Ring size in octreotide amide modulates differently agonist versus antagonist binding affinity and selectivity

H-DPhe (2)-c[Cys (3)-Phe (7)-DTrp (8)-Lys (9)-Thr (10)-Cys (14)]-Thr (15)-NH2 (1) (a somatostatin agonist, SRIF numbering) and H-Cpa (2)-c[DCys (3)-Tyr (7)-DTrp (8)-Lys (9)-Thr (10)-Cys (14)]-Nal (15)-NH2 (4) (a somatostatin antagonist) are based on the structure of octreotide that binds to three somatostatin receptor subtypes (sst 2/3/5) with significant binding affinity. Analogues of 1 and 4 were synthesized with norcysteine (Ncy), homocysteine (Hcy), or D-homocysteine (DHcy) at positions 3 and/or 14. Introducing Ncy at positions 3 and 14 constrained the backbone flexibility, resulting in loss of binding affinity at all sst s. The introduction of Hcy at positions 3 and 14 improved selectivity for sst 2 as a result of significant loss of binding affinity at the other sst s. Substitution by DHcy at position 3 in the antagonist scaffold (5), on the other hand, resulted in a significant loss of binding affinity at sst 2 and sst 3 as compared to the different affinities of the parent compound (4). The 3D NMR structures of the analogues in dimethylsulfoxide are consistent with the observed binding affinities.