Iodine-125 brachytherapy suppresses tumor growth and alters bone metabolism in a H1299 xenograft mouse model
Authors: Bai Jiangtao , Yu Qiquan , Wang Yuyang , Xu Linshan , Wang Jianping , Zhai Jianglong , Bao Qi , Guo Wentao , Wu Chunxiao , Zhang Kun , Shou Weizhen , Zhu Guoying 
Published: November 2022

Abstract
Objective: The present study aimed to investigate the efficacy of Iodine-125 (I-125) brachytherapy in a mouse model of non-small cell lung cancer, to further explore the efficacy and appropriate method of implantation of the I-125 radioactive seed. This study also aimed to determine the impact of brachytherapy on bone metabolism.
Methods:A total of 18 mice were used to establish H1299 xenograft models, and were randomly assigned to three groups. These included non-radioactive seed implantation (Sham IM), fractionated I-125 seed implantation (Fractionated IM) and single I-125 seed implantation (Single IM) groups. Mice were euthanized after 28 days of implantation. H&E staining, Ki67 immunohistochemistry, CD31 morphometric analysis and TUNEL immunofluorescence assays were respectively used to determinethe histopathological changes, proliferation, micro-angiogenesis and apoptosisof tumors. In addition, bone volume and microstructure were evaluated using trabecular bone area (Tb.Ar), trabecular thickness (Tb.Th), trabecular number (Tb.N) and cortical thickness. Bone metabolic status was analyzed using histomorphometric staining of tartrate-resistant acid phosphate (TRAP) and alkaline phosphatase (ALP) expression in the femur, and using an ELISA assay to determine the expression of C-telopeptide of type 1 collagen (CTX-1) and procollagen type 1 n-terminal propeptide (P1NP) in the serum. Moreover, reverse transcription-quantitative PCR and western blotting were carried out for the analysis of bone remodeling-related gene expression in the bone tissue.
Results: Results of the present study demonstrated that compared with the Sham IM group, both the I-125 seed implantation groups, including Fractionated IM and Single IM, demonstrated significant therapeutic effects in both tumor volume and weight. More specifically, the most significant therapeutic effects on tumor inhibition were observed in the Fractionated IM group. Results of Ki67 and CD31 immunohistochemical staining suggested a notable reduction in tumor cell proliferation and micro-angiogenesis, and results of the TUNEL assay demonstrated an increase in tumor cell apoptosis. Although the cortical bone appeared thinner and more fragile in both I-125 seed implantation groups, no notable adverse changes in the morphology of the cancellous bone were observed, and the index of Tb.Ar, Tb.Th and Tb.n was not significantly different among Sham IM and I-125 implantation groups. However, alterations in bone metabolism were characterized by a decrease in CTX-1 and P1NP expression, accompanied by an increase in TRAP activity and a decrease inALP activity. Results of the present study also demonstrated the notable suppression of osteocalcin and runt-related transcription factor 2.
Conclusions: I-125 seed implantation may be an effective and safe antitumor strategy. Moreover, the use of fractionated implantation patterns based on tumor shape exhibited improved therapeutic effect on tumor suppression when the total number of I-125 seeds was equivalent along with reduced complications associated with bone loss.