Structure–activity relationships of dynorphin analogs substituted in positions 2 and 3

Following up on the observation that the dynorphin analog [Pro 3 ]Dyn A(1–11)–NH 2 2 possesses high affinity and selectivity for the κ opioid receptor, a number of related peptides were prepared and characterized by radioligand binding and [ 35 S]GTPγS assays. While incorporation of 2‐azetidine carboxylic acid in position 3 led to the equally potent analog 3 , the corresponding analog containing piperidine‐2‐carboxylic acid showed a nearly 90‐fold reduction in κ affinity. Differential preferred bond angles ϕ in the three building blocks might account for these observations. Compounds 2 and 3 were κ antagonists with IC 50 values of 380 and 350 nM, respectively. The Sar 3 analog 7 and the Sar 2 analog 8 were κ agonists, with greater selectivity than Dyn A(1–11)–NH 2 1 . In view of their high κ affinities ( 8 : K i = 1.5 n M ; 2 : K i = 2.4 n M ), the new analogs were surprisingly weak κ agonists or antagonists, e.g., the EC 50 value for the agonist 8 was 280 n M . Different κ receptor subtypes in binding vs functional assays can not account for these results, since both assays were performed using the same membrane preparation. © 2003 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 71: 71–76, 2003