1. Peptide folding induces high and selective affinity of a linear and small beta-peptide to the human somatostatin receptor 4
K Gademann, T Kimmerlin, D Hoyer, D Seebach J Med Chem. 2001 Jul 19;44(15):2460-8. doi: 10.1021/jm010816q.
beta-Peptides with side chains in the 2- and 3-positions on neighboring residues (of (S) configuration) are known to fold and form a turn (similar to an alpha-peptidic beta-turn). Thus, we have synthesized an appropriately substituted beta-tetrapeptide derivative to mimic the hormone somatostatin in its binding to the human receptors hsst(1-5), which is known to rest upon a turn containing the amino acid residues Thr, Lys, Trp, and Phe. The N-acetyl-peptide amide Ac-beta(3)-HThr-beta(2)-HLys-beta(3)-HTrp-beta(3)-HPhe-NH(2) (1) indeed shows all characteristics of the targeted turn-mimic: Lys CH(2) groups are in the shielding cone of the Trp indole ring (by NMR analysis, Figure 2) and there is high and specific nanomolar affinity for hsst(4) receptor (Table 1). In contrast, the isomer 2 bearing the Lys side chain in 3-, rather than in the 2-position, has a 1000-fold smaller affinity to hsst(4). The syntheses of the required Fmoc-protected beta-amino acids (8-11, 17) are described (Schemes 1-3). Coupling of the beta-amino acids was achieved by the manual solid-phase technique, on Rink resin.
2. Design, preparation and biological evaluation of a 177Lu-labeled somatostatin receptor antagonist for targeted therapy of neuroendocrine tumors
Hossein Behnammanesh, et al. Bioorg Chem. 2020 Jan;94:103381. doi: 10.1016/j.bioorg.2019.103381. Epub 2019 Oct 22.
Somatostatin receptor-targeted radionuclide therapy has become an effective treatment in patients with neuroendocrine tumors. Recently, investigations on the development of antagonistic peptides are increasing with possible superior biological properties as opposed to the agonists. Herein, we have reported the development of a new somatostatin receptor peptide ligand labeled with 177Lu to achieve a therapeutic ligand for tumor treatment. The interactions of selected and drown ligands using Avogadro software were docked on somatostatin receptor by Dink algorithm. The best docked peptide-chelator conjugate (DOTA-p-Cl-Phe-Cyclo(d-Cys-l-BzThi-d-Aph-Lys-Thr-Cys)-d-Tyr-NH2) (DOTA-Peptide 2) was synthesized using the Fmoc solid-phase method. DOTA-Peptide 2 was radiolabeled with the 177Lu Trichloride (177LuCl3) solution at 95 °C for 30 min and radiochemical purity (RCP) of 177Lu-DOTA-Peptide 2 solution was monitored by radio-HPLC and radio-TLC procedures. The new radiolabeled peptide was evaluated for stability, receptor binding, internalization, biodistribution and single-photon emission computed tomography (SPECT) imaging using C6 glioma cells and C6 tumor-bearing rats. DOTA-Peptide 2 was obtained with 98% purity and efficiently labeled with 177Lu (RCP > 99%). 177Lu-DOTA-Peptide 2 showed a high value of stability in acetate buffer (91.4% at 312 h) and human plasma (>97% at 24 h). Radioconjugate exhibited low internalization (<5%) and high affinity for somatostatin receptors (Kd = 12.06 nM, Bmax = 0.20 pmol/106 cells) using saturation binding assay. Effective tumor uptake of 7.3% ID/g (percentage of injected dose per gram of tumor) at 4 h post-injection and fast clearance of radiopeptide from blood and other organs led to a high tumor-to-normal organ ratios. SPECT/CT imaging clearly showed the activity localization in tumor. The favorable antagonistic properties of 177Lu-DOTA-Peptide 2 on the somatostatin receptors can make it a suitable candidate for peptide receptor radionuclide therapy (PRRT). In the future study, the therapeutic application of this radiopeptide will be evaluated.
3. Functional characterisation of the putative somatostatin sst2 receptor antagonist CYN 154806
Caroline Nunn, Philippe Schoeffter, Daniel Langenegger, Daniel Hoyer Naunyn Schmiedebergs Arch Pharmacol. 2003 Jan;367(1):1-9. doi: 10.1007/s00210-002-0656-5. Epub 2002 Dec 20.
The two forms (DTyr8 and LTyr8) of the putative somatostatin sst2 receptor antagonist CYN 154806 (Ac-4NO2-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-D/LTyr-NH2) were investigated on recombinant human somatostatin receptors and endogenous guinea-pig ileum receptors. In radioligand binding studies using the agonist radioligands [125I]LTT-SRIF-28, [125I][Tyr10]cortistatin-14, [125I]CGP 23996 and [125I][Tyr3]octreotide in Chinese hamster lung fibroblast (CCL39) and Chinese hamster ovary (CHO) cells expressing human somatostatin receptors (hsst1-5), CYN 154806 binds to sst2 receptors with nanomolar affinity (pKD=8.14-8.89), 40- to 4500-fold higher than for sst1, sst3 or sst4. High affinity was also demonstrated for sst5 receptors, particularly for LTyr8CYN 154806 where the sst5 affinity was higher than for sst2 receptors when using [125I]CGP 23996 and [125I][Tyr3]octreotide. Functional properties of the compounds were examined in Chinese hamster ovary (CHO) cells expressing human sst2 receptors, in (1) inhibition of forskolin-stimulated adenylate cyclase, (2) stimulation of serum response element-driven luciferase expression and (3) [35S]guanosine 5'-O-(3-thiotriphosphate) ([35S]GTPS) binding. L- and DTyr8CYN 154806 showed full agonism at inhibition of forskolin-stimulated cAMP accumulation (pEC50=7.73 for both, Emax 104% and 78%, respectively), partial agonism at luciferase expression (pEC50=7.85 and 8.16, Emax=50% and 29%, respectively) and behaved as apparently silent antagonists at [35S]GTPS binding (no agonism observed, pKB=6.88 and 7.50, respectively). The agonist potential was confirmed in isolated guinea-pig ileum preparations via measurement of SRIF-induced inhibition of neurotransmission, where the L-isoform had marked agonism (pEC50=8.23, Emax=32%) whereas the D-isoform was apparently devoid of agonism. The present data suggest that CYN 154806 should be used with caution as an sst2 receptor antagonist tool, since it possesses intrinsic activity at sst2, and high affinity for both sst2 and sst5 receptors. The DTyr form, having lower intrinsic activity, especially in natural tissues, and greater selectivity for sst2 receptors, may be more reliable than LTyr CYN 154806.
