Design, synthesis, lipophilic properties, and binding affinities of potential ligands in positron emission tomography (PET) for visualization of brain dopamine D4 receptors

Article abstract of DOI:10.1002/cbdv.201300194

We report the synthesis of compounds structurally related to the high-affinity dopamine D₄ receptor ligand N-{2-[4-(3-cyanopyridin-2- yl)piperazin-1-yl]ethyl}-3-methoxybenzamide (1e). All compounds were specifically designed as potential PET radioligands for brain D₄ receptor visualization, having lipophilicity within a range for brain uptake and weak non-specific binding (0.75a substituent for easy access to labeling with the positron emitter isotope 11C or 18F. The best compound of the series, N-{2-[4-(4-chlorophenyl)piperazin-1-yl]ethyl}-6- fluoropyridine-3-carboxamide (7a), displayed excellent selectivity over D ₂ and D₃ receptors (>100-fold), but its D₄ receptor affinity was suboptimal for imaging of brain D₄ receptors (K_i=30 nm).

Full text of DOI:10.1002/cbdv.201300194

CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
299
Design, Synthesis, Lipophilic Properties, and Binding Affinities of Potential
Ligands in Positron Emission Tomography (PET) for Visualization of Brain
Dopamine D4 Receptors
by Enza Lacivita, Paola De Giorgio, Nicola A. Colabufo, Francesco Berardi, Roberto Perrone, Mauro
Niso, and Marcello Leopoldo*
`
Dipartimento di Farmacia–Scienze del Farmaco, Universita degli Studi di Bari Aldo Moro,
via Orabona, 4, IT-70125, Bari
(phone þ390805442798; fax þ390805442231; e-mail: marcello.leopoldo@uniba.it)
We report the synthesis of compounds structurally related to the high-affinity dopamine D4 receptor
ligand N-{2-[4-(3-cyanopyridin-2-yl)piperazin-1-yl]ethyl}-3-methoxybenzamide (1e). All compounds
were specifically designed as potential PET radioligands for brain D4 receptor visualization, having
lipophilicity within a range for brain uptake and weak non-specific binding (0.75<cLogP<3.15) and
bearing a substituent for easy access to labeling with the positron emitter isotope 11C or 18F. The best
compound of the series, N-{2-[4-(4-chlorophenyl)piperazin-1-yl]ethyl}-6-fluoropyridine-3-carboxamide
(7a), displayed excellent selectivity over D2 and D3 receptors (>100-fold), but its D4 receptor affinity
was suboptimal for imaging of brain D4 receptors (Ki ¼30 nm).
Introduction. – The dopamine D2-like receptor family is composed of three receptor
subtypes, D2, D3, and D4, and it is characterized by the ability to interact with Gi/o
proteins, resulting in inhibition of adenylyl cyclase. Among the D2-like receptors the D4
receptor exhibits a specific pharmacological profile [1][2], and an extensive poly-
morphism of the human dopamine receptor D4 gene is known [3][4]. Although the D4
receptor was first cloned in 1991, there is still ambiguity about its pathophysiological
functions, as well as about its exact localization and distribution density in the brain
[5][6]. In particular, to this receptor an extremely low density in the brain is attributed
[7][8]. Immunohistochemistry and hybridization studies revealed interspecies differ-
ences and contradictory findings, but suggested higher expression of D4 receptors in the
prefrontal cortex and the limbic system, and also in the temporal cortex, parts of
tectum, and cerebellum [5][6][9–11]. Early interest in the D4 receptor originated from
the finding that clozapine, an atypical antipsychotic drug with high antipsychotic
efficacy and reduced extrapyramidal and neuroendocrine side-effects, showed a tenfold
higher affinity for D4 than for D2 receptor [1][2]. On such basis, it was hypothesized
that the higher efficacy of clozapine to patients refractory to typical neuroleptics as well
as to therapy of negative symptomatology was a consequence of D4 binding. However,
further studies did not confirm a direct link between schizophrenia and the D4 receptor
[12][13], indicating that the role in psychiatric diseases is uncertain. Great interest in
the human dopamine D4 receptor was generated by studies that indicated an
association between D4 receptor gene polymorphism and various complex behaviors
including Attention Deficit Hyperactivity Disorder (ADHD), Touretteꢀs syndrome,
ꢁ 2014 Verlag Helvetica Chimica Acta AG, Zꢂrich

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CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
and the personality trait of novelty seeking. However, other studies did not confirm
these associations. Only a meta-analysis provided strong evidence that the seven-repeat
allele confers an increased risk for the development of ADHD, whereas the four-allele
is protective [14]. Recent studies have shown that a variety of D4 receptor agonists have
a proerectile effect in animal experiments [15][16], indicating a role of D4 receptors on
the physiology of sexual functions.
Table 1. Lipophilicity Values and Binding Affinities at Dopamine D4 Receptors of Reference Benzamides
R
Ar
ClogPa)
log k
Ki D4
[nm]b)
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
3-MeO
3-MeO
3-MeO
3-MeO
3-MeO
3-MeO
4-F
4-F
4-F
4-F
4-F
4-ClꢀC6H4
3.72
3.33
2.71
2.66
1.50
2.67
3.86
3.33
2.86
2.81
1.64
2.82
0.88
0.70
0.56
0.29
0.27
0.56
0.89
0.67
0.57
0.22
0.22
0.73
4.97
9.21
1.93
63.95
1.52
4-MeꢀC6H4
1,2-Benzoxazol-3-yl
4-CNꢀC6H4
3-Cyanopyridin-2-yl
5-Chloropyridin-2-yl
4-ClꢀC6H4
11.29
1.76
4-MeꢀC6H4
2.64
0.34
32.71
0.93
2.92
1,2-Benzoxazol-3-yl
4-CNꢀC6H4
3-Cyanopyridin-2-yl
5-Chloropyridin-2-yl
4-F
a) Calculated with ClogP Biobyte software [24]. b) Data taken from [29].
Noninvasive imaging using selective radioligands for the D4 receptor by positron
emission tomography (PET) provides the opportunity to investigate the D4-receptor
expression in the living brain with high sensitivity. Various attempts have been made to
identify a D4-selective PET radioligand [17–23], but a major problem was the
inadequate ratio of specific to nonspecific binding which is most obtrusive because of
the very low D4 receptor density. In particular, Langer et al. reported an attempt to
visualize the dopamine D4 receptor in primate brain with [11C]1a (Fig.) [19]. The
radioligand exhibited a very high background due to nonspecific binding. Similar
results were obtained by Zhang et al. who synthesized and tested [11C]1a and [11C]YM-
50001 (Fig.), with both radioligands being unsuitable for D4-receptor imaging with
PET [20]. In each case, it was suggested that the high nonspecific binding of [11C]1a
could be due to its relatively high lipophilicity (ClogP¼3.72) [24]. The nonspecific
binding of PET radioligand candidates, such as binding to brain lipids, is hardly
reversible and undisplaceable by non-radioactive ligands. Moreover, it is difficult to
predict, and it has been correlated with lipophilicity of the substance [25–28].

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301
Figure. Potential PET radiotracers for dopamine D4 receptor imaging
Recently we have reported the rational design of a set of benzamide derivatives
structurally related to the high-affinity D4 ligand 1a as potential PET radiotracers
(compounds 1a–1f and 2a–2f, Table 1) [29], which showed moderate lipophilicity in
the ClogP range of 2.5–3.5 that is considered optimal for adequate brain penetration
without an excessive level of nonspecific binding [30–32]. Among the target
compounds, derivative 1e (Table 1) was selected for PET studies in vivo, because it
showed a good compromise between high D4 receptor affinity and lipophilicity.
However, [11C]1e (Fig.) was not able to visualize D4 receptor in monkey brain, but it
showed fast kinetics, and it did not show nonspecific binding, suggesting that the
lipophilicity of 1e can be targeted to minimize nonspecific binding. It should be noted
that, for 1e, we observed a significant difference between experimental log P value
measured by the pH metric technique (log P¼2.55) [29] and the calculated one
(ClogP¼1.5). A recent study by Kꢀgler et al. [33] has reported a significant correlation
between the experimentally determined log P values of four potential D4 receptor PET
radiotracers and the extent of nonspecific binding as determined by competition
experiments in vitro. The most promising radiotracer candidate therein reported was
[18F]3 (Fig.) that showed D4 receptor affinity in the low nanomolar range, good brain
penetrance, and low nonspecific binding. The reported experimental log P value of
[18F]3 determined with the shake flask method was 1.81, whereas the calculated log P
value was 2.26 (ALOGPS 2.1 software). Collectively these data suggest that a new D4
receptor PETradiotracer should possess lipophilicity comparable to that of compounds
[11C]1e and [18F]3 and D4-receptor affinity lower than that of 1e (Ki ¼1.52 nm).
Thus, in keeping with the search of an effective D4-receptor PET radiotracer, we
modified further the structure of the compounds listed in Table 1 with the aim to obtain
new compounds that would have: a) high affinity for D4 receptor (Ki ꢁ1.5 nm) b) low
affinity for dopamine D2 and D3 receptors (Ki >500 nm) c) a good brain penetration as
predicted by log P values in an optimal range as compound 1e.

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Previous structureꢀactivity relationship studies on N-substituted-1-arylpiperazine
derivatives suggested that the arylpiperazine moiety was responsible for the high D4-
receptor affinity, while the other aromatic end tethered by a chain with variable length
to the protonatable N-atom of the N-substituted-1-arylpiperazine pharmacophore
promotes D4 selectivity over D2 receptors [34][35]. Therefore, in order to achieve the
aims a and b listed above, we left the arylpiperazine moieties of compounds 1a–1f and
2a–2f unchanged (also considering the restriction imposed by lipophilicity require-
ments), and, in order to achieve the aim c we modified the arenecarboxamide moiety
by replacing a ring CH with an aza group, preferring the most accessible isomers. Thus,
the 3-methoxybenzamide moieties of compounds 1a–1f were replaced with 5-
methoxypyridine-3-carboxamide moieties to give compounds 6a–6f, and the 4-
fluorobenzamide moieties of compounds 2a–2f with 6-fluoropyridine-3-carboxamide
moieties to give compounds 7a–7f, respectively. The selection of the carboxylic acids
was based on the stringent requirements for radiolabeling: [11C]6a–[11C]6f can be
prepared from the corresponding phenolic derivatives 5a–5f (see below), respectively,
whereas [18F]7a–[18F]7f can be synthesized starting from the corresponding 6-
bromopyridyl or 6-nitropyridyl derivatives via aromatic nucleophilic substitution with
[18F]Fꢀ. Other possible pyridinecarboxylic acids, yet commercially available, cannot be
radiolabelled.
Results and Discussion. – Chemistry. The target compounds were prepared as
outlined in the Scheme. The 5-methoxypyridine-3-carboxamide derivatives 6a–6f were
prepared by condensing 5-hydroxy-3-pyridinecarboxylic acid [36] with amines 4a–4f,
prepared according to literature methods as detailed in the Exper. Part, in the presence
of 1,1’-carbonyldiimidazole (CDI) to give the hydroxy derivatives 5a–5f, respectively.
The latters were methylated with CH2N2 to afford the target compounds 6a–6f,
respectively. The 6-fluoropyridine-3-carboxamide derivatives 7a–7f were prepared by
condensing the commercially available 6-fluoro-3-pyridinecarboxylic acid with amines
4a–4f, respectively, using CDI as condensing agent.
Pharmacology. The target compounds 6a–6f and 7a–7f were assessed for their
binding affinities at human cloned D4, D2, and D3 receptors using [3H]spiroperidol as
radioligand. The Ki values (Table 2) were calculated from the IC50 values according to
the equation of Cheng and Prusoff [37] by means of the Prism, v. 3.0, GraphPad
software.
StructureꢀActivity Relationships. Previous studies on N-substituted 1-arylpipera-
zine derivatives have revealed that polar features of the N-substituent were well
tolerated with respect to D4-receptor affinity [38][39]. Instead, comparing affinities of
the target compounds 6a–6f and 7a–7f, characterized by a pyridinecarboxamide
moiety (Table 2), with those of the parent non-heteroaromatic compounds 1a–1f and
2a–2f (Table 1), a clear decrease in D4-receptor affinity was evident. These data
indicate that the aim a illustrated in the Introduction was not achieved. While the 4-
fluorobenzamides 2a–2f possessed higher D4 affinities than their 3-methoxy counter-
parts 1a–1f, the same trend was not displayed by the corresponding pyridinecarbox-
amides 6a–6f and 7a–7f. Having observed that the presence of the pyridinecarbox-
amide fragment was detrimental for affinity at D4 receptor, we examined if there exists
a relationship between lipophilicity values of the ligands and D4-receptor affinities. A

CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
303
Scheme
a) 5-Hydroxypyridine-3-carboxylic acid, 1,1’-carbonyldiimidazole (CDI), THF, 10–12 h, r.t., 40–50%.
b) CH2N2, t-ButOH, 30 min, r.t. 40–50%. c) 6-Fluoropyridine-3-carboxylic acid, CDI, THF, 10–12 h,
r.t., 40–50%.
linear relationship was not found when plotting pKi vs. log k values (data not shown),
suggesting that the interaction between the ligands and the receptor binding site does
not rely solely on hydrophobic interactions. The amount of the decrease in affinities
varied significantly from case to case, ranging from elevenfold (1a vs. 6a) to over 2000-
fold (1e vs. 6e, 2c vs. 7c, and 2d vs. 7d). This suggests two observations. First, the
presence of the pyridinecarboxamide moiety almost completely disrupted the
interaction between compounds 6d, 6e, 7c, 7d and the D4-receptor binding site: it is
likely that the pyridinecarboxamide moiety interacts with the binding site in a way that
hinders the decisive interaction between the protonated basic N-atom of 6d, 6e, 7c, and
7d with the highly conserved aspartic acid in the third transmembrane domain (TM3)
of D4-receptor binding site [35]. Second, the presence of the pyridinecarboxamide
moiety exerted variable effects on the affinity of the ligands, because the local
orientation of the arenecarboxamide moiety in the D4-receptor binding site orients the
arylpiperazine fragment toward transmembrane microdomains in different ways,
resulting in acceptable or in poor binding affinity [35].
As far as the affinities for D2 and D3 receptors are concerned, the present data
confirmed previous findings about the N-[2-(4-arylpiperazin-1-yl)ethyl]arenecarbox-

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Table 2. Lipophilicity Values and Binding Affinities at Dopamine D4, D2, and D3 Receptors of Target
Arenecarboxamides
R
Ar
ClogPa) log k Ki [nm]ꢂS.E.M.b)
D4
D2
D3
6a 5-MeO 4-ClꢀC6H4
6b 5-MeO 4-MeꢀC6H4
3.15
2.77
0.64
0.49
0.3
0.10
0.06 >3000
0.47
0.61
0.47
0.28
56.0ꢂ5
528ꢂ13
61.0ꢂ8
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
>1000
44.0ꢂ3
89.0ꢂ7
>1000
27.0ꢂ2
>1000
6c
5-MeO 1,2-Benzoxazol-3-yl 2.15
6d 5-MeO 4-CNꢀC6H4
2.10
2236ꢂ210
6e 5-MeO 3-Cyanopyridin-2-yl 0.93
6f
5-MeO 5-Chloroyridin-2-yl 2.11
1384ꢂ32
>1000
7a 6-F
7b 6-F
4-ClꢀC6H4
2.97
2.44
30.0ꢂ0.6 >1000
4-MeꢀC6H4
301ꢂ19
>1000
>1000
>1000
7c
6-F
1,2-Benzoxazol-3-yl 1.97
1537ꢂ135
7d 6-F
7e 6-F
4-CNꢀC6H4
1.92
ꢀ0.30 >3000
3-Cyanopyridin-2-yl 0.75
ꢀ0.02
80.6ꢂ13
372ꢂ65
4.2ꢂ0.2
357ꢂ20
7f
6-F
5-Chloroyridin-2-yl 1.93
0.41
–
>1000
Haloperidol
Quinpirole
–
–
–
2.9ꢂ0.7
–
–
12.1ꢂ 1.9
–
a) Calculated with ClogP Biobyte software [24]. b) Binding data are the means of three independent
experiments using standard displacement assays with [3H]spiroperidol as the competitive ligand and
human cloned receptor subtypes.
amide framework [40]: all the compounds were devoid of dopamine D2 receptor
affinity (except 7e), while a few more compounds showed remarkable affinity for D3
receptor (i.e., 7b, 7c, and 7e). These data indicate that the aim b mentioned in the
Introduction was substantially achieved.
Collectively, these data confirmed that the N-[2-(4-arylpiperazin-1-yl)ethyl]arene-
carboxamide structure can deliver selective D4-receptor ligands if properly decorated,
as in the case of compounds 6a, 6b, and 7a.
Lipophilicity Evaluation. Lipophilicity can be evaluated in various theoretical and
experimental ways. Theoretical partition coefficients are routinely calculated with
various softwares that provide values generally well correlated with the experimental
determinations. However, differences between the calculated log P values and the
experimental ones have been reported as in the case described above compounds 1e
and [18F]3. Therefore, we compared experimentally the lipophilicity values of the newly
synthesized compounds 6a–6f and 7a–7f with those of the benzamides 1a–1f and 2a–
2f studied previously. For this purpose, we adopted a reversed-phase HPLC method
that we had previously used for arylpiperazine derivatives [41]. Plotting ClogP and log
k values of compounds 1a–1f, 2a–2f, 6a–6f, and 7a–7f (Tables 1 and 2) led to a
modest correlation (r¼0.67). 4-Cyano derivatives 1d, 2d, 6d, and 7d appeared more

CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
305
hydrophilic than predicted: it can be hypothesized that solvation of the 4-cyano
substituent results in higher solubility in the mobile phase. Instead, by excluding the 4-
CN derivatives 1d, 2d, 6d, and 7d, ClogP and log k values were well correlated (r¼
0.941). These observations confirm that calculated log P values reliably predict
physicochemical properties of the compounds, but when lipophilicity must fall within a
narrow range, and ClogP values are borderline, an experimental determination is
prudent. The experimental determinations confirmed that compounds 6a–6f and 7a–
7f were less lipophilic than the corresponding derivatives 1a–1f and 2a–2f, indicating
that the aim c mentioned in the Introduction was accomplished. Among these,
compound 7c showed the same lipophilicity as 1e.
Conclusions. – With the aim to obtain potential PET radiotracers for CNS
dopamine D4 receptors, we have structurally manipulated a set of previously reported
high-affinity dopamine D4 ligands characterized by the N-[2-(4-arylpiperazin-1-
yl)ethyl]arenecarboxamide framework, in order to obtain molecules endowed with
lipophilicity values that should account for low nonspecific binding. Although the
modification was targeted at the arenecarboxamide moiety that previous structureꢀ
activity realtionship studies had indicated as less sensitive with respect to the
interaction with the target receptor, we have observed a loss in D4-receptor affinity
that renders these compounds unsuitable candidates as PETradiotracers. On the other
hand, compounds 6a, 6c, and 7a display binding affinities and physicochemical
properties still attractive for future investigations of the role of dopamine D4 receptors.
Experimental Part
Chemistry. The purities of the tested compounds 1a–1f and 2a–2f have been assessed by RP-HPLC
and combustion analyses. All compounds showed ꢃ 95% purity. The following compounds were
synthesized according to published procedures: 2-[4-(4-chlorophenyl)piperazin-1-yl]ethanamine (4a)
[42], 2-[4-(4-methylphenyl)piperazin-1-yl]ethanamine (4b) [42], 2-[4-(1,2-benzoxazol-3-yl)piperazin-1-
yl]ethanamine (4c)[29], 4-[4-(2-aminoethyl)piperazin-1-yl]benzonitrile (4d) [29], 2-[4-(2-aminoethyl)pi-
perazin-1-yl]pyridine-3-carbonitrile (4e) [29], 2-[4-(5-chloropyridin-2-yl)piperazin-1-yl]ethanamine (4f)
[29], 5-hydroxynicotinic acid [36]. Column chromatography (CC): Merck silica gel 60A (SiO2; 63–
200 mm) as the stationary phase. M.p.: in open capillaries on a Gallenkamp electrothermal apparatus.
1H-NMR Spectra: at 300 MHz on a Varian Mercury-VX spectrometer; all spectra were recorded on free
bases; all chemical shift values in ppm (d). MS: HP6890–5973 MSD gas chromatograph/mass
spectrometer; only significant m/z peaks, with their percentage of relative intensity in parentheses, are
reported. ESIþ/MS/MS: Agilent 1100 Series LC-MSD trap System VL workstation. All spectra were in
accordance with the assigned structures. When necessary, a standard procedure was used to transform
final compounds into their HCl salts. Elemental analyses (C, H, N): Eurovector Euro EA 3000 analyzer;
results within ꢂ 0.4% of the theoretical values.
General Procedure for Preparation of Compounds 5a–5f. A mixture of 5-hydroxypyridine-3-
carboxylic acid (0.07 g, 0.5 mmol) and 1,1’-carbonyldiimidazole (CDI; 0.10 g, 0.6 mmol) in 10 ml of anh.
THF was stirred for 8 h. A soln. of amine 4a–4f (0.5 mmol) in anh. THF (10 ml) was added, and then the
mixture was stirred until the acid disappeared (TLC). The mixture was partitioned between AcOEt
(20 ml) and H2O (20 ml). The separated org. layer was washed with a sat. aq. soln. of Na2CO3 (20 ml),
dried (Na2SO4), and concentrated in vacuo. The crude residue was chromatographed as detailed below to
afford the pure arenecarboxamide in 40–50% yield.
N-{2-[4-(4-Chlorophenyl)piperazin-1-yl]ethyl}-5-hydroxypyridine-3-carboxamide (5a). Eluted with
CHCl3/MeOH 19 :1. Yield: 60%. 1H-NMR ((D6)DMSO): 2.47–2.49 (m, 2 H); 2.52–2.56 (m, 2 H); 3.10

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CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
(app. t, 2 H); 3.33 (br. s, 4 H); 3.37–3.42 (m, 2 H); 6.89–6.94 (m, 2 H); 6.99 (br. s, 1 H, D2O exchanged);
7.17–7.22 (m, 2 H); 7.62 (br. s, 1 H); 8.21 (d, J¼2.8, 1 H); 8.43 (d, J¼1.9, 1 H); 8.53 (br. s, 1 H, D2O
exchanged). ESI-MS (pos.): 361 ([M þ H]þ ). ESI-MS/MS (pos.): 197 (19), 165 (100).
5-Hydroxy-N-{2-[4-(4-methylphenyl)piperazin-1-yl]ethyl}pyridine-3-carboxamide (5b). Eluted with
CHCl3/MeOH 19 :1. Yield: 36%. 1H-NMR (CDCl3): 2.26 (s, 3 H); 2.68 (app. t, 6 H); 3.14 (app. t, 4 H);
3.58 (q, J¼5.8, 2 H); 6.83 (d, J¼8.5, 2 H); 7.07 (d, J¼8.5, 2 H); 7.18 (br. t, 1 H, D2O-exchanged); 7.60–
7.62 (m, 1 H); 8.37 (d, J¼2.8, 1 H); 8.43 (d, J¼1.9, 1 H); 8.44 (br. s, 1 H, D2O-exchanged). ESI-MS
(pos.): 341 ([M þ H]þ ). ESI-MS/MS (pos.): 177 (69), 165 (100).
N-{2-[4-(1,2-Benzoxazol-3-yl)piperazin-1-yl]ethyl}-5-hydroxypyridine-3-carboxamide (5c). Eluted
with CHCl3/MeOH 19 :1. Yield: 52%. 1H-NMR (CDCl3): 2.67–2.72 (m, 6 H); 3.54–3.62 (m, 6 H); 7.20–
7.24 (m, 3 H, D2O-exchanged); 7.46–7.49 (m, 2 H); 7.60–7.64 (m, 1 H); 8.37 (d, J¼2.8, 1 H); 8.46 (d, J¼
1.7, 1 H); 8.56 (br. s, 1 H, D2O-exchanged). ESI-MS (pos.): 368 ([M þ H]þ ). ESI-MS/MS (pos.): 204
(35), 165 (100).
N-{2-[4-(4-Cyanophenyl)piperazin-1-yl]ethyl}-5-hydroxypyridine-3-carboxamide (5d). Eluted with
1
CHCl3/MeOH 9 :1. Yield: 29%. H-NMR (CDCl3): 2.62–2.68 (m, 6 H); 3.32 (app. t, 4 H); 3.58 (q, J¼
5.5, 2 H); 6.83–6.86 (m, 2 H); 7.02 (br. s, 1 H, D2O-exchanged); 7.46–7.50 (m, 2 H); 7.61–7.62 (m, 1 H);
8.40 (d, J¼2.8, 1 H,); 8.43 (d, J¼1.9, 1 H); 8.64 (br. s, 1 H, D2O-exchanged). ESI-MS (pos.): 352 ([M þ
H]þ ). ESI-MS/MS (pos.): 165 (100).
N-{2-[4-(3-Cyanopyridin-2-yl)piperazin-1-yl]ethyl}-5-hydroxypyridine-3-carboxamide (5e). Eluted
with CHCl3/MeOH 19 :1. Yield: 60% 1H-NMR (CDCl3): 2.63–2.69 (m, 6 H); 3.58 (q, J¼5.5, 2 H); 3.71
(app. t, 4 H); 6.76 (dd, J¼7.7, 1 H); 7.45 (br. s, 2 H, D2O-exchanged); 7.61–7.62 (m, 1 H); 8.33 (dd, J¼4.7,
1.9, 1 H); 8.38 (d, J¼2.8, 1 H); 8.44 (d, J¼1.7, 1 H); 8.64 (br. s, 1 H, D2O-exchanged). ESI-MS (pos.): 353
([M þ H]þ ). ESI-MS/MS (pos.): 189 (17), 165 (100).
N-{2-[4-(5-Chloropyridin-2-yl)piperazin-1-yl]ethyl}-5-hydroxypyridine-3-carboxamide (5f). Eluted
1
with CHCl3/MeOH 19 :1. Yield: 38%. H-NMR (CDCl3): 2.59 (app. t, 4 H); 2.65 (t, J¼6.1, 2 H); 3.50
(app. t, 4 H); 3.57 (q, J¼5.5, 2 H); 6.57 (d, J¼9.1, 1 H); 7.15 (br. t, 1 H, D2O-exchanged); 7.41 (dd, J¼6.3,
2.8, 1 H); 7.61 (app. t, 1 H); 8.10 (d, J¼2.5, 1 H); 8.37 (d, J¼2.5, 1 H); 8.43 (d, J¼1.7, 1 H); 8.47 (br. s,
1 H, D2O-exchanged). ESI-MS (pos.): 363 ([M þ H]þ ). ESI-MS/MS (pos.): 198 (41), 165 (100).
General Procedure for Preparation of Compounds 6a–6f. A soln. of derivatives 5a–5f (0.3 mmol) in
t-BuOH (5 ml) was treated with an excess of CH2N2. The mixture was stirred at r.t. for 30 min, until the
precursor disappeared (TLC). The solvents were removed in vacuo, and the residue was partitioned
between AcOEt (20 ml) and H2O (20 ml). The separated org. layer was washed with brine, dried
(Na2SO4), and concentrated in vacuo. The crude residue was chromatographed as detailed below to give
the pure arenecarboxamide in 40–50% yield.
N-{2-[4-(4-Chlorophenyl)piperazin-1-yl]ethyl}-5-methoxypyridine-3-carboxamide (6a). Eluted with
CHCl3/MeOH 98 :2. Yield: 70%. M.p. 145–1478. 1H-NMR (CDCl3): 2.65–2.70 (m, 6 H); 3.18 (app. t,
4 H); 3.59 (q, J¼5.7, 2 H); 3.90 (s, 3 H); 6.81–6.87 (m, 2 H); 6.89 (br. s, 1 H, D2O-exchanged); 7.18–7.23
(m, 2 H); 7.67–7.68 (m, 1 H); 8.41 (d, J¼3.0, 1 H); 8.50 (d, J¼1.9, 1 H). ESI-MS (pos.): 375 ([M þ H]þ ).
ESI-MS/MS (pos.) 179 (100). Anal. calc. for C19H23ClN4O2 (374.87): C 60.88, H 6.18, N 14.95; found: C
60.94, H 6.26, N 14.65.
5-Methoxy-N-{2-[4-(4-methylphenyl)piperazin-1-yl]ethyl}pyridine-3-carboxamide (6b). Eluted with
CHCl3/MeOH, 19 :1. Yield: 45%. M.p. 132–1358. 1H-NMR (CDCl3): 2.27 (s, 3 H); 2.75–2.79 (m, 6 H);
3.23 (app. t, 4 H); 3.65 (q, J¼5.2, 2 H); 3.90 (s, 3 H); 6.84 (d, J¼8.5, 2 H); 7.07 (d, J¼8.3, 2 H); 7.29 (br. s,
1 H, D2O exchanged); 7.72–7.74 (m, 1 H); 8.40 (d, J¼3.0, 1 H); 8.56 (d, J¼1.6, 1 H). GC-MS: 355 (3,
[M þ 1]þ ), 354 (14, Mþ ), 189 (100), 146 (20), 108 (14). Anal. calc. for C20H26N4O2 (354.45): C 67.77, H
7.39, N 15.81; found: C 67.50, H 7.34, N 15.44.
N-{2-[4-(1,2-Benzoxazol-3-yl)piperazin-1-yl]ethyl}-5-methoxypyridine-3-carboxamide (6c). Eluted
1
with CHCl3/MeOH, 19 :1. 52% Yield. M.p. 88–908. H-NMR (CDCl3): 2.76–2.83 (m, 6 H); 3.63–3.69
(m, 6 H); 3.90 (s, 3 H); 7.20–7.24 (m, 2 H, D2O exchanged); 7.46–7.49 (m, 2 H); 7.66 (d, 1 H); 7.70–7.72
(m, 1 H); 8.40 (d, J¼2.7, 1 H); 8.57 (br. s, 1 H). ESI-MS/MS (pos.) 382 ([M þ H]þ ). ESI-MS/MS (pos.):
204 (15), 179 (100). Anal. calc. for C20H23N5O3 (381.43): C 62.98, H 6.08, N 18.36; found: C 63.10, H 6.22,
N 18.11.

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N-{2-[4-(4-Cyanophenyl)piperazin-1-yl]ethyl}-5-methoxypyridine-3-carboxamide (6d). Eluted with
CHCl3/MeOH 19 :1. Yield: 48%. M.p. 147–1498.1H-NMR (CDCl3): 2.66–2.72 (m, 6 H); 3.36 (app. t,
4 H); 3.62 (q, J¼5.5, 2 H); 3.90 (s, 3 H); 6.86 (d, J¼9.1, 2 H); 6.94 (br. s, 1 H, D2O-exchanged); 7.48–7.51
(m, 2 H); 7.68–7.70 (m, 1 H); 8.40 (d, J¼3.0, 1 H); 8.51 (d, J¼1.6, 1 H). ESI-MS/MS (pos.): 366 ([M þ
H]þ ). ESI-MS/MS (pos.) 179 (100). Anal. calc. for C20H23N5O2 (365.43): C 65.73, H 6.34, N 19.16; found:
C 66.05, H 6.21, N 19.46.
N-{2-[4-(3-Cyanopyridin-2-yl)piperazin-1-yl]ethyl}-5-methoxypyridine-3-carboxamide (6e). Eluted
1
with CHCl3/MeOH 19 :1. Yield: 38%. M.p. 112–1158 (HCl salt). H-NMR (CDCl3): 2.86 (br. s, 6 H);
3.70 (q, J¼5.2, 2 H); 3.85 (app. t, 4 H); 3.92 (s, 3 H); 6.83 (dd, J¼7.7, 1 H); 7.45 (br. s, 1 H, D2O-
exchanged); 7.78–7.81 (m, 2 H); 8.36 (dd, J¼4.7, 1.9, 1 H); 8.41 (d, J¼2.7, 1 H); 8.60 (d, J¼1.1, 1 H).
ESI-MS/MS (pos.): 367 ([M þ H]þ ). ESI-MS/MS (pos.): 179 (100). Anal. calc. for C19H22N6O2 ·4 HCl·
0.5 H2O (521.27): C 43.78, H 5.22, N 16.12; found: C 43.61, H 5.55, N 15.80.
N-{2-[4-(5-Chloropyridin-2-yl)piperazin-1-yl]ethyl}-5-methoxypyridine-3-carboxamide (6f). Eluted
with CHCl3/MeOH 19 :1. Yield: 34%. M.p. 143–1458. 1H-NMR (CDCl3): 2.70–2.77 (m, 6 H); 3.59–3.68
(m, 6 H); 3.90 (s, 3 H); 6.59 (d, J¼9.1, 1 H); 7.19 (br. s, 1 H, D2O-exchanged); 7.43 (dd, J¼9.1, 2.8, 1 H);
7.72 (dd, J¼2.8, 1.9, 1 H); 8.11 (d, J¼2.2, 1 H); 8.40 (d, J¼3.0, 1 H); 8.56 (d, J¼1.6, 1 H). GC/MS: 377
(3, [M þ 2]þ ), 375 (9, Mþ ), 210 (100), 181 (35), 155 (38), 108 (24). Anal. calc. for C18H22ClN5O2
(375.85): C 57.52, H 5.90, N 18.63; found: C 57.15, H 5.82, N 18.33.
General Procedure for Preparation of Compounds 7a–7f. A mixture of 6-fluoro-pyridine-3-
carboxylic acid (0.07 g, 0.5 mmol) and CDI (0.10 g, 0.6 mmol) in 10 ml of anh. THF was stirred for 8 h. A
soln. of amine 4a–4f (0.5 mmol) in anh. THF (10 ml) was added, and then the mixture was stirred until
the acid disappeared (TLC). The mixture was partitioned between AcOEt (20 ml) and H2O (20 ml). The
separated org. layer was washed with a sat. aq. soln. of Na2CO3 (20 ml), dried (Na2SO4) and concentrated
in vacuo. The crude residue was chromatographed as detailed below to afford the pure arenecarbox-
amide in 40–50% yield.
N-{2-[4-(4-Chlorophenyl)piperazin-1-yl]ethyl}-6-fluoropyridine-3-carboxamide (7a). Eluted with
1
CHCl3/MeOH 19 :1. Yield: 56%. M.p. 158–1608. H-NMR (CDCl3): 2.65–2.68 (m, 6 H); 3.17 (app. t,
4 H); 3.58 (q, J¼5.7, 2 H); 6.82–6.85 (m, 2 H); 6.87 (br. s, 1 H, D2O-exchanged); 7.01 (dd, J¼ 8.4, 2.8,
1 H); 7.19–7.22 (m, 2 H); 8.22–8.27 (m, 1 H); 8.60 (d, J¼2.8, 1 H). GC/MS: 364 (4, [M þ 2]þ ), 362 (9,
Mþ ), 211 (34), 209 (100), 166 (19), 124 (22). Anal. calc. for C18H20ClFN4O (362.83): C 59.59, H 5.56, N
15.44; found: C 59.29, H 5.55, N 15.16.
6-Fluoro-N-{2-[4-(4-methylphenyl)piperazin-1-yl]ethyl}pyridine-3-carboxamide (7b). Eluted with
1
CHCl3/MeOH 19 :1. Yield: 35%. M.p. 149–1518. H-NMR (CDCl3): 2.27 (s, 3 H); 2.73–2.76 (m, 6 H);
3.21 (app. t, 4 H); 3.63 (q, J¼5.2, 2 H); 6.83–6.86 (m, 2 H); 7.01 (dd, J¼ 8.5, 2.8, 1 H); 7.06–7.10 (m,
2 H); 7.16 (br. s, 1 H, D2O-exchanged); 8.25–8.32 (m, 1 H); 8.65 (d, J¼2.7, 1 H). GC/MS: 343 (3, [M þ
1]þ ), 342 (14, Mþ ), 189 (100), 146 (22), 124 (20). Anal. calc. for C19H23FN4O (342.41): C 66.65, H 6.77, N
16.36; found: C 66.52, H 6.74, N 16.17.
N-{2-[4-(1,2-Benzoxazol-3-yl)piperazin-1-yl]ethyl}-6-fluoropyridine-3-carboxamide (7c). Eluted
with CHCl3/MeOH 19 :1. Yield: 50%. M.p. 152–1548. 1H-NMR (CDCl3): 2.78–2.85 (m, 6 H); 3.66–
3.70 (m, 6 H); 7.01 (dd, J¼8.5, 2.7, 1 H); 7.15 (br. s, 1 H, D2O-exchanged); 7.20–7.24 (m, 1 H); 7.45–7.53
(m, 2 H); 7.66 (d, J¼8.0, 1 H); 8.30 (dt, J¼8.5, 2.5, 1 H); 8.67 (d, J¼2.5, 1 H). ESI-MS/MS (pos.): 370
([M þ H]þ ). ESI-MS/MS (pos.) 167 (100). Anal. calc. for C19H20FN5O2 (369.39): C 61.78, H 5.46, N
18.96; found: C 61.45, H 5.39, N 18.69.
N-{2-[4-(4-Cyanophenyl)piperazin-1-yl]ethyl}-6-fluoropyridine-3-carboxamide (7d). Eluted with
CHCl3/MeOH 19 :1. Yield: 42%. M.p. 155–1578 (HCl salt). 1H-NMR (CDCl3): 2.73–2.76 (m, 6 H);
3.39 (app. t, 4 H); 3.63 (q, J¼5.5, 2 H); 6.84–6.89 (m, 2 H); 7.02 (dd, J¼8.5, 3.0, 1 H); 7.05 (br. s, 1 H,
D2O-exchanged); 7.48–7.53 (m, 2 H); 8.28 (dt, J¼8.5, 2.5, 1 H); 8.63 (d, J¼2.5, 1 H). GC/MS: 353 (0.4,
Mþ ), 200 (100), 157 (15), 124 (17). Anal. calc. for C19H20FN5O·2 HCl (426.32): C 53.53, H 5.20, N 16.43;
found: C 53.85, H 5.51, N 16.25.
N-{2-[4-(3-Cyanopyridin-2-yl)piperazin-1-yl]ethyl}-6-fluoropyridine-3-carboxamide (7e). Eluted
1
with CHCl3/MeOH 19 :1. Yield: 78%. M.p. 165–1668. H-NMR (CDCl3): 2.73–2.75 (m, 6 H); 3.63 (q,
J¼5.5, 2 H); 3.78 (app. t, 4 H); 6.81 (dd, J¼7.5, 1 H); 7.01 (dd, J¼8.5, 2.7, 1 H); 7.07 (br. s, 1 H, D2O-
exchanged); 7.78 (dd, J¼7.7, 1.9, 1 H); 8.29 (dt, J¼8.3, 2.5, 1 H); 8.35 (dd, J¼4.7, 1.9, 1 H); 8.65 (d, J¼

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CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
2.2, 1 H). GC/MS: 354 (1, Mþ ), 201 (100), 124 (20). Anal. calc. for C18H19FN6O (354.38): C 61.01, H 5.40,
N 23.71; found: C 60.85, H 5.40, N 23.54.
N-{2-[4-(5-Chloropyridin-2-yl)piperazin-1-yl]ethyl}-6-fluoropyridine-3-carboxamide (7f). Eluted
with CHCl3/MeOH 19 :1. Yield: 35%. M.p. 180–1828. 1H-NMR (CDCl3): 2.67–2.75 (m, 6 H); 3.57–
3.66 (m, 6 H); 6.59 (d, J¼9.1, 1 H); 7.01 (dd, J¼8.5, 2.2, 1 H); 7.09 (br. s, 1 H, D2O-exchanged); 7.43 (dd,
J¼9.1, 2.8, 1 H); 8.11 (d, J¼2.5, 1 H); 8.25–8.31 (m, 1 H); 8.64 (d, J¼2.5, 1 H). GC/MS: 365 (1, [M þ
2]þ ), 363 (5, Mþ ), 210 (100), 181 (34), 155 (37), 124 (39). Anal. calc. for C17H19ClFN5O (363.82): C
56.12, H 5.26, N 19.25; found: C 55.89, H 5.22, N 18.95.
Lipophilicity. Lipophilicity indices were determined by a reversed-phase (RP) HPLC method
consisting in a PerkinElmer series 200 LC apparatus equipped with a PerkinElmer 785A UV/VIS
detector set at 254 nm. UV signals were monitored and the peaks obtained were integrated with a
personal computer running PerkinElmer Turbochrom Software. The capacity factors (k) were measured
with a Phenomenex Kinetex C18-XB (250ꢄ4.6 mm, 5 mm particle size) as non-polar stationary phase and
with MeOH/0.01m phosphate buffer (pH 5.5) 60 :40 (v/v) as mobile phase. This mobile phase
composition was chosen for the analysis due to reasonable retention times for all compounds analyzed.
All compounds were dissolved in MeOH, and the measurements were conducted at a flow rate of 1 ml/
min. Capacity factors were calculated as k¼(tR –t0)/t0, where tR is the retention time of the solute, and t0 is
the column dead time, measured as the solvent front.
Biological Methods. General. Human cloned dopamine D2L receptors stably expressed in rat C6
`
glioma cells was kindly donated by Prof. Roberto Maggio (Universita di LꢀAquila, Italy). Cell culture
reagents were purchased from EuroClone (IT-Milan). Human recombinant D4.4 dopamine receptor
expressed in CHO-K1 cells, human recombinant D3 dopamine receptor expressed in CHO-K1 cells, and
[3H]spiroperidol were obtained from PerkinElmer Life and Analytical Sciences (Boston, MA, USA).
Haloperidol, was purchased from SigmaꢀAldrich (IT-Milan); quinpirole was obtained from Tocris
Bioscience (UK-Bristol). For receptor binding studies, the compounds were dissolved in DMSO.
Cells Culture. Rat C6 glioma cells expressing human dopamine D2L receptors were grown in DMEM
high glucose supplemented with 10% fetal bovine serum, 2 mm glutamine, 100 U/ml penicillin, 100 mg/ml
streptomycin, in a humidified incubator at 378 in a 5% CO2 atmosphere.
Radioligand-Binding Assay at Human Cloned D4.4 Dopaminergic Receptors. Binding of [3H]spi-
roperidol at human cloned D4.4 receptor was performed as described in [29] with minor modifications.
The reaction buffer consisted of 50 mm Tris·HCl, pH 7.4, including 100 ml of dopamine D4.4 receptor
membranes, 0.5 nm of [3H]spiroperidol (Kd ¼0.17 nm), and 100 ml of the drug soln. (six to nine
concentrations) for a total volume of 1 ml. Samples were incubated at 258 for 90 min, then the incubation
was stopped by rapid filtration through Whatman GF/C glass fiber filters (presoaked in 0.3%
polyethylenimine for 20 min). The filters were washed twice with 1 ml of ice-cold buffer (50 mm Tris·
HCl; pH 7.4). Nonspecific binding was defined in the presence of 10 mm haloperidol. The radioactivity
bound to the filters was measured by liquid scintillation using LS6500 Multi-Purpose scintillation
counter, Beckman.
Radioligand-Binding Assay at Human Cloned D2L Dopaminergic Receptors. Membranes of human
dopamine D2L receptors stably expressed in rat C6 glioma cells were prepared as described by Colabufo
et al. [43]. Dopamine D2L receptors were radiolabelled with [3H]spiroperidol according to Scarselli et al.
[44] with minor modifications. The incubation buffer (50 mm Tris·HCl pH 7.4, 120 mm NaCl, 5.0 mm
KCl, 5.0 mm MgCl2, 1 mm EDTA) contained 100 mg of dopamine D2L receptor membranes, 0.50 nm
[3H]spiroperidol (Kd ¼0.093 nm), and six to nine concentrations of drug soln. in a final volume of 500 ml.
The samples were incubated for 120 min at 258, then the incubation was stopped by rapid filtration
through Whatman GF/C glass fiber filters (presoaked in 0.5 % polyethylenimine for 30 min). The filters
were washed 3ꢄ1 ml of ice-cold 50 mm Tris, pH 7.4, 0.9% NaCl. Nonspecific binding was determined in
the presence of 10 mm haloperidol. The radioactivity bound to the filters was measured by liquid
scintillation using LS6500 Multi-Purpose scintillation counter, Beckman.
Radioligand-Binding Assay at Human Cloned D3 Dopaminergic Receptors. Binding of [3H]spiro-
peridol at human cloned D3 receptor was performed as described to Swarzenski et al. [45] with minor
modifications. The reaction buffer consisted of 50 mm Tris, 5 mm MgCl2, 10 mg/ml saponine (pH 7.4),
including 100 ml of dopamine D3 receptor membranes, 0.4 nm of [3H]spiroperidol (Kd ¼0.60 nm), and

CHEMISTRY & BIODIVERSITY – Vol. 11 (2014)
309
100 ml of the drug soln. (six to nine concentrations) for a total volume of 1 ml. Samples were incubated at
258 for 30 min, then the incubation was stopped by rapid filtration through Whatman GF/C glass fiber
filters (pre-soaked in 0.5% polyethylenimine for 2 h). The filters were washed twice with 1 ml of ice-cold
buffer (50 mm Tris, pH 7.4, 5 mm MgCl2). Nonspecific binding was defined in the presence of 10 mm
quinpirole. The radioactivity bound to the filters was measured by liquid scintillation using LS6500 Multi-
Purpose scintillation Counter, Beckman.
Statistical Analysis. The inhibition curves on the different binding sites of the compounds were
analyzed by nonlinear curve fitting utilizing the GraphPad Prism program. The value for the inhibition
constant, Ki, was calculated by using the ChengꢀPrusoff equation [38].
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Received June 14, 2013