HORIZANT, known by its generic name gabapentin enacarbil, is a medication primarily prescribed for two specific conditions in adults: moderate-to-severe primary restless legs syndrome (RLS) and postherpetic neuralgia (PHN). Unlike other forms of gabapentin, HORIZANT is an extended-release formula, which means it is absorbed and stays in the body longer. This unique pharmacokinetic profile allows for less frequent dosing and a more steady therapeutic effect, providing lasting relief for those suffering from these painful and disruptive neurological disorders.
How HORIZANT works: The science of gabapentin enacarbil
HORIZANT is a prodrug, a compound that is converted into an active drug—in this case, gabapentin—after it is absorbed into the body. This process is key to its extended-release profile. After oral administration, gabapentin enacarbil is absorbed in the gastrointestinal tract and then converted into gabapentin.
Although the exact mechanism of action for its effects on RLS and PHN is not fully understood, research indicates that gabapentin binds with high affinity to the alpha-2-delta ($\alpha_2\delta$) subunit of voltage-activated calcium channels. It is believed that this action helps to modulate the release of certain neurotransmitters, calming the nerves that cause the painful sensations of PHN and the involuntary movements of RLS.
Primary indications for HORIZANT
Treatment for Restless Legs Syndrome (RLS)
RLS is a neurological disorder characterized by an irresistible urge to move the legs, often accompanied by unpleasant sensations like tingling, itching, or aching. These symptoms typically occur or worsen during periods of rest, such as in the evening or at night, severely disrupting sleep.
For adults with moderate-to-severe primary RLS, HORIZANT is typically prescribed as a tablet taken once daily at approximately 5 p.m. with food. The timing is crucial to ensure the medication's effects coincide with the evening onset of RLS symptoms, and taking it with food helps to improve absorption. This once-daily dosing offers sustained relief, helping patients sleep more soundly and experience fewer RLS-related interruptions. It is not recommended for patients who need to sleep during the daytime and remain awake at night.
Management of Postherpetic Neuralgia (PHN)
PHN is a type of nerve pain that can persist for months or even years after the shingles rash has healed. This chronic pain can feel like burning, stabbing, or aching, and it is caused by nerve damage from the herpes zoster virus.
For PHN, HORIZANT is prescribed differently than for RLS. The dosage is typically taken twice daily, with the initiation of therapy often involving a gradual increase in the amount over the first few days. Clinical studies have demonstrated that HORIZANT significantly reduces PHN pain intensity for many patients.
Comparison of HORIZANT to other gabapentin products
HORIZANT's extended-release mechanism distinguishes it from other gabapentin products, which have different pharmacokinetic profiles and are not interchangeable.
| Feature | HORIZANT (Gabapentin Enacarbil) | Immediate-Release Gabapentin (e.g., Neurontin) |
|---|---|---|
| Release Profile | Extended-release | Immediate-release |
| Dosing Frequency | Once or twice daily (depending on indication) | Up to three times daily |
| Absorption | Enhanced and prolonged absorption, especially with food | Absorption can be limited and saturable |
| Indications | RLS, PHN | Seizures, PHN |
| Interchangeability | Not substitutable with other gabapentin products | Can be interchanged between generic brands, but not with extended-release forms |
Important warnings and side effects
Like all medications, HORIZANT carries a risk of side effects and should be used with caution under a doctor's supervision.
- Dizziness and drowsiness: These are among the most common side effects and can impair a person's ability to drive or operate machinery.
- Suicidal thoughts: All antiepileptic drugs, including HORIZANT's active metabolite gabapentin, carry an increased risk of suicidal thoughts or behavior. Patients should be monitored for any unusual mood or behavior changes.
- Respiratory depression: Life-threatening breathing problems can occur, especially when HORIZANT is combined with central nervous system depressants like opioids.
- Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS): A serious, potentially fatal allergic reaction that affects multiple organ systems can occur.
- Withdrawal symptoms: Abruptly stopping HORIZANT can increase the risk of withdrawal symptoms, including seizures. Dosage should be tapered gradually.
A note on discontinuing treatment
It is crucial that patients do not stop taking HORIZANT suddenly without first speaking to their healthcare provider. A gradual dose reduction is often necessary to minimize the risk of withdrawal seizures. For RLS patients on the standard once-daily dose, discontinuation may not require tapering, but a doctor should always be consulted first.
Conclusion
In summary, HORIZANT is a highly specialized extended-release version of gabapentin used to treat moderate-to-severe primary Restless Legs Syndrome and postherpetic neuralgia. Its unique formulation allows for a more consistent and sustained therapeutic effect compared to immediate-release gabapentin, particularly beneficial for nocturnal conditions like RLS. However, patients must be aware of the potential for significant side effects, including dizziness, drowsiness, and a risk of suicidal thoughts. As with any prescription medication, HORIZANT should only be taken under the guidance of a healthcare professional who can properly manage the dosage and monitor for adverse reactions. For more detailed information, you can review the official FDA prescribing information for HORIZANT.
