Humans have been using cannabis medicinally since ancient times. However, its use in the modern world is strictly controlled due to the psychoactive properties of THC (tetrahydrocannabinol).
Research focuses on the potential medicinal benefits of the chemically-similar CBD (cannabidiol), which does not have psychoactive effects. In June 2018, the US Food and Drug Administration approved a drug based on CBD to treat two rare and severe forms of epilepsy - Dravet Syndrome and Lennox-Gastaut Syndrome - in children. The European Medicines Agency (EMA) followed suit in September 2019. Both of these forms of epilepsy are associated with voltage-gated sodium channels. However, the structural details of how CBD interacts with sodium channels were not known.
In work recently published in eLife, researchers from Birkbeck College, University of London used high-resolution macromolecular crystallography to determine the molecular basis of CBD interactions with a voltage-gated sodium channel target. Combined with functional studies, their results help explain why CBD can be an effective treatment for these types of epilepsy, and suggest why THC is not. The detailed structures they solved will be an invaluable input to rational drug design for future treatments.
The role of voltage-gated sodium channels (Navs) is to enable the passage of sodium ions across cell membranes, which contributes to the electrical signalling in cells. Nine Navs have been identified in humans, and mutations in these human sodium channels (hNavs) are associated with epilepsy and other neurological disorders, heart disease and pain. They are, therefore, major targets for drug development.
Professor Bonnie Ann Wallace refers to her team at Birkbeck College, University of London, as "drug detectives".
Prof Wallace, said:
We trawl through the literature looking for anecdotal evidence for drugs that affect sodium channels, including side effects. We find this a very valuable research tool, and it's far easier (and cheaper) to repurpose an existing drug than it is to develop a new one.
Dravet Syndrome and Lennox-Gastaut Syndrome are rare early-onset epilepsies associated with Navs. With increasing evidence of the effectiveness of CBD for treating epileptic conditions, Prof Wallace and her team - PhD student Lily Goodyer Sait, senior post-doc Dr Altin Sula and researcher David Hollingworth - set out to investigate the structural basis of how CBD interacts with sodium channels. They examined a bacterial Nav (NavMs) with the same functional properties, using high-resolution macromolecular crystallography (MX).
To complete this study, the team collected MX data on nearly a thousand crystals, at Diamond beamlines I03, I04, and I24, as well as at DESY in Germany and Soleil in France. The high resolution, ~2.3 Å, allowed them to clearly define the binding sites for the CBD molecule.
Prof Wallace said:
I've had a close relationship with Diamond since the beginning, and I am pleased to be able to say that it has a wonderful reputation, and provides easy access, even during Covid lockdowns.
Diamond can provide remote access to its beamlines, where samples can be shipped to Diamond and users can control the experiment remotely.
The Birkbeck team worked in collaboration with electrophysiologists from the Department of Biomedical Physiology and Kinesiology at Simon Fraser University in Canada. Their functional studies have shown that CBD does bind to the Nav, and they have also created mutants that don't bind, and which don't have the same effect.
The new data show that CBD binds at a novel site on the Nav, and provides a molecular mechanism for inhibiting the channel. The CBD site is similar to, but distinct from, sites on Navs where other drugs bind, and thus may also provide the basis for development of future pharmaceutical drugs.
CBD and THC are almost identical chemically, however, CBD has an extra OH group that plays a role in its interaction with the Nav. Although licensing restrictions prevented the team from investigating the effect of THC experimentally, modelling studies show that - without the OH group - it is unlikely to have the same effect on the Nav. This confirms functional studies that show THC is not an effective treatment for these types of epilepsy.
The combination of high-resolution structural evidence and functional studies presented in this study provides new insights into a possible mechanism for CBD interactions with sodium channels. It adds to our understanding of the molecular interactions between CBD and Nav targets, and how these may be related to its use for treating epilepsy. It is part of a more extensive program looking at potential drug compounds, and the team are already working with some promising candidate compounds.
To learn more about Diamond's MX beamlines, please visit the Macromolecular Crystallography web page.
Sait L et al. Cannabidiol interactions with voltage-gated sodium channels. eLife, 9, e58593 (2020). DOI:10.7554/eLife.58593.
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