Recent Phase 3 clinical trials of selective kappa-opioid (KOP) receptor antagonists aticaprant and navacaprant failed to demonstrate sufficient clinical efficacy in treatment-resistant depression (TRD). This highlights a critical gap in current strategies that target opioid-mediated hedonic suppression. We propose two hypotheses to explain these setbacks: (1) neutral antagonists are inherently ineffective in blocking constitutively active KOP receptor hyperactivation and (2) the nociceptin opioid (NOP) receptor provides functional redundancy that compensates for KOP receptor blockade. Gaining insights from paralogous compensation in drug-resistant tumors, we argue for shifting from selective opioid antagonists to dual KOP/NOP receptor blockers to meaningfully improve reward function. This concept provides a theoretical framework for overcoming clinical resistance where selective KOP targeting with neutral antagonists has failed. Thus, we advocate for the development of opioid inverse agonists (such as nor-BNI, CAS: 105618-26-6), pan-antagonists (such as AT-076, CAS: 1657028-64-2), and combinations of selective blockers.
Shagiakhmetov et al. (2026) studied this question.