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 (sst(1-5))-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-Nal(3)]-octreotide (DOTA-NOC), different spacers (X) based on 8-amino-3,6-dioxaoctanoic acid (PEG(2)), 15-amino-4,7,10,13-tetraoxapentadecanoic acid (PEG(4)), N-acetyl glucosamine (GlcNAc), triglycine, beta-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 [In-111-DOTA]-NOC was achieved with the introduction of the mentioned spacers, except with triglycine and beta-alanine. The high affinity of [In-III-DOTA]-NOC for human sst(2) (hsst(2)) was preserved with the structural modifications, while an overall drop for hsst(3) affinity was observed, except in the case of [In-III-DOTA]-beta-Ala-NOC. The new conjugates preserved the good affinity for hsst(5), except for [In-III-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 (sst(2) receptor expression) at 4 h was achieved with the introduction of Asp as a spacer in the parent compound. In sst(3)-expressing HEK cells, the specific internalization rate at 4 h for [In-111-DOTA]-NOC (13.1% +/- 0.3%) was maintained with [In-111-DOTA]-beta-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 [In-111-DOTA]-NOC (2.96% +/- 0.48% IA/g), the specific uptake in the tumor at 4 h p.i. was significantly improved for the In-111-labeled sugar analogue (4.17% +/- 0.46% IA/g), which among all the new derivatives presented the best tumor-to-kidney ratio (1.9).