Preparation of PLGA nanoparticles
Solid-in-oil-in-water (s/o/w) dual emulsion solvent evaporation was used to prepare PLGA@5Fu/PFC NPs as previously described [18]. Briefly, 50 mg of PLGA was dissolved in 2 mL of chloroform containing PFCs (2 mg). 5 mg of 5Fu was dissolved in 0.5 mL of aqueous solvent and the solution was mixed with PLGA solution to generate the s/o primary solution. The emulsion was dispersed in 10 mL of aqueous solvent containing 2% w/v PVA to generate the final s/o/w emulsion. Free PLGA/PVA polymers were separated by centrifugation at 3,000 rpm for 15 min. The PLGA NPs were modified with EGF using N’-ethylcarbodiimide hydrochloride (EDC) [19].
Characterization of nanoparticles
The shapes of EGF-PLGA@5Fu/PFC NPs were examined using transmission electron microscopy (TEM) (Hitachi, Tokyo, Japan). A drop of NPs solution (0.5 mg/mL) was mounted on a carbon-coated copper grid. The samples were observed at an acceleration voltage of 75 kV. Size distribution and zeta potential were determined by dynamic light scattering (DLS, Zetasizer Nano ZS, Malvern Instruments Ltd, UK).
Drug encapsulation and in vitro release
The encapsulation efficiency of 5Fu in NPs was determined using a UV-Vis spectrophotometer (1800, Shimadzu, Kyoto, Japan). Briefly, 1 mg of nanoparticles was dispersed in 1 mL of distilled water to extract 5Fu. The solution was shaken gently for 12 h at 37°C, the obtained filtrates were diluted (1:10) with methanol and the solution was analyzed at λmax = 266 nm. Encapsulation efficiency and loading efficiency were calculated using the following equations: (see Equations in the Supplementary Files)
The release profile of 5Fu from NPs was accessed at two pH values (5.0 or 7.4). 10 mg of NPs was dispersed in 10 mL of PBS, then transferred to a dialysis bag, which was placed in 50 mL of media and stirred at 37°C. At predetermined time points, 2 mL of release medium was removed, with replacement. The amount of 5Fu released was determined using a UV-Vis spectrophotometer.
Cell lines and culture
The human colorectal cancer cell line SW620 was obtained from the Chinese Academy of Science (Shanghai, China). The SW620 cells were maintained in Dulbecco's Modified Eagle Medium (DMEM) supplemented with 10% fetal bovine serum (FBS). The cells were cultured in a humidified cell incubator in a 5% CO2 atmosphere at 37°C.
In vitro cell uptake
Human SW620 cells were cultured in 12-well plates at a density of 2 × 104 cells/well. After 24 h, fresh DMEM with Cy5-labeled NPs (EGF-PLGA@5Fu/PFC and PLGA@5Fu/PFC) was added and the cells were cultured at 37°C for 2 h, 4 h or 6 h. The cells were then washed with PBS, fixed using 4% paraformaldehyde and stained using DAPI. Finally, the cells were visualized using a confocal laser scanning microscope (CLSM).
Cell cytotoxicity
Human SW620 cells were cultured in 96-well plates at a density of 2 × 103 cells/well. After 24 h, different formulations of NPs were added to the wells and the cells were cultured for 48 h at 37°C. Then, 10 μL of CCK-8 was added to each well, and the cells were incubated for an additional 4 h. Optical density was determined at 450 nm using a microplate reader (BioRad, Hercules, CA, USA).
Hoechst 33342 staining
Human SW620 cells (1×104 cells/well) were seeded in 24-well plates, then treated with different formulations of NPs for 48 h. Then, the cells were washed with PBS three times and stained with Hoechst (2 μg/mL) for 20 min at room temperature. Stained cells were visualized using a fluorescence microscope (Nikon TE2000; Nikon Corporation, Tokyo, Japan) (magnification, ×100).
Cell apoptosis
Human SW620 cells (1×105 cells/well) were cultured in 6-well plate with different formulations of NPs for 48 h. Then, the cells were washed twice with PBS, and suspended in staining buffer containing propidium iodide (PI) (1 μg/mL) and annexin V-FITC (0.025 μg/mL) for 15 min at room temperature. Apoptotic cells were evaluated using a FACScalibur flow cytometer (BD Bioscience, Franklin lakes, NJ).
Animal model
Female BALB/c mice (6-8 weeks, 20-22 g) were obtained from the Animal Laboratory of Nanjing University and were kept in standard conditions with humidity maintained at 50%-60%, temperature controlled at 25 ± 2°C, a 12-h dark/light cycle and free access to water and food. All animal experiments were performed in accordance with the Guidelines for Care and Use of Laboratory Animals of the University of Science and Technology of China and approved by the Animal Ethics Committee of Nanjing Medical University.
Human SW620 cells (3×106) were resuspended in 100 μL of PBS and injected into the right flanks of the mice. When the average tumor volume reached about 100 mm3, the mice were randomly assigned to 6 groups (n=8) and administered with saline, blank NPs (100 mg/kg), 5Fu (8 mg/kg), PLGA@5Fu (8 mg/kg of 5Fu), PLGA@5Fu&PFC (8 mg/kg of 5Fu) or EGF-PLGA@5FU&PFC (8 mg/kg of 5Fu) once every two days. Tumor size was measured and tumor volume was calculated as follows: volume (cm3) = Length (L) × Width2 (W2)/2. At the end of the study, the mice were sacrificed by intraperitoneal injection of sodium pentobarbital (100 mg/kg), and the tumor and major organs were isolated for further analysis.
In vivo fluorescence imaging
At 24 h after injection, the mice were sacrificed and the biodistribution of the different formulations of drugs in the tumors and major organs was determined using a fluorescence imaging system.
Histological analysis
Mouse tissues from the mice were fixed in 10% formalin, embedded in paraffin and cut into 5-μm sections. The sections were stained with hematoxylin and eosin (H&E) to evaluate the histological changes in the tumors and major organs. For TUNEL assay, tumor tissues were stained using an in-situ apoptosis detection kit (Thermo Fisher Scientific) according to the manufacturer’s instructions. For immunohistochemical staining (IHC), tumor tissues were incubated with the primary antibody against Ki-67 (ab15580, Abcam). Visualization of H&E and IHC staining was performed using a light microscope, and for TdT-mediated dUTP nick end labeling (TUNEL) was evaluated using a fluorescent microscope.
Immunofluorescence staining
At the end of the study, the mice were intravenously injected with pimonidazole (60 mg/kg) for hypoxia staining. After 90 min, tumors were collected, embedded in paraffin, and cut into 8-μm sections. The tumor tissues were then incubated with the primary antibody against pimonidazole (1:200, Hypoxyprobe-1 Plus Kit, Hypoxyprobe, Burlington) at 4°C overnight, then incubated with Alexa Fluo® 488 conjugated goat-anti-mouse antibody (1:500, ab150113, Abcam) for 1 h at 37°C. Nuclei were counterstained with DAPI for 3 min. Images were captured using a fluorescence microscope (magnification, ×200).
Statistical analysis
All data were analyzed using GraphPad Prism 5.0 and presented as mean ± standard deviation. One-way ANOVA analysis followed by Tukey’s post hoc test was used to compare differences between among multiple groups. P<0.05 was considered statistically significant.