Clickable photoaffinity ligands for the human serotonin transporter based on the selective serotonin reuptake inhibitor (S)-citalopram.

To date, the development of photoaffinity ligands targeting the human serotonin transporter (hSERT), a key protein involved in disease states such as depression and anxiety, have been radioisotope-based (i.e., 3H or 125I). This letter instead highlights three derivatives of the selective serotonin reuptake inhibitor (SSRI) (S)-citalopram that were rationally designed and synthesized to contain a photoreactive benzophenone or an aryl azide for protein target capture via photoaffinity labeling and a terminal alkyne or an aliphatic azide for click chemistry-based proteomics. Specifically, clickable benzophenone-based (S)-citalopram photoprobe 6 (hSERT Ki = 0.16 nM) displayed 11-fold higher binding affinity at hSERT when compared to (S)-citalopram (hSERT Ki = 1.77 nM), and was subsequently shown to successfully undergo tandem photoaffinity labeling-biorthogonal conjugation using purified hSERT. Given clickable photoprobes can be used for various applications depending on which reporter is attached by click chemistry subsequent to photoaffinity labeling, photoprobe 6 is expected to find value in structure-function studies and other research applications involving hSERT (e.g., imaging).

X-ray structure based evaluation of analogs of citalopram: Compounds with increased affinity and selectivity compared with R-citalopram for the allosteric site (S2) on hSERT.

The recent publication of X-ray structures of SERT includes structures with the potent antidepressant S-Citalopram (S-Cit). Earlier predictions of ligand binding at both a primary (S1) and an allosteric modulator site (S2), were confirmed. We provide herein examples of a series of Citalopram analogs, showing distinct structure-activity relationship (SAR) at both sites that is independent of the SAR at the other site. Analogs with a higher affinity and selectivity than benchmark R-Citalopram (R-Cit) for the S2 versus the S1 site were identified. We deploy structural and computational analyses to explain this SAR and demonstrate the potential utility of the newly emerging X-ray structures within the neurotransmitter:sodium Symporter family for drug design.

Pharmaceutical-Oriented Selective Synthesis of Mononitriles and Dinitriles Directly from Methyl(hetero)arenes: Access to Chiral Nitriles and Citalopram.

A pharmaceutical-oriented, transition-metal-free, cyanide-free one-step direct transformation of methylarenes to aryl nitriles is described. For the dimethylarenes, the selectivity can be well-controlled to form mononitriles or dinitriles. Enantioenriched nitriles can also be synthesized by this method. As a pharmaceutically practical method, the antidepressant drug citalopram was synthesized from cheap and commercially abundant m-xylene on a gram scale in high yield, avoiding transition-metal residues and toxic cyanides.

Highly Enantioselective Rhodium-Catalyzed Addition of Arylboroxines to Simple Aryl Ketones: Efficient Synthesis of Escitalopram.

Highly enantioselective additions of arylboroxines to simple aryl ketones have been achieved for the first time with a Rh/(R,R,R,R)-WingPhos catalyst, thus providing a range of chiral diaryl alkyl carbinols with excellent ee values and yields. (R,R,R,R)-WingPhos has been proven to be crucial for the high reactivity and enantioselectivity. The method has enabled a new, concise, and enantioselective synthesis of the antidepressant drug escitalopram.

Synthesis of novel triazoles and a tetrazole of escitalopram as cholinesterase inhibitors.

A novel serie of escitalopram triazoles (60-88) and a tetrazole (89) have been synthesized and subjected to a study to establish the inhibitory potential of these compounds toward acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). Some selectivity in inhibition has been observed. The 4-chlorophenyl- (75, IC50, 6.71 ± 0.25 µM) and 2-methylphenyl- (70, IC50, 9.52 ± 0.23 µM) escitalopram triazole derivatives depicted high AChE inhibition, while 2-fluorophenyl- (76, IC50 = 4.52 ± 0.17 µM) and 4-fluorophenyl- (78, IC50 = 5.31 ± 0.43 µM) have found to be excellent BChE inhibitors. It has also been observed that ortho, meta and para substituted electron donating groups increase the inhibition, while electron withdrawing groups reduce the inhibition. Docking analyses of inhibitors with AChE have depicted the binding energies for 70 and 75 as ΔG(bind) -6.42 and -6.93 kcal/mol, respectively, while ligands 76 and 78 have shown the binding affinity ΔG(bind) -9.04 and -8.51 kcal/mol, respectively, for BChE.

Design and synthesis of S-citalopram-imprinted polymeric sorbent: Characterization and application in enantioselective separation.

The current work describes the efficient creation and employment of a new S-citalopram selective polymeric sorbent, made from poly(divinylbenzene-maleic anhydride-styrene). The process began by using suspension polymerization technique in the synthesis of poly(styrene-maleic anhydride-divinylbenzene) microparticles. These were then modified with ethylenediamine, developing an amido-succinic acid-based polymer derivative. The S-citalopram, a cationic molecule, was loaded onto these developed anionic polymer particles. Subsequently, the particles were post-crosslinked using glyoxal, which reacts with the amino group residues of ethylenediamine. S-citalopram was extracted from this matrix using an acidic solution, which also left behind stereo-selective cavities in the S-citalopram imprinted polymer, allowing for the selective re-adsorption of S-citalopram. The attributes of the polymer were examined through methods such as 13C NMR, FTIR, thermogravemetric and elemental analyses. SEM was used to observe the shapes and structures of the particles. The imprinted polymers demonstrated a significant ability to adsorb S-citalopram, achieving a capacity of 878 mmol/g at a preferred pH level of 8. It proved efficient in separating enantiomers of (±)-citalopram via column methods, achieving an enantiomeric purity of 97 % for R-citalopram upon introduction and 92 % for S-citalopram upon release.

A rhodamine-labeled citalopram analogue as a high-affinity fluorescent probe for the serotonin transporter.

A novel fluorescent ligand was synthesized as a high-affinity, high specificity probe for visualizing the serotonin transporter (SERT). The rhodamine fluorophore was extended from an aniline substitution on the 5-position of the dihydroisobenzofuran ring of citalopram (2, 1-(3-(dimethylamino)propyl)-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile), using an ethylamino linker. The resulting rhodamine-labeled ligand 8 inhibited [(3)H]5-HT uptake in COS-7 cells (K(i)=225 nM) with similar potency to the tropane-based JHC 1-064 (1), but with higher specificity towards the SERT relative to the transporters for dopamine and norepinephrine. Visualization of the SERT with compound 8 was demonstrated by confocal microscopy in HEK293 cells stably expressing EGFP-SERT.

Chemo-enzymatic asymmetric synthesis of S-citalopram by lipase-catalyzed cyclic resolution and stereoinversion of quaternary stereogenic center.

A chemo-enzymatic synthesis method of S-citalopram was developed to overcome the disadvantage of relatively low selectivity of enzyme towards tertiary alcohols. The combination of kinetic resolution, cyclic resolution and stereoinversion synthesis was successfully applied in the asymmetric synthesis of the S-citalopram. Using the kinetic model to predict the cyclic resolution, R-diol with high ee value was obtained by controlling the conversion rate. Subsequently, the unwanted R-diol was inverted to S-citalopram by stereoinversion of chiral quaternary center with 98.0% yield and ee value of 91.0%. Based on dynamic simulation and experiments, the kinetic resolution was scaled up from 10 mL to 1 L and 14 L, gradually. There was no significant scale-up effect and the dynamic simulation result fitted the experimental data well, with an error of 12.5 and 14.0%, respectively. This chemo-enzymatic synthesis route is a promising model system for the production of pharmaceuticals with the chiral tertiary alcohols intermediate.

Natural and semisynthetic polymers blended orodispersible films of citalopram.

This study was aimed at developing orodispersible films of citalopram using combination of natural and semisynthetic polymers for patients with swallowing problem. Okra biopolymer and moringa gum were utilized in combination with hydroxypropyl methylcellulose (HPMC) and pullulan. The disintegration time was less than 30 seconds and the drug content uniformity was 97.89-102.05% for all film formulations. Films formulated with HPMC (K15 and K4M) combination (F1) and combination of okra and HPMC K15 (F2) had superior mechanical properties as compared with F3 (okra and pullulan) and F4 (moringa gum and HPMC). Thermal analysis revealed stable formulations over the studied temperature range and the crystalline citalopram was completely or partially transformed into amorphous form as revealed by the differential thermal analysis, X-ray diffraction and scanning electron microscopy images. In conclusion, okra biopolymer could be used in combination with HPMC for the development of orodispersible films.

From the selective serotonin transporter inhibitor citalopram to the selective norepinephrine transporter inhibitor talopram: synthesis and structure-activity relationship studies.

Citalopram and talopram are structurally closely related, but they have very distinct pharmacological profiles as selective inhibitors of the serotonin and norepinephrine transporters, respectively. A systematic structure-activity relationship study was performed, in which each of the four positions distinguishing the two compounds were varied. The inhibitory potencies of the resulting 16 compounds were tested at both serotonin and norepinephrine transporters. This showed that particularly two of the four positions are determinants for the biological activity.

Synthesis of some tropane derivatives of anticipated activity on the reuptake of norepinephrine and/or serotonin.

A variety of tropane derivatives 14a-g were prepared via the reaction of the alcohol analogs 12a and 12b with substituted fluorobenzenes 13a-f. The prepared compounds were tested for their activity and selectivity toward the norepinephrine transporter (NET) and serotonin transporter (SERT) using yohimbine-induced mortality and 5-hydroxytryptophan-induced neurotoxicity in mice, respectively. All the tested compounds were found to be NE and 5-HT reuptake inhibitors except 14d which exhibited selective 5-HT reuptake inhibition activity.

Synthesis of novel 5-(aroylhydrazinocarbonyl)escitalopram as cholinesterase inhibitors.

A novel series of 5-(aroylhydrazinocarbonyl)escitalopram (58-84) have been designed, synthesized and tested for their inhibitory potential against cholinesterases. 3-Chlorobenzoyl- (71) was found to be the most potent compound of this series having IC50 1.80 ± 0.11 µM for acetylcholinesterase (AChE) inhibition. For the butyrylcholinesterase (BChE) inhibition, 2-bromobenzoyl- (76) was the most active compound of the series with IC50 2.11 ± 0.31 µM. Structure-activity relationship illustrated that mild electron donating groups enhanced enzyme inhibition while electron withdrawing groups reduced the inhibition except o-NO2. However, size and position of the substituents affected enzyme inhibitions. . In docking study of AChE, the ligands 71, 72 and 76 showed the scores of 5874, 5756 and 5666 and ACE of -64.92,-203.25 and -140.29 kcal/mol, respectively. In case of BChE, ligands 71, 76 and 81 depicted high scores 6016, 6150 and 5994 with ACE values -170.91, -256.84 and -235.97 kcal/mol, respectively.

Novel Azido-Iodo Photoaffinity Ligands for the Human Serotonin Transporter Based on the Selective Serotonin Reuptake Inhibitor (S)-Citalopram.

Three photoaffinity ligands (PALs) for the human serotonin transporter (hSERT) were synthesized based on the selective serotonin reuptake inhibitor (SSRI), (S)-citalopram (1). The classic 4-azido-3-iodo-phenyl group was appended to either the C-1 or C-5 position of the parent molecule, with variable-length linkers, to generate ligands 15, 22, and 26. These ligands retained high to moderate affinity binding (K(i) = 24-227 nM) for hSERT, as assessed by [(3)H]5-HT transport inhibition. When tested against Ser438Thr hSERT, all three PALs showed dramatic rightward shifts in inhibitory potency, with Ki values ranging from 3.8 to 9.9 µM, consistent with the role of Ser438 as a key residue for high-affinity binding of many SSRIs, including (S)-citalopram. Photoactivation studies demonstrated irreversible adduction to hSERT by all ligands, but the reduced (S)-citalopram inhibition of labeling by [(125)I]15 compared to that by [(125)I]22 and [(125)I]26 suggests differences in binding mode(s). These radioligands will be useful for characterizing the drug-protein binding interactions for (S)-citalopram at hSERT.

A new alternative synthesis of 5-cyanophthalide, a versatile intermediate in the preparation of the antidepressant drug citalopram.

An alternative versatile synthesis of 5-cyanophthalide, a key synthetic intermediate in the preparation of the antidepressant drug Citalopram, is presented. The synthesis reported here allows the preparation of this important intermediate in three steps, avoiding the manipulation of environmentally detrimental cyanides.

Design and synthesis of 1-(3-(dimethylamino)propyl)-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile (citalopram) analogues as novel probes for the serotonin transporter S1 and S2 binding sites.

The serotonin transporter (SERT) is the primary target for antidepressant drugs. The existence of a high affinity primary orthosteric binding site (S1) and a low affinity secondary site (S2) has been described, and their relation to antidepressant pharmacology has been debated. Herein, structural modifications to the N, 4, 5, and 4' positions of (±)citalopram (1) are reported. All of the analogues were SERT-selective and demonstrated that steric bulk was tolerated at the SERT S1 site, including two dimeric ligands (15 and 51). In addition, eight analogues were identified with similar potencies to S-1 for decreasing the dissociation of [(3)H]S-1 from the S1 site via allosteric modulation at S2. Both dimeric compounds had similar affinities for the SERT S1 site (Ki = 19.7 and 30.2 nM, respectively), whereas only the N-substituted analogue, 51, was as effective as S-1 in allosterically modulating the binding of [(3)H]S-1 via S2.

Structure-activity relationships for a novel series of citalopram (1-(3-(dimethylamino)propyl)-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile) analogues at monoamine transporters.

(+/-)-Citalopram (1, 1-(3-(dimethylamino)propyl)-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile), and its eutomer, escitalopram (S-(+)-1) are selective serotonin reuptake inhibitors (SSRIs) that are used clinically to treat anxiety and depression. To further explore structure-activity relationships at the serotonin transporter (SERT), a series of (+/-)-4- and 5-substituted citalopram analogues were designed, synthesized, and evaluated for binding at the SERT, dopamine transporter (DAT) and norepinephrine transporter (NET) in native rodent tissue. Many of these analogues showed high SERT binding affinities (Ki=1-40 nM) and selectivities over both NET and DAT. Selected enantiomeric pairs of analogues were synthesized and both retained enantioselectivity as with S- and R-1, wherein S>R at the SERT. In addition, the enantiomeric pairs of 1 and 5 were tested for binding at the homologous bacterial leucine transporter (LeuT), wherein low affinities and the absence of enantioselectivity suggested distinctive binding sites for these compounds at SERT as compared to LeuT. These novel ligands will provide molecular tools to elucidate drug-protein interactions at the SERT and to relate those to behavioral actions in vivo.

Synthesis and biological evaluation of novel carbon-11-labelled analogues of citalopram as potential radioligands for the serotonin transporter.

Three serotonin reuptake inhibitors where the 5-cyano group in citalopram [1-(3-dimethylamino-propyl)-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile (1)] was replaced with a methyl, acetyl and piperidinyl carbonyl group, respectively, were synthesized. In a Stille reaction applying [(11)C]methyl iodide the labelled compound [5-methyl-(11)C][3-[1-(4-fluorophenyl)-5-methyl-1,3-dihydroisobenzofuran-1-yl]-propyl]-dimethylamine ([(11)C]-2) was synthesized in 60-90% radiochemical yield. [5-carbonyl-(11)C][1-[1-(3-dimethylaminopropyl)-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-yl]-1-piperidin-1-yl-methanone] ([(11)C]-3) was synthesized in 62% radiochemical yield in a palladium mediated cross-coupling reaction utilizing [(11)C]carbon monoxide. The specific activity of [(11)C]-2 was highly dependent on whether the corresponding trimethyltin or tributyltin precursor was applied. In ex vivo rodent studies compound [(11)C]-2 exhibited a good blood-brain barrier (BBB) penetration whereas [(11)C]-3 did not. The brain distribution of [(11)C]-2 was investigated in a non-human primate using PET. There was a rapid uptake of radioactivity into the brain. Accumulation of the radiotracer was in agreement with the known distribution of serotonin transporters. The maximal thalamus to cerebellum ratio of 1.3 was reached after 85 min and the specific binding was partly blocked after pre-treatment with citalopram. Thus, [(11)C]-2 does not exhibit appropriate properties as radioligand for visualization of the serotonin transporter in vivo.