The effects of (2R,6R)-hydroxynorketamine on oxycodone withdrawal and reinstatement

(2R,6R)-hydroxynorketamine prevents opioid abstinence-related negative affect and stress-induced reinstatement in mice

BACKGROUND AND PURPOSE

Opioid use disorder (OUD) is a pressing public health concern marked by frequent relapse during periods of abstinence, perpetuated by negative affective states. Classical antidepressants or the currently prescribed opioid pharmacotherapies have limited efficacy to reverse the negative affect or prevent relapse.

EXPERIMENTAL APPROACH

Using mouse models, we investigated the effects of ketamine's metabolite (2R,6R)-hydroxynorketamine (HNK) on reversing conditioning to sub-effective doses of morphine in stress-susceptible mice, preventing conditioned-place aversion and alleviating acute somatic abstinence symptoms in opioid-dependent mice. Additionally, we evaluated its effects on anhedonia, anxiety-like behaviours and cognitive impairment during protracted opioid abstinence, while mechanistic studies examined cortical EEG oscillations and synaptic plasticity markers.

KEY RESULTS

(2R,6R)-HNK reversed conditioning to sub-effective doses of morphine in stress-susceptible mice and prevented conditioned-place aversion and acute somatic abstinence symptoms in opioid-dependent mice. In addition, (2R,6R)-HNK reversed anhedonia, anxiety-like behaviours and cognitive impairment emerging during protracted opioid abstinence plausibly via a restoration of impaired cortical high-frequency EEG oscillations, through a GluN2A-NMDA receptor-dependent mechanism. Notably, (2R,6R)-HNK facilitated the extinction of opioid conditioning, prevented stress-induced reinstatement of opioid-seeking behaviours and reduced the propensity for enhanced morphine self-consumption in mice previously exposed to opioids.

CONCLUSIONS AND IMPLICATIONS

These findings emphasize the therapeutic potential of (2R,6R)-HNK, which is currently in Phase II clinical trials, in addressing stress-related opioid responses. Reducing the time and cost required for development of new medications for the treatment of OUDs via drug repurposing is critical due to the opioid crisis we currently face.

Ketamine and Hydroxynorketamine as Novel Pharmacotherapies for the Treatment of Opioid Use Disorders

Opioid use disorder (OUD) has reached epidemic proportions, with many countries facing high levels of opioid use and related fatalities. Although currently prescribed medications for OUD are considered lifesaving, they inadequately address negative affect and cognitive impairment, resulting in high relapse rates to nonmedical opioid use even years after drug cessation (protracted abstinence). Evidence supports the notion that ketamine, an anesthetic and rapid-acting antidepressant drug, holds promise as a candidate for OUD treatment, including the management of acute withdrawal somatic symptoms, negative affect during protracted opioid abstinence, and prevention of retaking nonmedical opioids. In this review, we comprehensively discuss preclinical and clinical research that has evaluated ketamine and its metabolites as potential novel therapeutic strategies for treating OUD. Furthermore, we examine evidence that supports the relevance of the molecular targets of ketamine and its metabolites in relation to their potential effects and therapeutic outcomes in OUD. Overall, existing evidence demonstrates that ketamine and its metabolites can effectively modulate pathophysiological processes affected in OUD, suggesting a promising therapeutic role in the treatment of OUD and the prevention of return to opioid use during abstinence.

Ketamine and Esketamine in Clinical Trials: FDA-Approved and Emerging Indications, Trial Trends With Putative Mechanistic Explanations

Ketamine has a long and very eventful pharmacological history. Its enantiomer, esketamine ((S)-ketamine), was approved by the US Food and Drug Administration (FDA) and EMA for patients with treatment-resistant depression (TRD) in 2019. The number of approved indications for ketamine and esketamine continues to increase, as well as the number of clinical trials. This analysis provides a quantitative overview of the use of ketamine and its enantiomers in clinical trials during 2014-2024. A total of 363 trials were manually assessed from clinicaltrial.gov with the search term "Ketamine." The highest number of trials were found for the FDA-approved indications: anesthesia (~22%) and pain management (~28%) for ketamine and TRD for esketamine (~29%). Clinical trials on TRD for both ketamine and esketamine also comprised a large proportion of these trials, and interestingly, have reached phase III and phase IV status. Combinatorial treatment of psychiatric disorders and non-psychiatric conditions with pharmacological and non-pharmacological combinations (electroconvulsive therapy, psychotherapeutic techniques, virtual reality, and transcranial magnetic stimulation) is prevalent. Sub-anesthetic doses of ketamine may represent novel therapeutic avenues in neuropsychiatric conditions, that is, major depression, schizophrenia, and bipolar disorder, where glutamate excitotoxicity and oxidative stress are likely to be involved. The study suggests that the number of ketamine studies will continue to grow and possible ketamine variants can be approved for treatment of additional indications.

Endogenous opiates and behavior: 2023

This paper is the forty-sixth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2023 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 and alcohol abuse (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).

Comparing withdrawal- and anxiety-like behaviors following oral and subcutaneous oxycodone administration in C57BL/6 mice

Excessive prescribing and misuse of prescription opioids, such as oxycodone, significantly contributed to the current opioid crisis. Although oxycodone is typically consumed orally by humans, parenteral routes of administration have primarily been used in preclinical models of oxycodone dependence. To address this issue, more recent studies have used oral self-administration procedures to study oxycodone seeking and withdrawal in rodents. Behavioral differences, however, following oral oxycodone intake versus parenteral oxycodone administration remain unclear. Thus, the goal of the current studies was to compare anxiety- and withdrawal-like behaviors using established opioid dependence models of either home cage oral intake of oxycodone (0.5 mg/ml) or repeated subcutaneous (s.c.) injections of oxycodone (10 mg/kg) in male and female mice. Here, mice received 10 days of oral or s.c. oxycodone administration, and following 72 h of forced abstinence, anxiety- and withdrawal-like behaviors were measured using elevated zero maze, open field, and naloxone-induced precipitated withdrawal procedures. Global withdrawal scores were increased to a similar degree following oral and s.c. oxycodone use, while both routes of oxycodone administration had minimal effects on anxiety-like behaviors. When examining individual withdrawal-like behaviors, mice receiving s.c. oxycodone exhibited more paw tremors and jumps during naloxone-induced precipitated withdrawal compared with oral oxycodone mice. These results indicate that both models of oxycodone administration are sufficient to elevate global withdrawal scores, but, when compared with oral consumption, s.c. oxycodone injections yielded more pronounced effects on some withdrawal-like behaviors.

Loss of the sustained antidepressant-like effect of (2R,6R)-hydroxynorketamine in NMDA receptor GluN2D subunit knockout mice

Ketamine, an N-methyl-d-aspartate (NMDA) receptor antagonist, has attracted attention for its acute and sustained antidepressant effects in patients with depression. Hydroxynorketamine (HNK), a metabolite of ketamine, exerts antidepressant effects without exerting ketamine's side effects and has attracted much attention in recent years. However, the detailed pharmacological mechanism of action of HNK remains unclear. We previously showed that the GluN2D NMDA receptor subunit is important for sustained antidepressant-like effects of (R)-ketamine. Therefore, we investigated whether the GluN2D subunit is involved in antidepressant-like effects of (2R,6R)-HNK and (2S,6S)-HNK. Treatment with (2R,6R)-HNK but not (2S,6S)-HNK exerted acute and sustained antidepressant-like effects in the tail-suspension test in wildtype mice. Interestingly, sustained antidepressant-like effects of (2R,6R)-HNK were abolished in GluN2D-knockout mice, whereas acute antidepressant-like effects were maintained in GluN2D-knockout mice. When expression levels of GluN2A and GluN2B subunits were evaluated, a decrease in GluN2B protein expression in the nucleus accumbens was found in stressed wildtype mice but not in stressed GluN2D-knockout mice. These results suggest that the GluN2D subunit and possibly the GluN2B subunit are involved in the sustained antidepressant-like effect of (2R,6R)-HNK.