Desmetramadol Is Identified as a G-Protein Biased µ Opioid Receptor Agonist

The anomalous pharmacology of fentanyl

Fentanyl is a key therapeutic, used in anaesthesia and pain management. It is also increasingly used illicitly and is responsible for a large and growing number of opioid overdose deaths, especially in North America. A number of factors have been suggested to contribute to fentanyl's lethality, including rapid onset of action, in vivo potency, ligand bias, induction of muscle rigidity and reduced sensitivity to reversal by naloxone. Some of these factors can be considered to represent 'anomalous' pharmacological properties of fentanyl when compared with prototypical opioid agonists such as morphine. In this review, we examine the nature of fentanyl's 'anomalous' properties, to determine whether there is really a pharmacological basis to support the existence of such properties, and also discuss whether such properties are likely to contribute to overdose deaths involving fentanyls. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Increase of high-risk tramadol use and harmful consequences in France from 2013-2018: evidence from the triangulation of addictovigilance data

Aims

The aim of this paper is to assess recent developments in non‐medical tramadol use, tramadol use disorder, illegal procurement and deaths.

Methods

This study used repeated cross‐sectional analysis of data collected nationwide from 2013 to 2018. Analysis was conducted through multisource monitoring of the French Addictovigilance Network of: (1) validated reports of high‐risk tramadol use, (2) record systems collecting information from toxicology experts investigating analgesic‐related deaths (DTA) and deaths related to substance abuse (DRAMES), and pharmacists for forged prescriptions (OSIAP), and (3) survey of drug users, with investigation of patterns of use while visiting addiction‐specialised institutions (OPPIDUM).

Results

Despite a plateauing level of tramadol exposure in the French population, the proportion of tramadol reports increased 1.7‐fold (187 cases in 2018, 3.2% (95% confidence interval [CI]: 2.74–3.63%), versus 1.9% (95% CI: 1.49–2.42% in 2013). Trends were similar in OSIAP: 11.9% of forged prescriptions in 2018 (95% CI: 10.56–13.45%); 1.7‐fold increase; in OPPIDUM: 0.76% (95% CI: 0.55–1.02); 2.2‐fold increase; and DRAMES: 3.2% of drug abuse‐related deaths in 2018 (95% CI: 1.89–5.16) versus 1.7% in 2013 (95% CI: 0.65–3.84). Tramadol was the primary opioid in analgesic‐related deaths in DTA (45% in 2018). Two profiles of high‐risk tramadol users were identified: (1) patients treated for pain or with tramadol persistence when pain disappeared (mainly women; mean age 44 years), and (2) individuals with non‐medical use for psychoactive effects (mainly men; mean age 36 years).

Conclusion

The triangulation of the data obtained through addictovigilance monitoring evidenced a recent increase in high‐risk tramadol use. These findings have a practical impact on the limitation of the maximal duration of tramadol prescriptions.

Interaction With the Lipid Membrane Influences Fentanyl Pharmacology

Overdose deaths from fentanyl have reached epidemic proportions in the USA and are increasing worldwide. Fentanyl is a potent opioid agonist that is less well reversed by naloxone than morphine. Due to fentanyl’s high lipophilicity and elongated structure we hypothesised that its unusual pharmacology may be explained by its interactions with the lipid membrane on route to binding to the μ-opioid receptor (MOPr). Through coarse-grained molecular dynamics simulations, electrophysiological recordings and cell signalling assays, we determined how fentanyl and morphine access the orthosteric pocket of MOPr. Morphine accesses MOPr via the aqueous pathway; first binding to an extracellular vestibule, then diffusing into the orthosteric pocket. In contrast, fentanyl may take a novel route; first partitioning into the membrane, before accessing the orthosteric site by diffusing through a ligand-induced gap between the transmembrane helices. In electrophysiological recordings fentanyl-induced currents returned after washout, suggesting fentanyl deposits in the lipid membrane. However, mutation of residues forming the potential MOPr transmembrane access site did not alter fentanyl’s pharmacological profile in vitro. A high local concentration of fentanyl in the lipid membrane, possibly in combination with a novel lipophilic binding route, may explain the high potency and lower susceptibility of fentanyl to reversal by naloxone.

Sex-specific role for serotonin 5-HT2A receptor in modulation of opioid-induced antinociception and reward in mice

Opioids are among the most effective analgesics and the mainstay of pain management. However, concerns about safety and abuse liability have challenged their widespread use by the medical community. Opioid-sparing therapies include drugs that in combination with opioids have the ability to enhance analgesia while decreasing opioid requirement as well as their side effects. Sex differences in antinociceptive responses to opioids have received increasing attention in recent years. However, the molecular mechanisms underlying sex differences related to opioid-sparing adjuncts remain largely unexplored. Using warm water tail-withdrawal as a mouse model of acute thermal nociception, our data suggest that adjunctive administration of the serotonin 5-HT_2A receptor (5-HT_2AR) antagonist volinanserin dose-dependently enhanced potency of the opioid analgesic oxycodone in male, but not female, mice. This antinociceptive-like response induced by oxycodone was also augmented in 5-HT_2AR knockout (5-HT_2AR^-/-) male, but not female mice; an effect that was reversed by Cre-loxP-mediated selective expression of 5-HT_2AR in dorsal root ganglion (DRG) neurons of 5-HT_2AR^-/- littermates. Pharmacological inhibition with volinanserin or genetic deletion in 5-HT_2AR^-/- animals potentiated the ability of oxycodone to reduce DRG excitability in male mice. Adjunctive volinanserin did not affect oxycodone-induced conditioned place preference (CPP), whereas it reduced oxycodone-induced locomotor sensitization in male and female mice. Together, these results suggest that adjunctive volinanserin augments opioid-induced antinociception, but not abuse-related behavior, through a sex-specific signaling crosstalk mechanism that requires 5-HT_2AR expression in mouse DRG neurons. Ultimately, our results may pave the way for the clinical evaluation of volinanserin as a potential sex-specific opioid adjuvant.

Postoperative opioid administration characteristics associated with opioid-induced respiratory depression: Results from the PRODIGY trial

STUDY OBJECTIVE

Opioid administration for pain in general care floor patients remains common, and can lead to adverse outcomes, including respiratory compromise. The PRODIGY trial found that among ward patients receiving parenteral opioids, 46% experienced ≥1 respiratory depression episode. The objective of this analysis was to evaluate the geographic differences of opioid administration and examine the association between opioid administration characteristics and the occurrence of respiratory depression.

DESIGN

Prospective observational trial.

SETTING

16 general care medical and surgical wards in Asia, Europe, and the United States.

PATIENTS

1335 patients receiving parenteral opioids.

INTERVENTIONS

Blinded, alarm-silenced continuous capnography and pulse oximetry monitoring.

MEASUREMENTS

Opioid-induced respiratory depression, defined as respiratory rate ≤ 5 bpm, SpO₂ ≤ 85%, or ETCO₂ ≤ 15 or ≥ 60 mmHg for ≥3 min; apnea episode lasting >30 s; or any respiratory opioid-related adverse event.

RESULTS

Across all patients, 58% received only long-acting opioids, 16% received only short-acting (<3 h) opioids, and 21% received a combination of short- and long-acting (≥3 h) opioids. The type and median total morphine milligram equivalent (MME) of opioid administered varied significantly by region, with 31.5 (12.5-76.7) MME, 31.0 (6.2-99.0) MME, and 7.2 (1.7-18.7) MME in the United States, Europe, and Asia, respectively (p < 0.001). Considering only postoperative opioids, 54% (N = 119/220) and 45% (N = 347/779) of patients receiving only short-acting opioids or only long-acting opioids experienced ≥1 episode of opioid-induced respiratory depression, respectively. Multivariable analysis identified post-procedure tramadol (OR 0.62, 95% CI 0.424-0.905, p = 0.0133) and post-procedure epidural opioids (OR 0.485, 95% CI 0.322-0.731, p = 0.0005) being associated with a significant reduction in opioid-induced respiratory depression.

CONCLUSIONS

Despite varying opioid administration characteristics between Asia, Europe, and the United States, opioid-induced respiratory depression remains a common global problem on general care medical and surgical wards. While the use of post-procedure tramadol or post-procedure epidural opioids may reduce the incidence of respiratory depression, continuous monitoring is also necessary to ensure patient safety when receiving postoperative opioids. REGISTRATION NUMBER: www.clinicaltrials.gov, ID: NCT02811302.

G protein-coupled receptors: structure- and function-based drug discovery

As one of the most successful therapeutic target families, G protein-coupled receptors (GPCRs) have experienced a transformation from random ligand screening to knowledge-driven drug design. We are eye-witnessing tremendous progresses made recently in the understanding of their structure-function relationships that facilitated drug development at an unprecedented pace. This article intends to provide a comprehensive overview of this important field to a broader readership that shares some common interests in drug discovery.

Critical Assessment of G Protein-Biased Agonism at the μ-Opioid Receptor

G protein-biased agonists of the μ-opioid receptor (MOPr) have been proposed as an improved class of opioid analgesics. Recent studies have been unable to reproduce the original experiments in the β-arrestin2-knockout mouse that led to this proposal, and alternative genetic models do not support the G protein-biased MOPr agonist hypothesis. Furthermore, assessment of putatively biased ligands has been confounded by several factors, including assay amplification. As such, the extent to which current lead compounds represent mechanistically novel, extremely G protein-biased agonists is in question, as is the underlying assumption that β-arrestin2 mediates deleterious opioid effects. Addressing these current challenges represents a pressing issue to successfully advance drug development at this receptor and improve upon current opioid analgesics.