Molecular determinants of selective agonist and antagonist binding to the histamine H₄ receptor

The deorphanization of the histamine H? receptor (H?R) has led to a significant number of scientific publications and patent applications. Whereas some histamine H?, H? and H? receptor ligands were found to have significant affinity for H?R, several agonists and antagonists with high affinity for H?R and selectivity over the other histamine receptors were successfully designed and synthesized. Moreover, site-directed mutation studies on H?R have been performed and reveal detailed information on receptor-ligand interactions. This review will focus on the most important H?R ligand scaffolds and their structure-activity relationships and selectivity over other histamine receptors and specific H?R functional activity. Experimental data are used to construct and validate high resolution three-dimensional receptor-ligand models and, vice versa, in silico models are used to design and rationalize experimental studies to probe receptor-ligand interactions.     

Strain-dependent effects of the histamine H₄ receptor antagonist JNJ7777120 in a murine model of acute skin inflammation

The effects of the histamine H(4) receptor antagonist JNJ7777120 were evaluated in a model of acute skin inflammation induced by local application of croton oil. The influence of strain on the effect of JNJ7777120 was investigated in four different mouse strains (CD-1, NMRI, BALB/c and C57BL/6J). In CD-1 mice, JNJ777720 (30-100 mg/kg subcutaneously, s.c.) exerted a dose-dependent inhibition of croton oil-induced ear inflammation and polymorphonuclear leucocyte infiltration, as confirmed by histological evaluation of ear tissues. JNJ7777120 (30-100 mg/kg) did not reduce ear oedema in NMRI, BALB/c or C57BL/6J mice. The positive control, dexamethasone (2 mg/kg s.c.) induced significant anti-inflammatory effects only in CD-1 and NMRI mice. In these strains, also the histamine H(1) -receptor blocker pyrilamine (30 mg/kg s.c.) significantly reduced ear oedema at 2 h after croton oil challenge, being as effective as JNJ7777120 in CD-1 mice. Taken together, these data demonstrate that the H(4) receptor antagonist JNJ7777120 may reduce acute croton oil-induced skin inflammation as effectively as H(1) receptor blockade. However, present experiments evidenced for the first time marked strain-related differences in the JNJ7777120 pharmacological activity, which have to be carefully considered when using this ligand to characterize histamine H(4) receptor functions in murine models and translating preclinical data to clinical human settings.     

Constitutive activity of histamine h(3) receptors stably expressed in SK-N-MC cells: display of agonism and inverse agonism by H(3) antagonists.

Agonist-independent activity of G-protein-coupled receptor, also referred to as constitutive activity, is a well-documented phenomenon and has been reported recently for both the histamine H(1) and H(2) receptors. Using SK-N-MC cell lines stably expressing the human and rat H(3) receptors at physiological receptor densities (500-600 fmol/mg of protein), we show that both the rat and human H(3) receptors show a high degree of constitutive activity. The forskolin-mediated cAMP production in SK-N-MC cells is inhibited strongly upon expression of the G(i)-coupled H(3) receptor. The cAMP production can be further inhibited upon agonist stimulation of the H(3) receptor and can be enhanced by a variety of H(3) antagonists acting as inverse agonists at the H(3) receptor. Thioperamide, clobenpropit, and iodophenpropit raise the cAMP levels in SK-N-MC cells with potencies that match their receptor binding affinities. Surprisingly, impentamine and burimamide act as effective H(3) agonists. Modification of the amine group of impentamine dramatically affected the pharmacological activity of the ligand. Receptor affinity was reduced slightly for most impentamine analogs, but the functional activity of the ligands varied from agonist to neutral antagonist and inverse agonist, indicating that subtle changes in the chemical structures of impentamine analogs have major impact on the (de)activation steps of the H(3) receptor. In conclusion, upon stable expression of the rat and human H(3) receptor in SK-N-MC cells constitutive receptor activity is detected. In this experimental system, H(3) receptors ligands, previously identified as H(3) antagonists, cover the whole spectrum of pharmacological activities, ranging from full inverse agonists to agonists.     

Radiosynthesis and biodistribution of 123I-labeled antagonists of the histamine H3 receptor as potential SPECT ligands

We have synthesized three 123I-labeled histamine H3 receptor ligands, i.e., [123I]GR 190028, [123I]FUB 271, and [123I]iodoproxyfan, in moderate to good radiochemical yields via a Cu+-assisted I-for-123I exchange method. Biodistribution in the rat of these compounds revealed high hepatic and pulmonary uptake. Brain uptake was moderate, but for [123I]iodoproxyfan, brain uptake was high enough for a pilot single photon emission computed tomography (SPECT) study in the rabbit. However, for this compound, the cerebral uptake could not be blocked by a pretreatment with [R]-alpha-methylhistamine, a selective, high-affinity histamine H3 receptor agonist, both in the SPECT study in the rabbit and in the biodistribution study in the rat. Apparently, [123I]iodoproxyfan is binding to a non-H3 receptor binding site. None of the three investigated compounds is suitable for use as a SPECT ligand for the H3 receptor in the brain.     

Drug-induced thrombocytopenia is associated with increased binding of IgG to platelets both in vivo and in vitro

Thrombocytopenia is a common serious adverse effect of drug treatment. A variety of in vitro diagnostic techniques to confirm the diagnosis are available, but the majority lack sufficient sensitivity to detect all cases of drug-induced thrombocytopenia. We studied 19 patients with suspected drug-induced thrombocytopenia and demonstrated that platelet- associated IgG (PAIgG) was elevated in all at the time of thrombocytopenia, and PAIgG returned to normal levels as the thrombocytopenia resolved. In the majority of patients, the platelet count rapidly returned to normal after the drug was discontinued; however, in six patients, the thrombocytopenia persisted well beyond the period of time that the offending drug would be expected to be cleared from the blood. In 13 patients, serum obtained after recovery was used to identify the drug responsible for the thrombocytopenia in an in vitro assay. In all cases, the addition of the drug historically associated with the thrombocytopenic episode was associated with an increased binding of IgG to control platelets. For uncertain reasons, the concentration of drug required to increase the in vitro binding of IgG to test platelets was often more than the concentration usually achieved in vivo. Wider application of these techniques may provide better understanding of the clinical characteristics and mechanisms responsible for drug-induce thrombocytopenia.     

Identification ofβ 2-adrenoceptors on guinea pig alveolar macrophages using (−)-3-[125I]iodocyanopindolol

The -adrenoceptor antagonist (–)-3-[¹²⁵I]iodocyanopindolol ([¹²⁵I]ICYP) binds with high affinity and in a saturable way to membranes of guinea pig alveolar macrophages. The equilibrium dissociation constant for [¹²⁵I]ICYP is 24.3 ± 1.2 pM, and the number of binding sites is 166.3 ± 13.7 fmol/mg protein (N=4, ±SEM). Displacement studies with selective antagonists showed that [¹²⁵I]ICYP labels ₂-adrenoceptors on guinea pig alveolar macrophages.     

Homologous histamine H1 receptor desensitization results in reduction of H1 receptor agonist efficacy.

Prolonged exposure of the guinea-pig intestinal longitudinal smooth muscle to histamine caused homologous desensitization of the H1 receptor, which led to reduced H1 receptor-mediated production of [3H]inositol phosphates as well as to reduced H1 agonist-induced contractions. [3H]Mepyramine binding studies showed that desensitization affected neither the agonist affinity nor the number of H1 receptors. Combining the data from the binding studies and the contraction measurements it was found that desensitization results in a selective reduction of agonist efficacy.