponding to infancy, childhood, and adolescence resulted in toxicities similar to those in adult animals (decreased food consumption with decreased body weight gain, centrolobullar hepatocellular hypertrophy). There were no relevant effects on long bone (tibia) growth or bone (femur) mineral density, preweaning and reproductive development, neurological development (including assessments on memory and learning), mating and fertility or hysterotomy parameters.
In a battery of in vitro and in vivo mutagenicity assays, topiramate did not show genotoxic potential.
Not applicable.
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