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As part of our interest into the bioinorganic chemistry of gallium, gallium(III) complexes of the azole ligands 2,1,3-benzothiadiazole (btd), 1,2,3-benzotriazole (btaH), and 1-methyl-4,5-diphenylimidazole (L) have been isolated. Reaction of btaH or btd with GaBr3 or GaCl3 resulted in the mononuclear complexes [GaBr3(btaH)2] (1) and [GaCl3(btd)2] (2), respectively, while treatment of GaCl3 with L resulted in the anionic complex (LH)2[GaCl4] (3). All three complexes were characterized by single-crystal X-ray crystallography and IR spectroscopy, while their antiproliferative activities were investigated against a series of human and mouse cancer cell lines.
The coordination chemistry of gallium(III) has become an area of increasing research activity due to its relevance with both materials science [1–6] and biomedical developments [7–21]. In the area of materials science, for example, complex [Ga2(saph)2q2], where saph2− is the Schiff-base ligand bis(salicylidene-o-aminophenolate)(-2) and q− is 8-quinolinate(-1), is a very good candidate as a novel electron-transporting and emitting material for organic light-emitting diodes (OLEDs) . [Gaq3] is also a promising electroluminescence (EL) material, exhibiting higher power efficiency than the aluminum analogue, [Alq3] [5, 6]. The biological interest of gallium(III) complexes originates from the incorporation of gallium(III) radionuclides (67Ga3+, 68Ga3+) into diagnostic radiopharmaceuticals . In addition, the gallium salts GaCl3 and Ga(NO3)3 as well as few gallium(III) complexes [8–19] have exhibited antitumour activity, while Ga(NO3)3 and some GaCl3/L complexes (L = various azoles) showed in vitro anti-HIV (HIV = human immunodeficiency virus) activity . The biological activity of gallium(III) complexes has often been attributed to the fact that gallium(III) is the diamagnetic biological mimic of iron(III) . It is worth mentioning that [Gaq3], which is of current interest in materials science [5, 6], is also being evaluated in clinical trials, along with other Ga(III) complexes, such as gallium maltolate [tris(3-hydroxy-2-methyl-4H-pyran-4-onato)gallium(III)], for anticancer activity [22–24].
Following our interest in the coordination chemistry of gallium(III) [25–31] which is focused on the synthesis, structural characterization, physical/spectroscopic study and evaluation of the biological (antitumour and antiviral) activity of Ga(III) complexes with biologically relevant and nonrelevant ligands, we report herein the synthesis, structural characterization, and antiproliferative activity of three gallium complexes based on the azole ligands 2,1,3-benzothiadiazole (btd), 1,2,3-benzotriazole (btaH), and 1-methyl-4,5-diphenylimidazole (L).
All manipulations were performed under a dinitrogen atmosphere, using standard inert atmosphere techniques and purified solvents unless otherwise noted. All other chemicals were purchased from commercial sources and used without further purification. L was synthesized as described elsewhere . Microanalyses (C, H, and N) were performed by the University of Ioannina Microanalytical Laboratory using an EA 1108 Carlo Erba analyzer. IR spectra (4000–450cm−1) were recorded on a Perkin-Elmer 16 PC spectrometer with samples prepared as KBr pellets. Far-IR spectra (500–50cm−1) were recorded on a Bruker IFS 113v FT spectrometer as polyethylene pellets.
A solution of GaBr3 (0.3g, 0.9mmol) in 3ml of toluene/diethyl ether (80:20, v/v) was added dropwise to a stirred solution of btaH (0.3g, 2.5mmol) in toluene (20ml). The resultant solution was refluxed for about 3 hours and then left undisturbed at room temperature. Upon standing, X-ray quality colorless crystals of 1 formed over a period of 3 days. The crystals were collected by filtration, washed with toluene and dried in vacuum. Yield: 0.31g (63%); Anal. Calc. for C12H10N6Br3Ga: C, 26.32; H, 1.84; N, 15.34. Found: C, 26.28; H, 1.82; N, 15.33%. Selected IR data (cm−1): 3238m [ν(N–H)], 1222mb [ν(N=N)], 1116s [ν(N–N)], 291s [ν(Ga–Br)], and 224w [ν(Ga–N)].
A solution of GaCl3 (0.25g, 1.40mmol) in 5ml of toluene/diethyl ether (80:20, v/v) was added dropwise to a stirred solution of btd (0.6g, 4.4mmol) in toluene/diethyl ether (60:40, v/v) (10ml). The resultant solution was refluxed for about 2 hours and then left undisturbed at −10°C. Upon standing at low temperature for several days, X-ray quality yellowish crystals of 2 formed. The crystals were collected by filtration, washed with diethyl ether and dried in vacuum. Yield: 0.60g (95%); m.p.: 112°C. Anal. Calc. for C12H8N4S2Cl3Ga: C, 32.14; H, 1.80; N, 12.49. Found: C, 32.13; H, 1.78; N, 12.49%. Selected IR data (cm−1): 1612s and 1528s [ν(C=C)], 1482s [ν(C=N)], 961m and 922s [ν(S–N)], 382s [ν(Ga–Cl)], and 207w [ν(Ga–N)].
A solution of GaCl3 (0.2g, 1.13mmol) in 5ml of toluene/diethyl ether (80:20, v/v) was added dropwise to a stirred mixture of L (0.6g, 2.6mmol) in diethyl ether (1ml). The resultant mixture was stirred until a clear yellowish solution was obtained. Slow evaporation of the resultant solution afforded a microcrystalline solid. The solid was collected by filtration, washed with toluene and diethyl ether, and dried in vacuum. The product was recrystallised three times from toluene to give crystals of 3 suitable for X-ray structural analysis. The crystals were collected by filtration, washed with toluene and dried in vacuum. Yield: 0.18g (45%); Anal. Calc. for C32H30N4Cl5Ga: C, 53.56; H, 4.21; N, 7.81. Found: C, 53.36; H, 4.17; N, 7.78%. Selected IR data (cm−1): 3146–2620 sb [ν(N–H)], 1622m [ν(C=N)], 1578w [ν(C=C)], and 369s [ν(Ga–Cl)].
Crystals of 1 and 2 were mounted in air, while crystals of 3 were mounted in air and covered with epoxy glue. Diffraction measurements for 1 and 2 were made on a Crystal Logic Dual Goniometer diffractometer using graphite-monochromated Mo radiation, while those for 3 were made on a P21 Nicolet diffractometer using graphite-monochromated Cu radiation. Complete crystal data and parameters for data collection and processing are reported in Table 1. Unit cell dimensions were determined and refined by using the angular settings of 25 automatically centred reflections in the ranges 11 < 2θ < 23° for 1 and 2 and 22 < 2θ < 54° for 3. Three standard reflections monitoring every 97 reflections showed less than 3% variation and no decay. Lorentz, polarization and ψ-scan (only for 1) corrections were applied using CRYSTAL LOGIC software. The structures were solved by direct methods using SHELXS-86  and refined by full-matrix least squares techniques on F 2 with SHELXL-97 . All hydrogen atoms were located by difference maps and refined isotropically, except those on the methyl groups of 3 which were introduced at calculated positions as riding on bonded atoms. For all the three structures, all nonhydrogen atoms were refined using anisotropic thermal parameters.
All test substances (complexes 1, 2, and 3) were diluted in methanol at a concentration of 200mM. Final concentration of methanol in culture was always less than 0.5%, a concentration that produced no effects on cell growth and proliferation, as was experimentally confirmed.
Cell lines used were HeLa  (human cervical cancer), OAW-42  (human ovarian cancer), HT29  (human colon cancer), MCF-7  (human breast cancer), T47D  (human breast cancer), and L929 (929 is a clone isolated  from the parental strain L derived from normal subcutaneous areolar and adipose tissues of a mouse ). Cells were grown as monolayer cultures in T-75 flasks (Costar), were subcultured twice a week at 37°C in an atmosphere containing 5% CO2 in air and 100% relative humidity. Culture medium used was Dulbecco's modified Eagle's medium (DMEM, Gibco Glasgow, UK), supplemented with 10% Fetal Bovine Serum (FBS, Gibco, Glasgow, UK), 100μg/ml streptomycin and 100IU/ml penicillin.
Adherent cells at a logarithmic growth phase were detached by addition of 2-3ml of a 0.05% trypsin (Gibco, 1:250) −0.02% EDTA mixture and incubation for 2–5 min at 37°C. Cells were plated (100μl per well) in 96-well flat-bottom microtiter plates (Costar-Corning, Cambridge) at a density of 5,000 (HeLa and L929) or 10,000 (HT-29, OAW-42, MCF-7 and T47D) cells per well. Cells were left for 24 h at 37°C to resume exponential growth. An equal volume (100μl) of either complete culture medium (control wells), or twice the final substance concentration diluted in complete culture medium, was added 24 h later. Six replicate wells for each concentration were used for the sulforhodamine B (SRB) assay and three replicate wells for the bromodeoxyuridine (BrdU) assay. Background control wells (n = 8), containing the same volume of complete culture medium, were included in each experiment. Cell growth or DNA-synthesis was evaluated 48 h later by means of the SRB or BrdU assays. All experiments were performed at least twice.
The SRB assay was carried out by a modification  of the previously reported method . In brief, culture medium was aspirated prior to fixation using a microplate-multiwash device (Tri-Continent Scientific, Inc. Grass Valley, CA) and 50μl of 10% cold (4°C) TCA were gently added to the wells. Microplates were left for 30 min at 4°C, washed 5 times with deionized water and left to dry at room temperature for at least 24 hr. Subsequently, 70μl 0.4% (w/v) sulforhodamine B (Sigma) in 1% acetic acid solution were added to each well and left at room temperature for 20 min. SRB was removed and the plates were washed 5 times with 1% acetic acid before air drying. Bound SRB was solubilized with 200μl 10mM unbuffered Tris-base solution (E. Merck, Darmstadt, Germany) and plates were left on a plate shaker for at least 10 min. Absorbance was read in a 96-well plate reader (Anthos-2001, Anthos labteck instruments, A-5022, Salzburg) at 492nm subtracting the background measurement at 620nm. The test optical density (OD) value was defined as the absorbance of each individual well, minus the blank value (“blank” is the mean optical density of the background control wells, n = 8). Mean values and CV from six replicate wells were calculated automatically. Results were expressed as the “survival fraction” (sf), derived from the following equation: sf = ODx/ODc, (where ODx and ODc represent the test and the control optical density, resp.).
DNA-synthesis was estimated by the BrdU assay  using a standard colorimetric ELISA (Boehringer Mannheim). After 47h exposure to test substances, cells were incubated at 37°C for further 60 min in the presence of 10μM BrdU. Subsequently, cells were fixed with an ethanol-containing fixative, an anti-BrdU mouse monoclonal antibody conjugated with peroxidase was added and plates were incubated at 37°C for 60 min. After washing, peroxidase substrate (tetramethylbenzidine) was added, the reaction was stopped 10 min later by 1M H2SO4 and absorbance was read at 450nm subtracting the background measurement at 620nm. Results from each triplicate well (ODBrdUx/ODBrdUc) were divided by the results of a parallel experiment estimated with the SRB assay (ODSRBx/ODSRBc) and they were expressed as the "DNA synthesis fraction" (fDNA) (derived from the following equation: fDNA = (ODBrdUx×ODSRBc)/(ODBrdUc×ODSRBx), where ODx and ODc represent the test and the control optical density resp.), resulting in an estimation of the DNA synthesis per cell number.
For cell cycle experiments 1.5 × 106 (HeLa and L929) or 2.5 × 106 (HT-29, OAW-42, MCF-7 and T47D) cells were seeded in 75cm2 flasks and left for 24h in incubator to resume exponential growth. Cells were exposed to test substances (at concentrations that produced 50% inhibition of cell growth—estimated by the SRB assay) and after 48h they were harvested (using trypsin/EDTA as above), washed in PBS and counted in a hemocytometer chamber; 3×106 cells were resuspended in 125μl cold “Saline GM” (g/L: glucose 1.1; NaCl 8.0; KCl 0.4; Na2HPO4·12H2O 0.39; KH2PO4 0.15; and 0.5mM EDTA) followed by the addition of 375μl of 95% nondenatured, ice-cold ethanol . Cells were kept in 4°C for a maximum period of 3 days (short-term storage does not alter results, as was experimentally confirmed) until analysis was performed.
For cell cycle analysis a 10% of standard chicken erythrocyte nuclei were added as a control. The samples were processed in a DNA-preparation Epics Workstation (Coulter, El). By this method the content of cellular DNA is assessed using Propidium Iodide [46, 47]. To avoid an increased signal by staining artifact on double stranded RNA, cells were digested with DNase-free RNase A .
Cellular DNA content was measured using an Epics II flow cytometer (Coulter, El). The fluorescent signals from 10,000–20,000 cells were collected and the result was displayed as a frequency-distribution histogram (DNA histogram). The mean channel, cell count, standard deviation (SD), coefficient of variation (CV), DNA index (DI), and cell cycle distribution were calculated for each sample using the Multicycle Cell Cycle Analysis Software (Phoenix Flow Systems Inc.). Care was taken to exclude any doublets or cell debris noise from the assessment.
Complexes 1 and 2 were prepared by the simple reactions of GaBr3 or GaCl3 and btaH or btd in toluene/diethyl ether under nitrogen employing 1:3 molar ratios, respectively. A similar reaction involving GaCl3 and btaH has yielded [GaCl3(btaH)2] . An 1:1 complex of GaCl3/btaH has also been isolated and structurally characterized . An attempt to isolate the 1:1 GaBr3/btaH complex was unsuccessful resulting in 1 in a lower yield. Complex 2 is also the only product resulting from the GaCl3/btd reaction mixtures in various molar ratios. Complex 3 might be regarded as a product of hydrolysis which is pretty usual in Ga(III) chemistry in water or water containing solutions .
The IR spectrum of 1 exhibits a medium intensity band at ~3238cm−1, assignable to ν(N–H). The bands at 1222 and 1116cm−1 are attributed to the ν(N=N) and ν(N–N) vibrations, respectively, and are shifted to higher wavenumbers with respect to the spectrum of the free ligand (1208 versus and 1084m, resp.). The IR spectrum of 2 exhibits three strong intensity bands at 1612, 1528 and 1482cm−1 assignable to stretching carbon-carbon and carbon-nitrogen vibrations. These bands are not shifted significantly with respect to the spectrum of the free ligand [1608w, 1518s and 1476 s]. The bands at 950 and 916cm−1 in the spectrum of btd, which are assigned to the ν(S–N) mode, have been shifted to higher wavenumbers in the spectrum of 2 [961 and 922cm−1]. A set of broad bands in the region of 3146–2620cm−1 in the spectrum of 3 can be assigned to the ν(N–H) of the protonated ligand, LH+. The ν(C=N) and ν(C=C) of the free L at 1602 and 1575cm−1 have shifted to 1622 and 1578cm−1 in the spectrum of 3 due to protonation.
The far-IR spectra of all three complexes are expected to show one Ga–X (X = Cl or Br) stretching mode  and these modes appear at 291s [ν(Ga–Br) in 1], 382s [ν(Ga–Cl) in 2], and 369s [ν(Ga–Cl) in 3]. The far-IR spectra of complexes 1 and 2 exhibit one more band at 224 and 207cm−1, respectively, which are attributed to the ν(Ga–N) mode .
An ORTEP diagram of 1 is shown in Figure 1. Selected bond distances and angles are given in Table 2. Complex 1 is isostructural with [GaCl3(btaH)2] . Its structure consists of the monomeric discrete [GaBr3(btaH)2] units. The gallium coordination geometry is trigonalbipyramidal with the bromo ligands defining the equatorial plane. There is a two-fold crystallographic axis along the Ga–Br2 bond. The Ga–N bond length in complex 1 [2.212(3)Å] is longer than that of [GaCl3(btaH)2] [2.169(2)Å]. The dihedral angle between the best planes of the btaH molecules is 10.90Å and is larger than that of [GaCl3(btaH)2] [7.4°]. The N1 proton is hydrogen bonded to atom Br1 of a neighboring molecule [N1Br1′ (1 − x, −y, 1 − z) 3.425(4)Å, HN1Br1′ 2.64(7)Å and N1–HN1Br1′ 149(6)°] creating a hydrogen-bonded tape running parallel to the a axis (Figure 2). These tapes are hold together in the crystal lattice through π-π interactions. Those interactions form between the phenyl groups of the coordinated btaH molecules of neighboring tapes [centroidcentroid′ (1 − x, 0.5 + y, 1.5 − z) 3.658(4) and 3.906(4)Å] (Figure 2).
Complex 2 crystallizes in the monoclinic space group C2/c. An ORTEP diagram of 2 is shown in Figure 3, while selected bond distances and angles are listed in Table 3. Its structure consists of monomeric discrete [GaCl3(btd)2] units. The gallium coordination geometry is again trigonalbipyramidal with the choro ligands defining the equatorial plane. There is a two-fold crystallographic axis along the Ga–Cl1 bond. The Ga–Cl bond lengths in complex 2 [2.171(2) and 2.180(1)Å] compare favourably with those of [GaCl3(btaH)2] [2.204(1) and 2.178(2)Å]. The Ga–N bond length in complex 2 [2.201(3)Å] is longer than that of [GaCl3(btaH)2] [2.169(2)Å], but compares well with that of 1 [2.212(3)Å]. The dihedral angle between the best planes of the btd molecules is 52.51Å and is much larger than that of 1 and [GaCl3(btaH)2] (10.90 and 7.4°, resp.). There appear to be intermolecular stacking interactions between the nearly parallel btd ligands. Those interactions involve both the thiadiazole and the phenyl groups of the btd ligands as shown in Figure 4.
An ORTEP diagram of the asymmetric unit of 3 is shown in Figure 5. Selected bond distances and angles are listed in Table 4. The crystal of 3 consists of protonated LH+ ligand cations, tetrachlorogallate(III) anions and Cl− anions. The Ga–Cl distances in the tetrahedral [GaCl4]− ion are in the narrow range 2.152(1)–2.173(1) A° with the Cl–Ga–Cl angles varying from 107.1(1)° to 110.9(1)°. These values are similar to those observed for other complexes containing the tetrachlorogallate(-1) ion [13, 29]. The crystal structure of (LH)2[GaCl4]Cl is dominated by an intermixture of N–HCl and CMe–Hπ hydrogen bonds, Ga–Clπ azole and π-π interactions (Figure 6). The organic moieties LH+ are connected through N–HCl and C–Hπ phenyl interactions to form a chain; data are as follows N3Cl5′ (2 − x, −y, 2 − z) 3.088(3)Å, HN3Cl5′ 2.24(4)Å and N3–HN3Cl5′ 160(4)°; N13Cl5′′ (x − 1, 1 + y, z − 1) 3.066(4)Å, HN13Cl5′′ 2.19(5)Å and N13–HN13Cl5′′ 177(5)°; C38Centroid′ (1 − x, −y, 2 − z) 3.691(5)Å, H38ACentroid′ 2.85(1)Å and C38–H38ACentroid′ 147(1)°. The organic chains are bridged through Ga–Clπ azole interactions to form layers [Cl1Centroid′′ (−x, 1 − y, 1 − z) 3.455(2)Å and Cl4Centroid′′′ (1 − x, 1 − y, 1 − z) 3.550(2)Å], which are further bridged through π-π interactions in the third dimension [centroidcentroid′ (1 − x, −y, 2 − z) 3.778(3) and centroidcentroid′′′′ (−x, 2 − y, 1 − z) 3.878(3)].
Complexes 1 (Figure 7) and 2 (Figure 8) had no significant inhibition on cellular proliferation against HeLa, HT29 and OAW-42 cancer cell lines and a small effect against L929 normal fibroblastic cell line. In contrast, complex 3 inhibited cellular growth of all cell lines, with IC50 concentrations varying between 75 and 125μM (Figure 9).
DNA synthesis was not inhibited in HT29, HeLa, MCF-7 or L929 cell lines when they were exposed to 3 at concentrations up to 100μM. Higher concentrations exhibited an inhibition of DNA synthesis per cell number only in HeLa and at a lower level in L929 cells (Figure 10).
Treatment with IC50 concentrations of 3 for 48fh had no effects on cell cycle distribution of HeLa and T47D cells (Table 5). HT29 and MCF-7 were partially arrested at the G1 phase, OAW-42 were arrested at the G1 phase with a percentage of 87.2% and L929 fibroblasts exhibited a partial G2-phase arrest. However, the overall effect of 3 on cell cycle distribution (except with OAW-42 cells) was not significant, an observation in concert with the results of the BrdU assay, where no inhibition of DNA-synthesis was observed.
In this study, three gallium(III) azole complexes were synthesized and structurally characterized, while their antiproliferative activities were studied. The three different azole ligands were chosen in order to be able to draw structure-properties relations. In two of the complexes (1 and 2) the Ga(III) atom is in a trigonal-bipyramidal coordination environment where the terminal azole ligands occupy the axial positions. The third complex (3) consists of [GaCl4]− anions, chlorine anions and protonated imidazole cations. From the three complexes tested only 3 exhibited a potent anti-proliferative activity against all cell lines tested. The order of cell lines in respect to their sensitivity to 3 (at IC50 values) is as follows: HeLa > MCF-7 > T47D > L929 > HT29 > OAW-42. Complex 3 does not inhibit DNA synthesis at concentrations that exert antiproliferative activity (IC50s) and does not produce major disturbances in cell cycle distribution (with the exception of OAW-42 cells that, notably, are the most resistant to its anti-proliferative activity).
CCDC 717554, 717555, and 717553 contain the supplementary crystallographic data for 1, 2, and 3. These data can be obtained free of charge from the Cambridge Crystallographic Data Center via http://www.ccdc.cam.ac.uk/data_request/cif.
The authors acknowledge the Research Committee of the University of Patras (Grant “K. Caratheodory”, 2004–2007) for support of this work. G. S. Papaefstathiou thanks the Special Account for Research Grants (SARG) of the National and Kapodistrian University of Athens for support. This work is dedicated to Professor Nick Hadjiliadis for his retirement.