Synthesis and Pharmacological Evaluation of 5-Pyrrolidinylquinoxalines as a Novel Class of Peripherally Restricted κ-Opioid Receptor Agonists

5-Pyrrolidinyl substituted perhydroquinoxalines were designed as conformationally restricted κ-opioid receptor agonists restricted to the periphery. The additional N atom of the quinoxaline system located outside the ethylenediamine κ pharmacophore allows the fine-tuning of the pharmacodynamic and pharmacokinetic properties. The perhydroquinoxalines were synthesized stereoselectively using the concept of late stage diversification of the central building blocks 14. In addition to high κ-opioid receptor affinity they demonstrate high selectivity over μ, δ, σ1, σ2, and NMDA receptors. In the [35S]GTPγS assay full agonism was observed. Because of their high polarity, the secondary amines 14a (log D7.4 = 0.26) and 14b (log D7.4 = 0.21) did not penetrate an artificial blood–brain barrier. 14b was able to inhibit the spontaneous pain reaction after rectal mustard oil application to mice (ED50 = 2.35 mg/kg). This analgesic effect is attributed to activation of peripherally located κ receptors, since 14b did not ...