Icatibant

Icatibant competitively inhibits bradykinin at B2 receptors with high affinity and selectivity over B1 receptors. B2 receptors couple through Gq to activate PLC, generating IP3 and increasing intracellular Ca2+ in endothelial and smooth muscle cells. B2-mediated Ca2+ elevation activates eNOS (releasing NO) and stimulates prostaglandin production, collectively causing vasodilation and increased vascular permeability. Icatibant blockade of this cascade prevents the vascular leak that drives HAE swelling.

In HAE, C1-inhibitor deficiency allows unregulated activation of kallikrein, which cleaves high-molecular-weight kininogen to release bradykinin. Bradykinin acts on B2 receptors in submucosal tissue to increase vascular permeability, causing fluid extravasation and angioedema. HAE attacks in the larynx carry a 30% mortality if untreated. Icatibant's B2 blockade interrupts this final step, resolving swelling independent of whether the initial kallikrein-kinin activation is blocked.

ACE inhibitors block bradykinin degradation (bradykinin is normally metabolized by ACE), leading to bradykinin accumulation. In susceptible individuals, this causes ACE inhibitor-induced angioedema (estimated 1-2% incidence), which is not C1-inhibitor-mediated. Icatibant directly blocks the accumulating bradykinin at B2 receptors. Several RCTs investigated icatibant for ACE inhibitor angioedema with mixed results, some trials showed benefit while others did not, likely due to the complex interplay of bradykinin and histamine mechanisms in different patients.

FAST-1 and FAST-3 trials demonstrated icatibant reduces time to significant symptom relief from ~28 hours (placebo) to ~2 hours in HAE attacks, with 90% of patients responding within 4 hours. It is approved for adults and adolescents (>=12 years), can be self-injected, and is stored at room temperature, major advantages for emergency home use. Current HAE guidelines position icatibant alongside C1-inhibitor concentrate and kallikrein inhibitors as first-line acute therapy.

The CAMEO trial (n=121) found icatibant did not significantly reduce time to resolution of ACE inhibitor angioedema versus placebo, though there was a trend toward benefit in severely affected patients. The RADICAL trial showed benefit over supportive care. Current evidence suggests icatibant may benefit patients with severe or rapidly progressive ACE inhibitor angioedema but is not consistently effective across all patients in this setting.

Analysis of COVID-19 transcriptomics suggested a "bradykinin storm" with upregulation of ACE2 and kallikrein pathways in lung tissue, reducing bradykinin clearance and increasing B2R expression. Case series of severe COVID-19 pneumonia with icatibant showed potential benefit in oxygenation. Controlled trials have been initiated but results are inconclusive. This application demonstrates the broader relevance of bradykinin biology beyond HAE.