Horizant gabapentin enacarbil (extended release) 

Medication class: Analgesic, anticonvulsant
Similar drugs: Neurontin (gabapentin)
Manufacturer: GlaxoSmithKline
Approval Date: April 6, 2011

What is Horizant and its mechanism of action?

Horizant is a prodrug that contains gabapentin enacarbil  which has to be converted into gabapentin in the body before it can have its effects. The exact mechanism of action of gabapentin in the treatment of restless legs syndrome and postherpetic neuralgia (nerve pain due to damage from herpes zoster or shingles) is unknown. Gabapentin prevents pain response in laboratory models for pain. Gabapentin's structure is very similar to a neurotransmitter (a chemical used to transmit signals in brain cells) called gamma-aminobutyric acid (GABA). GABA reduces the transmission of nerve impulses and calms excitable nerve cells in the nervous system. 

What is Horizant used for treating?

Horizant is a medication approved for treatment of restless legs syndrome (RLS) and management of postherpetic neuralgia (nerve pain due to damage from herpes zoster, also known as shingles)

How effective is Horizant?

The effectiveness of Horizant in the treatment of restless legs syndrome was demonstrated in two 12-weeks clinical trial. In one clinical study, 76% of patients treated with Horizant 1200 mg showed improvement (much improved or very much improved) while 39% of patients on placebo showed improvements. In another clinical trial, patients were treated with Horizant 600 mg or 1200 mg or a placebo pill. Improvements were shown in 73% of patients treated with Horizant 600 mg, 77% of patients treated with Horizant 1200 mg, 45% of patients on placebo.

In a 12-week study of patients with postherpetic neuralgia (PHN) treatment with Horizant reduced pain and increased the proportion of patients who reported at least 50% reduction in pain.  

Interesting facts about Horizant

• Horizant is the extended release and prodrug form of gabapentin
• Horizant is not interchangeable with other gabapentin products even though they share the same active chemical.
• Horizant is not for someone who needs to sleep in the daytime and stay awake at night.
• For the treatment of restless legs syndrome, Horizant should be taken at 5 PM because the drug has a peak effect of about  5-7 hours allowing it to be most effective towards bedtime.

What are side effects of Horizant?

Some side effects include:

• Somnolence (drowsiness)
• Dizziness
• Headache
• Nausea
• Dry mouth
• Fatigue
• Irritability
• Increased weight
• Increased appetite

What is the dosage of Horizant?

Horizant is available in 300 mg and 600 mg extended-release tablets. For the treatment of restless legs syndrome the recommended dosage is 600 mg once daily with food at about 5PM.

For postherpetic neuralgia, the recommended dosage is 600 mg once in the morning with food for 3 days, then increased to 600 mg twice daily. Horizant should be taken as a whole tablet. Do not cut, crush, or chew the tablets.

The maximum dose of Horizant is 1200 mg daily. Larger doses do not provide additional benefit for RLS or PHN but cause more side effects. 

What are Horizant drug interactions?

In clinical trials, Horizant was tested with cimetidine and naproxen. There were no significant changes in Horizant blood levels.  Combining morphine with Horizant caused more sedation, dizziness, and nausea than either drug alone. Orlistat can decrease the effectiveness of gabapentin.

Alcohol  consumption while taking Horizant is not recommended because it may increase the release of Horizant in the body, increasing the risk of adverse effects.

What are warnings and precaution for Horizant?

• Horizant may affect a patient's ability to drive a car. Patients should not drive a vehicle until they know how Horizant will affect them.
• Horizant may cause suicidal thoughts or behavior. Monitor patients for any unusual behavior or suicidal thoughts.
• Horizant may impair a patient's ability to operate machinery.
• Horizant should be tapered to 600 mg daily for 1 week prior to discontinuation to reduce the risk of withdrawal seizures. 

Pregnancy: Horizant is pregnancy category C which means it has not been adequately evaluated in pregnant women. Horizant should only be used in pregnancy if the potential benefits outweighs the risk to the fetus.

Nursing Mothers: Gabapentin is excreted in human milk. Since there is potential for adverse reactions in nursing infants, mothers should either discontinue nursing or discontinue Horizant.

Cost of Horizant

The average wholesale price for 30 tablets of Horizant 300 mg and 600 mg is about $252. It is about $8.42 per tablet.

Prescription assistance

For more information on reducing cost of Horizant prescription visit Copay Savings Card.

References

Horizant Prescribing Information