Cyclo(PheN2-Tyr-D-Trp-Lys-Val-PheC3)-Thr-NH2 (PTR 3046), a backbone-cyclic somatostatin analogue, was synthesized by solid-phase methodology. The binding characteristics of PTR 3046 to the different somatostatin receptors, expressed in CHO cells, indicate high selectivity to the SSTR5 receptor. PTR 3046 is highly stable against enzymatic degradation as determined in vitro by incubation with rat renal homogenate and human serum. The biological activity of PTR 3046 in vivo was determined in rats. PTR 3046 inhibits bombesin- and caerulein-induced amylase and lipase release from the pancreas without inhibiting growth hormone or glucagon release. The major conformation of PTR 3046 in CD3OH, as determined by NMR, is defined by a type II' beta-turn at D-Trp-Lys and a cis amide bond at Val-PheC3.

We report the synthesis, bioactivity, and structure-activity relationship studies of compounds related to the Merck cyclic hexapeptide c[Pro6-Phe7-d-Trp8-Lys9-Thr10-Phe11], L-363,301 (the numbering in the sequence refers to the position of the residues in native somatostatin). The Pro residue in this compound is replaced with arylalkyl peptoid residues. We present a novel approach utilizing beta-methyl chiral substitutions to constrain the peptoid side-chain conformation. Our studies led to molecules which show potent binding and increased selectivity to the hsst2 receptor (weaker binding to the hsst3 and hsst5 receptors compared to L-363, 301). In vivo, these peptoid analogues selectively inhibit the release of growth hormone but have no effect on the inhibition of insulin. The biological assays which include binding to five recombinant human somatostatin receptors carried out in two independent laboratories and in vivo inhibition of growth hormone and insulin provide insight into the relationship between structure and biological activity of somatostatin analogues. Our results have important implications for the study of other peptide hormones and neurotransmitters.

In the present investigation, the rationale for the design, synthesis, and biological evaluation of potent inhibitors of neuronal Na+ channels is described. N,N'-diaryl- and N-aryl-N-aralkylguanidine templates were locked in conformations mimicking the permissible conformations of the flexible diarylguanidinium ion (AS+, AA+, SS+). The resulting set of constrained guanidines termed "lockamers" (cyclophane, quinazoline, aminopyrimidazolines, aminoimidazolines, azocino- and tetrahydroquinolinocarboximidamides) was examined for neuronal Na+ channel blockade properties. Inhibition of [14C]guanidinium ion influx in CHO cells expressing type IIA Na+ channels showed that the aminopyrimidazoline 9b and aminoimidazoline 9d, compounds proposed to lock the N,N'-diarylguanidinium in an SS+ conformation, were the most potent Na+ channel blockers with IC50's of 0.06 microM, a value 17 times lower than that of the parent flexible compound 18d. The rest of the restricted analogues with 4-p-alkyl substituents retained potency with IC50 values ranging between 0.46 and 2.9 microM. Evaluation in a synaptosomal 45Ca2+ influx assay showed that 9b did not exhibit high selectivity for neuronal Na+ vs Ca2+ channels. The retention of significant neuronal Na+ blockade in all types of semirigid conformers gives evidence for a multiple mode of binding in this class of compounds and can possibly be attributed to a poor structural specificity of the site(s) of action. Compound 9b was also found to be the most active compound in vivo based on the high level of inhibition of seizures exhibited in the DBA/2 mouse model. The pKa value of 9b indicates that 9b binds to the channel in its protonated form, and log D vs pH measurements suggest that ion-pair partitioning contributes to membrane transport. This compound stands out as an interesting lead for further development of neurotherapeutic agents.