In vitro structure–activity relationship of Re-cyclized octreotide analogues

Dannoon, Shorouk F.; Bigott-Hennkens, Heather M.; Ma, Lixin; Gallazzi, Fabio; Lewis, Michael R.; Jurisson, Silvia S.

doi:10.1016/j.nucmedbio.2010.03.008

« Previous Next »Nuclear Medicine and Biology
Volume 37, Issue 5 , Pages 527-537, July 2010

In vitro structure–activity relationship of Re-cyclized octreotide analogues☆

Shorouk F. Dannoon
- Department of Chemistry, University of Missouri, Columbia, MO 65211, USA
- S. Dannoon and H. Bigott-Hennkens contributed equally to this work.
Heather M. Bigott-Hennkens
- Department of Veterinary Medicine and Surgery, University of Missouri, Columbia, MO 65211, USA
- S. Dannoon and H. Bigott-Hennkens contributed equally to this work.
Lixin Ma
- Department of Radiology, University of Missouri, Columbia, MO 65211, USA
- International Institute of Nano and Molecular Medicine, University of Missouri, Columbia, MO 65211, USA
- Nuclear Science and Engineering Institute, University of Missouri, Columbia, MO 65211, USA
- Research Service, Harry S. Truman Memorial Veterans' Hospital, Columbia, MO 65201, USA
Fabio Gallazzi
- Structural Biology Core, University of Missouri, Columbia, MO 65211, USA
Michael R. Lewis
- Department of Veterinary Medicine and Surgery, University of Missouri, Columbia, MO 65211, USA
- Department of Radiology, University of Missouri, Columbia, MO 65211, USA
- Nuclear Science and Engineering Institute, University of Missouri, Columbia, MO 65211, USA
- Research Service, Harry S. Truman Memorial Veterans' Hospital, Columbia, MO 65201, USA
- Corresponding authors. Silvia S. Jurisson is to be contacted at the Department of Chemistry, University of Missouri, Columbia, MO 65211, USA. Tel.: +1 573 882 2107; fax: +1 573 882 2754. Michael R. Lewis, Department of Veterinary Medicine and Surgery, University of Missouri, MO 65211, USA. Tel.: +1 573 814 6000x53703; fax: +1 573 814 6551.
Silvia S. Jurisson
- Department of Chemistry, University of Missouri, Columbia, MO 65211, USA
- Department of Radiology, University of Missouri, Columbia, MO 65211, USA
- Nuclear Science and Engineering Institute, University of Missouri, Columbia, MO 65211, USA
- Corresponding authors. Silvia S. Jurisson is to be contacted at the Department of Chemistry, University of Missouri, Columbia, MO 65211, USA. Tel.: +1 573 882 2107; fax: +1 573 882 2754. Michael R. Lewis, Department of Veterinary Medicine and Surgery, University of Missouri, MO 65211, USA. Tel.: +1 573 814 6000x53703; fax: +1 573 814 6551.

Received 20 January 2010; received in revised form 2 March 2010; accepted 27 March 2010.

Abstract

Introduction

Development of radiolabeled octreotide analogues is of interest for targeting somatostatin receptor (SSTR)-positive tumors for diagnostic and therapeutic purposes. We are investigating a direct labeling approach for incorporation of a Re ion into octreotide analogues, where the peptide sequences are cyclized via coordination to Re rather than through a disulfide bridge.

Methods

Various octreotide analogue sequences and coordination systems (e.g., S₂N₂ and S₃N) were synthesized and cyclized with nonradioactive Re. In vitro competitive binding assays with ¹¹¹In-DOTA-Tyr³-octreotide in AR42J rat pancreatic tumor cells yielded IC₅₀ values as a measure of SSTR affinity of the Re-cyclized analogues. Three-dimensional structures of Re-cyclized Tyr³-octreotate and its disulfide-bridged analogue were calculated from two-dimensional NMR experiments to visualize the effect of metal cyclization on the analogue's pharmacophore.

Results

Only two of the 11 Re-cyclized analogues investigated showed moderate in vitro binding affinity toward somatostatin subtype 2 receptors. Three-dimensional molecular structures of Re- and disulfide-cyclized Tyr³-octreotate were calculated, and both of their pharmacophore turns appear to be very similar with minor differences due to metal coordination to the amide nitrogen of one of the pharmacophore amino acids.

Conclusions

Various Re-cyclized analogues were developed and analogue 4 had moderate affinity toward somatostatin subtype 2 receptors. In vitro stable studies that are in progress showed stable radiometal cyclization of octreotide analogues via NS₃ and N₂S₂ coordination forming five- and six-membered chelate rings. In vivo biodistribution studies are underway of ^99mTc-cyclized analogue 4.

Keywords: Octreotide, Tyr³-octreotate, Metal cyclization, Re, Two-dimensional NMR