Triazolo-Peptidomimetics: Novel Radiolabeled Minigastrin Analogs for Improved Tumor Targeting

Abstract

MG11 is a truncated analog of minigastrin, a peptide with high affinity and specificity toward the cholecystokinin-2 receptor (CCK2R), which is overexpressed by different tumors. Thus, radiolabeled MG11 derivatives have great potential for use in cancer diagnosis and therapy. A drawback of MG11 is its fast degradation by proteases, leading to moderate tumor uptake in vivo. We introduced 1,4-disubstituted 1,2,3-triazoles as metabolically stable bioisosteres to replace labile amide bonds of the peptide. The "triazole scan" yielded peptidomimetics with improved resistance to enzymatic degradation and/or enhanced affinity toward the CCK2R. Remarkably, our lead compound achieved a 10-fold increase in receptor affinity, resulting in a 2.6-fold improved tumor uptake in vivo. Modeling of the ligand−CCK2R complex suggests that an additional cation−π interaction of the aromatic triazole moiety with the Arg 356 residue of the receptor is accountable for these observations. We show for the first time that the amide-to-triazole substitution strategy offers new opportunities in drug development that go beyond the metabolic stabilization of bioactive peptides.

1,4-Disubstituted 1,2,3-Triazoles as Amide Bond Surrogates for the Stabilisation of Linear Peptides with Biological Activity

“…It has been shown that the amide-to-triazole switch leads to peptidomimetics with maintained lipophilicity. In the studied cases, single or multiple substitutions of (up to three) amide bonds with triazoles did not change the logD 7.4 values of peptides consisting of at least six amino acids [ 77 , 82 , 93 , 112 ]. Even though 1,4-disubstituted 1,2,3-triazoles are good bioisosteres of trans -amide bonds by mimicking electronic and steric features, a few essential differences exist.…”

“…Besides, the Corey-Fuchs reaction for the transformation of aldehydes to alkynes can be employed for the synthesis of Boc-protected α-amino alkynes [ 81 ]. However, it is known that this approach may lead to partial racemisation of the α-amino alkyne [ 52 , 56 , 82 ]. The use of α-amino alkynes as a mixture of enantiomers results in the formation of diastereomers when used for peptide synthesis and additional measures (i.e., purification of the alkynes using chiral HPLC or separation of the resulting diastereomeric peptide products by HPLC) have to be taken into account.…”

“…Minigastrin analogues have been extensively explored as potential tumour-targeting vectors for CCK2R-positive cancer cells and a screening test resulted in the truncated DOTA-conjugated octapeptide MG11 as a promising candidate with a high receptor affinity but poor plasma stability [ 111 ]. Recently, Grob et al reported the successful application of the amide-to-triazole switch methodology to MG11 in order to increase its stability [ 82 ]. The authors focussed on the use of MG11 as a radiotracer.…”

Initial In Vitro and In Vivo Evaluation of a Novel CCK2R Targeting Peptide Analog Labeled with Lutetium-177

“…The stability was not considerably further improved by additional triazole insertion at Ala-Tyr or Tyr-Gly. On the other hand, the insertion of triazoles at Tyr-Gly, Ala-Tyr, DGlu-Ala had a remarkable effect on tumor uptake, which was however clearly inferior to DOTA-MGS5 [ 28 , 30 , 44 ]. The introduction of an N -methylated peptide bond at Nle-Trp showed both effects of increased stability and improved receptor interaction.…”

“…With the aim of further improving the in vivo stability of DOTA-MGS5, we have explored additional modification of the peptide sequence. The amino acid proline (Pro) is a promising candidate for amino acid exchange and forms a tertiary amide bond which similarly to N -methylated peptide bonds and triazoles may improve the stability in vivo [ 27 , 30 ]. In this study, a preliminary preclinical characterization of the new MG analog DOTA-DGlu-Pro-Tyr-Gly-Trp-( N -Me)Nle-Asp-1Nal-NH 2 ( 1 ) was carried out focusing on the enzymatic stability of the 177 Lu-labeled radiopeptide in vivo.…”