Abstract
The overexpression of one or more somatostatin receptors (SST(1-5)R) in human tumors has provided an opportunity for diagnosis and therapy with somatostatin-like radionuclide carriers. The application of "pansomatostatin" analogs is expected to broaden the clinical indications and upgrade the diagnostic/therapeutic efficacy of currently applied SST(2)R-prefering radioligands. In pursuit of this goal, we now introduce two bicyclic somatostatin-14 (SS14) analogs, AT5S (DOTA-Ala(1)-Gly(2)-c[Cys(3)-Lys(4)-Asn(5)-c[Cys(6)-Phe(7)-DTrp(8)-Lys(9)-Thr(10)-Cys(11)]-Thr(12)-Ser(13)-Cys(14)]) and AT6S (DOTA-Ala(1)-Gly(2)-c[Cys(3)-Lys(4)-c[Cys(5)-Phe(6)-Phe(7)-DTrp(8)-Lys(9)-Thr(10)-Phe(11)-Cys(12)]-Ser(13)-Cys(14)]), suitable for labeling with trivalent radiometals and designed to sustain in vivo degradation. Both AT5S and AT6S and the respective [(111)In]In-AT5S and [(111)In]In-AT6S were evaluated in a series of in vitro assays, while radioligand stability and biodistribution were studied in mice. The 8/12-mer bicyclic AT6S showed expanded affinity for all SST(1-5)R and agonistic properties at the SST(2)R, whereas AT5S lost all affinity to SST(1-5)R. Both [(111)In]In-AT5S and [(111)In]In-AT6S remained stable in the peripheral blood of mice, while [(111)In]In-AT6S displayed low, but specific uptake in AR4-2J tumors and higher uptake in HEK293-SST(3)R tumors in mice. In summary, high radioligand stability was acquired by the two disulfide bridges introduced into the SS14 motif, but only the 8/12-mer ring AT6S retained a pansomatostatin profile. In consequence, [(111)In]In-AT6S targeted SST(2)R-/SST(3)R-positive xenografts in mice. These results call for further research on pansomatostatin-like radioligands for cancer theranostics.