Histamine and Antihistaminics

 Chapter 11

Histamine and Antihistaminics

HISTAMINE

• Histamine, meaning ‘tissue amine’ (histos— tissue) is almost ubiquitously present in animal tissues and in certain plants, e.g. stinging nettle. Its pharmacology was studied in detail by Dale in the beginning of the 20th century when close parallelism was noted between its actions and the manifestations of certain allergic reactions. It was implicated as a mediator of hypersensitivity phenomena and tissue injury reactions. It is now known to play important physiological roles.

• Histamine is present mostly within storage granules of mast cells. Tissues rich in histamine are skin, gastric and intestinal mucosa, lungs, liver and placenta. Nonmeat cell histamine occurs in brain, epidermis, gastric mucosa and growing regions. Turnover of mast cell histamine is slow, while that of nonmast cell histamine is fast.

Histamine is also present in blood, most body secretions, venoms and pathological fluids.

Synthesis, storage and destruction 

• Histamine is β imidazolyl ethylamine. It is synthesized locally from the amino acid histidine and degraded rapidly by oxidation and methylation. In mast cells, histamine (positively charged) is held by an acidic protein and heparin (negatively charged) within intracellular granules. When the granules are extruded by exocytosis, Na+ ions in e.c.f. exchange with histamine to release it free. Increase in intracellular cAMP inhibits histamine release. Histamine is inactive orally because liver degrades all histamine that is absorbed from the intestines.

• Histamine receptors Analogous to adrenergic α and β receptors, histaminergic receptors were classified by Asch and Schild (1966) into H1 and H2 : those blocked by then available antihistamines were labelled H1. Sir James Black (1972) developed the first H2 blocker burimamide and confirmed this classification. Both H1 and H2 receptors have now been cloned. A third H3 receptor, which serves primarily as an autoreceptor controlling histamine release from neurons in brain was identified in 1983. Though some selective H3 agonists and antagonists have been produced, none has found any clinical application. Features of these 3 types of histaminergic receptor are compared.

• In sensitized atopic individual, specific reaginic (IgE) antibody is produced and gets bound to Fc epsilon receptor I (FcεRI) on the surface of mast cells. On challenge, the antigen bridges IgE molecules resulting in transmembrane activation of a tyrosine-protein kinase (t-Pr-K) which phosphorylates and activates phospholipaseCγ. Phosphatidyl inositol bisphosphate (PIP2) is hydrolysed and inositol trisphosphate (IP3) is generated which triggers intracellular release of Ca2+. The Ca2+ ions induce fusion of granule membrane with plasma membrane of the mast cell resulting in exocytotic release of granule contents. In the granule, positively charged histamine (Hist+ ) is held complexed with negatively charged protein (Prot– ) and heparin (Hep– ) molecules. Cationic exchange with extracellular Na+ (and Ca2+) sets histamine free to act on the target cells.

• Molecular cloning has revealed yet another (H4) receptor in 2001. It has considerable homology with H3 and binds many H3 ligands. Eosinophils, mast cells and basophils are the primary cells expressing H4 receptors; activation enhances chemotaxis of these cells. The H4 receptor may be playing a role in allergic inflammation: H4 antagonists are being explored as potential drugs for allergic inflammatory conditions like rhinitis and asthma. Intestines and brain are the other sites where H4 receptors have been located.

PHARMACOLOGICAL ACTIONS

• Blood vessels Histamine causes marked dilatation of smaller blood vessels, including arterioles, capillaries and venules. On s.c. injection flushing, especially in the blush area, heat, increased heart rate and cardiac output, with little or no fall in BP are produced. Rapid i.v. injection causes fall in BP which has an early short lasting H1 and a slow but more persistent H2 component. With low doses only the H1 component is manifest since H1 receptors have higher affinity. Fall in BP due to large doses is completely blocked only by a combination of H1 and H2 antagonists. Dilatation of cranial vessels causes pulsatile headache.

• Like ACh and many other autacoids, vasodilatation caused by histamine is partly (H1 component) indirect, mediated through ‘endothelium dependent relaxing factor’ (EDRF): the receptor being located on the endothelial cells. H2 receptors mediating vasodilatation are located directly on the vascular smooth muscle.

• Larger arteries and veins are constricted by histamine: mediated by H1 receptor on vascular smooth muscle. Histamine also causes increased capillary permeability due to separation of endothelial cells → exudation of plasma. This is primarily a H1 response.

• Injected intradermally, it elicits the triple response consisting of:

1. Red spot: due to intense capillary dilatation.
2. Wheal: due to exudation of fluid from capillaries and venules.
3. Flare: i.e., redness in the surrounding area due to arteriolar dilatation mediated by axon reflex.

• Heart Direct effects of histamine on in situ heart are not prominent, but the isolated heart, especially of guinea pig, is stimulated rate as well as force of contraction is increased. These are primarily H2 responses but a H1 mediated negative dromotropic (slowing of A-V conduction) effect has also been demonstrated.

• Visceral smooth muscle Histamine causes bronchoconstriction; guinea pigs and patients of asthma are highly sensitive. Large doses cause abdominal cramps and colic by increasing intestinal contractions. Guineapig uterus is contracted while that or rat is relaxed; human uterus is not much affected as are most other visceral smooth muscles. Smooth muscle contraction is a H1 response. In few instances H2 mediated relaxation is also seen, e.g., bronchial muscle of sheep, human bronchi after H1 blockade.

• Glands Histamine causes marked increase in gastric secretion—primarily of acid but also of pepsin. This is a direct action exerted on parietal cells through H2 receptors and is mediated by increased cAMP generation, which in turn activates the membrane proton pump (H+ K+ ATPase). Histamine can increase other secretions also, but the effect is hardly discernable.

• Sensory nerve endings Itching occurs when histamine is injected i.v. or intracutaneously. Higher concentrations injected more deeply cause pain. These are reflections of the capacity of histamine to stimulate nerve endings.

• Autonomic ganglia and adrenal medulla These are stimulated and release of Adr occurs, which can cause a secondary rise in BP.

• CNS Histamine does not penetrate bloodbrain barrier—no central effects are seen on i.v. injection. However, intracerebroventricular administration produces rise in BP, cardiac stimulation, behavioural arousal, hypothermia, vomiting and ADH release. These effects are mediated through both H1 and H2 receptors.

PATHOPHYSIOLOGICAL ROLES

• Gastric secretion Histamine has dominant physiological role in mediating secretion of HCl in the stomach. Nonmast cell histamine occurs in gastric mucosa, possibly in cells called ‘histaminocytes’; situated close to the parietal cells and has high turnover rate. It is released locally under the influence of all stimuli. that evoke gastric secretion (feeding, vagal stimulation, cholinergic drugs and gastrin) and activates the proton pump (H+K+ ATPase) through H2 receptors.
• H2 blockers not only suppress acid secretion induced by histamine but also markedly diminish that in response to ACh and gastrin. By a mutually synergistic interaction the three secretagogues amplify responses to each other with histamine playing the dominant role. As such, antimuscarinic drugs dampen the response to histamine and gastrin also. All three secretagogues activate the same proton pump (H+K+ATPase) in the parietal cell membrane, but through their own receptors.

• Allergic phenomena Mediation of hypersensitivity reactions has been the first role ascribed to histamine. However, histamine is only one of the mediators of such phenomena. Released from mast cells following AG: AB reaction on their surface (involving IgE type of reaginic antibodies; in immediate type of hypersensitivity reactions, histamine is causative in urticaria, angioedema, bronchoconstriction and anaphylactic shock. The H1 antagonists are effective in controlling these manifestations to a considerable extent, except asthma and to a lesser extent anaphylactic fall in BP in which leukotrienes (especially LTD4) and PAF appear to be more important. Histamine is not involved in delayed or retarded type of allergic reactions.

• As transmitter Histamine is believed to be the afferent transmitter which initiates the sensation of itch and pain at sensory nerve endings. Nonmast cell histamine occurs in brain, especially hypothalamus and midbrain. It is involved in maintaining wakefulness; H1 antihistaminics owe their sedative action to blockade of this function. In the brain H1 agonism suppresses appetite; certain H1 antagonists stimulate appetite. Histamine also appears to act as a transmitter regulating body temperature, cardiovascular function, thirst, hormone release from anterior pituitary and possibly other functions.

• Inflammation Histamine has been implicated as a mediator of vasodilatation and other changes that occur during inflammation. It promotes adhesion of leukocytes to vascular endothelium by expressing adhesion molecule Pselectin on endothelial cell surface, sequestrating leukocytes at the inflammatory site. It may also regulate microcirculation according to local needs.

• Tissue growth and repair Because growing and regenerating tissues contain high concentrations of histamine, it has been suggested to play an essential role in the process of growth and repair.

• Headache Histamine has been implicated in certain vascular headaches, but there is no conclusive evidence.

USES

• Histamine has no therapeutic use. In the past it has been used to test acid secreting capacity of stomach, bronchial hyperreactivity in asthmatics, and for diagnosis of pheochromocytoma, but these pharmacological tests are risky and obsolete now.

• Betahistine It is an orally active, somewhat H1 selective histamine analogue, which is used to control vertigo in patients of Meniéré’s disease: possibly acts by causing vasodilatation in the internal ear. It is contraindicated in asthmatics and ulcer patients.

HISTAMINE RELEASERS

A variety of mechanical, chemical and immunological stimuli are capable of releasing histamine from mast cells. 

1. Tissue damage: trauma, stings and venoms, proteolytic enzymes, phospholipase A.
2.  Antigen: antibody reaction involving IgE antibodies.
3.  Polymers like dextran, polyvinyl pyrrolidone (PVP).
4. Some basic drugs—tubocurarine, morphine, atropine, stilbamidine, polymyxin B, vancomycin and even some antihistaminics directly release histamine without an immunological reaction.
5. Surface acting agents like Tween 80, compound 48/ 80 etc. The primary action of these substances is release of histamine from mast cells, therefore they are called histamine liberators. They produce an ‘anaphylactoid’ reaction—itching and burning sensation, flushing, urticaria, fall in BP, tachycardia, headache, colic and asthma. Most of these symptoms are controlled by a H1 antihistaminic, better still if H2 blocker is given together.

H1 ANTAGONISTS (Conventional antihistaminics)

• These drugs competitively antagonize actions of histamine at the H1 receptors. The first compounds of this type were introduced in the late 1930s and have subsequently proliferated into an unnecessary motley of drugs. Nevertheless, they are frequently used for a variety of purposes. More commonly employed now are the less sedating second generation H1 antihistamines that have been added after 1980. Seemingly, H1 antihistaminics have diverse chemical structures, but majority have a substituted ethylamine side chain.

PHARMACOLOGICAL ACTIONS

Qualitatively all H1 antihistaminics have similar actions, but there are quantitative differences, especially in the sedative property.

• Antagonism of histamine They effectively block histamine induced bronchoconstriction, contraction of intestinal and other smooth muscle and triple response—especially wheal, flare and itch. Fall in BP produced by low doses of histamine is blocked, but additional H2 antagonists are required for complete blockade of higher doses. Pretreatment with these drugs protects animals from death caused by i.v. injection of large doses of histamine. Release of Adr from adrenal medulla in response to histamine is abolished. Constriction of larger blood vessel by histamine is also antagonized. Action of histamine on gastric secretion is singularly not affected by these drugs.

• Antiallergic action Many manifestations of immediate hypersensitivity (type I reactions) are suppressed. Urticaria, itching and angioedema are well controlled. Anaphylactic fall in BP is only partially prevented. Asthma in man is practically unaffected though anaphylactic bronchoconstriction in guinea pig is largely prevented. This tissue and species dependence of response probably reflects extent of involvement of histamine in the reaction. It is now well established that leukotrienes (C4 and D4) and PAF are more important mediators for human asthma.

• CNS The older antihistamines produce variable degree of CNS depression. This appears to depend on the compound’s ability to penetrate blood-brain barrier and its affinity for the central (compared to peripheral) H1 receptors. Individual susceptibility to different agents varies considerably, but an overall grading of the sedative property is presented. Some individuals also experience stimulant effects like restlessness and insomnia. Excitement and convulsions are frequently seen at toxic doses. The second generation antihistaminics are practically nonsedating.

• Certain (see below) H1 antihistamines are effective in preventing motion sickness. It is not certain whether this is due to antagonism of histamine in the brain or reflects antimuscarinic property of these drugs. Promethazine also controls vomiting of pregnancy and other causes.

• Promethazine and few other antihistaminics reduce tremor, rigidity and sialorrhoea of parkinsonism. Anticholinergic and sedative properties underlie the benefit.

• Anticholinergic action Many H1 blockers in addition antagonize muscarinic actions of ACh. The anticholinergic action can be graded as:

If used concurrently with atropine or its substitutes, phenothiazines, tricyclic antidepressants or disopyramide, the anticholinergic action adds up.

•  Local anaesthetic Some drugs like pheniramine, have strong while others have weak membrane stabilizing property. However, they are not used clinically as local anaesthetic because they cause irritation when injected s.c. Membrane stabilizing activity also confers antiarrhythmic property to these compounds.

• BP Most antihistaminics cause a fall in BP on i.v. injection (direct smooth muscle relaxation). However, this is not evident on oral administration.

PHARMACOKINETICS

The classical H1 antihistaminics are well absorbed from oral and parenteral routes, metabolized in the liver and excreted in urine. They are widely distributed in the body and enter brain. The newer compounds penetrate brain poorly. Duration of action of most agents is 4–6 hours, except meclizine, loratadine, cetirizine and fexofenadine which act for 12–24 hours or more.

SIDE EFFECTS AND TOXICITY

• Side effects with first generation H1 antihistaminics are frequent but are generally mild. Individuals show marked differences in susceptibility to side effects with different drugs. Some tolerance to side effects develops on repeated use.

• Sedation, diminished alertness and concentration, light headedness, motor incoordination, fatigue and tendency to fall asleep are the most common. Objective testing shows impairment of psychomotor performance. Patients should be cautioned not to operate motor vehicles or machinery requiring constant attention. Alcohol synergises in producing these effects as do other CNS depressants. Few individuals become restless, nervous and are unable to sleep. Second generation compounds are largely free of CNS effects.

• Dryness of mouth, alteration of bowel movement, urinary hesitancy and blurring of vision can be ascribed to anticholinergic property. Epigastric distress and headache are also common. Local application can cause contact dermatitis.

• Some like cyclizine and fexofenadine are teratogenic in animals; but not in humans; caution is nevertheless to be exercised during pregnancy. Acute overdose produces central excitation, tremors, hallucinations, muscular incordination, convulsions, flushing, hypotension, fever and some other features of belladonna poisoning. Death is due to respiratory and cardiovascular failure.

SECOND GENERATION ANTIHISTAMINICS

• The second generation antihistaminics (SGAs) may be defined as those H1 receptor blockers marketed after 1980 which have one or more of the following properties:

1. Higher H1 selectivitiy: no anticholinergic side effects. 
2. Absence of CNS depressant property. 
3. Additional antiallergic mechanisms apart from histamine blockade: some also inhibit late phase allergic reaction by acting on leukotrienes or by antiplatelet activating factor effect.

• Some recent compounds like fexofenadine and cetirizine that are active metabolites of earlier drugs have also been referred as ‘third generation antihistamines’, but this has not been accepted by an international consensus group of experts.

• These newer drugs have the advantage of not impairing psychomotor performance (driving etc. need not be contraindicated), produce no subjective effects, no sleepiness, do not potentiate alcohol or benzodiazepines. Some patients do complain of sedation, but incidence is similar to placebo. However, they have a narrow spectrum of therapeutic usefulness which is limited by the extent of involvement of histamine (acting through H1 receptors) in the disease state. Their principal indications are:

1. Allergic rhinitis and conjunctivitis, hay fever, pollinosis—control sneezing, runny but not blocked nose, and red, watering, itchy eyes. 
2. Urticaria, dermographism, atopic eczema. 
3. Acute allergic reactions to drugs and foods. They have poor antipruritic, antiemetic and antitussive actions.

• Fexofenadine It is the active metabolite of terfenadine, the first nonsedating SGA that was withdrawn because of several deaths due to polymorphic ventricular tachycarida (Torsades de pointes) occurring with its higher doses or when it was coadministered with CYP3A4 inhibitors (erythromycin, clarithromycin, ketoconazole, itraconazole, etc.). This toxicity is based on blockade of delayed rectifier K+ channels in the heart at higher concentrations. Astemizole is another SGA banned for the same reason. Fexofenadine has a low propensity to block delayed rectifier K+ channels, does not prolong QTc interval; no interaction with CYP3A4 inhibitors have been reported. It is largely free of arrhythmogenic potential, but some cases of ventricular arrhythmia in patients with preexisting long QT interval have been reported. Thus, it is not entirely safe in patients with long QT, bradycardia or hypokalemia.

• Fexofenadine does not cross blood-brain barrier—does not produce sedation or impair psychomotor performance and is free of atropinic side effects. It is rapidly absorbed, excreted unchanged in urine and bile, has plasma t½ 11– 16 hours and duration of action 24 hours. Though erythromycin and ketoconazole increase its blood levels, but no arrhythmias have been observed. Dose: For allergic rhinitis 120 mg OD; for urticaria and other skin allergies 180 mg OD.

• Loratadine Another long-acting selective peripheral H1 antagonist which lacks CNS depressant effects and is fast acting. It is partly metabolized by CYP3A4 to an active metabolite with a longer t½ of 17 hr, but has not produced cardiac arrhythmia in overdose, though seizures are reported. No interaction with macrolides or antifungals has been noted. Good efficacy has been reported in urticaria and atopic dermatitis.

• Desloratadine It is the major active metabolite of loratadine effective at half the dose. Noninterference with psychomotor performance and cardiac safety are documented.

• Cetirizine It is a metabolite of hydroxyzine with marked affinity for peripheral H1 receptors; penetrates brain poorly, but subjective somnolence has been experienced at higher doses. It is not metabolized; does not prolong cardiac action potential or produce arrhythmias when given with erythromycin/ketoconazole.

• Cetirizine in addition inhibits release of histamine and of cytotoxic mediators from platelets as well as eosinophil chemotaxis during the secondary phase of the allergic response. Thus, it may benefit allergic disorders by other actions as well. It attains high and longer lasting concentration in skin, which may be responsible for superior efficacy in urticaria/atopic dermatitis, as well as for once daily dosing despite elimination t½ of 7- 10 hr. It is indicated in upper respiratory allergies, pollinosis, urticaria and atopic dermatitis; also used as adjuvant in seasonal asthma.

• Levocetirizine is the active R(–) enantiomer of cetirizine. It is effective at half the dose and appears to produce few side effects.

• Azelastine This newer H1 blocker has good topical activity; in addition, inhibits histamine release and inflammatory reaction triggered by LTs and PAF; and has bronchodilator property. After intranasal application it has been shown to down regulate intracellular adhesion molecule-1 (ICAM-1) expression on nasal mucosa. Its t½ is 24 hr, but action lasts longer due to active metabolite. Its metabolism is inhibited by CYP 3A4 inhibitors. Given by nasal spray for seasonal and perennial allergic rhinitis it provides quick symptomatic relief lasting 12 hr. Stinging in the nose and altered taste perception are the local side effects. Some somnolence has been reported on nasal application and a tendency to weight gain noted after oral use.

• Mizolastine This nonsedating antihistaminic is effective in allergic rhinitis and urticaria by single daily dosing despite a t½ of 8–10 hr and no active metabolite.

• Ebastine Another newer SGA that rapidly gets converted to the active metabolite carbastine having a t½ of 10–16 hr. It is nonsedating and active in nasal and skin allergies. Animal studies. have found it to prolong Q-Tc interval which makes it liable to arrhythmogenic potential and CYP3A4 interaction, but actual reports are still few.

• Rupatadine This recently introduced antihistaminic has additional PAF antagonistic property and is indicated in allergic rhinitis.

USES

• The uses of H1 antihistaminics are based on their ability to block certain effects of histamine released endogenously, as well as on sedative and anticholinergic properties.

• Allergic disorders Antihistaminics do not suppress AG: AB reaction but block the effects of released histamine—are only palliative. They effectively control certain immediate type of allergies, e.g., itching, urticaria, seasonal hay fever, allergic conjunctivitis and angioedema of lips, eyelids, etc. However, their action is slow—Adr alone is lifesaving in laryngeal angioedema. Similarly, they cannot be relied upon in anaphylactic shock and have a secondary place to Adr. Benefits are less marked in perennial vasomotor rhinitis, atopic dermatitis and chronic urticarias; combination with an H2 antagonist succeeds in some cases of chronic urticaria not responding to H1 antagonist alone.

1. Leukotrienes (C4, D4) and PAF are more important mediators than histamine. (ii) 
2. Concentration of antihistamines attained at the site may not be sufficient to block high concentration of histamine released locally in the bronchi.

• Certain newer compounds like cetirizine have adjuvant role in seasonal asthma.

• Antihistaminics are also ineffective in other types of humoral and cell mediated allergies because histamine is not involved. They do suppress urticaria and swellings in serum sickness, but not other components of the syndrome.

• Type I hypersensitivity reactions to drugs (except asthma and anaphylaxis) are suppressed. Some skin rashes also respond.

• Other conditions involving histamine Antihistaminics block symptoms produced by histamine liberators; afford symptomatic relief in insect bite and ivy poisoning. Abnormal dermographism is suppressed. They have prophylactic value in blood/saline infusion induced rigor.

• Pruritides Many conventional antihistamines have antipruritic action independent of H1 antagonism. Though relief is often incomplete, older antihistaminics remain the first-choice drugs for idiopathic pruritus.

• Common cold Antihistaminics do not affect the course of the illness but may afford symptomatic relief by anticholinergic (reduce rhinorrhoea) and sedative actions. The newer nonsedating antihistamines are less effective in this respect.

• Motion sickness Promethazine, diphenhydramine, dimenhydrinate and cyclizine have prophylactic value in milder types of motion sickness; should be taken one hour before starting journey. Promethazine can also be used in morning sickness, drug induced and postoperative vomiting, radiation sickness. Cyproheptadine has appetite stimulating effect; has been used in underweight children.

• Vertigo Cinnarizine is the H1 antihistamine having additional anticholinergic, anti-5-HT, sedative and vasodilator properties which has been widely used in vertigo. It modulates Ca2+ fluxes and attenuates vasoconstrictor action of many endogenous substances. Cinnarizine inhibits vestibular sensory nuclei in the inner ear, suppresses postrotatory labyrinthine reflexes, possibly by reducing stimulated influx of Ca2+ from endolymph into the vestibular sensory cells. Beneficial effects have been reported in Méniére’s disease and other types of vertigo. Side effects are sedation and mild g.i. upset.

DRUGS FOR VERTIGO

• The therapy of vertigo occurring in Méniére’s disease and other conditions is imperfect. A variety of approaches have been tried and have met with partial success.

• Labyrinthine suppressants They suppress endorgan receptors or inhibit central cholinergic pathway (in vestibular nuclei).

1. Antihistaminics (with anticholinergic action)— cinnarizine, cyclizine, dimenhydrinate, diphenhydramine, promethazine.
2. Anticholinergics—atropine, hyoscine. 
3. Antiemetic phenothiazines—prochlorperazine, trimethylpyridine.

• Vasodilators They improve blood flow to labyrinth and brainstem—betahistine, codergocrine, nicotinic acid, naftidrofuryl.

• Diuretics They decrease labyrinthine fluid pressure —acetazolamide, thiazides, furosemide.

• Anxiolytics, antidepressants These drugs appear to modify the sensation of vertigo—diazepam, amitriptyline.

• Corticosteroids They suppress intralabyrinthine edema due to viral infection or other causes. Parenteral prochlorperazine is the most effective drug for controlling violent vertigo and vomiting.

• Preanaesthetic medication Promethazine has been used for its anticholinergic and sedative properties.

• Cough Antihistaminics like chlorpheniramine, diphenhydramine and promethazine are constituents of many popular cough remedies. They have no selective cough suppressant action but may afford symptomatic relief by sedative and anticholinergic property.

• Parkinsonism Promethazine and some others afford mild symptomatic relief in early cases— based on anticholinergic and sedative property.

• Acute muscle dystonia Caused by antidopaminergic-antipsychotic drugs is promptly relieved by parenteral promethazine or hydroxyzine. This is again based on central anticholinergic action of the drugs.

• As sedative, hypnotic, anxiolytic Antihistamines with CNS depressant action have been used as sedative and to induce sleep, especially in children. However, promethazine has produced serious respiratory depression in young children; few deaths are on record; it is not indicated in children aged 2 years or less. For promoting sleep, antihistaminics are not as dependable as benzodiazepines. Hydroxyzine has been used in anxiety associated with autonomic manifestations.

• H2 antagonist the first H2 blocker Burimamide was developed by Black in 1972. Metiamide was the next, but both were not found suitable for clinical use. Cimetidine was introduced in 1977 and gained wide usage. Ranitidine, famotidine, roxatidine, and many others have been added subsequently. They are primarily used in peptic ulcer and other gastric hypersecretory states and are described.

• H3 antagonist Though a selective H3 antagonist thioperamide has been produced, it has not found any clinical utility