Somatostatin receptor 1 selective analogues: 3. Dicyclic peptides

Detailed Insight into the Interaction of Bicyclic Somatostatin Analogue with Cu(II) Ions

Somatostatin analogues are useful pharmaceuticals in peptide receptor radionuclide therapy. In previous studies, we analyzed a new bicyclic somatostatin analogue (BCS) in connection with Cu(II) ions. Two characteristic sites were present in the peptide chain: the receptor- and the metal-binding site. We have already shown that this ligand can form very stable imidazole complexes with the metal ion. In this work, our aim was to characterize the intramolecular interaction that occurs in the peptide molecule. Therefore, we analyzed the coordination abilities of two cyclic ligands, i.e., P1 only with the metal binding site and P2 with both sites, but without the disulfide bond. Furthermore, we used magnetic circular dichroism (MCD) spectroscopy to better understand the coordination process. We applied this method to analyze spectra of P1, P2, and BCS, which we have described previously. Additionally, we analyzed the MCD spectra of P3 ligand, which has only the receptor binding site in its structure. We have unequivocally shown that the presence of the Phe-Trp-Lys-Thr motif and the disulfide bond significantly increases the metal binding efficiency.

The Binding Ability of a Bicyclic Somatostatin Analogue Towards Cu(II) Ions

Somatostatin (SST) analogues have aroused the interest of scientists for years. This group of compounds is used in the diagnosis and treatment of neuroendocrine tumors. However, new molecules useful as radiopharmaceuticals in targeted therapy are still searched for. Bicyclic peptides seem to be very interesting in this context. These molecules are associated with beneficial properties. In this work, we present studies on the binding ability of the bicyclic analogue of somatostatin toward Cu(II) ions which could potentially be a chelator for copper radionuclides. The research is focused on the analysis of Cu(II) interactions with the metal binding cycle of the ligand and the influence of the receptor binding site on the coordination process. This is a novelty in comparison to the SST analogues used in medicine, where a metal ion is coordinated by a chelator and connected with a bioactive molecule by the linker. In this work, we present the first coordination study for a bicyclic ligand. The obtained results showed that the complexes with only imidazole donors are characterized by significantly higher stability in comparison to the other peptides.

The diversity of (68)Ga-based imaging agents

Development of new radiopharmaceuticals and their availability are crucial factors influencing the expansion of clinical nuclear medicine. The number of new (68)Ga-based imaging agents for positron emission tomography (PET) is increasing greatly. (68)Ga has been used for labeling of a broad range of molecules (small organic molecules, peptides, proteins, and oligonucleotides) as well as particles, thus demonstrating its potential to become a PET analog of the legendary generator-produced gamma-emitting (99m)Tc but with added value of higher sensitivity and resolution as well as quantitation and dynamic scanning. Further, the availability of technology for GMP-compliant automated tracer production can facilitate the introduction of new radiopharmaceuticals and enable standardized, harmonized multicenter studies to be conducted for regulatory approval. This chapter presents some examples of tracers for targeted, pretargeted, and nontargeted imaging with emphasis on the potential of (68)Ga to facilitate clinically practical PET development and to promote the PET technique worldwide for earlier and better diagnostics, and personalized medicine with the ultimate objective of improved therapeutic outcome.

Novel, potent, and radio-iodinatable somatostatin receptor 1 (sst1) selective analogues

The proposed sst(1) pharmacophore (J. Med. Chem. 2005, 48, 523-533) derived from the NMR structures of a family of mono- and dicyclic undecamers was used to design octa-, hepta-, and hexamers with high affinity and selectivity for the somatostatin sst(1) receptor. These compounds were tested for their in vitro binding properties to all five somatostatin (SRIF) receptors using receptor autoradiography; those with high SRIF receptor subtype 1 (sst(1)) affinity and selectivity were shown to be agonists when tested functionally in a luciferase reporter gene assay. Des-AA(1,4-6,10,12,13)-[DTyr(2),DAgl(NMe,2naphthoyl)(8),IAmp(9)]-SRIF-Thr-NH(2) (25) was radio-iodinated ((125)I-25) and specifically labeled sst(1)-expressing cells and tissues. 3D NMR structures were calculated for des-AA(1,4-6,10,12,13)-[DPhe(2),DTrp(8),IAmp(9)]-SRIF-Thr-NH(2) (16), des-AA(1,2,4-6,10,12,13)-[DAgl(NMe,2naphthoyl)(8),IAmp(9)]-SRIF-Thr-NH(2) (23), and des-AA(1,2,4-6,10,12,13)-[DAgl(NMe,2naphthoyl)(8),IAmp(9),Tyr(11)]-SRIF-NH(2) (27) in DMSO. Though the analogues have the sst(1) pharmacophore residues at the previously determined distances from each other, the positioning of the aromatic residues in 16, 23, and 27 is different from that described earlier, suggesting an induced fit mechanism for sst(1) binding of these novel, less constrained sst(1)-selective family members.

Ring size of somatostatin analogues (ODT-8) modulates receptor selectivity and binding affinity

The synthesis, biological testing, and NMR studies of several analogues of H-c[Cys (3)-Phe (6)-Phe (7)-DTrp (8)-Lys (9)-Thr (10)-Phe (11)-Cys (14)]-OH (ODT-8, a pan-somatostatin analogue, 1) have been performed to assess the effect of changing the stereochemistry and the number of atoms in the disulfide bridge on binding affinity. Cysteine at positions 3 and/or 14 (somatostatin numbering) were/was substituted with d-cysteine, norcysteine, D-norcysteine, homocysteine, and/or D-homocysteine. The 3D structure analysis of selected partially selective, bioactive analogues (3, 18, 19, and 21) was carried out in dimethylsulfoxide. Interestingly and not unexpectedly, the 3D structures of these analogues comprised the pharmacophore for which the analogues had the highest binding affinities (i.e., sst 4 in all cases).

Molecular modeling suggests conformational scaffolds specifically targeting five subtypes of somatostatin receptors

Several analogs of somatostatin with conformational constraints in their peptide backbones have been modeled to determine energetically feasible conformations. Comparison of low-energy backbone structures of these peptides suggested unique conformations of the central Phe/Ala(i)-D-Trp(i+1)-Lys(i+2)-Thr(i+3) fragment characteristic for specific interactions of somatostatin with each of the five distinct subtypes of somatostatin receptors (SSTRs). The conformations obtained were in good agreement with experimental data obtained earlier by NMR measurements and/or X-ray crystallography. The results help rationalize experimental observations on the specificity of binding of various somatostatin analogs with different subtypes of the SSTRs. They also serve as templates for the design of conformationally constrained non-peptide scaffolds that effectively and selectively interact with different subtypes of SSTRs. Such scaffolds can be convenient carriers of radiolabels and near-infrared labels in specific agents for imaging tumors expressing different SSTR subtypes.

Somatostatin receptor 1 selective analogues: 2. N(alpha)-Methylated scan

Des-AA(1,2,5)-[d-Trp(8)/d-Nal(8),IAmp(9)]SRIF (AA = amino acid, Nal = 3-(2-naphthyl)-alanine, IAmp = 4-(N-isopropyl)-aminomethylphenylalanine, SRIF = somatostatin), with or without a tyrosine or monoiodotyrosine, were scanned with the introduction of a backbone N-methyl group and tested for binding affinity at the five human somatostatin receptors (sst(1)(-)(5)). N(alpha)-Methylation resulted in loss of sst affinity (2- to >5-fold) when introduced at residues Lys(4) (6), Phe(6) (7), Phe(7) (8), Thr(10) (11), and Phe(11) (12) of the parent compound Des-AA(1,2,5)-[d-Nal(8),IAmp(9)]SRIF (4). N(alpha)-Methylation was tolerated at residues Cys(3) (5), d-Nal(8) (9), Thr(12) (13), and Cys(14) (15) with retention of binding sst affinity and selectivity and resulted in an increase in sst binding affinity at positions IAmp(9) (10) and Ser(13) (14). In these series, the d-Trp(8) substitution versus d-Nal(8) is clearly superior. C-Terminally lysine-extended analogues (21-25) retained sst(1) selectivity and binding affinity when compared to their d-Nal(8)- (4) or d-Trp(8)- (3) containing parent. Des-AA(1,2,5)-[d-Trp(8), (N(alpha)Me)IAmp(9)]SRIF (17), Des-AA(1,2,5)-[d-Trp(8),IAmp(9),(N(alpha)Me)Ser(13)]SRIF (19), Des-AA(1,2,5)-[d-Trp(8),IAmp(9),(N(alpha)Me)Cys(14)]SRIF (20), Des-AA(1,2,5)-[d-Trp(8),(N(alpha)Me)IAmp(9),Tyr(11)]SRIF (34), and Des-AA(1,2,5)-[d-Agl(8)(N(beta)Me,2-naphthoyl),IAmp(9),Tyr(11)]SRIF (42) (Agl = aminoglycine) are sst(1) agonists in their ability to inhibit forskolin-induced cAMP production.

Somatostatin Receptor 1 Selective Analogues: 4. Three-Dimensional Consensus Structure by NMR

The three-dimensional NMR structures of six analogues of somatostatin (SRIF) are described. These analogues with the amino acid 4-(N-isopropyl)-aminomethylphenylalanine (IAmp) at position 9 exhibit potent and highly selective binding to human SRIF subtype 1 receptors (sst(1)). The conformations reveal that the backbones of these analogues have a hairpin-like structure similar to the sst(2)-subtype-selective analogues. This structure serves as a scaffold for retaining a unique arrangement of the side chains of d-Trp(8), IAmp(9), Phe(7), and Phe(11) or m-I-Tyr(11) (m-I-Tyr = mono-iodo-tyrosine). The conformational preferences and results from biological analyses of these analogues(1,2) allow a detailed study of the structure-activity relationship of SRIF. The proposed consensus pharmacophore of the sst(1)-selective analogues requires a unique set of distances between an indole/2-naphthyl ring, an IAmp side chain, and two aromatic rings. This motif is necessary and sufficient to explain the binding affinities of all of the analogues studied and is distinct from the existing models suggested for sst(4) as well as sst(2)/sst(5) selectivity.