Design, synthesis, and opioid activity of arodyn analogs cyclized by ring-closing metathesis involving Tyr(allyl)

Abstract

Kappa (κ) opioid receptor selective antagonists are useful pharmacological tools in studying κ opioid receptors and have potential to be used as therapeutic agents for the treatment of a variety of diseases including mood disorders and drug addiction. Arodyn (Ac[Phe,Arg,d-Ala]Dyn A-(1-11)NH) is a linear acetylated dynorphin A (Dyn A) analog that is a potent and selective κ opioid receptor antagonist (Bennett et al. J Med Chem 2002;45:5617-5619) and prevents stress-induced reinstatement of cocaine-seeking behavior following central administration (Carey et al. Eur J Pharmacol 2007;569:84-89). To restrict its conformational mobility, explore possible bioactive conformations and potentially increase its metabolic stability we synthesized cyclic arodyn analogs on solid phase utilizing a novel ring-closing metathesis (RCM) reaction involving allyl-protected Tyr (Tyr(All)) residues. This approach preserves the aromatic functionality and directly constrains the side chains of one or more of the Phe residues. The novel cyclic arodyn analog 4 cyclized between Tyr(All) residues incorporated in positions 2 and 3 exhibited potent κ opioid receptor antagonism in the [S]GTPγS assay (K = 3.2 nM) similar to arodyn. Analog 3 cyclized between Tyr(All) residues in positions 1 and 2 also exhibited nanomolar κ opioid receptor antagonist potency (K = 27.5 nM) in this assay. These are the first opioid peptides cyclized via RCM involving aromatic residues, and given their promising pharmacological activity represent novel lead peptides for further exploration.

Optimized Ring Closing Metathesis Reaction Conditions To Suppress Desallyl Side Products in the Solid-Phase Synthesis of Cyclic Peptides Involving Tyrosine(O-allyl)

“…The initial RCM reaction between Tyr­(All) residues in the model dipeptide under the same reaction conditions used for arodyn analogs , (3 mM Grubbs II (G II) catalyst, 40 mol %, DCM/DMF (4:1), 60 °C) resulted in a low yield of the cyclic peptide (20%), as expected, due in part to formation of desallyl products (>35%, Figure ). Higher catalyst loading can increase catalyst degradation products and result in isomerization, so we explored the use of a lower catalyst loading (15 mol %) as well as a lower temperature.…”

“…This study stems from initial attempts to prepare novel constrained analogs of the acetylated dynorphin A (Dyn A) analog arodyn (Ac­[Phe 1,2,3 ,Arg 4 , d -Ala 8 ]­Dyn A­(1-11)-NH 2 , Figure ) via cyclizations involving aromatic residues. − Arodyn is a potent and selective kappa opioid receptor (KOR) antagonist which has high affinity ( K i = 10 nM) and selectivity for KOR ( K i ratio (κ/μ/δ) = 1/174/583) . KOR antagonists, used classically as pharmacological tools, have recently shown potential for the treatment of depression and drug addiction. , Unlike cyclization of Dyn A analogs via RCM utilizing allylglycine residues, attempts to cyclize arodyn analogs containing Tyr­(All) via RCM resulted in mixtures of the desired cyclic peptide plus desallyl side products (Figure ).…”

“…KOR antagonists, used classically as pharmacological tools, have recently shown potential for the treatment of depression and drug addiction. , Unlike cyclization of Dyn A analogs via RCM utilizing allylglycine residues, attempts to cyclize arodyn analogs containing Tyr­(All) via RCM resulted in mixtures of the desired cyclic peptide plus desallyl side products (Figure ). − These side products resulted from isomerization of the double bond to give the vinyl ether with subsequent cleavage to the free phenol. , The high percentage of the bis-desallyl side product 5 obtained highlights the extent of this side reaction, which limits the utility of RCM for cyclizations involving this residue. In this report, we describe the optimization of the reaction conditions to suppress desallyl product formation and enhance on-resin RCM between Tyr­(All) residues.…”

Macrocyclization strategies for cyclic peptides and peptidomimetics

“…224 The application of RCM to peptides has been reviewed previously, 209,219,225 including a perspective discussing all-hydrocarbon-stapled α-helical peptides in general. 208 Alkenes for peptide stapling have been introduced as modified sidechains on carbon 13,204,[226][227][228][229][230] as well as on the α-N, [231][232][233][234] side-chain aliphatic alcohols 206,229 and phenols, 235,236 C-terminal or side-chain acids, 46,237,238 N-terminal carbamates, 237 and cysteine thiol groups. 239 Often, when the alkene is introduced as a modified C-bound chain, α-methyl-αalkenyl sidechains are used for additional helix stabilisation.…”

“…Alkenes for peptide stapling have been introduced as modified sidechains on carbon 13,204,226–230 as well as on the α- N , 231–234 side-chain aliphatic alcohols 206,229 and phenols, 235,236 C-terminal or side-chain acids, 46,237,238 N-terminal carbamates, 237 and cysteine thiol groups. 239 Often, when the alkene is introduced as a modified C-bound chain, α-methyl-α-alkenyl sidechains are used for additional helix stabilisation.…”