ИСТИНА

Nowadays bismuth and rare earth elements (REE) radioisotopes are very perspective for application in therapy. In addition gadolinium is already used for magnetic resonance imaging and europium is well-known fluorescence probe. In this work radiolabeling of tetrapeptide by 207Bi and 152Eu (as representative of REE) radionuclides with DOTA as chelator was studied. All experiments were carried out with 152Eu and 207Bi. For labeling optimization variation of pH, temperature and concentration of ligand was performed. Thin layer chromatography with consequent autoradiography and gamma-spectrometry and high performance liquid chromatography were used for analysis of labeling efficiency. At lower pH values 2-3 the maximal labeling efficiency is 17% for Bi3+ and 11% for Eu3+. Increasing of pH facilitates ligand’s deprotonation and consequently leads to increase of labeling yield and rate of chelation. According to obtained results labeling takes 20-30 min at 90°С and 60-120 min at 60°С. Bismuth chelation is regularly slower that could be the sequence of its high tendency to hydrolysis (constants of Bi3+ hydroxide complexes are higher by 6-7 orders than that of REE) and competition of DOTA-tetrapeptide with hydroxide-ions. For both cations labeling of 90-95% at 37°С is achieved after 1 day of interaction. Under same conditions (pH, temperature, ligand concentration) DOTA itself chelates 98-99% of cations. The possible reason for difference between yield of labeling of DOTA and DOTA-tetrapeptide is lower stereochemical availability of chelator’s molecule for cation. Tetrapeptide sequence can partly shield macrocycle from cation. Obtained complexes with Eu3+ and Bi3+ were tested for stability in 100 times volume excess of fetal bovine serum. By precipitation of serum proteins it was shown that 15-20% of complex dissociate during first 30 min and further this value doesn’t increase for both cations.