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All about: Gabapentin

Big Image Pronouncation: (GAB-uh-PEN-tin)
Class: Anticonvulsant

Trade Names:
Neurontin
- Tablets 600 mg
- Tablets 800 mg
- Capsules 100 mg
- Capsules 300 mg
- Capsules 400 mg
- Oral solution 250 mg per 5 mL

Apo-Gabapentin (Canada)
Novo-Gabapentin (Canada)
PMS-Gabapentin (Canada)

Mechanism of Action

Pharmacology

Mechanism unknown; structurally related to GABA, but does not react with GABA receptors.

Pharmacokinetics

Absorption

Bioavailability decreases as dose increases: bioavailability is approximately 60%, 47%, 34%, 33%, and 27% following 900, 1,200, 2,400, 3,600, and 4,800 mg/day given in 3 divided doses, respectively. Food has only a slight effect on rate and extent of absorption (14% increase in AUC and C max ).

Distribution

Less than 3% bound to plasma proteins. Vd is about 58 L.

Metabolism

Not significantly metabolized in humans.

Elimination

Excreted unchanged in urine. The t ½ is 5 to 7 h.

Special Populations

Renal Function Impairment

In CrCl less than 30 mL/min, t ½ is about 52 h.

Elderly

Renal Cl decreases as result of age-related decline in renal function.

Hemodialysis

The t ½ is about 132 h on nondialysis days; gabapentin is significantly removed by hemodialysis.

Indications and Usage

Adjunctive therapy in treatment of partial seizures with or without secondary generalization in patients older than 12 yr of age with epilepsy; adjunctive therapy for partial seizures in children 3 to 12 yr of age; management of postherpetic neuralgia in adults.

Contraindications

Standard considerations.

Dosage and Administration

Epilepsy
Adults and children older than 12 yr of age

PO 900 to 1,800 mg/day (divided 3 times daily). Initial dose: 300 mg 3 times daily.

Children 5 to 12 yr of age

PO Initiate therapy at 10 to 15 mg/kg/day in divided doses (eg, 3 times daily) and titrate dose upward over a period of about 3 days to effective dose. Effective dose is 25 to 35 mg/kg/day in divided doses (eg, 3 times daily).

Children 3 to 4 yr of age

PO Initiate therapy at 10 to 15 mg/kg/day in divided doses (eg, 3 times daily) and titrate dose upward over a period of about 3 days to effective dose. Effective dose is 40 mg/kg/day in divided doses (eg, 3 times daily).

Postherpetic Neuralgia
Adults

PO Start with a single 300 mg dose on day 1, 600 mg on day 2 (divided twice daily), and 900 mg on day 3 (divided 3 times daily). Subsequently, titrate the dose upward as needed for pain relief to a daily dose of 1,800 mg (divided 3 times daily).

Dosage Adjustment for Renal Function Impairment
Adults and children older than 12 yr of age CrCl 60 mL/min or higher

Total daily dose range 900 to 3,600 mg/day.

CrCl 30 to 59 mL/min

Total daily dose range 400 to 1,400 mg/day.

CrCl 16 to 29 mL/min

Total daily dose range 200 to 700 mg/day.

CrCl 15 mL/min

Total daily dose range 100 to 300 mg/day.

CrCl less than 15 mL/min

Reduce dose in proportion to CrCl (eg, patients with CrCl 7.5 mL/min should receive ½ daily dose of patient with CrCl 15 mL/min).

Patients on hemodialysis

Maintenance doses based on CrCl as recommended plus a supplemental post-hemodialysis dose administered after each 4 h of hemodialysis as follows: if maintenance dose is 100 mg daily, postdialysis dose is 125 mg; if maintenance dose is 125 mg daily, postdialysis dose is 150 mg; if maintenance dose is 150 mg daily, postdialysis dose is 200 mg; if maintenance dose is 200 mg daily, postdialysis dose is 250 mg; if maintenance dose is 300 mg daily, postdialysis dose is 350 mg.

General Advice

  • Tablets, capsules, and oral solution are interchangeable on a mg-to-mg basis.
  • Max time between doses in 3 times daily schedule should not exceed 12 h.
  • Administer without regard to meals. Administer with food if GI upset occurs.
  • If 600 or 800 mg scored tablet is split to administer a half-tablet, administer unused half-tablet at next dose. Discard any half-tablet not used within several days of splitting.
  • Measure and administer prescribed dose of oral solution using dosing syringe, dosing spoon, or dosing cup.
  • Reduce dose gradually over a minimum of 1 wk if gabapentin is discontinued.

Storage/Stability

Store tablets and capsules at controlled room temperature (59° to 86°F). Store oral solution in refrigerator (36° to 46°F).

Drug Interactions

Antacids

May reduce bioavailability of gabapentin. Take gabapentin 2 h after antacid.

Cimetidine

Reduces renal Cl of gabapentin.

Hydrocodone

The AUC of gabapentin may be increased by 14%.

Morphine

The AUC of gabapentin may be increased 44%.

Naproxen

Gabapentin absorption may be increased 12% to 15% by naproxen.

Oral contraceptives

Norethindrone plasma levels may be increased 13%, which is not expected to be clinically important.

Laboratory Test Interactions

False-positive readings for Ames N-Multistix SG dipstick test when gabapentin is added to other antiepileptic drugs. Sulfosalicylic acid precipitation procedure is recommended to determine the presence of urine protein.

Adverse Reactions

Cardiovascular

Vasodilation (1%); hypertension (at least 1%).

CNS

Dizziness (28%); somnolence (21%); ataxia (12%); fatigue (11%); nystagmus (8%); tremor (7%); asthenia (6%); abnormal thinking, headache (3%); abnormal gait, abnormal thinking, amnesia, depression, dysarthria, incoordination, nervousness (2%); hypesthesia, twitching (1%); anxiety, confusion, hostility, hyperkinesia, malaise, paresthesia, vertigo (at least 1%); abnormal movements (postmarketing).

Children 12 yr of age and younger

Somnolence, hostility (8%); emotional lability (4%); dizziness, hyperkinesia, fatigue (3%).

Dermatologic

Abrasion, pruritus, rash (1%); erythema multiforme, Stevens-Johnson syndrome (postmarketing).

EENT

Diplopia (6%); amblyopia, rhinitis (4%); pharyngitis (3%); conjunctivitis, otitis media (1%); abnormal vision (at least 1%).

GI

Diarrhea (6%); dry mouth (5%); constipation, nausea (4%); abdominal pain, vomiting (3%); dental abnormalities, dyspepsia, flatulence (2%); increased appetite (1%); anorexia, gingivitis (at least 1%).

Children 12 yr of age and younger

Nausea, vomiting (8%).

Genitourinary

Impotence (2%).

Hematologic-Lymphatic

Decreased WBC, leukopenia (1%); purpura (at least 1%).

Hepatic

Elevated LFTs, jaundice (postmarketing).

Hypersensitivity

Angioedema (postmarketing)

Metabolic-Nutritional

Weight gain (3%); hyperglycemia (1%); blood glucose fluctuations, hyponatremia (postmarketing).

Children 12 yr of age and younger

Weight increase (3%).

Musculoskeletal

Back pain, myalgia (2%); fracture (1%); arthralgia, decreased or absent reflexes, increased reflexes (at least 1%).

Respiratory

Coughing (2%).

Children 12 yr of age and younger

Bronchitis, respiratory infection (3%); pneumonia (at least 1%).

Miscellaneous

Peripheral edema (8%); infection (5%); accidental injury (3%).

Children 12 yr of age and younger

Viral infection (11%); fever (10%); face edema (at least 1%).

Precautions

Pregnancy

Category C .

Lactation

Excreted into breast milk.

Children

Effectiveness as adjunctive therapy in the treatment of partial seizures in children younger than 3 yr of age not established; safety and efficacy in management of postherpetic neuralgia in children not established.

Elderly

Because of age-related renal function impairment, adjust dose based on CrCl.

Renal Function

Adjust dose based on CrCl.

Carcinogenesis

May have carcinogenic potential.

Hazardous Tasks

May cause dizziness, drowsiness, and other symptoms of CNS depression.

Neuropsychiatric events

Emotional lability, hostility, thought disorders, and hyperkinesia have been reported with increased frequency in patients 3 to 12 yr of age with epilepsy.

Serious adverse reactions

During clinical trials, some patients experienced status epilepticus, and 8 sudden, unexplained deaths occurred. The association of these reactions with gabapentin use is unclear.

Withdrawal

Do not discontinue antiepileptic drugs abruptly because of possible increased seizure frequency from drug withdrawal.

Overdosage

Symptoms

Diarrhea, double vision, drowsiness, lethargy, slurred speech.

Patient Information

  • Advise patient or caregiver to read the patient information leaflet before starting therapy and with each refill.
  • Instruct patient with epilepsy to continue to take other medications for seizures unless advised otherwise by health care provider.
  • Advise patient or caregiver that medication will be started at a low dose and then increased as tolerated until max benefit has been obtained.
  • Instruct patient or caregiver to take exactly as prescribed and not to change the dose or discontinue therapy unless advised by health care provider.
  • Advise patient or caregiver to take without regard to meals but to take with food if stomach upset occurs.
  • Advise patient or caregiver that max time between doses in 3 times daily schedule should not exceed 12 h.
  • Advise patient or caregiver splitting 600 or 800 mg scored tablet to administer a half-tablet dose, to administer unused half-tablet at the next dose, and to discard any half-tablet not used within several days of splitting.
  • Advise patient or caregiver using oral solution to measure and administer prescribed dose using dosing syringe, spoon, or cup.
  • Advise patient or caregiver that if medication needs to be discontinued it will be slowly withdrawn over a period of 1 wk or more unless safety concerns (eg, rash) require a more rapid withdrawal.
  • Caution patient that drug may cause dizziness, drowsiness, or incoordination and to use caution while driving or performing other tasks requiring mental alertness or coordination until tolerance is determined.
  • Instruct patient or caregiver to contact health care provider if seizures worsen or if new types of seizures occur.
  • Advise patient or caregiver to inform health care provider if any of the following occur: emotional lability, hostility, thought disorders or abnormal thinking, restlessness or hyperactivity, excessive dizziness or drowsiness, swelling in feet or ankles, any unexplained symptom or feeling.
  • Advise patient with epilepsy to carry medical identification (eg, card, bracelet) indicating medication usage and epilepsy.

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