ИСТИНА

Bioconjugates composed of radionuclide with peptide bound by linker-chelator are actively developed systems for target delivery of diagnostic or therapeutic emission to cancer cells in nuclear medicine. As chelators polyaminopolycarboxylates are the mostly used and studied class. Among them macrocyclic H4DOTA and acyclic H5DTPA have already found their application in radiopharmaceuticals with such radionuclides like Y-90 and Lu-177. However complexes with acyclic DTPA derivatives are not stable in vivo and kinetics of cations binding by H4DOTA at room temperature is comparable with half-lives of some radionuclides (e.g. Bi-213). Therefore the search for new ligands with fast complexation kinetics and high stability remains an important task. Present work considers complexation of Eu3+ as representative of rare earth elements (REE) cations by pyridine containing azacrown-ether. The fast rate of Eu3+ complexation by L was demonstrated by competitive spectrophotometry with Arsenazo III: complex forms during first minute similar to DTPA and by 2 orders faster than with DOTA at the same conditions. Constants of ligand’s protonation and complexation with Eu3+ were determined by potentiometric titration with consequent data treatment in Hyperquad and by competitive sorption on SiO2. The values calculated by different methods are in line with each other and equal to logK(EuL)=8.2±0.2, logK(EuLOH-)=2.3±0.1, logK(EuL(OH-)2)=-5.9±0.1, logK(EuL(OH-)3)=-16.9±0.1 in 0.1M NaClO4. For evaluation of stability of complex EuL in different media thin layer chromatography on SiO2 plates was used. Eluents with varied composition were tested for complex with azacrown ligand L (table), Eu-152 was used as tracer. For estimation of behavior of cation bound by serum peptides we used Eu-DOTATATE complex as a reference. All resulting TLC plates were counted on autoradiographic system and associated software (Cyclone, Perkin Elmer). Table. Retention factors for europium species with using of different eluents in TLC. Eluent 0.1M (H,Na)3cit pH5.2 CH3CN(CH3OH):H2O:NH4OH (1:1:2) Py:H2O:C2H5OH (1:4:2) CH3OH:10% NaOAc (1:1) Eu-DOTATATE Rf = 0-0.2 Rf = 1.0 Rf = 0.5-0.6 Rf = 0.5 Eu-L Rf = 0.6-0.9 Rf = 0-0.4 Rf = 0.7-1.0 Rf = 0.6-1.0 Eu3+ Rf = 0.6-0.9 Rf = 0 Rf = 0 Rf = 0 According to the obtained results for fetal bovine serum experiments 0.1M (H,Na)3cit was used since it provides better separation of Eu-L and Eu bound with high molecular weight compounds like DOTATATE and serum proteins. For study of stability in physiological solution Py:H2O:C2H5OH (1:4:2) was selected as optimal with better separation of spots of Eu3+ and Eu-L and fast and flat eluent development. By these techniques stability of EuL during 3 days in physiological solution and serum was shown. Since the constant of EuL complex formation is not high enough for complete labeling separation from unbound cation is required. In this work we carried out this separation by HPLC on C18 column in gradient mode using H2O and CH3OH as eluents. In this regime free Eu3+ elutes during first minute when flow is H2O/CH3OH=0.9/0.1 and consequent monotonous increase of CH3OH part in the flow extracts Eu-L on 6-8 minutes upon H2O/CH3OH=0.4/0.6-0.6/0.4. Eu3+ in fractions was measured by gamma spectrometry of Eu-152 and ligand was detected by UV-vis absorption spectroscopy at wavelengths 226 and 275 nm. This work was supported by RFBR project 16-33-00642.