A Unique Matched Quadruplet of Terbium Radioisotopes for PET and SPECT and for α- and β−-Radionuclide Therapy: An In Vivo Proof-of-Concept Study with a New Receptor-Targeted Folate Derivative

Terbium offers 4 clinically interesting radioisotopes with complementary physical decay characteristics: ¹⁴⁹Tb, ¹⁵²Tb, ¹⁵⁵Tb, and ¹⁶¹Tb. The identical chemical characteristics of these radioisotopes allow the preparation of radiopharmaceuticals with identical pharmacokinetics useful for PET (¹⁵²Tb) and SPECT diagnosis (¹⁵⁵Tb) and for α- (¹⁴⁹Tb) and β⁻-particle (¹⁶¹Tb) therapy. The goal of this proof-of-concept study was to produce all 4 terbium radioisotopes and assess their diagnostic and therapeutic features in vivo when labeled with a folate-based targeting agent. Methods:¹⁶¹Tb was produced by irradiation of ¹⁶⁰Gd targets with neutrons at Paul Scherrer Institute or Institut Laue-Langevin. After neutron capture, the short-lived ¹⁶¹Gd decays to ¹⁶¹Tb. ¹⁴⁹Tb, ¹⁵²Tb, and ¹⁵⁵Tb were produced by proton-induced spallation of tantalum targets, followed by an online isotope separation process at ISOLDE/CERN. The isotopes were purified by means of cation exchange chromatography. For the in vivo studies, we used the DOTA–folate conjugate cm09, which binds to folate receptor (FR)–positive KB tumor cells. Therapy experiments with ¹⁴⁹Tb-cm09 and ¹⁶¹Tb-cm09 were performed in KB tumor–bearing nude mice. Diagnostic PET/CT (¹⁵²Tb-cm09) and SPECT/CT (¹⁵⁵Tb-cm09 and ¹⁶¹Tb-cm09) studies were performed in the same tumor mouse model. Results: Carrier-free terbium radioisotopes were obtained after purification, with activities ranging from approximately 6 MBq (for ¹⁴⁹Tb) to approximately 15 GBq (for ¹⁶¹Tb). The radiolabeling of cm09 was achieved in a greater than 96% radiochemical yield for all terbium radioisotopes. Biodistribution studies showed high and specific uptake in FR-positive tumor xenografts (23.8% ± 2.5% at 4 h after injection, 22.0% ± 4.4% at 24 h after injection, and 18.4% ± 1.8% at 48 h after injection). Excellent tumor-to-background ratios at 24 h after injection (tumor to blood, ∼15; tumor to liver, ∼5.9; and tumor to kidney, ∼0.8) allowed the visualization of tumors in mice using PET (¹⁵²Tb-cm09) and SPECT (¹⁵⁵Tb-cm09 and ¹⁶¹Tb-cm09). Compared with no therapy, α- (¹⁴⁹Tb-cm09) and β⁻-particle therapy (¹⁶¹Tb-cm09) resulted in a marked delay in tumor growth or even complete remission (33% for ¹⁴⁹Tb-cm09 and 80% for ¹⁶¹Tb-cm09) and a significantly increased survival. Conclusion: For the first time, to our knowledge, 4 terbium radionuclides have been tested in parallel with tumor-bearing mice using an FR targeting agent. Along with excellent tumor visualization enabled by ¹⁵²Tb PET and ¹⁵⁵Tb SPECT, we demonstrated the therapeutic efficacy of the α-emitter ¹⁴⁹Tb and β⁻-emitter ¹⁶¹Tb.