Recent developments in neuronal nicotinic acetylcholine receptor modulators

First total synthesis of (+/-)-3-hydroxy-11-norcytisine: structure confirmation and biological characterization

The first total synthesis of the natural product 3-hydroxy-11-norcytisine (1), structurally related to cytisine (2), a benchmark ligand at neuronal nicotinic acetylcholine receptors (NNRs), has been achieved. The synthesis permits the unambiguous confirmation of the structure originally proposed for 1 and has enabled initial biological characterization of 1 and its related compounds against NNRs.

Unique spirocyclopiperazinium salt III: Further investigation of monospirocyclopiperazinium (MSPZ) salts as potential analgesics

Two novel classes of monospirocyclopiperazinium salts were designed, synthesized, and evaluated for their in vivo analgesic activities. Some interesting structure-activity relationships are revealed: (1) Spirocyclopiperazinium moiety is favorable to improve the analgesic activity; (2) The size and conformation of spirocyclopiperazinium moiety significantly affects the analgesic activity; (3) Phenylethyl group of 3d is a crucial pharmacophore. Among the compounds synthesized, 3d exhibited the most potent activity with low toxicity. Further antinociceptive mechanism studies of 3d showed that these compounds will be a new kind of analgesics.

Development of subtype-selective ligands as antagonists at nicotinic receptors mediating nicotine-evoked dopamine release

N-n-Alkylation of nicotine converts it from an agonist into an antagonist at neuronal nicotinic acetylcholine receptor subtypes mediating nicotine-evoked dopamine release. Conformationally restricted analogues exhibit both high affinity and selectivity at this site, and are able to access the brain due to their ability to act as substrates for the blood-brain barrier choline transporter.

Synthesis and analgesic activity of hydrochlorides and quaternary ammoniums of epibatidine incorporated with amino acid ester

Hydrochloride derivatives 5a-c and quaternary ammonium derivatives 6a-c of epibatidine incorporated with amino acid ester were synthesized and evaluated for their in vivo analgesic activity and toxicity. Among all tested compounds, compound 6c has the most potent analgesic activity. The quaternary ammonium salts 6a and 6c showed better analgesic activity than the corresponding hydrochlorides 5a and 5c. Both 5a-c and 6a-c showed significantly lower toxicity than epibatidine itself.

Synthesis of a [2-Pyridinyl-18F]-labelled fluoro derivative of (−)-Cytisine as a candidate radioligand for brain nicotinic α4β2 receptor imaging with PET

In recent years, there has been considerable effort to design and synthesize radiotracers suitable for use in Positron Emission Tomography (PET) imaging of the alpha4beta2 neuronal nicotinic acetylcholine receptor (nAChR) subtype. A new fluoropyridinyl derivative of (-)-cytisine (1), namely (-)-9-(2-fluoropyridinyl)cytisine (3, K(i) values of 24 and 3462 nM for the alpha4beta2 and alpha7 nAChRs subtypes, respectively) has been synthesized in four chemical steps from (-)-cytisine and labelled with fluorine-18 (T(1/2): 119.8 min) using an efficient two-step radiochemical process [(a). nucleophilic heteroaromatic ortho-radiofluorination using the corresponding N-Boc-protected nitro-derivative, (b). TFA removal of the Boc protective group]. Typically, 20-45 mCi (0.74-1.67 GBq) of (-)-9-(2-[18F]fluoropyridinyl)cytisine ([18F]-3, 2-3 Ci/micromol or 74-111 GBq/micromol) were easily obtained in 70-75 min starting from a 100 mCi (3.7 GBq) aliquot of a cyclotron-produced [18F]fluoride production batch (20-45% non decay-corrected yield based on the starting [18F]fluoride). The in vivo pharmacological profile of (-)-9-(2-[18F]fluoropyridinyl)cytisine ([18F]-3) was evaluated in rats with biodistribution studies and brain radioactivity monitoring using intracerebral radiosensitive beta-microprobes. The observed in vivo distribution of the radiotracer in brain was rather uniform, and did not match with the known regional densities of nAChRs. It was also significantly different from that of the parent compound (-)-[3H]cytisine. Moreover, competition studies with (-)-nicotine (5 mg/kg, 5 min before the radiotracer injection) did not reduce brain uptake of the radiotracer. These experiments clearly indicate that (-)-9-(2-[18F]fluoropyridinyl)cytisine ([18F]-3) does not have the required properties for imaging nAChRs using PET.

Unique spirocyclopiperazinium salt. Part 2: synthesis and structure-activity relationship of dispirocyclopiperazinium salts as analgesics

Three series of spirocyclopiperazinium derivatives 5a-d, 6a-f and 17a-d were synthesized and evaluated for their in vivo analgesic activities. Compounds 5a, 17a and 17b exhibited excellent analgesic activity. Two important structure-activity relationships were observed from this study: (1) the quaternary ammonium functionality is a critical pharmacophore for analgesic activity; (2) it is important to adjust the lipophilic property of compounds to improve analgesic activity.

Unique spirocyclopiperazinium salt I: synthesis and structure-activity relationship of spirocyclopiperazinium salts as analgesics

Based on the structure of compound 3, two series of spirocyclopiperazinium derivatives 7a-n and 10a-h were synthesized and evaluated for their in vivo analgesic and sedative activities. Compounds 7f and 10c were discovered to exhibit excellent analgesic activity. Structure-activity relationships revealed that anion of the quaternary salt affected the analgesic and sedative activity significantly; the allyl group is a most effective group among the compounds 7a-n; the electron-released substitute on the aromatic ring is favorable to increase the analgesic activity.

Cytisine derivatives as ligands for neuronal nicotine receptors and with various pharmacological activities

Neuronal nicotinic acetylcholine receptors (nAChRs) form a family of ACh-gated cation channels made up of different subtypes. They are widely distributed in peripheral and central nervous systems and are involved in complex cerebral processes as learning, memory, nociception, movement, etc. The possibility that subtype-selective ligands be used in the treatment of CNS disorders promoted the synthesis of a large number of structural analogues of nicotine and epibatidine, two very potent nAChR agonists. Pursuing our long standing research on the structural modification of quinolizidine alkaloids, we devoted our attention to cytisine, another very potent ligand for many nAChR subtypes. Thus a systematic structural modification of cytisine was undertaken in order to obtain compounds of potential therapeutic interest at peripheral as well as central level, with a particular concern for achieving nAChR subtype selective ligands. Up to the present more than 80 cytisine derivatives, mainly of N-substitution and a few by modifying the pyridone ring, have been prepared. The biological results, which concern so far about an half of the prepared compounds, indicate that the introduction of a nitro group in position 3 of the pyridone nucleus further enhances the high affinity of cytisine, while the introduction of substituents on the basic nitrogen, though reducing in different degrees the affinity, gives rise to compounds with a higher selectivity for central (alpha(4)beta(2)) versus gangliar (alpha(3)-containing) receptor subtype. On the other hand, the analgesic, antihypertensive and inotropic activities found in some N-substituted cytisines, represent an attractive starting point for the development of more active compounds.

bis-Azaaromatic quaternary ammonium analogues: ligands for alpha4beta2* and alpha7* subtypes of neuronal nicotinic receptors

A series of bis-nicotinium, bis-pyridinium, bis-picolinium, bis-quinolinium and bis-isoquinolinium compounds was evaluated for their binding affinity at nicotinic acetylcholine receptors (nAChRs) using rat brain membranes. N,N'-Decane-1,12-diyl-bis-nicotinium diiodide (bNDI) exhibited the highest affinity for [(3)H]nicotine binding sites (K(i)=330 nM), but did not inhibit [(3)H]methyllycaconitine binding (K(i) >100 microM), indicative of an interaction with alpha4beta2*, but not alpha7* receptor subtypes, respectively. Also, bNDI inhibited (IC(50)=3.76 microM) nicotine-evoked (86)Rb(+) efflux from rat thalamic synaptosomes, indicating antagonist activity at alpha4beta2* nAChRs. N,N'-Dodecane-1,12-diyl-bis-quinolinium dibromide (bQDDB) exhibited highest affinity for [(3)H]methyllycaconitine binding sites (K(i)=1.61 microM), but did not inhibit [(3)H]nicotine binding (K(i)>100 microM), demonstrating an interaction with alpha7*, but not alpha4beta2* nAChRs. Thus, variation of N-n-alkyl chain length together with structural modification of the azaaromatic quaternary ammonium moiety afforded selective antagonists for the alpha4beta2* nAChR subtype, as well as ligands with selectivity at alpha7* nAChRs.

Syntheses and evaluation of pyridazine and pyrimidine containing bioisosteres of (+/-)-pyrido[3.4-b]homotropane and pyrido-[3.4-b]tropane as novel nAChR ligands

Bioisosteric replacement of the pyridine pharmacophoric element in (+/-)-pyrido[3.4-b]homotropane (PHT) and pyrido[3.4-b]tropane with the pyridazine and pyrimidine nucleus resulted in hitherto unknown nAChR ligands such as 5-8. Inverse type Diels-Alder reactions constitute the key steps in the new routes to the pyridazine- or pyrimidine-annulated bioisosteres. The enantiopure (+)-2-tropinone (11) from the 'chiral pool' is transformed to the ring-expanded silyl enol ether 12 and to the enamine 15. Both proved to be highly dienophilic species in the inverse type [4+2] cycloaddition reactions with the 1,2,4,5-tetrazines 13 and 16a,b or with the 1,3,5-triazine 19 to provide the enantiopure target compounds 5-7. In the same way the racemic pyrimidine-annulated species 8 was obtained from 3-tropanone 21. The new ligands were tested for their in vitro affinity for (alpha4)2(beta2)3 and alpha7* nAChR subtype. In comparison to PHT, well known to exhibit affinity for agonist binding sites in rat brain approximately equivalent to that of (+)-anatoxin-a (1), replacement of the pyridine by the bioisosteric pyridazine resulted in 30-fold lower affinity at the (alpha4)2(beta2)3 subtype. The annulated diazinotropanes 6-8, ligands with ferruginine-like structures more or less retained the affinity of (-)-norferruginine (3) except of compound 7. Remarkably, all of the novel ligands are devoid of affinity at the alpha7* subtype.

New developments in psychopharmacology of attention deficit hyperactivity disorder

Stimulants have been the mainstay of the psychopharmacological treatment of attention deficit hyperactivity disorder (ADHD) for over 60 years. In the last 5 years, there have been a number of important developments in terms of potential new treatments for ADHD. Since stimulants have such a short half-life, considerable research has focused on the development of new delivery systems that will allow once-a-day dosing. New formulations of both amphetamine (AMP) and methylphenidate (MPH) have appeared which differ in terms of their optical isomers from the commonly used compounds. A wide variety of compounds are currently in development as therapeutic agents for ADHD. Some, like the stimulants, primarily impact the noradrenergic and dopaminergic neurotransmitter systems, while others have novel effects on the cholinergic, histaminergic and peptidergic systems. Advances in the pharmacogenetics of ADHD may lead to the development of yet more compounds in the near future.

Syntheses and evaluation of halogenated cytisine derivatives and of bioisosteric thiocytisine as potent and selective nAChR ligands

We have developed one-step syntheses of halogenated derivatives of (-)-cytisine featuring a halogen substituent at positions 3, 5 or 3 and 5 of the 2-pyridone fragment, and prepared the novel bioisosteric thiocytisine by oxygen-sulphur exchange. The affinities of these pyridone-modified analogs of (-)-cytisine for (alpha 4)(2)(beta 2)(3) and alpha 7* nAChRs in rat forebrain membranes were determined by competition with (+/-)-[(3)H]epibatidine and [(3)H]MLA, respectively. The 3-halocytisines 7 possess subnanomolar affinities for (alpha 4)(2)(beta 2)(3) nAChRs, higher than those found for (-)-cytisine as well as for the 5-halocytisines 8 and 3,5-dihalocytisines 6. In contrast to the parent alkaloid the 3-halogenated species display much a higher affinity for the alpha 7* nAChR subtype. The most potent molecule was 3-bromocytisine (7b) with preferential selectivity (200-fold) for the (alpha 4)(2)(beta 2)(3) subtype [K(i)=10 pM (alpha 4 beta 2) and 2.0 nM (alpha 7*)]. Replacement of the lactam with a thiolactam pharmacophore to thiocytisine (12) resulted in a subnanomolar affinity for the (alpha 4)(2)(beta 2)(3) nAChR subtype (K(i)=0.832 nM), but in a drastic decrease of affinity for the alpha 7* subtype; thiocytisine (12) has a K(i) value of 4000 nM (alpha 7*), giving a selectivity of 4800-fold for the neuronal (alpha 4)(2)(beta 2)(3)-nAChR and thus displaying the best affinity-selectivity profile in the series under consideration.