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Comparison of Four ⁶⁴Cu-Labeled Somatostatin Analogues in Vitro and in a Tumor-Bearing Rat Model: Evaluation of New Derivatives for Positron Emission Tomography Imaging and Targeted Radiotherapy

Supporting Info

Jason S. Lewis,^† Michael R. Lewis,^† Ananth Srinivasan,^‡ Michelle A. Schmidt,^‡ Jian Wang,^† and Carolyn J. Anderson*^†

Mallinckrodt Institute of Radiology, Washington University School of Medicine, 510 South Kingshighway Boulevard, Campus Box 8225, St. Louis, Missouri 63110, and Mallinckrodt, Inc., 675 McDonnell Boulevard, Hazelwood, Missouri 63042

J. Med. Chem., 1999, 42 (8), pp 1341–1347

DOI: 10.1021/jm980602h

Publication Date (Web): March 31, 1999

Abbreviations: DTPA, diethylenetriaminepentaacetic acid; DOTA, 1,4,7,10-tetraazacyclododecane-N,N‘,N‘‘,N‘‘‘-tetraacetic acid; TETA, 1,4,8,11-tetraazacyclotetradecane-N,N‘,N‘‘,N‘‘‘-tetraacetic acid; Y3, tyrosine-3; OC, octreotide; TATE, octreotate; MALDI FTMS, matrix-assisted laser desorption−ionization Fourier transform mass spectrometry.

†

Washington University School of Medicine.

‡

Mallinckrodt, Inc.

Correspondence to: Carolyn J. Anderson, Ph.D. Phone: (314) 362-8427. Fax: (314) 362-9940. E-mail: andersoncj@mirlink.wustl.edu.

Abstract

Previous studies have shown that modification of the somatostatin analogue octreotide (OC), by substitution of tyrosine for phenylalanine at position 3 and of a C-terminal carboxylic acid for an alcohol, to give Tyr³-octreotate (Y3-TATE) improved uptake of the peptide in somatostatin receptor-positive tissues. To determine which substitution best accounts for increased target tissue uptake, the peptides containing single modifications, Tyr³-octreotide (Y3-OC) and octreotate (TATE), were synthesized. These peptides were conjugated to the macrocyclic chelating agent 1,4,8,11-tetraazacyclotetradecane-N,N‘,N‘‘,N‘‘‘-tetraacetic acid (TETA) and radiolabeled with ⁶⁴Cu(II). The in vitro receptor binding, in vitro tumor cell uptake, and in vivo distribution properties of ⁶⁴Cu-labeled TETA-Y3-OC and TETA-TATE were compared to those of [⁶⁴Cu]TETA-OC and [⁶⁴Cu]TETA-Y3-TATE. Cu-TETA-TATE (IC₅₀ = 0.297 ± 0.0055 nM) and Cu-TETA-Y3-TATE (IC₅₀ = 0.308 ± 0.0375 nM) displayed significantly higher binding affinity to somatostatin receptors on CA20948 rat pancreatic tumor membranes than Cu-TETA-Y3-OC (IC₅₀ = 0.397 ± 0.0206 nM) and Cu-TETA-OC (IC₅₀ = 0.498 ± 0.039 nM). Similarly, the uptakes of [⁶⁴Cu]TETA-Y3-TATE (60.75 ± 1.21%) and [⁶⁴Cu]TETA-TATE (55.62 ± 0.16%) into AR42J rat pancreatic tumor cells over a 2-h time period were higher than those of [⁶⁴Cu]TETA-Y3-OC (47.20 ± 1.20%) and [⁶⁴Cu]TETA-OC (34.07 ± 2.24%). The in vitro results suggest that the C-terminal carboxylate may contribute more to enhanced receptor binding and tumor cell uptake than the substitution at the 3-position. Biodistributions in CA20948 tumor-bearing rats showed receptor-mediated uptake of the ⁶⁴Cu-labeled peptides in somatostatin-rich tissues, including the pituitary, adrenals, pancreas, and tumor. The structure−activity relationships of the four ⁶⁴Cu-labeled peptides did not show consistent trends in all target tissues, but [⁶⁴Cu]TETA-Y3-TATE exhibited tumor uptake 1.75−3.5 times higher than the other derivatives at 4 h postinjection. The greater tumor retention of [⁶⁴Cu]TETA-Y3-TATE justifies the selection of this agent for future PET imaging and targeted radiotherapy studies.