Brachytherapy is a type of radiotherapy wherein titanium capsules containing therapeutic radioisotopes are implanted within tumor tissues, enabling high-dose radio-irradiation to tumor tissues around the seeds. Although marked therapeutic effects have been demonstrated, brachytherapy needs complicated implantation technique under general anesthesia and the seeds could migrate to other organs. The aim of this study is to establish a novel brachytherapy using biocompatible, injectable thermo-responsive polymers (polyoxazoline; POZ) labeled with yttrium-90 (⁹⁰Y), which can self-aggregate above a specific transition temperature, resulting in long-term intratumoral retention of radioactivity and therapeutic effect. Therefore, we evaluated the tumor retention of radiolabeled POZ derivatives and their therapeutic effects. METHODS: Using oxazoline derivatives with ethyl (Et), isopropyl (Isp), and propyl (Pr) side chains, EtPOZ, IspPOZ, Isp-PrPOZ (heteropolymer), and PrPOZ were synthesized, and their characteristic transition temperature (Tt) was measured. The intratumoral retention of ¹¹¹In-labeled POZ was evaluated until 7 days post-injection in nude mice bearing PC-3 human prostate cancer. The intratumoral localization of ¹¹¹In-labeled POZ derivatives was investigated by an autoradiographic study. Furthermore, a therapeutic study using ⁹⁰Y-labeled Isp-PrPOZ was performed, and tumor growth and survival rate were evaluated. RESULTS: The Tts of EtPOZ, IspPOZ, Isp-PrPOZ, and PrPOZ (approximately 20 kDa) were >70°C, 34°C, 25°C, and 19°C, respectively. In the intratumoral injection study, Isp-PrPOZ and PrPOZ (2000 μM) with Tts lower than tumor temperature (33.5°C under anaesthesia) showed a significantly higher retention of radioactivity at 1 day post-injection (73.6% and 73.9%, respectively) than EtPOZ (5.6%) and IspPOZ (15.8%). Even at low injected dose (100 μM), Isp-PrPOZ exhibited high retention (68.3% at 1 day). The high level of radioactivity of Isp-PrPOZ was retained in the tumor 7 days post-injection (69.5%). The autoradiographic study demonstrated that the radioactivity of ¹¹¹In-labeled Isp-PrPOZ and PrPOZ was localized in a small area. In the therapeutic study using ⁹⁰Y-labeled Isp-PrPOZ, significant suppression of tumor growth and prolonged survival rate were achieved in an injection dose-dependent manner compared to that observed for the vehicle-injected group and non-radioactive Isp-PrPOZ-injected group. CONCLUSION: The injectable ⁹⁰Y-labeled Isp-PrPOZ was retained for a prolonged period within tumor tissues via self-aggregation and exhibited marked therapeutic effect, suggesting its usefulness for brachytherapy.

Since membrane type-1 matrix metalloproteinase (MT1-MMP) is exclusively expressed in tumors and is closely associated with metastasis and invasion, MT1-MMP is a potential target of radiotracers for the evaluation of tumor malignancy. In this study, we planned to visualize MT1-MMP in vivo by a two-step pre-targeting strategy using a streptavidin (SAv)-biotin system combined with anti-MT1-MMP monoclonal immunoglobulin (IgG) (anti-MT1-MMP monoclonal antibody (mAb)). Streptavidinylated anti-MT1-MMP mAb was synthesized by reacting biotinylated anti-MT1-MMP mAb with SAv. In the pre-targeting study, FM3A mouse breast carcinoma-implanted mice were injected with anti-MT1-MMP mAb-SAv, followed 72 h later with radioiodinated biotin, (3-[^123/125I]iodobenzoyl)norbiotinamide (^123/125I-IBB). Biodistribution and imaging (single photon emission computed tomography (SPECT)/CT) data were collected at several time points in the 24 h period following introduction of the tracer. The comparison groups were injected with ¹²⁵I-IBB alone or with ¹²⁵I-IBB pre-targeted with negative control IgG-SAv. In the pre-targeting study for MT1-MMP, within 1 h of tracer injection, rapid tumor uptake and abrupt clearance from the blood of radioactivity (2.22, 0.87% injected dose/g at 1 h) were observed. The tumor to blood (T/B) radioactivity ratios were significantly higher than those from mice dosed with the pre-targeting negative control (p<0.0001). ¹²⁵I-IBB alone did not accumulate in tumors. SPECT/CT image analysis of FM3A bearing mice showed high-contrast tumor images after 3 h with minimal blood-pool activity. The present study that uses a pre-targeting method showed high T/B radioactivity ratios and clear tumor images of MT1-MMP. This imaging method may be useful for the clinical diagnosis of malignant tumors.