**8. Bradykinin as common final mediator of "bradykininergic" angioedema**

#### **8.1. Formation of bradykinin**

**7. Classification of angioedema due to functionally active C1 esterase** 

the generation of bradykinin, a vasoactive peptide released from HMWK [9].

Functionally active C1‐INH deficiency can be hereditary or acquired. The hereditary form is a primary immunodeficiency [25] and is the most common genetic defect of the complement system [26]. The absence or malfunction of C1‐INH results in the presence of attacks of AE (subcutaneous or mucosal swelling) due to uncontrolled activation of the contact system, with

C1‐INH‐HAE is a genetic autosomal dominant disease characterized by a deficiency of the functionally active C1 esterase inhibitor (C1 inhibitor) protein. Initially, it was believed that it affected one individual per 10,000–150,000 people, but being a rare disease it makes an estimate of prevalence difficult to pinpoint [27]. It could affect around 2000–3000 people in the USA [28]. There is a register of patients in Spain where the minimum prevalence is 1.09 per 100,000 inhabitants [29], while another register in Denmark describes a prevalence rate of 1.41 per 100,000 inhabitants [30]. The highest published prevalence is in Norway with 1.75 per 100,000 inhabitants [31]. Delays in diagnosis (an average of 13.1 years in the Spanish study) [29] along with the possibility of misdiagnosis and lack of recognition of the disease may mean that the true prevalence may be higher than estimates suggest. To date, no studies have

Two phenotypic variants were described [32, 33]*. Type I* (HAE‐I) is the most common (85%), characterized by a quantitative decrease of C1‐INH, which results in a decrease in functional activity; *type II* (HAE‐II) (15%) is characterized by normal or elevated levels of dysfunctional C1‐INH. In both cases, the defect is transmitted as an autosomal dominant form, although with different genetic alterations. There is another estrogen‐dependent hereditary AE variant in which both levels and function of C1‐INH are normal and which has been called HAE

C1‐INH‐AAE is biochemically characterized by low C1‐INH concentrations and/or functions and no evidence of heredity. It is mainly associated with B cell lymphoproliferative disorders and occasionally with autoimmune, neoplastic, or infectious diseases [14]. Initially, it was classified into two types: type I, with most patients having an associated B cell line malignancy; type II, there were anti‐C1‐INH autoantibodies that interfered with C1‐INH functional activity [36]. C1‐INH production is normal or slightly increased. In many patients with type I, the paraproteinemia or M component actually behaves as an anti‐C1‐INH autoantibody, so some authors such as Cicardi suggest that the distinction between types I and II may be artificial [37]. Acquired C1‐INH deficiency is characterized by the activation of the classical complement pathway and accelerated catabolism of C1‐INH and the activation of the contact system [9]. This results in low C4 and C2 levels and normal C3 levels in plasma. C1q levels are frequently

**inhibitor protein (C1 inhibitor) deficiency**

164 A Comprehensive Review of Urticaria and Angioedema

shown differences in prevalence between ethnic groups.

**7.1. Hereditary angioedema**

*type III* [34, 35].

**7.2. Acquired angioedema**

BK is a linear nonapeptide (with sequence Arg1‐Pro2‐Pro3‐Gly4‐Phe5‐Ser6‐Pro7‐Phe8‐Arg9) produced endogenously in humans and other mammals as a result of the proteolytic activity of kallikrein on kininogens [38, 39].

Kallikreins belong to serine proteases and fall into two groups: tissue and plasma kallikreins. Within the tissue kallikreins, a family of 15 proteins is true kallikrein (hk1) and prostate‐ specific antigen (PSA or hk3) [39–41]. Plasma kallikrein is involved in processes that initiate coagulation especially during the activation phase due to contact with negatively charged surfaces. The plasma and tissue kallikreins release vasoactive peptides known as kinins implicated in biological processes such as the relaxation of vascular smooth muscle (hypoten‐ sion), increased vascular permeability, smooth muscle contraction of the bronchial tree, and pain [39–41]. This peptide family produces BK release due to plasma kallikrein action on the HMWK, while it also releases Lys‐bradykinin (Lys‐BK) by the action of tissue kallikrein hk1 on low‐molecular‐weight kininogen (LMWK) [39–41] (**Figure 8**).

#### **8.2. Other forms of angioedema with activation of the contact system**

HAE *type III*, described in 2000 by two independent research groups [35, 42], has been also named as hereditary angioedema with normal C1‐INH (nC1‐INH‐HAE) [7]. A subgroup of patients with nC1‐INH‐HAE (approximately 30%) has a mutation in exon 9 of *F12* gene [7] and this type of AE is known as FXII‐HAE [7]. The rest have not known mutation and are known as unknown‐HAE (U‐HAE) [7].

FXII is a protease involved in the activation of the coagulation and contact systems and these mutations found in *F12* gene in patients with FXII‐HAE have been shown to produce hyperac‐ tivability of coagulation factor FXII, with the consequent activation of the contact system [43].

#### **8.3. Inhibition of bradykinin‐metabolizing enzymes**

Once produced, kinins are rapidly metabolized by metallopeptidases: neutral endopepti‐ dase (NEP), angiotensin‐converting enzyme (ACE), dipeptidyl peptidase‐IV (DPP‐IV), ami‐ nopeptidase P (APP), carboxypeptidases (CPN, CPM), and endothelin‐converting enzyme‐1 (ECE‐1). Dendorfer et al. [44] described the metabolic pathways of BK degradation in murine models. In human plasma, BK is cleaved on the Pro7‐Phe8 and Phe8‐Arg9 bonds by the action of the two largest kininases: ACE and CPN [45]. Besides, in the 1960s it was reported that carboxypeptidase A cleaved the Pro7‐Phe8 bond [46], while carboxypeptidase B cleaved the Phe8‐Arg9 bond [46]. Generally, carboxypeptidases remove Arg9 (carboxyl terminus) from the kinin molecule. Although NEP plays an important role in the kidney and epithelium, unlike ACE it barely exerts its action in plasma. APP cleaves BK in the Arg1‐Pro2 bond [47]. NEP and ACE cleave BK at the Pro7‐Phe8 bond (releasing the dipeptide Phe8‐Arg9) [48]; NEP further cleaves the Gly4‐Phe5 bond and ACE in the Phe5‐Ser6 bond [48].

The following drug classes can cause acute AE by inhibition of the BK‐metabolizing pathway (**Figure 8**):


**Figure 8.** Formation of kinins in plasma and tissues. Each kinin is formed from kininogen by the action of a different enzyme.

#### **8.4. Bradykinin receptor ligands**

Phe8‐Arg9 bond [46]. Generally, carboxypeptidases remove Arg9 (carboxyl terminus) from the kinin molecule. Although NEP plays an important role in the kidney and epithelium, unlike ACE it barely exerts its action in plasma. APP cleaves BK in the Arg1‐Pro2 bond [47]. NEP and ACE cleave BK at the Pro7‐Phe8 bond (releasing the dipeptide Phe8‐Arg9) [48]; NEP

The following drug classes can cause acute AE by inhibition of the BK‐metabolizing pathway

• DPP‐IV (EC 3.4.14.5) inhibitors: sitagliptin, vildagliptin, saxagliptin, alogliptin, and

• NEP, also known as neprilysin (EC 3.4.24.11) inhibitor: SQ29072 [50], SCH39370 [51],

• Dual inhibitor of NEP and ACE: omapatrilat [57], fasidotril [58], sampatrilat [59], and

• Dual inhibitor of NEP and ECE‐1: SLV‐306 [61], S‐17162 [62], CGS 26303 [63, 64], CGS 26393 [65], CGS 31447 [66], WS 75624B [67], B‐90063 [68], CGS 34226 [69], and CGS 34043 [70].

**Figure 8.** Formation of kinins in plasma and tissues. Each kinin is formed from kininogen by the action of a different

• Triple inhibitor of ECE‐1, NEP, and ACE: CGS 35601 [71–73] and CGS 37808 [74].

further cleaves the Gly4‐Phe5 bond and ACE in the Phe5‐Ser6 bond [48].

• ACE (EC 3.4.15.1) inhibitors: lisinopril, captopril, enalapril, and ramipril.

candoxatrilat [52], phosphoramidon [53], BP102 [54], and ecadotril [55].

(**Figure 8**):

linagliptin.

mixanpril [60].

enzyme.

• CPN and CPM inhibitors.

• APP (EC 3.4.11.9) inhibitors: apstatin [49].

166 A Comprehensive Review of Urticaria and Angioedema

• ECE‐1 (EC 3.4.24.71) inhibitor: CGS35066 [56].

The biological effects of kinins involve the activation of specific receptors on the surface of the target cell. At least two different kinin receptors are known [75, 76]: BK receptor B1 (bradykinin receptor B1, also known as BDKRB1, B1R, BKR1, B1BKR, BKB1R, and BRADYB1) [77], which is coded in region 14q32.1‐q32.2, and BK receptor B2 (bradykinin receptor B2, also known as BDKRB2, B2R, BK2, BK‐2, BKR2, and BRB2) [78], which is coded in region 14q32.1‐q32.2.

BKR1 binds and is activated by des‐[Arg9]‐bradykinin (DBK) and des‐[Arg9]‐Lys‐bradykinin (Lys‐BK), formed by the action of carboxypeptidases on Lys‐BK and BK, respectively [79].

BKR1 is expressed in low amounts on normal physiological conditions in smooth muscle of blood vessels being regulated additively by inflammation [75, 79]. During stressful situations (trauma, tissue pressure, or inflammation with increase of IL1β or TNFα) [80, 81], the effects on BKR1 can predominate.

On the contrary, BKR2 binds selectively with BK and kallidin, mediating most of the effects of the contact system activation in the absence of inflammation.

Antagonists have been developed for both types of receptors, such as des‐[Arg9]‐bradykinin‐ Leu8 for BKR1 and HOE140 (icatibant acetate) for BKR2 [82]. Icatibant acetate has been shown to be effective for the treatment of acute AE attacks in C1‐INH‐HAE [7, 8, 83].

In summary, most of the biological effects of kinins are mediated by BKR2 and under conditions of inflammation or tissue damage there is induction of BKR1 [84] (**Figure 9**).

**Figure 9.** Bradykinin receptor ligands*.*

Both receptors belong to the superfamily of receptors that have seven transmembrane domains coupled to G proteins, differing both in primary structure, expression, and regulation of their tissue distribution [85, 86].

Two types of G protein‐coupled receptors have been found that bind to BK mediating its response in pathophysiological conditions. To summarize, there are stimulatory G proteins (Gs and Gq) and inhibitory G proteins (Gi). Gs binds to GTP and activates adenylate cyclase, increasing the amount of intracellular cAMP. Gi binds to GTP and inactivates adenylate cyclase, indirectly reducing the amount of intracellular cAMP. Gq binds to GTP and activates PLC, increasing the amount of DAG, IP, and intracellular Ca++. Transduction pathways stimu‐ lated by kinins have been extensively investigated in endothelial cells, where BKR1 interacts with Gq and Gi proteins, using the same signaling pathways as BKR2 (**Figure 10**).

BKR2 binds to G proteins and activates phospholipases A<sup>2</sup> and C. The kinin‐induced increase in phospholipase C (PLC) causes it to act on their specific substrate, phosphatidylinositol biphosphate (PIP2), hydrolyzing it generating the two metabolites: inositol triphosphate (IP<sup>3</sup> ) and diacylglycerol (DAG). IP3 binds to a specific receptor (IP3R) in the endoplasmic reticu‐ lum facilitating the release of intracellular Ca++. IP3, possibly together with its metabolite, IP4, can regulate calcium channels of the plasma membrane allowing the entry of extracellular calcium into the cell [87, 88]. The other metabolite of PIP2 hydrolysis, DAG, is responsible for the activation of protein kinase C (PKC) [89, 90]. PKC consists of one polypeptide chain with two functional domains: (a) a hydrophobic domain for binding to the cell membrane and (b) a hydrophilic domain, which possesses catalytic function. PKC at cellular rest is found in an inactive form in the cytosol, but once stimulated by DAG together with Ca++ ions it translocates to the cell membrane to exert its function of protein kinase in serine and threonine

**Figure 10.** Bradykinin receptors and G‐protein–coupled receptor‐signaling pathway.

amino acids. BK has been shown to activate a Ca++‐dependent PKC and PKC not dependent on this ion, as well as atypical isoforms [91]. The stimulation of phospholipase A<sup>2</sup> (PLA<sup>2</sup> ) releases arachidonic acid from membrane phospholipids [92], which can be metabolized in the form of powerful inflammatory mediators.

In addition, BKR2 transitorily promotes phosphorylation of tyrosine from tyrosine kinases such as MAP kinase ("mitogen‐activated protein kinase"), as well as the activation of the JAK/ STAT pathway. Activated BKR2 interacts directly with nitric oxide synthase (NOS) resulting in nitric oxide (NO) [93].
