Identification of the first trans-(3R,4R)- dimethyl-4-(3-hydroxyphenyl)piperidine derivative to possess highly potent and selective opioid kappa receptor antagonist activity

A structurally novel opioid kappa receptor selective ligand has been identified. This compound, (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, 10) demonstrated high affinity for the kappa receptor in the binding assay (kappa K(i) = 0.3 nM) and highly potent and selective kappa antagonism in the [(35)S]GTP-gamma-S assay using cloned opioid receptors (kappa K(i) = 0.006 nM, mu/kappa ratio = 570, delta/kappa ratio > 16600).

Structure-activity relationship of a series of diaminoalkyl substituted benzimidazole as neuropeptide Y Y1 receptor antagonists

A series of benzimidazoles (4) was synthesized and evaluated in vitro as potent and selective NPY Y1 receptor antagonists. Substitution of the piperidine nitrogen of 4 with appropriate R groups resulted in compounds with more than 80-fold higher affinity at the Y receptor compared to the parent compound 5 (R = H). The most potent benzimidazole in this series was 21 (Ki = 0.052 nM).

Identification of an opioid kappa receptor subtype-selective N-substituent for (+)-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine

A three-component library of compounds was prepared in parallel using multiple simultaneous solution-phase synthetic methodology. The compounds were biased toward opioid receptor antagonist activity by incorporating (+)-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine (a potent, nonselective opioid pure antagonist) as one of the monomers. The other two monomers, which included N-substituted or unsubstituted Boc-protected amino acids and a range of substituted aryl carboxylic acids, were selected to add chemical diversity. Screening of these compounds in competitive binding experiments with the kappa opioid receptor selective ligand [3H]U69,593 led to the discovery of a novel kappa opioid receptor selective ligand, N-¿(2'S)-[3-(4-hydroxyphenyl)propanamido]-3'-methylbutyl¿-(3R, 4R)-dimethyl-4-(3-hydroxyphenyl)piperidine (8, RTI-5989-29). Additional structure-activity relationship studies suggested that 8 possesses lipophilic and hydrogen-bonding sites that are important to its opioid receptor potency and selectivity. These sites appear to exist predominantly within the kappa receptor since the selectivity arises from a 530-fold loss of affinity of 8 for the mu receptor and an 18-fold increase in affinity for the kappa receptor relative to the mu-selective ligand, (+)-N-[trans-4-phenyl-2-butenyl]-(3R, 4R)-dimethyl-4-(3-hydroxyphenyl)piperidine (5a). The degree of selectivity observed in the radioligand binding experiments was not observed in the functional assay. According to its ability to inhibit agonist stimulated binding of [35S]GTPgammaS at all three opioid receptors, compound 8 behaves as a mu/kappa opioid receptor pure antagonist with negligible affinity for the delta receptor.

N-substituted octahydro-4a-(3-hydroxyphenyl)-10a-methyl-benzo[g]isoquinolines are opioid receptor pure antagonists

N-Methyl- and N-phenylethyl-(+/-)-1,2,3,4,4a,5,10,10a- octahydro-4a-(3-hydroxyphenyl)-10a-methyl-benzo[g]isoquinolines (4 and 5, respectively) were found to be pure opioid antagonists. These compounds were shown to share many of the characteristics identified with the N-methyl- and N-phenylethyl trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidine (1 and 2, respectively) including N-substituent mediated potency and a lack of N-substituent mediated antagonism. These data suggest that compounds 4 and 5 and the N-substituted trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidines (1 and 2) may interact with opioid receptors similarly.

N-Substituted 9beta-methyl-5-(3-hydroxyphenyl)morphans are opioid receptor pure antagonists

The inhibition of radioligand binding and [35S]GTPgammaS functional assay data for N-methyl- and N-phenethyl-9beta-methyl-5-(3-hydroxyphenyl)morphans (5b and 5c) show that these compounds are pure antagonists at the micro, delta, and kappa opioid receptors. Since 5b and 5c have the 5-(3-hydroxyphenyl) group locked in a conformation comparable to an equatorial group of a piperidine chair conformation, this information provides very strong evidence that opioid antagonists can interact with opioid receptors in this conformation. In addition, it suggests that the trans-3, 4-dimethyl-4-(3-hydroxyphenyl)piperidine class of antagonist operates via a phenyl equatorial piperidine chair conformation. Importantly, the close relationship between the 4-(3-hydroxyphenyl)piperidines and 5-(3-hydroxyphenyl)morphan antagonists shows that the latter class of compound provides a rigid platform on which to build a novel series of opioid antagonists.

Synthesis and evaluation of a series of novel 2-[(4-chlorophenoxy)methyl]benzimidazoles as selective neuropeptide Y Y1 receptor antagonists

A series of novel benzimidazoles (BI) derived from the indole 2 was synthesized and evaluated as selective neuropeptide Y (NPY) Y1 receptor antagonists with the aim of developing antiobesity drugs. In our SAR approach, the (4-chlorophenoxy)methyl group at C-2 was kept constant and a series of BIs substituted with various piperidinylalkyl groups at N-1 was synthesized to identify the optimal spacing and orientation of the piperidine ring nitrogen relative to the benzimidazole. The 3-(3-piperidinyl)propyl in 33 was found to maximize affinity for the Y1 receptor. Because of the critical importance of Arg33 and Arg35 of NPY binding to the Y1 receptor, the incorporation of an additional aminoalkyl functionality to the structure of 33 was explored. Methyl substitution was used to probe where substitution on the aromatic ring was best tolerated. In this fashion, the C-4 was chosen for the substitution of the second aminoalkyl functionality. Synthesis of such compounds with a phenoxy tether using the 4-hydroxybenzimidazole 11 was pursued because of their relative ease of synthesis. Functionalization of the hydroxy group of 45 with a series of piperidinylalkyl groups provided the dibasic benzimidazoles 55-62. Among them, BI 56 demonstrated a Ki of 0.0017 microM, which was 400-fold more potent than 33. To evaluate if there was a stereoselective effect on affinity for these BIs, the four constituent stereoisomers (69-72) of the BI 60 were prepared using the S- and R-isomers of bromide 17. Antagonist activity of these BIs was confirmed by measuring the ability of selected compounds to reverse NPY-induced forskolin-stimulated cyclic AMP. The high selectivity of several BI antagonists for the Y1 versus Y2, Y4, and Y5 receptors was also shown.

Investigation of the N-substituent conformation governing potency and mu receptor subtype-selectivity in (+)-(3R, 4R)-dimethyl-4-(3-hydroxyphenyl)piperidine opioid antagonists

A study of the binding site requirements associated with the N-substituent of (+)-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine (4) derivatives was undertaken using a set of rigid vs flexible N-substituents. The study showed that compounds 7-9 bearing the trans-cinnamyl N-substituent most closely reproduced the potency at the opioid receptor of the flexible N-propylphenyl or N-propylcyclohexyl analogues previously reported. Neither the N-substituted cis-cinnamyl nor the cis-phenylcyclopropylmethyl compounds 10 and 11, respectively, showed high affinity for the opioid receptor. However, the N-trans-phenylcyclopropylmethyl compound 12 closely approximated the affinity of compounds 7-9. Additionally, we found that free rotation of the phenyl ring is necessary for high affinity binding and mu receptor subtype selectivity as the planar N-substituted thianaphthylmethyl and benzofuranylmethyl compounds 13 and 14 had significantly lower binding affinities. Altogether, these findings suggest that the high binding affinity, selectivity, and antagonist potency of N-propylphenyl or N-propylcyclohexyl analogues of (+)-(3R, 4R)-dimethyl-4-(3-hydroxyphenyl)piperidine (4) are achieved via a conformation wherein the connecting chain of the N-substituents is extended away from piperidine nitrogen with the appended ring system rotated out-of-plane relative to the connecting chain atoms. This conformation is quite similar to that observed in the solid state for 5, as determined by single crystal X-ray analysis. Additionally, it was found that, unlike naltrexone, N-substituents bearing secondary carbons attached directly to the piperidine nitrogen of 4 suffer dramatic losses of potency vs analogues not substituted in this manner. Using a functional assay which measured stimulation or inhibition of [35S]GTP-gamma-S binding, we show that the trans-cinnamyl analogues of (+)-(3R, 4R)-dimethyl-4-(3-hydroxyphenyl)piperidine (4) retain opioid pure antagonist activity and possess picomolar antagonist potency at the mu receptor.

Structure-activity relationships of a series of 1-substituted-4-methylbenzimidazole neuropeptide Y-1 receptor antagonists

The characterization of a novel series of NPY-1 receptor antagonists derived from the 4-methylbenzimidazole 4 is described. Appropriate substitution on the piperidyl nitrogen of 4 led to systematic increases in Y-1 receptor affinity, to approximately 50-fold, and to the discovery of the importance of a second basic substituent.

Synthesis of a series of aryl kainic acid analogs and evaluation in cells stably expressing the kainate receptor humGluR6

The synthesis and pharmacological characterization of a novel series of 4-aryl-substituted kainic acid analogs are described. Receptor affinities were determined on recombinantly expressed humGluR6 kainate receptors and on [3H]kainate binding to rat forebrain kainate receptors. Functional agonist potencies were assessed using whole cell voltage clamp recordings in cells expressing humGluR6 receptors. Substitution of phenyl for the methyl at the C-4 position of kainic acid produced 11 which has high affinity and agonist potency at the GluR6 receptor. Substitution on phenyl led to a series of compounds with varying affinity for this kainate receptor. Agonist potency correlated with receptor affinity and with no derivative could antagonist activity be identified. Affinities for the humGluR6 kainate receptor were approximately 10-50 less than the observed affinities at rat forebrain kainate receptors. Furthermore, within the series of 4-aryl-substituted kainic acid analogs, there was a high degree of correlation between binding affinities for humGluR6 receptors and competition with kainate binding to rat forebrain kainate receptors.

Discovery of a potent, peripherally selective trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidine opioid antagonist for the treatment of gastrointestinal motility disorders

Structure-activity relationship studies were pursued within N-substituted-trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidines in an effort to discover a peripherally selective opioid antagonist with high activity following systemic administration. Altering the size and the polarity of the N-substituent led to the discovery of 3 (LY246736). Compound 3 has high affinity for opioid receptors (Ki = 0.77, 40, and 4.4 nM for mu, kappa, and delta receptors, respectively). It is a potent mu receptor antagonist following parenteral and oral administration and distributes selectively (> 200-fold selectivity) to peripheral receptors. Thus, 3 has properties suitable for the clinical investigation of mu opioid receptor involvement in GI motility disorders.

3,4-Dimethyl-4-(3-hydroxyphenyl)piperidines: opioid antagonists with potent anorectant activity

A series of (3R*,4R*)-3,4-dimethyl-4-(3-hydroxyphenyl)piperidine opioid antagonists with varying substituents on the nitrogen were evaluated for their effect on food consumption in obese Zucker rats. Opioid affinity (mu, kappa, and delta for selected compounds) and opioid antagonist activity (mu and kappa) were characterized and compared to effects on food consumption. No compounds with high selectivity for either mu or kappa receptors were discovered. However, compounds in the series had exceptional potency as opioid antagonists and in reducing food consumption in the obese Zucker rat. In contrast, a few compounds with high potency as opioid antagonists had much weaker potency for inhibiting food consumption. (3R,4R)-3,4-Dimethyl-1-[(3S)-3- hydroxy-3-cyclohexyl-propyl]-4-(3-hydroxyphenyl)piperidine (11,LY255582) emerged as having the best activity profile, both in reducing food consumption and as an opioid antagonist. Compound 11 is a highly potent mu, kappa-, and delta-opioid antagonist with possible clinical utility as an appetite suppressant for weight loss.

Structure-activity relationships of trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidine antagonists for mu- and kappa-opioid receptors

A series of racemic N-substituted trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidines were evaluated for opioid agonist and antagonist activity at mu and kappa receptors. Several highly potent mu and kappa antagonists were discovered; however, no compounds with high selectivity for either the mu or kappa receptor were identified. Importantly, no derivative was found to have significant opioid agonist activity. Two derivatives were resolved, and the activities of the enantiomers were investigated. Only a limited stereochemical effect on opioid receptor selectivities was observed. The structure-activity relationships described establish the existence of an important lipophilic binding site distal to the nitrogen for both mu and kappa receptors and confirm the pure opioid antagonist pharmacophore nature of the trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidine structure.

Synthesis and analgesic properties of N-substituted trans-4a-aryldecahydroisoquinolines

A representative series of N-substituted derivatives of the morphine-based trans-4a-aryldecahydroisoquinoline were synthesized and evaluated for opioid analgesic activities. Compounds with potent analgesic activity and high affinities for the mu and kappa opioid receptors were discovered. The effect of varying the N-substituent in the trans-4a-aryldecahydroisoquinoline paralleled, to a certain extent, previous findings with other morphine part structures. Replacement of the N-methyl with a phenethyl group significantly increased analgesic potency. The N-cyclopropylmethyl analogue was found in rodents to have mixed agonist-antagonist properties; however, its antagonist activity was far weaker than those reported for the N-(cyclopropylmethyl)morphinan and -benzomorphan derivatives. Resolution of the stereoisomers and determination of their absolute configuration by X-ray crystallography showed that the opioid receptor effects were predominantly found with the 4aR,8aR isomer, the same relative absolute configuration of morphine. Unexpectedly, the 4aR,8aR N-cyclopropylmethyl analogue (compound 30), which in rodents had mixed agonist-antagonist properties similar to those of pentazocine, was found in rhesus monkeys to behave as a full morphine-like agonist.

Characterization of the neurotoxic potential of m-methoxy-MPTP and the use of its N-ethyl analogue as a means of avoiding exposure to a possible Parkinsonism-causing agent

1-Methyl-4-(3-methoxyphenyl)-1,2,3,6-tetrahydropyridine (2) produced persistent depletion of striatal dopamine in mice after four daily injections, although it was less potent than 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). MPTP has been implicated as a cause of Parkinsonism in drug abusers who inadvertently self-administered it and in industrial chemists who were exposed to it. Our results suggest that the m-methoxy compound has the same neurotoxic potential to cause destruction of nigrostriatal dopamine neurons that would lead to Parkinsonian symptoms in humans. In contrast, 1-ethyl-4-(3-methoxyphenyl)-1,2,3,6-tetrahydropyridine (11) had no effects on striatal dopamine in mice, even at doses 8 times those of MPTP. A method of preparing 11 and using it as an intermediate in the synthesis of potential analgesic drugs, thus avoiding a potentially neurotoxic intermediate, is described.