What Is Icatibant (Firazyr)? The Bradykinin Blocking Peptide for Hereditary Angioedema Attacks and What the FAST Trials Found

Icatibant is a synthetic peptide that blocks the bradykinin B2 receptor, the switch that drives the swelling of hereditary angioedema (HAE). Sold as Firazyr, icatibant is an injection given under the skin to treat an HAE attack once it has started, and it is not used to prevent attacks. The [Firazyr label](https://dailymed.nlm.nih.gov/dailymed/fda/fdaDrugXsl.cfm?setid=ed6657ca-ab68-477a-9968-e12dc928b540&type=display) lists an initial U.S. approval in 2011 and an indication for acute HAE attacks in adults 18 years of age and older.

Hereditary angioedema is a rare inherited condition in which your skin, your gut or your throat can swell suddenly, sometimes for days. Antihistamines and steroids, the usual tools for allergic swelling, do not work well in HAE because the swelling is driven by bradykinin rather than histamine, and icatibant was built to sit on the bradykinin receptor and shut that signal off. If you follow peptide science, icatibant is a clean example of how chemists turn a fragile natural peptide signal into a drug. This article explains how icatibant works, what the three FAST trials and a pediatric study found, how the drug is dosed and self injected, and why it did not hold up for swelling caused by blood pressure pills. It is educational content about published research, not medical advice.

Hereditary angioedema is an inherited disorder in which people lack enough working C1 inhibitor, a protein that keeps the body's contact system in check. Without that brake, the system overproduces bradykinin, a small peptide that makes blood vessels leaky, so fluid seeps into the deep layers of the skin, the wall of the gut or the airway and produces a swelling attack.

The FAST-1 and FAST-2 investigators described HAE as a condition of recurrent swelling attacks of the skin, larynx and gastrointestinal tract, and they named bradykinin as the key mediator of symptoms ([Cicardi and colleagues, New England Journal of Medicine, 2010](https://doi.org/10.1056/NEJMoa0906393)). That is why HAE swelling behaves differently from hives or allergic swelling, and why antihistamines do little for it. Throat swelling can block the airway, which is why laryngeal attacks are the ones clinicians worry about most.

Icatibant works by occupying the bradykinin B2 receptor so that bradykinin cannot switch it on. The Firazyr label describes icatibant as "a competitive antagonist selective for the bradykinin B2 receptor, with an affinity similar to bradykinin." Because icatibant binds the receptor about as tightly as bradykinin itself, a dose given during an attack can compete with the natural peptide for the receptor and stop the leak that drives the swelling.

Icatibant does not replace the missing C1 inhibitor, and it does not lower bradykinin levels, because it acts only at the downstream receptor. That makes icatibant an on demand rescue treatment, which means it treats the attack in front of you but does nothing to fix the underlying shortage, so people with HAE still work with their specialist on a separate plan for prevention where that is appropriate.

Icatibant is a synthetic decapeptide, which means a chain of 10 amino acids. According to the label, five of those amino acids are nonproteinogenic, meaning they are building blocks that do not occur in normal human proteins. Swapping in unnatural amino acids is a common peptide engineering strategy, because the enzymes that quickly break down natural bradykinin do not recognize the modified chain well, so icatibant lasts long enough to work as a drug.

Icatibant is still a short lived peptide. The label reports bioavailability of approximately 97% after a 30 mg dose under the skin, peak blood levels of 974 ng/mL at approximately 0.75 hours and a mean elimination half life of 1.4 hours. That quick in, quick out profile suits a rescue drug, since icatibant reaches the receptor fast during an attack, and the repeat dose rule exists for attacks that outlast the first injection.

If you want the broader background on why peptide half life shapes dosing schedules, our guide to [peptide half life and dosing frequency](/articles/peptide-half-life-dosing-frequency) covers the general principles.

FAST-3 showed that a single 30 mg icatibant injection relieved HAE attacks many hours faster than placebo. In the phase 3 FAST-3 trial ([Lumry and colleagues, Annals of Allergy, Asthma and Immunology, 2011](https://doi.org/10.1016/j.anai.2011.08.015)), adults with HAE type I or II who had moderate to very severe cutaneous or abdominal attacks received icatibant (n = 43) or placebo (n = 45).

The median time to a 50% or greater reduction in symptom severity, the primary endpoint, was 2.0 hours with icatibant and 19.8 hours with placebo. Onset of primary symptom relief came at 1.5 versus 18.5 hours, almost complete symptom relief at 8.0 versus 36.0 hours and initial symptom relief at 0.8 versus 3.5 hours.

In practical terms, the median patient on icatibant felt meaningfully better in about 2 hours, while the median patient on placebo waited most of a day. The authors also reported that no icatibant treated subject needed rescue medication before symptom relief. Adverse events were reported in 41% of the icatibant group and 52% of the placebo group, and every icatibant treated subject had an injection site reaction.

FAST-3 included only a handful of laryngeal attacks, so the throat data from the controlled phase are thin. For mild to moderate laryngeal first attacks, the median time to a 50% or greater reduction in symptom severity was 2.5 hours on icatibant and 3.2 hours on placebo, and five people with severe laryngeal attacks received open label icatibant. The extension analysis described below adds more laryngeal experience, but the label is clear that anyone treating a laryngeal attack should still seek immediate medical attention after the injection.

FAST-1 missed its primary endpoint because the placebo group was rescued early, which blurred the comparison. In the paired FAST-1 and FAST-2 trials ([Cicardi and colleagues, New England Journal of Medicine, 2010](https://doi.org/10.1056/NEJMoa0906393)), 56 and 74 patients were randomized, and each received a single 30 mg icatibant dose or a comparator. In FAST-1, the primary endpoint was reached in 2.5 hours with icatibant versus 4.6 hours with placebo, a difference that was not statistically significant (P = 0.14).

The authors pointed to rescue medication, since 3 icatibant recipients and 13 placebo recipients needed rescue treatment, and they wrote that early rescue use may have obscured the benefit of icatibant in the placebo trial. FAST-2 compared icatibant with oral tranexamic acid at 3 g daily for 2 days, and there icatibant reached the primary endpoint in 2.0 hours versus 12.0 hours. FAST-3 was then designed to settle the placebo question, and its result was clear.

Icatibant kept working across repeated attacks in the FAST-3 extension. In the open label extension of FAST-3 ([Lumry and colleagues, International Archives of Allergy and Immunology, 2015](https://doi.org/10.1159/000441060)), patients could receive up to three icatibant injections per attack. In groups of patients with one to five icatibant treated attacks at any location (n = 88), the median time to onset of symptom relief stayed in a narrow band of 1.9 to 2.1 hours.

For laryngeal attacks (n = 25), the median time to onset of symptom relief was 1.0 to 2.0 hours. The most frequent adverse events were worsening or recurrence of the HAE attack, headache and nasopharyngitis, and two serious adverse events, an arrhythmia and noncardiac chest pain, were judged related to icatibant. A consistent response from one attack to the next is the property that matters most for a rescue drug that a person may use many times a year.

Icatibant is given as a single 30 mg injection under the skin of the abdomen, and the Firazyr prefilled syringe delivers 3 mL at a concentration of 10 mg per mL. If the response is not enough or symptoms come back, the label allows additional 30 mg doses at intervals of at least 6 hours, with no more than 3 injections in 24 hours.

The label states that patients may self administer the injection upon recognition of an HAE attack, after training from a healthcare professional, and its safety database included 56 patients in a separate self administration trial. Self injection matters because HAE attacks can start anywhere, far from a clinic. It does not replace emergency care for a throat attack, though, because after injecting for a laryngeal attack the label says to seek immediate medical attention. For general technique questions about injections under the skin, see our article on [peptide injection sites and subcutaneous technique](/articles/peptide-injection-site-subcutaneous-technique).

Icatibant has been studied in children in a phase 3 trial, although the U.S. Firazyr label indication covers adults 18 and older. In the pediatric study ([Farkas and colleagues, Journal of Allergy and Clinical Immunology: In Practice, 2017](https://doi.org/10.1016/j.jaip.2017.04.010)), patients aged 2 years to younger than 18 received a single weight based dose of 0.4 mg/kg, capped at 30 mg. In total, 32 patients received icatibant, and the efficacy population was 11 children and 11 adolescents with attacks.

The median time to onset of symptom relief was 1.0 hour, the same for children and adolescents. Treatment emergent adverse events occurred in 28.1% of patients, all mild or moderate, and injection site reactions affected 90.6%, with almost all resolving within 6 hours. Time to peak concentration was about 0.5 hours. None of the treated attacks were laryngeal, so the pediatric study does not answer how icatibant performs in a child's throat attack.

Injection site reactions are by far the most common side effect of icatibant, typically redness, swelling, burning or itching where the shot goes in. The label reports that injection site reactions occurred in almost all patients (97%) in clinical trials, and the other reactions that were more common than placebo were small in size.

The label's overall safety experience comes from three controlled trials that included 223 patients who received Firazyr 30 mg. In the FAST-3 extension, two serious adverse events, an arrhythmia and noncardiac chest pain, were judged related to icatibant, so if you have a heart condition it is worth raising with the clinician who manages your HAE. For a wider view of how to read peptide side effect data, see our [beginner's guide to peptide side effects](/articles/peptide-side-effects-beginners).

Icatibant has not been shown to work reliably for ACE inhibitor angioedema, and it is not approved for that use. ACE inhibitors, a common class of blood pressure medicines, slow the breakdown of bradykinin, so it seemed logical that a bradykinin blocker would help, but the trials disagree.

A German phase 2 trial ([Baş and colleagues, New England Journal of Medicine, 2015](https://doi.org/10.1056/NEJMoa1312524)) randomized patients with ACE inhibitor angioedema of the upper airway to 30 mg icatibant or intravenous prednisolone plus clemastine. Among 27 patients, the median time to complete resolution was 8.0 hours with icatibant versus 27.1 hours with standard therapy, and 5 of 13 icatibant patients versus 0 of 14 were fully resolved within 4 hours.

A U.S. placebo controlled trial ([Straka and colleagues, Journal of Allergy and Clinical Immunology, 2017](https://doi.org/10.1016/j.jaci.2016.09.051)) treated 31 patients, 13 with icatibant and 18 with placebo, and found that time to resolution was similar between the groups. The authors concluded that their study did not support clinical efficacy of a bradykinin B2 receptor antagonist in this kind of angioedema. With two small trials pointing in opposite directions, the honest summary is that the question remains unsettled.

Icatibant is not a wellness, recovery or antiaging peptide, because it is a prescription rescue drug with one approved job, which is treating acute attacks of hereditary angioedema. The evidence base consists of controlled trials in people with a diagnosed, rare genetic condition, and none of that research tested icatibant in healthy people.

Icatibant is still a useful case study if you are interested in peptide science. It shows how chemists turn a fragile natural signal into a usable drug by inserting unnatural amino acids, and the FAST program shows how a trial design choice, early rescue medication in FAST-1, can hide a real effect until a better designed trial reveals it. Other approved peptides in this series, such as [octreotide](/articles/octreotide-somatostatin-analog-acromegaly-neuroendocrine-tumors), follow the same pattern of a stabilized copy of a natural signal.

Icatibant is a 10 amino acid peptide that blocks the bradykinin B2 receptor, and it is approved in the U.S. for treating acute hereditary angioedema attacks in adults. In FAST-3, a single 30 mg injection brought the median time to a 50% or greater drop in symptoms down to 2.0 hours, compared with 19.8 hours on placebo, and the response held across repeated attacks in the extension study. Injection site reactions are nearly universal but usually short lived, a throat attack still calls for emergency care after the injection, and the evidence does not support using icatibant for ACE inhibitor swelling. If you live with HAE, icatibant is one of the on demand options to discuss with your specialist, alongside a prevention plan that fits your attack pattern.