Modulating oxytocin activity and plasma stability by disulfide bond engineering

Disulfide bond engineering is an important approach to improve the metabolic half-life of cysteine-containing peptides. Eleven analogues of oxytocin were synthesized including disulfide bond replacements by thioether, selenylsulfide, diselenide, and ditelluride bridges, and their stabilities in human plasma and activity at the human oxytocin receptor were assessed. The cystathionine (K(i) = 1.5 nM, and EC?? = 32 nM), selenylsulfide (K(i) = 0.29/0.72 nM, and EC?? = 2.6/154 nM), diselenide (K(i) = 11.8 nM, and EC?? = 18 nM), and ditelluride analogues (K(i) = 7.6 nM, and EC?? = 27.3 nM) retained considerable affinity and functional potency as compared to oxytocin (K(i) = 0.79 nM, and EC?? = 15 nM), while shortening the disulfide bridge abolished binding and functional activity. The mimetics showed a 1.5-3-fold enhancement of plasma stability as compared to oxytocin (t(?) = 12 h). By contrast, the all-D-oxytocin and head to tail cyclic oxytocin analogues, while significantly more stable with half-lives greater than 48 h, had little or no detectable binding or functional activity.     

1-[2-methoxy-5-(3-phenylpropyl)]-2-aminopropane unexpectedly shows 5-HT(2A) serotonin receptor affinity and antagonist character

Certain phenylethylamines, such as 1-(4-bromo-2,5-dimethoxyphenyl)-2-aminopropane (DOB; 1a), are high-affinity 5-HT(2) agonists. Previous structure-affinity studies have concluded that both the 2,5-dimethoxy substitution pattern and the nature of substituents at the 4-position are important determinants of high affinity. We recently demonstrated that replacement of the bromo group of DOB with a 3-(phenyl)propyl substituent results in retention of affinity and that, counter to established structure-affinity relationships, the 2,5-dimethoxy substitution pattern is no longer a requirement for the binding. The present investigation extends these findings by examining a series of analogues, 3, lacking a 5-methoxy group. It was additionally found that shifting the phenylalkyl substituent from the 4- to the 5-position (e.g., 4i) also results in retention of affinity. For example, 1-(2-methoxy-5-(3-phenylpropyl)-2-aminopropane (6; the alpha-methyl derivative of 4i) binds at 5-HT(2A) receptors with high affinity (K(i) = 13 nM) and possesses 5-HT(2A) antagonist character. Thus, not only is the 2,5-dimethoxy substitution pattern not a requirement for the binding of certain phenylethylamines at 5-HT(2A) receptors, the presence of a 4-position substituent (previously thought to serve as a modulator of affinity of DOB-like agents) is also not required. Striking differences in the 5-HT(2A) binding requirements of the present compounds as compared to DOB-like agents suggest multiple substituent-dependent modes of binding.     

2-Pyrazolyl-N(6)-substituted adenosine derivatives as high affinity and selective adenosine A(3) receptor agonists

We describe the synthesis of new high affinity and selective A(3)-adenosine receptor (A(3)-AdoR) agonists. Introduction of a methyl group at the N(6)-position of the A(2A)-AdoR selective 2-pyrazolyl-adenosine analogues (Figure 2) brought about a substantial increase in the A(3)-AdoR binding affinity and selectivity. While the N(6)-desmethyl analogues 3a and 4 were inactive at the A(3)-AdoR (K(i) > 10 microM), the corresponding N(6)-methyl analogues 5 and 22 showed good binding affinity at the A(3)-AdoR (K(i) = 73 and 97 nM, respectively). Replacement of the carboxamide group in 5 with different heteroaryl groups resulted in analogues with high affinities and selectivity for the A(3)-AdoR. (2R,3S,4R)-tetrahydro-2-(hydroxymethyl)-5-(6-(methylamino)-2-(4-(pyridin-2-yl)-1H-pyrazol-1-yl)-9H-purin-9-yl)furan-3,4-diol (15, K(i) = 2 nM) displayed high selectivity for the A(3)-AdoR versus A(1)- and A(2A)-AdoRs (selectivity ratios of 1900 and >2000, respectively).     

Structure-activity relationship studies on a novel series of (S)-2beta-substituted 3alpha-[bis(4-fluoro- or 4-chlorophenyl)methoxy]tropane analogues for in vivo investigation

In general, 3alpha-(diphenylmethoxy)tropane (benztropine)-based dopamine uptake inhibitors do not demonstrate cocaine-like pharmacological activity in models of psychostimulant abuse and have been proposed as potential medications for the treatment of cocaine addiction. However, several (S)-2-carboalkoxy-substituted-3alpha-[bis(4-fluorophenyl)methoxy]tropane analogues were discovered to stimulate locomotor activity and substitute in subjects trained to discriminate cocaine, suggesting a role of the 2-position substituent in mediating these cocaine-like actions. Herein, we describe the synthesis of a series of novel N- and 2-substituted-3alpha-[bis(4-fluoro- or 4-chlorophenyl)methoxy]tropane analogues. Most of these analogues demonstrated high affinity binding to the dopamine transporter (DAT; K(i) = 1.8-40 nM), and selectivity over the other monoamine transporters and muscarinic M(1) receptors. When the (S)-2-carboalkoxy substituent was replaced with (S)-2-ethenyl, the resulting analogue 11 demonstrated the highest DAT binding affinity in the series (K(i) = 1.81 nM) with DAT selectivity over serotonin transporters (SERT; 989-fold), norepinephrine transporters (NET; 261-fold) and muscarinic receptors (90-fold). When the 4'-F groups of compounds 5 (K(i) = 2.94 nM) and 8 (K(i) = 6.87 nM) were replaced with 4'-Cl in the (S)-2-carboalkoxy series, DAT binding affinities were slightly reduced (K(i) = 12.6 and 14.6 nM for 6 and 7, respectively), yet inhibition of dopamine uptake potency remained comparably high (IC(50) range = 1.5-2.5 nM). Interestingly, the 4'-Cl analogue (+/-)-6 substituted less in rats trained to discriminate cocaine than the 4'-F analogue (+/-)-5. These studies demonstrate that manipulation of the 2-, N-, and 3-position substituents in the 3alpha-(diphenylmethoxy)tropane class of dopamine uptake inhibitors can result in ligands with high affinity and selectivity for the DAT, and distinctive in vivo pharmacological profiles that cannot be predicted by their effects in vitro.     

2-Substituted tryptamines: agents with selectivity for 5-HT(6) serotonin receptors

Several 2-alkyl-5-methoxytryptamine analogues were designed and prepared as potential 5-HT(6) serotonin agonists. It was found that 5-HT(6) receptors accommodate small alkyl substituents at the indole 2-position and that the resulting compounds can bind with affinities comparable to that of serotonin. In particular, 2-ethyl-5-methoxy-N, N-dimethyltryptamine (8) binds with high affinity at human 5-HT(6) receptors (K(i) = 16 nM) relative to 5-HT (K(i) = 75 nM) and was a full agonist, at least as potent (8: K(act) = 3.6 nM) as serotonin (K(act) = 5.0 nM), in activating adenylate cyclase. Compound 8 displays modest affinity for several other populations of 5-HT receptors, notably h5-HT(1A) (K(i) = 170 nM), h5-HT(1D) (K(i) = 290 nM), and h5-HT(7) (K(i) = 300 nM) receptors, but is otherwise quite selective. Compound 8 represents the first and most selective 5-HT(6) agonist reported to date. Replacing the 2-ethyl substituent with a phenyl group results in a compound that retains 5-HT(6) receptor affinity (i.e., 10: K(i) = 20 nM) but lacks agonist character. 2-Substituted tryptamines, then, might allow entry to a novel class of 5-HT(6) agonists and antagonists.     

Synthesis and evaluation of 11C-labeled 6-substituted 2-arylbenzothiazoles as amyloid imaging agents

The synthesis and evaluation of a series of neutral analogues of thioflavin-T (termed BTA's) with high affinities for aggregated amyloid and a wide range of lipophilicities are reported. Radiolabeling with high specific activity [(11)C]methyl iodide provided derivatives for in vivo evaluation. Brain entry in control mice and baboons was high for nearly all of the analogues at early times after injection, but the clearance rate of radioactivity from brain tissue varied by more than 1 order of magnitude. Upon the basis of its rapid clearance from normal mouse and baboon brain tissues, [N-methyl-(11)C]2-(4'-methylaminophenyl)-6-hydroxybenzothiazole (or [(11)C]6-OH-BTA-1) was selected as the lead compound for further evaluation. The radiolabeled metabolites of [(11)C]6-OH-BTA-1 were polar and did not enter brain. The binding affinities of [N-methyl-(3)H]6-OH-BTA-1 for homogenates of postmortem AD frontal cortex and synthetic Abeta(1-40) fibrils were similar (K(d) = 1.4 nM and 4.7 nM, respectively), but the ligand-to-Abeta peptide binding stoichiometry was approximately 400-fold higher for AD brain than Abeta(1-40) fibrils. Staining of AD frontal cortex tissue sections with 6-OH-BTA-1 indicated the selective binding of the compound to amyloid plaques and cerebrovascular amyloid. The encouraging in vitro and in vivo properties of [(11)C]6-OH-BTA-1 support the choice of this derivative for further evaluation in human subject studies of brain Abeta deposition.     

Chemo-enzymatic synthesis of a series of 2,4-syn-functionalized (S)-glutamate analogues: new insight into the structure-activity relation of ionotropic glutamate receptor subtypes 5, 6, and 7

( S)-Glutamic acid (Glu) is the major excitatory neurotransmitter in the central nervous system (CNS) activating the plethora of ionotropic Glu receptors (iGluRs) and metabotropic Glu receptors (mGluRs). In this paper, we present a chemo-enzymatic strategy for the enantioselective synthesis of five new Glu analogues 2a- f ( 2d is exempt) holding a functionalized substituent in the 4-position. Nine Glu analogues 2a- j are characterized pharmacologically at native 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA), kainic acid (KA), and N-methyl- d-aspartic acid (NMDA) receptors in rat synaptosomes as well as in binding assays at cloned rat iGluR5-7 subtypes. A detailed in silico study address as to why 2h is a high-affinity ligand at iGluR5-7 ( K i = 3.81, 123, 57.3 nM, respectively), while 2e is only a high affinity ligand at iGluR5 ( K i = 42.8 nM). Furthermore, a small series of commercially available iGluR ligands are characterized in iGluR5-7 binding.     

Synthesis and 5-HT1A/5-HT2A receptor activity of N-(4-arylpiperazin-1-yl)alkyl derivatives of 2-azaspiro([4.4]nonane and [4.5]decane-1,3-dione

Two series of N-[(4-arylpiperazin-1-yl)-alkyl]-2-azaspiro[4.4]nonane (5-10) and [4.5]decane-1,3-dione (11-16) derivatives were synthesized and their serotonin 5-HT1A and 5-HT2A receptor affinities were determined. Compounds with the methylene spacer (5-7 and 11-13) exhibited low 5-HT1A/5-HT2A receptor affinity, in contrast to their ethylene analogues regarded as potent 5-HT1A ligands, especially those containing a cyclohexane moiety (14-16; Ki = 5.1, 2.7 and 4.3 nM, respectively) in the 3-position of the pyrrolidine-2,5-dione ring. Moreover, derivatives with 3-chloro substituent (10 and 14) showed distinct affinity for 5-HT2A receptors. The functional activity of compounds 10, 14, 15 and 16 was tested in vivo in the commonly used animal models. In those experiments, the tested compounds showed features of agonists of pre- and postsynaptic (14), agonists of presynaptic and antagonists of postsynaptic (10, 15), or agonists of postsynaptic (16) 5-HT1A receptors. Additionally, 10 and 16 exhibited properties of potential 5-HT2A receptor antagonists. The above results suggested a crucial role of the spacer between the amide fragment and 4-arylpiperazine moiety, as well as of the size of the cycloalkyl ring at the 3-position of pyrrolidine-2,5-dione ring in functional 5-HT1A/5-HT2A properties.     

Synthesis and binding affinities of cyclic and related linear analogues of CCK8 selective for central receptors

To investigate the role of the sulfate group and the influence of cyclization on the biological properties of conformationally constrained CCK8 analogues, three series of compounds were synthesized: Boc-Glu-Tyr-Nle-D-Lys-Trp-Nle-Asp-Phe-NH2 (1), Boc-Glu-Tyr(SO3H)-Nle-D-Lys-Trp-Nle-Asp-Phe-NH2 (2), and Boc-Glu-Tyr-(SO3H)-Nle-D-Lys-Trp-Nle-Asp-Phe-NH2 (3) (series A); Boc-D-Glu-Tyr-Nle-D-Lys-Trp-Nle-Asp-Phe-NH2 (4), Boc-D-Glu-Tyr(SO3H)-Nle-D-Lys-Trp-Nle-Asp-Phe-NH2 (5), Boc-D-Glu-Tyr(SO3H)-Nle-D-Lys-Trp-Nle-Asp-Phe-NH2 (6), and Boc-D-Glu-Tyr(SO3H)-Nle-D-Nle-Trp-Asp-Phe-NH2 (7) (series B); and Boc-gamma-D-Glu-Tyr-Nle-D-Lys-Trp-Nle-Asp-Phe-NH2 (8), Boc-gamma-D-Glu-Tyr(SO3H)-Nle-D-Lys-Trp-Nle-Asp-Phe-NH2 (9), and Boc-gamma-D-Glu-Tyr-(SO3H)-Nle-D-Lys-Trp-Nle-Asp-Phe-NH2 (10) (series C). The selectivity of these peptides was studied by measuring their ability to displace [3H]propionyl-CCK8 from guinea pig brain and pancreatic membranes. All the peptides displayed low affinities (KI values around 10(-6) M) for the pancreatic receptors (A type). In contrast, both sulfated and nonsulfated cyclic analogues displayed high affinities for central-type binding sites (B type), especially compounds belonging to series C [KI(8) = 4.7 nM and KI(9) = 0.56 nM]. In all series the linear analogues had relatively poor affinities (KI approximately 300 nM) for B-type receptors. Compound 9 was the most potent (KI = 0.56 nM) and selective [KI(pancreas)/KI(brain) = 4464] for central-type CCK receptors of guinea pig. The cyclization of the N-terminal region of CCK8 permits one therefore to obtain probes for central receptors, and small changes directed toward the cyclic part modulate the affinity for these receptors.     

Synthetic and computer-assisted analyses of the pharmacophore for the benzodiazepine receptor inverse agonist site

The structural requirements for ligand binding to the benzodiazepine receptor (BzR) inverse agonist site were probed through the synthesis and in vitro evaluation of 3-substituted beta-carbolines 6, 7, 11, 12, gamma-carboline 13, and diindoles 18-21, 23-25, 27, 28, and 34. On the basis of the apparent binding affinities of these and other analogues, a hydrogen bond acceptor site (A2) on the receptor is proposed to interact with the N(9) hydrogen atom of the beta-carbolines or the N(7) hydrogen nuclei of the diindoles. Likewise, a proposed hydrogen bond donating site (H1) interacts with the N(2) nitrogen atom of the beta-carbolines or the N(5) nitrogen atom of the diindoles. It appears that interaction with both sites is a prerequisite for high affinity since analogues which have either one or both of these positions blocked exhibit substantial reduction in affinity. Moreover, H1 appears to be capable of engaging in a three-centered hydrogen bond with appropriately functionalized ligands, which explains the increase in potency observed in the following series of 3-substituted beta-carbolines: the n-butyl (12, IC50 = 245 nM), n-propoxy (9, IC50 = 11 nM), and propyl ketone (11, IC50 = 2.8 nM) congeners. In addition to H1 and A2, there appears to be a relatively narrow hydrophobic pocket in the binding cleft that can accommodate substituents at the 3-position of the beta-carbolines which have chain lengths less than or equal to C5. There is a 1 order of magnitude decrease in affinity between n-propoxy analogue 9 (IC50 = 11 nM, chain length = 4) and n-butoxy derivative 7 (IC50 = 98 nM, chain length = 5). Furthermore, alpha- and gamma-branching [e.g. ethoxycarbonyl (2), IC50 = 5 nM and tert-butoxycarbonyl (31) IC50 = 10 nM] but not beta- and delta-branching [e.g. isopropoxy (6), IC50 = 500 nM and (neopentyloxy) carbonyl (48), IC50 = 750 nM] at position 3 are tolerated. Occupation of this hydrophobic pocket is clearly important for high affinity as evidenced by the relatively low affinity of 30, a beta-carboline which possesses a hydrogen atom at the 3-position. This same hydrophobic pocket is partially filled by the D and E rings of the diindoles, which accounts for the high affinity of several members of this series. An excluded volume analysis using selected 3-substituted beta-carbolines and ring-E substituted pyridodiindoles is consistent with the presence of this hydrophobic pocket (see Figure 1).(ABSTRACT TRUNCATED AT 400 WORDS)     

Isoquinoline and quinazoline urea analogues as antagonists for the human adenosine A(3) receptor

Isoquinoline and quinazoline urea derivatives were found to bind to human adenosine A(3) receptors. Series of N-phenyl-N'-quinazolin-4-ylurea derivatives and N-phenyl-N'-isoquinolin-1-ylurea derivatives were synthesized and tested in radioligand binding assays on their adenosine receptor affinities. A structure-affinity analysis indicated that on the 2-position of the quinazoline ring or the equivalent 3-position of the isoquinoline ring a phenyl or heteroaryl substituent increased the adenosine A(3) receptor affinity in comparison to unsubstituted or aliphatic derivatives. Furthermore, the structure-affinity relationship of substituted phenylurea analogues was investigated. Substituents such as electron-withdrawing or electron-donating groups were introduced at different positions of the benzene ring to probe electronic and positional effects of substitution. Substitution on the 3- or 4-position of the phenyl ring decreased the adenosine A(3) receptor affinity. Substitution at position 2 with an electron-donating substituent, such as methyl or methoxy, increased human adenosine A(3) receptor affinity, whereas substitution on the 2-position with an electron-withdrawing substituent did not influence affinity. Combination of the optimal substituents in the two series had an additive effect, which led to the potent human adenosine A(3) receptor antagonist N-(2-methoxyphenyl)-N'-(2-(3-pyridyl)quinazolin-4-yl)urea (VUF5574, 10a) showing a K(i) value of 4 nM and being at least 2500-fold selective vs A(1) and A(2A) receptors. Compound 10a competitively antagonized the effect of an agonist in a functional A(3) receptor assay, i.e., inhibition of cAMP production in cells expressing the human adenosine A(3) receptor; a pA(2) value of 8.1 was derived from a Schild plot. In conclusion, compound 10a is a potent and selective human adenosine A(3) receptor antagonist and might be a useful tool in further characterization of the human A(3) receptor.     

N6-cyclopentyl-2-(3-phenylaminocarbonyltriazene-1-yl)adenosine (TCPA), a very selective agonist with high affinity for the human adenosine A1 receptor

Four subtypes of adenosine receptors are currently known, that is, A(1), A(2A), A(2B), and A(3) receptors. Interestingly, quite substantial species differences exist especially between human and rat A(3) receptors. As a result, ligands such as CCPA, which are very selective for the rat A(1) receptor versus the human A(3) receptor, are substantially less selective when the human A(1) and A(3) receptors are compared. New 2-substituted and 2,N(6)-disubstituted adenosines were synthesized, and their affinities for the human adenosine A(1), A(2A), A(2B), and A(3) receptors were determined. Although large substituents on the C2-position are generally thought to yield adenosine A(2A) receptor selective ligands, the reported series of 2-triazeno-substituted adenosines had a very high affinity for the A(1) receptor. For example, 2-(3-phenylaminocarbonyltriazene-1-yl)adenosine had an affinity of 6.1 +/- 1.3 nM for the human adenosine A(1) receptor. Introduction of a diphenethyl substituent at the N(6)-position of this compound resulted in a high-affinity agonist, 3.1 +/- 0.9 nM, for the human adenosine A(1) receptor with 316- and 45-fold selectivity versus the human A(2A) and human A(3) receptors, respectively. The most selective, high-affinity human adenosine A(1) receptor agonist was the disubstituted compound N(6)-cyclopentyl-2-(3-phenylaminocarbonyltriazene-1-yl)adenosine (TCPA). TCPA had an affinity of 2.8 +/- 0.8 nM for the human adenosine A(1) receptor and was 75-fold and 214-fold selective versus the human A(2A) and human A(3) receptors, respectively. In addition, TCPA was a full agonist and inhibited the forskolin-induced cAMP production of CHO cells stably transfected with the human adenosine A(1) receptor with an IC(50) of 1.5 +/- 0.5 nM.     

7-Substituted 5-amino-2-(2-furyl)pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidines as A2A adenosine receptor antagonists: a study on the importance of modifications at the side chain on the activity and solubility.

It was demonstrated in the early 1990s that adenosine exerts many physiological functions through the interaction with four different receptors, named A1, A2A, A2B, and A3. In the past few years, our group has been involved in the development of A2A antagonists, which led to the synthesis of SCH 58261 (1), the first potent and selective adenosine A2A antagonist, which has been widely used as a reference compound. In this paper, we present an extended series of pyrazolotriazolopyrimidines synthesized with the aim to investigate the influence of the substitutions on the pyrazole ring. The choice of the substituents was based on their capability to improve water solubility while retaining high affinity and selectivity at the human A2A adenosine receptor subtype. In this series, some structural characteristics that are important for activity, i.e., tricyclic structure, free amino group at 5-position, furan ring, and substituent at 7-position on the pyrazole moiety, have been maintained. We focused our attention on the nature of the phenyl ring substituent to improve water solubility. Following this strategy, we developed new compounds with good affinity and selectivity for A2A adenosine receptors, such as 8d (K(i) 0.12 nM; hA1/hA2A ratio = 1025; R(m) = 2.8), 8h (K(i) 0.22; hA1/hA2A ratio = 9818; R(m) = 3.4), 8i (K(i) 0.18 nM; hA1/hA2A ratio = 994; R(m) = 2.8), 8k (K(i) 0.13 nM; hA1/hA2A ratio = 4430; R(m) = 3.6), and 14b (K(i) 0.19 nM; hA1/hA2A ratio = 2273; R(m) = 2.7). All the new synthesized compounds have no significant interaction with either A2B or A3 receptor subtypes. This new series of compounds deeply enlightens some structural requirements to display high affinity and selectivity for the A2A adenosine receptor subtype, although our goal of identifying new compounds with increased water solubility was not completely achieved. On this basis, other strategies will be devised to improve this class of compounds with a profile that appears to be promising for treatment of neurodegenerative disorders, such as Parkinson's disease.     

Structure-activity relationships of bisphosphate nucleotide derivatives as P2Y1 receptor antagonists and partial agonists

The P2Y1 receptor is present in the heart, in skeletal and various smooth muscles, and in platelets, where its activation is linked to aggregation. Adenosine 3',5'- and 2',5'-bisphosphates have been identified as selective antagonists at the P2Y1 receptor (Boyer et al. Mol. Pharmacol. 1996, 50, 1323-1329) and have been modified structurally to increase receptor affinity (Camaioni et al. J. Med. Chem. 1998, 41, 183-190). We have extended the structure-activity relationships to a new series of deoxyadenosine bisphosphates with substitutions in the adenine base, ribose moiety, and phosphate groups. The activity of each analogue at P2Y1 receptors was determined by measuring its capacity to stimulate phospholipase C in turkey erythrocyte membranes (agonist effect) and to inhibit phospholipase C stimulation elicited by 10 nM 2-(methylthio)adenosine 5'-diphosphate (antagonist effect). 2'-Deoxyadenosine bisphosphate analogues containing halo, amino, and thioether groups at the 2-position of the adenine ring were more potent P2Y1 receptor antagonists than analogues containing various heteroatom substitutions at the 8-position. An N6-methyl-2-chloro analogue, 6, was a full antagonist and displayed an IC50 of 206 nM. Similarly, N6-methyl-2-alkylthio derivatives 10, 14, and 15 were nearly full antagonists of IC50 < 0.5 microM. On the ribose moiety, 2'-hydroxy, 4'-thio, carbocyclic, and six-membered anhydrohexitol ring modifications have been prepared and resulted in enhanced agonist properties. The 1,5-anhydrohexitol analogue 36 was a pure agonist with an EC50 of 3 microM, i.e., similar in potency to ATP. 5'-Phosphate groups have been modified in the form of triphosphate, methyl phosphate, and cyclic 3',5'-diphosphate derivatives. The carbocyclic analogue had enhanced agonist efficacy, and the 5'-O-phosphonylmethyl modification was tolerated, suggesting that deviations from the nucleotide structure may result in improved utility as pharmacological probes. The N6-methoxy modification eliminated receptor affinity. Pyrimidine nucleoside 3', 5'-bisphosphate derivatives were inactive as agonists or antagonists at P2Y receptor subtypes.     

New diphenylacetylenes as probes for positron emission tomographic imaging of amyloid plaques

A series of 18F fluoropegylated diphenylacetylenes as probes for binding to Abeta plaques were successfully prepared. These relatively rigid acetylenes, 12a, 12b, 14a, and 14b, displayed high binding affinities in postmortem AD brain homogenates (Ki ranging from 1.2 to 2.9 nM). In vivo biodistribution in normal mice exhibited excellent initial brain penetrations (4.42, 4.55, 5.41, and 6.78% dose/g at 2 min for [18F]12a, 12b, 14a, and 14b, respectively). [18F]12b and [18F]14b, with a longer fluoropegylated unit, that is, n=3, showed faster brain washout at 30 min postinjection (0.42 and 1.57% dose/g) as compared to the shorter fluoropegylated chain ligands, that is, [18F]12a and [18F]14a (1.03 and 3.69% dose/g). Autoradiography and homogenate binding confirmed the high binding signal due to Abeta plaques. These preliminary results suggest that the novel diphenylacetylenes may be potentially useful for imaging of Abeta plaques in the brain of patients with Alzheimer's disease.     

Potent 4-aryl- or 4-arylalkyl-substituted 3-isoxazolol GABA(A) antagonists: synthesis, pharmacology, and molecular modeling

We have previously described a series of competitive GABA(A) antagonists derived from the low-efficacy partial agonist 5-(4-piperidyl)-3-isoxazolol (4-PIOL, 4). The 2-naphthylmethyl analogue, 4-(2-naphthylmethyl)-5-(4-piperidyl)-3-isoxazolol (5), provided affinity for the GABA(A) receptor site higher than that of the standard GABA(A) receptor antagonist, SR 95531 (3). Molecular modeling studies of these compounds exposed a cavity at the receptor recognition site capable of accommodating aromatic groups of substantial size in the 4-position in the 3-isoxazolol ring. Here we present a series of analogues of 5, with various substituents in different positions in the naphthyl ring system (6a-k), and compounds with aromatic substituents directly attached to the 4-position of the 3-isoxazolol ring (7l-n). The compounds have been pharmacologically characterized using receptor-binding assays and electrophysiological whole-cell patch-clamp techniques. All of the tested compounds show affinity for the GABA(A) receptor site. While the 5-, 7-, and 8-bromo analogues, 6b-d, showed receptor affinities (K(i) = 45, 109, and 80 nM, respectively) comparable with that of 5 (K(i) = 49 nM), the 1-bromo analogue, 6a, provided the highest receptor affinity of the series (K(i) = 10 nM). Introduction of a series of different substituents in the 1-position in the 2-naphthyl ring system led to compounds, 6e-k, with retained high affinity for the GABA(A) receptor (K(i) = 16-250 nM). Introduction of a phenyl ring directly into the 4-position on the 3-isoxazolol ring gave a 41-fold increase in affinity relative to that of 4-PIOL. In whole-cell patch-clamp recordings from cultured cerebral cortical neurons, all of the tested compounds were able to inhibit the effect of the specific GABA(A) agonist isoguvacine, 6a showing antagonist potency (IC(50) = 42 nM) markedly higher than that of 3 (IC(50) = 240 nM). Molecular modeling studies, based on the compounds described, emphasized the importance of the distal ring in 5 for receptor affinity and the considerable dimensions of the proposed receptor cavity. Furthermore, the phenyl rings in 7l and in 6k were shown to represent highly favorable positions for an aromatic ring in previously unexplored receptor regions in terms of a pharmacophore model.     

Novel sst(4)-selective somatostatin (SRIF) agonists. 2. Analogues with beta-methyl-3-(2-naphthyl)alanine substitutions at position 8

We present a family of human sst(4)-selective, high-affinity (IC(50) = 2-4 nM) cyclic somatostatin (SRIF) octapeptides. These peptides result from the substitution of dTrp(8) in H-c[Cys(3)-Phe(6)-Phe(7)-DTrp(8)-Lys(9)-Thr(10)-Phe(11)-Cys(14)]-OH (SRIF numbering) (ODT-8) by one of the four conformationally biased stereoisomers of beta-methyl-3-(2-naphthyl)alanine (beta-Me2Nal). Whereas H-c[Cys-Phe-Phe-DNal-Lys-Thr-Phe-Cys]-OH (ODN-8, 2) has high affinity and marginal selectivity for human sst(3) (Reubi et al., Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 13973-13978), H-c[Cys-Phe-Tyr-D-threo-beta-Me2Nal-Lys-Thr-Phe-Cys]-OH (5) has high affinity for all sst's except for sst(1); H-c[Cys-Phe-Tyr-L-threo-beta-Me2Nal-Lys-Thr-Phe-Cys]-OH (6) has high affinity for sst(4) (IC(50) = 2.1 nM), with more than 50-fold selectivity toward the other receptors. Analogues 7 and 8, containing d- and l-erythro-beta-Me2Nal instead of the corresponding threo derivatives at position 8, are essentially inactive at all receptors. Substitution of Tyr(7) in 5 and 6 by Aph(7) resulted in 9 and 10 with similar affinity patterns overall yet lowered affinity. The substitution of DCys(3) for Cys(3) in 5 and 6 yielded H-c[DCys-Phe-Tyr-D-threo-beta-Me2Nal-Lys-Thr-Phe-Cys]-OH (11) and H-c[DCys-Phe-Tyr-L-threo-beta-Me2Nal-Lys-Thr-Phe-Cys]-OH (12), with biological profiles almost identical to those of their parents 5 and 6 (i.e., high affinity for sst(2-5) for 11 and high affinity and selectivity for sst(4) for 12). Analogue 12, with high sst(4) affinity combined with the highest sst(4) selectivity among all tested compounds, is an agonist in the cAMP accumulation assay (EC(50) = 1.29 nM). Cold monoiodination of 12 yielded 14, with loss of sst(4) selectivity and loss of high affinity (IC(50) = 21 nM). Introduction of Tyr(2) in 9 and 10 and substitution of Cys(3) by dCys(3), to yield 15 and 16 (IC(50) = 9.8 and 61 nM, respectively, for sst(4) and limited selectivity), failed to generate a high-affinity (125)iodinatable sst(4)-selective ligand. Substitution of Phe by Tyr at position 11 in H-c[DCys-Phe-Phe-L-threo-beta-Me2Nal-Lys-Thr-Phe-Cys]-OH yielded 18 (IC(50) = 11.8 nM at sst(4)), with limited sst(4) selectivity (30-fold or greater at the other receptors) yet only slightly improved affinity over that of 14. Cold monoiodination of 18 yielded 20 (IC(50) = 30 nM at sst(4) and high selectivity). Whereas we were able, in this study, to identify a new family of sst(4)-selective, high-affinity compounds, our additional goal, to identify highly potent and sst(4)-selective ligands amenable to (125)iodination, could not be achieved satisfactorily. On the other hand, some of the diastereomers identified in this study, such as 5, 11, 17, and 19, are very potent ligands at all receptors but sst(1).     

(8 beta)-6-methylergoline amide derivatives as serotonin antagonists: N1-substituent effects on vascular 5HT2 receptor activity

A series of (8 beta)-6-methylergoline amide derivatives was synthesized with various alkyl substituents in the N1-position in order to evaluate their effectiveness in blocking vascular 5HT2 receptors. The influence of both the N1 substituent and amide derivative proved to be of great importance on binding affinities to vascular 5HT2 receptors. Within each series of amides, however, maximum affinity was achieved with an N1-isopropyl substituent (14, 18, 26, 38, and 41; all with 2.7-50 times greater affinity than their N1-H analogues), with the exception of two cases (22 and 37) in the cyclohexylamide derivatives wherein N1-methyl equalled the isopropyl in potency. Other than these exceptions, affinities followed the pattern of H less than Me less than Et less than iPr, with potencies falling off with larger alkyl substituents.     

2-(Alkylamino)tetralin derivatives: interaction with 5-HT1A serotonin binding sites

8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) is a selective 5-HT1A serotonin agonist. Derivatives of 8-OH-DPAT with amine substituents larger or more bulky than n-propyl appear to be inactive in a presynaptic biochemical assay measuring agonist-induced feedback inhibition of 5-HT synthesis but have never been examined in brain binding assays. A series of N-phenylalkyl derivatives of 8-methoxy-2-aminotetralin was evaluated at [3H]-8-OH-DPAT-labeled 5-HT1A sites in rat brain hippocampal membranes. All of the phenylalkyl derivatives displayed significant affinity for these sites and, of the agents examined, the 3-phenylpropyl 8-hydroxy analogue appears to be optimal and had an affinity (Ki = 1.9 nM) comparable to that of 8-OH-DPAT (Ki = 1.2 nM). In addition, the presence of an oxygen-containing substituent at the 8-position of the tetralin ring is not necessary for good affinity, and secondary amines and tertiary amines displayed equal affinity at central 5-HT1A binding sites. 5-HT1A sites are found both pre- and postsynaptically; thus, differences observed in the biochemical assay as compared to the results of the present binding study could be due to different structural requirements of these two receptors. This seems unlikely, however, because there was little difference in the affinities of several selected analogues for striatal versus hippocampal binding sites. Because we have now demonstrated that amine substituents larger than propyl, and an unsubstituted 8-position, are well tolerated by central 5-HT1A sites, future studies aimed at the development of new serotonergic tetralin analogues need not be limited to N-propyl or 8-hydroxy derivatives of 2-aminotetralin.     

Structure-activity relationships at the monoamine transporters and sigma receptors for a novel series of 9-[3-(cis-3, 5-dimethyl-1-piperazinyl)propyl]carbazole (rimcazole) analogues.

9-[3-(cis-3,5-Dimethyl-1-piperazinyl)propyl]carbazole (rimcazole) has been characterized as a sigma receptor antagonist that binds to the dopamine transporter with moderate affinity (K(i) = 224 nM). Although the binding affinities at the dopamine transporter of rimcazole and cocaine are comparable, rimcazole only depressed locomotor activity in mice and antagonized the stimulant effects produced by cocaine. The neurochemical mechanisms underlying the attenuation of cocaine's effects are not understood, although interaction at a low affinity site/state of the dopamine transporter has been suggested. To explore further this class of compounds, a series of rimcazole analogues was designed and synthesized. Displacement of [(3)H]WIN 35,428 binding at the dopamine transporter in rat caudate-putamen revealed that aromatic substitutions on rimcazole were not well tolerated, generally, with significant reductions in affinity for the 3,6-dibromo (5; K(i) = 3890 nM), 1,3, 6-tribromo (6; K(i) = 30300 nM), 3-amino (8; K(i) = 2400 nM), and 3, 6-dinitro (9; K(i) = 174000 nM) analogues. The N-phenylpropyl group was the only terminal piperazine nitrogen substituent that retained moderate affinity at the dopamine transporter (11; K(i) = 263 nM). Analogues in which the carbazole ring was replaced with a freely rotating diphenylamine moiety were also prepared. Although the diphenylamino analogue in which the terminal piperazine nitrogen was unsubstituted, as in rimcazole, demonstrated relatively low binding affinity at the dopamine transporter (24; K(i) = 813 nM), the N-phenylpropyl analogue was found to have the highest affinity for the dopamine transporter within the series (25; K(i) = 61.0 nM). All of the analogues that had affinity for the dopamine transporter inhibited [(3)H]dopamine uptake in synaptosomes, and potencies for these two effects showed a positive correlation (r(2) = 0.7731, p = 0.0018). Several of the analogues displaced [(3)H]paroxetine from serotonin transporters with moderate to high affinity, with the N-phenylpropyl derivative (11) having the highest affinity (K(i) = 44.5 nM). In contrast, none of the analogues recognized the norepinephrine transporter with an affinity of <1.3 microM. Binding affinities for sigma(1) and sigma(2) receptors were also determined, and several of the compounds were more potent than rimcazole with affinities ranging from 97 nM to >6 microM at sigma(1) sites and 145 to 1990 nM at sigma(2) sites. The compound with the highest affinity (25) at sigma(1) sites was also the compound with highest affinity at the dopamine transporter. These novel rimcazole analogues may provide important tools with which to characterize the relationship between the low affinity site or state of the dopamine transporter, sigma receptors, and their potential roles in modulating cocaine's psychostimulant actions.