Opioid Analgesics: Rise and Fall of Ligand Biased Signaling and Future Perspectives in the Quest for the Holy Grail

Opioid analgesics have been used for more than 5000 years and remain the main pain medications prescribed today. Although morphine is considered the gold standard of pain relief, this selective µ-opioid receptor (MOP) agonist provides only moderate relief for many chronic pain conditions and produces a number of unwanted effects that can affect the patient's quality of life, prevent adherence to treatment or lead to addiction. In addition to the lack of progress in developing better analgesics, there have been no significant breakthroughs to date in combating the above-mentioned side effects. Fortunately, a better understanding of opioid pharmacology has given renewed hope for the development of better and safer pain medications. In this review, we describe how clinically approved opioids were initially characterized as biased ligands and what impact this approach might have on clinical practice. We also look at the preclinical and clinical development of biased MOP agonists, focusing on the history of oliceridine, the first specifically designed biased analgesic. In addition, we explore the discrepancies between ligands with low intrinsic efficacy and those with biased properties. Finally, we examine the rationale behind the development of biased ligands during the opioid crisis.