Inverse agonists of H1-receptors as promising antiallergy agents (a review)

In Silico Analysis, Synthesis, and Biological Evaluation of Triazole Derivatives as H1 Receptor Antagonist

INTRODUCTION

Histamine, a biological amine, is considered as a principal mediator of many pathological processes regulating several essential events in allergies and autoimmune diseases. Numerous derivatives have been developed that strive with histamine at the H1 receptor and prevent binding of histamine at the H1 receptor, thereby preventing allergic reactions. Molecules containing a triazole ring fused with six-membered ring systems are found to possess broad applications in the field of medicine and industry. The present study is an attempt to characterize the impact of the nature of the substituent introduced at 5 positions of the-4H-1,2,4-triazole-3-thiol on their capacities to bind with the H1 receptor.

METHODS

Molecular docking (PDB ID: 3RZE) revealed that synthesized derivatives and target proteins were actively involved in binding with Tyr-108, Thr-112, Ala-216, and Phe-432 subunits. A pharmacophore model, new 5-(4-substituted phenyl)-4-(phenylamino)-4-H-1,2,4-triazole-3- thiols (5a-5h) were designed and evaluated for H1-blocking activity using isolated segments from the guinea pig ileum.

RESULTS

According to in silico analysis, all the compounds have a topological polar surface area (TPSA) less than 140 Å squared, so they tend to easily penetrate cell membranes. The results show that most of the compounds are non-inhibitors of CYP450 substrates that play a fundamental role in drug metabolism. Compounds 5d (50.53±12.03), 5h (50.62±12.33) and 7a (55.07±12.41) are more active than others.

CONCLUSION

Finally, these derivatives were screened for H1 receptor antagonist activity using guinea pig ileum, taking chlorpheniramine maleate as a standard. Most of the compounds were found to possess better antihistamine activity.

Design of Phthalazinone Amide Histamine H1 Receptor Antagonists for Use in Rhinitis

The synthesis of potent amide-containing phthalazinone H₁ histamine receptor antagonists is described. Three analogues 3e, 3g, and 9g were equipotent with azelastine and were longer-acting in vitro. Amide 3g had low oral bioavailability, low brain-penetration, high metabolic clearance, and long duration of action in vivo, and it was suitable for once-daily dosing intranasally, with a predicted dose for humans of approximately 0.5 mg per day.

Why are second-generation H1-antihistamines minimally sedating?

H1-antihistamines are widely used in treating allergic disorders, e.g., conjunctivitis, urticaria, dermatitis and asthma. The first-generation H1-antihistamines have a much greater sedative effect than the second-generation H1-antihistamines. Researchers could not offer a satisfactory explanations until late 1990s when studies showed that second-generation H1-antihistamines were substrates of P-glycoprotein. P-glycoprotein, expressed in the blood-brain barrier, acts as an efflux pump to decrease the concentration of H1-antihistamines in the brain, which minimizes drug effects on the central nervous system and results in less sedation. P-glycoprotein is found in the apical side of the epithelium. It consists of transmembrane domains that bind substrates/drugs and nucleotide-binding domains that bind and hydrolyze ATP to generate energy for the drug efflux. This review mainly discusses interactions between P-glycoprotein and commonly used second-generation H1-antihistamines. In addition, it describes other possible determining factors of minimal sedating properties of second-generation H1-antihistamines.

Synthesis and pharmacological investigation of azaphthalazinone human histamine H(1) receptor antagonists

5-Aza, 6-aza, 7-aza and 8-aza-phthalazinone, and 5,8-diazaphthalazinone templates were synthesised by stereoselective routes starting from the appropriate pyridine/pyrazine dicarboxylic acids by activation with CDI, reaction with 4-chlorophenyl acetate ester enolate to give a β-ketoester, which was hydrolysed, and decarboxylated. The resulting ketone was condensed with hydrazine to form the azaphthalazinone core. The azaphthalazinone cores were alkylated with N-Boc-D-prolinol at N-2 by Mitsunobu reaction, de-protected, and then alkylated at the pyrrolidine nitrogen to provide the target H(1) receptor antagonists. All four mono-azaphthalazinone series had higher affinity (pK(i)) for the human H(1) receptor than azelastine, but were not as potent as the parent non-aza phthalazinone. The 5,8-diazaphthalazinone was equipotent with azelastine. The least potent series were the 7-azaphthalazinones, whereas the 5-azaphthalazinones were the most lipophilic. The more hydrophilic series were the 8-aza series. Replacement of the N-methyl substituent on the pyrrolidine with the n-butyl group caused an increase in potency (pA(2)) and a corresponding increase in lipophilicity. Introduction of a β-ether oxygen in the n-butyl analogues (2-methoxyethyl group) decreased the H(1) pA(2) slightly, and increased the selectivity against hERG. The duration of action in vitro was longer in the 6-azaphthalazinone series. The more potent and selective 6-azaphthalazinone core was used to append an H(3) receptor antagonist fragment, and to convert the series into the long acting single-ligand, dual H(1) H(3) receptor antagonist 44. The pharmacological profile of 44 was very similar to our intranasal clinical candidate 1.