Fentanyl Overdose Deaths: Underlying Mechanisms

在过去的5年里，英国和美国的急性阿片类药物过量死亡人数都有所上升。一个新的危险是非法芬太尼（芬太尼及其结构类似物）在毒品领域的出现。在美国，芬太尼导致的死亡人数已达到流行水平（2016年超过20，000人），现在超过了海洛因或处方阿片类药物（如羟考酮）。在英国和欧洲，最近发生了几起与芬太尼有关的死亡事件。我们研究的目的是确定为什么芬太尼是如此致命的药物。阿片类药物过量死亡主要是由于呼吸抑制。在大脑中，芬太尼与μ阿片受体（MOPr）相互作用产生呼吸抑制，是一种非常有效的阿片类药物，比吗啡强50 - 100倍。可能导致芬太尼过量死亡的因素是-作为强效药物，即使是服用量的一个小错误也可能导致严重的呼吸抑制。- 芬太尼引起的呼吸抑制不太容易被拮抗剂纳洛酮逆转。静脉注射芬太尼可引起呼吸肌强直（被称为“木胸”综合征）使呼吸更加困难。为了促进有效干预措施的发展，以打击芬太尼过量，我们需要充分了解为什么这些药物是如此强大，他们如何抑制呼吸，为什么它们对纳洛酮逆转有抗性，以及芬太尼是否对其他阿片类药物表现出较少的交叉耐受性。我们建议研究芬太尼与MOPr. (i)芬太尼如何结合并激活MOPr我们假设-更高的亲脂性导致芬太尼更大的质膜结合，-未结合的芬太尼可以非常迅速地与MOPr重新结合以重新激活它，而其他阿片样物质激动剂不能迅速地重新结合受体，而是在重新结合发生之前从受体附近扩散开。- 芬太尼可以通过疏水途径和亲水途径直接从膜脂质进入MOPr的结合口袋，这种新的性质将增强芬太尼抑制呼吸的体内激动剂效力，并使其不易被纳洛酮逆转。我们将确定芬太尼到磷脂单层的分配系数，利用新型荧光技术研究快速药物-受体缔合和解离动力学，并使用MOPr蛋白的晶体结构进行配体-受体相互作用的高级分子动力学模拟。(ii)肌肉强直对芬太尼呼吸抑制的作用。我们将使用阿片类药物诱导的呼吸抑制小鼠模型来检查呼吸肌强直对芬太尼衍生物呼吸抑制的贡献。(iii)亲脂性拮抗剂对芬太尼的抑制作用。我们将研究高亲脂性MOPr拮抗剂是否比纳洛酮更容易逆转芬太尼呼吸抑制。(iv)耐受性突破。我们将确定芬太尼的“顶部”使用是否可以突破对呼吸抑制的耐受性，这种耐受性是由长期使用海洛因或长期使用美沙酮或丁丙诺啡的阿片类药物替代治疗引起的。我们假设，使芬太尼更有效的过程及其诱导呼吸肌强直的能力可能会降低它们对其他阿片类药物的交叉耐受程度。我们研究的重要性在于，它解决了当前主要的国际健康问题-芬太尼类阿片类药物过量死亡的增加。

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 MOPrWe 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.