Development of novel bi-modal agents for combination use of Auger electron therapy and photodynamic therapy

Introduction: Auger electrons have high linear energy transfer (LET) and a short distribution range. Therefore, compared to targeted radionuclide therapy with α- or β-particles, Auger electron therapy is expected to have similar therapeutic effects and reduced side effects. On the other hand, photodynamic therapy (PDT) is one of the well-established approaches in cancer treatment, but it requires higher doses of photosensitizer (PS) than radiopharmaceuticals. Therefore, PS causes unwilling side effects including photosensitivity. Combination therapy using PDT and another conventional cancer treatment is one of the useful strategies to avoid high doses of PS, but the combination of PDT and Auger electron therapy has not been investigated. We have reported bi-modal agents (BH-1 and BH-2), which comprised Hoechst and iodo-BODIPY, that can be applied to Auger electron therapy and PDT. However, the short absorption wavelengths of these agents have limited their in vivo applications. In this study, we newly designed and synthesized NBH-1 and NBH-2 to develop bi-modal agents with absorption wavelengths in the near-infrared region, and evaluated their utility in in vitro and in vivo.

Methods: [¹²⁵I]NBH-1 and [¹²⁵I]NBH-2 were prepared in the presence of N-chlorosuccinimide and acetic acid. Intracellular and nuclear uptake of [¹²⁵I]NBH-1 and [¹²⁵I]NBH-2 were evaluated by incubation with HeLa cells after 2, 4, and 6 h incubation.HeLa cell viabilities after incubation with [¹²⁵I]NBH-1, [¹²⁵I]NBH-2, NBH-1, or [¹²⁵I]NBH-1/NBH-1 were determined using MTT assay. In vivo therapeutic efficacy derived from Auger electron therapy, PDT, and combination of these treatments was assessed using tumor-bearing nude mice.

Results: [¹²⁵I]NBH-1 and [¹²⁵I]NBH-2 were successfully obtained in radiochemical yields of 5.4 and 5.8%, respectively, and radiochemical purities of more than >99%. In in vitro intracellular and nuclear uptake studies, [¹²⁵I]NBH-1 showed higher intracellular and nuclear uptakes than [¹²⁵I]NBH-2 after 2, 4, and 6 h incubation. In the MTT assay, cell viability treated with [¹²⁵I]NBH-1 decreased in a radioactivity-dependent manner. On the other hand, little cytotoxicity was observed in [¹²⁵I]NBH-2 treated cells. In the MTT assay after light irradiation, cell viability treated with NBH-1 decreased in a dose-dependent manner. These results suggested that [¹²⁵I]NBH-1 has fundamental properties as a bi-modal agent. Furthermore, the result of combination therapy using [¹²⁵I]NBH-1 and NBH-1 with light irradiation in in vitro exhibited significantly higher cytotoxicity than those compounds alone. In addition, in vivo treatment study, combination therapy with [¹²⁵I]NBH-1 and NBH-1 under light irradiation resulted in minimal tumor volume after 12 days of injection in mice.

Conclusions: The results in the present study suggested that [¹²⁵I]NBH-1/NBH-1 is a potent bi-modal agent for Auger electron therapy and PDT and that the combination therapy using [¹²⁵I]NBH-1 and NBH-1 showed favorable in vivo antitumor effects.