Novel sst₄-Selective Somatostatin (SRIF) Agonists. 2. Analogues with β-Methyl-3-(2-naphthyl)alanine Substitutions at Position 8

Abstract

We present a family of human sst(4)-selective, high-affinity (IC(50) = 2-4 nM) cyclic somatostatin (SRIF) octapeptides. These peptides result from the substitution of dTrp(8) in H-c[Cys(3)-Phe(6)-Phe(7)-DTrp(8)-Lys(9)-Thr(10)-Phe(11)-Cys(14)]-OH (SRIF numbering) (ODT-8) by one of the four conformationally biased stereoisomers of beta-methyl-3-(2-naphthyl)alanine (beta-Me2Nal). Whereas H-c[Cys-Phe-Phe-DNal-Lys-Thr-Phe-Cys]-OH (ODN-8, 2) has high affinity and marginal selectivity for human sst(3) (Reubi et al., Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 13973-13978), H-c[Cys-Phe-Tyr-D-threo-beta-Me2Nal-Lys-Thr-Phe-Cys]-OH (5) has high affinity for all sst's except for sst(1); H-c[Cys-Phe-Tyr-L-threo-beta-Me2Nal-Lys-Thr-Phe-Cys]-OH (6) has high affinity for sst(4) (IC(50) = 2.1 nM), with more than 50-fold selectivity toward the other receptors. Analogues 7 and 8, containing d- and l-erythro-beta-Me2Nal instead of the corresponding threo derivatives at position 8, are essentially inactive at all receptors. Substitution of Tyr(7) in 5 and 6 by Aph(7) resulted in 9 and 10 with similar affinity patterns overall yet lowered affinity. The substitution of DCys(3) for Cys(3) in 5 and 6 yielded H-c[DCys-Phe-Tyr-D-threo-beta-Me2Nal-Lys-Thr-Phe-Cys]-OH (11) and H-c[DCys-Phe-Tyr-L-threo-beta-Me2Nal-Lys-Thr-Phe-Cys]-OH (12), with biological profiles almost identical to those of their parents 5 and 6 (i.e., high affinity for sst(2-5) for 11 and high affinity and selectivity for sst(4) for 12). Analogue 12, with high sst(4) affinity combined with the highest sst(4) selectivity among all tested compounds, is an agonist in the cAMP accumulation assay (EC(50) = 1.29 nM). Cold monoiodination of 12 yielded 14, with loss of sst(4) selectivity and loss of high affinity (IC(50) = 21 nM). Introduction of Tyr(2) in 9 and 10 and substitution of Cys(3) by dCys(3), to yield 15 and 16 (IC(50) = 9.8 and 61 nM, respectively, for sst(4) and limited selectivity), failed to generate a high-affinity (125)iodinatable sst(4)-selective ligand. Substitution of Phe by Tyr at position 11 in H-c[DCys-Phe-Phe-L-threo-beta-Me2Nal-Lys-Thr-Phe-Cys]-OH yielded 18 (IC(50) = 11.8 nM at sst(4)), with limited sst(4) selectivity (30-fold or greater at the other receptors) yet only slightly improved affinity over that of 14. Cold monoiodination of 18 yielded 20 (IC(50) = 30 nM at sst(4) and high selectivity). Whereas we were able, in this study, to identify a new family of sst(4)-selective, high-affinity compounds, our additional goal, to identify highly potent and sst(4)-selective ligands amenable to (125)iodination, could not be achieved satisfactorily. On the other hand, some of the diastereomers identified in this study, such as 5, 11, 17, and 19, are very potent ligands at all receptors but sst(1).

Novel sst2-Selective Somatostatin Agonists. Three-Dimensional Consensus Structure by NMR

“…For example, the position and the number of aromatic rings involved in binding are different for the two ligands, suggesting that these hydrophobic residues interact with different regions of the sst 1 and sst 4 receptors, respectively ( Figure 3D and 3E). On the other hand, most of the previously published analogues binding with high affinity to sst 2 receptor were also binding to sst 5 receptors with nM affinity and sometimes to sst 3 receptors as well. Melacini et al, proposed a pharmacophore model for these analogues binding nonselectively to all of the three receptors ( Figure 3C).…”

“…Hence, it can be concluded that this aromatic ring is not crucial for sst 2 binding, and is crucial for sst 3 and sst 5 binding, as the binding to receptors 3 and 5 decreased ( Table 1). Replacement of Phe at position 7 by an amino acid having a longer side chain, such as Aph has also decreased the binding to sst 3 and sst 5 receptors, retaining high binding affinity to sst 2 receptors. Table 4).…”

“…1,2 Cells containing SRIF are typically neurons or endocrine-like cells which are found in high density throughout the central and peripheral nervous systems, in the endocrine pancreas, and in the gut and in small numbers in the thyroid, adrenals, submandibular glands, kidneys, prostrate and placenta. 1,2 The activities of SRIF are mediated through a family of five different high affinity membrane receptors, sst 1 -sst 5 (sst s ). Due to its broad spectrum of physiological activities and its short duration of action due to rapid proteolytic degradation in vivo, 3 SRIF continues to be a target for the development of subtype-specific analogues.…”

Receptor-Mediated Tumor Targeting with Radiopeptides. Part 1. General Concepts and Methods: Applications to Somatostatin Receptor-Expressing Tumors

“…The same is true for the cyclohexapeptide TT-232 (264). Because some tumors express sst 1 and/or sst 4 but lack sst 2 and sst 5 , efforts have been put into the development of sst 1 and sst 4 subtype-specific analogues (167,168). More recently, high-affinity subtype-selective nonpeptide agonists of sst 1 -sst 5 were identified in combinatorial libraries, based on molecular modeling of known peptide agonists (459); these structure are, however, less suitable as carriers for targeting sst receptors with radioisotopes.…”

Enantioseparation of β-methyl-substituted amino acids with cyclodextrins by capillary zone electrophoresis☆

“…Recently amino acid analysis by CE have been reviewed by Rizzi [22] and Chankvetadze [23], but very few papers dealing with capillary electrophoresis (CE) of ␤-methyl-amino acids have appeared. Gübitz and coworkers [17] applied ligand-exchange CE,Érchegyi et al [24] separated erythro and threo stereoisomers of ␤-methyl-3-(2-naphthylalanine) by CE and very recently Jiang et al [25] resolved some racemic erythroand threo-␤-methyl-amino acid enantiomers. Survey of literature data revealed that the baseline separation of all four stereoisomers in one run is rarely attainable.…”