Bradykinin
Bradykinin is a vasoactive nonapeptide generated from high-molecular-weight kininogen by the serine protease kallikrein. Acting through constitutive B2 and inducible B1 receptors, it is a potent mediator of vasodilation, vascular permeability, pain, and inflammation, with major clinical relevance in ACE inhibitor side effects, hereditary angioedema, and inflammatory disease.
Bradykinin is a nine-amino acid vasoactive peptide (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) that serves as one of the most potent endogenous mediators of vasodilation, vascular permeability, and pain signaling. Generated from high-molecular-weight kininogen (HMWK) by the serine protease kallikrein, bradykinin is a central effector of the kallikrein-kinin system (KKS) and plays critical roles in cardiovascular regulation, inflammatory responses, and nociception.
Overview
Bradykinin was first identified in 1949 by Mauricio Rocha e Silva, Wilson Teixeira Beraldo, and Gastao Rosenfeld, who observed that trypsin-treated blood plasma contained a substance that caused slow contraction of guinea pig ileum — hence the name "bradykinin" (from Greek: bradys = slow, kinein = to move). Since then, it has been recognized as a pivotal mediator in virtually every inflammatory process and a key regulator of vascular tone.
The kallikrein-kinin system operates in parallel with the RAAS and coagulation cascades. Contact activation of factor XII (Hageman factor) on negatively charged surfaces activates prekallikrein to kallikrein, which cleaves HMWK to release bradykinin. Tissue kallikrein cleaves low-molecular-weight kininogen (LMWK) to produce kallidin (Lys-bradykinin), which is subsequently converted to bradykinin by aminopeptidases. Bradykinin is rapidly degraded by kininases, most notably angiotensin-converting enzyme (ACE, also called kininase II), which is the critical intersection between the KKS and RAAS systems.
The dual role of ACE — converting angiotensin I to angiotensin II while simultaneously degrading bradykinin — explains why ACE inhibitors not only reduce Ang II but also increase bradykinin levels, producing both therapeutic benefits (additional vasodilation, cardioprotection) and adverse effects (cough, angioedema). This pharmacological intersection makes bradykinin one of the most clinically relevant endogenous peptides in medicine.
Mechanism of Action
Bradykinin exerts its biological effects through two G protein-coupled receptors — B1 and B2 — with distinct expression patterns, pharmacology, and physiological roles:
B2 Receptor (BDKRB2) — Constitutive: The B2 receptor is constitutively expressed on endothelial cells, smooth muscle cells, sensory neurons, epithelial cells, and fibroblasts. It is the primary receptor for intact bradykinin and mediates the majority of acute kinin effects. B2 receptor activation triggers Gq-mediated phospholipase C activation, producing IP3 (intracellular calcium release) and DAG (protein kinase C activation). In endothelial cells, the resulting calcium rise activates eNOS and cyclooxygenase, producing NO and prostacyclin (PGI2) — the principal mediators of bradykinin-induced vasodilation. B2 receptors undergo rapid desensitization and internalization upon sustained agonist exposure.
B1 Receptor (BDKRB1) — Inducible: The B1 receptor is normally expressed at very low levels but is dramatically upregulated during tissue injury, inflammation, and infection by cytokines (IL-1beta, TNF-alpha) and bacterial endotoxin via NF-kB-dependent transcription. B1 receptors are activated by des-Arg9-bradykinin and des-Arg10-kallidin (metabolites of bradykinin and kallidin produced by carboxypeptidase M/N). Unlike B2, B1 receptors do not desensitize and provide sustained signaling during chronic inflammation. This makes B1 a key mediator of chronic inflammatory pain and persistent edema.
Vasodilation Pathway: In the vasculature, bradykinin/B2 receptor activation on endothelial cells stimulates production of three vasodilatory mediators: nitric oxide (via eNOS), prostacyclin (via COX-1/2), and endothelium-derived hyperpolarizing factor (EDHF). The relative contribution of each varies by vascular bed — NO predominates in large arteries, while EDHF is more important in resistance vessels.
Vascular Permeability: Bradykinin increases vascular permeability by inducing endothelial cell contraction, widening inter-endothelial gaps in postcapillary venules. This involves B2-mediated activation of myosin light chain kinase through calcium/calmodulin signaling, as well as phosphorylation of VE-cadherin and disruption of adherens junctions. The resulting plasma extravasation causes edema — the pathological hallmark of angioedema.
Pain and Nociception: Bradykinin is one of the most potent endogenous algogenic (pain-producing) substances. B2 receptor activation on primary afferent sensory neurons (C-fibers and A-delta fibers) sensitizes transient receptor potential vanilloid 1 (TRPV1) channels through PKC-dependent phosphorylation, lowering the thermal activation threshold and producing hyperalgesia. B1 receptors on sensory neurons mediate sustained inflammatory pain through similar but non-desensitizing mechanisms.
Bronchoconstriction: In the airways, bradykinin stimulates bronchial smooth muscle contraction (via B2 receptors), mucus secretion, and neurogenic inflammation through sensory nerve stimulation. This is the primary mechanism of ACE inhibitor-induced cough, where elevated bradykinin activates sensory C-fibers in the bronchial epithelium.
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Bradykinin
Bradykinin is a nine-amino acid vasoactive peptide (Arg-Pro-Pro-Gly-Phe-Ser-Pro-
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Research
Hereditary Angioedema (HAE)
Hereditary angioedema is a rare autosomal dominant disorder characterized by recurrent episodes of severe subcutaneous and submucosal edema. HAE types I and II result from deficiency or dysfunction of C1-inhibitor (C1-INH), the primary plasma inhibitor of kallikrein and factor XIIa. C1-INH deficiency leads to uncontrolled kallikrein activity, excessive bradykinin generation, and episodes of potentially life-threatening angioedema (particularly laryngeal edema). HAE type III involves mutations in factor XII (Hageman factor) or other KKS components. The identification of bradykinin as the primary mediator of HAE attacks — rather than complement-derived mediators as previously assumed — was a paradigm shift that led to development of targeted therapies. Nussberger et al. (1998) demonstrated elevated bradykinin levels during HAE attacks, establishing the mechanistic basis for kinin-targeted therapy.
Icatibant (B2 Receptor Antagonist)
Icatibant (HOE 140, Firazyr) is a synthetic decapeptide and selective B2 receptor antagonist approved for treatment of acute HAE attacks. It contains five non-natural amino acids that confer resistance to proteolytic degradation while maintaining high B2 receptor affinity. Cicardi et al. (2010) — N. Engl. J. Med. demonstrated that icatibant 30 mg subcutaneous injection provided significantly faster symptom relief compared to tranexamic acid in HAE attacks, with a median time to symptom relief of 2.0 hours versus 12.0 hours. Icatibant has also been investigated off-label for ACE inhibitor-induced angioedema, with case series suggesting efficacy in refractory cases.
Cardiovascular Protection
Beyond its role in ACE inhibitor pharmacology, bradykinin has direct cardioprotective properties. Bradykinin is a key mediator of ischemic preconditioning — the phenomenon whereby brief episodes of ischemia protect the myocardium against subsequent prolonged ischemic injury. Wall et al. (1994) demonstrated that B2 receptor activation triggers preconditioning through PKC-dependent opening of mitochondrial KATP channels. In clinical studies, the B2 receptor T-58C polymorphism (associated with higher B2 receptor expression) correlates with reduced risk of left ventricular hypertrophy and heart failure. Bradykinin also promotes tissue-type plasminogen activator (t-PA) release from endothelial cells, contributing to fibrinolytic activity.
Inflammatory Pain and Nociception
Bradykinin is one of the most intensively studied endogenous pain mediators. Intradermal injection of bradykinin produces immediate, intense pain at nanomolar concentrations, making it among the most potent naturally occurring algogenic substances. Dray & Perkins (1993) characterized the B2 receptor-mediated sensitization of sensory neurons, demonstrating that bradykinin lowers the activation threshold of TRPV1 channels and promotes release of substance P and CGRP from peripheral nerve terminals, amplifying neurogenic inflammation. The transition from B2 to B1 receptor-mediated pain signaling during chronic inflammation — driven by cytokine-induced B1 upregulation — represents a shift from acute to chronic pain states and has made B1 antagonists attractive targets for chronic pain therapy.
Diabetic Complications
The KKS plays a complex role in diabetes. Bradykinin enhances insulin-stimulated glucose uptake in skeletal muscle through B2 receptor-mediated GLUT4 translocation, and kinin receptor knockout mice show impaired glucose tolerance. Duka et al. (2001) demonstrated that B2 receptor knockout mice developed insulin resistance and glucose intolerance. However, bradykinin also mediates some complications of diabetes, particularly diabetic retinopathy, where increased vascular permeability contributes to macular edema. This dual role complicates therapeutic targeting of the KKS in diabetes.
ACE Inhibitor Pharmacology
The relationship between bradykinin and ACE inhibitor therapy is one of the most clinically significant drug-peptide interactions in medicine. ACE (kininase II) is the primary enzyme responsible for bradykinin degradation, cleaving the Phe5-Ser6 and Pro7-Phe8 bonds. ACE inhibitors (enalapril, lisinopril, ramipril, etc.) prevent this degradation, resulting in elevated tissue and plasma bradykinin levels. Israili & Hall (1992) established that bradykinin accumulation contributes both to the therapeutic benefits of ACE inhibitors (enhanced vasodilation, cardioprotection, improved endothelial function) and to their side effects. ACE inhibitor-induced cough occurs in 5-35% of patients (higher incidence in women and East Asian populations) and is mediated by bradykinin activation of pulmonary C-fiber sensory nerves through B2 receptors and subsequent tachykinin (substance P) release. ACE inhibitor-induced angioedema, affecting approximately 0.1-0.7% of patients, represents excessive bradykinin-mediated vascular permeability, predominantly in the head and neck region.
Safety Profile
Bradykinin is not administered therapeutically due to its extremely short half-life, potent vasoactive effects, and capacity to cause pain, bronchoconstriction, and edema. In research settings, intradermal bradykinin injection produces immediate local pain, erythema (flare), and wheal formation — the classic Lewis triple response — which resolves within 30-60 minutes. Intravenous bradykinin infusion at low doses (100-400 ng/kg/min) causes dose-dependent hypotension, flushing, tachycardia, and headache. At higher doses, severe hypotension and reflex tachycardia can occur. Bradykinin's effects are self-limiting due to rapid enzymatic degradation. The clinical safety concern with bradykinin relates primarily to pathological overproduction or impaired degradation: HAE attacks, ACE inhibitor-induced angioedema, and the "bradykinin storm" hypothesis in severe COVID-19 all represent states of excessive bradykinin activity with potentially life-threatening consequences.
Clinical Research Protocols
- Research infusion protocols: IV bradykinin 100-400 ng/kg/min has been used to assess endothelial function and vascular reactivity in clinical research.
- Intradermal injection: 10-100 nmol bradykinin intradermal injection used in pain research and assessment of flare responses.
- B2 antagonist (Icatibant/Firazyr): 30 mg SC, single injection for acute HAE attacks. May repeat at 6-hour intervals (maximum 3 injections/24 hours). Self-administration approved.
- Kallikrein inhibitors: Lanadelumab (Takhzyro) 300 mg SC every 2 weeks for HAE prophylaxis. Berotralstat (Orladeyo) 150 mg oral daily.
- C1-INH replacement: Plasma-derived C1-INH (Cinryze, Berinert, Haegarda) for acute treatment and prophylaxis of HAE.
- Key clinical trials: FAST-1/FAST-2/FAST-3 (icatibant in HAE), HELP (lanadelumab in HAE), APeX-2 (berotralstat in HAE).
Subpopulation Research
- ACE inhibitor users: Approximately 5-35% develop cough (higher in women, East Asians due to genetic variation in bradykinin metabolism and B2 receptor expression). Angioedema risk is 0.1-0.7% overall but 3-4x higher in Black patients.
- HAE patients (C1-INH deficiency): Estimated prevalence 1:50,000. Bradykinin levels increase 5-10 fold during attacks. Laryngeal edema is the most dangerous manifestation, causing asphyxiation in untreated patients (historical mortality ~30% without treatment).
- COVID-19 patients: The "bradykinin storm" hypothesis proposes that SARS-CoV-2-induced ACE2 downregulation and enhanced des-Arg9-bradykinin/B1 signaling contributes to pulmonary edema and ARDS in severe COVID-19 (PMID: 32633718).
- Diabetic patients: B2 receptor activation improves insulin-mediated glucose uptake. B2 receptor gene polymorphisms are associated with diabetes susceptibility and ACE inhibitor response in diabetic nephropathy.
- Sepsis patients: Severe bradykinin-mediated vasodilation contributes to septic shock pathophysiology. Kinin generation is activated through contact pathway activation by bacterial components.
- Chronic pain populations: B1 receptor upregulation in chronic inflammatory conditions (osteoarthritis, neuropathic pain) creates sustained pain signaling. B1 antagonists are in preclinical/early clinical development for chronic pain.
Pharmacokinetic Profile
Bradykinin — Pharmacokinetic Curve
Not administered therapeutically (research: IV infusion, intradermal)Ongoing & Future Research
- B1 receptor antagonists for chronic pain: Selective B1 antagonists are being developed for chronic inflammatory and neuropathic pain, based on the rationale that B1 upregulation during chronic inflammation provides a disease-specific target without affecting constitutive B2-mediated physiological functions.
- Oral kallikrein inhibitors: Next-generation oral plasma kallikrein inhibitors for HAE prophylaxis — berotralstat (approved), sebetralstat (on-demand oral), and others in development aim to replace injectable therapies.
- FXII-targeted therapies: Monoclonal antibodies against activated factor XII (garadacimab) target the upstream trigger of contact pathway-mediated bradykinin generation for HAE prophylaxis.
- Bradykinin in cancer: Emerging research on the role of kinin receptors in tumor microenvironment, angiogenesis, and metastasis. B1 receptor expression is upregulated in several tumor types and may promote tumor invasion.
- Neuroinflammation: B1 receptor upregulation in the CNS during neuroinflammatory conditions (multiple sclerosis, Alzheimer's disease) suggests a role in blood-brain barrier disruption and neurodegeneration.
- Gene therapy for HAE: Long-term strategies using AAV-mediated C1-INH expression or CRISPR-based correction of SERPING1 mutations are in early development.
- Biomarker applications: Plasma kallikrein activity and cleaved HMWK as biomarkers for contact pathway activation in sepsis, trauma, and inflammatory conditions.
Quick Start
- Route
- Not administered therapeutically (research: IV infusion, intradermal)
Molecular Structure
- Formula
- C50H73N15O11
- Weight
- 1060.21 Da
- CAS
- 58-82-2
- PubChem CID
- 439201
- Exact Mass
- 1059.5614 Da
- LogP
- -4.8
- TPSA
- 419 Ų
- H-Bond Donors
- 12
- H-Bond Acceptors
- 14
- Rotatable Bonds
- 27
- Complexity
- 2080
Identifiers (SMILES, InChI)
InChI=1S/C50H73N15O11/c51-32(16-7-21-56-49(52)53)45(72)65-25-11-20-39(65)47(74)64-24-9-18-37(64)43(70)58-28-40(67)59-34(26-30-12-3-1-4-13-30)41(68)62-36(29-66)46(73)63-23-10-19-38(63)44(71)61-35(27-31-14-5-2-6-15-31)42(69)60-33(48(75)76)17-8-22-57-50(54)55/h1-6,12-15,32-39,66H,7-11,16-29,51H2,(H,58,70)(H,59,67)(H,60,69)(H,61,71)(H,62,68)(H,75,76)(H4,52,53,56)(H4,54,55,57)/t32-,33-,34-,35-,36-,37-,38-,39-/m0/s1
QXZGBUJJYSLZLT-FDISYFBBSA-NResearch Indications
Cardiovascular
Bradykinin is a potent vasodilator released by the kallikrein-kinin system. ACE inhibitors partly exert their antihypertensive effect by preventing bradykinin degradation, enhancing its vasodilatory action.
Bradykinin mediates ischemic preconditioning in the heart. Activation of B2 receptors triggers protective signaling cascades that reduce infarct size in experimental models.
Inflammation
Excess bradykinin is the primary mediator of hereditary angioedema attacks. Icatibant (B2 receptor antagonist) and ecallantide (kallikrein inhibitor) are FDA-approved treatments targeting this pathway.
Bradykinin is one of the most potent endogenous pain-producing substances. It sensitizes nociceptors and promotes inflammation via prostaglandin release, making its pathway a therapeutic target for analgesics.
Bradykinin accumulation is responsible for the dry cough (5-20% of patients) and rare angioedema associated with ACE inhibitor therapy, due to reduced enzymatic degradation.
Research Protocols
intravenous Injection
Intravenous bradykinin infusion at low doses (100-400 ng/kg/min) causes dose-dependent hypotension, flushing, tachycardia, and headache.
| Goal | Dose | Frequency | Duration |
|---|---|---|---|
| Acute HAE attacks | 30 mg | Per protocol | — |
| Inhibitors | 300 mg | Per protocol | 2 weeks |
oral
Berotralstat (Orladeyo) 150 mg oral daily. - Oral kallikrein inhibitors: Next-generation oral plasma kallikrein inhibitors for HAE prophylaxis — berotralstat (approved), sebetralstat (on-demand oral), and others in development aim to replace injectable therapies.
| Goal | Dose | Frequency | Duration |
|---|---|---|---|
| Kallikrein inhibitors | 300 mg, 150 mg | Daily | 2 weeks(Route: Oral) |
intradermal Injection
Intradermal injection of bradykinin produces immediate, intense pain at nanomolar concentrations, making it among the most potent naturally occurring algogenic substances. In research settings, intradermal bradykinin injection produces immediate local pain, erythema (flare), and wheal formation — th
| Goal | Dose | Frequency | Duration |
|---|---|---|---|
| B2 antagonist (Icatibant/Firazyr) | 30 mg | Per protocol | — |
subcutaneous Injection
Hereditary Angioedema (HAE) Hereditary angioedema is a rare autosomal dominant disorder characterized by recurrent episodes of severe subcutaneous and submucosal edema. Med.] demonstrated that icatibant 30 mg subcutaneous injection provided significantly f
| Goal | Dose | Frequency | Duration |
|---|---|---|---|
| HAE attacks | 30 mg | Per protocol | — |
| Inhibitors | 300 mg | Per protocol | 2 weeks |
Interactions
Peptide Interactions
Bradykinin stimulates release of substance P from sensory nerve terminals, creating a positive feedback loop in neurogenic inflammation. Dual blockade of kinin and neurokinin pathways may provide superior anti-inflammatory effects.
Bradykinin activates sensory C-fibers which release CGRP as part of the neurogenic inflammatory response. CGRP provides potent vasodilation while bradykinin increases vascular permeability. Both are key mediators of inflammatory pain and edema. Bradykinin sensitizes CGRP-containing nociceptors, enhancing peptide release. This synergy is clinically relevant in migraine pathophysiology where both kinin and CGRP pathways are activated (Geppetti et al., 2012).
Bradykinin and neurotensin both contribute to pain modulation but through independent mechanisms. Bradykinin activates B1/B2 kinin receptors on nociceptors, while neurotensin provides opioid-independent analgesia via NTS1/NTS2 receptors. Neurotensin does not interfere with bradykinin-mediated vascular effects. No documented adverse pharmacological interactions between these two peptide systems.
What to Expect
What to Expect
In research settings, intradermal bradykinin injection produces immediate local pain, erythema (flare), and wheal formation — the classic Lewis...
Kallikrein inhibitors: Lanadelumab (Takhzyro) 300 mg SC every 2 weeks for HAE prophylaxis.
May repeat at 6-hour intervals (maximum 3 injections/24 hours).
Continued use as directed
Quality Indicators
What to look for
- Naturally occurring compounds
- Multiple peer-reviewed studies available
Caution
- Short half-life may require frequent dosing
- Injection site reactions reported
- Commonly used off-label
Frequently Asked Questions
References (10)
- [2]
- [7]Marceau, F. et al Bradykinin receptors: Agonists, antagonists, expression, signaling, and adaptation to sustained stimulation Int. Immunopharmacol. (2020)
- [1]Israili, Z. H. & Hall, W. D Cough and angioneurotic edema associated with angiotensin-converting enzyme inhibitor therapy Ann. Intern. Med. (1992)
- [4]
- [5]
- [6]Garvin, M. R. et al A mechanistic model and therapeutic interventions for COVID-19 involving a RAS-mediated bradykinin storm eLife (2020)
- [8]van de Veerdonk, F. L. et al Kallikrein-kinin blockade in patients with COVID-19 to prevent acute respiratory distress syndrome eLife (2020)
- [9]Longhurst, H. et al Lanadelumab for the prevention of hereditary angioedema attacks: long-term extension study J. Allergy Clin. Immunol. Pract. (2022)
- [10]
- [3]Cicardi, M. et al Icatibant, a new bradykinin-receptor antagonist, in hereditary angioedema N. Engl. J. Med. (2010)
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