2,3-Dihydro-1H-cyclopenta[b]quinoline Derivatives as Acetylcholinesterase Inhibitors—Synthesis, Radiolabeling and Biodistribution

In the present study we describe the synthesis and biological assessment of new tacrine analogs in the course of inhibition of acetylcholinesterase. The obtained molecules were synthesized in a condensation reaction between activated 6-BOC-hydrazinopyridine-3-carboxylic acid and 8-aminoalkyl derivatives of 2,3-dihydro-1H-cyclopenta[b]quinoline. Activities of the newly synthesized compounds were estimated by means of Ellman’s method. Compound 6h (IC50 = 3.65 nM) was found to be most active. All obtained novel compounds present comparable activity to that of tacrine towards acetylcholinesterase (AChE) and, simultaneously, lower activity towards butyrylcholinesterase (BChE). Apart from 6a, all synthesized compounds are characterized by a higher affinity for AChE and a lower affinity for BChE in comparison with tacrine. Among all obtained molecules, compound 6h presented the highest selectivity towards inhibition of acetylcholinesterase. Molecular modeling showed that all compounds demonstrated a similar binding mode with AChE and interacted with catalytic and peripheral sites of AChE. Also, a biodistribution study of compound 6a radiolabeled with 99mTc was performed.

Nevertheless, the search for new candidates-tacrine analogues is still of interest to scientists involved in AD research [15,16]. For example, 6-fluorotacrin-1-ol and 6-chlorotacrin-1-ol were found to be more potent than tacrine [17]. In the last decade, analogues containing two tacrine moieties linked by an alkylene chain were synthesized. It was reported that these dimeric molecules of tacrine were characterized by a stronger potency and a higher selectivity towards AChE [18,19]. It was proven that bis-(7)-tacrine had several modes of action, such as inhibition of AChE, N-Methyl-D-aspartate (NMDA) receptors, and nitric oxide synthase signaling. Furthermore, several homodimeric tacrine-based AChE inhibitors were synthesized. Their increased inhibitory potency was believed to be derived from the simultaneous binding of the units to the active and peripheral anionic sites of AChE [19]. One of these novel compounds, heptylene-linked bis-tacrine, was found to be 150-fold more active against rat AChE than tacrine and 250-fold more selective for acetylcholinesterase (AChE) than for butyrylcholinesterase (BChE) [20].
Second generation AChEIs (donepezil, rivastigmine and galantamine) demonstrate greater efficacy in AD treatment. Furthermore, when compared to tacrine, these drugs have fewer side effects and longer half-lives [6]. Therefore, synthesis of the analogues of the approved drugs: donepezil, rivastigmine, and galantamine is still of interest for many research groups [21,22].
The purpose of this study was to synthesize new derivatives of 2,3-dihydro-1H-cyclopenta [b]quinoline and nicotinic acid and to determine their activity towards inhibition of AChE and BChE. Furthermore, the selectivity of synthesized compounds was determined. This is significant for further phases of our study related to complexation of radioactive isotopes because during the design of novel potential radiopharmaceuticals selectivity and capability of binding with specified biological targets are more important than activity. Molecular modeling studies for synthesized compounds were also performed in order to elucidate the interactions between the enzymes and synthesized compounds. Furthermore, one of the synthesized molecules (compound 6a) was labeled with 99m Tc and a biodistribution study of radioactivity was conducted following intravenous administration of 99m Tc-6a to rats in order to estimate its potential as diagnostic marker in AD.

Chemistry
The first step of the synthesis was the preparation of 6-BOC-hydrazinopyridine-3-carboxylic acid (2). The substrate for this synthesis was 6-chloronicotinic acid. This compound reacted with hydrazine to give 6-hydrazinenicotinic acid (1). Subsequently, it was treated with di-tert-butyl dicarbonate and triethylamine in the presence of dimethylformamide (Scheme 1). Scheme 1. Synthesis of 6-BOC-hydrazinopyridine-3-carboxylic acid. Reagents: (a) 85% NH 2 NH 2 ; (b) (t-BuOCO) 2 O, triethyl amine, DMF. These syntheses were described earlier by Abrams and co-workers [26]. 8-chloro-2,3-dihydro-1Hcyclopenta[b]quinoline (3) was prepared according to a procedure mentioned in a previous paper [17]. This reaction involved cyclization of anthranilic acid with cyclopentanone in POCl 3 . Subsequently, the obtained compound (3) was coupled with the appropriate alkyldiamine (number of carbon atoms ranging from 2 to 9) and giving 8-amino-2,3-dihydro-1H-cyclopenta[b]quinolines (Scheme 2) [27][28][29][30][31]. (3), 4a-4h. Reagents: (a) cyclopentanone, POCl 3 , reflux; (b) diamine, phenol, NaI, reflux. Novel compounds were obtained via the synthesis between 6-(N'-tertbutoxycarbonylhydrazino)nicotinic acid (2), previously activated by 2-chloro-4,6-dimethoxy-1,3,5-triazine (CDMT), and N-methylmorpholine in solvent and reacted with compounds 4a-4h. The best results (75% yield) in this reaction were achieved with drop by drop addition of N-methylmorpholine to the solution in tetrahydrofuran at −5 °C. Monitoring the reaction by TLC showed that the reaction of activation of the carboxylic group was usually completed within 1-4 h; subsequently, a mixture of the appropriate reactant 4a-4h in the respective solvent was added at −5 °C. The last step of the synthesis involved conversion of the obtained compounds 5a-5h into hydrochlorides 6a-6h (Scheme 3) in the presence of hydrochloric acid with recrystallization from HCl in ether. In this step, the BOC group split off and the new compound precipitated.  The activity of the synthesized compounds towards inhibition of both enzymes (AChE and BChE) was estimated by Ellman's spectrophotometric method [32][33][34]. Table 1 presents IC 50 values of the newly obtained compounds with respect to AChE and BChE inhibition. Among these molecules, the most active towards inhibition of AChE appears to be molecule 6h (IC 50 = 3.65 nM). Compound 6g (IC 50 = 5.17 nM) exhibits a similar value of IC 50 to tacrine. Obtained data shows that all synthesized molecules were characterized by lower BChE inhibitory activity in comparison with tacrine. Table 1 also lists values of relative inhibitory effects towards acetylcholinesterase (ratio IC 50 BChE/AChE) and butyrylcholinesterase (ratio IC 50 AChE/BChE). The most active molecule, compound 6h, was more selective for AChE than tacrine. Derivative 6g, with similar activity with regard to tacrine, was characterized by a higher selectivity for AChE in comparison with tacrine. All acquired compounds possessed lower affinity for BChE than tacrine; among all synthesized compounds, compound 6a presented the highest activity for BChE. These findings are significant in view of the pathological processes involved in this type of neurodegenerative disease [35].

Studies of Molecular Modeling
A novel series of compounds was docked to acetyl-and butyrylcholinesterase to show the possible interactions between inhibitors and enzymes. All ligands demonstrated a similar binding mode with AChE. They were extended along the active gorge and interacted with catalytic and peripheral sites. The most active molecule (compound 6h) and its binding mode are presented in Figure 1. The fragment of the tacrine analogue with a cyclopentane ring created a characteristic sandwich due to π−π stacking with Trp84 and Phe330. The linker was located in the middle of the gorge, where it formed hydrophobic interactions with aromatic rings of Tyr121 and Tyr334. The nicotinamide moiety, in particular the amide bond, was located between two aromatic residues of the peripheral anionic site-Tyr70 and Trp279. Hydrazine interacted by H-bonding with the carbonyl group of the Asp276 backbone which was why the pyridine ring was shifted and was not able to create classical π−π stacking; it was engaged in some hydrophobic interactions. In the case of butyrylcholinesterase, the binding mode of the cyclopentaquinoline moiety was similar. The small differences concerned the location of the hydrazinenicotinic fragment in the reduced peripheral anionic site of BChE. The binding mode of the most potent butyrylcholinesterase inhibitor 6a is shown in Figure 2. A fragment of the tacrine analogue created π-π stacking with Trp82 and CH-π interactions with Trp430. The carbonyl group of the inhibitor amide bond formed H-bonding with OH Thr120 and the hydrazine moiety with the C=O of the Ile69 backbone.

Radiolabeling with 99m Tc and Biodistribution Studies in Rats
Compound 6a was designed for radiolabeling. Also, spectrophotometric experiments were performed to determine its stability in water ( Figure 3). Taking into consideration all synthesized compounds, compound 6a is characterized by the highest selectivity towards BChE. Assessment of the level of this enzyme is very important as it is different in various stages of Alzheimer's disease. Quality control of 99m Tc-labeled HYNIC-compounds with tricine as coligand with HPLC confirmed the purity of the product, which was found to be without unbound technetium (pertechnetate or hydrolyzed form, which would have appeared after a short elution time) [36]. Figure 4 presents radiochromatograms of complexes formed by technetium-99m with tricine and HYNIC.  Table 2 presents the tissue distribution of radioactivity after intravenous administration of 99m Tc-6a to rats. Collectively, compound 6a exhibited relatively rapid blood radioactivity clearance; a large percentage of 99m Tc-radioactivity was located in the liver, but also partly in the kidney, lung, and the gastrointestinal tract. Whereas the liver radioactivity uptake persisted for a long time for 99m Tc-6a (Table 2), radioactivity found in the kidney and the gastrointestinal tract is more likely connected with elimination of the parent compounds and/or their metabolites from the body. Radioactivity concentrations in the brain were very low; probably, as a result of the hydrophilicity of radiolabelled compounds (the effect of technetium and co-ligands attached to the 2,3-dihydro-1H-cyclopenta [b]quinoline analogues), which are unable to cross the blood-brain barrier. This behavior is disadvantageous for the intended use of the agents for diagnosis of Alzheimer's disease.

Chemistry
During conducted syntheses dry organic solutions were used. This was achieved by employing anhydrous Na 2 SO 4 . Solvents were removed with a rotary evaporator. Melting points were determined by using an Electrothermal apparatus with open capillaries and were uncorrected. For monitoring of conducted reactions, TLC with 25 DC-Alufolien Kieselgel 60F 254 (Merck) was used; detection was carried out with a UV Lamp (254 nm). Column chromatography was executed using silica gel 60 (200-400 mesh, Merck). For determining IR spectra, Mattson Infinity Series FT-IR spectrophotometer was used. IR spectra were recorded in KBr. 1 H NMR spectra were recorded with Varian Mercury 300 MHz spectrometer, with tetramethylsilane as an internal standard. Mass spectra were performed by the Centre of Molecular and Macromolecular Studies in Lodz (Polish Academy of Sciences). (1) 6-Chloronicotinic acid (8.0 g, 50.77 mmol) was dissolved in 80% hydrazine hydrate (35 mL, 930.0 mmol) and placed in a 100 °C oil bath for 4 h. The homogeneous reaction mixture was cooled to room temperature and concentrated to dryness to give a white solid. The solid was dissolved in water and on acidification to pH 5.5 with concentrated hydrochloric acid, a precipitate was formed. The precipitate was isolated by filtration; the solid was washed with 95% ethanol and ether, and dried in vacuum. Compound 1: yield 85%; mp 292-293 °C; 1  3.1.2. 6-BOC-hydrazinopyridine-3-carboxylic Acid (2) To a solution of 1 (1.4 g, 9.8 mmol) and triethylamine (1.2 mL, 11.8 mmol) in DMF (10 mL) was added di-tert-butyl dicarbonate (2.13 g, 9.8 mmol). The reaction mixture became homogeneous over 1 h and stirring was continued for 16 h at room temperature. The reaction mixture was concentrated to dryness under reduced pressure to give a brown solid. Recrystallization from ethyl acetate gave the desired product 2 as a white solid.  (3) To a mixture of anthranilic acid (7.4 g, 53.9 mmol) and cyclopentanone (1.64 mL, 53.9 mmol) was carefully added 30 mL of POCl 3 in an ice bath. The mixture was heated under reflux for 2 h, then cooled to room temperature, and concentrated under reduced pressure to give a slurry. The residue was diluted with ethyl acetate (50 mL), neutralized with aqueous K 2 CO 3 (30 mL), and washed with brine (2 × 20 mL). The organic layer was dried over MgSO 4 and concentrated to dryness under reduced pressure to give a brown solid. Recrystallization from acetone gave the desired product 1 as a yellow solid. Compound 1: yield 54%; mp 85-87 °C; 1 H NMR (CDCl 3 ) (δ ppm.): 8.1 (1H, d, J = 8.3 Hz, ArH), 7.9 (1H, d, J = 8.5 Hz, ArH), 7.6 (1H, t, J = 6.9 Hz, ArH), 7.4 (1H, t, J = 6.9 Hz, ArH), 3.0 (2H, d, J = 5.9 Hz, CH 2 ), 2.9 (2H, d, J = 5.0 Hz, CH 2 ), 1.8 (2H, t, J = 6.9 CH 2 ); IR (KBr) ν (cm −1 ): 766.2, 1607.2, 2955.1, 2920.7, 3417.8.

Biochemical Studies
Determination of the inhibitory activity of all synthesized compounds towards AChE and BChE was performed by means of Ellman's spectrophotometric method with our own modifications. Every sample contained 5,5'-dithiobisnitrobenzoic acid (DTNB, 0.05 mL, 0.5 M), acetylthiocholine iodide (substrate), newly synthesized inhibitor, and AChE (5 units/mL) or BChE (5 units/mL), respectively, in cholinesterases activity measurements. Every measurement was conducted in the presence of phosphate buffer (0.1 M, pH 8.0) at 37 °C. The total volume of every sample amounted to 3 mL. Measurement procedure was identical for all samples. All ingredients were incubated for 1 min and then the absorbance was recorded at 412 nm. Seven concentrations of acetylthiocholine iodide were used in order to obtain the inhibition curves for every compound. Every measurement was conducted three times. Also, samples without inhibitor were evaluated in order to obtain absolute AChE and BChE activity. The value of IC 50 , defined as the drug concentration that contributes to the inhibition of 50% AChE or BChE activity, was determined by non-linear and linear regression.

Molecular Modeling
The three-dimensional structures of inhibitors were created by Corina on-line (Molecular Networks) and subsequently prepared with Sybyl 8.0 (Tripos). Atom types were checked, hydrogen atoms were added, and, then, Gasteiger-Marsili charges were assigned. Ligands were bound to acetylcholinesterase from 2CKM and butyrylcholinesterase from 1P0I crystal complex. Protein was prepared before binding with GoldSuite 5.0.1 (CCDC). All histidine residues were protonated at Nε, the hydrogen atoms were added, ligand and water molecules were removed, and the binding site was defined as all amino acid residues within 10 Å from bis-(7)-tacrine for AChE and 20 Å from the glycerol molecule present in the active center of BChE. A standard set of genetic algorithms with population size of 100, number of operations 100,000, and clustering with a tolerance of 1Å was applied. As a result 10 ligand poses, sorted by GoldScore (AChE) and ChemScore (BChE) function value were obtained. The results were visualized by PyMOL 0.99rc6 (DeLano Scientific LLC).

Spectrophotometric Experiments
The absorption spectrum was obtained by scanning the sample between 200 and 350 nm with a Perkin Elmer spectrophotometer. The experiment was started with a solution of pure water at room temperature. The stability of compound 6a was assessed by monitoring the variability of the spectrum at regular intervals (15 min) over 4 h.

Radiolabelling
For radiolabeling with Tc-99m, 1 mg of the ligand 6a was dissolved in 150 µL of water. Subsequently, 100 mg of tricine and 1.5 mL of technetium eluate were added together with 25 µL of SnCl 2 in ethanol (1 mg/mL). After 30 min incubation at room temperature, quality control was performed by HPLC analysis. HPLC analysis was performed on an Agilent System 1100 Series with UV and radiometric detection, with LiChrocart column 250-3 Luichrospher 100 RP-18 (5 µm). Flow rate was 1 mL/min.

Animals
For biological experiments, the radiolabelled compound was dissolved in saline to a concentration of the ligand equal to 100 µg/mL. Intravenous dose was 20 µg per animal For biodistribution studies, male Wistar rats weighing 190-260 g were used. Prior to the experiment, the animals were fasted overnight (to empty the bowels), but had free access to water. All animal experiments were approved by the Ethics Committee of the Faculty of Pharmacy, Charles University, Hradec Kralove.

Biodistribution in Rats
The agent was administered to rats intravenously in a volume of 0.2 mL. During the course of the experiments, each animal was placed in an individual cage. At various time points after injection, the carotid artery was exposed under ether anesthesia and a blood sample was collected in glass tubes containing dry heparin. The rats were sacrificed and dissected. The organs of interest were weighed and counted for radioactivity in an automatic gamma counter (1480 Wizard 3).
The results were expressed as mean ± standard deviations of at least four animals.

Conclusions
As a consequence of a limited number of efficacious drugs in the treatment of AD, many scientific teams are aiming to discover novel compounds able to improve cholinergic neurotransmission. Currently approved AChE inhibitors, such as galantamine, rivastigmine, and donepezil, provide comparatively little chance for a prolonged improvement in cognitive functions. Thus, the search for novel compounds with anticholinesterase activity continues as numerous scientists are focused on the development of novel compounds such as cystamine-tacrine dimers [37], bisquaternary isoquinolinium derivatives [38], diversely substituted furo[2,3-b]quinolin-4-amine and pyrrolo[2,3-b]quinolin-4-amine derivatives [39], and tacrine-8-hydroxyquinoline hybrid [40] that might increase the level of ACh.
Several teams have also utilized derivatives of tacrine as potential radiopharmaceuticals. Tacrine and its modified structure derivatives were marked with radioactive isotope in order to determine the level of AChE or BChE as a function of tacrine analogue labeled isotope accumulated in the target site [41,42]. Also, other AChE inhibitors such as donepezil and huperzine were modified and marked with radioisotopes [43,44].
In our previous papers, we presented the synthesis and biological evaluation of derivatives of tetrahydroacridine derivatives with hydrazine nicotinate (HYNIC) moiety [45,46]. The fragment of tetrahydroacridine that possesses the possibility to inhibit both the moieties of cholinesterase and HYNIC has the potential to be utilized as a co-ligand for radiolabeling.
As described in this article, eight synthesized compounds differed from each other only in the length of the aliphatic chain between the tetrahydroacridine and the hydrazine nicotinate moiety. According to the results of the studies, compounds showing the highest activity with regard to AChE inhibition were those with six and eight carbon atoms in the aliphatic chain; which were approximately 7-and 9-fold more active than tacrine, respectively. This data suggests that altering the molecule by incorporating a longer aliphatic chain proved to be a good choice. All novel compounds, apart from the one with six carbon atoms in the aliphatic chain, described in these studies were characterized, in comparison to tacrine, by lower inhibitory activity towards BChE [45,46].
Within this work, compared to our previous studies, the six membered ring of tetrahydroacridine was exchanged with a five membered ring; this influenced the activity and interaction with active sites.
Of all the synthesized compounds, the most active was 6h (IC 50 = 3.65 nM). This compound was about 1.5-fold more active than tacrine and, as compared to this reference compound, more selective towards AChE. Compound 6g exhibited a similar value of IC 50 to that of tacrine. Similarly to our previous studies, obtained data suggests that the activity of the synthesized compounds increases simultaneously with the length of the aliphatic chain between the hydrazine nicotinate moiety and 2,3-dihydro-1H-cyclopenta[b]quinolone. Our data showed that all synthesized molecules were characterized by lower BChE inhibitory activity in comparison to tacrine.
On the other hand, selectivity of the obtained compounds is very promising because the moiety of 6-hydrazinenicotinic acid (HYNIC) is thought to be responsible for the binding of technetium-99m radiotracer ( 99m Tc) (as reported previously by Abrams et al.) [26].
According to molecular modeling studies, all ligands were extended along the active gorge and interacted with both the catalytic and peripheral site of AChE. This dual type of binding to AChE is responsible for an additional function related to the interaction with β-amyloid. The mode of binding with BChE was similar; the main difference was the location of the hydrazinenicotinic fragment in the reduced peripheral anionic site of BChE. These results are of vital importance as it has been established that AChE not only plays a crucial role in cholinergic dysfunction, but also is involved in the β-amyloid cascade via the AChE peripheral anionic site (PAS), such as mediating the adhesion, differentiation and deposition of β-amyloid in AD. It has been reported that AChE is a protein associated with the amyloid core of mature senile plaques, pre-amyloid diffuse deposits, and cerebral blood vessels in AD brain. Alvares et al., in in vitro studies showed that AChE is incorporated into β-amyloid aggregates by forming macromolecular complexes with the growing β-amyloid fibrils. The following scientific work of Alvares demonstrated that these complexes accelerate the maturation of β-amyloid plaques and are more toxic at the cellular level than the amyloid fibrils alone [47,48]. Molecular modeling studies presented within this work revealed that synthesized compounds are dual binding site inhibitors; thus, there is a possibility to influence non-cholinergic functions of AChE including AChE-induced aggregation of β-amyloid.
Biodistribution studies in rats revealed that compound 6a exhibited comparatively rapid blood radioactivity clearance. 99m Tc-radioactivity was mainly located in the liver, and to a lesser extent in the kidney, lung, and the gastrointestinal tract. Low radioactivity concentrations in the brain suggest that these agents do not cross the blood-brain barrier, and, therefore, cannot be regarded as potential agents for diagnosis of Alzheimer's disease. However, similarly to [ 11 C]choline, the synthesized compounds might be further evaluated as molecules suitable for the detection of cancers in certain organs (e.g., liver, kidney, lungs) or to monitor the response to various therapies. For example, there are scientific reports which prove that [ 11 C]choline, developed by Hara et al., as an oncologic PET (positron emission tomography) radiopharmaceutical, might be utilized with good results in the diagnosis of lung cancer [49], colon cancer [50], or prostate cancer [51]. Furthermore, [ 11 C]choline and multimodality fusion imaging with integrated PET and contrast-enhanced CT (PET/CT) could be used to monitor the response to anti-androgenic therapy [52].
Results of our synthesis and analysis suggest that the obtained hybrids of 2,3-dihydro-1H-cyclopenta[b]quinolone and 6-hydrazinonicotynic acid may be considered as novel potential anti-Alzheimer's drugs. Conversely, these compounds, following radiolabeling, could be used in the detection of the cholinergic deficit occurring in the peripheral nervous system or in the diagnosis of various types of cancers or to correct physiological functions.