The mechanism of action for lamotrigine is not entirely understood. It is a triazine, and it has been shown that lamotrigine selectively binds sodium channels, stabilizing presynaptic neuronal membranes and inhibiting glutamate release. Lamotrigine has not been proven to have significant effects on other neurotransmitters such as serotonin, norepinephrine or dopamine. It is thought that lamotrigine may interact with voltage-activated calcium-gated channels, contributing to its broad range of activity. In vitro studies have also shown that lamotrigine inhibited dihydrofolate reductase which may contribute to concerns for its teratogenicity. Lamotrigine follows first-order kinetics with a half-life of 29 hours.
Lamotrigine is available as tablets, chewable tablets, and orally disintegrating tablets. In a tablet form, it is available in formulations of as 25 mg, 100 mg, 150 mg and 200 mg tablets. In a chewable, dispersible tablet form, it is available in formulations of 2 mg, 5 mg, and 25 mg dispersible tablets. The orally disintegrating tablets are available in formulations of 25 mg, 50 mg, 100 mg and 200 mg. All formulations should be stored at room temperature and should be protected from light.
Lamotrigine dosing is altered if it is given concurrently with carbamazepine, phenytoin, phenobarbital, primidone, rifampin, lopinavir/ritonavir, and atazanavir, ritonavir and valproic acid.
If it is necessary to discontinue lamotrigine, it should be done in a step-wise fashion over a period of 2 weeks, if possible. There is a possibility of withdrawals seizures when lamotrigine is discontinued, which is lowered if the drug is tapered rather than stopped quickly.
If not being used concurrently with carbamazepine, phenytoin, phenobarbital, primidone, rifampin, lopinavir/ritonavir, atazanavir, ritonavir and valproic acid, dosing instructions are as follows. Initially, 25 mg should be given daily. At week three the dose should be increased to 50 mg daily. At week 5 increase by an additional 50 mg every week or every other week. Typical maintenance ranges from 225 mg to 375 mg to two divided doses.
If being used concurrently with valproic acid, dosing instructions are as follows. Initially, 25 mg should be given every other day. At week 3 the dose should be increased to 25 mg daily. At week 5 increase by an additional 25 mg to 50 mg every week or every other week. Typical maintenance varies from 100 mg to 200 mg daily in one or two divided doses if it is given with valproic acid alone, or 100 mg to 400 mg in one or two divided doses if it is given with other medications that induce glucuronidation.
If being used concurrently with carbamazepine, phenytoin, phenobarbital, primidone, rifampin, lopinavir/ritonavir, and atazanavir, or ritonavir, dosing instructions are as follows. Initially, 50 mg should be given daily. At week three the dose should be increased to 100 mg daily in 2 divided doses. At week five increase by an additional 100 mg every week or every other week. Typical maintenance ranges from 300,g to 500 mg to two divided doses.
For Bipolar I: Maintenance is 200 mg to 400 mg, with consideration given to medication given concurrently with lamotrigine.
United States Boxed Warning: Lamotrigine can cause serious rashes requiring hospitalization and discontinuation of this medication. Rash severity varies but includes a risk for Stevens-Johnson syndrome. The incidence of Stevens-Johnson syndrome in the pediatric population is 0.3% to 0.8% and 0.03% to 0.08% in adult populations. The number of cases associated with toxic epidermal necrolysis is too low to report an estimated incidence. Nearly all cases of rash occur 2 to 8 weeks after the initiation of lamotrigine. It should also be noted that the discontinuation of lamotrigine may not prevent a rash from becoming life-threatening and patient education should include continuous monitoring of the rash for improvement after the discontinuation of the medication.
Side effects include:
Hypersensitivity to lamotrigine or its ingredients is the primary contraindication for the administration of lamotrigine.
Evaluating gender, age and contraceptive use are essential for the consideration of starting lamotrigine. While some studies in humans have not shown an increased risk for congenital malformations during lamotrigine therapy during pregnancy, animal studies have shown there is an increased risk. Pregnancy risk factor C, animal studies have shown risk for congenital malformations.
Lamotrigine is present in breast milk and is detected in the blood of breast-fed infants. Symptoms of lamotrigine in infants include poor feeding, drowsiness, rash, and apnea. These symptoms can improve with the discontinuation of lamotrigine.
Consideration for other drugs effects on glucuronidation must be considered, as glucuronic acid conjugation primarily metabolizes lamotrigine.
Drugs that induce lamotrigine glucuronidation include carbamazepine, phenytoin, phenobarbital, rifampin, lopinavir/ritonavir, atazanavir/ritonavir, and primidone.
Valproic acid inhibits lamotrigine glucouronidation.
Concurrent use with central nervous system (CNS) depressants may increase the potency of CNS depression.
Lamotrigine is reported to interfere with urine drug screen and to cause false-positive readings phencyclidine.
The value of monitoring lamotrigine concentrations has not been established. Due to pharmacokinetics between lamotrigine and other drugs and their effect on lamotrigine concentration, clinical judgment must be exercised during concomitant use if there are concerns in regards to lamotrigine levels.
Labs should include pertinent serum levels of concurrent anticonvulsants, as well as liver function testing, and renal function assessments. Ample time should be spent educating patients on monitoring themselves for hypersensitivity, particularly rashes or other skin changes occurring near or on the mucosa. Patient education should also include discussing how to monitor for changes in seizures and their frequency and duration. Patients should also be informed to monitor for changes in suicidality including suicidal thoughts and increased desire to commit suicide. Finally, patients should be educated to monitor for signs/symptoms of aseptic meningitis.
In excessive lamotrigine overdoses, some reported to be as high as 16g; fatalities have been observed following complications including seizures, coma and conduction abnormalities. Immediate-release lamotrigine is rapidly absorbed, and emesis may not be indicated in this instance. However, hospitalization and supportive care are indicated, as well as the usual precautions to protect the airway. There is no known specific antidote for lamotrigine toxicity.
Lamotrigine is often prescribed by the primary care provider, nurse practitioner, internist, neurologist and the pain specialist for the treatment of several other disorders besides seizures. However, all prescribers should be aware of the United States Boxed Warning which states that Lamotrigine can cause serious rashes requiring hospitalization and discontinuation of this medication. Rash severity varies but includes a risk for Stevens-Johnson syndrome. It should also be noted that the discontinuation of lamotrigine may not prevent a rash from becoming life-threatening and patient education should include continuous monitoring of the rash for improvement after the discontinuation of the medication.
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