Internal radiotherapy of tumors using radiolabeled thermoresponsive polymers via self-aggregation under hyperthermic conditions

Introduction: In the development of radiopharmaceuticals for internal radiotherapy, the basic requirements are (i) marked accumulation specific to tumors and (ii) long-term intratumoral retention of radioactivity. As a radiopharmaceutical satisfying these conditions, we have previously reported a radiolabeled thermoresponsive polymer (polyoxazoline; POZ) soluble at room and body temperature with a rapid clearance from normal tissues, but self-aggregate in the tumor upon heat treatment (hyperthermia, 42-43°C) after intravenous injection. The radiolabeled POZ was highly accumulated in the tumor via self-aggregation under hyperthermic conditions and retained after stopping exposure to heat.⁽¹⁾ In this study, we investigated the therapeutic effect of tumors by internal radiotherapy using yttrium-90-labeled POZ (⁹⁰Y-POZ) combined with hyperthermia.

Methods: Using oxazoline derivatives with ethyl (Et) and isopropyl (Ipr) side chains, Et-IprPOZ (heteropolymer) was synthesized by microwave irradiation, and the aggregation temperature was measured. Et-IprPOZ was conjugated with a chelate agent DOTA derivative, followed by ⁹⁰Y-labeling. For therapeutic studies, colon26 tumor-bearing mice were randomized into 7 groups for the following treatments. (A) PBS, (B) non-radiolabeled POZ + hyperthermia, (C) ⁹⁰Y-POZ (3.7 MBq), (D) ⁹⁰Y-POZ (7.4 MBq), (E) ⁹⁰Y-POZ (1.85 MBq) + hyperthermia, (F) ⁹⁰Y-POZ (3.7 MBq) + hyperthermia, and (G) ⁹⁰Y-POZ (7.4 MBq) + hyperthermia. In the hyperthermia-treated groups, after the tumors were pre-heated in warm water (42°C) for 15 min, the agents were intravenously administered, followed by heating for the additional 60 min. Tumor size and body weight were measured twice a week. The number of white blood cells was counted for the evaluation of side effects.

Results: The molecular weight of Et-IprPOZ was adjusted to approximately 27 kDa. The aggregation temperature of Et-IprPOZ was 38°C. The radiochemical yield and purity of ⁹⁰Y-POZ was 25% and more than 93%, respectively. In the group treated with nonradiolabeled POZ plus hyperthermia (group B), a temporal suppression of tumor growth was observed at an early phase (1-5 days) after treatment, while there was no significant difference on tumor size compared to the control group (group A). On the other hand, ⁹⁰Y-POZ (without hyperthermia treatment) significantly reduced tumor size compared to the control group at 13 days after treatment, although any dose-dependency was not observed (group C and D). A marked and radioactivity dose-dependent therapeutic effect was exhibited in the groups treated with ⁹⁰Y-POZ combined with hyperthermia (groups E-G). No obvious loss of body weight and systemic side effects were observed. Conclusion: These results demonstrated that thermoresponsive ⁹⁰Y-POZ showed a synergic therapeutic effect by combination with hyperthermia, suggesting the usefulness for internal radiotherapy of tumors. Reference: 1. Sano K. et al., Mol Pharm. 15(9), 3997-4003 (2018).