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Three Million to One, Homing in on a Winner with Few Side Effects

Breathing slows, oxygen levels plummet and, all too often, lungs cease to function at all. An opioid again takes a life, and another person is lost to an overdose. Deaths from opioids are happening with increasing frequency throughout the world.

Since long before Dorothy fell asleep in a field of poppies alongside the Yellow Brick Road while off to see The Wizard of Oz, the powers of poppies have been legend. The sleep-inducing and pain-reducing properties stem from the natural opiates found in the seed pods of the opium poppy. Too often, morphine prescribed to control pain leads to an out-of-control addiction and, in the most serious situations, a fatal overdose.

Researchers have sought out alternatives to morphine, codeine and other opiates, options that might effectively control pain without the risk of addiction and respiratory distress. Recently, teams from the United States and Germany announced that a search built around the structure of the opioid receptor yielded a promising replacement candidate. They described their process and findings in, “Structure-based discovery of opioid analgesics with reduced side effects,” in Nature.

Planning a New Scaffold

When morphine reaches the mu-opioid receptor (μOR), it combines with two signaling proteins. One, beta-arrestin, is linked to troublesome and potentially dangerous opioid side effects, such as addiction and respiratory failure. The other, G-protein, is linked to pain relief. To reduce risks of addiction and respiratory failure, some research has focused on altering morphine, aiming to change it just enough to avoid the risks while retaining the benefits.

Aashish Manglik, MD, PhD, Stanford University School of Medicine, and his team explored a fundamentally different approach. They weren’t looking for a compound like morphine. In fact, they wanted a novel molecule constructed on a new scaffold, one dissimilar in structure to opioids but that nonetheless would fit into the stereotactic pocket of the mu-opioid receptor.

As in virtually all research, the team’s current study builds on previous findings, in this case, their own. In 2012, they identified the crystal structure of several opioid receptors. Based on those structures, the researchers created a computational screening program to find non-opioid molecules that would dock to — in essence “fit” into —μOR in a biased way, setting in motion the pathway for pain control but not the pathway linked to dangerous side effects. That program screened 3 million virtual molecules to illustrate how each might bind to the docking site of the μOR. Based on computer analysis and direct observation, the researchers winnowed down millions of possibilities to the most promising candidate, which they optimized to construct PZM21. Then they moved from computer models and bench results to in vivo tests.

Of Mice and Monitoring

The researchers employed several key tests to gather information on pain relief, respiratory depression and indicators of addiction.

One test demonstrated that mice who received PZM21 didn’t exhibit signs of pain when exposed to heat. In fact, the pain protection from PZM21 was just about as strong as that of morphine. PZM21 also kicked in more quickly and lasted longer.

In another experiment, the mice on PZM21 flicked their tails as frequently as if they hadn’t received any medication. Importantly, this showed that, although PZM21 alleviated pain, it did not dull spinal responses, an encouraging sign that PZM21 would not fatally depress respiration. Direct measurements of breathing confirmed that respiration remained close to normal on PZM21 in stark contrast to the radical respiratory depression in mice on morphine.

Finally, although there are inherent limits to watching the activity of mice to predict the behavior of people, the rodent response offered reason for optimism. Mice did not demonstrate the classic signs of addiction, drug-seeking behavior and hyperactivity. They moved around a normal amount and didn’t show a preference for PZM21 solution over saline. They weren’t revved up in search of their next high.

Complex Analyses Using Complementary Technologies

The rigorous evaluation of compounds throughout the study required the complementary abilities of a collection of spectroscopy techniques. For high-resolution electrospray ionization mass spectrometry, the scientists used either a Bruker microTOF II or a Bruker maXis MS, a quadrupole-time-of-flight machine. To determine mass, the team used the Bruker Esquire 2000 ion-trap mass spectrometer or a Bruker amaZon SL mass spectrometer. The researchers recorded 1H and 13C spectra on a Bruker Avance 360 or 600 NMR-Spectrometer.

Shaping What’s Next

Along with the importance of PZM21 as a potential substitute for dangerous opioid pain relievers, its very structure might reveal new information about the mechanisms of opioid receptors and signaling systems.

The methods used in this experiment — from virtual screening and docking to optimizing and testing — can serve as a template for employing a structure-based approach to developing other molecules that are a good fit for any number of important receptors.

Getting PZM21 from the research lab to the prescription pad will require extensive clinical trials. In the meantime, Manglik and his colleagues believe what they learned about the mu-opioid receptor specifically and structure-based methods, in general, will spark research into receptors critical to other treatments and diseases.

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研究者は、中毒および呼吸窮迫のリスクなしに有効な疼痛コントロールを達成する選択肢として、モルヒネ、コデインおよびその他のオピエートに代わる薬剤を探索しています。最近、米国およびドイツのチームがオピオイド受容体の構造に基づく研究によって有望な代用薬候補を発見したと発表しました。彼らは、このプロセスおよび結果をStructure-based discovery of opioid analgesics with reduced side effects(構造に基づく副作用の少ないオピオイド鎮痛薬の発見)としてNature誌に発表しました。



スタンフォード大学医学部のAashish Manglik(MD, PhD)氏のチームは、根本的に異なるアプローチを探索しました。彼らはモルヒネに似た化合物を探すのではなく、オピオイドの構造に似ていないもののμオピオイド受容体の結合ポケットにフィットする新しい骨格からなる新規分子を探し求めました。

ほぼすべての研究と同様に、このチームの研究も過去の知見(ここでは自身の研究)に基づいて構築されたものです。2012年に彼らはいくつかのオピオイド受容体の結晶構造を明らかにしました。これらの構造に基づいて、危険な副作用に関わる経路ではなく疼痛コントロールの経路に着目し、研究チームは異なる様式でμORに結合(実質的に「フィット」)する非オピオイド分子のコンピュータ式スクリーニングプログラムを作成しました。このプログラムによって300万のバーチャル分子のスクリーニングを行い、各分子がμORの結合部位にどのように結合するかを評価しました。コンピュータ解析および直接観察に基づいて、研究チームは数百万の可能性から最も有望な候補物質を選定し、それを最適化してPZM21を作り出しました。彼らはその後、コンピュータモデルおよびベンチテストからin vivo試験に進みました。







研究を通じて化合物の厳密な評価を行うために、様々な分光法の相補的能力が必要でした。高分解能エレクトロスプレーイオン化質量分析のために、研究チームは四重極時間飛行型質量分析計であるブルカーmicroTOF IIまたはブルカーmaXis MSを使用しました。質量を決定するために、このチームはブルカーEsquire 2000イオントラップ型質量分析計またはブルカーamaZon SL 質量分析計を使用しました。研究チームはブルカーAvance360または600 NMRを使用して1Hおよび13Cのスペクトルを記録しました。






  • Manglik, A. et al. Structure-based discovery of opioid analgesics with reduced side effects. Nature 537, 185–190 (08 September 2016).
  • Manglik, A. et al. Crystal structure of the μ-opioid receptor bound to a morphinan antagonist. Nature 485, 321–326 (21 March 2012).
  • “Making Better Opioids”. Nature Audio Podcast. N.p., 2016. Web, 18 Aug. 2016.
  • Reporter Adam Levy talks to Peter Gmeiner, whose team sorted through 3 million compounds to find a promising painkiller. Interview starts at 14:18. https://www.acast.com/nature/naturepodcast-18august2016?utm_source=feedburner&utm_medium=feed utm_campaign=Feed%3A+nature%2Fpodcast%2Fcurrent+%28Nature+Podcast%29
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