Conformational characteristics of the interaction of SR141716A with the CB1 cannabinoid receptor as determined through the use of conformationally constrained analogs

Interest in cannabinoid pharmacology increased dramatically upon the identification of the first cannabinoid receptor (CB1) in 1998 and continues to expand as additional endocannabinoids and cannabinoid receptors are discovered. Using CB1 receptor (CB1R) systems, medicinal chemistry programs began screening libraries searching for cannabinoid ligands, ultimately leading to the discovery of the first potent cannabinoid receptor antagonist, SR141716A (Rimonabant). Its demonstrated efficacy in treating obesity and facilitating smoking cessation, among other impressive pharmacological activities, has furthered the interest in cannabinoid receptor antagonists as therapeutics, such that the number of patents and publications covering this class of compounds continues to grow at an impressive rate. At this time, medicinal chemistry approaches including combinatorial chemistry, conformational constraint, and scaffold hopping are continuing to generate a large number of cannabinoid antagonists. These molecules provide an opportunity to gain insight into the 3-dimensional structure-activity relationships that appear crucial for CB1R-ligand interaction. In particular, studies in which conformational constraints have been imposed on the various pyrazole ring substituents of SR141716A provide a direct opportunity to characterize changes in conformation/conformational freedom within a single class of compounds. While relatively few conformationally constrained molecules have been synthesized to date, the structure-activity information is often more readily interpreted than in studies where entire substituents are replaced. Thus, it is the focus of this mini-review to examine the structural properties of SR141716A, and to use conformationally constrained molecules to illustrate the importance of conformation and conformational freedom to CB1R affinity, selectivity, and efficacy.

Synthesis of long-chain amide analogs of the cannabinoid CB1 receptor antagonist N-(piperidinyl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716) with unique binding selectivities and pharmacological activities

An extended series of alkyl carboxamide analogs of N-(piperidinyl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl- 1H-pyrazole-3-carboxamide (SR141716; 5) was synthesized. Each compound was tested for its ability to displace the prototypical cannabinoid ligands ([3H]CP-55,940, [3H]2; [3H]SR141716, [3H]5; and [3H]WIN55212-2, [3H]3), and selected compounds were further characterized by determining their ability to affect guanosine 5'-triphosphate (GTP)-gamma-[35S] binding and their effects in the mouse vas deferens assay. This systematic evaluation has resulted in the discovery of novel compounds with unique binding properties at the central cannabinoid receptor (CB1) and distinctive pharmacological activities in CB1 receptor tissue preparations. Specifically, compounds with nanomolar affinity which are able to fully displace [3H]5 and [3H]2, but unable to displace [3H]3 at similar concentrations, have been synthesized. This selectivity in ligand displacement is unprecedented, in that previously, compounds in every structural class of cannabinoid ligands had always been shown to displace each of these radioligands in a competitive fashion. Furthermore, the selectivity of these compounds appears to impart unique pharmacological properties when tested in a mouse vas deferens assay for CB1 receptor antagonism.

Preparation of monoclonal antibodies reactive to the endogenous small molecule, anandamide

The development of an easy and inexpensive immunoassay to measure the limited quantities of endogenous cannabinoids found in the body would be beneficial for both cannabinoid researchers and clinicians. This report describes the hapten design and carrier molecule strategy that we used to generate a panel of monoclonal antibodies (mAB) to the endogenous cannabinoid anandamide (N-arachidonylethanolamide, AEA). We designed and successfully prepared a hapten, N-arachidonyl-7-amino-6-hydroxy-heptanoic acid (AHA), which retained the basic characteristic features of anandamide--the carboxamide, the hydroxyl and the lipophilic arachidonyl moiety with its skipped double bond system, while still allowing attachment to protein. In addition, a secondary alcohol structure was added to reduce the potential for biological hydrolysis of the hapten. Because of the diverse responses obtained after coupling this hapten to four different carriers, we determined that the type of carrier molecule used was particularly important for generating anti-anandamide antibodies. Described in this report are the characteristics of a panel of 11 mAB, generated from four separate fusions, with a range of relative affinities and cross reactivities. Excellent selectivity for anandamide vs. two other endogenous cannabinoids and arachidonic acid was achieved this strategy (cross-reactivities <5%). In addition, at least one mAB maintained specificity for anandamide compared to two very closely related fatty acid amide molecules. However, the IC50 values in a standard enzyme-linked immunosorbent assay (ELISA) format (ca. 2-3 microM) indicate that improvement in antibody affinities or assay format will be required for an immunoassay to measure endogenous levels. Such work is underway.

N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A) interaction with LYS 3.28(192) is crucial for its inverse agonism at the cannabinoid CB1 receptor

In superior cervical ganglion neurons, N-(piperidiny-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A) competitively antagonizes the Ca(2+) current effect of the cannabinoid (CB) agonist (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone (WIN55212-2), and behaves as an inverse agonist by producing opposite current effects when applied alone. In contrast, in neurons expressing CB1 with a K-->A mutation at residue 3.28(192) (i.e., K3.28A), SR141716A competitively antagonizes the effects of WIN55212-2, but behaves as a neutral antagonist by producing no current effects itself. Receptor modeling studies suggested that in the CB1 inactive (R) state, SR1417A16A stabilizes transmembrane helix 6 in its inactive conformation via aromatic stacking with F3.36/W6.48. In this binding site, SR141716A would exhibit higher affinity for CB1 R due to a hydrogen bond between the SR141716A C3 substituent and K3.28(192), a residue available to SR141716A only in R. To test this hypothesis, a "mutant thermodynamic cycle" was constructed that combined the evaluation of SR141716A affinity at WT CB1 and K3.28A with an evaluation of the wild-type CB1 and K3.28A affinities of an SR141716A analog, 5-(4-chlorophenyl)-3-[(E)-2-cyclohexylethenyl]-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole (VCHSR), that lacks hydrogen bonding potential at C3. Binding affinities suggested that K3.28 is involved in a strong interaction with SR141716A in WT CB1, but does not interact with VCHSR. Thermodynamic cycle calculations indicated that a direct interaction occurs between the C3 substituent of SR141716A and K3.28 in WT CB1. Consistent with these results, VCHSR acted as a neutral antagonist at WT CB1. These results support the hypothesis that hydrogen bonding of the SR141716A C3 substituent with K3.28 is responsible for its higher affinity for the inactive R state, leading to its inverse agonism.

Synthesis and structure-activity relationships of amide and hydrazide analogues of the cannabinoid CB(1) receptor antagonist N-(piperidinyl)- 5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716)

Analogues of the biaryl pyrazole N-(piperidinyl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716; 5) were synthesized to investigate the structure-activity relationship (SAR) of the aminopiperidine region. The structural modifications include the substitution of alkyl hydrazines, amines, and hydroxyalkylamines of varying lengths for the aminopiperidinyl moiety. Proximity and steric requirements at the aminopiperidine region were probed by the synthesis of analogues that substitute alkyl hydrazines of increasing chain length and branching. The corresponding amide analogues were compared to the hydrazides to determine the effect of the second nitrogen on receptor binding affinity. The N-cyclohexyl amide 14 represents a direct methine for nitrogen substitution for 5, reducing the potential for heteroatom interaction, while the morpholino analogue 15 adds the potential for an additional heteroatom interaction. The series of hydroxyalkyl amides of increasing chain length was synthesized to investigate the existence of additional receptor hydrogen binding sites. In displacement assays using the cannabinoid agonist [(3)H](1R,3R,4R)-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-4-(3-hydroxypropyl) cyclohexan-1-ol (CP 55 940; 2) or the antagonist [(3)H]5, 14 exhibited the highest CB(1) affinity. In general, increasing the length and bulk of the substituent was associated with increased receptor affinity and efficacy (as measured in a guanosine 5'-triphosphate-gamma-[(35)S] assay). However, in most instances, receptor affinity and efficacy increases were no longer observed after a certain chain length was reached. A quantitative SAR study was carried out to characterize the pharmacophoric requirements of the aminopiperidine region. This model indicates that ligands that exceed 3 A in length would have reduced potency and affinity with respect to 5 and that substituents with a positive charge density in the aminopiperidine region would be predicted to possess increased pharmacological activity.

Structure and receptor activity for classical cannabinoids

In this decade, classical cannabinoid SAR has increasingly been interpreted in terms of ligand-receptor interaction. Membrane receptor binding affinity, inhibition of receptor coupled processes, functional in vitro assays that distinguish between agonists and antagonists, transfected receptor systems, and distinction among receptor subtypes have been applied to cannabinoid pharmacology studies. These studies have revealed information about the pivotal receptor interaction that mediates the neurochemical system characterized by the receptor. The positive correlation between binding affinity and behavioral effects validates ligand-receptor interaction as a focus for developing new cannabinoids with the potential for selective pharmacological effects. Added to these direct receptor studies are the computational methods that compare active and inactive ligands for steric, electronic, and conformational similarities that reveal an SAR for activity with a sophistication and a perspective not possible before computer methodology. Merging these studies with knowledge of the receptor sequence, consequences of mutations of the receptor, and a calculated structure of the receptor are evolving a physical picture of the interaction of cannabinoids with its receptor(s). This picture of a ligand-receptor interaction guides medicinal chemists in their interpretation of classical cannabinoid SAR and the design of new analogs for selective pharmacological activity and as probes of the cannabinoid receptor. It is from a receptor perspective that this review of the SAR of classical cannabinoids is presented.

The bioactive conformation of aminoalkylindoles at the cannabinoid CB1 and CB2 receptors: insights gained from (E)- and (Z)-naphthylidene indenes

The aminoalkylindoles (AAIs) are agonists at both the cannabinoid CB1 and CB2 receptors. To determine whether the s-trans or s-cis form of AAIs is their receptor-appropriate conformation, two pairs of rigid AAI analogues were studied. These rigid analogues are naphthylidene-substituted aminoalkylindenes that lack the carbonyl oxygen of the AAIs. Two pairs of (E)- and (Z)-naphthylidene indenes (C-2 H and C-2 Me) were considered. In each pair, the E geometric isomer is intended to mimic the s-trans form of the AAIs, while the Z geometric isomer is intended to mimic the s-cis form. Complete conformational analyses of two AAIs, pravadoline (2) and WIN-55, 212-2 (1), and of each indene were performed using the semiempirical method AM1. S-trans and s-cis conformations of 1 and 2 were identified. AM1 single-point energy calculations revealed that when 1 and each indene were overlayed at their corresponding indole/indene rings, the (E)- and (Z)-indenes were able to overlay naphthyl rings with the corresponding s-trans or s-cis conformer of 1 with an energy expense of 1.13/0.69 kcal/mol for the C-2 H (E/Z)-indenes and 0.82/0.74 kcal/mol for the C-2 Me (E/Z)-indenes. On the basis of the hypothesis that aromatic stacking is the predominant interaction of AAIs such as 1 at the CB receptors and on the demonstration that the C-2 H (E/Z)- and C-2 Me (E/Z)-indene isomers can mimic the positions of the aromatic systems in the s-trans and s-cis conformers of 1, the modeling results support the previously established use of indenes as rigid analogues of the AAIs. A synthesis of the naphthylidene indenes was developed using Horner-Wittig chemistry that afforded the Z isomer in the C-2 H series, which was not produced in significant amounts from an earlier reported indene/aldehyde condensation reaction. This approach was extended to the C-2 Me series as well. Photochemical interconversions in both the C-2 H and C-2 Me series were also successful in obtaining the less favored isomer. Thus, the photochemical process can be used to provide quantities of the minor isomers C-2 H/Z and C-2 Me/E. The CB1 and CB2 affinities as well as the activity of each compound in the twitch response of the guinea pig ileum (GPI) assay were assessed. The E isomer in each series was found to have the higher affinity for both the CB1 and CB2 receptors. In the rat brain membrane assay versus [3H]CP-55,940, the Ki's for the C-2 H/C-2 Me series were 2.72/2.89 nM (E isomer) and 148/1945 nM (Z isomer). In membrane assays versus [3H]SR141716A, a two-site model was indicated for the C-2 H/C-2 Me (E isomers) with Ki's of 10. 8/9.44 nM for the higher-affinity site and 611/602 nM for the lower-affinity site. For the Z isomers, a one-site model was indicated with Ki's of 928/2178 nM obtained for the C2 H/C-2 Me analogues, respectively. For the C-2 H/C-2 Me series, the CB2 Ki's obtained using a cloned cell line were 2.72/2.05 nM (E isomer) and 132/658 nM (Z isomer). In the GPI assay, the relative order of potency was C-2 H E > C-2 Me E > C-2 H Z > C-2 Me Z. The C-2 H E isomer was found to be equipotent with 1, while the C-2 Me Z isomer was inactive at concentrations up to 3.16 microM. Thus, results indicate that the E geometric isomer in each pair of analogues is the isomer with the higher CB1 and CB2 affinities and the higher pharmacological potency. Taken together, results reported here support the hypothesis that the s-trans conformation of AAIs such as 1 is the preferred conformation for interaction at both the CB1 and CB2 receptors and that aromatic stacking may be an important interaction for AAIs at these receptors.

Comparative receptor binding analyses of cannabinoid agonists and antagonists

To further characterize neuronal cannabinoid receptors, we compared the ability of known and novel cannabinoid analogs to compete for receptor sites labeled with either [3H]SR141716A or [3H]CP-55,940. These efforts were also directed toward extending the structure-activity relationships for cannabinoid agonists and antagonists. A series of alternatively halogenated analogs of SR141716A were synthesized and tested in rat brain membrane binding assays along with the classical cannabinoids, Delta9-tetrahydrocannabinol, cannabinol, cannabidiol, the nonclassical cannabinoid CP-55,940, the aminoalkylindole WIN55212-2 and the endogenous fatty acid ethanolamide, anandamide. Saturation binding isotherms were performed with both radioligands, as were displacement studies, allowing an accurate comparison to be made between the binding of these various compounds. Competition studies demonstrated that all of the compounds were able to displace the binding of [3H]CP-55,940 with rank order potencies that agreed with previous studies. However, the rank order potencies of these compounds in competition studies with [3H]SR141716A differed significantly from those determined with [3H]CP-55,940. These results suggest that CP-55,940, WIN55212-2 and other agonists interact with cannabinoid binding sites within the brain which are distinguishable from the population of binding sites for SR141716A, its analogs and cannabidiol. Structural modification of SR141716A significantly altered the affinity of the compound and its relative ability to displace either [3H]CP-55,940 or [3H]SR141716A preferentially within the rat brain receptor membrane preparation.

Synthesis and pharmacological comparison of dimethylheptyl and pentyl analogs of anandamide

(Dimethylheptyl)anandamide [(16,16-dimethyldocosa-cis-5,8,11,14-tetraenoyl)ethanolamine ] (17a) and its amide analogs were synthesized by Wittig coupling of a ylide derived from a fragment of arachidonic acid. These amides were compared to the endogenous cannabinoid receptor ligand arachidonylethanolamide (anandamide, 2a) and its amide analogs in pharmacological assays for potential enhancement of cannabimimetic activities. The receptor affinity to rat brain membranes of the dimethylheptyl (DMH) analogs increased by an order of magnitude in most comparisons to the corresponding anandamides in displacement assays versus the cannabinoid agonist [3H]CP 55,940 or antagonist [3H]SR141716A, for which rank order differences in affinity were observed. An order of magnitude enhancement of potency with comparable or higher efficacy in behavioral assays in the mouse tetrad of tests of cannabinoid activity was observed in 17a versus 2a. In contrast, no enhancement in potency for the pentyl to DMH side chain exchange was seen in the mouse vas deferens assay. The data indicate a structural equivalence between classical plant cannabinoids and 2a as well as different receptor-ligand interactions that characterize multiple receptor sites or binding modes.

Importance of the C-1 substituent in classical cannabinoids to CB2 receptor selectivity: synthesis and characterization of a series of O,2-propano-delta 8-tetrahydrocannabinol analogs

The separation of the mood-altering effects of cannabinoids from their therapeutic effects has been long sought. Results reported here for a series of C-9 analogs of the cyclic ether O,2-propano-delta 8-tetrahydrocannabinol (O,2-propano-delta 8-THC) point to the C-1 position in classical cannabinoids as a position for which CB2 subtype selectivity occurs within the cannabinoid receptors. O,2-Propano-11-delta 8-THC, O,2-propano delta 9,11-THC, O,2-propano-9-oxo-11-nor-hexahydrocannabinol (O,2-propano-9-oxo-11-nor-HHC), and O,2-propano-9 alpha- and O,2-propano-9 beta-OH-11-nor-HHC were synthesized and evaluated in radioligand displacement assays for affinity at the CB1 and CB2 receptors and in the mouse vas deferens in vitro assay and the mouse tetrad in vivo assay for cannabinoid activity. Evaluation of binding affinity at the CB1 and CB2 receptors revealed that each compound possesses a modest increased affinity for the CB2 receptor. Analogs which contained an oxygen attached to C-9 (i.e., oxo and hydroxy derivatives) showed the highest affinity and selectivity for CB2 (for O,2-propano-9-oxo-11-nor-HHC, Ki(CB1) = 90 nM, Ki(CB2) = 23 nM, selectivity ratio 3.9; for O,2-propano-9 beta-OH-11-nor-HHC, Ki(CB1) = 26 nM, Ki(CB2) = 5.8 nM, selectivity ratio 4.5). Each compound was found to produce a dose-dependent inhibition of electrically-evoked contractions of the mouse isolated vas deferens when administered at submicromolar concentrations. This inhibition could readily be prevented by the selective CB1 cannabinoid receptor antagonist SR-141716A. The analogs exhibited unique in vivo profiles with O,2-propano-delta 9,11-THC exhibiting antinociception with reduced activity in three other in vivo measures and O,2-propano-9 beta-OH-HHC exhibiting lack of dose responsiveness in all measures. The CB2 selectivities in the O,2-propano analogs may be due to differences in solvation/desolvation that occur when the ligands enter the CB1 vs CB2 binding site. Alternatively, the CB2 selectivities may be a results of an amino acid change from a hydrogen bond-accepting residue in CB1 to a hydrogen bond-donating residue in CB2.

Synthesis and in vivo studies of a selective ligand for the dopamine transporter: 3 beta-(4-[125I]iodophenyl) tropan-2 beta-carboxylic acid isopropyl ester ([125I]RTI-121)

A selective ligand for the dopamine transporter 3 beta-(4-iodophenyl)tropan-2 beta-carboxylic acid isopropyl ester (RTI-121) has been labeled with iodine-125 by electrophilic radioiododestannylation. The [125I]RTI-121 was obtained in good yield (86 +/- 7%, n = 3) with high radiochemical purity (> 99%) and specific radioactivity (1210-1950 mCi/mumol). After i.v. administration of [125I]RTI-121 to mice, the rank order of regional brain tissue radioactivity (striatum > olfactory tubercles > > cortex, hippocampus, thalamus, hypothalamus, cerebellum) was consistent with dopamine transporter labeling. Specific in vivo binding in striatum and olfactory tubercles was saturable, and was blocked by the dopamine transporter ligands GBR 12,909 and (+/-)-nomifensine. By contrast, binding was not reduced by paroxetine, a serotonin transporter inhibitor, or desipramine, a norepinephrine transporter inhibitor. A variety of additional drugs having high affinities for recognition sites other than the neuronal dopamine transporter also had no effect. The [125I]RTI-121 binding in striatum and olfactory tubercles was inhibited by d-amphetamine in dose-dependent fashion. Nonmetabolized radioligand represents 85% of the signal observed in extracts of whole mouse brain. Thus, [125I]RTI-121 is readily prepared, and is a useful tracer for dopamine transporter studies in vivo.

Mapping dopamine transporters in the human brain with novel selective cocaine analog [125I]RTI-121

The novel cocaine analog RTI-121 [3 beta-(4-iodophenyl)tropane-2 beta-carboxylic acid isopropyl ester] was evaluated as a probe for the in vitro labeling and localization of the dopamine transporter in the human brain. Saturation binding experiments conducted in sucrose-phosphate buffer (10 mM sodium phosphate, pH 7.4, 0.32 M sucrose) revealed high- and low-affinity binding components with affinity values (KD) of 0.25 +/- 0.04 and 4.9 +/- 1.6 nM (mean +/- SE) and densities (Bmax) of 56.8 +/- 13.8 and 147.7 +/- 23.4 pmol/g tissue, respectively. In contrast, when saturation binding experiments were performed in phosphate-buffered saline (10 mM Na2HPO4, 1.8 mM KH2PO4, 136 mM NaCl, 2.8 mM KCl, 10 mM NaI, pH 7.4), a 9-fold decrease in the density of the low-affinity component was noted, suggesting that the low-affinity RTI-121 binding site is associated with the region of the transporter involved in the ionic dependence of substrate recognition and/or uptake. The rank order of potency for inhibition of [125I]RTI-121 binding to human caudate membranes demonstrates that the radioligand selectively labels the dopamine transporter (GBR 12909 > RTI-121 > mazindol > nomifensine > (-) cocaine > desipramine > citalopram). Autoradiographic mapping of [125I]RTI-121 revealed very high densities of cocaine recognition sites over areas known to be rich in dopaminergic innervation, including the caudate, putamen, and nucleus accumbens. Moderate densities were also observed over the substantia nigra and the ventral tegmental area. Low-to-background labeling of [125]RTI-121 was seen throughout the cerebral cortex, amygdaloid nuclei, globus pallidus, and thalamus. In comparison with the autoradiographic distribution of the cocaine analogs [3H]WIN 35,428 (or CFT) and [125I]RTI-55 (or beta-CIT), the labeling pattern for [125I]RTI-121 was more restricted. These studies demonstrate that [125I]RTI-121 labels dopamine-rich brain regions with greater selectivity than other currently available cocaine analogs, which makes it a potentially superior imaging probe for mapping the dopamine transporter in the human brain.