Dual-Acting Peripherally Restricted Delta/Kappa Opioid (CAV1001) Produces Antinociception in Animal Models of Sub-Acute and Chronic Pain

Blocking Aδ- and C-fiber neural transmission by sub-kilohertz peripheral nerve stimulation

Introduction

We recently showed that sub-kilohertz electrical stimulation of the afferent somata in the dorsal root ganglia (DRG) reversibly blocks afferent transmission. Here, we further investigated whether similar conduction block can be achieved by stimulating the nerve trunk with electrical peripheral nerve stimulation (ePNS).

Methods

We explored the mechanisms and parameters of conduction block by ePNS via ex vivo single-fiber recordings from two somatic (sciatic and saphenous) and one autonomic (vagal) nerves harvested from mice. Action potentials were evoked on one end of the nerve and recorded on the other end from teased nerve filaments, i.e., single-fiber recordings. ePNS was delivered in the middle of the nerve trunk using a glass suction electrode at frequencies of 5, 10, 50, 100, 500, and 1000 Hz.

Results

Suprathreshold ePNS reversibly blocks axonal neural transmission of both thinly myelinated Aδ-fiber axons and unmyelinated C-fiber axons. ePNS leads to a progressive decrease in conduction velocity (CV) until transmission blockage, suggesting activity-dependent conduction slowing. The blocking efficiency is dependent on the axonal conduction velocity, with Aδ-fibers efficiently blocked by 50-1000 Hz stimulation and C-fibers blocked by 10-50 Hz. The corresponding NEURON simulation of action potential transmission indicates that the disrupted transmembrane sodium and potassium concentration gradients underly the transmission block by the ePNS.

Discussion

The current study provides direct evidence of reversible Aδ- and C-fiber transmission blockage by low-frequency (<100 Hz) electrical stimulation of the nerve trunk, a previously overlooked mechanism that can be harnessed to enhance the therapeutic effect of ePNS in treating neurological disorders.

Systematic Structure-Activity Relationship Study of Nalfurafine Analogues toward Development of Potentially Nonaddictive Pain Management Treatments

Despite the availability of numerous pain medications, the current array of Food and Drug Administration-approved options falls short in adequately addressing pain states for numerous patients and consequently worsens the opioid crisis. Thus, it is imperative for basic research to develop novel and nonaddictive pain medications. Toward addressing this clinical goal, nalfurafine (NLF) was chosen as a lead and its structure-activity relationship (SAR) systematically studied through design, syntheses, and in vivo characterization of 24 analogues. Two analogues, 21 and 23, showed longer durations of action than NLF in a warm-water tail immersion assay, produced in vivo effects primarily mediated by KOR and DOR, penetrated the blood-brain barrier, and did not function as reinforcers. Additionally, 21 produced fewer sedative effects than NLF. Taken together, these results aid the understanding of NLF SAR and provide insights for future endeavors in developing novel nonaddictive therapeutics to treat pain.

Targeting sensory neuron GPCRs for peripheral neuropathic pain

Despite the high prevalence of peripheral neuropathic pain (NP) conditions and significant progress in understanding its underlying mechanisms, the management of peripheral NP remains inadequate. Existing pharmacotherapies for NP act primarily on the central nervous system (CNS) and are often associated with CNS-related adverse effects, limiting their clinical effectiveness. Mounting preclinical evidence indicates that reducing the heightened activity in primary sensory neurons by targeting G-protein-coupled receptors (GPCRs), without activating these receptors in the CNS, relieves pain without central adverse effects. In this review, we focus on recent advancements in GPCR-mediated peripheral pain relief and discuss strategies to advance the development of more effective and safer therapies for peripheral NP by shifting from traditional CNS modulatory approaches toward selective targeting of GPCRs on primary sensory neurons.

Characterization of a Potential KOR/DOR Dual Agonist with No Apparent Abuse Liability via a Complementary Structure-Activity Relationship Study on Nalfurafine Analogues

Discovery of analgesics void of abuse liability is critical to battle the opioid crisis in the United States. Among many strategies to achieve this goal, targeting more than one opioid receptor seems promising to minimize this unwanted side effect while achieving a reasonable therapeutic profile. In the process of understanding the structure-activity relationship of nalfurafine, we identified a potential analgesic agent, NMF, as a dual kappa opioid receptor/delta opioid receptor agonist with minimum abuse liability. Further characterizations, including primary in vitro ADMET studies (hERG toxicity, plasma protein binding, permeability, and hepatic metabolism), and in vivo pharmacodynamic and toxicity profiling (time course, abuse liability, tolerance, withdrawal, respiratory depression, body weight, and locomotor activity) further confirmed NMF as a promising drug candidate for future development.

Endogenous opiates and behavior: 2020

This paper is the forty-third consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2020 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Morphinan derivatives with an oxabicyclo[3.2.1]octane structure as dual agonists toward δ and κ opioid receptors

The κ opioid receptor (KOR) is one of the promising targets to develop analgesics lacking morphine like side effects. To seek a novel KOR agonist we designed 6-amide derivatives with an oxabicyclo[3.2.1]octane structure based on a proposed active conformation of a selective KOR agonist nalfurafine. All the synthesized compounds strongly bound to the KOR and some compound showed KOR selectivities. 6R-Amides were more potent and efficacious KOR agonists than the corresponding 6S-isomers. However, most 6-amide derivatives were partial KOR agonist. Conformational analyses of 6R- and 6S-amide derivatives and nalfurafine well accounted for the difference of KOR agonistic activities between two diastereomers. Surprisingly, the tested N-H amides were full δ opioid receptor (DOR) agonists. Among the tested compounds 7a with benzamide moiety was the most potent dual DOR/KOR agonist. On the other hand, 6S-phenylacetamide 8b was potent full DOR agonist with less efficacious agonist activity for the μ receptor and KOR. 6-Amide derivatives with an oxabicyclo[3.2.1]octane structure were expected to be a promising fundamental skeleton for the dual DOR/KOR agonists and/or selective DOR agonists.

Discovery of μ,δ-Opioid Receptor Dual-Biased Agonists That Overcome the Limitation of Prior Biased Agonists

Morphine is widely used in pain management although the risk of side effects is significant. The use of biased agonists to the G protein of μ-opioid receptors has been suggested as a potential solution, although oliceridine and PZM21 have previously failed to demonstrate benefits in clinical studies. An amplification-induced confusion in the process of comparing G protein and beta-arrestin pathways may account for previously biased agonist misidentification. Here, we have devised a strategy to discover biased agonists with intrinsic efficacy. We computationally simulated 430 000 molecular dockings to the μ-opioid receptor to construct a compound library. Hits were then verified experimentally. Using the verified compounds, we performed simulations to build a second library with a common scaffold and selected compounds that showed a bias to μ- and δ-opioid receptors in a cell-based assay. Three compounds (ID110460001, ID110460002, and ID110460003) with a dual-biased agonistic effect for μ- and δ-opioid receptors were identified. These candidates are full agonists for the μ-opioid receptor and show specific binding modes. On the basis of our findings, we expect our novel compounds to act as more biased agonists compared to existing drugs, including oliceridine.