2.1 Materials
PTX, 2-nitroimidazole and N-Boc-6-bromohexylamine were purchased from Aladdin Reagent (Shanghai) Co., Ltd. GEM, N-hydroxysuccinimide and N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride were purchased from Maclean's Biochemical Technology Co., Ltd. Dimethyl sulfoxide was purchased from Sinopharm Reagent (Shanghai) Co., Ltd. Phosphate buffered salt solution (PBS) and DMEM were purchased from Gibco (USA). Fetal bovine serum was purchased from BI.
MDA-MB-231 cells were obtained from Prof. Xiyun Deng's group at the School of Medicine, Hunan Normal University in September 2021.
2.2 Synthesis of ANI
2-Nitroimidazole (150 mg; 1.33 mmol) and K2CO3 (280 mg; 2.03 mmol) were dissolved in dimethylformamide (DMF). Then, N-Boc-6-bromohexylamine (390 mg; 1.39 mmol) was added to the DMF solution, and the reaction was stirred at 80°C for 4 h. The solid impurities were removed by filtration and washed with methanol. The solid product was obtained by washing with methanol and evaporating the residual solvent. The above solid was suspended in deionized water and extracted with ethyl acetate, and the organic layer was collected and spun to obtain Boc-protected 6-(2-nitroimidazolyl)hexylamine (QANI). This product was redissolved in methanol, and 5 mL of 1.25 M HCl was added to the methanol solution and stirred at room temperature for 24 h. Afterward, the product ANI was obtained by removing the solvent from the reaction mixture using a rotary evaporator.
2.3 Synthesis of HA-ANI
Sixty milligrams of HA (molecular weight of approximately 30 kDa) was dissolved in water, to which 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDCI) (107.35 mg; 0.56 mmol) and N-hydroxysuccinimide (NHS) (64.40 mg; 0.56 mmol) were added and stirred at room temperature for 1 h. ANI (30 mg; 0.14 mmol) was added to the mixture, and the reaction was carried out at room temperature for 24 h. The reaction mixture was transferred to a dialysis bag (MW = 3000) for dialysis, and the product HA-ANI was obtained by lyophilization after clean solvent dialysis and prefreezing at -20 ℃.
2.4 Preparation of P/G NPs
Approximately 10 mg of HA-ANI polymer was dissolved in DMSO and stirred at 40°C for 2 h. Approximately 5 mg of PTX and 2 mg of GEM were predissolved in appropriate amounts of DMSO and then mixed with HA-ANI solution and stirred at 40°C for 4 h. The liquid mixture was then transferred to a dialysis bag (MW = 3000) and dialyzed with deionized water to completely remove DMSO. After prefreezing at -20 ℃, the lyophilization was performed in a freeze-dryer to obtain P/G NPs. HA-ANI NPs and HA-ANI@Cy5 NPs were prepared by the same method.
2.5 Characterization of P/G NPs
The structures of HA, ANI and HA-ANI were characterized by infrared spectroscopy and nuclear magnetic resonance hydrogen spectroscopy (1H-NMR); the size, potential and polymer dispersity index (PDI) of the nanoparticles were detected by dynamic light scattering (DLS); and the morphology of the nanoparticles was observed by transmission electron microscopy.
2.6 Stability of P/G NPs
The P/G NPs were stored at room temperature and monitored for changes in particle size, potential, and PDI on days 1, 3, 5, 7, 14, and 21.
2.7 Drug loading capacity and encapsulation efficiency
Five milligrams of paclitaxel and gemcitabine were dissolved in 25 mL of DMSO to obtain a standard stock solution of 200 µg/mL. Then, a series of solutions with different concentrations were prepared using the dilution method to draw the standard curve. The absorbance of P/G NPs was measured by UV spectrophotometry, and the drug loading and encapsulation rate were calculated using the standard curve equation.
Drug loading capacity[\(\text{L}\text{C}\left(\text{%}\right)]=\frac{{\text{W}}_{\text{l}\text{o}\text{a}\text{d}\text{e}\text{d}}}{{\text{W}}_{\text{P}/\text{G} \text{N}\text{P}\text{s}}}\times 1\)00%
\(\text{E}\text{n}\text{c}\text{a}\text{p}\text{s}\text{u}\text{l}\text{a}\text{t}\text{i}\text{o}\text{n} \text{e}\text{f}\text{f}\text{i}\text{c}\text{i}\text{e}\text{n}\text{c}\text{y}\left[\text{E}\text{E}\right(\text{%}\left)\right]=\frac{{\text{W}}_{\text{l}\text{o}\text{a}\text{d}\text{e}\text{d}}}{{\text{W}}_{\text{a}\text{d}\text{d}\text{e}\text{d}}}\times 1\) 00%
2.8 In vitro drug release
First, 1 mL of 1 mg/mL P/G nanoparticles was precisely measured into each dialysis bags (MW = 3000). The bags were then placed in pH 7.4 PBS, and pH 7.4 PBS containing 1 mM NADPH, 1 mL of 1 mg/mL PTX and 1 mL of 1 mg/mL GEM was placed in dialysis bags (MW = 3000). The dialysis bags were then placed in pH 7.4 PBS and shaken at 37°C and 100 rpm, protected from light. A 3 mL sample of release medium was taken at 0, 0.083, 0.25, 0.5, 1, 2, 4, 8, 12, 24, 36, 48, and 72 h, while the same volume of release medium was used to replenish the tube. A UV‒Vis spectrophotometer was used to determine the release of PTX and GEM, and the release rates were calculated separately according to the following equations.
$$\text{Q}\left(\text{%}\right)=\left({C}_{n}\times V+{V}_{n}\sum _{t=0}^{n}{C}_{i}\right)/\text{W}\ast \text{L}\text{C}\left(\text{%}\right)$$
W is the total weight of nanoparticles, Cn is the sample concentration at Tn, V is the total volume of release medium, Vn is the sample volume, and Ci is the sample concentration at Ti (i = 0,0.083,0.25...... hours, V0, C0 is equal to 0).
2.9 Cell culture
MDA-MB-231 cells were selected as the model cell line, and the cells were cultured in DMEM containing 10% FBS and 1% PS in an incubator at 37 ℃ and 5% CO2.
2.10 Cell uptake
MDA-MB-231 cells at the exponential growth stage were seeded in 6-well plates (2 mL, 5×104 cells/well) and incubated at 37°C and 5% CO2 for 24 h until the cell wall fusion rate reached 80%. HA-ANI@Cy5 NP-treated cells were placed in a cell culture incubator at 37°C and 5% CO2 for 2 h, 4 h and 6 h, and then the culture medium was aspirated and discarded. The cells were washed gently with PBS 3 times and fixed with 4% paraformaldehyde for 15 min, after which they were stained with DAPI staining solution for 5 min, and images were taken under an inverted fluorescence microscope.
2.11 In vitro cytotoxicity
Cytotoxicity studies were performed by the MTT method. MDA-MB-231 cells at the exponential growth stage were grown in 96-well plates (100 µL, 5000 cells/well) and incubated at 37°C and 5% CO2 for 24 hours until the cells were completely adhered to the wall. The cells were treated with serial concentrations of free PTX, free GEM, free PTX/GEM or P/G NPs and incubated in a cell incubator at 37°C and 5% CO2 for 48 hours. For the control group, an equal volume of medium was added to continue the culture. Forty-eight hours later, 20 µL of 5 mg/mL MTT solution was added to each well, and the culture was continued for 4 h. After careful aspiration and discarding of the culture solution, 150 µL of DMSO was added to each well, and the cell survival rate was calculated by determining the OD value of each well at 570 nm with a multifunctional enzyme marker after shaking for 15 min at room temperature.
2.12 Cell migration
Cell migration was studied by scratch assay. MDA-MB-231 cells at the exponential growth phase were seeded in 6-well plates (2 mL, 1×105 cells/well) and incubated at 37°C with 5% CO2 for 24 h until the cell wall fusion rate reached 100%. Cells were incubated in the wells with a 10 µL pipette tip, and the scratched cells were rinsed with PBS, followed by the addition of serum-free DMEM containing P/G NPs (CPTX=0.16 µM; CGEM=0.057 µM), PTX (0.16 µM), GEM (0.057 µM), and PTX/GEM (CPTX=0.16 µM; CGEM=0.057 µM). The cells were incubated in a constant temperature incubator. Cell images were taken at 0, 6, 12 and 24 hours.
2.13 Cell apoptosis analysis
Apoptosis was studied by the AO/EB double-staining method. MDA-MB-231 cells at the exponential growth phase were seeded in 6-well plates (2 mL, 5×104 cells/well) and incubated at 37°C with 5% CO2 for 24 hours. Then, 200 µl of P/G NPs (CPTX=0.16 µM; CGEM=0.057 µM), PTX (0.16 µM), GEM (0.057 µM), or PTX/GEM (CPTX=0.16 µM; CGEM=0.057 µM) was used to treat cells and the same volume of PBS was added to the control group. Forty-eight hours later, the cells were collected, the density was adjusted to 1×106 cells/mL, the prepared AO/EB staining solution was added and incubated at room temperature for 15 min, and images were obtained by inverted fluorescence microscopy.
2.14 Statistical analyses
Prism software was used for statistical significance analysis. The signal indicated a significant difference (*: P < 0.01, **: P < 0.005, ***: P < 0.001, ****: P < 0.0001).