Abstract
Background213Bismuth (213Bi, T1/2 = 45.6 min) is one of the most frequently used α-emitters in cancer research. High specific activity radioligands are required for peptide receptor radionuclide therapy. The use of generators containing less than 222 MBq 225Ac (actinium), due to limited availability and the high cost to produce large-scale 225Ac/213Bi generators, might complicate in vitro and in vivo applications though.Here we present optimized labelling conditions of a DOTA-peptide with an 225Ac/213Bi generator (< 222 MBq) for preclinical applications using DOTA-Tyr3-octreotate (DOTATATE), a somatostatin analogue. The following labelling conditions of DOTATATE with 213Bi were investigated; peptide mass was varied from 1.7 to 7.0 nmol, concentration of TRIS buffer from 0.15 mol.L-1 to 0.34 mol.L-1, and ascorbic acid from 0 to 71 mmol.L-1 in 800 μL. All reactions were performed at 95 °C for 5 min. After incubation, DTPA (50 nmol) was added to stop the labelling reaction. Besides optimizing the labelling conditions, incorporation yield was determined by ITLC-SG and radiochemical purity (RCP) was monitored by RP-HPLC up to 120 min after labelling. Dosimetry studies in the reaction vial were performed using Monte Carlo and in vitro clonogenic assay was performed with a rat pancreatic tumour cell line, CA20948.ResultsAt least 3.5 nmol DOTATATE was required to obtain incorporation ≥ 99 % with 100 MBq 213Bi (at optimized pH conditions, pH 8.3 with 0.15 mol.L-1 TRIS) in a reaction volume of 800 μL. The cumulative absorbed dose in the reaction vial was 230 Gy/100 MBq in 30 min. A minimal final concentration of 0.9 mmol.L-1 ascorbic acid was required for ~100 MBq (t = 0) to minimize radiation damage of DOTATATE. The osmolarity was decreased to 0.45 Osmol/L.Under optimized labelling conditions, 213Bi-DOTATATE remained stable up to 2 h after labelling, RCP was ≥ 85 %. In vitro showed a negative correlation between ascorbic acid concentration and cell survival.Conclusion213Bismuth-DOTA-peptide labelling conditions including peptide amount, quencher and pH were optimized to meet the requirements needed for preclinical applications in peptide receptor radionuclide therapy.
Highlights
213Bismuth (213Bi, T1/2 = 45.6 min) is one of the most frequently used α-emitters in cancer research
Labelling The standard labelling protocol 213Bi-DOTATATE resulted in an incorporation > 99 % and a radiochemical purity (RCP) > 85 % at pH 8.7 for up to 2 h after labelling
Labelling with 1.7 nmol DOTATATE in 800 μL resulted in low incorporation of 6 ± 4 % and poor RCP of 5 ± 5 % (median: 6.9 %), see Fig. 2
Summary
High specific activity radioligands are required for peptide receptor radionuclide therapy. The number of receptors available on the cell membrane is limited, so high specific activity (SA, expressed in MBq per nmol of peptide) of labelled peptides is advantageous for administration in small animals e.g. mice (~25 g) (Breeman et al 2001; Breeman et al 1995; de Jong et al 1999). Besides high SA, other requirements for preclinical applications include high stability of the radio-peptide at physiological conditions in vitro (~0.3 Osmol.L−1, pH ~7.4) and in vivo. A high osmolarity of the drug-containing solution is inconsistent with maintenance of physiological conditions for in vitro and in vivo applications
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
