Anticonvulsants Flashcards
Describe the variety of mechanisms of anticonvulsants and which drugs are typically considered in this class?
These drugs have a variety of mechanisms including increasing γ-aminobutyric acid (GABA)ergic function or decreasing glutamatergic function. This chapter includes six of the newer anticonvulsants: gabapentin (Neurontin), levetiracetam (Keppra), pregabalin (Lyrica), tiagabine (Gabitril), topiramate (Topamax), and zonisamide (Zonegran), as well as one of the first used anticonvulsants, phenytoin (Dilantin).
Carbamazepine (Tegretol), valproate (Depakene, Depakote), lamotrigine (Lamictal), and oxcarbazepine (Trileptal) are discussed in separate sections.
Describe the mechanism of Gabapentin?
Appears to increase cerebral GABA and may inhibit glutamate synthesis as well.
It also increases human whole blood serotonin concentrations and modulates calcium channels to reduce monoamine release.
It has anti seizure as well as anti spastic and antinociceptive effects in pain.
Therapeutic indications for Gabapentin?
General and partial seizures, neuropathic pain, fibromyalgia, headache, sometimes chronic pruritus. In Psychiatry, used in social anxiety and panic disorder.
May also reduce alcohol cravings.
Precautions and Adverse effects of Gabapentin?
Adverse effects are mild, with the most common being daytime somnolence, ataxia, and fatigue, which are usually dose related.
The drug is classified as pregnancy category C and is excreted in breast milk, so it is best to avoid it in pregnant women and nursing mothers.
Drug interactions with Gabapentin?
Gabapentin bioavailability may decrease as much as 20% when administered with antacids. In general, there are no drug interactions. Chronic use does not interfere with lithium administration.
Lab interferences with Gabapentin?
Gabapentin does not interfere with any laboratory tests, although spontaneous reports of false-positive or positive drug toxicology screenings for amphetamines, barbiturates, benzodiazepines, and marijuana have been reported.
Dosage, formulation and clinical guidelines for Gabapentin?
Gabapentin is well tolerated, and the dosage can be increased to the maintenance range within a few days. A general approach is to start with 300 mg on day 1, increase to 600 mg on day 2, 900 mg on day 3, and subsequently increase up to 1,800 mg per day in divided doses as needed to relieve symptoms.
Final total daily doses tend to be between 1,200 and 2,400 mg per day but occasionally results may be achieved with dosages as low as 200 to 300 mg per day, especially in elderly persons.
Gabapentin is available as 100, 300, and 400 mg capsules and as 600 and 800 mg tablets. A 250 mg/5 mL oral solution is also available. Although abrupt discontinuation of gabapentin does not cause withdrawal effects, use of all anticonvulsant drugs should be gradually tapered.
What are some indications for Topomax?
It was developed as an anticonvulsant, but is also used for migraine prevention, treatment of obesity, bulimia, binge eating , alcohol dependence, smoking cessation, pain syndromes, PTSD and essential tremor.
What is the mechanism of Topiramate?
Increases cerebral Gaba in humans.
Pharmacokinetics and dynacmics of Topomax?
It has 80% oral bioavailability and is not significantly altered by food. It is 15% protein bound, and about 70% of the drug is eliminated by renal excretion. With renal insufficiency topiramate clearance decreases about 50%, so the dosage needs to be decreased. It has a half-life of around 24 hours.
Precautions and adverse reactions with Topomax?
The most common adverse effects of topiramate include paresthesias, weight loss, somnolence, anorexia, dizziness, and memory problems.
The drug affects acid–base balance (low serum bicarbonate), which can be associated with cardiac arrhythmias, and the formation of renal calculi in about 1.5% of cases. Patients should be encouraged to drink plenty of fluids.
It also is renally excreted (~70%) so dose reductions and caution in renal insufficiency.
Drug interactions with Topomax?
Topiramate may increase phenytoin concentrations up to 25% and valproic acid 11%;
Topiramate concentrations are decreased by 40% to 48%, with concomitant administration of carbamazepine or phenytoin.
Topiramate should not be combined with other carbonic anhydrase inhibitors, such as acetazolamide (Diamox) or dichlorphenamide (Daranide), because this could increase the risk of nephrolithiasis or heat-related problems (oligohidrosis and hyperthermia). Laboratory Interferences
Dosage, formulation and clinical guidelines for Topomax?
Topiramate is available as unscored 25, 100, and 200 mg tablets. To reduce the risk of adverse cognitive and sedative effects, topiramate dosage is titrated gradually over 8 weeks to a maximum of 200 mg twice a day. Doses higher than 400 mg have not been shown to increase efficacy.
All of the dose can be given at bedtime to take advantage of the sedative effects. Persons with renal insufficiency should reduce doses by half.
