Synthetic glucagon antagonists and partial agonists

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 [Lys^17.18,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, Lys^17.18,Glu²¹]glucagon (2); [Tip¹,d‐Phe⁴,Tyr⁵,Arg¹²,Lys^17.18,Glu²¹ glucagon (3); [dHis¹,d‐Phe⁴,Tyr⁵,Arg¹², Lys^17.18,Glu²¹ glucagon (4); [dHis¹,Asp³,d‐Phe⁴,Tyr⁵,Arg¹²,Lys^17.18,Glu²¹]glucagon (5); des‐Phe⁶‐[Tip¹,D‐Phe⁴,Tyr⁵Arg¹²,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 10⁻⁵m. In addition, in competition experiments, analogues 3‐7 caused a right‐shift of the glucagon stimulated adenylate cyclase dose‐response curve. Hence these compounds are glucagon receptor antagonists with respect to the glucagon receptor coupled to the adenylate cyclase system.