Synthesis and Monoamine Transporter Binding Properties of 3α-(Substituted phenyl)nortropane-2β-carboxylic Acid Methyl Esters. Norepinephrine Transporter Selective Compounds

3alpha-(substituted phenyl)nortropane-2beta-carboxylic acid methyl esters (8a-h) showed high affinity for the norepinephrine transporter (NET). The most potent and selective compound was 3alpha-(3-fluoro-4-methylphenyl)nortropane-2beta-carboxylic acid methyl ester (8d), with a Ki of 0.43 nM at the NET and 21- and 55-fold selectivity relative to binding at the dopamine and serotonin transporters. The development of 8d makes available compounds selective for all three transporters from the same structural class.

Synthesis, nicotinic acetylcholine receptor binding, and antinociceptive properties of 3'-substituted deschloroepibatidine analogues. Novel nicotinic antagonists

A series of 3'-substituted deschloroepibatidine analogues (3a-g and 4) showed high affinity for alpha4beta2 binding and relatively weak affinity for alpha7 nAChRs. The 3'-ethynyl (3g) and 3'-fluoro (3a) analogues with K(i) values of 0.02 and 0.037 nM, respectively, were the most potent. Even though the alpha4beta2 binding affinity of several of the analogues were equal to that of epibatidine, all of the compounds were weak agonists in the antinociceptive, hypothermia, and spontaneous activity test in mice. In contrast, all of the compounds were functional antagonists of nicotine-induced antinociception. In general, compounds 3a-g and 4 were more potent in the tail-flick assay than the hot-plate test. For example, the 3'-fluoro analogue 3a and the N-methyl-3'-iodo analogue 4 showed AD(50) values of 0.07 and 0.04 microg/kg, respectively, in the tail flick test and only 35 and 0% inhibition at 20 and 10 microg/kg in the hot-plate assay, respectively. These results suggest that these compounds will be highly useful for identifying which specific receptor subtypes are involved in each of nicotine's pharmacological effects. The high affinity of the N-methyl-3'-iodo analogue 4 combined with its weak agonist and potent antagonist activity suggests that carbon-11 and iodine-123 analogues may be useful as PET and SPECT ligands, respectively, for studying nAChRs in vivo.

Synthesis and Monoamine Transporter Binding Properties of 3β-(3‘,4‘-Disubstituted phenyl)tropane-2β-carboxylic Acid Methyl Esters

3beta-(3'-Methyl-4'-chlorophenyl)tropane-2-carboxylic acid methyl ester (3b, RTI-112) is a 3-phenyltropane analogue that has high affinity for both the dopamine and serotonin transporters (DAT and 5-HTT, respectively). Compound 3b shows significant reduction of cocaine self-administration in rhesus monkeys, yet fails to maintain robust drug self-administration. PET studies revealed that unlike more DAT selective analogues such as GBR 12 909 and 3-(4-chlorophenyl)tropane-2-carboxylic acid phenyl ester (RTI-113), 3b shows no detectible DAT occupancy when dosed at its ED(50) for reduction of cocaine self-administration. In contrast, it highly occupies the 5-HTT at this dose. In this study, we report the synthesis and monoamine transporter binding potency of several new 3-(3',4'-disubstituted phenyl)tropane-2-carboxylic acid methyl esters (3c-k), which have binding properties very similar to 3b. With the exception of the 3',4'-dimethyl analogue 3k, all of the compounds possess subnanomolar IC(50) and K(i) values at the DAT and 5-HTT, respectively. The 3'-chloro-4'-bromo analogue 3e (IC(50) = 0.12 nM) and the 3'-bromo-4'-iodo analogue 3i (K(i) = 0.14 nM) are the most potent analogues at the DAT and 5-HTT, respectively. These compounds will be useful to further characterize the highly interesting behavioral profile of 3b.

Synthesis, structural identification, and ligand binding of tropane ring analogs of paroxetine and an unexpected aza-bicyclo[3.2.2]nonane rearrangement product

The structural requirements for high affinity at the serotonin transporter (5-HTT) have been investigated through the preparation of rigid paroxetine analogs. Tropane-derived analogs (4a-i) of paroxetine (2) were designed and synthesized as potential inhibitors of serotonin reuptake based on the structural and biological similarity between the two compound classes. Overall, the affinity of tropane-derived analogs at the 5-HTT was found to be at least an order of magnitude lower than that of paroxetine and ranged from 2-400nM. The reduced affinity at the 5-HTT may be attributed to the inability of the rigid tropane-derived analogs to adopt conformations favored by the 5-HTT. Within the series of tropane analogs, the 2beta,3beta- and 2beta,3alpha-isomers, 4a and 4d, were the most potent at the DAT and NET and are also significantly more potent than paroxetine (2) suggesting that their reduced conformational flexibility maximizes residence time in conformations favored by these transporters. Examination of the previously published preparation and structural assignment of 4a by additional NMR and X-ray crystallographic data has established that nucleophilic addition to the intermediate 2beta-methanesulfonyloxymethyl-3beta-(4-fluorophenyl)tropane unexpectedly provided the aza-bicyclo[3.2.2]nonane derivative 10a.

Monoamine Transporter Binding, Locomotor Activity, and Drug Discrimination Properties of 3-(4-Substituted-phenyl)tropane-2-carboxylic Acid Methyl Ester Isomers

The monoamine transporter binding properties, gross behavior, and locomotor activity effects in mice and drug discrimination results in cocaine-trained rats of the 2 beta,3beta-, 2 beta,3 alpha-, 2 alpha,3beta-, and 2 alpha,3 alpha-isomers of several 3-(4-substituted-phenyl)tropane carboxylic acid methyl esters were compared (2a-f, 3a-f, 4a-f, and 5b,c). The 2 beta,3beta-isomer showed the highest affinity for the dopamine transporter (DAT), and the 2 beta,3 alpha-isomer showed the next highest affinity. The order of potency for the 2 beta,3beta-isomer is 4'-chloro (2c) = 4'-iodo (2e) > 4'-bromo (2d) = 4'-methyl (2f) > 4'-fluoro (2b) > 4'-hydrogen (2a). In the case of the 2 beta,3 alpha-isomer, the order of affinity was 4'-bromo (3d) > 4'-iodo (3e) = 4'- chloro (3c) > 4'-methyl (3f) > 4'-fluoro (3b) > 4'-hydrogen (3a). The 4'-hydrogen, 4'-fluoro, and 4'-methyl 2 alpha,3beta-isomers, 4a, 4b, and 4f, had the lowest affinity for the DAT. While most of the compounds showed their highest affinity at the DAT, none were selective relative to the other two monoamine transporters. In general, the 2 alpha,3 alpha- and 2 alpha,3beta-isomers were more toxic (death and convulsions) than the 2 beta,3beta- and 2 beta,3 alpha-isomers. With the exception of the 2 alpha,3 alpha-isomers, all compounds produced the locomotor activity stimulation typical of dopaminergic drugs. The ED(50) ranges for the 2 beta,3beta- (2a-f), 2 beta,3 alpha- (3a-f), and 2 alpha,3beta-isomers (4a-f) in the locomotor assay were 0.1-1.2, 6.6-21.8, and 2.4-11.7 mg/kg, respectively. With the exception of the 2 alpha,3 alpha-isomer, all compounds generalized to cocaine. The 2 beta,3beta-isomers were at least 10-fold more potent than cocaine and the other three sets of isomers in this test.

Enantioselective effects of hydroxy metabolites of bupropion on behavior and on function of monoamine transporters and nicotinic receptors

Bupropion is an atypical antidepressant that also has usefulness as a smoking-cessation aid. Because hydroxybupropion, a major metabolite of bupropion, is believed to contribute to its antidepressant activity, this metabolite may also contribute to the smoking-cessation properties of bupropion. This study investigated the effects of hydrobupropion enantiomers on monoamine transporters and nicotinic acetylcholine receptor (nAChR) subtypes. Racemic bupropion and hydroxybupropion inhibit [(3)H]norepinephrine (NE) uptake with similar potency (IC(50) values of 1.9 and 1.7 microM, respectively), but most of the latter activity resides in the (2S,3S)-hydroxy isomer (IC(50) = 520 nM) rather than (2S,3R)-hydroxybupropion (IC(50) > 10,000 nM). Similar results were found with [(3)H]dopamine (DA) uptake. The effects of bupropion and enantiomers of hydroxybupropion on human nAChR subtypes indicate that the (2S,3S) isomer is more potent than the (2S,3R) isomer or racemic bupropion as an antagonist of alpha(4)beta(2) (functional IC(50) = 3.3 microM). In addition, (2S,3S)-hyroxybupropion and bupropion were considerably more potent than (2R, -3R)-hydroxybupropion in a mouse depression model (forced swimming test) and in antagonism of acute nicotine effects in mice. Together, our results suggest that clinical and behavioral effects of bupropion arise from actions at nAChR as well as DA and NE transporters. Furthermore, our data suggest that the (2S,3S)-hydroxybupropion isomer may be a better drug candidate for smoking cessation than bupropion because of its higher potency at the relevant targets.

Synthesis, Nicotinic Acetylcholine Receptor Binding, and Antinociceptive Properties of 2‘-Fluoro-3‘-(substituted phenyl)deschloroepibatidine Analogues. Novel Nicotinic Antagonist

A series of 2'-fluoro-3'-(substituted phenyl)deschloroepibatidine analogues (5a-k) showed high affinity for alpha4beta2 binding with no affinity at alpha7 nAChRs. The most potent compound was 2'-fluoro-3'-(4-nitrophenyl)deschloroepibatidine (5g) which possessed a Ki value of 0.009 nM. Surprisingly, none of the compounds showed agonist effects in pain tests and body temperature changes in mice even when tested at 10-15 mg/kg with the exception of 5b, which showed only very weak agonist effects. In contrast, all the compounds were potent functional antagonists of nicotine-induced antinociception. Interestingly, the 3'-substituted phenyl analogues 5b-k were 10-870-fold more effective as antagonists in the tail-flick test versus the hot-plate procedure. They failed to antagonize nicotine-induced hypothermia. The 4-chlorophenyl analogue (5e) (AD50 = 0.0003 in the tail-flick test) was the most potent and selective analogue. These results suggest that these compounds will be highly useful for identifying which specific receptor subtypes are involved in each of nicotine's pharmacological effects. These compounds also deserve consideration as potential pharmacotherapies for treatment of smoking cessation.

Structural and functional probing of the biogenic amine transporters by fluorescence spectroscopy

Fluorescence spectroscopy techniques have proven extremely powerful for probing the molecular structure and function of membrane proteins. In this review, it will be described how we have applied a series of these techniques to the biogenic amine transporters, which are responsible for the clearance of dopamine, norepinephrine, and serotonin from the synaptic cleft. In our studies, we have focused on the serotonin transporter (SERT) for which we have established a purification procedure upon expression of the transporter in Sf-9 insect cells. Importantly, the purified transporter displays pharmacological properties in detergent micelles similar to that observed in membranes suggesting that the overall tertiary structure is preserved upon purification. Using this purified SERT preparation and the fluorescent cocaine analogue RTI-233 as a molecular reporter, we have been able to characterize the microenvironment of the cocaine-binding pocket. In current follow-up studies, we are attempting to map the relative position of this binding pocket using fluorescence resonance energy transfer (FRET) between RTI-233 and an acceptor fluorophore covalently attached to endogenous cysteines in the transporter. Finally, it will be described how we recently initiated the implementation of single-molecule confocal fluorescence spectroscopy techniques in our studies of the SERT.

Importance of phenolic address groups in opioid kappa receptor selective antagonists

In vitro characterization and comparison of JDTic, its dehydroxy analogue and nor-BNI, and its dehydroxy analogue demonstrates that the N-substituted 3,4-dimethyl-(3-hydroxyphenyl)piperidine-derived antagonist, JDTic, relies more heavily on its phenol address group for affinity and antagonist activity relative to the corresponding naltrexone derived antagonists, nor-BNI. The structural flexibility of the former class of compound relative to the latter is postulated to underlie the difference.

Epibatidine structure–activity relationships

Epibatidine is a potent but nonselective nAChR agonist. Its biological effects appear to be mediated largely by alpha4beta2 nAChRs. Surprisingly, only a limited number of epibatidine analogues have been synthesized and evaluated in in vitro assays. Even fewer analogues have received in vivo pharmacological evaluation. In this paper, SAR studies directed toward epibatidine analogues will be reviewed.

Synthesis, monoamine transporter binding properties, and behavioral pharmacology of a series of 3beta-(substituted phenyl)-2beta-(3'-substituted isoxazol-5-yl)tropanes

Several 3beta-(substituted phenyl)-2beta-(3-substituted isoxazol-5-yl)tropanes (3a-t) were evaluated for their ability to inhibit radioligand binding at the DAT, 5-HTT, and NET as well as in gross observation and locomotor activity in mice and in rats trained to discriminate cocaine. All compounds showed high affinity for the DAT. The IC(50) values ranged from 0.5 to 26 nM. With the exception of 3e and 3f, which have no substituent on the 2beta-(1,2-isoxazole) ring, all compounds were selective for the DAT relative to the 5-HTT and NET. No compound showed death when dosed at 100 mg/kg; however, most compounds did show signs typical of dopamine activity. The ED(50) values for 2beta-(1,2-isoxazoles) that caused locomotor stimulation ranged from 0.2 to 12.8 mg/kg. Most compounds showed slower on-set and longer duration of action relative to cocaine. Surprisingly, 3beta-(4-methylphenyl)-2beta-[3-(4'-chlorophenyl)isoxazol-5-yl]tropane (3p) and 3beta-(4-methylphenyl)-2beta-[3-(4'-methylphenyl)isoxazol-5-yl]tropane (3r) did not produce significant increases in locomotor activity. In the cocaine discrimination test, all analogues showed full or at least 50% generalization with the exception of 3p, which did not show generalization. Importantly, both the locomotor activity and the cocaine discrimination ED(50)values were correlated with the DAT binding but not 5-HTT and NET binding. This provides further support for the dopamine hypothesis of cocaine abuse. High DAT affinity and selectivity, increased locomotor activity with slow onset and long duration of action, and generalization to cocaine shown by the 3beta-(substituted phenyl)-2beta-(3-substituted isoxazol-5-yl)tropanes are properties thought necessary for a pharmacotherapy for treating cocaine abuse.

Orphanin FQ/nociceptin suppresses motor activity through an action along the mesoaccumbens axis in rats

OBJECTIVE

Intracerebroventricular administration of orphanin FQ/nociceptin (OFQ/N), the endogenous agonist ligand of the opioid receptor-like (ORL-1) receptor, decreases extracellular levels of dopamine and suppresses motor activity. The presence of the ORL-1 receptor on mesoaccumbal and nigrostriatal dopaminergic neurons raises the possibility that an action along these pathways may be one means by which OFQ/N produces motor suppression. Thus, the present study used local administration of OFQ/N into the ventral tegmental area (VTA), the substantia nigra, the nucleus accumbens and the striatum to determine the contribution of cell-body regions and terminal fields of the dopaminergic neurons to the motor-suppressant effect of OFQ/N.

METHODS

Rats were implanted bilaterally with guide cannulae into one of the brain regions and tested 4 days later. First, the effect of a single dose of OFQ/N (30 microg/0.5 microL per side) on motor activity was determined after direct injection into the VTA, substantia nigra, nucleus accumbens or striatum. Rats were habituated to activity chambers for 1 hour and then injected with either artificial cerebrospinal fluid or OFQ/N into one of the brain regions, and motor activity was recorded for a further 1 hour. Next, the dose-response effect of intra-VTA or intranigral OFQ/N (3 microg or 30 microg/0.5 microL per side) on motor activity was examined. Finally, the effect of intra-VTA OFQ/N (3 microg or 30 microg/0.5 microL per side) on motor activity was determined in the presence of J-113397, an ORL-1 receptor antagonist.

RESULTS

OFQ/N suppressed motor activity when injected into the VTA and to a lesser extent after direct injection into the nucleus accumbens. However, OFQ/N failed to attenuate motor activity significantly after injection into the substantia nigra or the striatum. Subsequent dose-response studies showed that OFQ/N suppressed motor activity even at a 10-fold-lower dose after intrategmental but not intranigral administration. The motor-suppressant action of intra-VTA OFQ/N was attenuated by J-113397 (1.5 microg/0.5 microL per side) administered into the VTA 10 minutes before administration of OFQ/N.

CONCLUSION

Our results indicate that OFQ/N suppresses motor activity through activation of the ORL-1 receptor primarily through an action in the VTA.

Effects of murine-derived anti-methamphetamine monoclonal antibodies on (+)-methamphetamine self-administration in the rat

Two murine-derived anti-methamphetamine monoclonal antibodies were studied as potential pharmacokinetic antagonists of (+)-methamphetamine self-administration by rats. Intravenous administration of a 1 g/kg dose of the lower affinity [antibody equilibrium dissociation constant (K(d)) = 250 nM] monoclonal antibody (mAb) designated mAb6H8, 1 day before the start of several daily 2-h self-administration sessions produced effects that depended on the dose of (+)-methamphetamine. mAb6H8 increased the rate of self-administration of a unit dose of 0.06 mg/kg (+)-methamphetamine, had little effect on the rate of self-administration of a unit dose of 0.03 mg/kg (+)methamphetamine, and lowered the rate of self-administration of a unit dose of 0.01 mg/kg (+)-methamphetamine to a level similar to that after saline substitution. mAb-induced changes in rates of self-administration occurred very early in self-administration sessions and lasted for 3 to 7 days. Intravenous administration of a 1 or a 0.6 g/kg dose of a higher affinity (K(d) = 11 nM) mAb designated mAb6H4, 24 h before the first of several self-administration sessions, produced very similar effects to the lower affinity mAb, despite the more than 20-fold greater affinity for (+)-methamphetamine. It is proposed that these anti-methamphetamine antibodies bind some of the self-administered (+)-methamphetamine before it can penetrate into brain, thereby reducing the amount of free drug available to function as a reinforcer. Although neither of these mAb medications are optimal antibodies for treating (+)-methamphetamine abuse, the experiments demonstrate that anti-(+)-methamphetamine monoclonal antibodies can attenuate the self-administration of the drug and suggest the potential of using monoclonal antibodies as pharmacokinetic antagonists of (+)-methamphetamine.

Effects of Dopamine Transporter Inhibitors on Cocaine Self-Administration in Rhesus Monkeys: Relationship to Transporter Occupancy Determined by Positron Emission Tomography Neuroimaging

The dopamine transporter (DAT) is a critical recognition site for cocaine and contributes to its significant abuse liability. Accordingly, the development of compounds that target the DAT represents a logical approach in the pharmacological treatment of cocaine abuse. The present study characterized the effects of DAT inhibitors as pretreatments in rhesus monkeys trained to self-administer cocaine under a second-order schedule of i.v. drug delivery. The drugs also were substituted for cocaine to characterize their effectiveness in maintaining drug self-administration. Positron emission tomography neuroimaging with [(18)F]8-(2-[(18)F]fluoroethyl)-2beta-carbomethoxy-3beta-(4-chlorophenyl) nortropane established the DAT occupancy associated with behaviorally relevant doses of each drug. The drugs studied included a selective DAT inhibitor, [1-(2[bis(4-fluorophenyl-) methoxy]ethyl)-4-(3-phenylpropyl)piperazine] bimesylate hydrate (GBR 12909); an inhibitor with equal potency at dopamine and norepinephrine transporters, [3beta-(4-chlorophenyl)tropane-2beta-(3-phenylisoxazol-5-yl)] HCl (RTI-177); and a nonselective inhibitor of dopamine, norepinephrinem and serotonin transporters, [(-)-3beta-(3'-methyl-4-chlorophenyl)tropane-2beta-carboxylic acid methyl ester] tartrate (RTI-112). All drugs produced dose-related reductions in cocaine self-administration. Doses of GBR 12909 and RTI-177 that reduced responding by 50% (ED(50)) resulted in DAT occupancies of 67 +/- 5 and 73 +/- 5%, respectively. In contrast, DAT occupancy was below the limit of detection for the ED(50) dose of RTI-112. Both GBR 12909 and RTI-177 reliably maintained drug self-administration, and DAT occupancies at doses that maintained peak rates of responding were 57 +/- 1 and 92 +/- 7%, respectively. In contrast, RTI-112 failed to maintain robust drug self-administration in any subject. The results indicate that selective DAT inhibitors may require high DAT occupancy to reduce cocaine self-administration and maintain drug self-administration. Moreover, the behavioral profile of DAT inhibitors may be influenced by actions at other monoamine transporters.

Discovery of the First N-Substituted 4β-Methyl-5-(3-hydroxyphenyl)morphan To Possess Highly Potent and Selective Opioid δ Receptor Antagonist Activity

A structurally novel opioid delta receptor selective antagonist has been identified. This compound, (+)-5-(3-hydroxyphenyl)-4-methyl-2-(3-phenylpropyl)-2-azabicyclo[3.3.1]non-7-yl-(1-phenyl-1-cyclopentane)carboxamide [(+)-KF4, (+)-4], showed a K(e) value of 0.15 nM in the [(35)S]GTPgammaS functional assay. (+)-KF4 is also a delta inverse agonist with an IC(50) value of 1.8 nM. To our knowledge, this is the first potent and selective delta opioid receptor antagonist from the 5-phenylmorphan class of opioids.

Buprenorphine-induced antinociception is mediated by mu-opioid receptors and compromised by concomitant activation of opioid receptor-like receptors

Buprenorphine is a mixed opioid receptor agonist-antagonist used clinically for maintenance therapy in opiate addicts and pain management. Dose-response curves for buprenorphine-induced antinociception display ceiling effects or are bell shaped, which have been attributed to the partial agonist activity of buprenorphine at opioid receptors. Recently, buprenorphine has been shown to activate opioid receptor-like (ORL-1) receptors, also known as OP4 receptors. Here we demonstrate that buprenorphine, but not morphine, activates mitogen-activated protein kinase and Akt via ORL-1 receptors. Because the ORL-1 receptor agonist orphanin FQ/nociceptin blocks opioid-induced antinociception, we tested the hypothesis that buprenorphine-induced antinociception might be compromised by concomitant activation of ORL-1 receptors. In support of this hypothesis, the antinociceptive effect of buprenorphine, but not morphine, was markedly enhanced in mice lacking ORL-1 receptors using the tail-flick assay. Additional support for a modulatory role for ORL-1 receptors in buprenorphine-induced antinociception was that coadministration of J-113397, an ORL-1 receptor antagonist, enhanced the antinociceptive efficacy of buprenorphine in wild-type mice but not in mice lacking ORL-1 receptors. The ORL-1 antagonist also eliminated the bell-shaped dose-response curve for buprenorphine-induced antinociception in wild-type mice. Although buprenorphine has been shown to interact with multiple opioid receptors, mice lacking micro-opioid receptors failed to exhibit antinociception after buprenorphine administration. Our results indicate that the antinociceptive effect of buprenorphine in mice is micro-opioid receptor-mediated yet severely compromised by concomitant activation of ORL-1 receptors.

Distinct molecular recognition of psychostimulants by human and Drosophila serotonin transporters

In this study, human embryonic kidney (HEK)-293 cells stably expressing human, Drosophila, or a chimeric serotonin (5-hydroxytryptamine, 5-HT) transporter (hSERT, dSERT, and H(1-281)D(282-476)H(477-638), respectively) were used to explore the ability of two libraries of structurally distinct psychostimulants to inhibit 5-HT uptake. One library consisted of 3-phenyltropane analogs, whereas the second library consisted of several substituted amphetamines. hSERT exhibited a lower K(i) value for all the compounds in both libraries compared with dSERT, whereas the chimeric SERT exhibited properties more closely resembling those of dSERT. This species selectivity was explored using computer-generated comparative molecular field analysis to model the interactions of the cocaine analogs and substituted amphetamines at hSERT, dSERT, and the cross-species chimera. Models for the 3-phenyltropane analogs indicate that a region exists around the aromatic ring where decreased electron density is favored, particularly for hSERT. This finding may indicate pi-pi stacking with an aromatic amino acid residue in SERT. Also, electronegative substituents in the 4'-position provide favorable interactions. This structural feature was demonstrated by increased potency of analogs with electronegative substituents on the aromatic ring that withdraw electron density. For the substituted amphetamines, key areas for interaction exist around the amine, an electrostatic component surrounding the 3-position on the aromatic ring, and a steric component surrounding the 4-position.

Identification of (3R)-7-hydroxy-N-((1S)-1-[[(3R,4R)-4-(3-hydroxyphenyl)- 3,4-dimethyl-1-piperidinyl]methyl]-2-methylpropyl)-1,2,3,4-tetrahydro- 3-isoquinolinecarboxamide as a novel potent and selective opioid kappa receptor antagonist

(3R)-7-Hydroxy-N-((1S)-1-[[(3R,4R)-4-(3-hydroxyphenyl)-3,4-dimethyl-1-piperidinyl]methyl]-2-methylpropyl)-1,2,3,4-tetrahydro-3-isoquinolinecarboxamide (JDTic) was identified as a potent and selective kappa opioid receptor antagonist. Structure-activity relationship (SAR) studies on JDTic analogues revealed that the 3R,4R stereochemistry of the 3,4-dimethyl-4-(3-hydroxyphenyl)piperidine core structure, the 3R attachment of the 7-hydroxy-1,2,3,4-tetrahydroisoquinoline group, and the 1S configuration of the 2-methylpropyl (isopropyl) group were all important to its kappa potency and selectivity. The results suggest that, like other kappa opioid antagonists such as nor-BNI and GNTI, JDTic requires a second basic amino group to express potent and selective kappa antagonist activity in the [(35)S]GTPgammaS functional assay. However, unlike previously reported kappa antagonists, JDTic also requires a second phenol group in rigid proximity to this second basic amino group. The potent and selective kappa antagonist properties of JDTic can be rationalized using the "message-address" concept wherein the (3R,4R)-3,4-dimethyl-4-(hydroxyphenyl)piperidinyl group represents the message, and the basic amino and phenol group in the N substituent constitutes the address. It is interesting to note the structural commonality (an amino and phenol groups) in both the message and address components of JDTic. The unique structural features of JDTic will make this compound highly useful in further characterization of the kappa receptor.

Reversing the action of noncompetitive inhibitors (MK-801 and cocaine) on a protein (nicotinic acetylcholine receptor)-mediated reaction

The nicotinic acetylcholine receptor (nAChR) is one of five structurally related membrane proteins required for communication between approximately 10(12) cells of the mammalian nervous system. The receptor is inhibited by both therapeutic agents and abused drugs. Understanding the mechanism of noncompetitive allosteric inhibitors of the nicotinic acetylcholine receptor is a long-standing and intensely investigated problem. During the past two decades, many attempts have been made to find drugs that prevent cocaine inhibition, including the synthesis of hundreds of cocaine analogues and derivatives, so far without success. The use of newly developed transient kinetic techniques in investigations of the inhibition of the receptor by the anticonvulsant MK-801 [(+)-dizocilpine] and the abused drug cocaine led to an inhibition mechanism not previously proposed. This mechanism indicates the properties of compounds that would prevent allosteric inhibition of the receptor and how to test for such compounds. Here we present the first evidence that small organic compounds (cocaine derivatives) exist that prevent cocaine and MK-801 inhibition of this receptor. These compounds are RTI-4229-70, a previously synthesized cocaine derivative, and based on its structure four newly synthesized cocaine derivatives, RCS-III-143, RCS-III-140A, RCS-III-218, and RCS-III-202A. Because the nAChR desensitizes rapidly, to make the required measurements a cell-flow technique with a time resolution of 10 ms was used to equilibrate BCH(3) cells containing the fetal mouse muscle-type nAChR with carbamoylcholine. The resulting whole-cell current pertaining to the nondesensitized nAChR was determined. Inhibitors and compounds that alleviate inhibition were tested by their effect on the whole-cell current.

Pharmacodynamic mechanisms of monoclonal antibody-based antagonism of (+)-methamphetamine in rats

Our studies examined pharmacokinetic mechanisms involved in high-affinity (K(d) approximately 11 nM) monoclonal antibody-based antagonism of (+)-methamphetamine-induced locomotor effects. Male rats received (+)-methamphetamine (0.3, 1, or 3 mg/kg i.v.) followed 30 min later by saline or anti-(+)-methamphetamine monoclonal antibody. All groups received a constant dose of monoclonal antibody that was equimolar in binding sites to the body burden of a 1 mg/kg i.v. (+)-methamphetamine dose 30 min after administration. The monoclonal antibody antagonized locomotor effects due to 0.3 and 1 mg/kg (+)-methamphetamine. In contrast, monoclonal antibody treatment increased locomotor activity due to 3 mg/kg (+)-methamphetamine. We also investigated the serum and brain pharmacokinetics of (+)-methamphetamine without and with the monoclonal antibody. Rats received (+)-methamphetamine (1 mg/kg i.v.) followed by saline or monoclonal antibody treatment at 30 min. The monoclonal antibody significantly increased serum methamphetamine concentrations and significantly decreased brain methamphetamine concentrations. These data indicate that anti-(+)-methamphetamine monoclonal antibody-induced pharmacodynamics are complex, but are related to time-dependent changes in (+)-methamphetamine brain distribution.

Withdrawal from repeated cocaine alters dopamine transporter protein turnover in the rat striatum

Several studies have shown that repeated cocaine administration, followed by withdrawal, alters dopamine transporter (DAT) levels in the rat. These changes must arise from changes in either transporter protein production or degradation, or both. Previously, our laboratory developed an approach to measure the synthesis rate, degradation rate constant, and half-life of DAT in the rat striatum and nucleus accumbens after administration of the irreversible dopamine transporter ligand, RTI-76 [3beta-(3-p-chlorophenyl)tropan-2beta-carboxylic acid p-isothiocyanatophenylethyl ester hydrochloride]. Transporter binding was measured with [(3)H]GBR12935 [1-(2-[diphenylmethoxy]ethyl)-4-[3-phenylpropyl]piperazine]. These initial studies showed that: 1) the half-life of the transporter was between 2 and 3 days in these two brain regions; 2) pretreatment with dopamine D1 and D2 receptor agonists and antagonists over several days differentially altered DAT half-lives in the striatum and nucleus accumbens; and 3) pretreatment with cocaine for several days increased the half-life of DAT by decreasing the degradation rate constant in both brain regions. In the present study, we determined that repeated pretreatment (10 days) with 20 mg/kg cocaine (i.p.) and a subsequent withdrawal period (10 days) alters the dopamine transporter turnover in the rat striatum, but not in the nucleus accumbens. Cocaine pretreatment and withdrawal reduced the half-life of the transporter protein from 2.1 days to 0.94 day in the striatum, but did not alter the half-life of 2.2 days in the nucleus accumbens. The results indicate the complex and long-lasting effects of cocaine administration on cellular processes. The mechanism(s) of these effects remains to be elucidated.