Drugs presently available for the emergency treatment of acute severe pain are opioids, ketamine, and the inhalational anaesthetics methoxyflurane and nitrous oxide. All these drugs have substantial side effects that severely limit their use. In particular, the deployment of ketamine is limited by a high incidence of severe dysphoric psychotic states that occur around the same dosage as is required for analgesia. It is commonly believed that ketamine’s actions are caused by its interaction with a specific protein in the brain called the NMDA receptor, which controls how the neurons (nerve cells) within the brain communicate and send information to each other. However, there is growing evidence that ketamine’s main analgesic effects could in fact be due to actions on other target molecules, rather than binding to NMDA receptors. It is quite possible that the binding of ketamine to NMDA might only cause ketamine’s adverse hallucinogenic side effects. In a rodent model, the investigators have shown that cerebral intraventricular injection of a potassium channel blocker – barium chloride – antagonises the analgesia. This would strongly suggest that positive modulation of one or more potassium channels, plays a pivotal role in mediating the ketamine-analogue analgesia.
Using computational simulations, we can make a model of a protein sitting in the cell membrane, and see how the ions pass down the protein channel. We can then add ketamine molecules in various concentrations, and see where they might bind to the target protein, and how the ion flow is disrupted. The principles of quantum physics are used to describe the shapes of the molecules; and the movements for each of these thousands of molecules (explicitly including water molecules) are calculated using known intermolecular forces. We can then use this virtual molecular world to explore which ion channels are likely to be important for producing ketamine’s analgesic and anaesthetic effects.
The next step is to test how accurately these simulations represent the real molecular world. The investigators will compare the computer-predicted strength of binding to (and channel opening of) the various potassium channels for more than 20 different ketamine-like compounds, with their known actual analgesic potency – as has been previously measured in the laboratory. If a particular potassium channel is actually important in causing analgesia, we would expect to see a good correlation of known experimental analgesic potency for each of the compounds, with its computer derived binding strength/channel opening for the channel of interest – this is known as a Meyer-Overton relationship. The channel(s) with the strongest Meyer-Overton relationship are likely to be the primary molecular targets for these ketamine analgesic compounds. This information will help us to find a compound with strong analgesia (high potassium channel binding/opening) but minimal hallucinogenic side effects (low NMDA binding/blocking).
The detailed understanding of the interaction between ketamine and its various molecular targets could pave the way for the development of more specific ketamine analogue analgesia drugs that are devoid of ketamine’s serious side effects.
Professor Jamie Sleigh, Waikato Hospital, New Zealand.
The project was awarded $A55,000 through the ANZCA research grants program for 2022.