Switch from antagonist to agonist after addition of a DOTA chelator to a somatostatin analog

Peptide receptor targeting has become an increasingly attractive method to target tumors diagnostically and radiotherapeutically. Peptides linked to a variety of chelators have been developed for this purpose. They have, however, rarely been tested for their agonistic or antagonistic properties. We report here on a somatostatin antagonist that switched to an agonist upon coupling to a DOTA chelator.

Two novel somatostatin analogs, 406-040-15 and its DOTA-coupled counterpart 406-051-20, with and without cold Indium labeling, were tested for their somatostatin receptor subtypes 1-5 (sst(1)-sst(5)) binding affinity using receptor autoradiography. Moreover, they were tested functionally for their ability to affect sst(2) and sst(3) internalization in vitro in HEK293 cells stably expressing the human sst(2) or sst(3) receptor, using an immunofluorescence microscopy-based internalization assay.

All three compounds were characterized as pan-somatostatin analogs having a high affinity for all five sst. In the sst(2) internalization assay, all three compounds showed an identical behavior, namely, a weak agonistic effect complemented by a weak antagonistic effect, compatible with the behavior of a partial agonist. Conversely, in the sst(3) internalization assay, 406-040-15 was a full antagonist whereas its DOTA-coupled counterpart, 406-051-20, with and without Indium labeling, switched to a full agonist.

Adding the DOTA chelator to the somatostatin analog 406-040-15 triggers a switch at sst(3) receptor from an antagonist to an agonist. This indicates that potential radioligands for tumor targeting should always be tested functionally before further development, in particular if a chelator is added.