Novel d-amino acid tetrapeptides produce potent antinociception by selectively acting at peripheral κ-opioid receptors^,

Kappa-(κ) opioid receptors are widely distributed in the periphery and activation results in antinociception; however supraspinal acting κ-agonists result in unwanted side effects. Two novel, all d-amino acid, tetrapeptide κ-opioid receptor agonists, FE 200665 and FE 200666, were identified and compared to brain penetrating (enadoline) and peripherally selective (asimadoline) κ-agonists as potential analgesics lacking unwanted central nervous system (CNS) side effects. In vitro characterization was performed using radioligand binding and GTPγS binding. Antinociception was evaluated in both mice and rats. Rotarod tests were performed to determine motor impairment effects of the κ-agonists. FE 200665 and FE 200666 showed high affinity for human κ-opioid receptor 1 (K_i of 0.24 nM and 0.08 nM, respectively) and selectivity for human κ-opioid receptor 1 (human κ-opioid receptor 1/human μ-opioid receptor/humanδ-opioid receptor selectivity ratios of 1/16,900/84,600 and 1/88,600/> 1,250,000, respectively). Both compounds demonstrated agonist activity in the human κ-opioid receptor 1 [³⁵S]GTPγS binding assay (EC₅₀ of 0.08 nM and 0.03 nM) and resulted in dose-related antinociception in the mouse writhing test (A₅₀: 0.007 and 0.013 mg/kg, i.v., respectively). Markedly higher doses of FE 200665 and FE 200666 were required to induce centrally-mediated effects in the rotarod assay (548- and 182-fold higher doses, respectively), and antinociception determined in the mouse tail-flick assay (> 1429- and 430-fold fold higher doses, respectively) after peripheral administration supporting a peripheral site of action. The potency ratios between central and peripheral activity suggest a therapeutic window significantly higher than previous κ-agonists. Furthermore, FE 200665 has entered into clinical trials with great promise as a novel analgesic lacking unwanted side effects seen with current therapeutics.