Nocaine was first reported in 1998 in the guise of a cocaine mimic. Infact, phenylpiperidine derives from the same article in which phenyltropane was first published, but it was essentially impotent in tests conducted on mice. The (3R,4S) isomer of nocaine only manages to elicit weakly reinforcing effects, although it is several-fold less dangerous than cocaine. Thus, it is hypothesized that nocaine might be a good substitute type of agent with potential uses in treating cocaine addiction. Clearly, these compounds don't contain the necessary tropane 2C linker, required to deliver high intensity ambulatory counts, although they are still able to interact with MA transporters in an inhibitory fashion. In order to simply the situation in an a priori sense, it was chosen first to tune the focus into the DAT, since this is a critical recognition site, in the field of cocaine addiction (A. Kozikowski, et al. 1998). Given the similarity & innovation of these analogs, compared to PT based cocaine derivatives, steps were taken to explore their pharmacological activity (W. Woolverton, et al. 2002), (A. Kozikowski, et al. 2003).
Nocaine: Basic Pharmacology
Like cocaine, SS and 3R,4S bind to the DAT and inhibit DA uptake, stimulate LMA in rodents and completely substitute for cocaine in 2D tests. Pretreatment with SS or 3R,4S enhances the cocaine discriminative stimulus in rats. However, the LMA effects of the piperidine based compounds are much less than those induced by cocaine. Pretreating mice with SS or 3R,4S does not increase cocaine induced convulsions in mice. Furthermore, pretreating mice with 3R,4S actually attenuated cocaine-induced locomotor stimulation. With regard to reinforcing effects, SS is similar to cocaine as revealed by their nearly identical inverted U-shaped dose-response curves in fixed-ratio self-administration tests in rats. 3R,4S, however, has a flat dose-response curve in FR SA tests. Similarly, SS and cocaine had nearly identical break points in a PR SA test, whereas 3R,4S has a lower break point than either of these two drugs.
|Compound||[3H]NE nM||[3H]5-HT nM||[3H]DA nM||5-HT/NE||DA/5-HT||DA/NE|
|Cocaine||119 ± 38||177 ± 13||275 ± 24||1.487||1.554||2.311|
|SS||98 ± 7||390 ± 27||67 ± 24||3.980||.1718||.6837|
|(3R,4S)||90 ± 5||5900 ± 400||276 ± 33||65.56||.0468||3.067|
The generally lower efficacy of 3R,4S in locomotor and methamphetamine discrimination tests could result from the differential selectivity of the two isomers for the DAT relative to the SERT. That is, if serotonin receptor activation is requisite for maximal efficacy, the difference SERT affinity between SS and 3R,4S might play a contributory role in accounting for the differences in the observed pharmacology. Catecholamine selective drugs, like TMP (methylphenidate), are reported to possess decent abuse potential though, so it is not easy to gauge why the trans analog, does not entice a strong SA propensity. In the specific case of Indatraline, the trans layout is actually desirable.
A possible explanation might be 3R,4S nocaine binds to the DAT in a different kind of fashion to SS and cocaine. If this is the case, it shouldn't be viewed as surprising that it is less reinforcing and possibly even aversive (S. Lomenzo, et al. 2005).
Some sort of cholinergic effect might also be aversive. For example, muscarinic activity of benztropine analogs is known to limit their reinforcing potential (M. Zou, et al. 2006). Ion-channel activity is another factor that can be used to explain certain differences in pharmacology.
Another possible explanation may lie in the fact that these compounds lack activity at the σ-receptors (R. Matsumoto, et al.). See also, (Ping and Teruo, rev 2003). Sigma receptor activity is characteristic of cocaine, methamphetamine and phencyclidine. Administering sigma-receptor antagonists can reverse cocaine-induced lethality, and also suppresses locomotor activity. Since the piperidine based ligands do not have sigma receptor activity, it can be hypothesized that this may be one of the possible reasons in accounting for their lack of locomotor activity, lessened probability to induce convulsions at high doses, etc.
Also, GABA activity is another neural target, cf. benzodiazapine and baclofen.
In summary, 3R,4S has lower potency and efficacy than than cocaine in increasing LMA in rodents. 3R,4S only manages to produce partial methamphetamine-like discriminative stimulus effects, although it is fully cocaine-like in cocaine-trained animals. 3R,4S has lower reinforcing potential than cocaine as assessed by fixed and progressive ratio IV self-administration tests in rats, with its reinforcing effects confirmed by rhesus monkeys. Furthermore, 3R,4S dose dependently antagonizes cocaine-induced LMA and potentiates the discriminative stimulus effects of a low dose of cocaine. 3R,4S, unlike cocaine, does not enhance cocaine-induced convulsions.
Clearly, SS is much better than (3R,4S), although maybe not as desirable as cocaine.
Triple Monoamine QSAR
Based upon the results reported in the tropane series, it became desirable to modify the aryl-arecoline nucleus, in ways that were predicted to improve SERT affinity, and also maintain/strengthen DAT/NET binding (A. Tamiz, et al. 2000). A series of Nocaine analogs were tested for their ability to inhibit the high affinity synaptic re/uptake of tritium radiolabelled biogenic monoamines, at DA/NE/5HT neurotransporters. The uptake data and selectivity profiles of these compounds are listed in the table. The 4-(β-naphthyl) 3-CO2Me compound is related to RTI-318. The p-allyl compound is a piperidine based mimic of RTI-301. It is depicted as the terminal alkene, although it should be emphasized that the olefin will internalize upon exposure to light. Then there are two isomers, each with a different code.
|Identification Marker||SERT / DAT / NET IC50, nM (K, nM)||IC50 ÷ Ki||IC50 Ratio|
|SS||p-Vinyl||Me||155 ± 3.9 (138 ± 3.5)||144 ± 20 (131 ± 18)||204 ± 5.6 (175 ± 4.8)||1.123||1.099||1.166||0.9493||1.268||1.336|
|SS||p-Ethyl||Me||275 ± 39 (255 ± 37)||>1800 (>1700)||>1300 (>1100)||1.078||1.059||1.182||>6.667||>4.314||0.6471|
|SS||p-Allyl||Me||334 ± 48 (309 ± 44)||>1000 (964 ± 100)||>1200 (>1000)||1.081||>1.037||1.2||3.120||>3.236||1.037|
|SS||p-Ethynyl||Me||189 ± 37 (175 ± 34)||213 ± 30 (187 ± 26)||399 ± 12 (364 ± 9.2)||1.080||1.139||1.096||1.069||2.080||1.947|
|SS||p-Phenyl||Me||67 ± 4.5 (62 ± 4.1)||184 ± 30 (173 ± 26)||239 ± 42 (203 ± 36)||1.081||1.064||1.177||2.790||3.274||1.173|
|SS||β-Naphthyl||Me||8.2 ± 0.3 (7.6 ± 0.2)||23 ± 1.0 (21 ± 0.9)||n.d. (34 ± 0.8)||1.079||1.095||2.763||4.474||1.619|
|3R,4S||β-Naphthyl||Me||46 ± 4.4 (42 ± 4.0)||>1000 (947 ± 135)||n.d. (241 ± 1.7)||1.095||>1.056||22.55||5.738||0.2545|
|RR||β-Naphthyl||Me||209 ± 17 (192 ± 16)||94 ± 9.6 (87 ± 8.9)||n.d. (27 ± 1.6)||1.089||1.080||0.4531||0.1406||0.3103|
|3S,4R||β-Naphthyl||Me||13 ± 0.7 (12 ± 0.7)||293 ± 6.4 (271 ± 5.9)||n.d. (38 ± 4.0)||1.083||1.081||22.58||3.167||0.140|
|3S,4R||β-Naphthyl||H2Cl||3.9 ± 0.5 (3.5 ± 0.5)||97 ± 8.6 (90 ± 8.0)||34 ± 2.5 (30 ± 2.3)||1.114||1.078||1.133||25.71||8.571||0.3333|
|SS/RR||α-Naphthyl||Me||113 ± 4.3 (101 ± 3.8)||326 ± 1.2 (304 ± 1.1)||337 ± 37 (281 ± 30)||1.119||1.072||1.199||3.010||2.782||0.9243|
|All data are mean values ± range or SEM of 2–5 separate experiments each conducted with 6 drug concentrations in triplicate.|
The vinyl compound was picked to represent this series of compounds in LMA studies. Both cocaine and the vinyl compound stimulated LMA. However, cocaine is ~2.5 x more potent in increasing the distance traveled. In contrast, the vinyl compound is about ~2.4 x more potent in enhancing stereotypic movements. Both cocaine and vinyl-Nocaine had a similar time-course on locomotor effects, which was ~2 h.
It is important to point out, in the case of the β-naphthyl isomers, although SERT activity is tolerated in both cis and trans cases, DAT activity is better tolerated for the cis isomers, meaning that these can be honored, whereas the trans isomers should be discarded. Enantiomeric separations may or may not be desirable, dependent on the exact identity of the aromatic entity.
Nocaine: Ester and Amine Modifications
A series of novel N- and 3α-modified Nocaine analogs were synthesized and tested for their SNDRI activity and behavioral properties in mice (Petukhov, et al. 2002).
The rational design of ligands with a predetermined potency at and selectivivity for DA/NE/5HT transporters is hindered by the lack of knowledge about the 3D structure (U. Gether, et al. 2001), (N. Chen, et al. 2000).
"In cases where the 3D structure of the binding site in a target protein is not well defined, as is the case for the MATs, one can perform ligand-based design to develop a pharmacophore. That is, by studying the conformational properties of a series of pharmacologically similar compounds, one can form hypotheses regarding the pharmacophore" (M. Froimowitz, et al. 2007).
In other words, the logical design of novel drugs is made possible, using ab initio principles, or at least by semi-empirical methods.
|Identification Marker||Ki (nM)||Uptake Ratio|
|Tag||R||N||[3H]NE||[3H]DA||[3H]5-HT||DA ÷ NE||5-HT ÷ DA||5-HT ÷ NE|
|1a||CO2Me||Me||252 ± 43||233 ± 62||8490 ± 1430||.9246||36.44||33.69|
|1b||H||7.9 ± 3.0||279 ± 98||434 ± 50||35.32||1.556||54.94|
|2a||CH2OH||Me||198 ± 53||497 ± 58||1550 ± 360||2.510||3.119||7.828|
|2b||H||69 ± 6||836 ± 35||239 ± 28||12.12||.2859||3.464|
|3a||Oxadiazole||Me||256 ± 17||187 ± 3||5960 ± 80||.7305||31.87||23.28|
|3b||H||34 ± 6||189 ± 24||373 ± 4||5.559||1.974||10.97|
N-demethylation led to positive enhancements in strength at the NET and the SERT in vitro. However, convulsions were witnessed at the highest dose tested, which could undermine the safety expectancy of these compounds. Interestingly, the demethyl ester also had increased duration span, which was unexpected, although it functioned as an unreliable behavioral enhancer, in contrast to the other compounds.
The alcohol is a better SERT blocker, than either the ester or the oxadiazole, which seem to discriminate against this receptor.
Both the alcohol and the oxadiazole had increased duration relative to the ester, although the alcohol had a delayed onset of action. This is accounted for on the basis of its increased polarity, meaning that it is likely to take longer to pass the BBB.
3α-Substituted Nocaine Ligand Design
In an earlier study, it was found that 3α-amido and bulky 3α-oxadiazoyl nocaine ligands, which possess greater stability relative to the ester FG, and are therefore more attractive as potential therapies, are inactive (P. Petukhov, et al. 2001). This result led to the hypothesis that the binding site of the DAT and NET in close proximity to the 3α-position of the piperidine ring is compact and cannot accomodate bulky, sterically occluded substituents, like the 3-substituted 1,2,4-oxadiazolyl groups. Supplied with this information, it was reasoned that introduction of a methylene spacer would confer improved MAT binding affinity upon the resultant molecules (P. Petukhov, et al. 2004).
|Identification Marker||Ki (nM)||Uptake Ratio||Physicochemical|
|Heterocycle = 3-[(1,3-benzodioxol-5-yl)-1,2,4-oxadiazol-5-yl]|
One of the possible reasons that the C2–C3 compounds are more active than the C1 compounds is that the polar group present in the more flexible 3α-appendage of the C2–C3 ligands is able to avoid unfavorable interactions with the binding site in close proximity to the piperidine ring. For the same reason the appendage in the C2–C3 series may more closely, but not precisely, mimic the binding mode of the more active SS based ligands, and possibly even transfer over to tropane based compounds.
To better understand the difference between the C1 and the C2–C3 series, the compounds were energy minimized and flexibly superimposed on WIN35428. Additionally, the conformation of the 3α-substituents were adjusted to maintain the closest possible overlap with the 2β-substituent of WIN35428, which helps avoid the SEA discussed earlier.
The resulting overlay shows that only the C2–C3 ligands are able to adopt a conformation in which the polar group of the 3α-substituent occupies the position proximal to that of the 2β-polar group in WIN35428.
DAT Arylpiperidine CoMFA Study
In this study a host of (3R,4S) Nocaine ligands were used as a test bed to do molecular modeling, using in vitro DAT binding data from earlier work (H. Yuan, et al. 2004). Two highly predictive and statistically significant CoMFA models were constructed.
Nocaine: Sulfur Appendage
The carboxymethyl locus of (d)-(3R,4S) Nocaine was used to generate a cluster of new side-chains, each imbuing various different shapes and sizes etcetera. One such example is a rigorously functionalized thioalkyl chain. The eugeroic, "wakefulness promoting agent" modafinil, was used as a punitative lead, to fuel these compounds discovery, although it turns out that the SAR of the pharmacophoric elements are, infact, only fleetingly related to one another (J. Zhou, et al. 2004). The NRI selective molecules discovered in this study were employed as brain imaging agents, to unravel details about the NE transporter (J. Musachio, et al, 2006). The other isomers of these sulfur-appendage "modafinil hybrids" were prepared, although it appears that it was no mistake that the (3R,4S) isomers yielded the most potent MAT inhibitor activity, in the preliminary study (R. He, et al. 2005).
|Identification Marker||NET / DAT / SERT Ki (nM)||Ki Ratio|
|Me||Ester||OMe||25 ± 6||80 ± 23||208 ± 47||3.2||2.6||8.32|
|H||.56 ± .09||51 ± 16||13 ± 3||91.07||.2549||23.21|
|Me||Amide||HNH||39 ± 5||159 ± 19||557 ± 150||4.077||3.503||14.28|
|H||10 ± .1||114 ± 32||170 ± 6||11.4||1.491||17|
|Me||HNOH||15 ± 2||85 ± 19||227 ± 7||5.667||2.671||15.13|
|HNMe||25 ± 2||13 ± 3||110 ± 45||.52||8.462||4.4|
|MeNMe||27 ± 7||116 ± 46||88 ± 22||4.296||.7586||3.259|
|isopropyl-NH||.8 ± .1||1.0 ± .2||1.1 ± .4||1.25||1.1||1.375|
|pyrrolidino||.68 ± .25||83 ± 1||4.5 ± .8||122.1||.05422||6.618|
|H2||OH||.94 ± .27||16 ± 5||158 ± 5||17.02||9.875||168.1|
|OMe||6 ± 2||50 ± 15||191 ± 57||8.333||3.82||31.83|
|OAc||3.6 ± 1.5||35 ± 11||57 ± 18||9.722||1.623||15.83|
|OBz||4.5 ± 1.2||68 ± 22||6.7 ± 1.5||15.11||.09853||1.489|
NRIs are important probes, but they are not thought to function as robust or powerful reinforcers (S. Wee, et al. 2006).
However, more focus has been tuned in to the ligand that has low nanomolar affinity at all three monoamine transporters, the first broadcasted piperidine compound developed to show such potent "triple reuptake inhibition".
It is also of note, that upon treatment of the ligands with H2O2 (hydrogen peroxide), the sulfur atom is oxidized. The resultant molecules then lose their ability to interact, with MA transporters, in an inhibitory fashion. Since all of these were less active than the unoxidized ligands, they were deliberately omitted from the above table. Thus, oxidation can be thought to sabbotage the biological activity of these ligands.
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