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

Abstract

Neurotensin(8-13) (NTS(8-13)) analogs with C- and/or N-terminal β-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 (Table 2). Most of the β-amino acid-containing NTS(8-13) analogs (Table 1 and Fig.2), including the (68) Ga complexes of the DOTA-substituted ones (6; Figs.2 and 5), are stable for ca. 1 h in human serum and plasma, and in murine plasma. The biodistributions of two (68) Ga 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 60 min (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 1 h) with the ex vivo blood content (after 10-15 min) of the two (68) Ga complexes shows that they are rapidly cleaved in the animals (Fig.5).

Evaluation of DOTA-chelated neurotensin analogs with spacer-enhanced biological performance for neurotensin-receptor-1-positive tumor targeting

“…While no in vitro degradation was found for the [N-α-Me 8 ,Dmt 11 ,Tle 12 ]NT(6–13) peptide, Gruaz-Guyon and co-workers have demonstrated that this pharmacophore does exhibit significant in vivo metabolism in as little as 15 min post-injection[11]. This is an important consideration, as others have pointed out, when evaluating in vivo data[36]. Although the stability studies were not carried out with 177 Lu-N0, 177 Lu-N2 and 177 Lu-N3, we assume similar stabilities given the relative similarity of the radioconjugates.…”

1,2,3-Triazole Stabilized Neurotensin-Based Radiopeptidomimetics for Improved Tumor Targeting

“…9,12,13,34,35,40 Strikingly, tumor uptake of triazole-stabilized [ 177 Lu]-NT VI and IX was increased up to 2-fold (approximately 2% injected dose (ID)/g) in comparison to that of the corresponding reference compounds [ 177 Lu]-NT I and VIII, respectively (each approximately 1% ID/g). These results suggest that the introduction of a 1,2,3-triazole moiety between the Arg 9 -Arg 8 residues of NT(8−13) may represent a general approach to improve the tumor targeting properties of the peptide.…”

18F- and 68Ga-Labeled Neurotensin Peptides for PET Imaging of Neurotensin Receptor 1

“…In another study, NT peptides with C-and/or N-terminal β-amino acid residues for metabolic stabilization were synthesized and labeled with gallium-68. 20 Again, fast degradation occurred in vivo and only low specific uptake in HT29 tumors was observed, making these radioligands unsuitable for PET imaging. The group of Shively et al 31 This prosthetic group was prepared in 35% isolated radiochemical yield (decay-corrected) and after coupling to the Cysbearing NT peptide the end product was isolated in 31% yield.…”

“…by the introduction of non-natural amino acids or by variations of the amino bonds. [12][13][14][15][16][17][18][19][20][21][22][23] In principle, PET allows non-invasive in vivo imaging of receptors expressed on tumors with excellent sensitivity and precise quantification of receptor densities and therefore PET is a highly powerful imaging modality in nuclear medicine, provided that suitable radioligands for in vivo use are available. 24 Most of the previously developed radiolabeled NT derivatives were labeled with nuclides suitable for single photon emission tomography (SPECT) or radiotherapy, [25][26][27] however, only a limited number of NT derivatives were radiolabeled with an isotope suitable for PET imaging studies, such as fluorine-18 or gallium-68.…”