Generally, Pt-1 was prepared by simply mixing cisplatin and porphyrin in two days [26]. The as-prepared cationic Pt-1 owns one porphyrin core for ROS generation and PDT as well as four cisplatin-like units for chemotherapy. The successful synthesis of Pt-1 was confirmed by 1H NMR (Additional file 1: Fig. S1–S3). To promote drug loading and intracellular-triggered release of Pt-1, P1 was designed with ROS sensitive thioketal linkages in the polymer main chain [27]. Subsequently, P1 was used to encapsulate Pt-1 via electrostatic interactions. As shown in Fig. 1A, NP@Pt-1 had a spherical morphology and a comparable hydrodynamic diameter at∼150 nm with a narrow particle size distribution (polydispersity index (PDI): 0.089) (Fig. 1B).
To find out whether the drug encapsulation could change the light responsiveness of Pt-1, UV−vis spectra of Pt-1 and NP@Pt-1 were further recorded. Compared with free Pt-1, NP@Pt-1 showed an 8 nm blue shift and a broader peak width, possibly indicating the aggregation of Pt-1 inside the polymeric core of NP@Pt-1 (Fig. 1C). Furthermore, to visualize and quantify the uptake of nanoparticles by tumor cells, we then investigated the intracellular uptake of NP@Pt-1 via confocal laser scanning microscope (CLSM). CT26 colon cancer cells, a widely used murine cancer cell line of colon adenocarcinoma, were treated with Cy5.5 labelled NP@Pt-1. Subsequently, the cells were visualized via CLSM. As shown in Fig. 1D, the blue color came from the nuclei stained with DAPI, while the red color and green color came from Cy 5.5 labeled NP@Pt-1 and the cytoskeleton stained with Alexa 488, respectively, indicating the distribution of NP@Pt-1 in the cell cytosol. Within 1 h of treatment, NP@Pt-1 was internalized by CT26 cancer cells as shown by the intracellular red fluorescence. Moreover, the red fluorescence was found to be steadily increased within 7 h, which indicated a time-and energy-depended cellular uptake of NP@Pt-1. This was further confirmed by flow cytometry (Fig. 1E, F). As there are Pt atoms in NP@Pt-1, it is possible to quantify the intracellular uptake by ICP-MS. Therefore, CT26 cells treated with Pt-containing drugs at various time points was extracted and the Pt in the cells was tested. As shown in Fig. 1G, after 7 h treatment, the Pt uptake in the CT26 cells increased from 3.8 ng Pt/million cells to 9.5 ng Pt/million cells (Fig. 1G), which was also in accordance with the flow cytometry results, suggesting the time-dependent efficient cellular uptake of the NP@Pt-1. Interestingly, Pt-1 exhibited higher intracellular colocalization than NP@Pt-1 possibly due to the positive charge after chelating with cationic porphyrin, which may result in undesirable toxic effect in vivo [28].
To further investigate the anticancer efficacy of NP@Pt-1 in vitro, the cytotoxicity of the different formulations of NP@Pt-1 was tested via an MTT assay. As shown in Fig. 2A, the viability of CT26 cells remained nearly 80% after incubation with Pt-1 and NP@Pt-1 at a concentration ranging from 0.0025 to 20 μM without light irradiation, while 50% of CT-26 cells were killed at 20 μM of cisplatin, indicating lower toxicity of the Pt-1 itself [26]. Notably, only after 15 min of light irritation (420 nm, 6.95 J cm−2), NP@Pt-1 + L (with light irradiation) and Pt-1 + L (with light irradiation) exhibited significantly stronger anticancer efficacy as above 90% of cells were killed. Additionally, the ability of NP@Pt-1 to induce cancer cell apoptosis was studied by an Annexin V-FITC/PI assay. As shown in Fig. 2B and C, the apoptosis rates induced by cisplatin, Pt-1 and NP@Pt-1 were 22.31%, 9.95%, and 13.28%, respectively. In contrast, Pt-1 + L and NP@Pt-1 + L induced significantly augmented apoptosis rate up to 90.09% and 95.41%, respectively, which was clearly in accordance with their anti-cancer activity by the above MTT assay (Fig. 2A). Moreover, CLSM were applied to visualize both the dead and live cells after treatment with various Pt-containing formulations via a calcein-AM and PI double staining live-dead assay (Fig. 2D). Theoretically, live cells could enzymatically hydrolyze the non-fluorescent calcein-AM to the green fluorescent calcein, while PI can penetrate the cell membrane of dead cells and bind to DNA to emit red fluorescence [25]. As shown in Fig. 2D, cells treated with Pt-1 + L and NP@Pt-1 + L showed the highest red fluorescence among all the other groups, indicating the strongest cancer cell killing efficiency of NP@Pt-1 + L. Finally, the toxicity of NP@Pt-1 was further confirmed on a 3D tumor cell spheroid with Calcein-AM /PI staining. As shown in Fig. 2E, the spheroid of the untreated cells were stained green (live cell) by Calcein-AM, while the spheroid of NP@Pt-1 + L had the most red fluorescence dots (dead cell), indicating that synergistic anti-cancer effect generated by ROS and cisplatin-like drugs from NP@Pt-1.
The ICD effect could be characterized by the presence of various DAMPs, such as release ATP and HMGB and translocation of CRT [12] (Fig. 3A). Therefore, to prove this, the release of ATP into the dying CT26 cells treated with NP@Pt-1 was studied by an ATP Assay Kit. After 12 h, cancer cells treated with NP@Pt-1 + L secreted 3 times more ATP than cells treated with Pt-1 + L in vitro (178 nmol versus 47 nmol, respectively) (Fig. 3B). Moreover, NP@Pt-1 + L and Pt-1 + L treatment significantly increased the translocation of CRT and HMGB1 release by CLSM (Fig. 3C, D). To analyze the membrane surface exposed DAMPs, i.e., CRT (ecto-CRT), cells were firstly treated with NP@Pt-1 and then labeled with anti-CRT antibodies and fluorescent secondary antibodies for CLSM observation. As shown in Fig. 3C, NP@Pt-1 + L showed enhanced effects on triggering CRT exposure compared to that of Pt-1 + L.Notably, both HMGB1 release and ATP secretion in cells treated with NP@Pt-1 + L were dramatically enhanced over those treated with Pt-1 + L (Fig. 3D), which was proved by the quantification results of CRT translocation (Fig. 3C) and HMGB1 released (Fig. 3D) from cell culture (Additional file 1: Fig. S4). Taken together, these results indicated that NP@Pt-1 increased DAMPs exposure for ICD effect.
Next, the anticancer efficacy of NP@Pt-1 in CT-26 tumor-bearing mice were examined. BALB/c mice were subcutaneously injected with CT26 colon carcinoma cells in the flank on day 0 and the intravenous administration of Cy7.5 labelled NP@Pt-1 was performed. Firstly, the noninvasive whole-animal imaging over time was conducted and the results revealed that the mice administered with NP@Pt-1 had the maximized fluorescence signal at tumor sites during the first 3–9 h and the fluorescence remained up to 36 h (Fig. 4A). The average radiance was shown the same (Fig. 4B, defined as fluorescence intensity/area/time). Notably, the ex vivo study of the major organs and tissues was performed and the results showed that after administration of NP@Pt-1 at 36 h, at least eight-fold greater red fluorescence intensity coming from Cy7.5 in tumor tissues was detected than that in the intestine (Fig. 4C, D). Moreover, the red fluorescence of NP@Pt-1 was strong in the liver and kidneys. Secondly, the therapeutic study of NP@Pt-1 and its effect on antitumor immune responses in vivo were examined. BALB/c mice were inoculated subcutaneously with CT26 cells. When the tumor size reached ~ 100 mm3, the mice were treated with cisplatin (3.5 mg/kg), NP@Pt-1(3.5 mg/kg) and NP@Pt-1(3.5 mg/kg) + L (420 nm laser irritation). At this dose, NP@Pt-1 was found to have moderate impact on the overall tumor growth, compared with PBS (Fig. 4E–G). In contrast, NP@Pt-1 + L significantly inhibited the tumor growth on a CT-26 mouse model (Fig. 4E–G).
In the ICD cascade, promoting the maturation of DC cells and improving the tumor infiltration of T cells is essential for effective cancer immunotherapy [29]. Therefore, to prove this, the immune cell population in desirable tissues and the spleen was investigated. The DC maturation, T cell proliferation and infiltration in tumor-draining lymph nodes (TDLN), spleen and tumor after various treatments were studied.
To understand the changes in immune cell population within the TME after different treatments, the residual tumors were first collected and analyzed by flow cytometry (Fig. 5). The gating strategies for flow cytometric analysis of tumor, peripheral lymphocytes and spleen was shown in Fig. 5A. Subsequently, the mature DCs in the TDLN (Fig. 5B), the effector killer T cells in spleen and tumors (Fig. 5C, D) were studied, which were further quantified thereafter (Fig. 5B–F). As shown in Fig. 5B and F, the mature DCs (CD11C+CD80+CD86+) [29] in the TDLN of mice treated with NP@Pt-1 + L were the highest at 30%, which were much higher than those of mice treated with cisplatin and NP@Pt-1without light irradiation at 20% and 22% respectively. Moreover, NP@Pt-1 + L demonstrated a potent ability to promote DC maturation compared to the other treatment groups, suggesting the necessity of light irritation of NP@Pt-1 for DCs maturation. To evaluate the side effect, the amount of proliferative effector killer T cells (CD3+CD8+) in the spleen were measured [30, 31]. Notably, the results showed there was no significant increase in the amount of CD8+ T cells in the spleen of mice treated with NP@Pt-1 + L (Fig. 5C–E). Previous study indicated that the eliciting of the tumor T cell infiltration via ICD could induce a strong antitumor immune response [18]. Results showed that CD8+ T cells in tumor tissues of mice treated with NP@Pt-1 + L were about 3 -fold more than those of all other groups (15% vs. 4%) (Fig. 5A, D, E). Therefore, we can conclude that NP@Pt-1 + L significantly increased the proportion of CD8+ T cells. Taken together, the above results confirmed that NP@Pt-1 effectively induced ICD cascade and improved the cancer immunotherapy.