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

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.