200,000+ products from a single source!
sales@angenechem.com
Home > Synthesis and characterization of PTS nanomicelles
Synthesis and characterization of PTS nanomicelles
1. Synthesis and characterization of PTS nanomicelles
A ROS-sensitive PEG conjugate, PTS, was synthesized using a series of chemical reactions. First, the TL linker with terminal carboxylate end groups was synthesized in a reaction between anhydrous 3-mercaptopropionic acid and acetone. The synthesized TL linker was then conjugated to PEG-AM using the EDC/NHS reaction between the carboxylic group of TL and the amino group of PEG-AM.
Finally, PEG-TL was conjugated to stearamine (C18) through the EDC/ NHS reaction between the carboxylic group of PEG-TL and the amino group of C18. The composition of resulting PTS was characterized by 1H NMR and FT-IR spectroscopy. Fig. 1a showed the 1H NMR spectra of PEG-AM, TL, PEG-TL, C18, and PTS polymer. Characteristic 1H NMR peaks at δ 0.83 and 1.23 ppm corresponded to terminal methyl and methylene protons of C18, respectively. The proton peaks of δ 2.3 and 3.60 ppm were attributed to –SCH2- of TL and O–CH2–CH2- of PEG block, respectively. The intensities of C18 peak and PEG peak were calculated to determine degree of substitution (DS %) of C18 in PTS polymer, which was found to be 72.79%. Conjugation of C18 and PEG on both terminal ends of TL was further confirmed by FT-IR analysis. As shown in Fig. S1, FT-IR spectra of PTS polymer demonstrated the characteristic peak at 1250 cm−1, 1527 cm−1, and 2680 cm−1corresponding to S–C bond (stretching), C]O double bonds (stretching) of the amide bond, and H–S bond (stretching), respectively. The peak at 2923 cm−1 belongs to the C–H bond (stretching). The MWs of PEG-AM and PTS were compared by GPC measurements using PEG standard samples. The MW of PEG-AM (Mn = 1800) increased to Mn = 2164 after conjugation with TL and C18. The CMC of PTS was
calculated to be 0.2 mg mL−1. The formation of nanomicelles was further confirmed by 1H NMR (Fig. S2). The disappearance of the δ (ppm) 0.83 and δ (ppm) 1.23, corresponding to -CH2CH2CH2- and -CH2CH3 of C18, respectively, in D2O clearly demonstrated the self- assembled structure of PTS that consisted of a hydrophobic C18 core domain and a highly solubilized PEG shell.
2. Preparation of DOX/PhA PTS nanomicelles
Different feed ratios of DOX and PhA to PTS were used to prepare PTS-DP. The drug loading content and encapsulation efficiency were calculated using the spectrophotometric analysis and summarized in Table S1. Average hydrodynamic diameter and zeta potential of PTS-DP were determined using the DLS analysis (Fig. 1b). The hydrodynamic diameters of PTS-DP (1:1), PTS-DP (1:2), and PTS-DP (2:1) were found to be 112.58 ± 19.23 nm, 104.65 ± 27.36 nm, and 140.50 ± 15.19 nm, respectively, whereas the average zeta potential values were calculated to be −5.772 ± 2.14 mV, −3.033 ± 2.17 mV, and −1.61 ± 1.17 mV, respectively. The DOX- loaded nanomicelles (PTS-D 1 and PTS-D 2), were observed to have relatively larger particle sizes and poor colloidal stabilities as well. It should be noted that PTS could more efficiently self-assemble in the presence of both hydrophobic DOX and PhA drugs, making it possible to achieve nanomicelles with smaller size and better colloidal stabi- lities. Although the reason for the enhanced stability and encapsulation of DOX in presence of PhA is not clearly understood, it might be due to the presence of π-π stacking between DOX and PhA. PTS-DP (1:1) (Drug loading content: DOX = 17.24 ± 0.53%, PhA = 13.43 ± 2.04%) represented as PTS-DP had the best homogeneity of size distribution and was chosen as optimal nanoparticles for further studies. The ana- lysis of FE-TEM images of PTS-DP (Fig. 1c) demonstrated isolated and mono-dispersed spherical nanoparticles with a smallest size of 85.26 ± 3.61 nm. The UV spectrum (Fig. 1d) of PTS-DP exhibited the absorption peaks at 670 nm and 480 nm, which are the characteristic peaks for PhA and DOX, respectively, indicating that both drugs, PhA and DOX, were successfully encapuslated into PTS nanomicelles with good stability in DW.
DMA was used as a probe to detect singlet oxygen generated by PTS- DP upon laser irradiation. As shown in Fig. (S3a), PTS-DP induced a significant decline in the fluorescence intensity of DMA as a function of irradiation time. However, there was no observed change in the fluor- escence intensity of DMA without irradiation (Fig. S3b). It can be considered that the irradiated PTS-DP could actively generate the singlet oxygen (1O2) that reacts irreversibly with DMA [24]. Ellman's test was performed to evaluate the photo-triggered degradation of TL linker in presence of PTS-DP. The ROS-mediated cleavage of TL bond converts it into two thiol terminal groups [32]. As shown in Fig. (S4a), the concentration of free thiol increased with the increasing irradiation time. To further investigate the photo-triggered release of DOX, we studied fluorescence spectra of DOX at different irradiation time points (Fig. S4b) and observed an increase in the fluorescence intensity with an increase in the laser irradiation time. FE-TEM assisted in analyzing the morphology changes (Fig. S5) and revealed the degradation of PTS- DP upon laser irradiation. From the above results, it could be speculated that laser irradiation of PTS-DP causes generation of ROS from PhA, which, in turn, facilitates the release of DOX via cleavage of TL bond within the PTS-DP.
We next performed an HPLC analysis to investigate the photo-trig- gered release of DOX and PhA from nanomicelles at different time in- tervals upon laser irradiation in the presence of H2O2. As shown in Fig. 1e and f, DOX and PhA release from PTS-DP occurred in an ROS and irradiation-dependent manner. This is also evident from the fact that as compared with the non-irradiated sample, combination of H2O2 and laser irradiation resulted in a faster release of DOX and PhA from PTS-DP. At 100 μM H2O2, a concentration similar to the intracellular ROS level within cancer cells [33], a higher release of DOX and PhA from PTS-DP compared to PTS-DP without H2O2 was observed. The combination treatment of PTS with 100 μM H2O2 and laser irradiation, an enhanced release of both DOX and PhA was observed. The in vitro drug analysis suggest that PTS-DP demonstrates both ROS cascade and laser irradiation responsive release of DOX and PhA specifically in tumor microenvironment. The higher intracellular ROS levels within tumor facilitated the initial release of DOX and PhA. Subsequent laser irradiation further increased the ROS level, which in turn causes rapid dissociation of ROS-responsive TL linkage in situ, resulting in in- stantaneous destabilization of nanomicelles leading to an enhanced release of DOX and PhA via degradation of the TL linkage [27]. Therefore, it can be speculated that encapsulation of PhA is a pre- requisite for the photo-triggered drug release through the degradation of TL.
3. In vitro chemo-photodynamic therapy and ROS generation
We observed that the enhanced release of DOX from PTS-DP upon laser irradiation resulted in an amplified synergistic anti-cancer effect. To demonstrate this, CT26 cells were incubated with PTS-DP in various concentrations of DOX and PhA for 4 h. After incubation and removal of the non-internalized nanomicelles, cell viability was determined using the CCK-8 assay (Fig. 2a). We observed a decline in the cell viability upon laser irradiation of PTS-DP implying that exposure of en- capsulated PhA to laser irradiation for 5 min, heightened ROS pro- duction, accelerated the disassembly of nanomicelles and release of DOX, resulting in a synergistic anti-cancer effect. In order to further confirm the synergistic anti-cancer effect C.I was calculated. The CI value was < 1 in cells treated PTS-DP + Laser containing 0.53 μg mL−1 doxorubicin and 0.41 μg mL−1 PhA (CI = 0.127), 1.06 μg mL−1 dox- orubicin and 0.81 μg mL−1 PhA (CI = 0.252), 2.13 μg mL−1 doxor- ubicin and 1.63 μg mL−1 PhA (CI = 0.439), 4.25 μg mL−1 doxorubicin and 3.25 μg mL−1 PhA (CI = 0.619) and 8.5 μg mL−1 doxorubicin and 6.5 μg mL−1 (CI = 0.828) PhA (Fig. S6). These results indicated that PTS-DP have a synergistic effect upon laser irradiation. To further confirm the highly effective ROS generation by encapsulated PhA, we employed the fluorescent probe DCFDA to quantify intracellular ROS generation (Fig. 2b). Upon laser irradiation for 2 min, a significant amount of ROS was generated in CT26 cells as compared to control cells clearly demonstrating the ROS-dependent antitumor potential of PTS- DP.
As caspase 3 serves as a marker for cellular apoptosis, it has been often been used to identify the increase in programmed cell death during PDT [34]. To confirm the increase in the caspase-3 levels during PDT and to uncover the mechanism of combined chemo-photodynamic therapy, Western blot analysis was performed. As shown in Fig. 2 c & d, caspase-3 protein levels were significantly up-regulated in the PTS- DP + Laser group than DOX + Laser and PhA + Laser. All these suggest that PTS-DP upon laser irradiation increases the ROS level with rapid micelle disassembly causing higher DOX release, leading to greater cellular toxicity through apoptosis.
4. Cellular uptake
To examine the endocytic pathway and intracellular DOX release from PTS-DP, CLSM was employed (Fig. 3a). We observed that free DOX rapidly diffused into the nuclei, as evident from the strong fluorescence intensity in the nuclei. In non-irradiated PTS-DP, a low fluorescence intensity of DOX was observed before irradiation. The initial low DOX fluorescence intensity could be due to the disassembly of endocytosed nanomicelles in response to intrinsic level of ROS, whereas laser irra- diation resulted in enhanced ROS production from PhA, accelerating the disassembly of nanomicelles and the rapid release of DOX and PhA (Fig. 3b and c). In order to investigate the lysosomal escape of PTS-DP upon laser irradiation, live cell microscopy was performed using lyso- some tracker. Lysotracker green DND- 26 stains the acidic lysosomal compartments in live cells. As shown in (Fig. 4), the absence of lyso- tracker green DND- 26 stains in the cells treated with PTS-DP and ir- radiated with laser demonstrate that destabilization of lysosome and the lysosomal escape of PTS-DP upon laser irradiation. Thus, the phe- nomenon of disassembly of nanomicelles and release of DOX could be summarized as follows: firstly, endocytosis-mediated internalization of drug-loaded nanomicelles is followed by their encapsulation into lyso- some/endosome. Secondly, elevated cellular ROS levels both from natural origin and laser irradiation cause the internalized nanomicelles to disassemble via degradation of TL, further accelerating the release of DOX.
5. Biodistribution, biocompatibility and pharmacokinetics
To monitor the in vivo distribution of PTS-DP were injected in- travenously injected into CT26 tumor-bearing mice. When the tumor volume reached 100 mm3, free PhA and PTS-DP were injected, and time-dependent accumulations were monitored using a bio-imaging system. As shown in Fig. 5 a, till 3 h post injection, PTS-DP was dis- tributed throughout the whole body. At 6 h post injection, it was ob- served that the PTS-DP started to accumulate with the tumor region. The fluorescence intensity continuously increased up to 12 h post-in- jection in a time-dependent manner, demonstrating a higher accumu- lation of PTS-DP at the tumor region than PhA. The enhanced accu- mulation of PTS-DP might be attributed to the prolonged circulation stability of PEG and nanometer-scale of the nanomicelles, promoting preferential accumulation inside the tumor via an enhanced perme- ability and retention (EPR) effect [35]. Twenty-four-hour post-injec- tion, ex vivo analysis of the excised organs was performed. Ex vivo fluorescence imaging of tumor showed a high fluorescence signal compared to other off-target organs (Fig. 5b). As shown in Fig. 5c and d, compared to positive control, PTS-DP does not any hemolysis sug- gesting that PTS-DP possesses excellent biocompatibility in mice blood. Next we determined quantitatively the DOX concentration in the plasma of mice i.v. injected with DOX or PTS-DP. As shown in Fig. 5e, PTS-DP displayed a prolonged circulation time in blood than free DOX, which could be attributed to the presence of the PEG outer layer in PTS- DP. We believe that once the nanoparticles are delivered to the tumor interstitial compartments, the illumination of PTS-DP with a 670-nm laser will generate ROS. The locally produced ROS in turn cleaves the TL linker on the PTS backbone, resulting in the rapid disassembly of the PTS-DP and rapid release of DOX from the nanomicelles. As shown in Fig. 5f, mice in the PTS-DP + laser group displayed a higher DOX ac- cumulation in the tumor compared to those in DOX- and PTS-DP- treated groups. This finding could be a result of enhanced disassembly of nanomicelles upon laser irradiation, leading to the rapid release of DOX. These results suggest that PTS-DP possesses prolonged stability in blood circulation, enhanced accumulation in the tumor region, and specific release of DOX in the tumor region, thus causing reduced toxicity in other major organs.
6. In vivo chemo-photothermal therapy and antitumor immunity
We further investigated the locoregional antitumor potential of PTS- DP. CT26 tumor-bearing mice were randomly grouped into eight groups (n = 4) and intravenously injected with PBS, DOX, PhA, and PTS-DP. Of these, four groups (PBS + laser, DOX + Laser, PhA + L aser, and PTS-DP + Laser) were subjected to laser irradiation (670 nm laser, 100 mW cm−2 for 15 min). Tumor volume of CT26 tumor-bearing mice were monitored in each group for 18 days (Fig. 6a). We observed no- ticeable inhibition of tumor volume in mice in the PTS-DP + Laser group compared to those in other groups. The tumor volume of mice in the PBS group rapidly increased to 20-fold compared to the volume at day 0. Compared to free DOX and PTS-DP, the mice in the group with laser irradiation (PTS-DP + Laser) demonstrated an inhibition of tumor growth, which illustrated the synergistic effect of PTS-DP + Laser treatment due to the combined effect of the rapid release of DOX upon nanomicelles destabilization and enhanced ROS production by PhA (Fig. S7). Eighteen-day post-injection, all mice were sacrificed, and tumors from each group were collected and average tumor weights were measured (Fig. 6b). The average tumor weight and tumor images further confirmed that PTS-DP could significantly inhibit the tumor growth upon laser irradiation. We also evaluated the biosafety of PTS- DP after day 18 post treatment. No significant reduction in the body weight was observed in PTS-DP-treated samples, confirming its bio- compatibility and non-toxicity (Fig. 6c), as was also evident from the tumor images (Fig. S8).
Among different types of immune cells, T cells play a major role in immune surveillance. To checking the T cell-mediated response in the tumor region, frozen sections of the tumor region were stained with anti-CD8+ T cell antibody and anti-CD4+ T cell antibody, followed by visualization by fluorescence microscopy. As shown in Figs. 6d and S9, a significant increase in the number of CD 8+ T cells and CD 4+ T cells was observed in mice in the PTS-DP + laser group as compared to those in other groups. We hypothesized that laser irradiation generated an anti-tumor immune response in the irradiated region as a result of acute inflammation, thereby enhancing the antigen presentation to T cells. Apart from tumor cells, macrophages are a key component of stroma and these macrophages play a central role in tumor progression, inva- sion, and metastasis. Various signals in the tumor microenvironment can induce the polarization of tumor-associated macrophages (TAMs) into M1 and M2 macrophages. Inflammatory M1 macrophages are tu- moricidal, whereas non-inflammatory M2 macrophages are im- munosuppressive and promote tumor progression. A study reported that the inherent phagocytic property of TAM led to phagocytosis- mediated accumulation of nanoparticles in TAM [36]. To investigate the infiltration of tumoricidal M1 macrophages into the tumor region, immunofluorescence analysis was performed. The number of M1 mac- rophages significantly increased in mice in the PTS-DP + laser group compared to those in other groups. Together, these observations sug- gest that enhanced chemo-PDT therapy not only eliminates primary tumor cells, but also generates tumor-associated antigens, if engulfment by the antigen-presenting cells triggers robust antitumor immunity.
To investigate therapy-induced histological changes, im- munohistochemical analysis of the tumor region was performed (Fig. 7). Samples in all groups, except in the PTS-DP + laser group, exhibited intense Ki-67 staining, indicating higher cellular proliferation as compared to those in the PTS-DP + laser group. Similarly, in tumor tissues stained with H&E, greater tumor cell death was found in the PTS-DP + laser group compared with other groups. Thus, the H&E staining confirmed the locoregional therapeutic efficacy of the nano- carrier.
After treatment, major organs (heart, lungs, liver, spleen, and kid- neys), the tissue samples were collected, sectioned, and analyzed using H&E staining (Fig. S10). Normal histological structures were observed in the PBS-treated group; however, DOX-treated samples demonstrated significant myocardial damage. No significant myocardial damage was observed in the PTS-DP and PTS-DP + laser groups. All other organs showed similar morphology with that of the PBS groups, suggesting that PTS nanomicelles could specifically reduce the DOX-induced car- diac toxicity.
Angene offers products at competitive prices:
© 2019 Angene International Limited. All rights Reserved.