Radiolabeling of Nanoparticles with Astatine-211 or Palladium-103 for Cancer Therapy by Bioorthogonal Pretargeting or Nanobrachytherapy

Targeted radiotherapy, as a promising treatment method, has not been comprehensively investigated as of yet. The majority of clinically used radioactive compounds are still related to diagnostics. In our opinion, however, radiotherapy shows high potential, as demonstrated for instance by clinical studies with ²²⁵Ac labeled PSMA-617. Therefore, it is worthwhile to further explore this versatile field and some approaches were investigated within this work:
Chapter 1: Brachytherapy is one of the radiotherapeutic methods commonly used in clinics. It has a long history with a variety of investigated radionuclides. For brachytherapy, radioactive dosage is administered locally, which is conventionally achieved by implantation of metal seeds into the tumor. This procedure has not changed within the last century, is painful for the patient, and it is difficult to distribute the seeds accurately within the tumor. Therefore, a novel way was considered. The most commonly employed radionuclide ¹⁰³Pd was used and incorporated into intrinsically labeled nanoparticles and into a chelator, connected to a sucrose molecule. Insertion into a biocompatible gel, which can be injected intratumorally via syringe, allows for a more convenient delivery method than the traditional metal seeds. The nanoparticle gel was used to perform efficacy studies in tumor bearing mice, where an increase in survival time of the mice could be observed. Due to these findings, higher activity gels were designed and successfully synthesized. The stability of these gels was validated by release studies.
Chapter 2: The focus here was shifted towards the promising α-emitting radionuclide ²¹¹At. The main problem related to this molecule with excellent properties for targeted radiotherapy, is the stability of the ²¹¹At-aryl bond. As solution, gold nanoparticle and polymeric micelle approaches were chosen. Embedding of ²¹¹At within gold nanoparticles was tested but could not be confirmed. Nevertheless, surface adsorption of ²¹¹At on gold nanoparticles shows excellent stability in vitro and in vivo. In the case of the polymeric micelles, a core labeling strategy for ²¹¹At was pursued and successfully designed. Satisfactory in vitro stability could be established. However, when tested in vivo, decomposition in the liver occurred.
Chapter 3: Within this chapter, the focus was shifted towards a targeting strategy, by investigation of a pretargeting approach. The arguably best-suited reaction to that end is the bioorthotogonal tetrazine ligation, which is the fastest known biocompatible reaction, with excellent specificity. This reaction has already been used for successful pretargeting of small molecules. Thus, the combination with nanoparticles was investigated within this work. Again, two different nanoparticles were considered. The surface of 20 nm gold nanoparticles was modified with tetrazines, which could be verified by reaction with a radiolabeled TCO. The successful in vivo click-reaction could be shown by a blocking assay. However, no increase in tumor accumulation could be observed. Therefore, polymeric micelles, with a size of around 100 nm were labeled with ⁶⁴Cu, core labeled with ¹²⁵I, and modified on the surface with tetrazines. They were tested in the same manner as the gold nanoparticles. Very similar results could be observed, namely that the in vitro and in vivo click reaction could be confirmed. However, no increased accumulation of the nanoparticles in the tumor compared to passive targeting could be observed.