Fentanyl Overdose Deaths: Underlying Mechanisms

Over the past 5 years acute opioid overdose deaths have risen both in the UK and the USA. A new danger is the emergence on the drug scene of illicit fentanyls (fentanyl and its structural analogues). In the USA deaths due to fentanyls have reached epidemic levels (over 20,000 in 2016) and now exceed those involving either heroin or prescription opioids such as oxycodone. In the UK and Europe there have recently been several alarming outbreaks of fentanyl-related deaths. The purpose of our research is to determine why fentanyls are such deadly drugs.

Opioid overdose deaths are due primarily to respiratory depression. In the brain fentanyl interacts with the mu opioid receptor (MOPr) to produce respiratory depression and is an extremely potent opioid, being 50 - 100 times more potent than morphine. Factors that likely contribute to fentanyl overdose deaths are
- being highly potent drugs, even a small error in the amount taken could lead to profound respiratory depression.
- respiratory depression by fentanyls is less readily reversible by the antagonist naloxone.
- on intravenous administration fentanyl can induce respiratory muscle rigidity (referred to as 'wooden chest' syndrome) making it even more difficult to breathe.

To facilitate the development of effective interventions to combat fentanyl overdose we need to understand fully why these drugs are so potent, how they depress respiration, why they are resistant to naloxone reversal and whether fentanyls exhibit less cross tolerance to other opioid drugs.

We propose to study several novel facets of the interactions of fentanyls with MOPr.
(i) How fentanyls bind to and activate MOPr
We hypothesise that
- higher lipophilicity results in greater plasma membrane association of fentanyls,
- on unbinding fentanyls can very rapidly re-associate with MOPr to reactivate it whereas other opioid agonists do not rapidly re-bind to the receptor but diffuse away from the vicinity of the receptors before re-binding can occur.
- fentanyls can access the binding pocket of MOPr directly from the membrane lipid through a hydrophobic pathway as well as via a hydrophilic aqueous pathway.
Such novel properties would enhance the in vivo agonist potency of fentanyls to depress respiration and render them less susceptible to reversal by naloxone. We will determine the partition coefficient of fentanyls into phospholipid monolayers, utilise novel fluorescence technologies to study rapid drug-receptor association and dissociation kinetics, and perform advanced molecular dynamic simulations of ligand-receptor interaction using the crystal structure of the MOPr protein.
(ii) Contribution of muscle rigidity to respiratory depression by fentanyls. We will use a mouse model of opioid-induced respiratory depression to examine the contribution of respiratory muscle rigidity to respiratory depression by fentanyl derivatives.
(iii) Reversal of fentanyls by lipophilic antagonists. We will examine whether highly lipophilic MOPr antagonists more readily reverse fentanyl respiratory depression than naloxone.
(iv) Tolerance breakthrough. We will determine whether 'on top' use of the fentanyls can break through the tolerance to respiratory depression that has been induced by prolonged heroin use or from prolonged opioid substitution therapy with methadone or buprenorphine. We hypothesise that the processes which make the fentanyls more potent and their abilities to induce respiratory muscle rigidity may reduce the degree of cross tolerance they exhibit to other opioids.

The importance of our research is that it addresses a current major international health problem - the rise in overdose deaths due to the fentanyl class of opioid drugs.

Fentanyls have become deadly drugs of abuse in the USA. Fentanyl interacts primarily with mu opioid receptors (MOPr) and is 50 - 100 times more potent than morphine in depressing respiration.
In vitro studies on recombinant MOPrs do not predict that fentanyl would be more potent in vivo than other opioids such as morphine. We hypothesise that fentanyls interact with MOPr in novel ways that do not conform to classical theories of ligand-receptor binding. To facilitate the development of effective interventions to combat overdose we need a full understanding of why fentanyls are so potent, why they are more resistant to naloxone reversal and whether they are still effective in individuals tolerant to other opioid drugs.
We propose
- to utilise time-resolved Forster Resonance Energy Transfer to study whether fentanyls exhibit rapid drug-receptor association kinetics allowing them to rebind and reactivate MOPrs rather than diffuse away from the vicinity of the
receptors.
- to assess the degree to which increased lipophilicity promotes plasma membrane interactions, increasing the local concentrations of fentanyls in the vicinity of the MOPr.
- to perform Molecular Dynamics simulations of ligand-MOPr interactions to determine whether fentanyls can access the binding pocket of the receptor directly from the membrane lipid through a hydrophobic pathway.
- to use the data gathered above to develop a mathematical model of agonist rebinding in situations of limited diffusion (e.g. interstitial spaces).
- to monitor respiration and respiratory muscle rigidity in mice to examine how fentanyls depress respiration and to determine the contribution of chest wall rigidity.
- to examine whether highly lipophilic MOPr antagonists more readily reverse respiratory depression by fentanyls.
- to determine whether 'on top' use of fentanyls can break through tolerance to respiratory depression that has been induced by prolonged exposure to heroin, methadone or buprenorphin

Our research will have significant impact in the following areas:
1. Academic research
(i) Our research aims to discover the molecular mechanisms that make the fentanyl drugs so much more potent and deadly than other opioid agonists, why they are less sensitive to reversal by the overdose antidote naloxone than other opioids such as heroin and methadone, and how we could better treat overdose involving the fentanyls. This knowledge will impact upon the future direction of research by other UK and international academic research groups working in this area.

(ii) At a local level, the PDRAs employed on this project, will gain experience of using cutting edge research technologies and be equipped with essential skills for research or related jobs in academia, education, healthcare, or industry in the longer term.

2. Wealth creation.
The findings of this work will be of interest to the pharmaceutical industry, a very important component of the UK economy. Industry will be interested in the novel ideas concerning drug-receptor interactions. This will influence their search for new ways in which to analyse drug action and their design of new drugs that interact with other, non-opioid receptors.

3. Evidence based policy making.
By providing information on why the fentanyls are so dangerous and how we could better treat overdose involving the fentanyls and limit deaths, our research will help to inform future drug policy and may potentially change Government policy in relation to the social and health problems associated with opioid drug abuse.

4. Public Awareness.
In more general terms our research will be of benefit to the wider community in the UK through our commitment to public engagement. All important discoveries will be communicated in non-specialist language to the general public via our university web sites, by press releases and by our involvement in public engagement. This includes talking with those involved in harm reduction, those going through drug rehabilitation, giving public lectures and speaking in schools. Being more aware of the dangers of opioid overdose is important in the wider community.