Copper Complexes of New Benzodioxotetraaza Macrocycles with Potential Applications in Nuclear Medicine

Two novel benzodioxotetraaza macrocycles [2,9-dioxo-1,4,7,10-tetraazabicyclo[10.4.0]1,11-hexadeca-1(11),13,15-triene (H2L1) and 2,10-dioxo-1,4,8,11-tetraazabicyclo[11.4.0]1,12-heptadeca-1(12),14,16-triene (H2L2)] were synthesized by a [1 + 1] crablike cyclization. The protonation constants of both ligands were determined by 1H NMR titration and by potentiometry at 25.0 °C in 0.10 M ionic strength in KNO3. The latter method was also used to ascertain the stability constants of their copper(II) complexes. These studies showed that the CuL1 complex has a much lower thermodynamic stability than the CuL2, and the H2L2 displays an excellent affinity for copper(II), due to the good fit of copper(II) into its cavity. The copper complexes of the novel ligands were characterized by electronic spectroscopy in solution and by crystal X-ray diffraction. These studies indicated that the copper center in the CuL1 complex adopts a square−pyramidal geometry with the four nitrogen atoms of the macrocycle forming the equatorial plane and a water molecule at axial position, and the copper in the CuL2 complex is square−planar. Several labeling conditions were tested, and only H2L2 could be labeled with 67Cu efficiently (>98%) in mild conditions (39 °C, 15 min) to provide a slightly hydrophilic radioligand (log D = −0.19 ±0.03 at pH 7.4). The in vitro stability was studied in the presence of different buffers or with an excess of diethylenetriamine-pentaethanoic acid. Very high stability was shown under these conditions for over 5 days. The incubation of the radiocopper complex in human serum showed 6% protein binding

Influence of Different Spacers on the Biological Profile of a DOTA−Somatostatin Analogue

Radiolabeled somatostatin analogues have been successfully used for targeted radiotherapy and for imaging of somatostatin receptor (sst1-5)-positive tumors. Nevertheless, these analogues are subject to improving their tumor-to-nontarget ratio to enhance their diagnostic or therapeutic properties, preventing nephrotoxicity. In order to understand the influence of lipophilicity and charge on the pharmacokinetic profile of [1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)]−somatostatin-based radioligands such as [DOTA,1-Nal3]−octreotide (DOTA−NOC), different spacers (X) based on 8-amino-3,6-dioxaoctanoic acid (PEG2), 15-amino-4,7,10,13-tetraoxapentadecanoic acid (PEG4), N-acetyl glucosamine (GlcNAc), triglycine, β-alanine, aspartic acid, and lysine were introduced between the chelator DOTA and the peptide NOC. All DOTA−X−NOC conjugates were synthesized by Fmoc solid-phase synthesis. The partition coefficient (log D) at pH = 7.4 indicated that higher hydrophilicity than [111In-DOTA]−NOC was achieved with the introduction of the mentioned spacers, except with triglycine and β-alanine. The high affinity of [InIII-DOTA]−NOC for human sst2 (hsst2) was preserved with the structural modifications, while an overall drop for hsst3 affinity was observed, except in the case of [InIII-DOTA]−β-Ala−NOC. The new conjugates preserved the good affinity for hsst5, except for [InIII-DOTA]−Asn(GlcNAc)−NOC, which showed decreased affinity. A significant 1.2-fold improvement in the specific internalization rate in AR4-2J rat pancreatic tumor cells (sst2 receptor expression) at 4 h was achieved with the introduction of Asp as a spacer in the parent compound. In sst3-expressing HEK cells, the specific internalization rate at 4 h for [111In-DOTA]−NOC (13.1% ± 0.3%) was maintained with [111In-DOTA]−β-Ala−NOC (14.0% ± 1.8%), but the remaining derivatives showed <2% specific internalization. Biodistribution studies were performed with Lewis rats bearing the AR4-2J rat pancreatic tumor. In comparison to [111In-DOTA]−NOC (2.96% ± 0.48% IA/g), the specific uptake in the tumor at 4 h p.i. was significantly improved for the 111In-labeled sugar analogue (4.17% ± 0.46% IA/g), which among all the new derivatives presented the best tumor-to-kidney ratio (1.9)