Synthetic glucagon antagonists and partial agonists

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Abstract
This paper reports the synthesis and the biological activities of six new glucagon analogues. In these compounds N-terminal modifications of the glucagon sequence were made, in most cases combined with changes in the C-terminal region which had been shown previously to enhance receptor affinity. The design of these analogues was based on [Lys17,18,Glu21]glucagon,1 a superagonist, which binds five times better than glucagon to the glucagon receptor, and on the potent glucagon antagonist [D-Phe4,Tyr5,Arg12]glucagon, which does not stimulate adenylate cyclase system even at very high concentrations. The N-terminal modifications involved substitution of His1 by the unnatural conformationally constrained residue, 4,5,6,7-tetrahydro-1H-imidazo[c]pyridine-6-carboxylic acid (Tip) and by desaminohistidine (dHis). In addition we prepared two analogues (6 and 7), in which we deleted the Phe6 residue, which was suggested to be part of a hydrophobic patch and involved in receptor binding. The following compounds were synthesized: [Tip1, Lys17,18,Glu21]glucagon (2); [Tip1,D-Phe4,Tyr5,Arg12,Lys17,18,Glu21]glucagon (3); [dHis1,D-Phe4,Tyr5,Arg12,Lys17,18,Glu21]glucagon (4); [dHis1,Asp3,D-Phe4,Tyr5,Arg12,Lys17,18,Glu21+ ++]glucagon (5); des-Phe6-[Tip1,D-Phe4,Tyr5,Arg12,Glu21]glucagon (6); des-Phe6-[Asp3,D-Phe4,Tyr5,Arg12,Glu21]glucagon (7). The binding potencies of these new analogues relative to glucagon (= 100) are 3.2 (2), 2.9 (3), 10.0 (4), 1.0 (5), 8.5 (6), and 1.7 (7). Analogue 2 is a partial agonist (maximum stimulation of adenylate cyclase (AC) approximately 15% and a potency 8.9% that of glucagon, while the remaining compounds 3-7 are antagonists unable to activate the AC system even at concentrations as high as 10(-5) M. In addition, in competition experiments, analogues 3-7 caused a right-shift of the glucagon stimulated adenylate cyclase dose-response curve.(ABSTRACT TRUNCATED AT 250 WORDS)