ИСТИНА

Nowadays radioisotopes of various cations are considered as perspective for diagnostic and therapeutic purposes. Compounds for targeted delivery labeled by 64Cu, 213Bi, 212Pb, 90Y, 177Lu are undergoing preclinical and clinical trials for positron emission tomography and targeted alpha and beta therapy of oncological diseases. Among these compounds there are peptides and antibodies as well as their fragments with affinity to different types of malignancies. Basically labeling of these molecules by cations of radionuclides is carried out via bifunctional chelators – derivatives of such ligands as macrocyclic H4DOTA or acyclic H5DTPA. It is worth noting that H4DOTA forms very stable complexes with plenty of cations even in extreme conditions that is not the case of H5DTPA but the latter in contrast to former forms complexes much faster at room temperature that is very important for short-lived medical radionuclides and sensitive to heating biomolecules. In our study we systematically investigate ligands that to a certain degree combine acyclic and macrocyclic moieties to immediately form stable complexes at room temperature with cationic radionuclides promising for nuclear medicine. We considered newly synthesized different azacrown-ethers with bigger cavity than H4DOTA possesses and for Cu2+ bispidine type ligands were also evaluated (Fig. 1). All studied ligands were studied for their protonation ability (except a few bispidines that were not soluble in water) and complexation properties towards set of cations using potentiometric, spectrophotometric titration and radiochemical techniques. Fig. 1. Ligands, considered in this work. Among azacrown ethers not only number of heteroatoms in macrocycle was varied but also type of pendant arm: carboxylic, pyridine, picolinic or absence of any arm – to follow structure – property relationship. All azacrowns possessed benzo- or pyridine fragment in the cavity that could be further used for conjugation of ligand to biomolecule. Among them ligands with and without amide groups in the cavity were considered. Starting from determination of complexation constants in model solutions with Cu2+, Pb2+, Bi3+ and rare earth elements by this time it was shown that Cu2+ and Bi3+ form the most stable complexes (logK(ML)>14) with ligands possessing carboxylic arms. Their radiolabeling conditions by 64Cu and 207Bi were optimized. It should be noted that all labeled compounds were obtained at room temperature in a few minutes that was confirmed by thin layer chromatography. Indeed amide containing ligands demonstrated lower protonation as well as complexation ability but two of them: benzo-2 pic and pyridine-3 COOH – showed high enough complexation constants with Cu2+ (logK=15.6(1)) and Bi3+ (logK=21.3(2)). However they’ve been shown to dissociate in presence of competing cations and serum proteins. Among bispidines such pendant arms as carboxylic, triazole-containing with different length were varied. Presence of triazole affected on solubility in water and only decrease of logK(CuL) was observed for such type of bispidines. However 2 ligands with carboxylic groups without triazoles have shown logK(CuL)=14,2 and >20 and were further tested for serum stability. Radiolabeled ligands that have shown the lack of cation release in competing media (0-10%) in at least 2 hours (Cu2+) and 1 day (Bi3+) were further tested for their in vivo behavior and biodistribution in normal mice. The results of leading compound with Bi3+ were the same as obtained with DOTA-complex: fast clearance from the body and much lower accumulation of radioactivity in organs incl. liver and kidneys compared to blank experiment. Similar behavior was shown for copper-bispidine complex. These complexes are concluded to be perspective as components of copper and bismuth radiopharmaceuticals and bifunctional modifications of such ligands for their conjugation to peptides or antibodies are under development now. We thank Russian Science Foundation, project №18-73-10035, for financial support.