Firdapse

Pronunciation: FIR-dapse
Generic name: amifampridine phosphate
Dosage form: oral tablet
Drug class: Cholinergic muscle stimulants

Medically reviewed by Carmen Pope, BPharm. Last updated on Oct 26, 2023.

What is Firdapse?

Firdapse (amifampridine phosphate) is an oral, nonspecific, voltage-dependent, potassium (K+) channel blocker for the treatment of Lambert Eaton myasthenic syndrome (LEMS) in adults and pediatric patients six years of age and older.

LEMS is a rare autoimmune disease that affects approximately 1 in 100,000 people in the United States. In people with LEMS, the body’s immune system attacks the neuromuscular junction (the connection between nerves and muscles) and disrupts the ability of nerve cells to send signals to muscle cells. LEMS most commonly occurs in patients with cancer such as small cell lung cancer, where its onset precedes or coincides with the diagnosis of cancer, but it may be associated with other autoimmune diseases.

Firdapse was approved on November 28, 2018, and was the first FDA-approved treatment for LEMS.

Warnings

May cause hypersensitivity reactions, such as anaphylaxis. Discontinue Firdapse and initiate appropriate treatment. Do not use in people who are hypersensitive to amifampridine phosphate, other aminopyridines, or any of the inactive ingredients in the tablets.

Do not use in people with a history of seizures. May cause seizures. Consider discontinuation or a dose reduction of Firdapse in patients who have a seizure.

Dosing information

Adults and children weighing 45 kg or more: 15 to 30 mg daily, split into divided dosages (usually 3 to 4 times daily).

Increase dosage by 5mg daily every 3 to 4 days if necessary and tolerated.
20mg is the maximum single dose.
Maximum dose: Do not exceed a total dose of 80mg daily.

Children weighing less than 45 kg: 5 to 15 mg daily, split into divided dosages (usually 3 to 4 times daily).

Increase dosage by 2.5mg daily every 3 to 4 days if necessary and tolerated.
10mg is the maximum single dose.
Maximum dose: Do not exceed a total dose of 40mg daily.

Patients with renal or hepatic impairment or known N-acetyltransferase 2 (NAT2) poor metabolizers:

Start with the lowest recommended initial daily dosage.

Suspension

A 1 mg/ml suspension can be prepared as follows, for those with difficulty swallowing, with feeding tubes, or with dosage adjustments of less than 5mg increments.

Place the required number of tablets in a sterile container with a lid (there is no need to crush them)
Add 10 ml of sterile water for each tablet
Wait 5 minutes, then shake well for 30 seconds.
Use an oral syringe to draw up and administer the correct dose by mouth or feeding tube.
Refrigerate the suspension between 2°C-8°C (36°F-46°F) between doses for up to 24 hours. Discard any unused portion of the suspension after 24 hours.

What are the side effects of Firdapse?

The most common (> 10%) adverse reactions are paresthesias (burning or prickling sensations usually in the limbs), upper respiratory tract infections, abdominal pain, nausea, diarrhea, headache, elevated liver enzymes, back pain, hypertension, and muscle spasms.

Firdapse may cause seizures, even if you have never had a seizure in the past. Stop taking this medicine and call your doctor right away if you have a seizure.

Get emergency medical help if you have signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat.

Common side effects of Firdapse may include:

a tingly feeling in your hands, feet, face, mouth, or other parts of your body;
nausea, stomach pain, diarrhea;
headache, back pain;
increased blood pressure;
abnormal liver function tests;
muscle spasms; or
cold symptoms such as stuffy nose, sneezing, and sore throat.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Before taking this medicine

You should not use Firdapse if you are allergic to it, or if:

you have had a seizure in the past; or
you are allergic to amifampridine, dalfampridine, or other aminopyridines

Tell your doctor if you have ever had:

a seizure;
liver disease;
kidney disease; or
if you also take a form of Firdapse obtained from a compounding pharmacy (3,4-diaminopyridine or 3,4-DAP).

Tell your doctor if you are pregnant or breastfeeding.

Pregnancy

Based on animal data, Firdapse may cause fetal harm.

Lactation

There are no data regarding the presence of Firdapse in human milk but animal data show it reaches levels similar to maternal plasma.

How should I take Firdapse?

Follow all directions on your prescription label and read all medication guides or instruction sheets. Your doctor may occasionally change your dose. Take Firdapse exactly as directed.

You may take Firdapse with or without food.

Do not take more than 2 tablets at one time. Do not take more than 8 tablets in 24 hours.

The Firdapse tablet may be split in half if needed.

What happens if I miss a dose?

Skip the missed dose and use your next dose at the regular time. Do not use two doses at one time.

What happens if I overdose?

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222.

What should I avoid while taking Firdapse?

Follow your doctor's instructions about any restrictions on food, beverages, or activity.

What other drugs will affect Firdapse?

The risk of seizures with Firdapse may be increased by taking it with other medications that also lower the seizure threshold such as some used to treat depression, mental illness, attention deficit disorder, or severe pain.

Concomitant use of Firdapse with other medications with cholinergic effects, such as dicyclomine, hyoscyamine, or glycopyrrolate, may increase the risk of side effects.

Many drugs can affect Firdapse. This includes prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible interactions are listed here. Tell your doctor about all your current medicines and any medicine you start or stop using.