Establishment of NaLuF₄:15%Tb-based low dose X-PDT agent and its application on efficient antitumor therapy

X-ray excited photodynamic therapy (X-PDT), is the bravo answer of photodynamic therapy (PDT) for deep-seated tumors, as it employs X-ray as the irradiation source to overcome the limitation of light penetration depth. However, high X-ray irradiation dose caused organ lesions and side effects became the major barrier to X-PDT application. To address this issue, firstly, we employed a classical co-precipitation reaction to synthesize NaLuF₄: 15%Tb³⁺(NLF) with an average particle size of 23.48 ± 0.91 nm, which was then coupled with the photosensitizer merocyanine 540 (MC540) to form the X-PDT system NLF-MC540 with high production of singlet oxygen. Secondly, the system could induce antitumor efficacy to about 24% in relative low dose X-ray irradiation range (0.1-0.3 Gy). In vivo, when NLF-MC540 irradiated by 0.1 Gy X-ray, the tumor inhibition percentage reached 89.5 ± 5.7%. Finally, we try to find the therapeutic mechanism which may exist in low dose X-PDT. A significant increase of neutrophils in serum was found on the third day after X-PDT. By immunohistochemical staining of tumor sections, we studied the Ly6G⁺, CD8⁺, and CD11c⁺ cells infiltrated in the tumor microenvironment. Utilizing the bilateral tumor model, we found the NLF-MC540 with 0.1 Gy X-ray irradiation could inhibit both the primary tumor and the distant tumor growth. Detected by Elisa assay, two cytokines IFN-γ and TNF-α in serum were upregulated 7 times and 6 times than negative control. Detected by Elispot assay, the number of immune cells attributable to the IFN-γ and TNF-α levels in the group of low dose X-PDT were 14 times and 6 times greater than that in the negative control group. Thus, we could conclude that our low dose X-PDT system could successfully upregulate the levels of immune cells, stimulate the secretion of cytokines (especially IFN-γ and TNF-α), activate antitumor immunity, finally inhibit colon tumor growth.