derived from OIHS containing NSAID are scarce.42 .... The solution was stirred at 0-5 0C with N2 atmosphere for. 1 day. After one day NHS-activated Sulindac ...
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Rationally Developed Metallogelators Derived from Pyridyl Derivatives of NSAIDs Displaying Anti-inflammatory and Anti-cancer Activities Koushik Sarkar, Shaik Khasimbi, Souvik Mandal, and Parthasarathi Dastidar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09872 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018
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Rationally Developed Metallogelators Derived from Pyridyl Derivatives of NSAIDs Displaying Anti-inflammatory and Anti-cancer Activities Koushik Sarkar, Shaik Khasimbi, Souvik Mandal, and Parthasarathi Dastidar* Department of Organic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Kolkata-700032, India KEYWORDS: Metallogel, NSAID, Anti-inflammatory property, Anti-cancer agent, Drug delivery ABSTRACT: Metal-ligand coordination involving hydrogen bond functionalized ligands was employed rationally to get an easy access to a series of metallogelators derived from 3-pyridyl derivatives of non-steroidal-anti-inflammatory drugs (NSAIDs) [e.g. ibuprofen, sulindac and flurbiprofen designated as 3-pyIBU, 3-pySUL and 3-pyFLR, respectively] and biogenic metal centers [Zn(II), Cu(II), Mn(II) and Ag(I)]. Thirteen metallogels (MG1-MG13) were obtained by allowing the ligands and the metal salts to react in DMSO/water at room temperature. Slightly different solvent system (DMSO/water/MeOH) afforded four crystalline coordination complexes of 3-pyIBU namely [{Cu(3pyIBU)4(DMSO)2}(NO3)2] (CC1), [{Ag(3-pyIBU)2}(BF4)] (CC2), [{Ag(3-pyIBU)2}(ClO4)] (CC3) and [{Cu(3pyIBU)4(CH3OH)2}(OTf)] (CC4) that were fully characterized by single crystal X-ray diffraction (SXRD). However, none of these coordination complexes produced metallogels – the results corroborated well with the rationale based on which the metallogelators were obtained. Two selected metallogels (MG3 and MG9) could be leached out from the corresponding metallogels to the bulk solvent to the extent of 51 and 59 %, respectively after 24 h of incubation at 37 ° C indicating their plausible use in topical application. Moreover, one of the selected metallogelators i.e. MG9 displayed anti-inflammatory response and was able to inhibit the migration of highly aggressive human breast cancer cells MDA-MB-231 suggesting its plausible use as anti-cancer agent. cult to appreciate the fact that loading and controlled release of a drug from microporous materials such as porous coordination polymers (PCPs) or metal organic INTRODUCTION frameworks (MOFs) is nontrivial. On the other hand, if Delivering bioactive molecules (drug or prodrug) inside the drug molecule is incorporated as a part of the OIHS, the cells is an important and challenging area of research the need for microporous vehicle (such as PCPs or MOFs) towards the development of drug delivery systems (DDS). does not exist. This can be readily achieved by choosing For this purpose, conventionally, the drug is loaded onto drug molecule that can act as ligand so that it is a stoichia carrier and delivered to the target site. There exist both ometric part of the OIHS (either CPs or CCs). The resultorganic (liposomes, polymers, dendrimers, etc.)1-5 and ing OIHS can be delivered to the target site either as nainorganic (quantum dots, metal nanoparticles, etc.)6-10 noscopic material (nanoparticle) or in the form of topical systems to carry out such jobs. In this context, organicgel. inorganic-hybrid-systems (OIHS) such as coordination Gels are visco-elastic materials wherein small amount complexes (CCs), coordination polymers (CPs) etc. are of gelator form a 3D gel network which is capable of imalso becoming popular as drug delivery vehicles because mobilizing the gelling solvent triggered by capillary force of some unique features these compounds offer – a) unaction or surface tension. Depending on the nature of the matched structural and compositional diversities resultgel network, gels are classified as chemical or polymeric ing from virtually unlimited combination of ligands and gel (covalent network)24 or supramolecular gel (noncovametals allowing tuneable physico-chemical properties; b) lent network).25-30 Metallogels are special class of suprarelatively labile metal-ligand coordination bond facilitatmolecular gel wherein metal ion is a part of the gel neting speedy biodegradation of the vehicles. Ferey and work. Both discrete coordination complexes and coordicoworkers11 demonstrated that porous CPs namely MILnation polymers are reported to produce metallogels.31-34 100 and MIL-101 could be exploited to host and controlled Gel containing metal nanoparticle also belongs to this release of a NSAID namely ibuprofen. Since then, many class. Thus, if supramolecular gel such as coordination groups around the world have become interested in decomplex based metallogels can be developed from an signing such OIHS for developing new DDS.12-21 It has OIHS containing a drug molecule, the local concentration been demonstrated that absorption, distribution, metaboof the drug would be high if the gel is applied via topical lism, excretion, and toxicity (ADMET) parameter of a route to the target site. In addition, the 22-23 drug could be fine-tuned in such OIHSs. It is not diffi-
Scheme 1: Schematic representation of the strategy based on which the metallogelators were designed. labile nature of the metal-ligand (drug) coordination bond would help release the drug in a slow and sustained manner. NSAIDs being anti-inflammatory agents are popular target for developing anti-inflammatory topical gels.35-37 Thus, topical gel derived from OIHS containing NSAID is important to develop. Since there is a close relation between inflammation and cancer,38-39 such OIHS can also be evaluated as possible anti-cancer agent. Recently we have demonstrated that NSAID derived CPs in its nanoscale form or in solution are capable of displaying anti-inflammatory response (in vitro), cell imaging and inhibiting migration of cancer cells.40-41 However, to the best of our knowledge, reports on topical gel derived from OIHS containing NSAID are scarce.42 With this background, we hereby disclose the synthesis of a new series of OIHS derived from 3-pyridyl derivatives of ibuprofen, sulindac and flurbiprofen (namely, 3-pyIBU, 3-pySUL, 3-pyFLR, respectively) and biogenic metal ion Cu(II), Zn(II), Mn(II) and Ag(I) as potential metallogelators. The attempt to synthesize such OIHS was based on the structural aspects of gelator molecules reported by us43-45 and others;46,47 crystal structures of these gelators do have a large number of lattice occluded guests (solvents). Akin to lattice occluded crystalline solids (LOCS), gels do have a large amount of solvent molecules entrapped within the gel network. Therefore, the molecules having propensity to form LOCS might be gelators under suitable conditions. This is particularly most suited for aqueous gels as in such cases, the solvent molecules participate in hydrogen bonding with the hydrogen bonding capable moieties of the gelator molecules. Following such structural rationale, we successfully designed a large number of metallogelators.48-50 In the present work, while pyridyl N is expected to coordinate to the metal center, the amide functionality of the ligands is anticipated to provide crucial hydrogen
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bonding sites for guest occlusion thereby increasing the possibility of gelation under suitable conditions (Scheme 1). Reaction of these NSAID derived ligands produced coordination complexes (CC1-CC4) from 3-pyIBU and metallogels (MG1-MG13) from all the ligands. While SXRD was employed to characterize the coordination complexes, the metallogels were characterized by dynamic rheology and TEM. In vitro cytotoxicity assay (MTT) and anti-inflammatory response (PGE2 assay) in RAW 264.7 cell line was carried out for a few selected metallogelators (MG3 and MG9). Anti-cancer property of one such bioacceptable metallogelator (MG9) was evaluated by PGE2 and cell migration assay in human breast cancer cell line MDA-MB-231. Internalization of the metallogelator in MDA-MB-231 was probed by fluorescence microscopy. Finally, release profile of the metallogelators in PBS (pH 7.4) was also studied.
EXPERIMENTAL SECTION Materials and Physical measurements: All chemicals, including NSAIDs, DCC, NHS, 3aminopyridine, metal salts etc., were purchased from Sigma Aldrich and used without further purification. Solvents were of analytical-reagent (AR) grade and were used without any further purification. NMR spectra (both 1H and 13C) were recorded by using 400 and 500 MHz spectrometers (Bruker Ultrashield Plus-400 and 500). FTIR spectra were recorded by using FTIR-8300 instrument (Shimadzu). TEM experiments were performed by using a JEOL instrument with a 300 mesh copper TEM grid. Rheological experiments were conducted by using an Anton Paar MCR 102 rheometer. Mouse macrophage RAW 264.7 and human breast cancer MDA-MB-231 cells were purchased from the American Type Culture Collection (ATCC). MTT and PGE2 assays were carried out by using a multiplate ELISA reader (Varioskan Flash Elisa Reader, Thermo Fisher). For the PGE2 assay, Prostaglandin E2 EIA Kit—Monoclonal (Cayman Chemicals, Ann Arbor, MI) was used. UV−Vis spectroscopic measurements were car-
ried out on a Hewlett-Packard 8453 diode array spectrophotometer equipped with a Peltier temperature controller. Fluorescence measurements were performed by using a Horiba Jobin Yvon Fluoromax-4 spectrofluorometer. Cell images were taken in a Carl Zeiss axio observer z1 instrument fitted with a Hamamatsu orca flash 4.0 attachment. ImageJ software package has been used for measuring TEM fiber dimensions and several biological analyses. Synthesis of amides Synthesis of Ibuprofen-3-pyridyl amide (3-pyIBU) Ibuprofen Sodium (Mol. Wt. 228.27 g/mol) (sodium salt of ibuprofen) was dissolved in 200 ml of distilled water. Few drops of concentrated HCl were added to it until pH became 1 (confirmed by litmus paper). Then solution is scratched using glass rod and white precipitate (Ibuprofen) was formed. The precipitate was collected using high vacuum filtration technique. 6 mmol of Ibuprofen (Mol. Wt. 206.29 g/mol) and 6.6 mmol of N-hydroxy succinamide (NHS) (Mol wt. 115.09 g/mol) were dissolved in 50 ml of dry THF. 9 mmol of Dicyclohexylcarbodiimide (DCC) (Mol. Wt. 232.37 g/mol) was dissolved in 10 ml of dry THF and then added drop wise. The solution was stirred at 0-5 0C with N2 atmosphere for 1 day. After one day NHS-activated Ibuprofen was formed and Dicyclohexylurea (DCU) was precipitated out as by product. 6 mmol of 3-aminopyridine (Mol. Wt. 94.12 g/mol) was dissolved in 10 ml of dry THF and added drop wise to NHSactivated Ibuprofen (filtrate). The solution was stirred at room temperature for 3 days and purified by column chromatography (3.5% MeOH in DCM). Synthesis of Sulindac-3-pyridyl amide (3-pySUL) 3 mmol Sulindac (Mol. Wt. 356.41 g/mol) and 3.3 mmol of N-hydroxy succinamide (NHS) (Mol wt. 115.09 g/mol) were dissolved in 30 ml of dry THF. 4.5 mmol of Dicyclohexylcarbodiimide (DCC) (Mol. Wt. 232.37 g/mol) was dissolved in 10 ml of dry THF and then added drop wise. The solution was stirred at 0-5 0C with N2 atmosphere for 1 day. After one day NHS-activated Sulindac was formed and Dicyclohexylurea (DCU) was precipitated out as by product. 3 mmol of 3-aminopyridine (Mol. Wt. 94.12 g/mol) was dissolved in 10 ml of dry THF and added drop wise to NHS-activated Sulindac (filtrate). The solution was stirred at room temperature for 3 days and purified by column chromatography (2.5% MeOH in DCM). Synthesis of Flurbiprofen-3-pyridyl amide (3-pyFLR) 6 mmol Flurbiprofen (Mol. Wt. 244.26 g/mol) and 6.6 mmol N-hydroxy succinamide (NHS) (Mol wt. 115.09 g/mol) were dissolved in 50 ml of dry THF. 9 mmol Dicyclohexylcarbodiimide (DCC) (Mol. Wt. 232.37 g/mol) was dissolved in 10 ml of dry THF and then added drop wise. The solution was stirred at 0-5 0C with N2 atmosphere for 1 day. After one day NHS-activated Flurbiprofen was formed and Dicyclohexylurea (DCU) was precipitated out as by product. 6 mmol of 3-aminopyridine (Mol. Wt. 94.12 g/mol) was dissolved in 10 ml of dry THF and added drop wise to NHS-activated Flurbiprofen (filtrate). The solu-
CC4 complex: FT-IR (KBr pellet): 3286, 2954, 2868, 1694, 1673, 1610, 1585, 1542, 1485, 1429, 1366, 1280, 1200, 1059, 746, 695, 473 cm-1. Gelation and metallogelation experiment To prepare gel from the ligands, we first dissolved the ligands in 100 µL DMSO by heating and to the hot clear solution 300 µL water was added. It was then allowed to cool to room temperature. Gel formation was confirmed by test tube inversion method. In a typical metallogelation experiment, ligands were dissolved in 75 μL of the DMSO by heating until a clear hot solution was formed. Then, metal salt in 225 μL water was added into the hot clear solution of the ligand. Ligand and metal ratio was kept 2:1 for silver salts and 4:1 for all the other metal salts. The solution was then allowed to cool to room temperature for the formation of a stable gel which was confirmed by test tube inversion. Microscopy Microscopic measurement of the metallogel was done by TEM (transmission electron microscopy). The sample for TEM was prepared by smearing a small portion gel on a carbon coated Cu (300 mesh) TEM grid. The grid was dried under vacuum at room temperature for one day and used for recording TEM images using an accelerating voltage of 100 kV without staining. Single crystal X-ray diffraction SXRD data were collected at the DBT-funded X-ray diffraction facility under the CEIB program in the Department of Organic Chemistry, IACS, Kolkata. Data were collected using MoKα (λ = 0.7107 Å) radiation on a BRUKER APEX II diffractometer equipped with CCD area detector. Data collection, data reduction, structure solution/refinement were carried out using the software package of SMART APEX-II. All structures were solved by direct method and refined in a routine manner. Nonhydrogen atoms were treated anisotropically whenever possible. All the hydrogen atoms were geometrically fixed. MTT assay The cytotoxicity of the metallogelator in both RAW 264.7 cell and MDA-MB-231 was evaluated using standard MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. Cells were purchased from American Type Culture Collection (ATCC) and maintained following their guidelines. The cells were grown in Dulbecco’s Modified Eagle’s Medium (DMEM) supplemented with 10% Fetal Bovine Serum (FBS), 1% penicillin and streptomycin in a humidified incubator at 37° C and 5% CO2. The cells were seeded in 96-well plates at a density of 1 × 104 cells/well for 24 h. During the preparation of the solutions of the respective compounds, the solutions were shaken vigorously to homogenize the turbidity that appeared after the addition of DMEM to the DMSO solution of sulindac, MG3 and MG9 due to the poor solubility of the corresponding compounds in water. After 24 h of seeding, the cells were treated with various concentrations of the compounds in DMSO/DMEM (1:99) or DMEM alone in 1% DMSO for 72 h in humidified incubator at 37˚C and 5% CO2. Then, the culture medium was replaced with 100
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μg of MTT per well and kept for 4 h at 37˚C. To dissolve the formazan produced by mitochondrial reductase from live cells, DMSO (100 μL/ well) was added and incubated for 30 minutes at room temperature. The colour intensity of the formazan solution, which is positively correlated to the cell viability, was measured using a multiplate ELISA reader at 570 nm (Varioskan Flash Elisa Reader, Thermo Fisher). The percentage of live cells in salt-treated sample was calculated considering DMEM-treated sample as 100%. PGE2 assay Prostaglandin E2 EIA kit -monoclonal (Cayman Chemicals, MI) was employed to carry out PGE2 assay of MG3 and MG9 in normal macrophage cell line RAW 264.7 and PGE2 assay of MG9 human breast cancer cell line MDAMB-231. In this assay, inflammation in the cells was monitored by the production of PGE2 in the cell culture medium. Approximately ∼1 × 106 cells were seeded in a six-well plate and incubated for 24 hours in DMEM media. Inflammation inducing agents (e.g. IFN-γ, lipopolysaccharide) were applied for inducing inflammation in RAW cell. The amount of PGE2 produced in the culture medium with and without the treatment of MG3 and MG9 and NSAIDs was determined using the kit. In all the cases concentration of DMSO was maintained 1% (v/v) in DMEM media. It should be noted that there was already inflammation in MDA-MB-231 cell, so there was no need to add LPS and IFN-γ. Cell migration assay For the cell migration assay, MDA-MB-231 cells were seeded in a 6-well plate and kept overnight in incubator. A 200 µl sterile pipette tip was used to introduce a scratch on the plate. The cells were charged separately with 100 µg/ml concentration (IC50 of MG9 in MDA-MB-231) of sulindac and MG9 in MDA-MB-231 cell line. In the control experiment no drug was added. In all the cases concentration of DMSO was maintained 1% (v/v) in DMEM media. Still images were captured after different time intervals for 24 h to measure migration speed. Cell imaging For cell imaging, MDA-MB-231 cells were cultured by using DMEM supplemented with 10% FBS and 1% penicillin–streptomycin on ethanol etched cover slips kept in a 35 mm tissue culture dishes. The dishes were then kept in incubator at 37 °C overnight. Then the cells were washed with PBS and incubated in serum-free media (SFM) for half an hour. DMSO solution of metallogelator MG9 at IC50 concentration was made by mixing it in serumcontaining medium keeping Serum-containing medium: DMSO = 99:1 (v/v). These solutions were incubated for 30 min. After incubation, media was discarded and cells were washed with PBS. The cells were fixed by using 4% paraformaldehyde for 10 min at room temperature. Then the cells were washed with PBS and mounted on glass slides for microscopy.
Figure 1: Frequency sweep of the metallogels. Leaching experiment To demonstrate the sustained release of the two selected metallogelators MG3 and MG9, we placed 2 ml PBS (pH 7.4) over the metallogel beds at MGC in a test tube; five such test tubes for each case were incubated at 37 oC for various time intervals at 3h, 6h, 9h,12h and 24h and the
concentration of the gelator in the PBS layer was measured in triplicate by spectrophotometric method after appropriate dilution and interpolation from previously standard UV spectrophotometry calibration curves for MG3 and MG9, respectively.
The ligands were synthesized by simple amidation reactions following literature procedure (see experimental).51 Before studying the metallogelation ability of these ligands, we first evaluated the gelling property of the free ligands in fourteen different solvents that include polar, nonpolar and aqueous solvents. The results are recorded in Table S1; only 3-pyFLR produced gel with DMSO/water (1:3 v/v); the minimum gelator concentration (MGC) was 5.5 wt % (w/v) and gel-sol transition temperature was 53 ° C. However, the gel was found to be thermo-irreversible and phase separated within ~48 h. Reaction of these ligands with various metal salts in DMSO/water (1:3 v/v) produced several metallogels (MG1-MG13) when an aqueous solution of the metal salt was added to a hot solution of the ligand in DMSO and cooled to room temperature (Scheme 2). All the gels were thermoirreversible resulting in permanent phase separation when heated. It is interesting to note that both 3-pySUL and 3-pyFLR failed to provide any metallogel with Zn salts whereas 3-pyIBU and 3-pyFLR did not provide metallogel with Mn salt (Table 1, see experimental).
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the fact that the amide >C=O experienced strong D and H bonding interactions involving D2O and small amount of Figure 2: TEM images of the xerogel of A) MG3, B) MG5, C)
Rheology Study of flow characteristics of a non-Newtonian fluid such as supramolecular gels is carried out by rheology.52-53 Supramolecular gels being visco-elastic in nature, display both viscous (liquid-like) and elastic (solid-like) features which can easily be probed by dynamic rheology. In a frequency sweep experiment wherein both elastic modulus (Gˊ) and viscous modulus (Gˊˊ) are plotted against frequency (ω, s-1) with a constant strain (suggested by amplitude sweep wherein Gˊ and Gˊˊ are plotted against increasing strain), if Gˊ is found to be larger than Gˊˊ and remain nearly frequency invariant over the entire range of ω, the material under study is said to be visco-elastic or gel-like material. Figure 1 clearly indicated that G´s were much larger than G´´s (G´´/G´ = 0.02-0.19) and frequency invariant over the entire range of frequency ω meaning that the metallogels (MG1-MG13) under study displayed visco-elastic response. (Table S3). Microscopy
MG9 and D) MG11.
H2O present in the solvent mixture (DMSO-D6/D2O).54 The data clearly support the rationale based on which the metallogels were prepared (Table S2, Figure S8-S10). Single crystal X-ray diffraction studies To get an insight into the supramolecular structures of the expected metal complexes that might be responsible for metallogel formation, we undertook single crystal Xray diffraction (SXRD) studies. For this purpose, we tried to crystallize these complexes by reacting a hot solution of the ligand with a MeOH/water solution of the metal salt so that the final composition of the solvents remained 1:1:6 (DMSO/MeOH/water) and metal: ligand ratio was 1:4 (for Zn(II), Cu(II) and Mn(II)) and 1:2 (for Ag(I)). Slow evaporation of these solutions resulted single crystals of four metal complexes derived from 3-pyIBU and Cu(NO3)2, AgBF4, AgClO4 and Cu(OTf)2 designated as CC1, CC2, CC3 and CC4, respectively and were subjected
HR-TEM of some selected gels (MG3, MG5, MG9 and MG11) revealed the morphology of the gel fibers; while several micrometer long highly entangled fibers were observed in the cases of MG5 and MG9, colonies of highly aligned short fibers (~200 nm long) and aggregates of several micrometer long tapes were seen in the cases of MG3 and MG11, respectively (Figure 2). FT-IR studies To get an insight into the role of supramolecular interactions specially hydrogen bonding in gelation, we carried out FT-IR studies on DMSO/D2O metallogels. We particularly monitored the hydrogen bonding environment of the amide >C=O. The asymmetric stretching bands of amide >C=O of the ligands (1688-1694 cm-1) were found to be redshifted to 1638-1676 cm-1 in all the corresponding metallogels in DMSO-D6/D2O; this could be because of
to SXRD. We could also determine the crystal structures of 3-pyIBU and 3-pyFLR (Table 2). Figure 3: Crystal structure illustrations: hydrogen bonding interactions in A) 3-pyIBU and B) 3-pyFLR.
Crystal structures of the ligands 3-pyIBU and 3pyFLR
Asymmetric unit of both the ligands contained more than one molecule; in 3-pyIBU, two hydrogen bonded dimers of the ligand molecule sustained by N-Hamide…Npy were located in the asymmetric unit (Zˊ = 4) whereas in 3pyFLR, two crystallographically independent molecules of the ligand were observed (Zˊ = 2). Both the ligands displayed an overall 1D hydrogen bonded networks; while NHamide…Npy hydrogen bonding prevailed in 3-pyIBU, typical amide…amide interactions involving NHamide…O>C=O(amide) hydrogen bonding were observed in 3pyFLR (Figure 3).
was sustained by dispersion forces and π-π stacking involving the pyridyl moieties (Figure 5).
Crystal structures of the coordination complexes The molecular formulae of the coordination complexes were assigned as [{Cu(3-pyIBU)4(DMSO)2}(NO3)2] (CC1), [{Ag(3-pyIBU)2}(BF4)] (CC2), [{Ag(3-pyIBU)2}(ClO4)] (CC3) and [{Cu(3-pyIBU)4(CH3OH)2}(OTf)] (CC4). While CC1 and CC4 were found to be isostructural displaying similar crystal packing in different space group settings (Pca21 for CC1 and Ccca for CC4), CC2 and CC3 were isomorphous having identical space group (P1) and near identical cell dimensions. The metal center Cu(II) displayed octahedral coordination environment wherein the equatorial positions were coordinated by pyridyl N of the ligand and the axial sites were occupied by DMSO and MeOH for CC1 and CC4, respectively. In both the structures, the coordination complex molecules were arranged in 1D array sustained by bifurcated hydrogen bonding involving the counter anion nitrate O and amide N-H (N-H…O interactions) in CC1 and typical amide…amide hydrogen bonding in CC4; the disordered triflate counter anion in CC4 was found to be hydrogen bonded with the metal bound MeOH (Figure 4).
Figure 5: Crystal structure illustrations: π-π stacking in A) CC2 and B) CC3.
Thus, the crystal structures of the metal complexes (CC1, CC2, CC3 and CC4) revealed that none of these possessed any lattice occluded solvents and except for CC4, amide…amide hydrogen bonding was not observed in any of these metal complexes. Surprisingly, these metal complexes failed to occlude any solvent molecule in the crystal lattice despite having amide functionality that was expected to facilitate guest occlusion; thus, these are not conducive for gelation according to the structural rationale we discussed earlier (see above). Interestingly, these metal complexes when isolated and studied for metallogelation under the identical conditions described above failed to produce any metallogels; instead, in each instance, the corresponding crystalline complex was isolated. It may be mentioned here that crystallization is directed by equilibrium thermodynamics whereas gelation is a kinetic phenomenon.55 Thus, when the reactants (ligands and metal salts) were allowed to interact at room temperature in such solvent system (DMSO/water), kinetic phenomenon must have been favoured leading to metallogels. On the other hand, equilibrium thermodynamic parameters must be operating in DMSO/water/MeOH solvent system that favoured crystallization of CC1-CC4. Biological Studies
Figure 4: Crystal structure illustrations; 1D hydrogen bonded network in A) CC1 and B) CC4.
The metal center Ag(I) in the isomorphous complexes i.e. CC2 and CC3 displayed the expected linear coordination geometry wherein pyridyl N atoms coordinated to the metal center. Typical amide…amide hydrogen bonding was absent in these structures. The crystal packing
Successful conversion of the NSAID derivatives to metallogelators encouraged us to carry out studies pertaining to plausible applications in biomedicine. For this purpose, we selected MG3 and MG9; the selection was based on the following considerations: while MG3 was selected for its anti-inflammatory properties as both ibuprofen and Ag ion are known anti-inflammatory agents,5658 MG9 was studied for its anti-cancer property as sulindac is reported to have possessed anti-cancer activity.5960
Cytotoxicity of these metallogelators in its xerogel form towards RAW 264.7 cells was first assessed by 3-(4,5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay for 72 h at 37 °C in Dulbecco’s modified Eagle’s medium (DMEM).61 The data revealed that MG9 displayed much superior biocompatibility as compared to that of MG3 (IC50 = 50 µg/mL for MG9, 2.5 µg/mL for MG3) (see experimental and Figure S20).
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tive and safe in treating prostate cancer68 and naproxen is also reported to be effective in inhibiting migration of cancer cells in in vitro studies.69 More recently sulindac has been reported to mediate breast cancer inhibition as an immune modulator.59,60 Therefore, we chose to work with a highly aggressive human breast cancer cell line namely MDA-MB-231 to study the anti-cancer property of MG9 which is derived from sulindac.
Figure 7: PGE2 assay of sulindac and MG9 in MDA-MB-231 cell line.
The IC50 values for sulindac and MG9 xerogel in MDAMB-231 cell line (MTT assay) was 0.5 mM and 100 µg/mL, respectively (Figure S21).
Figure 6: PGE2 assay of a) Ibuprofen sodium and MG3 and b) sulindac and MG9 in RAW 264.7 cells. (L= lipopolysaccharide, Y=interferon-γ)
Anti-inflammatory response of these xerogels was evaluated in RAW 264.7 cells by PGE2 assay.62 Figure 6 clearly established that extra-cellular PGE2 concentration (which is a measure of inflammation) was significantly reduced to ~82 and ~87 % by the xerogels of MG3 and MG9, respectively, which were comparable to that of the parent drugs (ibuprofen sodium salt and sulindac) evaluated under identical conditions (see supporting information).63 The data clearly suggested that both the metallogelators displayed anti-inflammatory response as good as that of the corresponding parent drugs. Anti-cancer activity Chronic inflammation at a particular site may cause cancer.38,39 Inflammation upregulates the production of PGE2 catalyzed by cyclooxygenase enzymes (COX-1 and COX-2) in intracellular fluid and a recent report indicated increased level of PGE2 in breast cancer patient.64 Progression and metastasis of cancer cells are also related to PGE2 level.65 PGE2 is also found to promote cell migration stimulating tumour growth and metastasis.66 Since antiinflammatory response of NSAIDs follow COX pathway (COX inhibition),67 they are being targeted as anti-cancer agents; for example, naproxen – a well-known NSAID along with calcitriol (vitamin D3) was found to be effec-
Figure 8. Migration of the cell front observed at different time intervals in a scratch assay performed on MDA-MB-231 cells after treating them by sulindac, and MG9.
We then evaluated the anti-inflammatory response of MG9 xerogel in the same cell line in order to assess its
anti-cancer behavior. It was observed that the untreated cells produced 1258.8 pg/ml of PGE2, which was significantly reduced in the cell treated with sulindac and MG9 (209.8 and 112.6 pg/ml, respectively) indicating relatively better anti-cancer behaviour of MG9 as compared to that of the mother drug sulindac (Figure 7). Cell migration is an essential activity of normal cells as it helps in the development and maintenance of multicellular organisms. However, for the cancer cells, uncontrolled and directed migration leads to metastasis.70-71 Therefore, cell migration assay is a technique by which anti-cancer property of a compound can be assessed. Towards this goal, we, therefore, undertook cell migration assay of MG9 xerogel in the same cell line and compared the results with the mother drug sulindac.
maintaining a concentration of 100 µg/mL for 24 h. The cell migration was quantified by measuring the gap after 24 h; it was observed that the cells treated with sulindac could cover ~19 % of the gap whereas the value was 12 % in the case of the cells treated with MG9 xerogel. On the other hand, as expected, untreated cells migrated to full extent covering 100 % of the gap. Migration speed calculations revealed that MG9 xerogel slowed down the migration of the cells most effectively thereby, displaying effective anti-cancer activity in vitro (Figure 8, Figure S22 and Table S10). Since MG9 xerogel contains Mn(ClO4)2, both the metal ion and its counter anion ClO4- may pose possible toxicity. If metal salt is administered via intravenous/muscular route, there could be significant toxicity due to its high concentration in the blood stream. On the other hand, in vivo application via topical route is expected to reduce the toxicity level significantly as it is now applied locally on the target site and it has to cross many barriers (skin, veins etc.) to penetrate to reach blood stream. Moreover, concentration of the metal ion need be adjusted to reduce the toxicity level below acceptable range before real-life applications. Metal containing topical formulation for treating inflammation, infection, burn-wound are commercially available (e.g. concentrated Zn(NO3)2 solution for treating guminflammation in the gum, AgNO3 in gel matrix for preventing secondary infection in cut-injury, mafenide acetate cream for burn-would treatment). The present work may be considered as a proof-of-concept that aims to provide topical approach for cancer treatment (e.g. skin cancer). Thus, the concern about toxicity induced by the ingredient (the metal salt) should be handled accordingly in the light of the pertinent discussion given above. Cell imaging To evaluate the extent of internalization of MG9 xerogel in the same cell line, we carried out cell imaging studies. MG9 was best suited for such purpose as it was found to be significantly fluorescent (λex = 488 nm, λem = 522-575 nm) (Figure S23). MDA-MB-231 cells were incubated along with MG9 for 30 minutes in DMEM media in such a way that the final concentration of MG9 was in its IC50 and the final amount of DMSO did not exceed 1% (v/v). The incubated cells produced appreciable red fluorescence (λex = 650 nm) when excited with green light (λex = 560 nm) under a fluorescence microscope. Overlay image confirmed that that fluorescence was due to the internalization of MG9. Manual z-stacking also supported the conclusion. (Figure 9, for z-stacking video see supporting information). Drug release
Figure 9: Fluorescence microscopic images of MDA-MB-231 cells incubated with MG9: A) bright field, B) fluorescence and C) overlay.
The cells were cultured in a six-well plate in DMEM and a scratch was introduced on it with a sterile micro-tip to create gap. Cells were then treated with only media (control experiment), sulindac and MG9 xerogel in media
Leaching experiments of the metallogelators MG3 and MG9 were performed under physiological conditions (in phosphate buffered saline (PBS) of pH 7.4 at 37 °C) to establish the self-delivery application of these metallogelators. This experiment was performed by placing PBS (2 mL; pH 7.4) over the metallogels prepared at MGC in a test tube at 37 °C. UV data of the aliquot taken in regular interval revealed that MG3 and MG9 were released from the metallogels to the extent of 51% and 59%, respectively after 24 h. These data revealed that the drug could be released to the bulk thereby making it suitable for topical applications (Figure 10).
ACKNOWLEDGMENT KS thanks CSIR, New Delhi (CSIR Grant No. 09/080(0882)/2013-EMR-1) for research fellowship. The technical help in performing cell migation assays extended by Ms. Rumana Parveen is greatfully acknowledged. P.D thanks DST (EMR/2016/000894) for financial support. Single crystal X-ray diffraction facility at the Department of Organic Chemistry, IACS supported by CEIB program of DBT (BT/01/CEIB/11/V/13) was used for data collection. We thank Mr. Parijat Biswas for helping with cell imaging.
REFERENCES (1) Uhrich, K. E.; Cannizzaro, S. M.; Langer, R. S.; Shakesheff, K. M. Polymeric Systems for Controlled Drug Release. Chem. Rev. 1999, 99, 3181-3198. (2) Soppimath, K. S.; Aminabhavi, T. M.; Kulkarni, A. R.; Rudzinski, W. E. Biodegradable polymeric nanoparticles as drug delivery devices. J. Control. Release 2001, 70, 1-20. (3) Caminade, A. -M.; Turrin, C. -O. Dendrimers for drug delivery. J. Mater. Chem. B. 2014, 2, 4055-4066. (4) Mintzer, M. A.; Grinstaff, M. W. Biomedical applications of dendrimers: a tutorial. Chem. Soc. Rev. 2011, 40, 173-190. Figure 10: Release of the drugs form MG3 and MG9 at physiological conditions.
CONCLUSIONS The major highlights of the report are: 1) in most likelihood, it is the first report of rationally developed metallogelators derived from NSAID based metal complexes, 2) even modified NSAID in its metallogelator form (MG3 and MG9) displayed anti-inflammatory response as good as that of the parent NSAIDs (ibuprofen and sulindac), 3) NSAID based metallogelator MG9 derived from sulindac displayed interesting anticancer property which was comparable to that of the parent drug sulindac, 4) cell imaging studies established successful internalization of MG9 in cancer cells, 5) the metallogelator drugs (MG3 and MG9) could be released from the corresponding metallogels to the bulk indicating its plausible use in topical applications. Overall, the results presented herein open up possibilities to develop OIHS for important applications in biomedicine.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental section, gelation and metallogelation studies, ORTEP plots and biological studies.
AUTHOR INFORMATION
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