**3. Historical review (from C1 inhibitor to bradykinin)**

factor from serum other than C′3 (the mid-piece was called C′1 and the end-piece was renamed C′2). It should be noted at this point that C′1 and C′2 do not correspond to the current fractions C1 and C2, since both constitute the full complement including C′3 and C′4. Ueno [23] established the order of performance of the components known up to that time. Pillemer [24] managed to separate the four serum fractions into different components and set the activation sequence C′1, C′4, C′2, and C′3. It was not until the early 1960s, once chromatographic methods were developed, that the various components could be purified. Nelson [25, 26] showed that in reality the third component C′3 was formed by at least six factors (C′3c, C′3b, C′3e, C′3f, C′3a, and C′3d). Having established that these were proteins not related to C′3 acting at a later stage, he called them C′5, C′6, C′7, C′8, and C′9, respectively. As of 1968, World Health Organization (WHO) annulled the

The heavy reliance of the study of the classical complement pathway using erythrocytes sensitized with antibodies for activation did not even consider the possibility of activation by other substances [3]. However, since the early twentieth century, there were data suggesting that it was possible to lyse erythrocytes with cobra venom without antibodies and with the participation of various components other than those of the classical pathway. Pillemer [27] was the father of the discovery of the alternative pathway upon describing a protein or a new component called "properdin," which when absent diminished the bactericidal potency of

Green et al. [28] suggested that the cytolysis mediated by complement involved the production of pores in the cell membrane on the grounds that large molecules (dextrans and albumin) prevented cell lysis when present in high concentration in the reaction medium; on the contrary, but small molecules did not [3] (**Figure 2**). Cell rupture was thought to be due to a colloid-osmotic swelling process that finally finished by lysing the cell. Borsos et al. [29], with the use of electron microscopy, visualized ultrastructural lesions etched into cell membranes, showing that the lesions were associated with the cytolytic complement activity. Lachman [30] showed that the five terminal components C5, C6, C7, C8, and C9 were necessary and sufficient to cause such lesions. Haxby [31] and Kinsky [32] were the first to demonstrate that the lipid bilayer was the target of the "membrane attack complex" (MAC), noting that C5-C9 directly damaged the integrity of the bilayer without any enzymatic activity. Mayer [33] formulated the "donut hypothesis" where cell damage is achieved through the formation of a structure described as a donut, forming stable transmembrane pores. Lysis would be explained by the osmotic difference between the exterior and the interior cell through the transmembrane channel. Bhadki [34] and Podack [35] observed that the MAC was due to C5-C9 multimolecular assembly. Bieseker [36] initially postulated a dimeric structure (C5-9)2, but Bhakdi [37] suggested a monomeric complex with the same structure as the complex SC5b-9 ("S" was one of the proteins that control the MAC). The C9 alone forms

symbol "′" leaving it currently C1, C2, and so on.

210 A Comprehensive Review of Urticaria and Angioedema

complexes structurally similar to the full MAC [38].

serum against certain bacteria.

**2.2. Historical development of the alternative complement pathway**

**2.3. Historical development of the final common lytic complement pathway**

Hereditary angioedema (HAE) due to C1-inhibitor (C1-INH) deficiency (C1-INH-HAE), also known as "non-allergic angioneurotic edema," "AE without urticaria," or "Osler's hereditary edema" is a potentially fatal clinical entity, which in recent years has become an example to be followed because of the great progress made from the union of researchers, physicians, and patient associations worldwide (**Figure 3**).

It was first described in 1843 by Robert Graves in his clinical lectures. In 1882, Heinrich Quincke documented some cases of acute, circumscribed edema, involving two generations of the same family and coined the term angioneurotic edema [39]. Subsequently, Sir William Osler in 1888 first described in detail an inherited form of angioedema (AE) [40], from which in 1917 the hereditary type was identified [41]. The disease was defined biochemically in 1963 by Donaldson and Evans [42], as an absence of serum inhibitor of the first component of the complement. Dating from 1972 is the first case of acquired angioedema due to C1 inhibitor deficiency (C1-INH-AAE) in lymphosarcoma [43].

The main symptom of C1-INH-HAE is the attack of AE, the laryngeal location being the most serious. Landerman [44] reviewed all the medical literature published between 1888 and 1962 and found 28 publications of more than one case of death from fatal laryngeal attacks in more than one family with C1-INH-HAE. The total number of deaths due to C1-INH-HAE was 92.

In 1960, Spaulding demonstrated the efficacy of methyl testosterone in the treatment of C1-INH-HAE in a family [45]. In 1976, a double-blind placebo-controlled trial demonstrated the efficacy of danazol for the treatment of C1-INH-HAE [46]. It was then when stanozolol, another attenuated androgen, started to be used [47].

In 1968, the first case of C1-INH-HAE successfully treated with epsilon-aminocaproic acid (EACA) was published [48], although it was not until 1972 when the efficacy of anti-fibrinolytic agents (AFs), EACA, and tranexamic acid was demonstrated in double-blind clinical trials [49, 50]. AFs are reserved for those patients who cannot tolerate attenuated androgens or present contraindications for their administration.

An article published in 1973 described for the first time the administration of concentrated C1-INH (pdC1INH), partially purified from a mixture of human plasma, in two patients [51]. Previously, replacement therapy in patients with C1-INH-HAE in the attack phase had been attempted with fresh-frozen plasma [52], which was abandoned later because of the risk of viral transmission, although it was still used in case of pdC1INH being unavailable [53].

In the USA, two double-blind placebo-controlled clinical trials had been conducted with pdC1INH, which had proven its efficacy and safety [54]; however, the Food and Drug Administration (FDA) had not yet approved its use in the 2000s. At that time, Berinert-P® (Behring, Marburg, Germany) was commercialized in Germany and a few European countries [55] and was available in Spain, where it was imported through the Foreign Medicines service [56].

**Figure 3.** Historical review of angioedema due to C1-inhibitor deficiency.

In 1986, the *C1NH* gene was identified (Gene Bank X54486; Swiss-Prot P05155), which encodes the C1INH protein, also called *SERPING1*, located on chromosome 11 subregion q11-q13.1 [57–59].

Although the possible relationship between AE and estrogens in women was described as early as 1986 [60], it was not until the first decade of the twenty-first century when several series of patients with HAE were described with normal levels of the fractions of the complement system [61, 62]. It was originally called HAE type III [62]. Finally, a mutation was found in *F12* gene in some of the families [63–65].

Initially, C2-kinin, a vasoactive peptide generated by cleavage of the C2b fragment was thought to be involved in angioedema formation in C1-INH-HAE [66].

In 1998, there was growing support for another hypothesis in the generation of AE. It argued that BK was the most important mediator in the development of AE [67] and had been proven through clinical, *in vitro* studies and experiments in an experimental model of C1INHdeficient transgenic mice [68]. In 2002, a transgenic mouse with C1 inhibitor deficiency was developed by Professor Davis [69].

In the last decade, several drugs have been approved and marketed in Europe, in the United States, and in other countries, contributing to improved management of C1-INH-HAE and patient's quality of life.

First, icatibant acetate (Firazyr®, Shire HGT, Zug, Switzerland) [70, 71], a bradykinin B2 receptor blocker, was approved by the European Medicines Agency (EMA) in 2008 for the treatment of acute AE attacks in adult patients with C1-INH-HAE [72] and was marketed in Spain in March 2009.

In 2008, a new C1-esterase inhibitor formulation, Cinryze®, was approved by FDA for the long-term prophylaxis of C1-INH-HAE [73]. This drug incorporated a nanofiltration step as an extra safety procedure to reduce the transmission of enveloped and nonenveloped viruses and possible prions [74, 75] and had been shown to be effective in reducing the number of AE attacks per month [76, 77]. In 2011, the European Medicines Agency (EMA) approved the marketing of Cinryze® for long-term prophylaxis, but also for short-term prophylaxis and treatment of acute AE attacks in adults and adolescents with C1-INH-HAE [78].

Berinert®, which had been marketed in Germany in 1985, was approved in 2008–2009 in different European countries through a mutual recognition agreement for the treatment of acute AE attacks in children and adults with C1-INH-HAE. Later, it also incorporated the nanofiltration step and it was approved by the EMA for short-term prophylaxis in children and adults in 2013 [79]. In 2009, FDA approved Berinert® for the treatment of acute abdominal and facial AE attacks in adolescents and adults with C1-INH-HAE [80].

In December 2009, Ecallantide (DX-88, Kalbitor®, Dyax Corp, currently part of Shire HGT), a kallikrein inhibitor, was approved by the FDA for the treatment of acute AE attacks in patients >16 years with C1-INH-HAE [81]. It was later approved for adolescents (2014).

A recombinant C1 inhibitor (rhC1INH) (Ruconest®, Pharming Technologies BV®, Leiden, The Netherlands) produced in transgenic rabbits [82] was approved by EMA in 2010 for the treatment of acute AE attacks in adult patients with C1-INH-HAE [83]. It was in 2014 when the FDA approved it for the same indication by FDA [84].

Some European centers have developed training programs for self-administration of intravenous and subcutaneous specific drugs for the treatment of C1-INH-HAE [85–90].

The development of new drugs or new uses for old drugs changed the therapeutic approach in C1-INH-HAE in the last decade. However, the development of new drugs will even alter more therapeutic landscape for C1-INH-HAE in the next years.
