N-Alkylated derivatives of [D-Pro¹⁰]dynorphin A-(1-11) are high affinity partial agonists at the cloned rat κ-opioid receptor

As part of an effort to develop peptides with selective κ-opioid antagonist activity, a series of N-alkylated [D-Pro¹⁰]dynorphin A-(1-11) derivatives were made through solid-phase peptide synthesis: R-Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-D-Pro-LysOH, where R = N-benzyl, N-cyclopropylmethyl, N,N-dicyclopropylmethyl, or N,N-diallyl. These derivatives and dynorphin A-(1-13)NH₂ were evaluated for κ-opioid receptor binding affinity and potency as inhibitors of adenylyl cyclase. Equilibrium competition binding experiments using [³H]diprenorphine( ~ 600 pM) were performed on membranes prepared from cultured Chinese hamster ovary (CHO) cells stably expressing the rat κ-opioid receptor. Tissue prepared from this cell line was used to evaluate opioid peptide inhibition of forskolin-stimulated (50 μM) adenylyl cyclase activity. Displacement of [³H]diprenorphine specific binding by these peptides was observed with a rank order of affinity (K(i), nM)= [D-Pro¹⁰]dynorphin A-(1-11) (0.13)> dynorphin A-(1-13)NH₂ (0.34)> N-cyclopropylmethyl- (1.4)> N,N-dicyclopropylmethyl- (12.6) ~ N-benzyl- (18.3)~ N,N-diallyl-[D-Pro¹⁰]dynorphin A-(1-11) (26.0). A similar rank order was observed for potency of adenylyl cyclase inhibition (IC₅₀, nM): [D-Pro¹⁰]dynorphin A-(1-11) (0.12)~ dynorphin A-(1 -13)NH₂ (0.19) > N-cyclopropylmethyl- (2.7) > N,N-dicyclopropylmethyl- (13.2) ~ N,N-diallyl- (18.0) ~ N-benzyl-[D-Pro¹⁰]dynorphin A-(1-11) (36.4). The peptides differed in their percent maximal inhibition of adenylyl cyclase activity: dynorphin A-(1-13)NH₂ (100%)~ N-cyclopropylmethyl- (94.3%)~ [D-Pro¹⁰]dynorphin A-(1-11) (87.9%)> N-benzyl- (71.4%) >> N,N-dicyclopropylmethyl- (23.6%)~ N,N-diallyl-[D-Pro¹⁰]dynorphin A-(1-11) (18.9%), As the N,N-dicyclopropylmethyl- and N,N-diallyl [D-Pro¹⁰]dynorphin A-(1-11) derivatives were found to have only weak partial agonist activity with respect to adenylyl cyclase inhibition, they were evaluated for their ability to reverse dynorphin A-(1-13)NH₂ (10 nM) inhibition of adenylyl cyclase activity. N,N-dicyclopropylmetllyl- and N,N-diallyl-[D-Pro¹⁰] dynorphin A-(1-11) reversed dynorphin A-(1-13)NH₂ inhibition to levels equal to the maximal inhibition produced by N,N-dicyclopropylmethyl- and N,N-diallyl-[D-Pro¹⁰]dynorphin A-(1-11) alone. This weak partial agonism combined with nanomolar potency render the N,N-dicyclopropylmethyl- and N,N-diallyl-[D-Pro¹⁰]dynorphin A-(1-11) compounds promising leads for further attempts to synthesize peptide κ-opioid receptor antagonists.