Stereochemical basis for a unified structure activity theory of aromatic and heterocyclic rings in selected opioids and opioid peptides

Novel Dual-Target μ-Opioid Receptor and Dopamine D3 Receptor Ligands as Potential Nonaddictive Pharmacotherapeutics for Pain Management

The need for safer pain-management therapies with decreased abuse liability inspired a novel drug design that retains μ-opioid receptor (MOR)-mediated analgesia, while minimizing addictive liability. We recently demonstrated that targeting the dopamine D₃ receptor (D₃R) with highly selective antagonists/partial agonists can reduce opioid self-administration and reinstatement to drug seeking in rodent models without diminishing antinociceptive effects. The identification of the D₃R as a target for the treatment of opioid use disorders prompted the idea of generating a class of ligands presenting bitopic or bivalent structures, allowing the dual-target binding of the MOR and D₃R. Structure-activity relationship studies using computationally aided drug design and in vitro binding assays led to the identification of potent dual-target leads (23, 28, and 40), based on different structural templates and scaffolds, with moderate (sub-micromolar) to high (low nanomolar/sub-nanomolar) binding affinities. Bioluminescence resonance energy transfer-based functional studies revealed MOR agonist-D₃R antagonist/partial agonist efficacies that suggest potential for maintaining analgesia with reduced opioid-abuse liability.

Members of the same pharmacological family are not alike: Different opioids, different consequences, hope for the opioid crisis?

Pain management is the specialized medical practice of modulating pain perception and thus easing the suffering and improving the life quality of individuals suffering from painful conditions. Since this requires the modulation of the activity of endogenous systems involved in pain perception, and given the large role that the opioidergic system plays in pain perception, opioids are currently the most effective pain treatment available and are likely to remain relevant for the foreseeable future. This contributes to the rise in opioid use, misuse, and overdose death, which is currently characterized by public health officials in the United States as an epidemic. Historically, the majority of preclinical rodent studies were focused on morphine. This has resulted in our understanding of opioids in general being highly biased by our knowledge of morphine specifically. However, recent in vitro studies suggest that direct extrapolation of research findings from morphine to other opioids is likely to be flawed. Notably, these studies suggest that different opioid analgesics (opioid agonists) engage different downstream signaling effects within the cell, despite binding to and activating the same receptors. This recognition implies that, in contrast to the historical status quo, different opioids cannot be made equivalent by merely dose adjustment. Notably, even at equianalgesic doses, different opioids could result in different beneficial and risk outcomes. In order to foster further translational research regarding drug-specific differences among opioids, here we review basic research elucidating differences among opioids in pharmacokinetics, pharmacodynamics, their capacity for second messenger pathway activation, and their interactions with the immune system and the dopamine D2 receptors.

Nonopioid effect of β-endorphin

This review presents the generalized literature data and the results of our own research of the nonopioid effect of β-endorphin, an opioid neuropeptide interacting not only with opioid but also with nonopioid (insensitive to the opioid antagonist naloxone) receptors. The roles of the hormone and its receptors in regulation of the immune, nervous, and endocrine systems are discussed. The effect of neuromediator on the immune system mediated by both opioid and nonopioid receptors is considered in detail. The data on distribution and function of the nonopioid β-endorphin receptor in human and animal organisms are presented. All available data on the characteristics of the nonopioid β-endorphin receptor obtained by means of radioligand analysis are given. The discussed information is supposed to extend our conceptions of the role of β-endorphin in mammals and to be of extensive use in medicine and pharmacology.

Low Molecular Weight Opioid Peptide Esters Could be Developed as a New Class of Analgesics

Low molecular weight opioid peptide esters (OPE) could become a class of analgesics with different side effect profiles than current opiates. OPE may have sufficient plasma stability to cross the blood brain barrier (BBB), undergo ester hydrolysis and produce analgesia. OPE of dipeptides, tyr-pro and tyr-gly conjugated to ethanol have a structure similar to the anesthestic agent, etomidate. Based upon the analgesic activity of dipeptide opioids, Lipinski's criteria, and permeability of select GABA esters to cross the BBB, opioid peptides (OP) conjugated to ethanol, cholesterol or 3-glucose are lead recommendations. Preliminary animal data suggests that tyr-pro-ethyl ester crosses the BBB and unexpectedly produces hyperalgesia. Currently, there are no approved OP analgesics available for clinical use. Clinical trials of good manufacturing practice OP administered to patients suffering from chronic pain with indwelling intrathecal pumps could resolve the issue that OP may be superior to opiates and may redirect research.