Chapter 32: Histamine, Bradykinin, and Their Antagonists

History. The history of histamine (β-aminoethylimidazole) parallels that of acetylcholine (ACh). Both were chemically synthesized before their biological significance was recognized; they were first detected as uterine stimulants in, and isolated from, extracts of ergot, where they proved to be contaminants derived from bacterial action (Dale, 1953).

Dale and Laidlaw subjected histamine to intensive pharmacological study (Dale, 1953), discovering that it stimulated a host of smooth muscles and had an intense vasodepressor action. Importantly, they observed that when a sensitized animal was injected with a normally inert protein, the immediate responses closely resembled those of poisoning by histamine. These observations anticipated by many years the finding that endogenous histamine contributes to immediate hypersensitivity reactions and to responses to cellular injury. Best and colleagues (1927) isolated histamine from fresh samples of liver and lung, thereby establishing it as a natural constituent of mammalian tissues, hence the name histamine after the Greek word for tissue, histos. The presence of histamine in tissue extracts delayed the acceptance of the discovery of some peptide and protein hormones (e.g., gastrin) until the technology for separating the naturally occurring substances was sufficiently advanced (Grossman, 1966).

Lewis and colleagues (Lewis, 1927) proposed that a substance with the properties of histamine ("H substance") was liberated from the cells of the skin by injurious stimuli, including the reaction of antigen with antibody. We now know that endogenous histamine plays a role in the immediate allergic response and is an important regulator of gastric acid secretion. More recently, a role for histamine as a modulator of neurotransmitter release in the central and peripheral nervous systems has emerged.

Early suspicions that histamine acts through more than one receptor have been borne out by the elucidation of four classes of receptors, designated H₁ (Ash and Schild, 1966), H₂ (Black et al., 1972), H₃ (Arrang et al., 1987), and H₄ (Leurs et al., 2009). H₁ receptors are blocked selectively by the classical "antihistamines." Second-generation H₁ antagonists are collectively referred to as nonsedating antihistamines. The term third generation has been applied to some recently developed antihistamines, such as active metabolites of first- or second-generation antihistamines that are not further metabolized (e.g., cetirizine derived from hydroxyzine or fexofenadine from terfenadine) or to antihistamines that have additional therapeutic effects. However, the Consensus Group on New-Generation Antihistamines concluded that none of the currently available antihistamines can be classified as true third-generation drugs, defined as lacking in cardiotoxicity, drug-drug interactions, and CNS effects or with possible additional beneficial effects (e.g., anti-inflammatory) (Holgate et al., 2003). The discovery of H₂ antagonists and their ability to inhibit gastric secretion has contributed greatly to the resurgence of interest in histamine in biology and clinical medicine (Chapter 45). H₃ receptors were discovered as presynaptic autoreceptors on histamine-containing neurons that mediate feedback inhibition of the release and synthesis of histamine. The development of selective H₃ receptor agonists and antagonists has led to an increased understanding of the importance of H₃ receptors in histaminergic neurons in vivo. None of these H₃ agonists or antagonists has yet emerged as a therapeutic agent (Sander et al., 2008). The H₄ receptor is most similar to the H₃ receptor but is expressed in cells of hematopoietic lineage; the availability of H₄-specific antagonists with anti-inflammatory properties should help to define the biological roles of the H₄ receptor (Thurmond et al., 2008).

Chemistry. Histamine is a hydrophilic molecule consisting of an imidazole ring and an amino group connected by two methylene groups. The pharmacologically active form is the monocationic Nγ–H tautomer, which is the charged form of the species depicted in Figure 32–1 (Ganellin and Parsons, 1982). H₃ and H₄ receptors have a much higher affinity for histamine than H₁ and H₂ receptors, and the four histamine receptors can be activated differently by analogs of histamine (Venable and Thurmond, 2006) (Figure 32–1 and Table 32–1). The specificities of histamine analogs were re-evaluated after the discovery of the H₄ receptor. Thus, 4-methylhistamine and dimaprit, originally classified as H₂ receptors (Black et al., 1972), are full H₄ agonists with a ∼100-fold higher affinity for the H₄ receptor. A number of H₃ agonists also are weaker agonists of the H₄ receptor (Lim et al., 2005; Venable and Thurmond, 2006).

At the secretory granule pH of ∼5.5, histamine is positively charged and ionically complexed with negatively charged acidic groups on other granule constituents, primarily proteases and heparin or chondroitin sulfate proteoglycans. The turnover rate of histamine in secretory granules is slow, and when tissues rich in mast cells are depleted of their histamine stores, it may take weeks before concentrations return to normal levels. Non–mast cell sites of histamine formation include the epidermis, the gastric mucosa, neurons within the CNS, and cells in regenerating or rapidly growing tissues. Turnover is rapid at these non–mast cell sites because the histamine is released continuously rather than stored. Non–mast cell sites of histamine production contribute significantly to the daily excretion of histamine metabolites in the urine. Because L-histidine decarboxylase is an inducible enzyme, the histamine-forming capacity at such sites is subject to regulation. Histamine that is ingested or formed by bacteria in the gastrointestinal (GI) tract does not contribute to the body's stores; rather, it is rapidly metabolized, and the metabolites are eliminated in the urine.

There are two major paths of histamine metabolism in humans (Figure 32–2). The more important is ring methylation to form N-methylhistamine, catalyzed by histamine-N-methyltransferase, which is distributed widely. Most of the N-methylhistamine formed is then converted to N-methylimidazole acetic acid by monoamine oxidase (MAO), and this reaction can be blocked by MAO inhibitors (Chapters 8, 15, and 22). Alternatively, histamine may undergo oxidative deamination catalyzed mainly by the nonspecific enzyme diamine oxidase, yielding imidazole acetic acid, which is then converted to imidazole acetic acid riboside. These metabolites have little or no activity and are excreted in the urine. Measurement of N-methylhistamine in urine affords a more reliable index of histamine production than assessment of histamine itself. Artifactually elevated levels of histamine in urine arise from genitourinary tract bacteria that can decarboxylate histidine. In addition, the metabolism of histamine appears to be altered in patients with mastocytosis such that determination of histamine metabolites is a more sensitive diagnostic indicator of the disease than histamine.

Antigen bridges the IgE molecules and via Fc∊RI activates signaling pathways in mast cells or basophils involving tyrosine kinases and subsequent phosphorylation of multiple protein substrates within 5-15 seconds of contact with antigen. Protein kinases implicated include the Src-related kinases Lyn and Syk. Prominent among the phosphorylated proteins are the β and γ subunits of Fc∊RI, itself, and phospholipase C (PLC)γ₁ and PLCγ₂, with consequent production of inositol trisphosphate (IP₃) and mobilization of intracellular Ca²⁺ (Chapter 3). These events trigger the exocytosis of the contents of secretory granules.

In addition to therapeutic agents, certain experimental compounds stimulate the release of histamine as their dominant pharmacological characteristic. The archetype is the polybasic substance known as compound 48/80. This is a mixture of low-molecular-weight polymers of p-methoxy-N-methylphenethylamine, of which the hexamer is most active.

Basic polypeptides often are effective histamine releasers, and over a limited range, their potency generally increases with the number of basic groups. For example, bradykinin is a poor histamine releaser, whereas kallidin (Lys-bradykinin) and substance P, with more positively charged amino acids, are more active. Some venoms, such as that of the wasp, contain potent histamine-releasing peptides (Johnson and Erdös, 1973). Polymyxin B also is very active. Basic polypeptides released upon tissue injury constitute pathophysiological stimuli to secretion for mast cells and basophils.

Within seconds of the intravenous injection of a histamine liberator, human subjects experience a burning, itching sensation. This effect, most marked in the palms of the hand and in the face, scalp, and ears, is soon followed by a feeling of intense warmth. The skin reddens, and the color rapidly spreads over the trunk. Blood pressure falls, the heart rate accelerates, and the subject usually complains of headache. After a few minutes, blood pressure recovers, and crops of hives usually appear on the skin. Colic, nausea, hypersecretion of acid, and moderate bronchospasm also frequently occur. The effect becomes less intense with successive injections as the mast cell stores of histamine are depleted. Histamine liberators do not deplete tissues of non–mast cell histamine.

Histamine Release by Other Means. Clinical conditions related to histamine release include cold, cholinergic, and solar urticaria. Some of these involve specific secretory responses of the mast cells and cell-fixed IgE. However, nonspecific cellular damage from any cause can release histamine. The redness and urticaria that follow scratching of the skin is a familiar example.

Increased Proliferation of Mast Cells and Basophils and Gastric Carcinoid Tumors. In urticaria pigmentosa (cutaneous mastocytosis), mast cells aggregate in the upper corium and give rise to pigmented cutaneous lesions that sting when stroked. In systemic mastocytosis, overproliferation of mast cells also is found in other organs. Patients with these syndromes suffer a constellation of signs and symptoms attributable to excessive histamine release, including urticaria, dermographism, pruritus, headache, weakness, hypotension, flushing of the face, and a variety of GI effects, such as diarrhea or peptic ulceration. Episodes of mast cell activation with attendant systemic histamine release are precipitated by a variety of stimuli, including exertion, insect stings, exposure to heat, and exposure to drugs that release histamine directly or to which patients are allergic. In myelogenous leukemia, excessive numbers of basophils are present in the blood, raising its histamine content to high levels that may contribute to chronic pruritus. Gastric carcinoid tumors secrete histamine, which is responsible for episodes of vasodilation as part of the patchy "geographical" flush.

Gastric Acid Secretion. Histamine acting at H₂ receptors is a powerful gastric secretagogue, evoking a copious secretion of acid from parietal cells (see Figure 45–1); it also increases the output of pepsin and intrinsic factor. The secretion of gastric acid from parietal cells also is caused by stimulation of the vagus nerve and by the enteric hormone gastrin. However, histamine undoubtedly is the dominant physiological mediator of acid secretion; blockade of H₂ receptors not only antagonizes acid secretion in response to histamine but also inhibits responses to gastrin and vagal stimulation. (For regulation of gastric acid secretion and the clinical utility of H₂ antagonists, see Chapter 45.)

Central Nervous System. There is substantial evidence that histamine functions as a neurotransmitter in the CNS. Histamine-containing neurons control both homeostatic and higher brain functions, including regulation of the sleep-wake cycle, circadian and feeding rhythms, immunity, learning, memory, drinking, and body temperature (see Haas et al., 2008). However, knockout animals lacking histamine or its receptors exhibit only subtle defects unless challenged, and no human disease has yet been directly linked to dysfunction of the brain histamine system. Histamine, histidine decarboxylase, enzymes that metabolize histamine, and H₁, H₂, and H₃ receptors are distributed widely but non-uniformly in the CNS (see Haas et al., 2008). H₁ receptors are associated with both neuronal and non-neuronal elements (e.g., glia, blood cells, vessels) and are concentrated in regions that control neuroendocrine function, behavior, and nutritional state. Distribution of H₂ receptors is more consistent with histaminergic projections than H₁ receptors, suggesting that they mediate many of the postsynaptic actions of histamine. H₃ receptors also are heterogeneously concentrated in areas known to receive histaminergic projections, consistent with their function as presynaptic autoreceptors. Histamine inhibits appetite and increases wakefulness via H₁ receptors, explaining sedation by classical antihistamines (Haas et al., 2008).

The existence of multiple histamine receptors was predicted by the studies of Ash and Schild (1966) and Black and colleagues (1972) a generation before the cloning of histamine receptors. Similarly, heterogeneity of H₃ receptors, predicted by kinetic and radioligand-binding studies, has been confirmed by cloning, which revealed H₃ isoforms differing in the third intracellular loop, transmembrane helices 6 and 7, and C-terminal tail, and in their capacity to couple G_i, inhibit adenylyl cyclase, and activate MAP kinase. Molecular cloning studies also identified the H₄ receptor.

As Figure 32–1 and Table 32–1 indicate, the pharmacological definition of H₁, H₂, and H₃ receptors is clear because relatively specific agonists and antagonists are available. However, the H₄ receptor exhibits 35-40% homology to isoforms of the H₃ receptor, and the two were harder to distinguish pharmacologically because many high-affinity H₃ ligands also interact with H₄ receptors. Several non-imidazole compounds that are more selective H₃ antagonists have been developed (Sander et al., 2008), and there are now several selective H₄ antagonists (Leurs et al., 2009; Venable and Thurmond, 2006). 4-Methylhistamine and dimaprit, previously identified as specific H₂ agonists (Black et al., 1972), are actually more potent H₄ agonists (Venable and Thurmond, 2006).

Histamine Toxicity from Ingestion. Histamine is the toxin in food poisoning from spoiled scombroid fish such as tuna (Morrow et al., 1991). The high histidine content combines with a large bacterial capacity to decarboxylate histidine, generating a lot of histamine. Ingestion of the fish causes severe nausea, vomiting, headache, flushing, and sweating. Histamine toxicity, manifested by headache and other symptoms, also can follow red wine consumption in persons with a diminished ability to degrade histamine. The symptoms of histamine poisoning can be suppressed by H₁ antagonists.

The uterus of some species is contracted by histamine; in the human uterus, gravid or not, the response is negligible. Responses of intestinal muscle also vary with species and region, but the classical effect is contraction. Bladder, ureter, gallbladder, iris, and many other smooth muscle preparations are affected little or inconsistently by histamine.

Peripheral Nerve Endings: Pain, Itch, and Indirect Effects. Histamine stimulates various nerve endings and sensory effects. In the epidermis, it causes itch; in the dermis, it evokes pain, sometimes accompanied by itching. Stimulant actions on nerve endings, including autonomic afferents and efferents, contribute to the "flare" component of the triple response and to indirect effects of histamine on the bronchi and other organs. In the periphery, neuronal receptors for histamine are generally of the H₁ type (see Rocha e Silva, 1978; Ganellin and Parsons, 1982).

History. Antihistamine activity was first demonstrated by Bovet and Staub in 1937 with one of a series of amines with a phenolic ether moiety. The substance 2-isopropyl-5-methylphenoxy-ethyldiethyl-amine protected guinea pigs against several lethal doses of histamine but was too toxic for clinical use. By 1944, Bovet and his colleagues had described pyrilamine maleate, an effective histamine antagonist of this category. The discovery of the highly effective diphenhydramine and tripelennamine soon followed (Ganellin and Parsons, 1982). In the 1980s, nonsedating H₁ histamine receptor antagonists were developed for treatment of allergic diseases. Despite success in blocking allergic responses to histamine, the H₁ antihistamines failed to inhibit a number of other responses, notably gastric acid secretion. The discovery of H₂ receptors and H₂ antagonists by Black and colleagues provided a new class of agents that antagonized histamine-induced acid secretion (Black et al., 1972). The pharmacology of these drugs (e.g., cimetidine, famotidine) is described in Chapter 45.

Flare and Itch. H₁ antagonists suppress the action of histamine on nerve endings, including the flare component of the triple response and the itching caused by intradermal injection.

Exocrine Glands. H₁ antagonists do not suppress gastric secretion; they do inhibit histamine-evoked salivary, lacrimal, and other exocrine secretions, but with variable success. However, the antimuscarinic properties of many H₁ antagonists may contribute to lessened secretion in cholinergically innervated glands and reduce ongoing secretion in, e.g., the respiratory tree. Nasal sprays of some H₁ antagonists can be used to treat allergic rhinitis.

Anticholinergic Effects. Many of the first-generation H₁ antagonists tend to inhibit responses to ACh that are mediated by muscarinic receptors and may be manifest during clinical use (see below). Some H₁ antagonists also can be used to treat motion sickness (Chapters 9 and 46). Because scopolamine potently prevents motion sickness, the anticholinergic properties of H₁ antagonists may be largely responsible for this effect. Indeed, promethazine has perhaps the strongest muscarinic-blocking activity among these agents and is the most effective H₁ antagonist in combating motion sickness. The second-generation H₁ antagonists have no effect on muscarinic receptors.

Local Anesthetic Effect. Some H₁ antagonists have local anesthetic activity, and a few are more potent than procaine. Promethazine (phenergan) is especially active. However, the concentrations required for this effect are much higher than those that antagonize histamine's interactions with its receptors.

Studies of the metabolic fate of the older H₁ antagonists are limited. Diphenhydramine, given orally, reaches a maximal concentration in the blood in ∼2 hours, remains there for another 2 hours, then falls exponentially with a plasma elimination t_1/2 of ∼4-8 hours. The drug is distributed widely throughout the body, including the CNS. Little, if any, is excreted unchanged in the urine; most appears there as metabolites. Other first-generation H₁ antagonists can be eliminated in much the same way (see Paton and Webster, 1985). Peak concentrations of these drugs in the skin may persist after plasma levels have declined. Thus, inhibition of "wheal and flare" responses to the intradermal injection of histamine or allergen can persist for ≥36 hours after treatment, even when the concentration in plasma is low.

Like other extensively metabolized drugs, H₁ antagonists are eliminated more rapidly by children than by adults and more slowly in those with severe liver disease. H₁-receptor antagonists also induce hepatic CYPs and thus may facilitate their own metabolism (Paton and Webster, 1985).

The second-generation H₁ antagonist loratadine is absorbed rapidly from the GI tract and metabolized in the liver to an active metabolite by CYPs (Chapter 6). Consequently, metabolism of loratadine can be affected by other drugs that compete for the P450 enzymes. Two other second-generation H₁ antagonists that were marketed previously, terfenadine and astemizole, also were converted by CYPs to active metabolites. Both of these drugs were found in rare cases to induce a potentially fatal arrhythmia, torsades de pointes, when their metabolism was impaired, such as by liver disease or drugs that inhibit the CYP3A family (Chapter 29). This led to the withdrawal of terfenadine and astemizole from the market in 1998 and 1999, respectively. The active metabolite of terfenadine, fexofenadine, is its replacement. Fexofenadine lacks the toxic side effects of terfenadine, is not sedating, and retains the anti-allergic properties of the parent compound (Meeves and Appajosyula, 2003). Another antihistamine developed using this strategy is desloratadine, an active metabolite of loratadine. Cetirizine, loratadine, and fexofenadine are all well absorbed and are excreted mainly in the unmetabolized form. Cetirizine and loratadine are excreted primarily into the urine, whereas fexofenadine is excreted primarily in the feces. Levocetirizine represents the active enantiomer of cetirizine.

Other allergies of the respiratory tract are more amenable to therapy with H₁ antagonists. The best results are obtained in seasonal rhinitis and conjunctivitis (hay fever, pollinosis), in which these drugs relieve the sneezing, rhinorrhea, and itching of eyes, nose, and throat. A gratifying response is obtained in most patients, especially at the beginning of the season when pollen counts are low; however, the drugs are less effective when the allergens are most abundant, when exposure to them is prolonged, and when nasal congestion is prominent. Nasal sprays or topical ophthalmic preparations of antihistamines such as levocabastine (discontinued in the U.S.), azelastine (astelin, astepro, nasal spray; optivar, ophthalmic drops), emedastine (emadine, drops), epinastine (elestat, drops), ketotifen (zaditor, drops), and olopatadine (patanol, drops; patanase, spray) are effective in allergic conjunctivitis and rhinitis. H₁ antihistamines have been investigated for potential anti-inflammatory properties because histamine releases inflammatory cytokines and eicosanoids, increases expression of endothelial adhesion molecules, and activates the pro-inflammatory transcription factor NF-κB (Holgate et al., 2003; Thurmond et al., 2008). Although H₁ antihistamines do exhibit a variety of anti-inflammatory effects in vitro and in animal models, in many cases the doses required are higher than those normally achieved therapeutically, and clinical effectiveness has not been proven (Holgate et al., 2003; Thurmond et al., 2008). Rupatadine is a second-generation H₁ antagonist (available in many countries outside the U.S.) that also blocks receptors for the inflammatory phospholipid, PAF (Mullol et al., 2008). Although rupatadine appears to have broader anti-inflammatory effects than other antihistamines, the clinical relevance of these properties is unclear.

Certain allergic dermatoses respond favorably to H₁ antagonists. The benefit is most striking in acute urticaria. Chronic urticaria can be less responsive but also may require higher doses than has been approved for rhinitis (Siebenhaar et al., 2009; Kaplan, 2002). Furthermore, the combined use of H₁ and H₂ antagonists sometimes is effective when therapy with an H₁ antagonist alone has failed. Angioedema also responds to treatment with H₁ antagonists, but the paramount importance of epinephrine in the severe attack must be re-emphasized, especially in life-threatening laryngeal edema (Chapter 12). In this setting, it may be appropriate to also administer an H₁ antagonist intravenously.

H₁ antagonists have a place in the treatment of pruritus. Some relief may be obtained in many patients with atopic and contact dermatitis (although topical corticosteroids are more effective) and in such diverse conditions as insect bites and poison ivy. Pruritus without an allergic basis sometimes responds to antihistamine therapy. However, the possibility of producing allergic dermatitis with local application of H₁ antagonists must be recognized. The urticarial and edematous lesions of serum sickness respond to H₁ antagonists, but fever and arthralgia often do not.

Many drug reactions attributable to allergic phenomena respond to therapy with H₁ antagonists, particularly those characterized by itch, urticaria, and angioedema. However, explosive release of histamine generally calls for treatment with epinephrine, with H₁ antagonists being accorded a subsidiary role. Nevertheless, prophylactic treatment with an H₁ antagonist may reduce symptoms to a tolerable level when a drug known to be a histamine liberator is to be given.

Common Cold. H₁ antagonists are without value in combating the common cold. The weak anticholinergic effects of the older agents may tend to lessen rhinorrhea, but this drying effect may do more harm than good, as may their tendency to induce somnolence.

Motion Sickness, Vertigo, and Sedation. Scopolamine, the muscarinic antagonist, given orally, parenterally, or transdermally, is the most effective drug for the prophylaxis and treatment of motion sickness. Some H₁ antagonists are useful for milder cases and have fewer adverse effects. These drugs include dimenhydrinate and the piperazines (e.g., cyclizine, meclizine). Promethazine, a phenothiazine, is more potent and more effective; its additional antiemetic properties may be of value in reducing vomiting, but its pronounced sedative action usually is disadvantageous. Whenever possible, the various drugs should be administered ∼1 hour before the anticipated motion. Treatment after the onset of nausea and vomiting rarely is beneficial.

Some H₁ antagonists, notably dimenhydrinate and meclizine, often are of benefit in vestibular disturbances such as Meniere's disease and in other types of true vertigo. Only promethazine is useful in treating the nausea and vomiting subsequent to chemotherapy or radiation therapy for malignancies; however, other, more effective anti-emetic drugs (e.g., 5-HT₃ antagonists) are available (Chapter 46). Diphenhydramine can reverse the extrapyramidal side effects caused by phenothiazines (Chapter 16).

The tendency of some H₁ receptor antagonists to produce somnolence has led to their use as hypnotics. H₁ antagonists, principally diphenhydramine, often are present in various proprietary over-the-counter remedies for insomnia. Although these drugs generally are ineffective in the recommended doses, some sensitive individuals may derive benefit. The sedative and mild anti-anxiety activities of hydroxyzine contribute to its use as a weak anxiolytic.

The next most frequent side effects involve the digestive tract and include loss of appetite, nausea, vomiting, epigastric distress, and constipation or diarrhea. Taking the drug with meals may reduce their incidence. H₁ antagonists such as cyproheptadine may increase appetite and cause weight gain. Other side effects, owing to the antimuscarinic actions of some first-generation H₁ antagonists, include dryness of the mouth and respiratory passages (sometimes inducing cough), urinary retention or frequency, and dysuria. These effects are not observed with second-generation H₁ antagonists.

Drug allergy may develop when H₁ antagonists are given orally but occurs more commonly after topical application. Allergic dermatitis is not uncommon; other hypersensitivity reactions include drug fever and photosensitization. Hematological complications, such as leukopenia, agranulocytosis, and hemolytic anemia, are very rare. Because H₁ antihistamines cross the placenta, caution is advised for women who are or may become pregnant. Several antihistamines (e.g., azelastine, hydroxyzine, fexofenadine) had teratogenic effects in animal studies, whereas others (e.g., chlorpheniramine, diphenhydramine, cetirizine, loratadine) did not (see Simons, 2003). Antihistamines can be excreted in small amounts in breast milk, and first-generation antihistamines taken by lactating mothers may cause symptoms such as irritability, drowsiness, or respiratory depression in the nursing infant (Simons, 2003). Because H₁ antagonists interfere with skin tests for allergy, they must be withdrawn well before such tests are performed.

In acute poisoning with H₁ antagonists, their central excitatory effects constitute the greatest danger. The syndrome includes hallucinations, excitement, ataxia, incoordination, athetosis, and convulsions. Fixed, dilated pupils with a flushed face, together with sinus tachycardia, urinary retention, dry mouth, and fever, lend the syndrome a remarkable similarity to that of atropine poisoning. Terminally, there is deepening coma with cardiorespiratory collapse and death usually within 2-18 hours. Treatment is along general symptomatic and supportive lines.

Pediatric and Geriatric Indications and Problems. Although little clinical testing has been done, second-generation antihistamines are recommended for elderly patients (>65 years of age), especially those with impaired cognitive function, because of the sedative and anticholinergic effects of first-generation drugs. Therapy should be approached cautiously, possibly at reduced dosages, because of the greater likelihood of compromised renal and hepatic function in these patients, which can reduce drug elimination.

First-generation antihistamines are not recommended for use in children because their sedative effects can impair learning and school performance. The second-generation drugs loratadine, desloratadine, fexofenadine, cetirizine, levocetirizine, and azelastine (intranasal) have been approved by the FDA for use in children and are available in appropriate lower-dose formulations (e.g., chewable or rapidly dissolving tablets, syrup).

Use of over-the-counter cough and cold medicines (containing mixtures of antihistamines, decongestants, antitussives, expectorants) in young children has been associated with serious side effects and deaths (Kuehn, 2008). In 2008, the FDA recommended that they not be used in children <2 years of age, and drug manufacturers affiliated with the Consumer Healthcare Products Association voluntarily re-labeled products "do not use" for children <4 years of age. The FDA is reviewing the safety of these medicines in children aged 2-11 years.

Dibenzoxepin Tricyclics (Doxepin). Doxepin, the only drug in this class, is marketed as a tricyclic antidepressant (Chapter 16). It also is one of the most potent H₁ antagonists and has significant H₂ antagonist activity, but this does not translate into greater clinical effectiveness. It can cause drowsiness and is associated with anticholinergic effects. Doxepin is better tolerated by patients with depression than those who are not depressed, where even small doses (e.g., 20 mg) may cause disorientation and confusion.

Ethanolamines (Prototype: Diphenhydramine). These drugs possess significant antimuscarinic activity and have a pronounced tendency to induce sedation. About half of those treated acutely with conventional doses experience somnolence. The incidence of GI side effects, however, is low with this group.

Ethylenediamines (Prototype: Pyrilamine). These include some of the most specific H₁ antagonists. Although their central effects are relatively feeble, somnolence occurs in a fair proportion of patients. GI side effects are quite common.

Alkylamines (Prototype: Chlorpheniramine). These are among the most potent H₁ antagonists. The drugs are less prone to produce drowsiness and are more suitable for daytime use, but a significant proportion of patients do experience sedation. Side effects involving CNS stimulation are more common than with other groups.

First-Generation Piperazines. The oldest member of this group, chlorcyclizine, has a more prolonged action and produces a comparatively low incidence of drowsiness. Hydroxyzine is a long-acting compound that is used widely for skin allergies; its considerable CNS-depressant activity may contribute to its prominent anti-pruritic action. Cyclizine and meclizine have been used primarily to counter motion sickness, although promethazine and diphenhydramine are more effective (as is the antimuscarinic scopolamine; see "H₁ Antihistamines").

Second-Generation Piperazines (Cetirizine). Cetirizine is the only drug in this class. It has minimal anticholinergic effects. It also has negligible penetration into the brain but is associated with a somewhat higher incidence of drowsiness than the other second-generation H₁ antagonists. The active enantiomer levocetirizine has slightly greater potency and may be used at half the dose with less resultant sedation.

Phenothiazines (Prototype: Promethazine). Most drugs of this class are H₁ antagonists and also possess considerable anticholinergic activity. Promethazine, which has prominent sedative effects, and its many congeners are used primarily for their antiemetic effects (Chapter 37).

First-Generation Piperidines (Cyproheptadine, Phenindamine). Cyproheptadine uniquely has both antihistamine and anti-serotonin activity. Cyproheptadine and phenindamine cause drowsiness and also have significant anticholinergic effects and can increase appetite.

Second-Generation Piperidines (Prototype: Terfenadine). Terfenadine and astemizole were withdrawn from the market. Current drugs in this class include loratadine, desloratadine, and fexofenadine. These agents are highly selective for H₁ receptors, lack significant anticholinergic actions, and penetrate poorly into the CNS. Taken together, these properties appear to account for the low incidence of side effects of piperidine antihistamines.

H₃ receptors are presynaptic autoreceptors on histaminergic neurons that originate in the tuberomammillary nucleus in the hypothalamus and project throughout the CNS, most prominently to the hippocampus, amygdala, nucleus accumbens, globus pallidus, striatum, hypothalamus, and cortex (Haas et al., 2008; Sander et al., 2008). The activated H₃ receptor depresses neuronal firing at the level of cell bodies/dendrites and decreases histamine release from depolarized terminals. Thus, H₃ agonists decrease histaminergic transmission, and antagonists increase it. H₃ receptors also are presynaptic heteroreceptors on a variety of neurons in brain and peripheral tissues, and their activation inhibits release from noradrenergic, serotoninergic, GABA-ergic, cholinergic, and glutamatergic neurons, as well as pain-sensitive C fibers. H₃ receptors in the brain have significant constitutive activity in the absence of agonist, which varies according to splicing isoform, cell type, and signaling pathway stimulated; consequently, inverse agonists will activate these neurons.

In the enterochromaffin-like cells of the stomach, H₃ receptors inhibit gastrin-induced release of histamine and, therefore, decrease HCl secretion mediated by H₂ receptors, but the effect is not large enough to warrant development of therapeutic agents. H₃ agonists decrease tachykinin release from capsaicin-sensitive C-fiber terminals and thereby reduce capsaicin-induced plasma extravasation and are antinociceptive. H₃ agonists also depress exaggerated catecholamine release in the heart (e.g., during ischemia).

By blocking H₃ autoreceptors on histaminergic neurons and H₃ heteroreceptors on other neurons, H₃ antagonists/inverse agonists have a wide range of central effects; for example, they promote wakefulness, improve cognitive function (e.g., enhance memory, learning, and attention), and reduce food intake (Esbenshade et al., 2008; Sander et al., 2008). As a result, there is considerable interest in developing H₃ antagonists for possible treatment of sleeping disorders, attention-deficit hyperactivity disorder (ADHD), epilepsy, cognitive impairment, schizophrenia, obesity, neuropathic pain, and Alzheimer's disease (Esbenshade et al., 2008; Sander et al., 2008). However, this task is complicated by the heterogeneity in receptor isoforms and signaling pathways, varying levels of constitutive H₃ activity, species differences in ligand affinities, and the effects of H₃ antagonists/inverse agonists on the release of neurotransmitters in addition to histamine (e.g., DA, ACh, NE, GABA, glutamate, and substance P) (Sander et al., 2008; Esbenshade et al., 2008). Although drug development is focused on inverse agonists to block basal and agonist-stimulated H₃ activity, it is not yet clear that such drugs will necessarily be clinically superior to neutral antagonists.

Thioperamide was the first "specific" H₃ antagonist/inverse agonist available experimentally, but it turned out to be equally effective on the H₄ receptor. A number of other imidazole derivatives have been developed as H₃ antagonists, including clobenpropit, ciproxifan, and proxyfan. However, the imidazole group can bind to and inhibit CYPs, reduce bioavailability in and penetration into the CNS, and enhance binding to H₄ receptors, prompting efforts to develop more selective H₃-receptor antagonists using non-imidazole-based structures. This has resulted in the generation of numerous antagonists/inverse agonists representing a remarkably broad variety of structural classes (Sander et al., 2008; Esbenshade et al., 2008). Although none is approved for clinical use, several non-imidazole H₃ antagonists/inverse agonists are in phase II clinical trials for treating epilepsy, narcolepsy and other sleep disorders, cognitive impairment, Alzheimer's disease, schizophrenia, and ADHD; these include tiprolisant (BF2.649), GSK189254, GSK239512, JNJ-17216498, and MK0249.

Because the H₄ receptor is expressed on cells with inflammatory or immune functions (see the discussion earlier), there is great interest in developing H₄ antagonists to treat various inflammatory conditions (Thurmond et al., 2008). Indeed, H₄ receptors can mediate histamine-induced chemotaxis, induction of cell shape change, secretion of cytokines, and upregulation of adhesion molecules. For example, the H₄ receptor likely accounts for early reports of histamine-dependent eosinophil chemotaxis, which was independent of H₁ receptors and inhibited by H₂ receptors (Clark et al., 1977). The presence of H₄ receptors in the CNS and animal studies with H₄ agonists and antagonists also have indicated a role for this receptor in pruritus and neuropathic pain (Leurs et al., 2009). The first selective H₄ antagonist, JNJ7777120, exhibits ∼1000-fold selectivity over other H receptors, has acceptable oral bioavailability, and has in vivo activity; however, its short t_1/2 (∼0.8 hour) limits its potential clinical usefulness. In addition to derivatives of the benzimidazole JNJ7777120, promising H₄ antagonists have been developed using several different scaffolds, including methylpiperazine-substituted 2-quinoxalinones, quinazolines, and aminopyrimidines (Leurs et al., 2009). Although these are high-affinity ligands, relatively high doses are required for significant anti-inflammatory effects in vivo, possibly due to either their pharmacokinetic properties or competition with endogenous histamine, which also has a high affinity for the receptor (Leurs et al., 2009). No H₄ antagonists have yet been tested in clinical trials.

History. In the 1920s and 1930s, Frey, Kraut, and Werle characterized a hypotensive substance in urine and found a similar material in saliva, plasma, and a variety of tissues. The pancreas also was a rich source, so they named this material kallikrein after a Greek synonym for that organ, kallikréas. By 1937, Werle, Götze, and Keppler had established that kallikreins generate a pharmacologically active substance from an inactive precursor present in plasma; the active substance, kallidin, proved to be a polypeptide cleaved from a plasma globulin termed kallidinogen (see Werle, 1970).

Interest in the field intensified when Rocha e Silva, Beraldo, and associates reported that trypsin and certain snake venoms acted on plasma globulin to produce a substance that lowered blood pressure and caused a slowly developing contraction of the gut (Rocha e Silva et al., 1949; Beraldo and Andrade, 1997). Because of this slow response, they named the substance bradykinin, a term derived from the Greek words bradys, meaning "slow," and kinein, meaning "to move." In 1960, the nonapeptide bradykinin was isolated by Elliott and coworkers and synthesized by Boissonnas and associates. Shortly thereafter, kallidin was found to be a decapeptide bradykinin with an additional lysine residue at the amino terminus. These peptides, except for Lys¹, have identical chemical structures (Table 32–3) and quite similar pharmacological properties. For the whole group, the generic term kinins has been adopted. The kinins have short half-lives because they are destroyed by plasma and tissue enzymes originally called kininase I and kininase II. Kininase I releases a single C-terminal amino acid; kininase II releases a dipeptide (Skidgel and Erdös, 1998). Kininase II is identical to angiotensin-converting enzyme (ACE) (Yang et al., 1970).

Ferreira and colleagues (1970) reported the isolation of a bradykinin-potentiating factor from the venom of the Brazilian snake Bothrops jararaca. Ondetti and colleagues (1971) subsequently determined the structure of a peptide from the venom that inhibited ACE and lowered blood pressure when given intravenously to hypertensive patients. Synthetic ACE inhibitors, administered orally (Chapter 26), now are widely used in the treatment of hypertension, diabetic nephropathy, congestive heart failure, and after myocardial infarction (Pfeffer and Frolich, 2006).

Regoli and Barabé (1980) divided the kinin receptors into B₁ and B₂ classes based on the rank order of potency of kinin analogs, and this was validated at the molecular level by cloning of the B₁ and B₂ receptors (Bhoola et al., 1992; Hess, 1997). A primary feature that distinguishes peptide ligands of the B₁ and B₂ receptors is the presence of a C-terminal Arg residue; intact kinins (bradykinin and kallidin) are agonists of the B₂ receptor, whereas their des-Arg forms ([des-Arg⁹]-bradykinin and [des-Arg¹⁰]-kallidin) are agonists for the B₁ receptor. First-generation B₂ kinin–receptor antagonists were developed in the mid-1980s (Stewart, 2004), followed by second- and third-generation blockers. These antagonists revealed the role of kinins in the therapeutic effects of ACE/kininase II inhibitors and have led to increased acceptance of the importance of kinins. Studies involving B₁ and B₂ receptor knockout mice (Hess, 1997; Pesquero and Bader, 2006; Jiang et al., 2009) have furthered our understanding of the role of bradykinin in the regulation of cardiovascular homeostasis and inflammatory processes.

Kallikreins. Bradykinin and kallidin are cleaved from HMW or LMW kininogens by plasma or tissue kallikrein, respectively (Figure 32–4). Plasma kallikrein and tissue kallikrein are distinct enzymes that are activated by different mechanisms (Bhoola et al., 1992).

Plasma prekallikrein is an inactive protein of ∼88,000 Da that complexes with its substrate, HMW kininogen (Mandle et al., 1976). The ensuing proteolytic cascade is restrained by the protease inhibitors present in plasma. Among the most important of these are the inhibitor of the activated first component of complement (C1-INH) and α₂ macroglobulin. Under experimental conditions, the kallikrein–kinin system is activated by the binding of factor XII, also known as Hageman factor, to negatively charged surfaces. Factor XII, a protease that is common to both the kinin and the intrinsic coagulation cascades (Chapter 30), undergoes autoactivation (Silverberg et al., 1980) and, in turn, activates prekallikrein. Importantly, kallikrein further activates factor XII, thereby exerting a positive feedback on the system. In vivo, factor XII may not undergo autoactivation on binding to endothelial cells. Instead, the binding of the HMW kininogen–prekallikrein heterodimer to a multiprotein-receptor complex on endothelial cells leads to activation of the prekallikrein-HMW kininogen complex (but not free prekallikrein) by heat shock protein 90 (Joseph et al., 2002) and by a lysosomal enzyme designated prolylcarboxypeptidase, which also is present on endothelial cell membranes (Schmaier, 2008). Kallikrein activates factor XII, cleaves HMW kininogen, and activates prourokinase (Schmaier, 2008; Kaplan et al., 2002; Colman, 1999). Human tissue kallikrein is a member of a large multigene family of 15 members with high sequence identity that are clustered at chromosome 19q13.4 (Emami and Diamandis, 2007). Only the classical (or "true") tissue kallikrein, hK1, generates kinins from kininogen. Another member, hK3, better known as the prostate-specific antigen (PSA), is an important marker in diagnosing prostate cancer, and several other family members are promising tumor biomarkers (Emami and Diamandis, 2007).

Compared with plasma kallikrein, tissue kallikrein is a smaller protein (29,000 Da). It is synthesized as a preproprotein in the epithelial cells or secretory cells in several tissues, including salivary glands, pancreas, prostate, and distal nephron. Tissue kallikrein also is expressed in human neutrophils. It acts locally near its sites of origin. The synthesis of tissue prokallikrein is controlled by a number of factors, including aldosterone in the kidney and salivary gland and androgens in certain other glands. The secretion of the tissue prokallikrein also may be regulated; for example, its secretion from the pancreas is enhanced by stimulation of the vagus nerve. The activation of tissue prokallikrein to kallikrein requires proteolytic cleavage to remove a 7–amino acid propeptide (Takada et al., 1985).

Kininogens. The two substrates for the kallikreins, HMW kininogen and LMW kininogen, are derived from a single gene by alternative splicing. The HMW kininogen contains 626 amino acid residues: The N-terminal "heavy chain" sequence (362 amino acids) consists of domains 1-3, followed by 9 residue bradykinin sequence (domain 4), connected to a C-terminal "light chain" sequence (255 amino acids) containing domains D5 and D6. LMW kininogen is identical to domains 1-4 of HMW kininogen but is distinguished by its short C-terminal light chain (Takagaki et al., 1985). HMW kininogen is cleaved by plasma and tissue kallikrein to yield bradykinin and kallidin, respectively. LMW kininogen is a substrate only of tissue kallikrein, and the product is kallidin. The kininogens also inhibit cysteine proteinases and thrombin binding and have anti-adhesive and profibrinolytic properties.

The kinins have an evanescent existence—their t_1/2 in plasma is only ∼15 seconds, and some 80-90% of the kinins may be destroyed in a single passage through the pulmonary vascular bed. Plasma concentrations of bradykinin are difficult to measure because inadequate inhibition of kininogenases or kininases in the blood can lead to artifactual formation or degradation of bradykinin during blood collection. When care is taken to inhibit these processes, the reported physiological concentrations of bradykinin in blood are in the picomolar range.

The principal catabolizing enzyme in the lung and other vascular beds is kininase II, or ACE (Figure 32–4) (Chapter 26). Removal of the C-terminal dipeptide by ACE or neutral endopeptidase 24.11 (neprilysin) inactivates kinins (Skidgel and Erdös, 1998) (Figure 32–5). A slower-acting enzyme, carboxypeptidase N (lysine carboxypeptidase, kininase I), releases the C-terminal arginine residue, producing [desArg⁹]-bradykinin or [des-Arg¹⁰]-kallidin (Table 32–3 and Figures 32–4 and 32–5), which are potent B₁ receptor agonists that no longer bind B₂ receptors (Bhoola et al., 1992; Skidgel and Erdös, 1998). Carboxypeptidase N is expressed constitutively in blood plasma, where its level is ∼10^–7 M (Skidgel and Erdös, 1998; Skidgel and Erdös, 2007). Carboxypeptidase M, which also cleaves basic C-terminal amino acids, is a widely distributed plasma membrane–bound enzyme (Skidgel and Erdös, 1998). The recently established crystal structures of the active subunit of carboxypeptidases N and M revealed an overall similarity with unique features consistent with their different localizations and functions (Skidgel and Erdös, 2007). A familial carboxypeptidase N deficiency, due to mutations in the active subunit, causes low plasma levels of this enzyme and is associated with angioedema or urticaria (Skidgel and Erdös, 2007; Matthews et al., 2004). Finally, aminopeptidase P can cleave the N-terminal arginine, rendering bradykinin inactive and susceptible to cleavage by dipeptidyl peptidase IV (Figure 32–5).

The classical bradykinin B₂ receptor is constitutively expressed in most normal tissues, where it selectively binds intact bradykinin and kallidin (Table 32–3 and Figure 32–4) and mediates the majority of their effects. The B₁ receptor is activated by the des-Arg metabolites of bradykinin and kallidin produced by the actions of carboxypeptidases N and M (Table 32–3 and Figures 32–4 and 32–5). Interestingly, carboxypeptidase M and the B₁ receptor interact on the cell surface to form an efficient signaling complex; disruption of this interaction decreases the efficiency of generation and delivery of des-Arg-kinin agonist to the B₁ receptor and reduces subsequent B₁ signaling (Zhang et al., 2008). B₁ receptors are normally absent or expressed at low levels in most tissues. B₁ receptor expression is upregulated by tissue injury and inflammation and by cytokines, endotoxins, and growth factors (Bhoola et al., 1992; Leeb-Lundberg et al., 2005). Carboxypeptidase M expression also is increased by cytokines (Sangsree et al., 2003), to such a degree that B₁ receptor effects may predominate over B₂ effects.

The B₂ receptor activates PLA₂ and PLC via interaction with distinct G proteins. Kinin-induced PLC activation through G_q activates the IP₃–Ca²⁺ pathway, stimulating PKC activity and also enhancing NO synthesis by eNOS/NOS3. Bradykinin activates the pro-inflammatory transcription factor NF-κB through Gα_q and βγ subunits and also activates the MAP kinase pathway. Coupling of activated B₂ receptors to G_i leads to PLA₂ activation and the liberation of arachidonate from membrane-bound phospholipids, which is converted to a variety of derivatives, including inflammatory mediators and vasodilator epoxyeicosatrienoic acids (EETs) and prostacyclin (Campbell and Falck, 2007; Leeb-Lundberg et al., 2005) (Chapter 33).

B₁ receptors also couple through G_q and G_i to activate many of the same signal transduction pathways as the B₂ receptor (Leeb-Lundberg et al., 2005). However, B₁ receptor activation enhances NO production by stimulation of the inducible nitric oxide synthase (iNOS) rather than eNOS (Zhang et al., 2007). The B₁ and B₂ receptors also differ in their time courses of downregulation; the B₂ receptor response is rapidly desensitized, whereas the B₁ response is not. This likely is due to modification at a Ser/Thr-rich cluster present in the C-terminal tail of the B₂ receptor that is not conserved in the B₁ receptor sequence (Leeb-Lundberg et al., 2005).

Because B₂ receptors are distributed widely and couple to several G proteins, receptor agonists are employed frequently as tools to activate and study signal transduction pathways in a variety of cells. The antagonist icatibant (HOE-140) is commonly used to prove that cellular responses are mediated by B₂ receptors. Nevertheless, blocking increased signaling through the B₂ receptor does not necessarily indicate enhanced kinin generation because some proteases (e.g., kallikrein) can activate the B₂ receptor directly, and this also is blocked by HOE-140 (Biyashev et al., 2006).

Activation of the angiotensin AT₂ receptor results in responses (e.g., vasodilation) that oppose those of the activated angiotensin AT₁ receptor (e.g., vasoconstriction) (Chapter 26); this can be mediated in part by cross-talk through activation of B₂ receptors (Widdop et al., 2003).

In hereditary angioedema, bradykinin is formed, and there is depletion of the upstream components of the kinin cascade during episodes of swelling, laryngeal edema, and abdominal pain. B₁ receptors on inflammatory cells (e.g., macrophages) can elicit production of the inflammatory mediators IL-1 and TNF-α (Bhoola et al., 1992). Kinin levels are increased in a number of chronic inflammatory diseases and may be significant in gout, disseminated intravascular coagulation, inflammatory bowel disease, rheumatoid arthritis, or asthma. Kinins also may contribute to the skeletal changes seen in chronic inflammatory states. Kinins stimulate bone resorption through B₁ and possibly B₂ receptors, perhaps by osteoblast-mediated osteoclast activation (Chapter 44).

The kallikrein–kinin system is cardioprotective, as revealed by studies showing that many of the beneficial effects of ACE inhibitors on heart function are attributable to enhancement of bradykinin effects, such as their anti-proliferative activity or ability to increase glucose uptake in tissue (Pfeffer and Frolich, 2006; Henriksen et al., 1999; Heitsch, 2003). Bradykinin contributes to the beneficial effect of preconditioning to protect the heart against ischemia and reperfusion injury. Bradykinin also stimulates tissue plasminogen activator (tPA) release from the vascular endothelium (Leeb-Lundberg et al., 2005) and may contribute to the endogenous defense against some cardiovascular events, such as myocardial infarction and stroke. Stimulation of either B₁ or B₂ receptors produces cardioprotective effects after myocardial infarction (Leeb-Lundberg et al., 2005; Heitsch, 2003; Jiang et al., 2009).

Other Effects. Many initial discoveries in the kallikrein– kinin system were made using a bioassay measuring isotonic contractions of the isolated rat uterus in estrus, which is especially sensitive to kinins. Kinins promote dilation of the fetal pulmonary artery, closure of the ductus arteriosus, and constriction of the umbilical vessels, all of which occur in the transition from fetal to neonatal circulation. The kallikrein–kinin system also functions in many other areas in the body, serving to mediate edema formation and smooth muscle contraction. Kinins also affect the CNS, disrupting the blood-brain barrier and allowing increased CNS penetration.

Kallikrein Inhibitors

Aprotinin (trasylol) is a natural proteinase inhibitor obtained for commercial purposes from bovine lung and is identical to Kunitz's pancreatic trypsin inhibitor (Waxler and Rabito, 2003). Aprotinin inhibits mediators of the inflammatory response, fibrinolysis, and thrombin generation following cardiopulmonary bypass surgery, including kallikrein and plasmin. Administration of aprotinin reduces requirements for blood products in patients undergoing coronary artery bypass grafting (Waxler and Rabito, 2003). Recent retrospective meta-analyses and a prospective multicenter trial linked aprotinin use with higher mortality and renal morbidity (Dietrich, 2009). Although there is controversy regarding the validity of these conclusions, the manufacturer has withdrawn the drug from the market (Dietrich, 2009); it remains available through a special investigational treatment protocol. Ecallantide (DX-88), a synthetic plasma kallikrein inhibitor, inhibits acute episodes of angioedema in patients with hereditary angioedema (Schneider et al., 2007).

A rare side effect of ACE inhibitors is angioedema, which may occur shortly after initiating therapy. This is a class effect of ACE inhibitors and may be connected to the inhibition of kinin metabolism by ACE (Slater et al., 1988). ACE inhibitor–associated angioedema is more common in blacks than in Caucasians. A common side effect of ACE inhibitors is a chronic, nonproductive cough that dissipates when the drug is stopped. The finding that angiotensin AT₁ receptor antagonists do not cause cough supports the role of bradykinin in this effect, but the mechanism has not been clearly defined.

Bradykinin also may contribute to the effects of the AT₁-receptor antagonists. During AT₁ receptor blockade, Ang II concentrations increase, which enhances signaling through the unopposed AT₂ subtype receptor, causing an increase in renal bradykinin concentrations (Widdop et al., 2003).

A new class of antihypertensive agent was developed to inhibit two major kinin-degrading enzymes, ACE and neprilysin, and the added inhibition of bradykinin metabolism was expected to enhance therapeutic effectiveness. In phase III clinical trials, the prototype drug omapatrilat was an effective antihypertensive agent but was associated with a 3-fold higher incidence of angioedema than with an ACE inhibitor alone, causing withdrawal of the drug.

Kinin Receptor Antagonists. The substitution of a D-Phe for Pro at position 7 in bradykinin conferred weak antagonist activity at the B₂ receptor and blocked the action of ACE. The addition of an N-terminal, D-Arg substitution of hydroxyproline in position 3 and thienylalanine at positions 5 and 8 resulted in the first useful and potent B₂ antagonist with in vivo activity (NPC-349; Table 32–3) (Stewart, 2004). However, this antagonist had a short t_1/2 owing to enzymatic degradation by carboxypeptidase N in vivo, which converted it into a B₁ antagonist. Based on NPC-349, the longer-acting selective B₂ receptor antagonist HOE-140 (icatibant) was developed by substituting synthetic amino acids at positions 7 [D-tetrahydroisoquinoline-3-carboxylic acid (Tic)] and 8 [octahydroindole-2-carboxylic acid (Oic)] (Table 32–3). Icatibant (firazyr) has been approved in the E.U. for treatment of acute episodes of swelling in patients with hereditary angioedema (Bork et al., 2008). It is administered by subcutaneous injection.

Orally active non-peptide receptor antagonists would be preferable for therapeutic use. The first of these for the B₂ receptor, WIN64338, had significant muscarinic cholinergic activity. In animals, a newer, more specific non-peptide B₂ antagonist, FR173657 (Table 32–3), decreased bradykinin-induced edema, hypotension, pain, bronchoconstriction, and inflammatory responses (Leeb-Lundberg et al., 2005; Abraham et al., 2006). This has led to increased interest by the drug industry and the generation of several new non-peptide B₂ antagonists with good oral bioavailability and in vivo stability and activity (Leeb-Lundberg et al., 2005; Abraham et al., 2006). On the other hand, synthetic B₂ receptor agonists (such as FR190997; Table 32–3) may be cardioprotective. Synthetic small-molecule bradykinin agonists or antagonists would not necessarily bind to the same regions of the B₂ receptor as the peptide (Heitsch, 2003).

Peptide antagonists of the B₁ receptor include des-Arg¹⁰-[Leu⁸]-kallidin and [des-Arg¹⁰]-HOE-140 (Table 32–3), among others (Campos et al., 2006). More recently, several non-peptide B₁ antagonists with in vivo activity have been developed (Campos et al., 2006; Feng et al., 2008); the most promising of these is SSR240612 (Table 32–3), which is orally active and inhibits inflammation and neuropathic pain in animal studies. None has yet been tested clinically.