Evaluation of 3p-C-DEPA as potential 225Ac-chelator

Stephen Ahenkorah, Danni Ramdhani, Christophe Deroose, Thomas Cardinaels, Guy Bormans, Frederik Cleeren, Maarten Ooms

Research outputpeer-review


Objectives: Terbium has four radioisotopes suitable for theranostic applications in nuclear medicine. Terbium-152 (PET) and terbium-155 (SPECT) are diagnostic radionuclides, whereas terbium149 ( decay) and terbium-161 (– decay) are promising therapeutic radionuclides. Recently, we reported a protocol for radiolabeling heat-sensitive biomolecules with a promising bifunctional chelator, DOTA-GA.1 Here, we developed a DOTA-GA conjugate of a bivalent half-life extended Nanobody® (Nb), for radiopharmaceutical therapy, targeting c-Met and serum albumin. The Nb-conjugate was radiolabeled using both terbium-161 and terbium-155. Methods: Maleimide-DOTA-GA (1.3 molar eq.) was conjugated with the Nb (1 eq., 20 mM HEPES, 150 mM NaCl, 25°C, 1 h) and purified by size-exclusion chromatography. [161Tb]TbCl3 and [155Tb]TbCl3 were produced by SCK CEN and NPL, respectively [1,2]. Conjugates were radiolabeled with ~1.0 MBq [161Tb]TbCl3 or [155Tb]TbCl3 (0.1 M NaOAc, pH 4.7, 1 h, 40°C). Biodistribution of free [161Tb]TbCl3 and [161Tb]Tb-DOTA-GA-Nb construct were evaluated in healthy NMRI mice. Mice were injected with 0.9-1.5 MBq (10 nmol) via a tail vein under anesthesia (2.5% isoflurane in O2 at 1 L/min), and sacrificed 10 min, 1 h, 4 h, 24 h or 7 d p.i. (n=3/time point). Organs were harvested and the radioactivity was quantified in each organ. Finally, [155Tb] Tb-DOTA-GA-Nb (0.6 MBq) was injected i.v. In a healthy NMRI mouse, which was subsequently scanned for 1 h using a Molecubes® SPECT/ CT system at 4 and 24h p.i. Results: Maleimide-DOTA-GA was regio-selectively coupled to the Nb and purified (protein recovery: 51.9±2.0%). DOTA-GA-Nb was radiolabeled with [161Tb]TbCl3 or [155Tb]TbCl3 with radiochemical yields above 89% and radiochemical purity of 99.9% after purification. Biodistribution studies of [161Tb]Tb-DOTA-GA-Nb showed a decrease in bone uptake over time, an area known for TbCl3 accumulation (SUVmean = 0.5±0.1 and 0.2±0.0 at 4h and 7d, respectively), indicative of a stable terbium-conjugate bond. High kidney retention was observed (0-24 h: SUVmean = 4.7) and the plasma half-life of the radiolabeled conjugate was ca.10 hours, with >40% of the injected activity still present in blood after 4 hours, which was expected because of the half-life extending properties of this Nb. SPECT images of [155Tb] Tb-DOTA-GA-Nb after 4 h provided further insight to the distribution of the conjugate, with a large proportion of the activity observed in blood-rich organs (~75%), such as spleen, lungs and liver. Conclusions: We successfully radiolabeled a Nanobody-DOTA-GA conjugate with both terbium isotopes using a previously developed protocol. The radiolabeled conjugates were observed to be stable in vivo, with no immediate evidence of metal-conjugate bond degradation. Further evaluation of [161Tb]Tb-DOTA-GA-Nb in tumor models is currently ongoing
Original languageEnglish
Title of host publicationNuclear Medicine and Biology
Subtitle of host publicationAbstracts of the Fourth International Symposium on Technetium and Other Radiometals in Chemistry and Medicine (TERACHEM 2022)
Number of pages1
StatePublished - 8 Dec 2022
Event2022 - TERACHEM: The Fourth International Symposium on Technetium and Other Radiometals in Chemistry and Medicine - Bressanone
Duration: 14 Sep 202217 Sep 2022

Publication series

NameNuclear Medicine and Biology
ISSN (Print)0969-8051
ISSN (Electronic)1872-9614


Conference2022 - TERACHEM

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