Synthesis and dopaminergic activity of some 3-(1,2,3,6-tetrahydro-1-pyridylalkyl)indoles. A novel conformational model to explain structure-activity relationships.

The synthesis and dopaminergic properties of a novel type of dopamine agonist is described. The number and kind of essential structural elements differ significantly from that of the rigid apomorphine-type dopamine agonists. Using standard molecular modeling techniques, a conformational model is developed proposing a U-shaped conformation which might be energetically preferred through aromatic pi-pi-interactions between both of the electron rich aromatic structural elements of this class of compounds. Superimposition of conformations of the lead compound 28 with apomorphine yields a novel model explaining the atypical structure-activity relationships found in this class of indolealkylamines.

Conformation-based design of two cyclic physalaemin analogues.

Two new cyclic analogues of physalaemin were designed on the basis of the conformation found in DMSO solution. Glp-Ala-cyclo(-Asp-Pro-Asn-Lys-)-Phe-Tyr-Gly-Leu-Met-NH2 (1) was synthesized by cyclization of physalaemin. In 2 the Asp residue was replaced by Glu. The linear analogue of 2 was synthesized by the solid phase method and subsequently cyclized. Two-dimensional nmr methods were employed to assign the proton and carbon resonances. Proton-proton distances were extracted from rotating frame nuclear Overhauser effect spectra and used as restraints in the molecular dynamics calculations. Analogue 1 was found to have a similar conformation as physalaemin, whereas 2 did not form intramolecular hydrogen bonds. The pharmacological evaluation revealed that both peptides have similar potencies as physalaemin in the dog carotid artery (NK-1 receptor). Therefore, the charged side chains of physalaemin appear not essential for NK-1 activation. However, the other tachykinin receptors show good sensitivity to the cyclic peptides. It is concluded that the replacement of a salt bridge by an amide bond connecting the side chains of natural residues might provide useful information about the biological significance of some charged side chains of neurokinins.

4-Heterocyclyloxy-2H-1-benzopyran potassium channel activators.

The reaction of 2,4-dihydroxypyridine (2) with 3,4-epoxy-3,4-dihydro-2,2-dimethyl-2H-1-benzopyran-6-carbonitrile (1) yielded the 4-[(1,2-dihydro-2-oxo-4-pyridyl)oxy] compound 3a, accompanied by small amounts of the isomeric 4-(1,2-dihydro-4-hydroxy-2-oxo-1-pyridyl) compound 4. This could also be prepared by hydrogenation of the benzyloxy derivative 5. Reaction of 3,6-pyridazinediol (10) with 1 (R = CN) gave the 4-[(1,6-dihydro-6-oxo-3-pyridazinyl)oxy] compound 11a, which in turn rearranged on heating with NaH in DMSO into the 4-(1,6-dihydro-3-hydroxy-6-oxo-1-pyridazinyl) compound 12. A series of 6-substituted analogues (R = CO2Me, CSNH2, NO2, Br) of 3a and 11a were synthesized. N-Alkylation led to compounds 14a-c (R = Me, Et, CHMe2). The 4-heterocyclyloxychromenes 9 and 16a were obtained by alkaline hydrolysis of their 3-camphorsulfonates. The racemic pyridazinyloxy compounds 11a and 14a could be resolved via their diastereomeric camphorsulfonates or camphanates. The differences between the 4-heterocyclyloxychromanols and the isomeric N-substituted compounds 4 and 12 were elucidated in the course of extensive NMR investigations. While in DMSO the former appeared to be conformationally flexible molecules the latter were rigid. All compounds were tested for oral antihypertensive activity in spontaneously hypertensive rats, using doses of 1 mg/kg. High and long lasting activities were found for the pyridyloxy compounds 3a and 3d, the pyridazinyloxy compound 11a, and its N-alkylation products, as well as for the 3S,4R-enantiomers 20a and 22a. (-)-(3S,4R)-3,4-Dihydro-4-[(1,6-dihydro-1-methyl-6-oxo-3- pyridazinyl)oxy]-3-hydroxy-2,2-dimethyl-2H-1-benzopyran-6-carbonitrile (22a) was selected for further development.

Prevention of phalloidin-induced lesions on isolated rat hepatocytes by novel synthetic analogues of somatostatin.

Exposure of freshly isolated hepatocytes to phalloidin produced blebs on their surfaces: this phenomenon was time- and dose-dependent and irreversible. When hepatocytes were pretreated with somatostatin or with some of its synthetic analogues, formation of blebs was dramatically reduced. This cytoprotective effect was dose-dependent: the dose-response profiles enabled the determination of the CD50 values, i.e., the concentrations of analogues that yielded 50% cytoprotection. The analogues with sequences of amino acids in the retro form, compared to those in somatostatin-14, exhibited higher cytoprotection; the retro hexapeptide, cyclo(-Phe-Thr-Lys-D-Trp-Phe-D-Pro-), was 27 times more active than somatostatin-14. Specificity of cytoprotection was examined by pretreating hepatocytes with biologically active peptide hormones prior to exposure to phalloidin. On a molar basis, prolactin and thyroid-stimulating hormone possessed activity comparable to that of somatostatin-14, whereas glucagon was twice as active. Insulin, vitamin A and propranolol exercised less than 10% protection. The synthetic analogues of somatostatin are potent protective agents against cell lesions induced by phalloidin. Formation of blebs on hepatocytes by toxins and their prevention by agents of interest may serve as a suitable morphological assay for screening of cytotoxicity and cytoprotection.

Modified somatostatins as inhibitors of a multispecific transport system for bile acids and phallotoxins in isolated hepatocytes.

Somatostatin inhibits the uptake of phallotoxins and of cholic acid in isolated liver cells in a concentration-dependent manner. The inhibition is independent on the preincubation period and fully reversed by switching to a somatostatin-free buffer. Concentrations needed for 50% inhibition decreased 30-80-fold when somatostatin was modified by variation of its amino acid sequence. Some cyclic hexa- or penta-peptides inhibited both kinds of transport more strongly as the original (14 amino acid) somatostatin did. Three of the analogs showed a 2-3-fold higher potency than the others. The most potent compound (cyclo (Phe-Thr-Lys-Trp-Phe-D-Pro) 1 was studied in detail. The IC50 for the initial uptake of phallotoxin (6 microM) or of cholate (6 microM) was 1.5 or 3 microM, respectively. 1 inhibited the uptake of cholate in a competitive manner. The inhibition was independent on the preincubation time, but in contrast to somatostatin not fully reversible after a preincubation of 35 min. Somatostatin as well as its analogs prevented binding of isothiocyanatobenzamido [3H]cholate (an affinity label of the cholate transporter) to isolated plasma membranes from rat liver. The transport inhibition of cholate uptake is unlikely to be a hormonal effect of somatostatin, because the concentrations needed are approx. 1000-fold higher than circulating levels; however, it is apparently possible to increase the inhibitory potency on the tested transport system by modification of the sequence without increase of the well-known hormonal effects (Designing Activity and Receptor-Selectivity in Cyclic Peptide Hormone Analogs, Kessler, H., 18th Ervag Conference, Brussels, 1983).