Histamine and Bradykinin Pharmacology

Correct Answer

C — Icatibant

Rationale

Icatibant is a selective competitive antagonist at the bradykinin B2 receptor and is the drug of choice for acute hereditary angioedema attacks at the Step 1 level. It is administered subcutaneously and can be self-administered by the patient. Lanadelumab is a monoclonal antibody that inhibits plasma kallikrein and is used for long-term prophylaxis, not acute attacks. Danazol is an attenuated androgen used for long-term prophylaxis by upregulating C1 inhibitor synthesis. Ecallantide is a plasma kallikrein inhibitor used for acute attacks but requires healthcare provider administration due to anaphylaxis risk.

Correct Answer

A — Lanadelumab

Rationale

Lanadelumab is a monoclonal antibody that binds and inhibits plasma kallikrein, the enzyme responsible for cleaving high-molecular-weight kininogen to generate bradykinin. Administered subcutaneously every two to four weeks, it is the preferred agent for long-term prophylaxis of hereditary angioedema. Icatibant is a B2 receptor antagonist used for acute attacks. Ecallantide is a plasma kallikrein inhibitor used for acute attacks, but is not a monoclonal antibody and requires healthcare provider administration. Danazol is an attenuated androgen used for long-term prophylaxis through a different mechanism — upregulation of hepatic C1 inhibitor synthesis.

Correct Answer

D — Danazol

Rationale

Danazol is an attenuated androgen used historically for long-term prophylaxis of hereditary angioedema. It has largely been replaced by lanadelumab due to its dose-dependent adverse effects including virilization in women, hepatotoxicity, and lipid abnormalities. Icatibant is a B2 receptor antagonist used for acute attacks. Lanadelumab is a monoclonal antibody kallikrein inhibitor used for long-term prophylaxis. Ecallantide is a plasma kallikrein inhibitor used for acute attacks.

Correct Answer

B — C1 inhibitor concentrate

Rationale

C1 inhibitor concentrate is an intravenous replacement protein therapy available in plasma-derived and recombinant formulations. It is used for acute hereditary angioedema attacks and for short-term prophylaxis before surgical or dental procedures that might trigger an attack. Danazol is an oral attenuated androgen used for long-term prophylaxis. Icatibant is a subcutaneous B2 receptor antagonist for acute attacks, not a replacement protein. Lanadelumab is a subcutaneous monoclonal antibody for long-term prophylaxis and is not administered intravenously nor used for pre-procedure prophylaxis.

Correct Answer

A — Ecallantide

Rationale

Ecallantide is a plasma kallikrein inhibitor used for acute hereditary angioedema attacks. Because it carries a risk of anaphylaxis, it must be administered by a healthcare provider in a setting equipped to manage that reaction — unlike icatibant, which can be self-administered subcutaneously by the patient. Ecallantide is lower yield than icatibant at the Step 1 level; the key distinguishing facts are its drug class (kallikrein inhibitor, not B2 receptor antagonist) and its administration restriction. Icatibant is a B2 receptor antagonist approved for patient self-administration. C1 inhibitor concentrate is a replacement protein therapy administered intravenously. Danazol is an oral attenuated androgen for long-term prophylaxis.

Correct Answer

C — Angiotensin receptor blockers

Rationale

Angiotensin receptor blockers block the angiotensin II receptor directly without inhibiting angiotensin converting enzyme. Because angiotensin converting enzyme activity remains intact, bradykinin continues to be degraded at normal rates, and angiotensin receptor blockers do not cause the bradykinin-mediated cough or angioedema that occurs with angiotensin converting enzyme inhibitors. This pharmacological distinction is the basis for switching patients from an angiotensin converting enzyme inhibitor to an angiotensin receptor blocker when they develop these adverse effects. Direct renin inhibitors reduce renin activity but do not directly preserve bradykinin degradation through this mechanism. Aldosterone antagonists and beta-blockers act through different pathways entirely and are not classified by this property.

Correct Answer

A — Icatibant competitively blocks the B2 receptor on vascular endothelium, preventing bradykinin from triggering the increased vascular permeability responsible for submucosal edema

Rationale

In hereditary angioedema, unregulated kallikrein activity generates excess bradykinin, which acts on constitutively expressed B2 receptors on vascular endothelium to produce nitric oxide-mediated vasodilation and gap formation between endothelial cells, causing fluid to leak into submucosal and subcutaneous tissues. Icatibant blocks this B2 receptor, preventing bradykinin from triggering these permeability changes and allowing the attack to resolve. Kallikrein inhibition describes lanadelumab and ecallantide, not icatibant. C1 inhibitor replacement describes C1 inhibitor concentrate. Upregulation of C1 inhibitor synthesis through androgen receptor activation describes danazol.

Correct Answer

C — Angiotensin converting enzyme inhibition allows bradykinin to accumulate in the bronchial mucosa, where it sensitizes sensory C fibers via B2 receptor activation, producing a dry cough

Rationale

Angiotensin converting enzyme (kininase II) normally degrades bradykinin in the bronchial mucosa. When angiotensin converting enzyme is inhibited, bradykinin accumulates locally and activates B2 receptors on bronchial sensory C fibers, sensitizing them and lowering the threshold for cough. The cough is dry and nonproductive, begins within days to weeks of starting the angiotensin converting enzyme inhibitor, is not dose-dependent, and resolves within days to weeks of stopping the drug. It is more common in patients of East Asian ancestry. Angiotensin converting enzyme also degrades substance P, and its inhibition does allow substance P to accumulate — substance P can sensitize cough receptors and likely contributes to the cough in some patients. However, bradykinin acting via B2 receptor activation on bronchial sensory C fibers is the primary and dominant mechanism, which is why the cough resolves reliably when the angiotensin converting enzyme inhibitor is stopped and why switching to an angiotensin receptor blocker — which does not affect substance P degradation either — nonetheless eliminates the cough in most patients. The correct management is to switch to an angiotensin receptor blocker, which blocks the angiotensin II receptor without inhibiting angiotensin converting enzyme, thereby leaving bradykinin degradation intact. Reduced angiotensin II, mast cell histamine release, and substance P elevation are not the established mechanisms for this class-specific adverse effect.

Correct Answer

B — Angiotensin converting enzyme (kininase II) normally degrades bradykinin in addition to converting angiotensin I to angiotensin II; its inhibition removes the principal pathway for bradykinin clearance

Rationale

Angiotensin converting enzyme has two physiologically important substrates. First, it converts angiotensin I to the vasoconstrictor angiotensin II — the intended target of angiotensin converting enzyme inhibitor therapy. Second, it functions as kininase II, cleaving two amino acids from the C-terminus of bradykinin to render it inactive. When angiotensin converting enzyme is inhibited, both functions are lost simultaneously: angiotensin II levels fall (the desired effect) and bradykinin accumulates in tissues that depend on angiotensin converting enzyme for its clearance (the source of adverse effects). This dual role is the complete pharmacological explanation for why angiotensin converting enzyme inhibitors — and not angiotensin receptor blockers, which leave angiotensin converting enzyme intact — produce bradykinin-mediated cough and angioedema. Angiotensin converting enzyme inhibitors do not activate kallikrein, upregulate B2 receptors, or alter renal clearance of bradykinin metabolites.

Correct Answer

D — Danazol upregulates hepatic C1 inhibitor gene expression through its androgenic activity, raising plasma C1 inhibitor levels toward normal and restoring regulation of the kallikrein-kinin cascade

Rationale

In hereditary angioedema types I and II, patients have deficient or dysfunctional C1 inhibitor, which normally restrains plasma kallikrein and prevents unregulated bradykinin generation. Danazol is an attenuated androgen that acts on androgen response elements in the hepatic C1 inhibitor gene promoter, increasing C1 inhibitor transcription and raising plasma C1 inhibitor concentrations toward the normal range. This indirectly controls kallikrein activity and reduces the frequency of bradykinin-driven attacks. Danazol does not directly inhibit kallikrein, does not block B2 receptors, and does not activate angiotensin converting enzyme. Its mechanism is entirely upstream — restoring the endogenous regulatory protein rather than targeting the bradykinin pathway downstream. The dose-dependent adverse effects of danazol — virilization in women, hepatotoxicity, and lipid abnormalities — have led to its replacement by lanadelumab in most clinical settings.

Correct Answer

A — C1 inhibitor concentrate replaces the deficient regulatory protein, restoring direct inhibition of plasma kallikrein; administered before a procedure, it provides physiological protection against the kallikrein activation that surgical stress can trigger

Rationale

C1 inhibitor is the endogenous serine protease inhibitor that restrains plasma kallikrein activity and prevents unregulated bradykinin generation. In hereditary angioedema types I and II, C1 inhibitor is deficient or dysfunctional. C1 inhibitor concentrate works by directly replacing this missing protein, providing the missing inhibitory control of kallikrein. This mechanism explains both uses: it terminates an acute attack by stopping ongoing kallikrein-driven bradykinin production, and it provides pre-procedure prophylaxis by ensuring physiological kallikrein regulation is in place before surgical or dental procedures — which are known triggers for hereditary angioedema attacks through contact activation of the coagulation cascade and kallikrein. C1 inhibitor concentrate does not reduce B2 receptor sensitivity and does not stimulate new hepatic synthesis of C1 inhibitor — that mechanism describes danazol. C1 inhibitor concentrate is used for short-term prophylaxis only, not long-term; lanadelumab is the preferred long-term prophylactic agent.

Correct Answer

C — Lanadelumab is a monoclonal antibody that binds and inhibits plasma kallikrein, preventing the enzymatic cleavage of high-molecular-weight kininogen that generates bradykinin

Rationale

Lanadelumab targets plasma kallikrein — the enzyme responsible for cleaving high-molecular-weight kininogen to release bradykinin. By binding and inhibiting kallikrein as a monoclonal antibody, lanadelumab prevents bradykinin generation at the source, reducing attack frequency over its dosing interval. Its long half-life supports subcutaneous dosing every two to four weeks, making it practical for long-term prophylaxis. C1 inhibitor replacement describes C1 inhibitor concentrate. B2 receptor blockade describes icatibant. Upregulation of C1 inhibitor synthesis through androgenic activity describes danazol. Each of these agents targets a different step in the kallikrein-kinin cascade: C1 inhibitor concentrate restores the endogenous brake, lanadelumab directly inactivates kallikrein, and icatibant blocks the downstream receptor.

Correct Answer

B — Gq coupling — activates phospholipase C and raises intracellular calcium; stimulates nitric oxide production and increases vascular permeability in endothelium; produces pain sensitization in sensory neurons

Rationale

The B2 receptor couples to Gq, activating phospholipase C to generate inositol trisphosphate and raise intracellular calcium. In vascular endothelium, calcium activates nitric oxide synthase, producing nitric oxide that diffuses to vascular smooth muscle and causes vasodilation; simultaneously, B2 activation promotes gap formation between endothelial cells, increasing vascular permeability. In sensory neurons, B2 receptor activation produces pain and lowers the threshold of nociceptors for other stimuli. The B2 receptor is constitutively expressed throughout the body and mediates acute bradykinin effects — distinguishing it from the B1 receptor, which is inducible by inflammation and mediates chronic pain. Option A describes a Gs-coupled receptor mechanism. Option C describes Gi coupling seen with H3 receptors. Option D describes the B1 receptor, which is distinct from the B2 receptor in two key ways: B1 receptors are inducible — expressed at very low baseline levels and dramatically upregulated only after tissue injury and inflammation — whereas B2 receptors are constitutively expressed throughout the body under normal conditions; and the primary endogenous agonist at B1 receptors is des-Arg9-bradykinin, a metabolite generated when kininase I cleaves the C-terminal arginine from bradykinin, rather than bradykinin itself. Because the question asks about the constitutively expressed receptor mediating acute bradykinin effects, B2 is correct and B1 does not fit.

Correct Answer

D — Hereditary angioedema is driven by bradykinin excess, not histamine; epinephrine, antihistamines, and corticosteroids address histamine-mediated pathways and have no effect on bradykinin-mediated vascular permeability

Rationale

The failure of epinephrine, antihistamines, and corticosteroids in a patient with angioedema is a diagnostic signal that the mediator is bradykinin rather than histamine. In hereditary angioedema, deficient C1 inhibitor allows unregulated plasma kallikrein to generate bradykinin continuously. Bradykinin acts on B2 receptors to increase vascular permeability, but these receptors are not affected by adrenergic agonists, H1 receptor blockade, or corticosteroids. Continuing to administer standard allergy treatments while the airway is compromising delays definitive bradykinin-targeted therapy and is a recognized cause of preventable death. The correct response is immediate administration of icatibant, C1 inhibitor concentrate, or ecallantide — whichever is available. Recognizing treatment failure with standard agents as a diagnostic sign of bradykinin-mediated angioedema is one of the highest-yield clinical distinctions in this chapter. Complement activation contributes to the pathogenesis upstream but is not addressable by epinephrine or antihistamines; the proximate mediator is bradykinin. Dose escalation of ineffective agents would not change the outcome because the mechanism is pharmacologically unresponsive to these drug classes regardless of dose.

Correct Answer

D — Subcutaneous icatibant or intravenous C1 inhibitor concentrate, because hereditary angioedema is bradykinin-mediated and requires agents targeting the kallikrein-kinin pathway

Rationale

Hereditary angioedema is driven by excess bradykinin generated through unregulated kallikrein activity in the setting of C1 inhibitor deficiency. Epinephrine, antihistamines, and corticosteroids are ineffective because they target histamine-mediated pathways that are not involved in the mechanism. When standard allergy treatments fail in a patient with known or suspected hereditary angioedema, icatibant (B2 receptor antagonist, subcutaneous) or C1 inhibitor concentrate (intravenous replacement therapy) must be administered immediately. At the Step 1 level, icatibant is the named drug of choice. With laryngeal involvement and prior intubation history, airway management must proceed in parallel. Adding H2 blockade to H1 blockade is appropriate adjunctive therapy for histamine-mediated anaphylaxis but is entirely irrelevant to a bradykinin-mediated reaction. Dose escalation of epinephrine and corticosteroid addition cannot overcome the pharmacological mismatch between the mediator and the drug class.

Correct Answer

B — Switch to an angiotensin receptor blocker, which blocks the angiotensin II receptor without inhibiting angiotensin converting enzyme, leaving bradykinin degradation intact

Rationale

Lisinopril-induced cough results from bradykinin accumulation in the bronchial mucosa after angiotensin converting enzyme (kininase II) is inhibited. The cough is not dose-dependent — it reflects individual sensitivity to bradykinin — and does not resolve with dose reduction. Angiotensin receptor blockers such as losartan or valsartan block the angiotensin II receptor directly and do not inhibit angiotensin converting enzyme, so bradykinin continues to be degraded at normal rates and the cough does not occur. This is the established management for angiotensin converting enzyme inhibitor-induced cough and allows continuation of renin-angiotensin-aldosterone system blockade, which is clinically important in heart failure. H1 antihistamines are ineffective for bradykinin-mediated cough because the mechanism involves B2 receptor activation on sensory C fibers, not histamine. Direct renin inhibitors reduce angiotensin I but do not restore angiotensin converting enzyme-mediated bradykinin degradation, so they would not reliably prevent the cough.

Correct Answer

A — Lanadelumab, a monoclonal antibody kallikrein inhibitor that does not interact with androgen receptors or require hepatic metabolism for its mechanism of action

Rationale

The adverse effects this patient is experiencing — virilization (facial hair, voice deepening) and hepatotoxicity — are the established dose-dependent toxicities of danazol's androgenic activity and hepatic effects. Lanadelumab has replaced danazol as the preferred long-term prophylactic agent for most patients because it inhibits plasma kallikrein through monoclonal antibody binding rather than through any androgenic mechanism, and its administration does not involve hepatic metabolism in a way that produces hepatotoxicity. It is administered subcutaneously every two to four weeks. Icatibant is approved for acute attacks and is not the established agent for long-term prophylaxis. C1 inhibitor concentrate is used for acute attacks and short-term pre-procedure prophylaxis, not for regular long-term prophylaxis in ambulatory patients. Switching to a more potent synthetic androgen would worsen the same class of adverse effects rather than eliminate them, since virilization and hepatotoxicity are inherent to androgen receptor activation.

Correct Answer

C — Icatibant, which is formulated for subcutaneous injection and is approved for self-administration by the patient for acute hereditary angioedema attacks

Rationale

Icatibant is a subcutaneous B2 receptor antagonist specifically approved for patient self-administration, making it the preferred acute treatment when a healthcare provider is not immediately available. Patients with hereditary angioedema are trained to inject it themselves at attack onset, enabling rapid treatment and reducing dependence on emergency care access. Ecallantide is also a subcutaneous acute treatment but requires healthcare provider administration due to the risk of anaphylaxis — it is not approved for patient self-administration. C1 inhibitor concentrate is administered intravenously and, while some formulations can be self-infused by trained patients at home, it requires intravenous access rather than a simple subcutaneous injection and is not the standard self-administration agent for travel situations. Danazol is an oral attenuated androgen used for long-term prophylaxis — it is not an acute treatment and does not provide rapid symptom relief during an attack.