Histamine H₄ receptor: a novel target for inflammation therapy.

Histamine, a low molecular weight amine has been extensively studied for its various pharmacological profiles. Until recently histamine was thought to act on three receptors - H₁, H₂ and H₃. Merely a decade back, sequencing of human genome has revealed a new histamine receptor - H₄ receptor. This 390 amino acid sequenced receptor has around 38% homology with histamine H₃ receptor besides; the pharmacological profile of the protein is quite different from other histamine receptors. H₄ receptor is mainly expressed in mast cells and leukocytes and involves various physiological functions related to inflammation and allergy. Potent selective H₄ receptor agonists and antagonists have been synthesized and in vivo studies have indicated their action on H₄ receptor. In this review, structure, expression, homology sequence of H₄ receptor among the different species has been documented. Further, structure activity relationship (SAR) of H₄ ligands on the basis of their nucleus has been discussed in depth. In addition, anti-inflammatory effects of H₄ receptor antagonists, with special emphasis to JNJ7777120, a selective H₄ receptor antagonist have been focused exhaustively.