Design and in Vitro Characterization of Highly sst2-Selective Somatostatin Antagonists Suitable for Radiotargeting

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Abstract
Radiolabeled sst2 and sst3 antagonists are better candidates for tumor targeting than agonists with comparable binding characteristics (Ginj, M.; Zhang, H.; Waser, B.; Cescato, R.; Wild, D.; Erchegyi, J.; Rivier, J.; Mäcke, H. R.; Reubi, J. C. Proc. Natl. Acad. Sci. U.S.A.2006, 103, 16436−16441.). Because most of the neuroendocrine tumors express sst2, we used the known antagonists acetyl-pNO2Phe2-c[dCys3-Tyr7-dTrp8-Lys9-Thr10-Cys14]-dTyr15-NH2 (1) (Bass, R. T.; Buckwalter, B. L.; Patel, B. P.; Pausch, M. H.; Price, L. A.; Strnad, J.; Hadcock, J. R. Mol. Pharmacol. 1996, 50, 709-715. Bass, R. T.; Buckwalter, B. L.; Patel, B. P.; Pausch, M. H.; Price, L. A.; Strnad, J.; Hadcock, J. R. Mol. Pharmacol. 1997, 51, 170; Erratum.) and H-Cpa2-c[dCys3-Tyr7-dTrp8-Lys9-Thr10-Cys14]-2Nal15-NH2 (7) (Hocart, S. J.; Jain, R.; Murphy, W. A.; Taylor, J. E.; Coy, D. H. J. Med. Chem. 1999, 42, 1863−1871.) as leads for analogues with increased sst2 binding affinity and selectivity. Among the 32 analogues reported here, DOTA-pNO2Phe2-c[dCys3-Tyr7-dAph8(Cbm)-Lys9-Thr10-Cys14-dTyr15-NH2 (3) and DOTA-Cpa2-c[dCys3-Aph7(Hor)-dAph8(Cbm)-Lys9-Thr10-Cys14]-dTyr15-NH2 (31) had the highest sst2 binding affinity and selectivity. All of the analogues tested kept their sst2 antagonistic properties (i.e., did not affect calcium release in vitro and competitively antagonized the agonistic effect of [Tyr3]octreotide). Moreover, in an immunofluorescence-based internalization assay, the new analogues prevented sst2 internalization induced by the sst2 agonist [Tyr3]octreotide without being active by themselves. In conclusion, several analogues (in particular 3, 31, and 32) have outstanding sst2 binding and functional antagonistic properties and, because of their DOTA moiety, are excellent candidates for in vivo targeting of sst2-expressing cancers.