Synthesis and pharmacology of potential cocaine antagonists. 2. Structure-activity relationship studies of aromatic ring-substituted methylphenidate analogs

The Tungsten-Promoted Synthesis of Piperidyl-Modified erythro-Methylphenidate Derivatives

Due to its efficacy as a dopamine receptor agonist, methylphenidate (MPH) is of interest as a potential therapeutic for cocaine addiction. While numerous derivatives of MPH have been investigated for their potential medicinal value, functionalization of the piperidine ring has not been explored. The pyridine borane ligand in WTp(NO)(PMe3)(η2-pyBH3) is dearomatized by the metal and can be elaborated to the analogous η2-mesylpyridinium complex. Installing a methyl phenylacetate moiety at the C2' position via a Reformatsky reaction followed by a tandem protonation/nucleophilic addition sequence results in a library of erythro MPH analogues functionalized at the piperidyl C5' position. The functional group is added chemoselectively to C5', cis to the methyl phenylacetate. Repeating this procedure with an enantioenriched source of the tungsten reagent results in enantioenriched MPH derivatives. All identities of the newly reported compounds are supported by comprehensive 2D NMR and HRMS data or crystallographic data.

Analytical characterization and differentiation between threo- and erythro-4-fluoroethylphenidate

PURPOSE

Methylphenidate analogs appeared on the drug market during the last years. Its analogs contain two chiral centers and, thus, have potential varying configurations (i.e., threo and erythro forms). This study presents the analytical characterization of 4-fluoroethylphenidate (4-FEP) and its differentiation between threo- and erythro-4-FEP.

METHODS

Analysis of the samples included high-performance liquid chromatography (HPLC), gas chromatography-electron ionization-mass spectrometry (GC-EI-MS), high-resolution mass spectrometry (HRMS) analyses, nuclear magnetic resonance (NMR) spectroscopy and X-ray crystal structure analysis.

RESULTS

NMR spectroscopic investigations confirmed the differences between threo- and erythro-4-FEP, and demonstrated that both isomers could be separated using HPLC and GC methods. Two samples obtained from one vendor in 2019 consisted of threo-4-FEP, whereas the other two samples obtained from a different vendor in 2020 consisted of a mixture of threo- and erythro-4-FEP.

CONCLUSIONS

Several analytical approaches including HPLC, GC-EI-MS, HRMS analyses, NMR spectroscopy and X-ray crystal structure analysis enabled the unambiguous identification of threo- and erythro-4-FEP. The analytical data presented in this article will be useful for identifying threo- and erythro-4-FEP included in illicit products.

Do 2-(Benzoyl)piperidines Represent a Novel Class of hDAT Reuptake Inhibitors?

2-(Benzoyl)piperidines (analogues of 1a), structural hybrids of the clinically employed ADHD medication methylphenidate (2) and the abused synthetic cathinone pentedrone (3), have been previously reported to act as novel and selective reuptake inhibitors of the human dopamine transporter (hDAT). One of the more potent benzoylpiperidines, as is the case with methylphenidate analogues, is its 3,4-dichloroaryl counterpart. Here, we demonstrate using homology models that these compounds (i.e., benzoylpiperidines and methylphenidate analogues) likely bind in a comparable manner at hDAT. In addition, it is shown here that the 3,4-dichlorobenzoylpiperidine analogue of 1a is more potent than its 3,4-dimethyl counterpart, suggesting that the electronic character of the substituents might play a role in the potency of these hybrids. Furthermore, the 3,4-benz-fused (i.e., naphthyl) benzoylpiperidine analogue acts in the same manner as its corresponding methylphenidate counterpart at hDAT. As with its methylphenidate counterpart, the naphthyl compound also acts, rather uniquely (although with lower potency) relative to other members of the two series, at the human serotonin transporter (hSERT). In conclusion, the benzoylpiperidines represent a novel structural class of hDAT reuptake inhibitors that function in a manner similar to their methylphenidate counterparts.

Methylphenidate Analogues as a New Class of Potential Disease-Modifying Agents for Parkinson's Disease: Evidence from Cell Models and Alpha-Synuclein Transgenic Mice

Parkinson's disease (PD) is characterized by dopaminergic nigrostriatal neurons degeneration and Lewy body pathology, mainly composed of α-synuclein (αSyn) fibrillary aggregates. We recently described that the neuronal phosphoprotein Synapsin III (Syn III) participates in αSyn pathology in PD brains and is a permissive factor for αSyn aggregation. Moreover, we reported that the gene silencing of Syn III in a human αSyn transgenic (tg) mouse model of PD at a pathological stage, manifesting marked insoluble αSyn deposits and dopaminergic striatal synaptic dysfunction, could reduce αSyn aggregates, restore synaptic functions and motor activities and exert neuroprotective effects. Interestingly, we also described that the monoamine reuptake inhibitor methylphenidate (MPH) can recover the motor activity of human αSyn tg mice through a dopamine (DA) transporter-independent mechanism, which relies on the re-establishment of the functional interaction between Syn III and α-helical αSyn. These findings support that the pathological αSyn/Syn III interaction may constitute a therapeutic target for PD. Here, we studied MPH and some of its analogues as modulators of the pathological αSyn/Syn III interaction. We identified 4-methyl derivative I-threo as a lead candidate modulating αSyn/Syn III interaction and having the ability to reduce αSyn aggregation in vitro and to restore the motility of αSyn tg mice in vivo more efficiently than MPH. Our results support that MPH derivatives may represent a novel class of αSyn clearing agents for PD therapy.

Molecular Mechanisms of Action of Novel Psychoactive Substances (NPS). A New Threat for Young Drug Users with Forensic-Toxicological Implications

Novel psychoactive substances (NPS) represent a severe health risk for drug users. Even though the phenomenon has been growing since the early 2000s, the mechanisms of action of NPS at the receptors and beyond them are still scarcely understood. The aim of the present study was to provide a systematic review of the updated knowledge regarding the molecular mechanisms underlying the toxicity of synthetic opioids, cannabinoids, cathinones, and stimulants. The study was conducted on the PubMed database. Study eligibility criteria included relevance to the topic, English language, and time of publication (2010–2020). A combined Mesh and free-text protocols search was performed. Study selection was performed on the title/abstract and, in doubtful cases, on the full texts of papers. Of the 580 records identified through PubMed searching and reference checking, 307 were excluded by title/abstract and 78 additional papers were excluded after full-text reading, leaving a total of 155 included papers. Molecular mechanisms of synthetic opioids, synthetic cannabinoids, stimulants, psychedelics, and hallucinogens were reviewed and mostly involved both a receptor-mediated and non-receptor mediated cellular modulation with multiple neurotransmitters interactions. The molecular mechanisms underlying the action of NPS are more complex than expected, with a wide range of overlap among activated receptors and neurotransmitter systems. The peculiar action profile of single compounds does not necessarily reflect that of the structural class to which they belong, accounting for possible unexpected toxic reactions.

Designer drugs: mechanism of action and adverse effects

Psychoactive substances with chemical structures or pharmacological profiles that are similar to traditional drugs of abuse continue to emerge on the recreational drug market. Internet vendors may at least temporarily sell these so-called designer drugs without adhering to legal statutes or facing legal consequences. Overall, the mechanism of action and adverse effects of designer drugs are similar to traditional drugs of abuse. Stimulants, such as amphetamines and cathinones, primarily interact with monoamine transporters and mostly induce sympathomimetic adverse effects. Agonism at μ-opioid receptors and γ-aminobutyric acid-A (GABAA) or GABAB receptors mediates the pharmacological effects of sedatives, which may induce cardiorespiratory depression. Dissociative designer drugs primarily act as N-methyl-D-aspartate receptor antagonists and pose similar health risks as the medically approved dissociative anesthetic ketamine. The cannabinoid type 1 (CB1) receptor is thought to drive the psychoactive effects of synthetic cannabinoids, which are associated with a less desirable effect profile and more severe adverse effects compared with cannabis. Serotonergic 5-hydroxytryptamine-2A (5-HT2A) receptors mediate alterations of perception and cognition that are induced by serotonergic psychedelics. Because of their novelty, designer drugs may remain undetected by routine drug screening, thus hampering evaluations of adverse effects. Intoxication reports suggest that several designer drugs are used concurrently, posing a high risk for severe adverse effects and even death.

Synthetic Cathinone Analogues Structurally Related to the Central Stimulant Methylphenidate as Dopamine Reuptake Inhibitors

Synthetic cathinones are, primarily, stimulant drugs of abuse that act at monoamine transporters (e.g., the dopamine transporter or DAT) as releasing agents or as reuptake inhibitors. In the past few years, the emergence of >150 new synthetic cathinones has attracted considerable attention from medical and law enforcement communities. threo-Methylphenidate (tMP), used clinically for the treatment of ADHD and narcolepsy, is also a DAT reuptake inhibitor. tMP is somewhat structurally similar to abused cathinone stimulants, and the structure-activity relationships (SAR) of tMP have been well-defined. Hence, available tMP literature might assist in understanding the SAR of synthetic cathinones, about which less is known. In the present study, we synthesized and examined eight 2-benzoylpiperidine analogues (4, 6-12) to determine if tMP SAR might be applicable to cathinone SAR. The benzoylpiperidine analogues were evaluated in a competition assay using live-cell imaging against APP+ in HEK293 cells stably expressing hDAT and in cells coexpressing DAT and voltage-gated Ca2+ channels. All compounds were found to be DAT reuptake inhibitors, and a significant correlation was obtained between the potency of the benzoylpiperidines and tMP binding data (r = 0.91), suggesting that the SAR of tMP analogues might be directly applicable to certain synthetic cathinones as DAT reuptake inhibitors.

In Vitro Neurochemical Assessment of Methylphenidate and Its "Legal High" Analogs 3,4-CTMP and Ethylphenidate in Rat Nucleus Accumbens and Bed Nucleus of the Stria Terminalis

3,4-dichloromethylphenidate (3,4-CTMP) and ethylphenidate are new psychoactive substances and analogs of the attention deficit medication methylphenidate. Both drugs have been reported on online user fora to induce effects similar to cocaine. In the UK, 3,4-CTMP appeared on the drug market in 2013 and ethylphenidate has been sold since 2010. We aimed to explore the neurochemical effects of these drugs on brain dopamine and noradrenaline efflux. 3,4-CTMP and ethylphenidate, purchased from online vendors, were analyzed using gas chromatography and mass spectroscopy to confirm their identity. Drugs were then tested in adolescent male rat brain slices of the nucleus accumbens and stria terminalis for effects on dopamine and noradrenaline efflux respectively. Fast cyclic voltammetry was used to measure transmitter release. Methylphenidate (10 μM) increased evoked dopamine and noradrenaline efflux by 4- and 2-fold, respectively. 3,4-CTMP (0.1 and 1 μM) increased evoked dopamine and noradrenaline efflux by ~6-fold and 2-fold, respectively. Ethylphenidate (1 μM) doubled evoked dopamine and noradrenaline efflux in both cases. 3,4-CTMP's effect on dopamine efflux was greater than that of methylphenidate, but ethylphenidate appears to be a weaker dopamine transporter inhibitor. Experiments using the dopamine D₂ antagonist haloperidol, the noradrenaline α₂ receptor antagonist yohimbine, the dopamine transporter inhibitor GBR12909 and the noradrenaline transporter inhibitor desipramine confirmed that we were measuring dopamine in the accumbens and noradrenaline in the ventral BNST. All three psychostimulant drugs, through their effects on dopamine efflux, may have addictive liability although the effect of 3,4-CTMP on dopamine suggests that it might be most addictive and ethylphenidate least addictive.

Pharmacological profile of methylphenidate-based designer drugs

BACKGROUND

Methylphenidate-based designer drugs are new psychoactive substances (NPS) that are used outside medical settings and their pharmacology is largely unexplored. The aim of the present study was to characterize the pharmacology of methylphenidate-based substances in vitro.

METHODS

We determined the potencies of the methylphenidate-based NPS N-benzylethylphenidate, 3,4-dichloroethylphenidate, 3,4-dichloromethylphenidate, ethylnaphthidate, ethylphenidate, 4-fluoromethylphenidate, isopropylphenidate, 4-methylmethylphenidate, methylmorphenate, and propylphenidate and the potencies of the related compounds cocaine and modafinil with respect to norepinephrine, dopamine, and serotonin transporter inhibition in transporter-transfected human embryonic kidney 293 cells. We also investigated monoamine efflux and monoamine receptor and transporter binding affinities. Furthermore, we assessed the cell integrity under assay conditions.

RESULTS

All methylphenidate-based substances inhibited the norepinephrine and dopamine transporters 4 to >1000-fold more potently than the serotonin transporter. Similar to methylphenidate and cocaine, methylphenidate-based NPS did not elicit transporter-mediated efflux of monoamines. Besides binding to monoamine transporters, several test drugs had affinity for adrenergic, serotonergic, and rat trace amine-associated receptors but not for dopaminergic or mouse trace amine-associated receptors. No cytotoxicity was observed after drug treatment at assay concentrations.

CONCLUSION

Methylphenidate-based substances had pharmacological profiles similar to methylphenidate and cocaine. The predominant actions on dopamine transporters vs. serotonin transporters may be relevant when considering abuse liability. This article is part of the Special Issue entitled 'Designer Drugs and Legal Highs.'

Analytical characterization and pharmacological evaluation of the new psychoactive substance 4-fluoromethylphenidate (4F-MPH) and differentiation between the (±)-threo and (±)-erythro diastereomers

Misuse of (±)-threo-methylphenidate (methyl-2-phenyl-2-(piperidin-2-yl)acetate; Ritalin®; MPH) has long been acknowledged, but the appearance of MPH analogs in the form of 'research chemicals' has only emerged in more recent years. 4-Fluoromethylphenidate (4F-MPH) is one of these recent examples. This study presents the identification and analytical characterization of two powdered 4F-MPH products that were obtained from an online vendor in 2015. Interestingly, the products appeared to have originated from two distinct batches given that one product consisted of (±)-threo-4F-MPH isomers whereas the second sample consisted of a mixture of (±)-threo and (±)-erythro 4F-MPH. Monoamine transporter studies using rat brain synaptosomes revealed that the biological activity of the 4F-MPH mixture resided with the (±)-threo and not the (±)-erythro isomers based on higher potencies determined for blockage of dopamine uptake (IC₅₀ 4F-MPH_mixture  = 66 nM vs. IC₅₀ (±)-threo = 61 nM vs. IC₅₀ (±)-erythro = 8,528 nM) and norepinephrine uptake (IC₅₀ 4F-MPH_mixture  = 45 nM vs. (±)-threo = 31 nM vs. IC₅₀ (±)-erythro = 3,779 nM). In comparison, MPH was three times less potent than (±)-threo-4F-MPH at the dopamine transporter (IC₅₀  = 131 nM) and around 2.5 times less potent at the norepinephrine transporter (IC₅₀  = 83 nM). Both substances were catecholamine selective with IC₅₀ values of 8,805 nM and >10,000 nM for (±)-threo-4F-MPH and MPH at the serotonin transporter. These findings suggest that the psychostimulant properties of (±)-threo-4F-MPH might be more potent in humans than MPH. Copyright © 2017 John Wiley & Sons, Ltd.

Classics in Chemical Neuroscience: Methylphenidate

As the first drug to see widespread use for the treatment of attention deficit hyperactivity disorder (ADHD), methylphenidate was the forerunner and catalyst to the modern era of rapidly increasing diagnosis, treatment, and medication development for this condition. During its often controversial history, it has variously elucidated the importance of dopamine signaling in memory and attention, provoked concerns about pharmaceutical cognitive enhancement, driven innovation in controlled-release technologies and enantiospecific therapeutics, and stimulated debate about the impact of pharmaceutical sales techniques on the practice of medicine. In this Review, we will illustrate the history and importance of methylphenidate to ADHD treatment and neuroscience in general, as well as provide key information about its synthesis, structure-activity relationship, pharmacological activity, metabolism, manufacturing, FDA-approved indications, and adverse effects.

Dopamine reuptake transporter (DAT) "inverse agonism"--a novel hypothesis to explain the enigmatic pharmacology of cocaine

The long held view is cocaine's pharmacological effects are mediated by monoamine reuptake inhibition. However, drugs with rapid brain penetration like sibutramine, bupropion, mazindol and tesofensine, which are equal to or more potent than cocaine as dopamine reuptake inhibitors, produce no discernable subjective effects such as drug "highs" or euphoria in drug-experienced human volunteers. Moreover they are dysphoric and aversive when given at high doses. In vivo experiments in animals demonstrate that cocaine's monoaminergic pharmacology is profoundly different from that of other prescribed monoamine reuptake inhibitors, with the exception of methylphenidate. These findings led us to conclude that the highly unusual stimulant profile of cocaine and related compounds, eg methylphenidate, is not mediated by monoamine reuptake inhibition alone. We describe the experimental findings which suggest cocaine serves as a negative allosteric modulator to alter the function of the dopamine reuptake transporter (DAT) and reverse its direction of transport. This results in a firing-dependent, retro-transport of dopamine into the synaptic cleft. The proposed mechanism of cocaine is, therefore, different from other small molecule negative allostereric modulators of the monoamine reuptake transporters, eg SoRI-6238, which merely reduce the rate of inward transport. Because the physiological role of DAT is to remove dopamine from the synapse and the action of cocaine is the opposite of this, we have postulated that cocaine's effect is analogous to an inverse agonist. If this hypothesis is validated then cocaine is the prototypical compound that exemplifies a new class of monoaminergic drugs; DAT "inverse agonists". This article is part of the Special Issue entitled 'CNS Stimulants'.

Evolution of a Compact Photoprobe for the Dopamine Transporter Based on (±)-threo-Methylphenidate

The development of photoaffinity ligands for determining covalent points of attachment to the dopamine transporter (DAT) has predominantly focused on tropane-based compounds bearing variable-length linkers between the photoreactive group and inhibitor pharmacophore. In order to expand the array of photoprobes useful for mapping inhibitor-binding pockets within the DAT, a compact non-tropane ligand was synthesized featuring a photoreactive azide and iodine tag directly attached to the aromatic ring of (±)-threo-methylphenidate. (±)-threo-4-Azido-3-iodomethylphenidate ((±)-6); K(i) = 4.0 ± 0.8 nM) displayed high affinity for hDAT. Moreover, a radioiodinated analog of (±)-6 demonstrated covalent ligation to the DAT in cultured cells and rat striatal membranes, thus suggesting the potential utility of this photoprobe in DAT structure-function studies.

Methylphenidate: established and expanding roles in symptom management

Methylphenidate is a psychostimulant originally used for the treatment of attention-deficit disorder. Methylphenidate inhibits neuronal neurotransmitter transporters involved in the uptake of dopamine and norepinephrine at the level of the synapse. Inhibition of these transmitter transporters leads to increased concentrations of dopamine and norepinephrine in the synapse, which results in increasing alertness. The stimulant effect of methylphenidate has been used for the treatment of major depression, poststroke depression, cognitive enhancement in patients with brain tumors, neurodegenerative disorders, HIV disease, fatigue, and as a treatment for delirium and sedation associated with opioid use. Other areas where methylphenidate has been evaluated include gait disorders in the elderly individuals and the treatment of apathy in dementia. Analgesic effects have been demonstrated in preclinical models but true analgesic effects remain to be proven in humans. This article reviews the current use of methylphenidate for symptom management with a critical look at the evidence base for its efficacy in the conditions described.

One-pot synthesis of 2-amino-indole-3-carboxamide and analogous

An efficient one-pot, two-step solution-phase synthetic method was developed to synthesize twenty-three 2-amino-indole-3-carboxamides (3) from 2-halonitrobenzene (1) or heterocyclic analogous and cyanoacetamides (2). In this sequence, first, intermediate 2-cyano-2-(2-nitrophenyl)acetamide (4) was generated under basic condition via S(NAr) reaction; after direct addition of hydrochloric acid solution, FeCl(3), and Zn powder, indole 3 was generated via reduction/cyclization process.

Azido-iodo-N-benzyl derivatives of threo-methylphenidate (Ritalin, Concerta): Rational design, synthesis, pharmacological evaluation, and dopamine transporter photoaffinity labeling

In contrast to tropane-based compounds such as benztropine and cocaine, non-tropane-based photoaffinity ligands for the dopamine transporter (DAT) are relatively unexplored. Towards addressing this knowledge gap, ligands were synthesized in which the piperidine nitrogen of 3- and 4-iodomethylphenidate was substituted with a benzyl group bearing a photoreactive azide. Analog (±)-3a demonstrated modest DAT affinity and a radioiodinated version was shown to bind covalently to rat striatal DAT and hDAT expressed in cultured cells. Co-incubation of (±)-3a with nonradioactive d-(+)-methylphenidate or (-)-2-β-carbomethoxy-3-β-(4-fluorophenyl)tropane (β-CFT, WIN-35,428, a cocaine analog) blocked DAT labeling. Compound (±)-3a represents the first successful example of a DAT photoaffinity ligand based on the methylphenidate scaffold. Such ligands are expected to assist in mapping non-tropane ligand-binding pockets within plasma membrane monoamine transporters.

Dopamine transporter relation to levodopa-derived synaptic dopamine in a rat model of Parkinson's: an in vivo imaging study

Studies showed that the dopamine (DA) transporter (DAT) modulates changes in levodopa-derived synaptic dopamine levels (Delta(DA)) in Parkinson's disease (PD). Here we evaluate the relationship between DAT and Delta(DA) in the 6-hydroxydopamine model of Parkinson's disease to investigate these mechanisms as a function of dopaminergic denervation and in relation to other denervation-induced regulatory changes. 27 rats with a unilateral 6-hydroxydopamine lesion (denervation approximately 20-97%) were imaged with (11)C-dihydrotetrabenazine (VMAT2 marker), (11)C-methylphenidate (DAT marker) and (11)C-raclopride (D2-type receptor marker). For denervation <75%Delta(DA) was significantly correlated with a combination of relatively preserved terminal density and lower DAT. For denervation <90%, Delta(DA) was significantly negatively correlated with DAT with a weaker dependence on VMAT2. For the entire data set, no dependence on pre-synaptic markers was observed; Delta(DA) was significantly positively correlated with (11)C-raclopride binding-derived estimates of DA loss. These findings parallel observations in humans, and show that (i) regulatory changes attempt to normalize synaptic DA levels (ii) a lesion-induced functional dependence of Delta(DA) on DAT occurs up to approximately 90% denervation (iii) for denervation < 75% relative lower DAT levels may relate to effective compensation; for higher denervation, lower DAT levels likely contribute to oscillations in synaptic DA associated with dyskinesias.

Differential pharmacokinetics and pharmacodynamics of methylphenidate enantiomers: does chirality matter?

d,l-threo-methylphenidate (MPH) is an effective first-line treatment for the symptoms associated with attention-deficit/hyperactivity disorder. threo-methylphenidate inhibits the dopamine transporter and the norepinephrine transporter, resulting in elevations of these monoamines after impulse release. Although MPH has long been administered as a racemic mixture of the 2 enantiomers, d-MPH and l-MPH, converging lines of evidence drawn from investigations using in vitro systems, animal models, and humans indicate that it is predominantly, if not exclusively, d-MPH that mediates the pharmacological/therapeutic actions of MPH. In both rodent and primate animal models, the binding of radiolabeled d-MPH to dopamine transporter was found to be selective, saturable, and reversible, whereas binding of l-MPH was diffuse and nonspecific. The behavioral effects of the enantiomers of MPH have been tested in several animal models, and results indicate these observed behavioral changes are likewise mediated by d-MPH, whereas l-MPH has little or no effect.The contribution of the l-isomer to the overall pharmacological profile of the racemate remains unclear, owing to several studies suggesting that l-MPH may not be merely an inert isomeric ballast. For example, behavioral studies conducted in rats demonstrate an attenuation of the effect of d-MPH in animals pretreated with l-MPH, suggesting that l-MPH may interfere with the action of the active enantiomer. The importance of MPH chirality to central nervous system MPH receptor targeting has culminated in human imaging studies revealing that d-MPH binds specifically to striatal structures, whereas l-MPH binding is nonspecific. Taken together, data from in vitro, animal, and human studies support the premise that the d-enantiomer of MPH mediates the neurophysiological actions of MPH and therefore likely mediates its clinical efficacy.

Catalytic C-H functionalization by metal carbenoid and nitrenoid insertion

Novel reactions that can selectively functionalize carbon-hydrogen bonds are of intense interest to the chemical community because they offer new strategic approaches for synthesis. A very promising 'carbon-hydrogen functionalization' method involves the insertion of metal carbenes and nitrenes into C-H bonds. This area has experienced considerable growth in the past decade, particularly in the area of enantioselective intermolecular reactions. Here we discuss several facets of these kinds of C-H functionalization reactions and provide a perspective on how this methodology has affected the synthesis of complex natural products and potential pharmaceutical agents.

Synthesis and pharmacology of site-specific cocaine abuse treatment agents: restricted rotation analogues of methylphenidate

A series of threo-1-aza-3 or 4-substituted-5-phenyl[4.4.0]decanes (quinolizidines), which were envisioned as restricted rotational analogues (RRAs) of methylphenidate (MP), was synthesized and tested for inhibitory potency against [(3)H]WIN35,428, [3H]citalopram, and [3H]nisoxetine binding to the dopamine, serotonin, and norepinephrine transporters, respectively. Two different synthetic schemes were used; a Wittig reaction or acylation (followed by an intramolecular condensation) was a key feature of each scheme. The unsubstituted RRA, threo(trans)-1-aza-5-phenyl[4.4.0]decane (12a), was equipotent to unconstrained threo-MP against [(3)H]WIN35,428 binding. The extra ring in these RRAs (which reduces the conformational freedom) and the orientation and polarity of substituents at the 4-position on this extra ring are of critical importance to the biological activity. Generally, the RRAs paralleled the corresponding unconstrained MP derivatives in binding affinity to the three transporters. The results suggest that the conformation of MP in which the carbonyl group of the methyl ester is H-bonded to the piperidinyl N-H may be the bioactive form of the molecule.

Slow-onset, long-duration, alkyl analogues of methylphenidate with enhanced selectivity for the dopamine transporter

Methylphenidate analogues, in which the carbomethoxy has been replaced by an alkyl group and with different phenyl substituents, have been synthesized and tested in monoamine transporter assays. As predicted from a pharmacophore model, most of the RR/SS diastereomers showed high potency as dopamine reuptake inhibitors. Analogues with a 4-chlorophenyl group and an unbranched initial alkyl atom had consistently enhanced selectivity for the dopamine transporter. The most potent compounds were those with a three- or four-carbon chain. The "inactive" RS/SR diastereomers showed substantial activity when the phenyl substituent was 3,4-dichloro. On a locomotor assay, one compound was found to have a slow onset and a long duration of action. The activity of these compounds provides additional evidence for a conformational/superposition model of methylphenidate with cocaine-like structures. A ketone analogue, obtained by hydrogenating a previously described vinylogous amide, had activity similar to that of methylphenidate.

Methylphenidate and its isomers: their role in the treatment of attention-deficit hyperactivity disorder using a transdermal delivery system

dl-threo-Methylphenidate is a highly efficacious drug for treating attention-deficit hyperactivity disorder (ADHD) that is currently administered as immediate- or controlled-release and osmotically controlled-released formulations. The drug exists as two enantiomers, d-threo-methylphenidate and l-threo-methylphenidate, with the former having been developed as a medication to treat ADHD in its own right. dl-threo-Methylphenidate undergoes enantioselective metabolism in the liver, which results in marked differences in the plasma concentrations of its isomers, depending on the route of administration and formulation. When dl-threo-methylphenidate is orally administered, the plasma concentrations of d-threo-methylphenidate are higher than those of l-threo-methylphenidate. However, with the recently developed methylphenidate transdermal system (MTS), 'first-pass' metabolism is circumvented and, as a consequence, plasma concentrations of d-threo-methylphenidate are consistent with those produced by oral formulations, but the relative concentrations of l-threo-methylphenidate are much higher, i.e. 50-60% of those of d-threo-methylphenidate. In this article, we review the pharmacokinetics and pharmacology of dl-threo-methylphenidate and its isomers to assess the extent to which their mechanism of action as noradrenaline (norepinephrine) and dopamine reuptake inhibitors is responsible for their efficacy and commonly occurring adverse effects. The major findings are that d-threo-methylphenidate and l-threo-methylphenidate share the same pharmacological profile as the parent racemate, i.e. catecholamine-selective reuptake inhibition with higher potency against dopamine versus noradrenaline reuptake in vivo. However, d-threo-methylphenidate is approximately 10-fold more potent than the l-isomer in this regard. For these drugs, their abilities not only to ameliorate the behavioural and cognitive dysfunctions in ADHD, but also to induce the common adverse effects of reduced appetite, nausea/vomiting and stomach ache, are almost certainly due to their ability to potentiate noradrenergic and/or dopaminergic function in the central and peripheral nervous systems. The sympathomimetic actions of ADHD drugs on cardiovascular function are currently an issue of concern. Since noradrenaline reuptake inhibition is the likely mediator for the effects of dl-threo-methylphenidate on blood pressure and heart rate, the more potent d-isomer will therefore be predominantly responsible. Motor and vocal tics are the other important adverse event to be considered in the treatment of ADHD. It is now accepted that tics are a frequently occurring behavioural manifestation of ADHD itself and the evidence for or against their exacerbation by treatment with dl-threo-methylphenidate or other stimulants remains highly contradictory. Focusing on the enantiomers of dl-threo-methylphenidate, it can be concluded that d-threo-methylphenidate, which is the more potent and abundant of the two isomers, is the major contributor of both efficacy and adverse effects, irrespective of the formulation or route of administration of the racemate. Moreover, for the oral, extended-release formulations of dl-threo-methylphenidate, the d-isomer represents the only pharmacologically active moiety when these medications are used in the clinic. With the MTS, plasma concentrations of l-threo-methylphenidate are higher than are achieved using oral formulations, but even in this case, it is likely that the contribution of this enantiomer to the efficacy and adverse effects of the racemate is no greater than 5-10% of the total.

Vinylogous amide analogs of methylphenidate

In an effort to produce compounds with longer durations of action, we attempted to synthesize ketone analogs of methylphenidate which, however, appear to be highly unstable due to a highly acidic proton alpha to the ketone and phenyl groups. Nevertheless, vinylogous amide by products have been synthesized and tested for activity at dopamine, norepinephrine, and serotonin transporters. The compounds were found to be weak inhibitors of monoamine reuptake despite rigid three dimensional structures that are quite similar to the global minimum of threo-(R,R)-methylphenidate. The structures were confirmed by X-ray crystallography.

Conformational analysis of methylphenidate: comparison of molecular orbital and molecular mechanics methods

Methylphenidate (MP) binds to the cocaine binding site on the dopamine transporter and inhibits reuptake of dopamine, but does not appear to have the same abuse potential as cocaine. This study, part of a comprehensive effort to identify a drug treatment for cocaine abuse, investigates the effect of choice of calculation technique and of solvent model on the conformational potential energy surface (PES) of MP and a rigid methylphenidate (RMP) analogue which exhibits the same dopamine transporter binding affinity as MP. Conformational analysis was carried out by the AM1 and AM1/SM5.4 semiempirical molecular orbital methods, a molecular mechanics method (Tripos force field with the dielectric set equal to that of vacuum or water) and the HF/6-31G* molecular orbital method in vacuum phase. Although all three methods differ somewhat in the local details of the PES, the general trends are the same for neutral and protonated MP. In vacuum phase, protonation has a distinctive effect in decreasing the regions of space available to the local conformational minima. Solvent has little effect on the PES of the neutral molecule and tends to stabilize the protonated species. The random search (RS) conformational analysis technique using the Tripos force field was found to be capable of locating the minima found by the molecular orbital methods using systematic grid search. This suggests that the RS/Tripos force field/vacuum phase protocol is a reasonable choice for locating the local minima of MP. However, the Tripos force field gave significantly larger phenyl ring rotational barriers than the molecular orbital methods for MP and RMP. For both the neutral and protonated cases, all three methods found the phenyl ring rotational barriers for the RMP conformers/invertamers (denoted as cte, tte, and cta) to be: cte, tte > MP > cta. Solvation has negligible effect on the phenyl ring rotational barrier of RMP. The B3LYP/6-31G* density functional method was used to calculate the phenyl ring rotational barrier for neutral MP and gave results very similar to those of the HF/6-31G* method.

Structural requirements for 2,4- and 3,6-disubstituted pyran biomimetics of cis-(6-benzhydryl-piperidin-3-yl)-benzylamine compounds to interact with monoamine transporters

In our effort to delineate novel pharmacophoric configuration of bioisosteric pyran versions of cis-(6-benzhydryl-piperidin-3-yl)-benzylamine derivatives in interacting with the monoamine transporter, further structure-activity relationship study was carried out. Both cis and trans 2,4- and 3,6-disubstituted derivatives were synthesized to determine the positional importance of N-substitution on affinity for monoamine transporters, that is the dopamine transporter (DAT), the serotonin transporter (SERT), and the norepinephrine transporter (NET) in rat brain. For that purpose, the potency of compounds was determined in competing for the binding of [(3)H]WIN 35,428, [(3)H]citalopram, and [(3)H]nisoxetine, respectively. Selected compounds were also evaluated for their activity in inhibiting the uptake of [(3)H]DA by DAT. Our binding results demonstrated potency in 3,6-disubstituted derivatives while 2,4-disubstituted derivatives failed to exhibit any appreciable binding affinity. Further structural exploration of the exocyclic N-atom in 3,6-disubstituted derivatives produced compounds potent at both DAT and NET. Compounds 16h and 16o with hydroxyl and amino groups in the phenyl moiety of the benzyl group produced the highest activity for the NET. In this regard, compound 16e with a methoxy substituent produced weak affinity at NET, which upon conversion into a hydroxyl functionality as in 16h produced potent affinity for the NET. Various indole derivatives displayed different interactions; the 5-substituted indole derivative 16n exerted potent affinity for NET, confirming the bioisosteric equivalence between this indole moiety and the phenyl-4-hydroxy group in 16h.

Synthesis of methylphenidate analogues and their binding affinities at dopamine and serotonin transport sites

The rhodium(II)-catalyzed intermolecular C-H insertion of methyl aryldiazoacetates with either N-Boc-piperidine or N-Boc-pyrrolidine followed by deprotection with trifluoroacetic acid is a very direct method for the synthesis of methylphenidate analogues. By using either dirhodium tetraacetate or dirhodium tetraprolinate derivatives as catalyst, either the racemic or enantioenriched methylphenidate analogues can be prepared. The binding affinities of the methylphenidate analogues to both the dopamine and the serotonin transporters are described. The most notable compounds are the erythro-(2-naphthyl) analogues which display high binding affinity and selectivity for the serotonin transporter.

Cocaine pharmacology and current pharmacotherapies for its abuse

Cocaine abuse continues to be prevalent and effective therapies for cocaine craving and addiction remain elusive. In the last decade immunopharmacotherapy has been proposed as a promising means to alleviate this illness. By using the organism's natural immune response, an anti-cocaine vaccine promotes the production of cocaine-specific antibodies that sequester the drug before their passage into the brain, where it exerts its reinforcing and thus addictive effects. A series of studies demonstrating the cocaine-blocking properties of various immunogenic conjugates will be reviewed in the context of the neuropsychopharmacological profile of the drug.

Block of human NaV1.5 sodium channels by novel α-hydroxyphenylamide analogues of phenytoin

Voltage-gated sodium (Na) channels are a critical component of electrically excitable cells. Phenytoin (diphenylhydantoin, DPH) is an established sodium channel blocker and is a useful anticonvulsant and class 1b antiarrhythmic, and has been effectively used in the treatment of neuropathic pain. In this study, we have synthesized novel alpha-hydroxyphenylamide analogues of diphenylhydantoin and examined their ability to inhibit human Na(V)1.5 sodium channels expressed in Chinese Hamster Ovary (CHO-K1) cells. Phenyl ring substitutions were examined including para-methyl, para-fluoro, para-chloro, ortho-chloro and meta-chloro. We have found that phenyl ring substitutions with electron withdrawing properties resulted in compounds with greater activity. In comparison to diphenylhydantoin, the novel chloro-substituted alpha-hydroxyphenylamide compounds produced as much as a 20-fold greater tonic and frequency-dependent blockade of Na(V)1.5 channels with an IC(50) value of 14.5 microM. In addition, the chloro-substitutions have position specific state dependent blocking properties. The ortho-, meta- and para-chloro substitutions have an 8-, 13- and 3-fold increased affinity for the inactivated state, respectively. Molecular modeling suggests that these differences in affinity are due to a direct interaction with the receptor. Comparing models of diphenylhydantoin to the novel alpha-hydroxyphenlyamide compound suggests that the increased activity may be due to an optimized phenyl ring position and increased molecular volume. This information may be useful in the development of more potent sodium channel blockers.

Acetonitrile Derivatives as Carbonyl Synthons. One-Pot Preparation of Diheteroaryl Ketones via a Strategy of Sequential SNAr Substitution and Oxidation

The anion of 2-aryl acetonitrile derivatives reacted with a variety of heteroaryl chlorides or bromides in an S(N)Ar manifold to afford intermediate anions which were susceptible to oxidation. The addition of sodium peroxide and aqueous NH(4)OAc solution effected oxidation to afford aryl heteroaryl ketones in good yields. Aryl acetonitrile derivatives are thus umpolung-type synthons of the corresponding aryl carbonyl functionality.

Dopamine transporter as target for drug development of cocaine dependence medications

Because much evidence implicates the dopamine transporter in the reinforcing effects of cocaine, development of potential medications for cocaine dependence has included the dopamine transporter as a target. The present overview covers progress in the drug development area regarding several classes of dopamine uptake inhibitors, with an emphasis on structure-activity relationships that enhance potency and selectivity at transporters for dopamine compared with those for serotonin or norepinephrine. The following categories of compounds are covered: tropane, benztropine, 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine (GBR), methylphenidate, mazindol, and phencyclidine analogs. Activity at transporters as well as on behavior is highlighted.

Interaction of cis-(6-benzhydrylpiperidin-3-yl)benzylamine analogues with monoamine transporters: structure-activity relationship study of structurally constrained 3,6-disubstituted piperidine analogues of (2,2-diphenylethyl)-[1-(4-fluorobenzyl)piperidin-4-ylmethyl]amine

To explore structure-activity relationships (SAR) of a novel conformationally constrained lead cis-3,6-disubstituted piperidine derivative derived from (2,2-diphenylethyl)-[1-(4-fluorobenzyl)piperidine-4-ylmethyl]amine (I), a series of compounds was synthesized by derivatizing the exocyclic N-atom at the 3-position of the lead. This study led to the formation of substituted phenyl and heterocyclic derivatives. All novel compounds were tested for their affinity at the dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET) in the brain by measuring their potency in competing for the binding of [3H]WIN 35 428, [3H]citalopram, and [3H]nisoxetine, respectively. Selected compounds were also evaluated for their activity in inhibiting the uptake of [3H]DA. The SAR results demonstrated that the nature of substitutions on the phenyl ring is important in activity at the DAT with the presence of an electron-withdrawing group having the maximum effect on potency. Replacement of the phenyl ring in the benzyl group by heterocyclic moieties resulted in the development of compounds with moderate activity for the DAT. Two most potent racemic compounds were separated by a diastereoisomeric separation procedure, and differential affinities were observed for the enantiomers. Absolute configuration of the enantiomers was obtained unambiguously by X-ray crystal structural study. One of the enantiomers, compound S,S-(-)-19a, exhibited the highest potency for the DAT (IC50 = 11.3 nM) among all the compounds tested and was as potent as GBR 12909 (1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine). However, the compound (-)-19a was more selective than GBR 12909 in binding to the DAT compared with binding to the SERT and NET. The present results establish the newly developed 3,6-disubstituted piperidine derivatives as a novel template for high-affinity inhibitors of DAT. Structurally these molecules are more constrained compared to our earlier flexible piperidine molecules and, thus, should provide more insights about their bioactive conformations.

Synthesis and evaluation of dopamine and serotonin transporter inhibition by oxacyclic and carbacyclic analogues of methylphenidate

Methylphenidate (Ritalin) binds stereoselectively and enantioselectively to the dopamine transporter (DAT) and inhibits dopamine reuptake with in vitro and in vivo potency similar to that of cocaine. Unlike cocaine, it manifests little, if any, tolerance or addiction liability. Since this compound has a substantial clinical history, it provides an excellent template from which to design potential medications for cocaine abuse. It has long been assumed that a nitrogen, such as exists in cocaine and methylphenidate, is essential for interaction with monoamine transporters. We previously demonstrated that an amine nitrogen in phenyltropane analogues of cocaine is not necessary for conferring high DAT binding affinity. We now report the synthesis of oxacyclic and carbacyclic analogues of methylphenidate, including the four enantiomerically pure isomers of 2-(3,4-dichlorophenyl)-2-(tetrahydropyran-2-yl)acetic acid methyl ester. The threo isomers are potent and selective inhibitors of the DAT. This is the first generalization of the principle that the presence of nitrogen is not a necessity for DAT inhibition.

Synthesis and pharmacology of site specific cocaine abuse treatment agents: 8-substituted isotropane (3-azabicyclo[3.2.1]octane) dopamine uptake inhibitors

A series of 8-substituted-3-azabicyclo[3.2.1]octanes (isotropanes) were synthesized and tested for inhibitor potency using [(3)H]WIN 35,428 binding at the dopamine (DA) transporter, [(3)H]citalopram binding at the serotonin (5-HT) transporter, and [(3)H]DA uptake assays. The synthesis started with a Mannich condensation of cyclopentanone, benzylamine, and fomaldehyde to afford N-benzyl-3-azabicyclo[3.2.1]octan-8-one (6). The 8-phenyl group was introduced by Grignard addition to ketone 6 or nucleophilic displacement via a triflate of the corresponding alcohol 7a. The 8beta-phenyl-8alpha-alcohols from Grignard addition generally have low affinity for the two transporters and do not effectively inhibit the uptake of [(3)H]DA. The 8beta-phenyl compound (14) without the hydroxyl group at C-8 was much more potent (22-fold) for [(3)H]WIN 35,428 binding inhibition than the corresponding 8beta-phenyl-8alpha-hydroxy compound (7a). The 8alpha-phenyl compound 8a was almost as potent as cocaine in binding to the DA transporter (IC(50) = 234 nM vs 159 nM for cocaine), whereas the C-8 epimer, compound 14, was somewhat less potent (IC(50) = 785 nM). The lower potency of 14 (beta-orientation of 8-phenyl group) as compared to 8a (alpha-orientation) was unexpected, based on modeling studies comparing the new compounds to WIN 35,065-2, an analogue of cocaine. The benzhydryl ethers at C-8 (17), analogous to the benztropines, had better selectivity than the corresponding phenyl compounds, 8a and 14, for the DA transporter as compared to the 5-HT transporter. The isotropane and benzisotropine analogues seem to bind in a manner that is more similar to that of the benztropine compounds 5 rather than those of cocaine and WIN 35,065-2.

Design and synthesis of an irreversible dopamine-sparing cocaine antagonist

Cocaine is a powerful reinforcer and stimulant that binds to specific recognition sites associated with monoamine transporters in the mammalian brain. The search for a functional antagonist to the addictive properties of cocaine has focused on the discovery of a molecule that can inhibit cocaine binding to the dopamine transporter (DAT) but continue to allow dopamine transport by the DAT. No such dopamine-sparing cocaine antagonist has been reported and it is becoming evident that dopamine-sparing antagonism of the pharmacological effects of cocaine by a classical antagonist may not be possible. Herein we present a new concept for the design of dopamine-sparing cocaine antagonists. A unique approach is utilized to deliver an inhibitor that binds irreversibly to the DAT, then cleaves and leaves behind a small fragment attached to the DAT that blocks access by cocaine but permits dopamine transport. The design of these compounds takes advantage of a cysteinyl sulfhydryl group in the DAT. This group is hypothesized to attack the incoming inhibitor and lead to selective inhibition of the cocaine binding site while sparing dopamine transport. This concept of a mechanism based irreversible dopamine-sparing cocaine antagonist has now been demonstrated to be viable and, as example, the unsaturated 6 showed inhibition of cocaine (63%) at the DAT after 24h incubation, while at that point considerably less inhibition of dopamine is manifested (23%). In contrast, the epoxide 7 showed a greater inhibition of dopamine reuptake than cocaine binding at 24h (68% versus 18%).

Mazindol analogues as potential inhibitors of the cocaine binding site at the dopamine transporter

A series of mazindol (2) and homomazindol (3) analogues with a variety of electron-donating and electron-withdrawing groups in the pendant aryl group and the benzo ring C, as well as H, methoxy, and alkyl groups replacing the hydroxyl group were synthesized, and their binding affinities at the dopamine transporter (DAT) on rat or guinea pig striatal membranes were determined. Several active analogues were also evaluated for their ability to block uptake of DA, 5-HT, and NE and inhibit binding of [(125)I] RTI-55 at HEK-hDAT, HEK-hSERT, and HEK-hNET cells. Mazindane (26) was found to be a pro-drug, oxidizing (5-H --> 5-OH) to mazindol on rat striatal membranes and HEK-hDAT cells. The 4',7,8-trichloro analogue (38) of mazindol was the most potent and selective ligand for HEK-hDAT cells (DAT K(i) = 1.1 nM; SERT/DAT = 1283 and NET/DAT = 38). Experimental results strongly favor the cyclic or ol tautomers of 2 and 3 to bind more tightly at the DAT than the corresponding keto tautomers.

Identification of a novel partial inhibitor of dopamine transporter among 4-substituted 2-phenylquinazolines

In an attempt to identify novel ligands for the dopamine transporter, a series of 4-substituted-2-phenylquinazolines were synthesized and evaluated. Among the compounds studied, 4-[(diphenylmethyl)amino]-2-phenylquinazoline (4 g) was identified as a novel partial inhibitor of [(125)I]RTI-55 binding to the dopamine transporter and a partial inhibitor of [(3)H]dopamine uptake.