Student: Roxana Goga

Coordinators:

Teaching assistant Aura Rusu, PhD Pharm

LecturerGabriel Hancu, PhD Pharm

New directions for the development of the H1-antihistamines

Faculty of PharmacyUniversity of Medicine and Pharmacy from Targu Mures

General considerations

Histamine:

= imidazole-5-β-ethylamine, small molecule, obtained from histidine with histidine decarboxylase (diamine oxidase), PLP dependent enzyme.

- major mediator in allergic diseases and inflammatory processes.- physiological functions: differentiation and proliferation of cells, embryonic

development, hematopoiesis, wound healing.

N

NH

H2N-H2C-H2CN

NH

H2N-HC-H2C

COOH

- CO2

Histidine Histamine

General considerations

Histamine:o Role in immediate-type hypersensitivity responses and delayed-type allergic reactions;o Increased concentrations at sites of allergic reactions and inflammation;o Histamine interferes with the Th1/Th2 balance (via a reduction in Th1 cell proliferation

and an enhanced secretion of IL-10 in T cells);o Modulates the development of allergies by upregulation of Th2 cell proliferation and

increased production of Th2 cytokines IL-4, IL-5, IL-10 and IL-13.

Brief history

1910: scientists Henry Dale and Patrick Laidlaw discovered that the human body produces certain chemicals in the allergic reactions;

1937:Etienne Fourneau gave the first antihistamine “thymo-ethyl-diethylamine”; researchers Daniel Bovet and Anne-Marie Staub determined the substance was too weak to be effective and too toxic to test on humans;

1942:Bernard N. Halpern began the first clinical trials on humans of phenbenzamine. Indications: allergic rhinitis & asthma, urticaria, blood conservation;

1955: extensive research into different antihistamines and their possible uses; 1981: scientists release 2nd generation AHs; 1986: several types of AHs were linked with cardiac problems; 1990s:scientists cloned several antihistamine formulas to create more members of

the second generation.

Physico-chemical properties

The first and second generation AH - substantially more lipophilic than the endogenous agonist histamine (or the H2-antagonists); posses low molecular weight;

Improved pharmacology of the second-generation zwitterionic drugs: new carboxylic moiety, in combination with the protonated-amine significantly reduces brain permeability;

Introduction of the carboxyl moiety improves the H1R selectivity;

3.1 Å resolution structure of the H1R-T4 lysozyme fusion protein (H1R-T4L) complex with doxepin;

The crystal structure reveals the atomic details of doxepin binding and its inverse agonistic activity;

The H1R crystal structure and the models of second-generation H1R antagonists will be highly beneficial for guiding rational design of ligands that do not penetrate the BBB while maintaining H1selectivity;

Classification according to the chemical structure

Chemical classFunctional class

First (old) generation Second (new) generation

AlkylaminesBrompheniramine, Chlorpheniramine, Dimethindene, Pheniramine, Triprolidine

Acrivastine

Piperazines Buclizine, Cyclizine, Hydroxizine, Meclizine, Oxatomidet

Cetirizine, Levocetirizine

Piperidines Azatadine, Cyproheptadine, Diphenylpyraline, Ketotifen

Astemizole, Desloratadine,Ebastine, Terfenadine, Fenoxfenadine, Loratadine, Levocabastine, Mizolastine

EthanolaminesCarbinoxamine, Clemastine, Dimenhydrinate, Diphenhydramine, Doxylamine, Phenyltoloxamine

——

Ethylendiamines Antazoline, Pyrilamine, Tripelenamine ——

Phenohtiazines Methdilazine, Promethazine ——

Other DoxepinAzelastine, Emedastine, Epinestine, Olopatadine

Mechanism of actionClassic theory

Histamine receptor HR1

Cell tissue expression

Nerve cells, airway and vascular smooth muscles, endothelial cells,hepatocytes, epithelial cells, neutrophils, eosinophils, monocytes, DC, T and B cells.

Activated intracellular signal

Main signaling: enhanced Ca2+

Others: PhLC, PhLD, cGMP, PhLA, NFκB

G protein Gq/11

When histamine binds to HR1, a stimulus and conformational change of this receptor occurs:

1.PLC, activating PKC and separating the dimer formed by 1-ĸB and NFĸB. This results in the release of NFĸB, which enters the cell nucleus and stimulates the activation of the codifying genes of inflammatory mediators; 2. the pathway of Gβγ protein separates the dimer composed by 1-ĸB and NFĸB.

Mechanism of actionInverse agonists

The H1-antihistamines have recently been reclassified as inverse agonists, rather than H1-receptor antagonists;

According to the two-state model, the active and inactive states of a receptor exist in equilibrium, so antihistamines could act as inverse agonists that combine and stabilize the inactive form of H1R to shift the equilibrium toward the inactive state and down-regulate constitutive receptor activity, even in the absence of histamine . On the basis of the two-state model, ligands may now be reclassified as agonists, both full and partial, which stabilize the active state R* and increase receptor signaling. Inverse agonists, full and partial, stabilize the inactive state R and decrease basal receptor signaling.

Leurs R, Church MK, Taglialatela M. H1-antihistamines: inverse agonism, anti-inflammatory actions and cardiac effects. Clin Exp Allergy. 2002;32489-98

Neutral antagonists, which have equal affinity for both R and R* and do not affect the equilibrium between the two states, reduce the ability of both agonists and inverse agonists to bind to the receptor.

Mechanism of actionInverse agonists

The constitutional activity of H1receptors is not restricted to the activation of phospholipase C (PLC); it also activates the entire genetic transcription mediated by the kappa B nuclear factor (NFκB);

The constitutional activity of the H1 receptor mediating NFκB activation was inhibited by all the antihistamines, including cetirizine, ebastine, epinastine, fexofenadine, loratadine, and mezolastine. This indicates that all these agents act as inverse agonists.

1. Molimard M, Diquet B, Benedetti MS. Comparison of pharmacokinetics and metabolism of desloratadine, fexofenadine, levocetirizine and mizolastine in humans. Fundam Clin Pharmacol. 2004;18:399-411.2. Bakker RA, Schoonus SB, Smit MJ, Timmerman H, Leurs R. Histamine H(1)-receptor activation of nuclear factor-kappa B: roles for G beta gamma- and G alpha(q/11)-subunits in constitutive and agonistmediated signaling. Mol Pharmacol. 2001;60:1133-42.3.Hill SJ, Ganelin CR, Timmerman H, Schwartz JC, Shankley NP, Young JM, et al. International Union of Pharmacology. XIII. Classification of histamine receptors. Pharmacol Rev. 1997;49:253-78.4.Milligan G, Bond RA, Lee M. Inverse agonism: pharmacological curiosity or potential therapeutic strategy? Trends Pharmacol Sci. 1995;16:10-3.

Differences between first and second-generation H1 antihistamines

First-generation H1 antihistamines Second-generation H1 antihistamines

Usually administered in three to four daily doses Usually administrated once, twice a day

Cross the brain-blood barier (lipofilicity, low molecular weight, lack of recognition by the P-glycoprotein efflux pump)

Do not cross the brain-blood barrier (lipophobicity, high molecular weight, recognition by the P-glycoprotein efflux pump)

Potentially cause side effects (sedation/hyperactivity/ insomnia/convulsions)

Do not cause relevant side effects (sedation/ fatigue/hyperactivity/convulsions), in the absence of drug interactions

Case reports of toxicity are regularly published No reports of serious toxicity

No randomized, double-blind, placebo-controlled trials in children

Some randomized, double-blind, placebo controlled studies in children

Lethal dose identified for infants/young children Do not cause fatality in overdose

de Benedictis FM, de Benedictis D, Canonica GW. New oral H1 antihistamines in children: facts and unmeet needs. Allergy. 2008;63:1395-1404

First generationCyproheptadine

applicable as an H1 and 5-HT receptor antagonist for the treatment of

allergic reactions, prophylactic for migraines and in the symptomatic treatment of metastatic carcinoid syndrome;

is thought to resemble clozapine ;

Similar structure with antidepressants: amitriptyline, imipramine and N-methylamitriptyline CPH might play a role in CNS;

acts as antagonist for serotonergic, dopaminergic, histaminergic, adrenergic or muscarinic receptors due to its relatively high binding affinity;

directly inhibits ion channels: potassium (K+), sodium (Na+) and N-type and L-type calcium (Ca2+) channels in cardiac cells;

1.Droogmans S, Roosens B, Cosyns B, Degaillier C, Hernot S, et al. Cyproheptadine prevents pergolide-induced valvulopathy in rats: an echocardiographic and histopathological study. Am J Physiol Heart Circ Physiol 2009; 296:H1940–1948.2. Goudie AJ, Cooper GD, Cole JC, Sumnall HR. Cyproheptadine resembles clozapine in vivo following both acute and chronic administration in rats. J Psychopharmacol 2007; 21: 179–190.3.Spangler DL, Brunton S. Efficacy and central nervous system impairment of newer-generation prescription antihistamines in seasonal allergic rhinitis.South Med 2006; J 99: 593–599.

First generationCyproheptadine

Recent studies:

─ CPH induces apoptosis in leukemia cells, which is independent of its activity as a histamine and serotonin receptor antagonist;

─ the diabetogenic effect of CPH may be at least partially due to altered regulation of protein synthesis independent effect of serotonergic or histaminergic receptors;

─ CPH may act as an intracellular signaling molecule by binding to intracellular sites, such as one reported on sigma-1 receptor ;

─ CPH enhances the IK in cultured cortical neurons through a Gi dependent cAMP/PKA pathway via activation of the sigma-1 receptor.

1.Codd EE, Baker J, Brandt MR, Bryant S, Cai C, et al. Diabetogenic effect of a series of tricyclic delta opioid agonists structurally related to cyproheptadine. Toxicol Sci, 2010; 117: 493–504.2. Mao X, Liang SB, Hurren R, Gronda M, Chow S, et al. Cyproheptadine displays preclinical activity in myeloma and leukemia. Blood, 2008; 112: 760–769.3.Pal A, Hajipour AR, Fontanilla D, Ramachandran S, Chu UB, et al. Identification of regions of the sigma-1 receptor ligand binding site using a novel photoprobe. Mol Pharmacol, 2007; 72: 921–933.

First generationHydroxizine

A first generation AH of the diphenylmethane and piperazine class; used for the treatment of anxiety in children with equal efficacy to chlorodiazepoxide;

Recent studies:

relate the Hydroxizine efficacy in bruxism; bruxism leads to the destruction of teeth or some other parts; the etiology is not clear;

Although the mechanism of action is not clear, hydroxyzine is worthy of consideration for treatment of sleep bruxism.

the possible pharmacological link between hydroxizine’s antidopaminergic and disturbances in the central dopaminergic system explains its efficiency in the treatment of bruxism.

• LAVIGNEGJ, KHOURYS, ABES, YAMAGUCHI T, RAPHAEL K., Bruxism physiology and pathology: an overview, EuropeanReviewforMedical and Pharmacological Sciences, 2013;17:839-841

First generationCyclizine

Cyclizine (1-benzhydryl-4 methylpiperazine): piperazine derivative;

may affect the upregulation of the number and function of β2-receptors;

Recent studies: Antiinflammatory effects may occur as a consequence of

their ability to influence the activation of genes that regulate the production of pro-inflammatory cytokines and adherent proteins (barely clear mechanism).

The cationic amphophilic nature of most H1-antihistamines suggests that they may form an association with cell membranes and inhibit the calcium binding and the membrane associated enzymes activities.

Illicit activities: use of cyclizine combined with a methadone dose strong psychoactive effects; also been used recreationally for its anticholinergic effects to induce hallucinations.

1.Králová J, Nosál R, Drábiková K, Jancvinová V, Denev P and Moravcová A. Comparative investigations of the influence of H1-antihistamines on the generation of reactive oxygen species by phagocytes. Inflamm. Res. 2008; 57: S49-S502.ZarrindastMR, Malekmohamadi N, Fazli Tabaei S and Ahmadi S. Intracerebroventricular injection of histamine induces state-dependency through H1 receptors. Iranian J. Pharm. Res. 2006 5: 255-260.

Second generationAstemizole

second-generation selective histamine H1-receptor antagonist used to treat the benign of seasonal allergic rhinitis in a chronic setting;

possesses antifungal and antimalarial activity; can be used for in vivo brain imaging in Alzheimer’s disease because of its affinity for Tau fibrils and good brain penetration and persistence;

scientists proposed a mechanism of action in prion diseases: inhibits prion propagation by a mechanism other than cell surface PrP reduction (investigations were performed at the prion inhibitory doses);

stimulates autophagy (involved in several protein misfolding neurodegenerative diseases and enhances autophagy counteracts cellular prion infection );

antiprion activity is linked to its activity on autophagy;

1. Deeb KK, Trump DL, Johnson CS Vitamin D signalling pathways in cancer: potential for anticancer therapeutics. Nat Rev Cancer, 2007; 7: 684–700.2. Beer TM, Myrthue A Calcitriol in cancer treatment: from the lab to the clinic. Mol Cancer Ther, 2004; 3: 373–381.3. Hansen CM, Binderup L, Hamberg KJ, Carlberg C Vitamin D and cancer: effects of 1,25(OH)2D3 and its analogs on growth control and tumorigenesis. Front Biosci, 2001; 6: D820–84

Second generationAstemizole

Recent studies:

Combined with calcitriol new promising antineoplasic drug;

Synergizes calcitriol antiproliferative activity by: down regulating CYP24A1, up-regulating VDR and blocking Eag1 activity. Cell proliferation: stimulated by ion permeation through Eag1 channels. Eag1 gene expression and activity : blocked by calcitriol and astemizole;

Down-regulates CYP24A1 mRNA expression, allowing calcitriol to evade catabolism.

Up-regulates VDR expression favoring calcitriol biological effects. Upon activation by calcitriol, the VDR forms heterodimerswith the RXR, binds to vitamin D response elements (VDRE) on DNA resulting in expression or transrepression of specific gene products.

Second generationCetirizine and Levocetirine

Cetirizine (Zyrtec), is a major metabolite of hydroxyzine, a first generation antihistamine.

It is a D-stereoisomer.

L-stereoisomer is levocetirizine, a third generation antihistamine.

Loratadine and Desloratadine

Loratadine (Claritin), is an allergic medication for rhinitis, urticaria, runny nose;

Its active metabolite, desloratadine (Clarinex) is a third generation H1-antihistamine;

They are different from each other only by the COOC2H5 group.

Second and “Third” generation

More recent improvements, generally in the form of active metabolites, led to the use of the term Third-generation antihistamines. This term emerged spontaneously, with no clear definition of its meaning or clinical implications, creating great confusion among healthcare professionals.

On the basis of the evidence on H1 antihistamines, none of them deserve the title Third-generation antihistamine. � � As the Consensus Group on New Generation Antihistamines concluded that, to merit this definition, a new class of antihistamines would have to demonstrate distinct clinical advantages over existing compounds and fulfill at least three prerequisites: they should be free from cardiotoxicity, drug interactions and effects on the CNS.

Holgate ST, Canonica GW, Simons FE, Taglialatela M, Tharp M, Timmerman H, et al. Consensus Group on New-Generation Antihistamines (CONGA): present status and recommendations.Clin Exp Allergy. 2003;33:1305-24

Other pharmaceutical forms

BilastinaApproved in Europe, Mexico, BrasilEMEA status: approvedFDA status: approved

Rupatadine (fumarate)Pharmaceutica forms:- Tablets- Granules- Oral liquidEMEA status: approvedFDA status: approved

Azelastina HydrochlorideEMEA status: approvedFDA status: approved

Other pharmaceutical forms

Ebastine Ph. Eur. 8:Approved in Europe (Spain), but in some countries may only be approved for vetenary use.Pharmaceutical forms:- Tabletes- Oral solution

Olopatadine HydroclorideEMEA status: approvedFDA status: approved

Cyanoguanidine derivates

Cyanoguanidine derivatives of loratadine: synthesized/screened for antitumor/anti-inflammatory activity.

The most promising compound (R = n-C8H17) possessed at least twofold higher in vitro cytotoxicity than 5-fluorouracil against mammary as well as colon carcinoma cells.

mode of action: participation of COX-1/2 enzymes on the cytotoxicity - very unlikely. All compounds: stronger in vivo anti-inflammatory activity than ibuprofen in the xylene-induced

ear swelling assay in mice.

SynthesisSynthetic pathway for cyanoguanidines

- mepyramine (pA29.07 ± 0.03) is more potent than pheniramine-like H1R antagonists;

- The H1R antagonist pharmacophoric moiety is connected via a carbon chain (spacer) of variable length and a cyanoguanidine group with the H2R antagonist partial structure of roxatidine, tiotidine, and ranitidine.

SynthesisSynthetic pathway for cyanoguanidines

Properties of cyanoguanidine compounds

Compounds 14–35showed histamine H1R antagonistic activities with pKB from 6.8 (compound 14) to 8.6 (compounds 28, 32);

Increased activity with the length of the polymethylene spacer in case of the methoxy-substituted derivatives bearing a roxatidine-like H2-antagonist moiety (compound 14vs. 16, 18and 21);

Affinity-increasing effect was not evident; interactions with the H1R were not dramatically affected the H2R antagonist moiety;

The most potent H1R antagonists achieved KB values in the one-digit nanomolar-range corresponding to about 30% of the activity of mepyramineat.

1.Seifert, R.; Strasser, A.; Schneider, E.H.; Neumann, D.; Dove, S.; Buschauer, A. Molecular and cellular analysis of human histamine receptor subtypes. Trends Pharmacol. Sci. 2013, 34, 33–58. 2.Strasser, A.; Wittmann, H.-J.; Buschauer, A.; Schneider, E.H.; Seifert, R. Species-dependent activities of GPCR ligands: Lessons from histamine receptor orthologs. Trends Pharmacol. Sci. 2013, 34, 13–32.

Future directions

Task for future developments: optimize therapy by rational and combined use of available drugs (steroid sprays and non-sedating antihistamines).

Additional comparative PET studies of first/second generation H1 antihistamines in order to correlate BBB penetration and CNS H1-receptor occupancy with CNS functional effects.

For establishing guidelines in treatment of chronic urticaria, studies should focus on choosing clinically relevant and reproducible primary outcomes, long-term follow-up, limited use of placebo and avoiding spin strategies.

Norastemizole and Descarboethoxy loratadine are in clinical development comparable or superior clinical benefits.

AH are now considered to be anti-inflammatory; although less potent than glucocorticosteroids the beneficial anti-allergic effects related to their inverse agonist anti-inflammatory effects are increasingly being appreciated.

These results suggest that inverse agonists alleviate allergy symptoms not only by inhibiting stimulus-induced up-regulation of H1R gene expression but also by suppressing basal histamine signaling through their inverse agonistic activity.

Conclusion

Despite of the fact that the majority of the H1-antihistamines have similar efficacy in the allergic diseases, after extensively research, we can conclude that the second generation of antihistamines constitute the medication of choice for the treatment of allergic rhinitis, allergic conjunctivitis and chronic urticaria.

New H1-antihistamines continue to be developed and introduced in therapy. Some of these medications might also have the intrinsic ability to down-regulate histamine at H2, H3, or H4 receptors or to down-regulate leukotrienes or cytokines.

Thank you for your attention!!!