Modular synthesis and bioevaluation of novel fused-β-carbolines as kappa opioid receptor modulators

Opioids are widely used for the management of chronic pain; however, their analgesic effects are primarily mediated through activation of the μ-opioid receptor (MOR), which is associated with serious adverse effects. It has been postulated that κ-opioid receptor (KOR) agonists biased toward G-protein signaling may provide effective analgesia while minimizing these side effects. Fused β-carbolines have recently emerged as promising biased KOR agonists with demonstrated antinociceptive potential. Herein, we report a modular synthetic strategy employing the Ugi multicomponent reaction between β-carboline-based aldehydes, aliphatic isonitriles, primary amines, and haloacetic acids to generate a new library of fused β-carboline derivatives. Biological evaluation of these compounds revealed that 14 members of the series exhibited potent and selective G-protein-biased KOR agonist activity (IC₅₀ = 5-500 nM), whereas four compounds displayed modest MOR antagonist activity (IC₅₀ = 1-5 μM) and one compound showed potent DOR antagonist activity (IC₅₀ = 65 ± 30 nM). Notably, a clear positive correlation was observed between antinociceptive efficacy and the degree of G-protein bias. In contrast, compounds displaying reduced G-protein bias and a higher propensity for β-arrestin recruitment induced pronounced sedative effects and impaired neuromuscular coordination. Integrated analysis of G-protein bias, antinociceptive activity, and locomotor effects demonstrated that fused β-carbolines, compared with the unbiased KOR agonist U50488, achieved greater functional separation between antinociception and motor-suppressant side effects.