Syntheses, Receptor Bindings, in vitro and in vivo Stabilities and Biodistributions of DOTA-Neurotensin(8-13) Derivatives Containing [beta]-Amino Acid Residues - A Lesson about the Importance of Animal Experiments

Neurotensin(8-13) (NTS(8-13)) analogs with C- and/or N-terminal [beta]-amino acid residues and three DOTA derivatives thereof have been synthesized (i.e., 1-6). A virtual docking experiment showed almost perfect fit of one of the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) derivatives, 6a, into a crystallographically identified receptor NTSR1 (Fig.1). The affinities for the receptors of the NTS analogs and derivatives are low, when determined with cell-membrane homogenates, while, with NTSR1-exhibiting cancer tissues, affinities in the single-digit nanomolar range can be observed (Table2). Most of the [beta]-amino acid-containing NTS(8-13) analogs (Table1 and Fig.2), including the 68Ga complexes of the DOTA-substituted ones (6; Figs.2 and 5), are stable for ca. 1h in human serum and plasma, and in murine plasma. The biodistributions of two 68Ga complexes (of 6a and 6b) in HT29 tumor-bearing nude mice, in the absence and in the presence of a blocking compound, after 10, 30, and 60min (Figs.3 and 4) lead to the conclusion that the amount of specifically bound radioligand is rather low. This was confirmed by PET-imaging experiments with the tumor-bearing mice (Fig.6). Comparison of the in vitro plasma stability (after 1h) with the ex vivo blood content (after 10-15min) of the two 68Ga complexes shows that they are rapidly cleaved in the animals (Fig.5). [PUBLICATION ABSTRACT]