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The combination of antagonism at histamine H 3 receptors and inhibition of acetylcholinesterase has been recently proposed as an approach to devise putative new therapeutic agents for cognitive diseases. The 4,4′-biphenyl fragment has been reported by us as a rigid scaffold leading to potent and selective non-imidazole H 3 -antagonists. Starting from these premises, the current work presents an expanded series of histamine H 3 receptor antagonists, characterized by a central 4,4′-biphenyl scaffold, where the structure–activity profile of both mono-basic and di-basic compounds is further explored and their ability to inhibit rat brain cholinesterase activity is determined. The steric properties and basicity of the terminal groups were modulated in symmetrical compounds, carrying identical substituents, and in asymmetrical compounds, having a piperidine ring at one end and different groups at the other. The length of the linker connecting the biphenyl scaffold to the terminal groups was also modulated. Binding studies at rat and human H 3 receptors evidenced the highest binding affinities for di-basic compounds, in the order of nM concentrations, and that the steric requirements for the two terminal groups are different. Many potent compounds showed good selectivity profiles over the other histamine receptors. Interestingly, some derivatives displayed a moderate ability to inhibit rat brain cholinesterase, for example compound 12 (1-[2-(4′-piperidinomethyl-biphenyl-4-yl)ethyl]piperidine) has a pIC 50 = 5.96 for cholinesterase inhibition and high H 3 receptor binding affinity and antagonist potency (p K i = 8.70; p K B = 9.28). These compounds can be considered as rigid analogs of a recently reported class of dual-acting compounds and as a promising starting point for the design of new H 3 -antagonists with anti-cholinesterase activity.

A parallel synthesis of aryl azoles with neuroprotective activity is described. All compounds obtained were evaluated in an in vitro assay using a NMDA toxicity paradigm showing a neuroprotective activity between 15% and 40%. The potential biological target of the active compounds was investigated by extensive literature searches based around similar scaffolds with reported neuroprotective activity. The most interesting molecules active in the NMDA toxicity assay (3a and 2g) showed moderate but significant activity in the inhibition of the Site 2 Sodium Channel binding assay at 10 microM. To confirm our hypothesis compounds 3a, c, f and 2g were tested in the Veratridine assay which is one of the excitotoxicity assays of relevance to NaV channels. The compounds tested showed an activity between 40% and 70%. The identification of neuroprotective small molecules and the identification of NaV channels as the potential site of action were the most important goals of this work.

A novel series of non-imidazole H(3)-receptor antagonists was developed, by chemical modification of a potent lead H(3)-antagonist composed by an imidazole ring connected through an alkyl spacer to a 2-aminobenzimidazole moiety (e.g., 2-[[3-[4(5)-imidazolyl]propyl]amino]benzimidazole), previously reported by our research group. We investigated whether the removal of the imidazole ring could allow retaining high affinity for the H(3)-receptor, thanks to the interactions undertaken by the 2-aminobenzimidazole moiety at the binding site. The imidazole ring of the lead was replaced by a basic piperidine or by a lipophilic p-chlorophenoxy substituent, modulating the spacer length from three to eight methylene groups; moreover, the substituents were moved to the 5(6) position of the benzimidazole nucleus. Within both the 2-alkylaminobenzimidazole series and the 5(6)-alkoxy-2-aminobenzimidazole one, the greatest H(3)-receptor affinity was obtained for the piperidine-substituted compounds, while the presence of the p-chlorophenoxy group resulted in a drop in affinity. The optimal chain length was different in the two series. Even if the new compounds did not reach the high receptor affinity shown by the imidazole-containing lead compound, it was possible to get good H(3)-antagonist potencies with 2-aminobenzimidazoles having a tertiary amino group at appropriate distance.

Histamine H(3) receptor is a G protein-coupled receptor whose activation inhibits the synthesis and release of histamine and other neurotransmitters from nerve endings and is involved in the modulation of different central nervous system functions. H(3) antagonists have been proposed for their potential usefulness in diseases characterized by impaired neurotransmission and they have demonstrated beneficial effects on learning and food intake in animal models. In the present work, a 3D model of the rat histamine H(3) receptor, built by comparative modeling from the crystallographic coordinates of bovine rhodopsin, is presented with the discussion of its ability to predict the potency of known and new H(3) antagonists. A putative binding site for classical, imidazole-derived H(3) antagonists was identified by molecular docking. Comparison with a known pharmacophore model and the binding affinity of a new rigid H(3) antagonist (compound 1, pK(i)=8.02) allowed the characterization of a binding scheme which could also account for the different affinities observed in a recently reported series of potent H(3) antagonists, characterized by a 2-aminobenzimidazole moiety. Molecular dynamics simulations were employed to assess the stability and reliability of the proposed binding mode. Two new conformationally constrained benzimidazole derivatives were prepared and their binding affinity was tested on rat brain membranes; compound 9, designed to reproduce the conformation of a known potent H(3) antagonist, showed higher potency than compound 8, as expected from the binding scheme hypothesized.

Several indole analogues of melatonin (MLT) were obtained by moving the MLT side chain from C(3) to C(2) of the indole ring. Binding and in vitro functional assays were performed on cloned human MT1 and MT2 receptors, stably transfected in NIH3T3 cells. Quantitative structure-activity relationship studies showed that 4-methoxy-2-(N-acylaminomethyl)indoles, with a benzyl group in position 1, were selective MT2 antagonists and, in particular, N-[(1-p-chlorobenzyl-4-methoxy-1H-indol-2-yl)methyl]propanamide (12) behaved as a pure antagonist at MT1 and MT2 receptors, with a 148-fold selectivity for MT2. We present a topographical model that suggests a lipophilic group, located out of the plane of the indole ring of MLT, as the key feature of the MT2 selective antagonists.

A number of 6-methoxy-1-(2-propionylaminoethyl)indoles, carrying properly selected substituents at the C-2 indole position, were prepared and tested as melatonin receptor ligands. Affinities and intrinsic activities for the human cloned mt 1 and MT 2 receptors were examined and compared with those of some 2-substituted melatonin derivatives recently described by us. A quantitative structure–activity relationship (QSAR) study of the sixteen 2-substituted indole compounds, 5a – k , 1 , 8 – 11 , using partial least squares (PLS) and multiple regression analysis (MRA) revealed the existence of an optimal range of lipophilicity for the C-2 indole substituent. There are also indications that planar, electron-withdrawing substituents contribute to the affinity by establishing additional interactions with the binding pocket. No mt 1 /MT 2 subtype selectivity was observed, with the relevant exception of the 2-phenethyl derivative 5e , which exhibited the highest selectivity for the h-MT 2 receptor among all the compounds tested (MT 2 /mt 1 ratio of ca. 50). Conformational analysis and superposition of 5e to other reported selective MT 2 ligands revealed structural and conformational similarities that might account for the MT 2 /mt 1 selectivity of 5e .

New histamine H3-receptor antagonists were synthesised and tested on rat brain membranes and on electrically stimulated guinea-pig ileum. The new compounds have a central polar group represented by a 2-alkylimidazole or a 2-thioimidazoline nucleus. The effect of the polar group basicity on the optimal length of the alkyl chain, connecting this group to a 4(5)-imidazolyl ring, was investigated. The best affinity values, obtained by displacement of [3H]-RAMHA from rat brain, were obtained for the 2-alkylimidazole derivatives (2a-f) with tetramethylene chain (pKi 8.03-8.97), having an intermediate basicity between that of the previously reported 2-thioimidazoles (1a-i) and that of 2-alkylthioimidazolines (3a-h). In contrast, a general lowering of affinity (pKi 5.90-7.63) was observed for compounds of the last series (3a-h), with a complex dependence on the terminal lipophilic group and chain length.

The CoMFA methodology was applied to melatonin receptor ligands in order to establish quantitative structure-affinity relationships. One hundred thirty-three compounds were considered: they were either collected from literature or newly synthesized in order to gain information about the less explored positions. To this end, various melatonin derivatives were prepared and their affinity for quail optic tecta melatonin receptor was tested. Compounds were aligned on the putative active conformation of melatonin proposed by our previously reported pharmacophore search, and their relative affinities were calculated from the displacement of 2-[125I]-iodomelatonin on different tissues expressing aMT receptors. Compounds were grouped into three sets according to their topology. Subset A: melatonin-like compounds; subset B: N-acyl-2-amino-8-methoxytetralins and related compounds; subset C:N-acyl-phenylalkylamines and related compounds. CoMFA models were derived for each set, using the steric, electrostatic, and lipophilic fields as structural descriptors; the PLS analyses were characterized by good statistical parameters, taking into account the heterogeneity of the binding data, obtained with different experimental protocols. From the CoMFA model for the melatonin-like compounds, besides the well-known positive effect of 2-substitution, a low steric tolerance for substituents in 1, 6, and 7, and a negative effect of electron-rich 4-substituents were observed; the information provided by the newly synthesized compounds was essential for these results. Moreover, a comprehensive model for the 133 compounds, accounting for a common alignment and a common mode of interaction at the melatonin receptor, was derived (Q2 = 0.769, R2 = 0.905). This model validates our previously reported pharmacophore search and offers a clear depiction of the structure-affinity relationships for the melatonin receptor ligands.

The synthesis of several novel indole melatonin analogues substituted at the 2-position with acylaminomethyl (8-11), acylaminoethyl (5a-k), or acylaminopropyl (13) side chains is reported. On the basis of a novel in vitro functional assay (specific binding of [35S]GTPgammaS), which can discriminate agonist from partial agonist, antagonist, and inverse agonist ligands, 5a,g, h,j and 13 were shown to be partial agonists, 5d,e and 8-11 competitive antagonists, and 5b,c,k putative inverse agonists. Binding and functional assays were performed on cloned human MT1 receptor. Structure-activity relationship considerations indicate that N-[1-aryl-2-(4-methoxy-1H-indol-2-yl)(C1-C2)alkyl]alkanamides represent a lead structure for this type of ligands.

The binding of thioperamide, a known H3-receptor antagonist, to rat plasma proteins and its affinity for rat cerebral phospholipids are investigated. Thioperamide is strongly bound to plasma proteins (95−80% at plasma concentrations of 3.5−400 μg mL−1), and its binding can be resolved into two components: a high-affinity, saturable component and a non-specific component. The drug has a high affinity for cerebral phospholipids, with a partition coefficient of approximately 100 (log K = 2.06 ± 0.14), which should promote brain penetration and accumulation. Protein binding and cerebral phospholipid affinity can suggest the explanation of some differences reported in the literature on thioperamide distribution data: at low plasma concentrations of the drug, its protein binding (95% at 3.5 μg mL−1) can prevent brain accumulation, while at higher concentrations the free plasma fraction suddenly increases (> 10% at 18 μg mL−1) and it allows passive distribution to lipophilic tissues such as brain tissue.

A series of carbamate derivatives of the H(3) antagonist ROS203 (1) were prepared, and their lipophilicity and steric hindrance were modulated by introducing linear or branched alkyl chains of various lengths. In vitro stability studies were conducted to evaluate how structural modulations affect the intrinsic reactivity of the carbamoyl moiety and its recognition by metabolic enzymes. Linear alkyl carbamates were the most susceptible to enzymatic hydrolysis, with bioconversion rates being higher in rat liver and plasma. Chain ramification significantly enhanced the enzymatic stability of the set, with two derivatives (1g and 1h) being more stable by a factor of 8-40 than the ethyl carbamate 1a. Incubation with bovine serum albumin (BSA) showed a protective role of proteins on chemical and porcine-liver esterase (PLE)-catalyzed hydrolysis. Ex vivo binding data after i.v. administration of 1h revealed prolonged displacement of the labeled ligand [(3)H]-(R)-alpha-methylhistamine ([(3)H]RAMHA) from rat-brain cortical membranes, when compared to 1. However, the high rates of bioconversion in liver, as well as the chemical instability of 1h, suggest that further work is needed to optimize the enzymatic and chemical stability of these compounds.

A class of rigid, dibasic, non-imidazole H 3 antagonists was developed, starting from a series of previously described flexible compounds. The original polymethylene chain between two tertiary amine groups was replaced by a rigid scaffold, composed by a phenyl ring or a biphenyl fragment. Modulation of the distance between the two amine groups, and of their alkyl substituents, was driven by superposition of molecular models and docking into a receptor model, resulting in the identification of 1,1′-[biphenyl-4,4′-diylbis(methylene)]bis-piperidine ( 5 ) as a subtype-selective H 3 antagonist with high binding affinity (p K i = 9.47) at human H 3 histamine receptor.

5-Methoxy-2-(N-acetylaminoethyl)indole (5d), a melatonin analogue derived from the transposition of the acetylaminoethyl side chain from C3 to C2 of the indole nucleus, had been previously characterized as a low affinity antagonist at MT1 and MT2 membrane receptors; this molecule is endowed with good in vitro antioxidant and cytoprotective potency in rat cerebellar cell cultures, comparable to or better than those of melatonin. In order to further investigate the role of structure-antioxidant activity relationships in cytoprotection, the structure of 5d was systematically modulated to design a new series of compounds. The 5-methoxy group was replaced by substituents with different electronic and lipophilic properties and it was moved to a different position on the indole ring. Other modifications of the lead structure involved the methylation of the indole nitrogen or its replacement by a sulfur atom. The side chain was also modified either increasing its lipophilicity or introducing an ionisable acid group. The antioxidant activity of this set of compounds was evaluated by the ABTS and conjugated dienes (CD) assays, while their cytoprotection was evaluated against kainate-induced cytotoxicity in cultured cerebellar neurons. In both antioxidant assays, the shift of the 5-methoxy group to the 4-position of the indole nucleus led to the most active radical scavenger (9), more potent than the parent compound and melatonin in the antioxidant tests, but much less effective as a cytoprotectant. Sharp structure-activity relationships were registered for cytoprotection, where the maintenance of the 5-alkoxy-2-(N-acylaminoethyl)indole scaffold appeared as the key feature to confer both antioxidant and cytoprotective activity to the structure. Some derivatives of the set, however, together with the most potent 5d, maintained a significant antioxidant and cytoprotective effect and could be employed as tools for in vivo pharmacological investigations on neuroprotective efficacy of melatonin-related indoles.

Melatonin (MLT) is known for its radical scavenger activity, which had been related to its ability to protect neuronal cells from different kinds of oxidative stress. In particular, MLT protects rat cerebellum granular cells from kainate-induced necrosis at concentrations higher than 100 microM, and is able to reduce lipoperoxidation induced by radical stress in rat brain homogenate at similar concentrations. On the other hand, MLT has nanomolar affinity for its membrane receptors (MT1 and MT2), and these are completely saturated at the high concentrations employed when the cytoprotective effect is observed. Other indole derivatives are also known to possess antioxidant and cytoprotective activity. In order to dissociate the cytoprotective effect of MLT from its receptor affinity, and to investigate the structure-activity relationships (SAR) between this effect and some potentially relevant chemical properties, we prepared a series of indole derivatives, where the structure of MLT was gradually modulated, varying the 5-methoxy group nature and position, the acylaminoethyl chain position, and by the introduction of lipophilic groups. These modifications resulted in a set of compounds having different receptor affinity and intrinsic activity, different lipophilicity, and different substitution at the indole nucleus. The compounds were tested for their antioxidant potency by the ABTS test and by inhibition of rat brain homogenate lipoperoxidation; their cytoprotective effect was also estimated from the inhibition of kainate-induced cellular death on rat cerebellum granular cells, and the results were evaluated by SAR comparison and QSAR analysis. An isomer of MLT resulted more potent and effective than MLT itself in the cytoprotection test, although it showed similar potency in the peroxidation test, and it was devoid of the ability to stimulate MT1 and MT2 receptors. This compound was selected as the lead compound for a further SAR study, devoted to the optimization of the cytoprotective effect and to the investigation on its mechanism.

The known neuroprotective actions of melatonin could be due to its antioxidant or radical scavenging activity, or they could be due to specific interactions of the indole with its receptors. A study of structure-activity relationships may provide useful information when a validated macromolecular target has not been (or is not) identified. A set of indole derivatives, with changes in the 5-methoxy and acylamino groups, the side chain position and the lipophilic/hydrophilic balance, were selected and tested for their in vitro antioxidant potency in the ABTS (2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid disodium salt) and thiobarbituric acid reactive substances (TBARS) assays and for their cytoprotective activity against kainate excitotoxicity on cerebellar cell cultures. No quantitative model was able to relate the potencies obtained in the two antioxidant assays, probably because they are related to different physico-chemical properties. However, the lipophilicity of the compounds and the antioxidant potency in the TBARS assay were linearly correlated. This may be due to improved access to the lipidic substrate, where the antioxidant action occurs. In the cytoprotection assay, most compounds showed potencies comparable with or lower than melatonin. An exception was N-[2-(5-methoxy-1H-indol-2-yl)ethyl]acetamide (12), yielding, at 50 microM, percentages of cell vitality higher than 75%, while melatonin EC50 was 333 microM. No correlation was observed between cytoprotective and antioxidant potencies, nor with MT1 or MT(2) receptor affinity. Compound 12 is a low-affinity antagonist at melatonin membrane receptors, and one of the most potent compounds in the antioxidant assays; its cytoprotective potency and the absence of agonist activity at melatonin membrane receptors make it a valid candidate for further investigations.

H3-antagonists possess promising pharmacological effects on awakening, learning and memory, but few data on their access to the central nervous system (CNS) have been reported so far. The purpose of this work was to investigate the relationships between structure and brain penetration of a series of H3-antagonists, using ex vivo binding experiments in rats. H3-antagonists belonging to different chemical classes but all having an imidazole ring, an alkyl spacer, a polar fragment and a lipophilic ending group, were selected among the numerous H3-antagonists recently described by us. Ex vivo binding studies were performed by inhibiting specific [3H]-(R)-alpha-methylhistamine ([3H]-RAMHA) binding to rat cerebral cortical membranes following H3-antagonist peripheral administration. Ionization constants and partition coefficients in n-octanol/water and 1,2-dichloroethane/water were determined by the potentiometric pH-metric method and were compared to the ex vivo binding potencies to analyse structure-property relationships (SPR). In the ex vivo assay, the H3-antagonists showed different potencies (pED50) not correlated to their in vitro H3-receptor binding affinities (pKi). Compound 4a, having a benzothiazol-2-yl-thioethyl chain, showed high ex vivo potency (ED50=1.35 mg kg(-1) i.p.) and a fast brain penetration, eliciting maximal displacement of [3H]-RAMHA already 5 min after i.v. or i.p. administration. Ex vivo binding assays of three compounds, following i.v. and i.p. administration, showed that the observed i.p. ex vivo potencies were not significantly affected by biotransformation. Within the set of compounds, those having a better ability to reach the CNS had a logDoct(7.4) in the range 2-3.5, and a DeltalogPoct-dce < 2. The combined use of two easily measurable physicochemical descriptors, namely logDoct(7.4) (apparent lipophilicity at pH 7.4) and DeltalogPoct-dce (a descriptor of H-bond donor capacity) allowed to model brain permeation of the majority of the compounds examined.

We report the design, synthesis, QSPR and QSAR of a new class of H(3)-antagonists, having a 2-aminobenzimidazole moiety connected to the 4(5) position of an imidazole ring through di- or tri-methylene chains. Eleven substituents, selected by experimental design to obtain broad and non-correlated variation in their lipophilic, electronic and steric properties, were introduced at the 5(6) position of the benzimidazole nucleus. The compounds were tested for their H(3)-receptor affinity, by displacement of [(3)H]-(R)-alpha-methylhistamine ([(3)H]-RAMHA) binding to rat brain membranes (pK(i)), for intrinsic activity, evaluating their effect on [(35)S]GTPgammaS binding to rat brain membranes, and for H(3)-antagonist potency, on electrically stimulated guinea-pig ileum (pK(B)). The pK(i) values of the derivatives with longer chain (5a-k) ranged over 2 orders of magnitude, with the 5(6)-methoxy derivative 5d endowed with sub-nanomolar affinity (pK(i)=9.37). The series having two methylene groups in the chain spacer (4a-k), showing a small variation in affinity, revealed to be somewhat insensitive to ring substitution. Lipophilicity (log P) and basicity (pK(a)) of the newly synthesized compounds were measured and related to receptor affinity in a QSAR study. Multiple regression analysis (MRA) showed an approximate parabolic dependence of pK(i) on log P, while an additional electronic effect of the substituents on benzimidazole tautomerism is suspected.

It has been recently reported that compounds composed of an imidazole connected through an alkyl spacer to a 2-aminobenzimidazole showed high affinity towards the H(3)-receptor. The guanidine fragment of the 2-aminobenzimidazole is probably involved in hydrogen bond interactions at the binding site, and is referred to as the 'polar fragment'. In the present work, starting from 2-aminobenzimidazole derivatives with a di-methylene spacer 1 (pK(i)=7.25) or a tri-methylene one 2 (pK(i)=8.90), we investigated the importance of the hydrogen bond (HB) donor groups at the polar fragment in the interaction with the H(3)-receptor. The replacement of 2-aminobenzimidazoles with different moieties [2-aminobenzothiazole, 3, 4; 2-thiobenzimidazole, 5, 6; 2-thiobenzothiazole, 7, 8; 2-thio-4-phenyl- or 2-thio-5-phenyl-N-methylimidazoles, 9-12] highlighted the effect of the polar group basicity on the optimal length of the alkyl chain: longer spacers were preferred with polar groups of moderate basicity whereas, in the presence of neutral polar groups, the best affinity values were obtained with di-methylene chains. Moreover, N-methylation at the 2-aminobenzimidazole moiety 13-16 revealed different behaviour for compounds having different spacer lengths. In fact, methylation of the exocyclic NH group maintained high affinity for the tri-methylene 2-aminobenzimidazole derivative, while a drop in affinity was observed for the annular N-methylation. An opposite trend characterised di-methylene derivatives. These observed SAR suggest that, within this class of compounds, the number of HB-donor groups can be lowered while maintaining high receptor affinity. Since the presence of HB-donor groups strongly affects brain access, this observation could be useful to design and prepare new H(3)-antagonists.

We designed and synthesized a series of pyrrole derivatives with the aim of investigating the structure-activity relationship (SAR) for the binding of non-classical agonists to CB(1) and CB(2) cannabinoid receptors. Superposition of two pyrrole-containing cannabinoid agonists, JWH-007 and JWH-161, allowed us to identify positions 1, 3 and 4 of the pyrrole nucleus as amenable to additional investigation. We prepared the 1-alkyl-2,5-dimethyl-3,4-substituted pyrroles 10a-e, 11a-d, 17, 21, 25 and the tetrahydroindole 15, and evaluated their ability to bind to and activate cannabinoid receptors. Noteworthy in this set of compounds are the 4-bromopyrrole 11a, which has an affinity for CB(1) and CB(2) receptors comparable to that of well-characterized heterocyclic cannabimimetics such as Win-55,212-2; the amide 25, which, although possessing a moderate affinity for cannabinoid receptors, demonstrates that the 3-naphthoyl group, commonly present in indole and pyrrole cannabimimetics, can be substituted by alternative moieties; and compounds 10d, 11d, showing CB(1) partial agonist properties.

The binding of a series of H3-antagonists to rat plasma proteins was investigated by dialysis experiments, with RP-HPLC measurement of the free ligand. The series was composed of 4(5)-phenyl-2-[[2-[4(5)-imidazolyl]ethyl]thio]imidazoles having, on the phenyl ring, meta- and para-substituents, with different physico-chemical characteristics. As high protein binding had been proposed as being one of the features limiting brain access for the reference H3-antagonist thioperamide, the title series was employed to test the possibility of achieving lower protein binding by modulation of lipophilicity, while maintaining good receptor affinity. The compounds tested showed quotas of bound drug ranging from 60 to 97.5%, while for thioperamide a 78% bound drug quota was observed at high total concentrations, with a steep increase in bound percentage at lower concentrations. Two of the tested compounds, having a carboxamide substituent, showed lower protein binding compared to thioperamide over a wide range of total concentration, without a significant loss in affinity with respect to the parent compound. A strict dependence of protein binding on lipophilicity was observed, and a QSPR model was derived which could also account for the protein binding observed for thioperamide, while receptor affinity had been reported to be quite insensitive to phenyl ring substitution. It is therefore possible to modulate protein binding of these H3-antagonists, through lipophilicity adjustment, without losing receptor affinity; this finding could help in the design of new compounds with improved brain access.

The third histamine receptor, initially seen as a central presynaptic autoreceptor, is currently recognised as an auto and heteroreceptor with the function of a general inhibitory brake in the cellular communications in neuronal and extraneuronal tissues [1]. The evidence that the H3 receptors are widely distributed in the brain and that they are tonically activated by histamine opened important perspectives for potential applications of H3-ligands in the treatment of different central disorders. The most recent data seem to indicate that H3 antagonism can favourably influence cognitive and learning functions, can normalize motor disturbances and can represent a new approach for the development of antiepileptic and antiobesity drugs [2]. Several H3 antagonists have been developed, but there remains a need to identify new molecules combining improved brain penetration, tolerability and good receptor affinity and selectivity. Proceeding from the study evaluating the pharmacological properties of original H3 blockers [3], we characterized in vitro the potency, affinity and selectivity at H3 histamine receptors of a series of imidazole derivatives with different degrees of basicity due to the presence of distinct polar groups in the side chain of variable length (Table 1). Further in vitro tests were performed to assess the possible interactions of the compounds at the guinea-pig ileal 5-HT3, a2 and M3 receptors.

We report the synthesis, octanol/water partition coefficient (log P), dissociation constants (pKa), H3-receptor affinity (pKi in rat brain membranes, [3H]-N alpha-methylhistamine), and H3-antagonist potency (pA2 in guinea ileum, (R)-alpha-methylhistamine) of novel H3-receptor antagonists obtained by introducing a para or meta substituent on the phenyl ring of the lead compound 4(5)-phenyl-2-[[2-[4(5)-imidazolyl]ethyl]thio]imidazole (3a). The substituents were chosen to obtain broad and uncorrelated variation in their lipophilic, electronic, and steric properties. The log P values of the neutral species cover almost 3 orders of magnitude (from 1.40 to 4.11). The pKa,2 values (protonation of the 2-thioimidazole fragment) vary from 3.13 to 4.34, indicating that this fragment, which incorporates the so-called polar group common to many H3-receptor antagonists, is neutral at physiological pH. The compounds had pKi values in a range too narrow (from 7.28 to 8.03) to derive QSAR equations. In one case (3g), a biphasic displacement curve was observed (pKi,1 = 8.53; pKi,2 = 6.90). The pA2 values ranged 2 orders of magnitude (from 6.83 to 8.87) and yielded a QSAR model (PLS) indicating that antagonist potency depends parabolically on lipophilicity and is decreased by bulky para substituents. The compounds of this series, therefore, maintain a fair-to-good affinity for rat brain H3-receptor and a fair-to-good H3-antagonist potency on guinea pig ileum, although varying markedly in their lipophilicity. The series thus appears as a good candidate for pharmacokinetic optimization leading to brain-penetrating H3-receptor antagonists.

Antipyretic activity of new compounds, 4-(3-oxo-1,2-benzisothiazolin-2-yl)phenylalkanoic acids, their esters, amides and 1,1-dioxide derivatives has been studied. The acid compound of the benzoic series (I:a), tested at graded doses, exerted a noticeable antipyretic action; it had two times the "potency" of benzisothiazolone but an almost equal "efficacy". Its "potency" however was not proportional to the development of gastric lesions and to the acute toxicity. A decreased pharmacological activity has been observed in phenylalkanoic acids in the following order: R = COOH greater than CH2COOCH greater than CH(CH3)COOH greater than CH(C2H5)COOH, probably due to their increasing lipophilic character. By contrast among 1,1-dioxide derivatives the most effective in preventing pyrogen-induced fever was the ethyl ester (V:c) of benzoic series which appeared to be as active as paracetamol. The interest arising from these observations is here after discussed.

In the present study the effects of three structurally different H2-receptor antagonists (cimetidine, ranitidine and oxmetidine) have been investigated on gastric acid and pepsin secretion of eight cats provided with cannulated gastric fistulas. The maximum pepsin output obtained from a set of complete dose-response curves of dimaprit was not statistically different from basal values. In the presence of the H2-antagonists, while the gastric acid secretion induced by dimaprit was competitively antagonized, the pepsin secretion was differently affected. The data obtained on pepsin activity with cimetidine and ranitidine were quite similar to the control values. By contrast, oxmetidine induced a significant increase. The results suggest a very weak involvement of the H2-receptors in pepsin activity and that oxmetidine performance could not be attributable to an H2-receptor block.

The H2-antagonist activity of thiazole derivatives, substituted on position 5 with urea-equivalent groups, has been tested on guinea-pig isolated atria stimulated by dimaprit. By comparing the activities of the 2,5-disubstituted thiazole derivatives with those of the corresponding 2,4-disubstituted derivatives it can be seen that the side-chain position is critical to activity and differently influences activity in the various series. The heteroaromatic ring atom sequence N-C-S-C-side chain is always associated with a low antagonist activity.

A representative group of 2-aminothiazolylethylamine derivatives, in which the gastric acid secretion stimulating S-aminoalkylisothiourea moiety can be recognized, was tested. The quite different responses observed suggest that in vivo but not in vitro some events mimicking an H2-receptor agonist-like activity rather than a direct interaction with H2-receptors could take place. On the basis of structure-activity relationships, it can be speculated that the active conformation of dimaprit is not that resembled by these compounds which have been considered as cyclic models of one of its possible conformations.

A series of thiazole derivatives, in which a side-chain with different urea-equivalent groups was introduced in position 5 of the heterocycle, have been tested as inhibitors of dimaprit-induced gastric acid and pepsin secretion in the gastric fistula cat. By comparing the in vivo and the previously reported in vitro activity of these compounds, we can note a very close parallelism not only in the quality of their action but also in the estimates of pA2 values. These data support an interdependence between the molecular substructures and the affinity for the H2-receptor.

The pharmacological properties of 2-aminohistamine and 2-amino-5-methylhistamine were studied and compared with those of histamine, 5-methylhistamine and dimaprit. The introduction of an amino group in position 2 of the histamine imidazole ring caused a reduction of histamine potency, mostly with respect to H1 receptors. Such disactivation was much more evident in its corresponding 5-methyl derivative. The pharmacological activity related to the chemical structure will be discussed in the paper.