Histamine & Bradykinin Pharmacology · Module 4 of 4

Bradykinin Pharmacology and Angioedema

Kallikrein-kinin cascade, ACE inhibitor adverse effects, and hereditary angioedema treatment

Bradykinin is generated on demand by the kallikrein-kinin cascade and has a plasma half-life of only 15–30 seconds — Factor XII activation (contact activation) → plasma kallikrein (from prekallikrein) → cleaves HMW kininogen → bradykinin; degraded primarily by ACE (kininase II, cleaves C-terminal dipeptide) and kininase I (carboxypeptidase N, cleaves C-terminal arginine → produces des-Arg⁹-bradykinin, the B1 receptor agonist); ACE inhibitors block bradykinin degradation → bradykinin accumulates in airways (cough) and vasculature (angioedema) — the same enzyme that converts angiotensin I → II also degrades bradykinin.
B2 receptor (constitutive, Gq) mediates all acute effects of bradykinin — vascular endothelium: ↑NO (vasodilation) + ↑vascular permeability (edema); sensory neurons: pain sensitization and nociceptor sensitization; bronchial smooth muscle: bronchoconstriction; B1 receptor (inducible, upregulated by inflammation, agonist = des-Arg⁹-bradykinin): chronic pain sensitization and sustained inflammatory responses; icatibant targets B2; ARBs (angiotensin receptor blockers) do not inhibit ACE → bradykinin degradation intact → no cough or angioedema at increased rates.
ACE inhibitor cough and angioedema share the same bradykinin mechanism but differ in tissue — Cough: bradykinin accumulates in bronchial mucosa → B2 sensitization of sensory C fibers → dry nonproductive cough; onset days-to-weeks; not dose-dependent (individual sensitivity); more common in East Asian patients; resolves on discontinuation; switch to ARB; Angioedema: bradykinin in dermal/submucosal microvasculature → B2-mediated edema of face/lips/tongue/larynx WITHOUT urticaria (no mast cell involvement); higher risk in African American patients 3–5×; can occur after months-to-years of stable use; does NOT respond to epinephrine, antihistamines, or corticosteroids; stop ACE inhibitor permanently.
Hereditary angioedema (HAE) is bradykinin excess from C1 inhibitor deficiency — Types I and II: C1 inhibitor protein deficiency or dysfunction → unregulated plasma kallikrein → continuous bradykinin generation; attacks: extremities, abdomen (severe pain, mimics surgical emergency), face, larynx (life-threatening); no urticaria; antihistamines and epinephrine are completely ineffective; the failure to respond to standard allergy treatment is itself the diagnostic signal.
ACE inhibitor angioedema vs. HAE share treatment logic — both require bradykinin-specific therapy — Icatibant (B2 receptor antagonist, SC self-administered) = drug of choice for acute HAE attacks and also used in ACE inhibitor angioedema; C1 inhibitor concentrate (IV, plasma-derived or recombinant) = acute attacks + pre-procedure prophylaxis in HAE; Lanadelumab (anti-kallikrein monoclonal Ab, SC q2–4 weeks) = preferred long-term HAE prophylaxis; Danazol (attenuated androgen, ↑hepatic C1 inhibitor synthesis) = historical prophylaxis, largely replaced by lanadelumab; Ecallantide (kallikrein inhibitor) = acute attack, lower yield, requires HCP administration due to anaphylaxis risk.

Abbreviations: ACE = angiotensin-converting enzyme · ARB = angiotensin receptor blocker · HMW = high-molecular-weight · B1/B2 = bradykinin receptor subtypes 1 and 2 · HAE = hereditary angioedema · C1-INH = C1 inhibitor · SC = subcutaneous · IV = intravenous · NO = nitric oxide · des-Arg⁹-BK = des-Arg⁹-bradykinin (B1 receptor agonist)

ACE Inhibitor Bradykinin-Mediated Adverse Effects

Bradykinin Accumulation — Airways

ACE Inhibitor Cough

ACE normally degrades bradykinin in bronchial mucosa; ACE inhibitor → bradykinin accumulates → B2 receptor sensitizes sensory C fibers
Dry, nonproductive cough; onset days-to-weeks; not dose-dependent
More common in East Asian patients (individual sensitivity)
Resolves within days-to-weeks of discontinuation
Fix: switch to ARB — ARBs do not inhibit ACE; bradykinin degradation remains intact; cough does NOT occur

Bradykinin Accumulation — Vasculature

ACE Inhibitor Angioedema

Bradykinin in dermal/submucosal microvessels → B2-mediated ↑vascular permeability → tissue edema
Face, lips, tongue, pharynx, larynx — laryngeal involvement = medical emergency
WITHOUT urticaria (no mast cell involvement — key distinguishing feature)
Higher risk in African American patients (3–5×)
Can occur after months-to-years of stable use — not only at initiation
Does NOT respond to epinephrine, antihistamines, or corticosteroids
Fix: stop ACE inhibitor permanently; switch to ARB; never rechallenge with another ACE inhibitor

Histamine vs. Bradykinin Angioedema — Key Distinctions

Feature	Histamine-Mediated	Bradykinin-Mediated
Urticaria	Yes — almost always (mast cell co-release)	No — key distinguishing feature; no mast cell involvement
Causes	IgE-mediated allergic reaction; food/drug allergy	ACE inhibitor use; HAE (C1 inhibitor deficiency types I and II)
Responds to epinephrine	Yes — first-line for anaphylaxis	No
Responds to antihistamines	Yes — adjunctive	No
Responds to corticosteroids	Yes — adjunctive (delayed onset)	No
Specific treatment	Epinephrine (anaphylaxis); H1 + H2 antihistamines; corticosteroids	Icatibant (B2 antagonist); C1-INH concentrate; stop ACE inhibitor

Hereditary Angioedema Treatment

Acute Attack — B2 Receptor Antagonist

Icatibant

Selective competitive B2 receptor antagonist; blocks the receptor mediating vascular permeability
SC administration — can be self-administered by patient
Drug of choice for acute HAE attacks at Step 1 level
Effective for cutaneous, abdominal, and laryngeal attacks
Also used for ACE inhibitor-induced angioedema

Acute Attack / Prophylaxis — Replacement

C1 Inhibitor Concentrate

Replaces deficient C1-INH protein (root cause of HAE types I and II)
Restores physiological kallikrein control → stops bradykinin generation
IV for acute attacks; also short-term prophylaxis before procedures
Plasma-derived and recombinant formulations available

Long-Term Prophylaxis — Anti-Kallikrein

Lanadelumab

Monoclonal antibody targeting plasma kallikrein; prevents bradykinin generation at its source
SC every 2–4 weeks; preferred long-term prophylactic agent (replaced danazol)
Significantly reduces HAE attack frequency (HELP trial)

Long-Term Prophylaxis — Historical

Danazol

Attenuated androgen; upregulates hepatic C1-INH gene expression → ↑plasma C1-INH levels
Significant adverse effects: virilization in women, hepatotoxicity, lipid abnormalities
Largely replaced by lanadelumab; lower Step 1 yield; know: mechanism = ↑C1-INH synthesis
Ecallantide (plasma kallikrein inhibitor) = acute attack, lower yield, requires HCP due to anaphylaxis risk

Chapter Complete — Histamine & Bradykinin Pharmacology (HBRD) · Key Rules

The angioedema distinction is the highest-yield clinical principle in this chapter: angioedema with urticaria = histamine-mediated (mast cell co-release) → epinephrine/antihistamines/corticosteroids work; angioedema without urticaria = bradykinin-mediated (ACE inhibitor or HAE) → none of the standard allergy treatments work. Failure to respond to epinephrine and antihistamines is itself a diagnostic signal — do not keep administering them. The correct response to ACE inhibitor angioedema is to stop the ACE inhibitor permanently and switch to an ARB. Never rechallenge. The correct acute treatment of HAE is icatibant (B2 antagonist, SC) or C1 inhibitor concentrate (IV); danazol ↑C1-INH synthesis for historical prophylaxis.

The ACE inhibitor cough-angioedema unifying mechanism: ACE = kininase II; inhibiting ACE accumulates bradykinin in any tissue where ACE normally degrades it; cough = bronchial C fiber sensitization via B2; angioedema = dermal/submucosal B2-mediated ↑permeability; ARBs do not inhibit ACE, so bradykinin degradation remains intact and neither complication occurs at increased rates. ARBs carry a small residual angioedema risk but roughly 10-fold lower than ACE inhibitors.

HBRD chapter pharmacological core: histamine is preformed in granules (mast cells, basophils, ECL cells, histaminergic neurons), released by IgE crosslinking or non-immunological stimuli, and acts through 4 GPCRs (H1 Gq, H2 Gs, H3 Gi autoreceptor, H4 Gi immune); H1 antihistamines = inverse agonists stabilizing inactive receptor conformation; first-generation cross BBB (lipophilic + no P-gp efflux) → sedation + antimuscarinic; second-generation excluded by P-gp efflux; cimetidine = only H2 blocker with CYP450 inhibition (warfarin/phenytoin/theophylline) + androgen receptor antagonism (gynecomastia/impotence); famotidine = preferred H2 blocker (no CYP, no androgen effects, highest potency); ranitidine withdrawn 2020 (NDMA contamination); PPIs superior for erosive disease, ZES, and H. pylori eradication.

Suggested References

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Brunton L, Knollmann B, Hilal-Dandan R, eds.	Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 14th ed.	McGraw-Hill; 2023
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Banerji A, Riedl MA, Bernstein JA, et al.	Effect of lanadelumab compared with placebo on prevention of hereditary angioedema attacks: a randomized clinical trial	JAMA. 2018;320(20):2108–2121
Maurer M, Magerl M, Ansotegui I, et al.	The international WAO/EAACI guideline for the management of hereditary angioedema: the 2017 revision and update	Allergy. 2018;73(8):1575–1596
Kostis WJ, Shetty M, Chowdhury YS, Kostis JB.	ACE inhibitor-induced angioedema: a review	Curr Hypertens Rep. 2018;20(7):55
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