Inhibition of high affinity choline transport by stereoisomers of some 3-quinuclidinol derivatives

Competitive inhibition of the high-affinity choline transporter by tetrahydropyrimidine anthelmintics

The high-affinity choline transporter CHT1 mediates choline uptake, the rate-limiting and regulatory step in acetylcholine synthesis at cholinergic presynaptic terminals. CHT1-medated choline uptake is specifically inhibited by hemicholinium-3, which is a type of choline analog that acts as a competitive inhibitor. Although the substrate choline and the inhibitor hemicholinium-3 are well-established ligands of CHT1, few potent ligands other than choline analogs have been reported. Here we show that tetrahydropyrimidine anthelmintics, known as nicotinic acetylcholine receptor agonists, act as competitive inhibitors of CHT1. A ligand-dependent trafficking assay in cell lines expressing human CHT1 was designed to search for CHT1 ligands from a collection of biologically active compounds. We found that morantel as well as other tetrahydropyrimidines, pyrantel and oxantel, potently inhibits the high-affinity choline uptake activity of CHT1 in a competitive manner similar to the inhibitor hemicholinium-3. They also inhibit the high-affinity choline transporter from the nematode Caenorhabditis elegans. Finally, tetrahydropyrimidines potently inhibit the high-affinity choline uptake in rat brain synaptosomes at a low micromolar level, resulting in the inhibition of acetylcholine synthesis. The rank order of potency in synaptosomes is as follows: morantel > pyarantel > oxantel (K_i = 1.3, 5.7, and 8.3 μM, respectively). Our results reveal that tetrahydropyrimidine anthelmintics are novel CHT1 ligands that inhibit the high-affinity choline uptake for acetylcholine synthesis in cholinergic neurons.

Preparation of novel meta- and para-substituted N-benzyl protected quinuclidine esters and their resolution with butyrylcholinesterase

Since the optically active quinuclidin-3-ol is an important intermediate in the preparation of physiologically or pharmacologically active compounds, a new biocatalytic method for the production of chiral quinuclidin-3-ols was examined. Butyrylcholinesterase (BChE; EC 3.1.1.8) was chosen as a biocatalyst in a preparative kinetic resolution of enantiomers. A series of racemic, (R)- and (S)-esters of quinuclidin-3-ol and acetic, benzoic, phthalic and isonicotinic acids were synthesized, as well as their racemic quaternary N-benzyl, meta- and para-N-bromo and N-methylbenzyl derivatives. After the resolution, all N-benzyl protected groups were successfully removed by catalytic transfer hydrogenation with ammonium formate (10% Pd-C). Hydrolyses studies with BChE confirmed that (R)-enantiomers of the prepared esters are much better substrates for the enzyme than (S)-enantiomers. Introduction of bromine atom or methyl group in the meta or para position of the benzyl moiety resulted in a considerable improvement of the stereoselectivity compared to the non-substituted compounds. Optically pure quinuclidin-3-ols were prepared in high yields and enantiopurity by the usage of various N-benzyl protected groups and BChE as a biocatalyst.

Synthesis of new N-quaternary-3-benzamidoquinuclidinium salts

The synthesis of racemic and enantiomerically pure N-p-methylbenzyl-3- and N-p-chlorobenzylbenzamidoquinuclidinium bromides (6-8 and 9-11, respectively) is described. These compounds were prepared from racemic or enantiomerically pure 3-benzamidoquinuclidines 3-5 using the appropriate quaternization reagents: p-methyl- benzyl bromide (1) and p-chlorobenzyl bromide (2).

3-Amidoquinuclidine derivatives: Synthesis of compounds and inhibition of butyrylcholinesterase

The synthesis of racemic and enantiomerically pure 3-butanamidoquinuclidines ((+/-)-Bu, (R)-Bu and (S)-Bu), (1-3) and 3-benzamidoquinuclidines ((+/-)-Bz, (R)-Bz, and (S)-Bz), (4-6) is described. The N-quaternary derivatives, N-benzyl-3-butanamidoquinuclidinium bromides ((+/-)-BnlBu, (R)-BnlBu and (S)-BnlBu), (7-9) and N-benzyl-3-benzamidoquinuclidinium bromides ((+/-)-BnlBz, (R)-BnlBz and (S)-BnlBz), (10-12) were subsequently synthesized. The interaction of the four enantiomerically pure quaternary derivatives with horse serum butyrylcholinesterase (BChE) was tested. All tested compounds inhibited the enzyme. The best inhibitior of the enzyme was (S)-BnlBz with a K(i) = 3.7 microM. The inhibitor potency decreases in order (S)-BnlBz > (R)-BnlBz >> (R)-BnlBu > (S)-BnlBu.

3-Hydroxyquinuclidinium derivatives: synthesis of compounds and inhibition of acetylcholinesterase

Four compounds were prepared: 3-hydroxy-1-methylquinuclidinium iodide (I), 3-(N,N-dimethylcarbamoyloxy)-1-methylquinuclidinum iodide (II), and two conjugates of I and II with 2-hydroxyiminomethyl-3-methylimidazole in which two parts of the molecule were linked by -CH2-O-CH2- (III and IV). III and IV are new compounds and their synthesis and physical data were given. All compounds were tested as inhibitors of human erythrocyte acetylcholinesterase (EC 3.1.1.7, AChE). The enzyme activity was measured in 0.1 M phosphate buffer (pH 7.4) at 10 and 37 degrees C with acetylthiocholine (ATCh) as the substrate. The obtained enzyme/inhibitor dissociation constants were between 0.05 and 0.5 mM at 10 degrees C and between 0.2 and 0.6 mM at 37 degrees C. At both temperatures compounds III and IV had higher affinities for the enzyme than compounds I and II and this difference was more pronounced at 10 than at 37 degrees C. The carbamates II and IV were also progressive AChE inhibitors. For compound II the rate constants of inhibition were 6300 and 2020 M(-1) min(-1) at 37 and 10 degrees C, respectively. Compound IV was a very weak carbamoylating agent with rate constants of inhibition of 100 and 63 M(-1) min(-1) at 37 and 10 degrees C, respectively. The oxime group in compounds III and IV hydrolyzed ATCh at rates of 23 and 3.2 M(-1) min(-1) at 37 and 10 degrees C, respectively.

Stereospecificity of high- and low-affinity transport of choline analogues into rat cortical synaptosomes

The present experiments used methylcholines to examine the stereoselectivity of choline transport into rat synaptosomes. R(+)-alpha-methylcholine and S(+)-beta-methylcholine were significantly better inhibitors of the high-affinity choline transport system than were their enantiomers. Although both enantiomers of alpha- and of beta-methylcholine inhibited [3H]choline transport, only R(+)-alpha-methylcholine and S(+)-beta-methylcholine could be transported by the high-affinity choline uptake mechanism. Therefore, we conclude that the chiral requirements for recognition of and for transport by the high-affinity transporter are clearly different. In addition to high-affinity choline transport, Na(+)-independent low-affinity transport was measured. This process transported R(+)-alpha-methylcholine, but not S(-)-alpha-methylcholine; however, it showed no stereoselectivity for the enantiomers of beta-methylcholine. Thus, high- and low-affinity choline transport mechanisms exhibit distinct differences in their substrate selectivities. We suggest that the stereoselective properties of choline transport might present a unique opportunity to study choline uptake and metabolism.

Quaternary and tertiary quinuclidine derivatives as inhibitors of choline uptake

The uptake of choline into cholinergic neurons for acetylcholine (ACh) synthesis is by a specific, high-affinity, sodium- and temperature-dependent transport mechanism (HAChU). Of several quaternary quinuclidinol derivatives tested, the N-allyl derivative proved to be most potent. Though the methyl, ethyl, and isopropyl derivatives were less potent at comparable concentrations, at higher concentrations they also maximally inhibited HAChU. The benzyl, hydroxyethyl, and methoxyethyl derivatives failed to inhibit HAChU by greater than 50% at concentrations up to 100 microM. N-Allyl-3-quinuclidinol (NAQ) proved to be a specific inhibitor of HAChU (IC50 = 0.9 microM) and a poor inhibitor of both sodium-independent transport (IC50 = 680 microM) and choline acetyltransferase activity (Ki = 200 microM). The NAQ exhibited noncompetitive type inhibition compared with N-methyl-3-quinuclidinol, a competitive inhibitor of HAChU. Thus, substitution at the N-functional group not only alters potency, but may change the mechanism by which inhibition is produced. The optical isomers of NAQ and several derivatives were prepared and employed to examine the stereochemical selectivity for inhibition of choline uptake. The S(+)-isomer of NAQ (IC50 = 0.1 microM) had approximately 100-fold greater inhibitory activity for HAChU than the corresponding R(-)-isomer (IC50 = 10 microM). With all other quinuclidinols tested, the S(+)-isomers were also more potent than the corresponding R(-)-isomers. In an effort to obtain a tertiary inhibitor of HAChU that would be expected to cross the blood-brain barrier following peripheral administration, 3-biphenyl-3-quinuclidinol (BHQ) and 3-naphthyl-3-quinuclidinol (NHQ) were synthesized and evaluated.(ABSTRACT TRUNCATED AT 250 WORDS)

Choline transport specificity in animal cells and tissues

1. Beta carbolines inhibit choline transport in rat brain. 2. The aziridinium ring on the nitrogen of mustard analogs of choline causes irreversible binding to the carrier in rat brain. 3. The uptake system in rat brain is stereoselective, requires a quaternary nitrogen, and prefers analogs with a nitrogen-oxygen distance of about 3.26 A. 4. In mouse brain troxonium derivatives inhibit choline transport. 5. In cuttlefish optic lobes and torpedo electric organ pyrene derivatives potently inhibit choline transport. 6. In guinea pig placenta, the affinity of the choline carrier remains high even when this molecule lacks one or two methyl groups.

Structure-activity studies on inhibition of choline uptake by a mouse brain synaptosomal preparation: basic data

Over 80 substances were studied for their inhibition of high-affinity uptake of [3H]choline into a mouse brain synaptosomal fraction. Kinetic experiments tested a number of them for competitive behavior. A minimal provisional model for the choline uptake process is envisioned that is consistent with current data and with relevant observations in the literature. There are two hydrophilic anionic sites on the choline transporter that are separated from each other by a cationic hydrophobic domain. Association of choline in a Na+-dependent manner with one or both of the sites is necessary for the transport of choline to take place. The choline binding anionic sites are sufficiently large and/or flexible to allow attachment of cationic moieties larger than choline. The cationic hydrophobic domain of the transporter is flexible, probably tending to planarity. The length of the hydrophobic region between the anionic sites is approximately that of 10 extended methylene groups, and the minimal width is approximated by the distance across the condensed ring system of chlorpromazine. The probability of attachment of the highly hydrophilic choline to its binding sites is increased both by hydrogen-bonding to a proton-acceptor within the anionic sites and by repulsion from the cationic hydrophobic region. A number of substances that potently and competitively inhibit high affinity choline uptake possess quaternary ammonium groups and neutral or negatively charged lipophilic groups. In general, substances in which two quaternary ammonium groups are separated by an appropriately configured hydrophobic group and which can combine with both anionic sites and the hydrophobic region between them are more potent inhibitors than monoquaternary substance with the same or similar groups. However, substances with a single high-affinity quaternary group and an appropriately structured hydrophobic group, e.g. the trimethoxy-3-butynyl quaternary ammonium compounds, possess inhibitory efficacies similar to those shown by the most potent bisquaternaries. The above suggests that further delineation of the characteristics of the structures of the above sites of the transporter could lead to devisal of more potent reversible inhibitors of choline uptake than now are available.

Methylpiperidine analog of hemicholinium-3: a selective, high affinity non-competitive inhibitor of sodium dependent choline uptake system

The potency of hemicholinium-3 (HC-3) and its analogs to inhibit sodium dependent high affinity choline uptake were evaluated in rat striatal synaptosomal preparation. Hemicholinium-3 inhibited sodium dependent high affinity choline uptake (IC50 = 18 nM) while the half molecule of HC-3, HC-15, was inactive. The order of potency for choline uptake inhibition of piperidine substituted HC-3 molecule was as follows: 4-methylpiperidine (A-5 and CA-5) much greater than HC-3 much greater than unsubstituted piperidines (CA-1 and A-1) much greater than 2- or 3-methylpiperidine (A-2 and A-3) and 4-hydroxypiperidine (A-7). The tertiary amine derivative of 4-methylpiperidine substituted HC-3 (A-4) was nearly 10-fold less potent than its corresponding quaternary derivative (A-5). Choline uptake was inhibited competitively by HC-3 and non-competitively by A-5. The inhibition of choline uptake by A-5 was readily reversible by washing. A-5 did not inhibit the uptake of dopamine and gamma-aminobutyric acid. These findings suggest that the N-methyl,4-methylpiperidine analog of HC-3 (A-5) is the most potent of all known inhibitors of sodium dependent high affinity choline uptake and that the inhibition of choline uptake by this compound is mediated through a mechanism distinct from a simple competitive one.

Inhibition of high-affinity choline uptake and acetylcholine synthesis by quinuclidinyl and hemicholinium derivatives

Choline uptake into cholinergic neurons for acetylcholine (ACh) synthesis is by a specific, high-affinity, sodium- and temperature-dependent transport mechanism (HAChU). To assess the role of choline availability in regulation of ACh synthesis, the structure-activity relationships of several hemicholinium (HC) and quinuclidinyl analogs were evaluated in a dose response manner. As confirms previous studies, the HCs, e.g., HC-3, acetylsecohemicholinium, and HC-15 are potent inhibitors of HAChU, HC-3 being the most potent (I50 = 6.1 X 10(-8) M). In the present study, the most potent quinuclidinyl derivative was the N-methyl-3-quinuclidinone (I50 = 5.6 X 10(-7) M). This compound had approximately 100-fold greater inhibitory activity than the corresponding racemic alcohol, suggesting that the 3-hydroxyl functional group is not absolutely essential for activity. Increasing the size of the N-functional group from a methyl to an allyl in the alcohol led to a 10-fold increase in activity. However, removal of the quaternizing N-methyl group yielding the tertiary amine, 3-quinuclidinol hydrochloride, greatly reduced its capacity to inhibit HAChU. Of the 2-benzylidene-3-quinuclidinone derivatives studied, only the m-chloro derivative significantly reduced HAChU.