Polymeric Micelle of a Gelatin-Oleylamine Conjugate: A Prominent

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Polymeric Micelle of Gelatin-Oleylamine Conjugate: A Prominent Drug Delivery Carrier for Treating Triple Negative Breast Cancer Cells Iruthayapandi Selestin Raja, Ramar Thangam, and Nishter Nishad Fathima ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.8b00526 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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ACS Applied Bio Materials

Polymeric Micelle of Gelatin-Oleylamine Conjugate: A Prominent Drug Delivery Carrier for Treating Triple Negative Breast Cancer Cells Iruthayapandi Selestin Raja, Ramar Thangam, Nishter Nishad Fathima

Inorganic and Physical Chemistry Laboratory, Central Leather Research Institute, Council of Scientific and Industrial Research, Adyar, Chennai-600020, India

Author

to whom correspondence should be made. Tel: +91 44 244117137, 7188

E-mail: [email protected]; [email protected]

1 ACS Paragon Plus Environment

ACS Applied Bio Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Protein-based polymeric micelles are proven as effective colloidal drug carriers due to high drug loading efficiency, sustained release, biocompatibility and ease of permeation into the cell. Gelatin-based polymeric micelles find applications in treating rare cancerous cells like triple negative breast cancer cells (TNBC), which do not overexpress receptors on its surface. In the present work, we have modified hydrophilic nature of gelatin into amphiphilic by conjugating with oleylamine using genipin as a cross-linking agent. Owing to amphiphilicity, gelatin-oleylamine conjugate (GOC) self-assembles to form micelles in the aqueous medium. NMR, FTIR, and UVVis characterizations were used to identify cross-linkage between gelatin and oleylamine while the results of DLS, confocal and TEM confirmed aggregation of GOC monomers into micelles. Fluorescence measurement has revealed that the critical micellar concentration of GOC was 0.04 ± 0.01 mg/mL. According to DLS measurements, hydrodynamic size, zeta potential, and the polydispersity index of GOC micelles were 230.6 ± 0.4 d. nm, −23.4 ± 0.2 mV and 0.175 ± 0.008, respectively proving its colloidal stability in solution at pH 7.4. Catechin was taken as a model antioxidant drug, and drug encapsulation efficiency of GOC micelle was determined to be 62 ± 3%. The cytotoxicity, fluorescent cell imaging, and flow cytometry analyses revealed that TNBC type cells (MDA-MB-231) internalized drug bound GOC nanocarriers (CT-GOC) and involved in cell cycle arrest through G2/M phase inducing cellular apoptosis. Further, CT-GOC exhibited higher cellular toxicity to MDA-MB-231 cancerous cells but not in normal cells (NIH-3T3). The overall outcomes of physicochemical and biological measurements suggest that the prepared GOC micelles might be a promising drug carrier for novel anticancer agents in TNBC chemotherapy.

KEYWORDS: gelatin, oleylamine, polymeric micelles, TNBC cells


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INTRODUCTION Micelles are one of the nano-carriers, in the biomedical field, to deliver less orally bioavailable drugs preserving their nature, structure, and function.1 An ideal nano-carrier like micelle should have improved biocompatibility, biodegradability, cellular internalization, high drug payload, increased circulation time and targeted delivery.2-4 According to literature reports, polymeric micelles bear advantages over small molecular weight surfactant based micelles regarding better micellar stability, higher drug loading efficiency, and lower toxicity.5-8 The hydrophilic shell of polymeric micelles is a brush-like corona masking the hydrophobic core from the biological milieu. As the hydrophilic surface minimizes protein adsorption and cellular adhesion, polymeric micelles have the ability to evade non-specific capture by the reticuloendothelial system and hence the blood circulation halftime of the carrier is extended.9 The nano-order size of polymeric micelles enables them to accumulate into cell through passive enhanced permeability and retention effect and reverse multiple drug resistance.10,11 Though researchers are aiming for targeted drug delivery nowadays, still, some malignant type cells do not express receptors on its surface and show opposition to receptor-based targeted chemotherapy. For instance, triple negative breast cancer (TNBC), which claims many lives of women every year, do not express estrogen, progesterone and human epidermal growth factor 2 receptors on their cell surfaces like other breast cancer cell lines.12 In the present work, we have formulated gelatin based polymeric micelle from gelatinoleylamine conjugate to encapsulate less orally available and less water-soluble antioxidant drug, catechin and treat non-site specific cancerous cells like TNBC.

Polymeric micelle may be either natural or synthetic.13,14 Among the naturally available biomacromolecules, polysaccharides are the most frequently used biomaterial in designing a drug delivery system. Despite proving biocompatibility and biodegradation, elimination of



polysaccharides have become questionable as there are no enzymes such as chitinase to digest polysaccharides in the bloodstream, which would result in decreased renal filtration.15 Hence, the researchers sought to explore protein based nano-carrier, as they can be quickly degraded and eliminated through the renal route by the actions of systemic proteolytic enzymes.16 The rationale behind choosing gelatin as a protein source in our work is, gelatin is a hydrolyzed denatured product of collagen, which acts as extracellular matrix component of skin, cartilage, and bone in the human body. Also, gelatin is considered to be better than parent collagen in some aspects such as low cost, more solubility and stability in water at physiological pH and temperature.17-19

Even in the human body, a lipid-protein nano-carrier, namely chylomicron is secreted for the transportation of fatty acids and lipophilic compounds into the systemic circulation, forming biological nano or micro-sized mixed micelles, from which hydrolyzed products are taken up by peripheral tissues.20,21 As gelatin is hydrophilic, the macromolecules need to attain amphiphilic state to self-assemble in aqueous solution and hence oleylamine, a hydrophobic surfactant has been cross-linked with gelatin to form gelatin-oleylamine conjugate (GOC) by the use of genipin, an active biological crosslinking agent to conjugate amine-containing polymers.22 When amine head group of oleylamine cross-links with gelatin macromolecules, the remaining hydrophobic chain of oleylamine provides amphiphilic nature to the product, gelatin-oleylamine conjugate. Though many researchers have proposed different pathways to explain the chemical reaction of genipin with primary amine compounds, Toyuma et al. have demonstrated that genipin forms a blue color developing dimer when free radical polymerization occurs between nitrogen-containing iridoid intermediates in the presence of oxygen.23-25


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The model drug, catechin is a plant polyphenolic compound belonging to a subclass, known as flavan-3-ols, in the flavonoid family. Catechin has possessed significant biological functions such as antioxidant, anticancer and anti-inflammatory properties.26 Galloyl and hydroxyl groups of catechin play an essential role in antioxidant properties.27 When orally ingested, galloylation and methylation occur in intestine and liver so that the level of unchanged catechin in bloodstream reduces significantly challenging oral bioavailability.28 The intravenous supply of less watersoluble drugs would aggregate in biological environment that might lead to various complications such as overdose toxicity.29 When the drug molecules are bound into polymeric micelles and administered, the amount of non-aggregated active drugs can show its potential at the sustained level when released into the bloodstream. The primary goal of the current work is to gain a better understanding on physicochemical and biological properties of the gelatin-oleylamine conjugate, regarding conjugation, colloidal stability, in vitro drug encapsulation and release, cell internalization and cytotoxicity.

EXPERIMENTAL METHODS Materials All chemicals were used as received. Gelatin, type B for bacteriology (120 kDa) was purchased from Hi-Media. Oleylamine (≥ 98%), 1, 6-diphenyl-1, 3, 5-hexatriene (DPH), genipin (≥ 98%, (1R,

4aS,

7aS)-methyl

1,

4a,

5,

7a-tetrahydro-1-hydroxy-7-(hydroxymethyl)-4a,

7a-

dimethylcyclopenta [c] pyran-4-carboxylate) and dialysis tubing cellulose membrane (MWCO 2 kDa) were availed from Sigma Aldrich. The solvents used throughout the work were of analytical grade.



Optimization and Preparation of Amphiphilic GOC Amphiphilic gelatin-oleylamine conjugate (GOC) was synthesized through genipin dimer formation between primary amine groups of gelatin and oleylamine using genipin. Gelatin was dissolved in trifluoroethanol (TFE) to have a stock solution of 0.30 mM. Oleylamine (500 mM, EtOH) was added to gelatin at different concentrations under magnetic stirring. The ratios of gelatin and oleylamine in the obtained homogenized solution varied from 1:0 to 1:250 regarding millimolar concentration, as listed in Table S1. Genipin (EtOH) was added to the above solution to have a final concentration of 50 mM and stirred for 12 h at 30 °C. The color of the solution turns light green initially and appears with blue, ensuring completeness of reaction with the formation of the genipin dimer. At the end of the reaction, the conjugate was purified through dialysis process against 50% TFE for 2 h, and the amount of TFE was decreased until the surrounding medium reached 100% water for 24 h. When oleylamine predominantly cross-linked with gelatin macromolecules, the conjugated product self-assembles to settle down at the bottom of the tube during the dialysis process. We have optimized that sample 6, in which conjugated product of gelatin and oleylamine with the ratio of 1:200 is enough to aggregate together to form a lump in water medium as shown in Figure 1a. The aggregate was separated from the supernatant and washed three times using ethanol to eliminate unreacted genipin and oleylamine. The purified sample 6 (marked as GOC) was dried in open air to obtain the solid form. The yield of pure compound GOC was 72%. The synthesis and purification of gelatin-oleylamine conjugate have been shown schematically in Figure 1b. 10 mg of dried GOC was weighed and re-dissolved in TFE. 100 μL of GOC was added to 1.0 mL of bulk water phase to obtain O/W type GOC micelles, photographic image of which are shown in Figure 1c. GOC micelles were subsequently lyophilized for further characterizations.


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Figure 1. (a) The ratio of gelatin and oleylamine has been optimized to obtain the self-assembled product of gelatin-oleylamine conjugate in the presence of the biological cross-linking agent, genipin. When the ratio of the concentration of gelatin and oleylamine is 1:200, the modified gelatin in TFE (trifluoroethanol) has been found to induce self-assembly among amphiphilic gelatin-oleylamine conjugate (GOC) when transferred into the aqueous environment. (b) The cross-linking between gelatin and oleylamine mediated by genipin and subsequent purification through the dialysis process have been demonstrated schematically. The molecular structure of genipin dimeric adduct has also been drawn. (c) Digital photographs of purified GOC in TFE and its micellar solution in water have been indicated by L and R, respectively.

Characterization of GOC Solution 1H NMR measurement of the samples was carried out in deuterated solvent (D2O) using a Bruker Advance III 500 MHz spectrometer. UV-Vis spectra were obtained using Shimadzu UVVisible spectrophotometer UV-1800. Fourier Transform Infrared (FTIR) transmittance spectra were recorded using FTIR spectrophotometer ABB MB 3000 with the measured range of 400 – 4000 cm−1 and accumulation of 32 scans. Before the measurement, the samples were mixed with KBr and pelletized. The critical micellar concentration (CMC) of GOC was determined by fluorescence measurement using DPH as a probe. The strength of GOC varied from 0.001 to 2.0 mg/mL in double distilled water, and 0.004 mM of DPH was incubated with them. The mixture was then sonicated at 30% amplification for 20 s. After 24 h, the fluorescence intensities of the samples were measured at 355/428 nm using a Cary Eclipse Fluorescence Spectrophotometer. As a negative control, the intensities of the samples were measured in the absence of DPH. The differences between the samples with and without DPH were estimated based on the mean


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fluorescence intensities. Colloidal stability of the particles was determined from Dynamic Light Scattering (DLS) measurement using a Zetasizer (NanoZS, Malvern Instruments: Worcestershire, UK). Confocal Laser Scanning Microscope (CLSM) and Transmission Electron Microscope (TEM) images were captured to observe the size and morphology of GOC micelles in liquid and dried states, respectively. In the case of CLSM, the aqueous GOC micelles prepared at a concentration higher than CMC was incubated with fluorescein sodium salt (λem 515 nm). After 6 h, the solution was centrifuged to 12,000 rpm to obtain fluorescein bound GOC micelles separately at the bottom. The supernatant containing unbound fluorescein molecules was carefully removed. The pellet was re-dispersed in water and subsequently was dropped over a glass slide to visualize fluorescein bound GOC micelles using a confocal laser scanning microscopy IX81-FV 1000; Olympus Corporation, Tokyo, Japan. TEM images were captured using a TEM, Philips CM-30, Philips Electron Optics: Eindhoven, Netherlands. Blank GOC micelle solution was incubated with osmium tetroxide for 30 min, and the resulting staining solution was drop cast on a carbon-coated copper grid, before analysis. Drug Encapsulation and Drug loading For the preparation of catechin loaded micelles, 20 mg of amphiphilic GOC was dissolved in 4 mL of TFE. A 2 mL of catechin stock solution was prepared in the mixture of EtOH and TFE (1:1, v/v) and added dropwise to GOC with a final concentration of 4 mM. The mixture was subjected to dialysis against 20% TFE followed by 100% PBS buffer (1X) maintaining pH 7.4. After 24 h, the drug bound micelles (CT-GOC) were found remaining in the suspension. The nonencapsulated drug particles settled down as catechin is weakly soluble in water. Both nonencapsulated drug particles and CT-GOC were lyophilized and weighed using an electronic



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analytical balance, Shimadzu AUX-220. Encapsulation efficiency and drug loading of catechin drugs into the micelles were determined using the following eqns 1 and 2.30 x

Encapsulation efficiency, % = y × 100 x

Drug loading, % = z × 100

(1)

(2)

where x is weight of drugs encapsulated into micelles, which are measured subtracting the dry weight of the non-encapsulated drug from the weight of the total drug added (y). The total weight of polymer and drug, i.e., the amount of CT-GOC is indicated by z. The in vitro drug release experiment was carried out using a dialysis process. The drug bound GOC micelles were dispersed in PBS at a final concentration of 5 mg/mL. The solution was taken in a dialysis bag and immersed in a 10 mL beaker containing 5 mL of PBS buffer. The beaker was placed in a shaking incubator maintaining 100 rpm and 37 °C. The solution in the tube was periodically withdrawn and replaced with fresh PBS at pre-defined time intervals from 3 h to 96 h. To measure the concentration of drug at each data point, 0.5 mL of solution was mixed with the same volume of EtOH for complete dissolution. The absorbance of catechin was measured from UV-Vis absorbance at 278 nm, and the amount of released drug was quantified from the standard curve of catechin at different concentrations in the mixture of EtOH and PBS (1:1, v/v). The experiment was carried out in triplicate. The analysis was performed for the pure drug, catechin, to compare with the result of CT-GOC drug release profile.31,32 The concentration of the pure drug and experimental conditions were similar to that of CT-GOC.


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Cell Culture Conditions

MTT 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay was carried out to evaluate the cytotoxicity of blank GOC and drug bound GOC micelles against normal mouse embryonic fibroblast cells (NIH-3T3) as well as cancerous TNBC cells (MDA-MB-231). The cell lines were cultured and were maintained in Dulbecco’s Modified Eagles medium supplemented with L-glutamine (2 mM) and Earle’s balanced salt solution containing sodium bicarbonate (1.5 g/L), non-essential amino acids (0.1 mM), and sodium pyruvate (1.0 mM) in a humidified atmosphere containing 5% CO2 at 37 C. The exponentially growing cells were seeded into a 96 well plate (1 x 104 cells per well) and allowed to attach approximately 24 h. The cells were then treated with the prepared samples at the concentrations 50, 100, 250, 500, 750 and 1000 μg/mL. After 48 h of incubation with samples, the MTT assay was performed to assess the cellular viability against the prepared materials. The cell viability was determined by measuring absorbance at 620 nm using a spectrophotometer, and the values were expressed in percentage relative to cell controls.

Internalization of GOC into MDA-MB-231 Cells

The MDA-MB-231 cells were grown in 6-well plates (1×105 cells/well) for 24 h and were treated with Fluorescein bound-GOC nanocarriers at their IC50 concentration for 7 h, 24 h and 48 h and then the cells were washed three times with cold PBS. Fluorescein bound GOC micelles were prepared following the procedure as given in CLSM analysis. The accumulation of micelles into the cells corresponding to fluorescence intensities was noticed at different time intervals, which states their internalization behavior into the TNBC cells.



Cell Cycle Analysis

Flow cytometric analysis was carried out to measure intracellular DNA content in the presence of blank and drug bound GOC micelles. The MDA-MB-231 cells (1 × 105) were seeded in each tissue culture dish and allowed to attach for 24 h. Then, the cells were treated with the samples for 48 h. Subsequently, the cells were harvested using trypsin and centrifuged to obtain cells in pellet form at 2500 rpm for 5 min. The cells were re-suspended in 300 μL of PBS-EDTA followed by dropwise addition of 700 μL of chilled 70% ethanol on stirring condition. Subsequently, RNase (20 mg/mL) was added to avoid interference of RNA during measurement, and the mixture was incubated at 37 C for 1 h. Propidium iodide (50 μg/mL) was finally added to the mixture at 25 C. After incubating the mixture for 20 min, DNA histograms and cell cycle phase distribution using flow cytometry (BD, FACS Calibur, USA) were analyzed for the stained cells.

RESULTS AND DISCUSSIONS Confirmation of Cross Linkage 1H

NMR is a useful tool to elucidate conjugation of oleylamine with gelatin macromolecule. As

shown in Figure 2A, oleylamine micelle in D2O has been identified by its characteristic resonances at 5.23 ppm (vinyl, a), 1.93 ppm (allyl, c), 0.80 ppm (methyl, g) and 0.99 ppm (amine, f). Methylene protons have been traced at 2.51 (b), 1.35 (d) and 1.20 (e) ppm levels of the spectrum.33 Gelatin is generally characterized with two different segments, i.e., the aliphatic amino acid region in the range of 0.9 - 3.92 ppm (peaks from 1 to 11) and aromatic amino acid content including Phe and Tyr around 7.2 ppm (peak 12).34 The list of amino acids and corresponding chemical shifts of typical gelatin has been indicated with numbers in Figure 2B. The amino acids Gly (peak 11), Hyp


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(peaks 11 and 6) and Pro (peaks 10 and 5) abundantly present in gelatin have been labeled at their respective chemical shifts of the spectrum. Significant differences have been observed between the NMR spectra of neat gelatin and GOC micelles.

Figure 2. 1H NMR spectra of (A) oleylamine micelles, (B) neat gelatin, and (C) gelatin-oleylamine conjugate (GOC) micelles, D2O are shown with chemical shift (ppm) representing X-axis. The



protons of oleylamine are marked with a - g whereas the amino acids of gelatin are indicated by numbers, 1 - 12. The comparable integral intensities between gelatin and GOC micelles are shown under the spectral lines. The overlapping of oleylamine protons with the amino acid content of gelatin is indicated by the corresponding letter with an asterisk symbol. The spectral line of GOC has broadened in the range of 0.69 - 0.91 ppm (g*), 0.96 - 1.40 (e* and d*) 1.8 - 2.0 (c*) and 2.08 - 2.32 ppm (b*) reflecting protons of hydrophobic chain of oleylamine, which have been supported by the increase in integral intensities when compared to neat gelatin (Figure 2C). In particular, peak 2 indicating proton of Thr residue of gelatin has shown a significant increase in integral intensity (from 1.12 to 12.04), when compared to other proton peaks. Works of literature reveal that spectral line broadens when the micellar structure is formed from monomers.35,36 Nevertheless, amine protons of oleylamine has been disappeared in GOC micelle indicating that the amine group of oleylamine has been involved in chemical cross-linkage. These spectral results suggest that oleylamine groups have been chemically conjugated successfully with gelatin side chains and induced micellar aggregation in aqueous solution. All of these results have been found in close agreement with the previous literature report, which studied the conjugation of the hexanoic acid anhydride with gelatin.37 FTIR spectra have been recorded to investigate the cross-linkage between oleylamine and gelatin in GOC. Free oleylamine has been characterized from CH2 symmetric and CH3 asymmetric vibrations at 2828 cm−1 and 2923 cm−1, respectively, as shown in Figure 3a. A δ CH2 bending vibration mode was observed at 1476 cm−1. Further, the peaks appeared at 1333 cm−1, and 1570 cm−1 indicate υ C–N stretching mode and δ N–H bending mode, respectively.38 Gelatin has been characterized from the appearance of peaks at 1649 cm−1, 1535 cm−1 and 1191 cm−1, which correspond to amide I, II and III, respectively.39 In the case of GOC micelle, complete


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disappearance of amide III peak of gelatin has been observed. The anticipated shift of δ N–H bending of oleylamine, which involves in genipin dimeric cross-linkage, might have been overlapped in the amide I of GOC. These results indicate that gelatin has been cross-linked with oleylamine surfactants effectively.

Figure 3. (a) FTIR spectra of oleylamine, gelatin and gelatin-oleylamine micelle performed with the aid of KBr and (b) UV-Vis spectra of gelatin (H2O), oleylamine (EtOH) and amphiphilic gelatin-oleylamine conjugate (GOC) dissolved in TFE solvent have been shown.

UV-Vis spectra of oleylamine, gelatin and amphiphilic GOC monomer are shown in Figure 3b. The neat aqueous gelatin has shown a peak at 278 nm due to aromatic amino acid residues whereas oleylamine has displayed a peak of about 275 nm, as reported in previous works of literature.40,41



The amphiphilic gelatin (solv, TFE) has presented a peak in the visible range at 594 nm indicating the formation of genipin dimer between the reactive amine groups of gelatin and oleylamine. Micellar Aggregation

The amphiphilic nature of GOC drives them to assemble spontaneously into micelles in the aqueous medium. Determination of critical micellar concentration (CMC) is helpful to find out the strength, in which amphiphilic polymers tend to form micelles. The formation of a micelle is thermodynamically driven by aligning the hydrophobic segments away from the aqueous environment, and the interaction existing between the hydrophobic portions in the core is majorly due to Van der Waals force.5 As shown in Figure 4a, the fluorescence intensity has increased dramatically when the concentration of GOC increases by 0.04 ± 0.01 mg/mL, which corresponds to its CMC value. The literature reports reveal that the fluorophore 1, 6-diphenyl-1, 3, 5-hexatriene (DPH) does not fluoresce in water. When DPH incorporates into hydrophobic region of micelles, it shows strong fluorescence intensity as micelle provides a hydrophobic environment to solubilize them. Hence, there is a significant change in fluorescence intensity at CMC point in which amphiphilic monomers are aggregating into micelles.42


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Figure 4. (a) Determination of CMC through dramatic increase in fluorescence intensities due to accumulation of fluorescent probe 1, 6-diphenyl-1, 3, 5-hexatriene (DPH) into GOC micelles. (b) hydrodynamic size of GOC micelles by DLS measurement. Transmission Electron Microscope (c) and Confocal Laser Scanning Microscope images (d) of GOC micelle have been shown with scale bar 50 nm. Osmium tetroxide, which is a good fixative stain for lipids, darkens GOC micelle by intense black staining during TEM measurement. Fluorescein molecules adhere with hydrophobic tails of GOC micelle and fluoresce green. Due to accumulation into spherical micelle, the bound fluorescein molecules emit fluorescence brighter than unbound molecules. According to DLS measurement, hydrodynamic diameter (DH) of GOC micelle has been observed with 230.6 ± 0.4 d. nm, as shown in Figure 4b. Polydispersity index (PI) and zeta potential (ζ) value have been found 0.175 ± 0.008 and −23.4 ± 0.2 mV, respectively. Generally, colloidal stability of the particles in solution is contributed by the larger and the lesser PDI values, and hence GOC micelles prepared has shown moderate colloidal stability at physiological pH of the solution. The negative sign of zeta potential indicates gelatin macromolecules of GOC micelles are at deprotonated state at pH 7.4. The literature reports reveal that negatively charged drug



particle is preferred for clinical use, as positively charged drug delivery systems could form aggregates in the presence of negatively charged serum proteins. At the same time, drug delivery system without surface charge could reduce plasma protein adsorption and increase the rate of non-specific cellular uptake and hence GOC with more negative charge is a better option as a drug delivery candidate.43 TEM and CLSM measurement of GOC micelles have revealed that the size of the micelle has been around 125 nm, as seen in Figure 4c and 4d. TEM micrograph of GOC micelles in population has been shown in supporting information, Figure S1. Due to the incorporation of fluorescein into the hydrophobic segment of GOC micelles, CLSM images have shown intensified green fluorescence leaving the hollow core portion blank.

Drug Release Study Drug encapsulation efficiency (%) and drug loading (%) of GOC micelle have been studied taking catechin as a model drug. Catechin possessing hydrophobic aromatic moiety can easily adhere to the hydrophobic tail and reside in the inner core of the micelles. It has been reported that the interaction of encapsulated drugs into the micelles can occur through single or multiple noncovalent bonds, that are, hydrophobic, π - π stacking and hydrogen bonding.44 Drug encapsulation and drug loading of GOC micelles were determined to be 62 ± 3% and 5.9 ± 0.2%, respectively.

Du et al. has synthesized stearate-g-dextran at different ratio and has shown doxorubicin drug encapsulation in the range of 38 – 92.2% and drug loading in the range of 3.6 – 14.4%.30 Encapsulation of catechin into GOC micelles has been confirmed through FTIR and UVVis measurements.


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Figure 5. (a) FTIR and (b) UV-Vis absorbance spectra of catechin (25 μM) and drug-loaded gelatin-oleylamine micelles (CT-GOC, 200 μg/mL) have been shown. (c) Cumulative drug release profiles of pure drug catechin and CT-GOC have been represented with mean ± SD, n=3. FTIR measurement has revealed that catechin (CT) possesses bands at 1629, 1149 and 1030 cm−1 indicating aromatic C=C stretching, aromatic C–C stretching and (C–H) in-plane deformation, respectively (Figure 5a). A broad band around 3270 cm−1 represents O–H stretching.45 The drug bound micelle (CT-GOC) has possessed characteristic vibrational bands of GOC micelle (studied in Figure 3a) as well as bands of catechin.



As shown in UV-Vis spectra (Figure 5b), CT-GOC has possessed a peak at 278 nm, which is indicative of aromatic ring of catechin. In vitro drug release profile of pure drug catechin and CTGOC micelles have been shown in Figure 5c. Pure drug catechin has shown 52 ± 1% of drug release at 0.5 h, and there has been an almost complete release of drugs with 98 ± 1.3% at 4 h of study. As far as drug release from CT-GOC is concerned, a combination of both diffusioncontrolled and erosion-controlled mechanisms has been followed.16 The initial phase occurs rapidly while the second phase is a slow and sustained release. The amount of cumulative drug release from CT-GOC has been around 28 ± 0.7%, 42 ± 0.9%, 56 ± 0.3% and 69 ± 0.8% at 6, 12, 24 and 36 h, respectively. After 36 h, the initial phase of diffusion is over, and dissociation with the detachment of hydrophobic tail resumes resulting in sustained release of drugs. At the end of the trial (96 h), the total cumulative amount of drugs released has been 78.5 ± 0.3%. There have been about 9.5 ± 0.2% drug additionally released at a sustained level in the second phase of drug release from CT-GOC.

Evaluation of Cytotoxicity

MTT assay was carried out to evaluate the cytotoxicity of blank and drug-loaded GOC micelles using MDA-MB-231 and NIH-3T3 cell lines, respectively. As shown in Figure 6a and Figure 6b, CT-GOC has triggered more cytotoxic effect than blank GOC in the range of concentration investigated from 50 - 1000 μg/mL against cancerous TNBC cells (MDA-MB-231). GOC has accounted for about 75% cell viability, whereas CT-GOC has exhibited a significant reduction in cell viability to reach up to 10% at the final concentration of the studies, 1000 µg/mL. CT-GOC has produced its IC50 at 100 µg/mL after 48 h of treatment. While measuring cytotoxicity to normal cells, IC50 of CT-GOC has been found above 750 µg/mL, which is significantly higher


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than that found against cancerous cells. This clearly shows that CT-GOC induces more cytotoxicity to cancerous cells rather than normal cells with the presence of the antioxidant drug, catechin. Blank GOC has almost similar cell viability pattern without significant changes against both healthy and cancerous cells.

The cellular uptake efficiency of fluorescein encapsulated GOC (F-GOC) nanocarriers have been investigated in MDA-MB-231 cells using fluorescence microscopy. Significant cellular uptake of micelles into the cells has been observed with green fluorescence intensity due to staining of the cytoplasm by F-GOC (Figure 6c). More substantial the amount of accumulated micelles into the cells, stronger is the fluorescence intensities. Moreover, increase in green ﬂuorescence intensity has been observed in cells with increasing incubation time from 7 h to 48 h, which indicates that GOC is recognized and is being taken up by MDA-MB-231 cells on time-dependent manner.



Figure 6. Evaluation of cytotoxicity of GOC micelles and catechin loaded micelles (CT-GOC) at different concentration 50 - 1000 μg/mL against MDA-MB-231 breast cancer cells (a) and normal fibroblast cells, NIH-3T3 (b). The data was represented with mean ± S.D. of three independent experiments. **P < 0.001 Vs respective blank GOC. (c) Fluorescence microscope images demonstrating permeability of fluorescein bound GOC micelles into NIH-3T3 cells at different period of incubation (7, 24 and 48 h).

Two possible reasons can be attributed for the cellular uptake of GOC micelles, one is receptormediated active permeation, and another one is passive permeation by Enhanced Permeability and Retention effects, as shown in Figure 7. Malignant tumor cells, at the early stage, switch on


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angiogenesis to produce vascular endothelium around the cells for nutrient supply, which overexpress integrin receptors. There are eight types of integrin receptors identified to recognize RGD (Arg-Gly-Asp) motif, in particular, ανβ3 has been reported to be over-expressed on angiogenic endothelium surrounding TNBC cells.12 Gelatin, being an extracellular matrix protein, contains RGD motif numerously.46 As gelatin is present in the exterior portion of GOC micelles in the aqueous medium, it accesses way easily to reach tumor regions and attach onto the receptors and internalize. Subsequently, the micelles from the site of vascular endothelium pave the way entering into TNBC cells through passive permeation. With the capacity of sustained release of drugs, CT-GOC can show its functionality until it evades from the cell.



Figure 7. The fate of catechin loaded gelatin-oleylamine conjugate (CT-GOC) micelles into the microenvironment of malignant triple negative breast cancer (TNBC) cells has been demonstrated. Tumor vasculature is produced around the cancerous cell mass to access nutrients. Though TNBC cells do not overexpress receptors, the surrounding endothelium cells of tumor vasculature overexpress integrin receptors. In particular, the ανβ3 receptor has a specific binding site of RGD (Arg-Gly-Asp) sequence, which is present in gelatin of GOC micelles numerously. While passing into TNBC region, CT-GOC undergoes endocytosis passively to enter forming endosome around the CT-GOC micelles and releases drug at the sustained level.

Effect of CT-GOC in Cell Cycle Regulation

Flow cytometry has been used to analyze cell cycle progression in cells and percentage of each cell phase distribution in the presence of CT-GOC (Figure 8). It has been found that the number of cells in the G2/M phase has increased when the incubation period increases from 7, 24, 48 and 72 h. The number of cells in control has been found decreased in S phase when compared to the number of cells treated with samples. However, the control has shown abundant cells than sample treated cells in G0/G1 phase, which indicates cellular viability. These experimental findings suggest that the compound CT-GOC has induced apoptosis on MDA-MB-231 cells through G2/M cell cycle arrest.


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Figure 8. Cell cycle arrest (a) and cell phase distribution (b) have been demonstrated using flow cytometry analysis. In cell cycle analysis, MDA-MB-231 cells (1 × 106 cells/mL) were treated with drug-loaded GOC micelles (CT-GOC), and the measurement was done at the different period of incubation (7, 24, 48 and 72 h). Each cell phase distribution was provided with percentage (*P < 0.05 Vs control, **P < 0.001 Vs control). The control was the cell line without the addition of the sample.

The induced cell cycle arrest functionality of CT-GOC nano micelles in the regulation of cell cycle progression was indicated by their cellular accumulation at G2/M and S phase on the different period of 7, 24, 48 and 72 h. As determined by FACS results, the number of cells in S and G2/M phases has been markedly reduced inhibiting cancer cell proliferation owing to apoptosis. Moreover, the cellular accumulation of micelles on treated TNBC cells displayed numbers of cells in the G2/M phase as well as limited cells on S phase compared with control cells that might trigger inhibition of cell growth.

CONCLUSIONS

The conversion of amphiphilic GOC into micellar aggregation has been characterized with NMR, UV-Vis, and FTIR. CLSM and TEM measurements have aided to observe GOC micelle above its critical micellar concentration, 0.04 ± 0.01 mg/mL. The results of DLS parameters have revealed that GOC micelles can maintain colloidal stability against aggregation in solution at pH 7.4. Drug encapsulation efficiency of GOC micelles has been found to be more than 60%. The outcome of the present work reveals that GOC micelles have colloidal stability and have been opting for efficient loading of less water-soluble drugs such as catechin. While treating GOC micelles with TNBC cells, cellular uptake of micelle has been observed with the function of time and model


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drug catechin bound GOC micelles has exhibited significant cytotoxicity to the TNBC cells but not to the normal cells at the selected compound concentration, 50 – 1000 μg/mL. These results suggested that the prepared GOC can be helpful as an efficient drug delivery carrier in chemotherapy to treat TNBC type breast cancer. We believe that this preliminary works would promote us for further understanding during our in vivo applications and this will pave us for the way to develop a novel class of drug carriers using gelatin in future research directions.

Acknowledgment We are thankful to the Department of Biotechnology, Government of India, Ministry of Science and Technology for the financial support (project no. BT/PR4406/NNT/28/574/2011). Supporting Information Available A list of gelatin-oleylamine conjugate (GOC) at different ratio and Transmission Electron Micrograph of GOC micelles in population.



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GRAPHICAL ABSTRACT


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Polymeric Micelle of a Gelatin-Oleylamine Conjugate: A Prominent

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