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Article

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

Supporting Info

Patricia Antunes,^† Mihaela Ginj,^† Martin A. Walter,^† Jianhua Chen,^† Jean-Claude Reubi,^‡ and Helmut R. Maecke*^†

Division of Radiological Chemistry, Department of Radiology, University Hospital Basel, and Divison of Cell Biology and Experimental Cancer Research, Institute of Pathology, University of Berne, Switzerland

Bioconjugate Chem., 2007, 18 (1), pp 84–92

DOI: 10.1021/bc0601673

Publication Date (Web): December 15, 2006

†

University Hospital Basel.

‡

University of Berne.

Helmut R. Maecke, Division of Radiological Chemistry, University Hospital Basel, Petersgraben 4, CH-4031 Basel, Switzerland, E-mail: hmaecke@uhbs.ch, Tel.: +41-61-2654699, Fax: +41-61-2655559.

Abstract

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 [In^III-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 [In^III-DOTA]−β-Ala−NOC. The new conjugates preserved the good affinity for hsst₅, 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 (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).