H. Timmerman

Molecular pharmacological aspects of histamine receptors.

In this article, we review the recent developments in the field of histamine research. Besides the description of pharmacological tools for the H1, H2 and H3 receptor, specific attention is paid to both the molecular aspects of the receptor proteins, including the recent cloning of the receptor genes, and their respective signal transduction mechanisms.

Therapeutic potential of histamine H3 receptor agonists and antagonists

The histamine H3 receptor was discovered 15 years ago, and many potent and selective H3 receptor agonists and antagonists have since been developed. Currently, much attention is being focused on the therapeutic potential of H3 receptor ligands. In this review, Rob Leurs, Patrizio Blandina, Clark Tedford and Henk Timmerman describe the available H3 receptor agonists and antagonists and their effects in a variety of pharmacological models in vitro and in vivo. The possible therapeutic applications of the various compounds are discussed.

Isothiourea analogues of histamine as potent agonists or antagonists of the histamine H3-receptor

The synthesis and H3-activity of a series of isothiourea analogues of histamine have been described. It has been shown that S-[2-(4(5)-imidazolyl)ethylisothiourea (VUF 8325) is a potent H3-agonist measured as the electrically evoked contraction of the guinea-pig ileum. Upon methylation of the imidazole system or the isothiourea moiety a decrease in affinity was observed leading to potent H3-antagonists. Particularly the 4-chlorobenzyl group appeared to be favourable in the series described resulting in a histamine H3-antagonist with a pA2-value of 9.9.

Mutational Analysis of the Antagonist-binding Site of the Histamine H1 Receptor

We combined in a previously derived three-dimensional model of the histamine H1 receptor (Ter Laak, A. M., Timmerman, H., Leurs, H., Nederkoorn, P. H. J., Smit, M. J., and Donne-Op den Kelder, G. M. (1995) J. Comp. Aid. Mol. Design. 9, 319–330) a pharmacophore for the H1 antagonist binding site (Ter Laak, A. M., Venhorst, J., Timmerman, H., and Donné-Op de Kelder, G. M. (1994) J. Med. Chem. 38, 3351–3360) with the known interacting amino acid residue Asp116 (in transmembrane domain III) of the H1 receptor and verified the predicted receptor-ligand interactions by site-directed mutagenesis. This resulted in the identification of the aromatic amino acids Trp167, Phe433, and Phe436 in transmembrane domains IV and VI of the H1 receptor as probable interaction points for the trans-aromatic ring of the H1 antagonists. Subsequently, a specific interaction of carboxylate moieties of two therapeutically important, zwitterionic H1 antagonists with Lys200 in transmembrane domain V was predicted. A Lys200 → Ala mutation results in a 50- (acrivastine) to 8-fold (d-cetirizine) loss of affinity of these zwitterionic antagonists. In contrast, the affinities of structural analogs of acrivastine and cetirizine lacking the carboxylate group, triprolidine and meclozine, respectively, are unaffected by the Lys200 → Ala mutation. These data strongly suggest that Lys200, unique for the H1receptor, acts as a specific anchor point for these “second generation” H1 antagonists.

Synthesis of some symmetrical curcumin derivatives and their antiinflammatory activity

Summary Curcumin is not only a frequently used food additive, but it is also a well-known constituent of Indonesian traditional medicines. Several beneficial effects are ascribed to curcumin, eg, its antiinflammatory properties. In order to study the antiinflammatory activity, a series of curcumin derivatives were prepared and the inhibition of the carrageenin-induced oedema by these compounds was established. It appeared that the para hydroxy groups in curcumin are important for antiinflammatory activity. This activity is enhanced when, in combination with the para hydroxy groups, the meta positions are occupied with alkyl groups. Since the methyl derivatives are more active than the corresponding ethyl and tert -butyl derivatives, it is suggested that sterical hindrance is involved.

Lipophilicity and hydrogen-bonding capacity of H1-antihistaminic agents in relation to their central sedative side-effects

Abstract Modern non-sedating histamine H 1 -receptor antagonists (e.g. terfenadine, temelastine, cetirizine, astemizole) are considered to be devoid of CNS side-effects because, as a result of their physicochemical properties, they do not cross the blood-brain barrier (BBB) in sufficient amounts. In the present study lipophilicity parameters considered to be of importance for brain penetration capability (such as log P oct , log D oct,7.4 , Δ log P and ∧ alkane ) were determined for a series of structurally different sedating and non-sedating histamine H 1 -receptor antagonists. These parameters were obtained from log P oct and log P alk values measured by centrifugal partition chromatography (CPC), a new and efficient method for measuring partition coefficients. From the lipophilicity data obtained it appears that the (non)-sedative effects of antihistamines cannot be correctly accounted for by brain penetration models that use only H-bonding (Δ log P) or hydration capacity (∧ alkane ) as a parameter. Indeed, in this series of usually basic H 1 -blockers, ionization also appears to play an important role. We conclude that sedative effects displayed by antihistamines are better explained by the parameter log D oct,7.4 , the octanol/water distribution coefficient of both neutral and ionized species at pH 7.4. For neural organic compounds it was found that brain penetration is highest if they have a log P oct value of approximately 2 (‘principle of minimal hydrophobicity’). Our data suggest that this principle is also applicable to ionizable drugs when log D oct,7.4 is used instead of log P oct . A tentative qualitative model for designing antihistamines without CNS side-effects is presented.

Histamine receptors : subclasses and specific ligands

In this review the three main types of histamine receptors are discussed together with their specific ligands. For the classical H1-receptors much emphasis is put on the mechanism by which the receptor is stimulated. For the H1- and H2-receptor the review includes information on the several models available for establishing agonistic or antagonistic activity. In the section on the H3-receptor the ligands are discussed as well as the possible physiological role of this receptor. In the final paragraphs some less well defined activities are presented.

Pharmacological characterization of the human histamine H2 receptor stably expressed in Chinese hamster ovary cells

1 The gene for the human histamine H2 receptor was stably expressed in Chinese hamster ovary (CHO) cells and characterized by [125I]‐iodoaminopotentidine binding studies. In addition, the coupling of the expressed receptor protein to a variety of signal transduction pathways was investigated. 2 After cotransfection of CHO cells with pCMVhumH2 and pUT626, a phleomycine‐resistant clonal cell line (CHOhumH2) was isolated that expressed 565 ± 35 fmol kg−1 protein binding sites with high affinity (0.21 ± 0.02 nm) for the H2 antagonist, [125I]‐iodoaminopotentidine. 3 Displacement studies with a variety of H2 antagonists indicated that the encoded protein was indistinguishable from the H2 receptor identified in human brain membranes and guinea‐pig right atrium. The Ki‐values observed in the various preparations correlated very well (r2 = 0.996–0.920). 4 Displacement studies with histamine showed that a limited fraction (32 ± 6%) of the binding sites showed a high affinity for histamine (2 ± 1.2 μm); the shallow displacement curves were reflected by a Hill‐coefficient significantly different from unity (nH = 0.58 ± 0.09). The addition of 100 μm Gpp(NH)p resulted in a steepening of the displacement curve (nH = 0.79 ± 0.02) and a loss of high affinity sites for histamine. 5 Displacement studies with other agonists indicated that the recently developed specific H2 agonists, amthamine and amselamine, showed an approximately 4–5 fold higher affinity for the human H2 receptor than histamine. 6 Stimulation of CHOhumH2 cells with histamine resulted in a rapid rise of the intracellular cyclic AMP levels. After 10 min an approximately 10 fold increase in cyclic AMP could be measured. The EC50 value for this response was 7 ± 1 nm for histamine. This response was effectively blocked by tiotidine and cimetidine, resulting in Ki values of 8 ± 1 nm and 0.56 ± 0.24 μm respectively. 7 Stimulation of CHOhumH2 cells with histamine neither inhibited the A23187‐induced release of [3H]‐arachidonic acid nor changed the intracellular IP3 levels. 8 These results show that the cloned human gene encodes a histamine H2 receptor that is indistinguishable from the H2 receptor identified in human brain tissue. This receptor is functionally coupled to the adenylate cyclase in CHO cells, but does not influence the inositolphosphate turnover or arachidonic acid release.

1,5-Diphenyl-1,4-pentadiene-3-ones and cyclic analogues as antioxidative agents. Synthesis and structure-activity relationship.

A series of 1,5-diphenyl-1,4-pentadiene-3-ones and cyclic analogues with OH-groups in the para position of the phenyl rings and various meta substituents were prepared and their antioxidant activity compared with that of curcumin. Most of them exhibited potent antioxidative activity, especially when all the meta positions were substituted by methoxy groups.

Nordimaprit, homodimaprit, clobenpropit and imetit: affinities for H3 binding sites and potencies in a functional H3 receptor model

SummaryWe determined the affinities of nordimaprit, homodimaprit, clobenpropit and imetit for H3 binding sites (labelled by 3H-Nα-methylhistamine) in rat brain cortex homogenates and their potencies at presynaptic H3A receptors on noradrenergic nerve endings in mouse brain cortex slices. 3H-Nα-Methylhistamine bound saturably to rat brain cortex homogenates with a Kd of 0.70 nmol/l and a Bmax of 98 fmol/mg protein. Binding of 3H-Nα-methylhistamine was displaced monophasically by dimaprit (pKi 6.55), nordimaprit (5.94), homodimaprit (6.44), clobenpropit (9.16), imetit (9.83), R-(−)-α-methylhistamine (8.87) and histamine (8.20), and biphasically by burimamide (pKi high 7.73, pKi low 5.97). In superfused mouse brain cortex slices preincubated with 3H-noradrenaline, the electrically (0.3 Hz) evoked tritium overflow was inhibited by imetit (pIC35 8.93),R-(−)-α-methylhistamine (7.87) and histamine (7.03). The effect of histamine was attenuated by nordimaprit, homodimaprit, clobenpropit and N-ethoxycarbonyl-2- ethoxy-1,2-dihydroquinoline (EEDQ); EEDQ (but not nordimaprit, homodimaprit and clobenpropit) attenuated the effect of histamine also in slices pre-exposed to the drug 60–30 min prior to superfusion. The concentration-response curve of histamine was shifted to the right by homodimaprit and clobenpropit; Schild plots yielded straight lines with a slope of unity for both drugs (pA2 5.94 and 9.55, respectively). Nordimaprit depressed the maximum effect of histamine (pD′2 5.55) and also slightly increased the concentration of histamine producing the half-maximum effect.In conclusion, nordimaprit and homodimaprit possess similar affinities for H3 binding sites like dimaprit; nordimaprit and homodimaprit as well as clobenpropit and imetit do not differentiate between H3A and H3B binding sites. Nordimaprit is a reversible noncompetitive H3 receptor antagonist, homodimaprit and clobenpropit are reversible competitive H3 receptor antagonists and imetit is an H3 receptor agonist.