Could including deuterated fatty acids in our diets one day help us to lead healthier and longer lives with reduced risk of neurodegenerative diseases, asks Vadim Demidov
In the mid-2000s, Oxford University scientist Mikhail Shchepinov had the novel idea to use non-radioactive heavy isotopes to slow down ageing. His idea revolved around the slowing of free radical damage in the body caused by reactive oxygen species (ROS), one of the principal causes of ageing and age-associated diseases.
ROS oxidise and damage cellular substrates via hydrogen-atom transfer (HAT) reactions. However, isotopic substitution can substantially slow the rate of a chemical transformation, via the kinetic isotope effect (KIE). The heavy hydrogen isotope deuterium (2H or D) makes stronger bonds with carbon and other atoms than the lighter normal hydrogen atom (1H or just H). The rates of simple hydrogen atom transfer (HAT) reactions should typically slow six- to ten-fold when hydrogen is replaced by its non-radioactive isotope deuterium. Substituting hydrogen with heavier tritium (3H or T) should slow these reactions even more, because it is three times heavier. But tritium is unstable and highly radioactive, therefore being harmful to human tissues.
Deuteriation of a few selected sites on the ROS-targeted biomolecules in our bodies might therefore lead to resistance to ROS attacks that cause ageing, Shchepinov reasoned.1 Importantly, such an ‘isotopic swap’ will not fundamentally change these compounds – their bonding structure will be identical, and their chemical properties will be broadly unaltered. Even the size and the shape of deuterated molecules will not change since a deuterium atom has roughly the same diameter as hydrogen and they both form covalent bonds of the same length with other atoms. However, deuterium will be tougher to remove.
Shchepinov filed a patent application for his potential anti-ageing invention and in 2006, together with biotech entrepreneurs Robert Molinari and Charles Cantor, he founded California-based pharma start-up Retrotope.
The first thing the new company had to do was to prove the principle that isotopic substitution can extend longevity. To get some initial evidence, Shchepinov worked with collaborators to grow the roundworms Caenorhabditis elegans on isotopically enriched food to see if this increased their lifespan. These tiny non-parasitic nematodes are often used as model organisms in biomedical studies.
A thorough study published in 2019 indeed found that deuterium supplementation resulted in up to 15% extension in the worm’s lifetime.2 This study also proved that the increase in lifespan on deuterated food was due to a decreased accumulation of harmful ROS as the worms aged and, hence, to reduced oxidative stress. A research report from Stanford University also revealed significant extension of lifespan in yeast through uptake of either deuterated glucose or heavy water (D2O).3 If similar effects happened in humans, these findings suggest that comparable deuterium supplementation could add several years to our lives.
Thus, Shchepinov’s theories about eating heavy isotopes for long and healthy living have been verified in studies on primitive animals and lower eukaryotes. Next, Retrotope was interested to extend these studies to higher animals and humans, then to age-related illnesses and other conditions.
More recently, the company has turned its attention to degenerative neurological disorders. ‘Newly obtained data, which shed new light on molecular mechanisms of a variety of neuropathologies, are in line with the earlier suggested hypothesis that mainly ROS reactions are involved in the pathogenesis of these degenerative diseases,’ says Shchepinov, [now] the company’s Chief Scientific Officer.
A common ‘Achilles’ heel’ in all these different neurodegenerations was found to be neuronal membranes that are attacked by ROS, initiating a set of damaging oxidative chain reactions. Neurons are the building blocks of our nervous system. These highly specialised nerve cells are responsible for transmitting and receiving information using their unique elongated cellular membranes, which are particularly at risk of ROS damage because they have high levels of oxygen consumption and a very high content of lipids containing polyunsaturated fatty acids (PUFAs).
The rates of simple hydrogen atom transfer (HAT) reactions should typically slow six- to ten-fold when hydrogen (1H or just H) is replaced by its non-radioactive isotope deuterium (2H or D).
Deuterium supplementation produced a small but meaningful extension – up to 15% – in the lifetime of roundworm’s grown on isotopically enriched food.
The vulnerability of membranes to oxidative damage arises from their unique structure in which lipids are tightly packed together in a highly ordered bilayer array. An oxidative hit by single ROS triggers a cascade of lipid-damaging events that has catastrophic consequences for membrane integrity because the oxidised lipids cannot form an orderly bilayer. Instead, their arrangement is greatly disordered, resulting in a lipid bilayer with holes and leaks that destroy proper membrane function. Moreover, the oxidised lipids generate a variety of highly reactive byproducts that can attack proteins and nucleic acids to cause additional pathologies. Collectively, this damage results in neurodegeneration.
Retrotope’s latest idea is to employ the KIE to treat neurodegeneration by protecting neuronal membranes from ROS-initiated oxidative chain reactions. This will be done by selectively replacing the hydrogen atoms adjacent to unsaturated carbon atoms – the major sites of ROS attacks, termed ‘bis-allylic sites’ – by deuterium isotopes. Human neurons will be protected from ROS by feeding deuterium-reinforced essential PUFAs, linoleic and linolenic acids (the major precursors of membrane lipids) to patients with neurological diseases. These PUFAs are normally ingested as part of our diet because human bodies are not able to synthesise them.
Oral uptake of deuterated essential PUFAs (as pills or emulsions) will lead to their incorporation into neuronal membranes since the uptake and transport systems cannot distinguish between hydrogen and deuterium, but the presence of the latter will strongly protect membranes from oxidative damage. It is hoped that deuterium substitution should greatly inhibit initial peroxidation of lipids by ROS, and slow numerous subsequent branching oxidative reactions of the lipids in neuromembranes, without substantially changing normal cellular membrane functions.
‘I have been following the research into deuterated polyunsaturated fatty acids for many years and have great hopes for the idea of making cellular membranes “fireproof” against ROS,’ says Konstantin Khrapko from the Biology Department at Northeastern University in Boston, US.
To examine this idea, Retrotope scientists and their collaborators first studied the protective effect of deuterium-reinforced PUFAs (d-PUFAs) on yeast cells subjected to lethal oxidative stress. They found that yeast treated with linoleic acid, only one fifth deuterated at bis-allylic sites, were fully rescued from the harmful effects of lipid oxidation.4
This surprise finding was later explained by the fact that even partial substitution of lipid membranes with d-PUFAs resulted in their complete protection from oxidation.5 And it settled a major concern regarding the prospective therapeutic use of d-PUFAs. PUFAs are involved in various vital metabolic cellular pathways, but consumption of relatively small amounts of modified PUFAs should not disrupt these pathways since most of the PUFAs will be unmodified. The need for relatively low amounts of PUFAs to stabilise membranes against oxidation also makes it much easier to reach the necessary therapeutic levels in patients.
Encouraging results were also obtained in pre-clinical testing of d-PUFAs in cell and animal models of several human neurodegenerative diseases.6 In each case, the drug was safe and effective.
Buoyed by success in cells and animals, Retrotope began, a few years ago, to sponsor clinical studies in patients with different neuropathologies. Retrotope’s lead candidate drug RT001 is 11,11-di-deuterolinoleic acid. The treatment suggested by Retrotope is not disease specific. It does not target the root cause of disease, such as different mutated genes involved in inherited neurological diseases. Regardless of how these mutations, or other disease-causing factors, sensitise lipid membranes to ROS oxidation, RT001 treatment should protect them against damage.
‘The beauty of this idea to employ d-PUFAs for averting the ROS-inflicted damages is that instead of eliminating ROS, this approach robustly defends from them,’ explains Khrapko. ‘This makes the treatment a highly targeted one by shielding the most vulnerable cellular components. And not eliminating ROS is also advantageous because, as it has now become clear, they are important regulators and metabolic intermediates.’
Initial clinical trials and case studies of RT001 in young and adult patients with Friedreich’s ataxia, infantile neuroaxonal dystrophy (INAD) and late onset Tay-Sachs disease showed the drug arrested progression and even some signs of disease reversal.7-9
No toxicity or serious RT001-related adverse events were observed. RT001 trial treatments were initiated, in 2018 and 2020, with patients having other neurodegenerative conditions, such as amyotrophic lateral sclerosis and progressive supranuclear palsy.
There are no currently approved drugs for any of these disorders, and all other experimental treatments of neurodegenerative diseases, such as antioxidant therapy, antisense oligonucleotides and gene editing so far have had limited clinical efficacy or have shown toxic side-effects when tested on human cells or humans. RT001 could become the first drug of choice to treat patients with various neurological diseases, including common diseases like Alzheimer’s and Parkinson’s where the role of oxidative stress and lipid peroxidation as major executioners in disease pathology has long been known.
‘Lipid oxidation is also one of the initiators of apoptosis – a type of programmed cell death, so the attenuation of lipid oxidation may potentially interfere with removal of the dysfunctional cells,’ remarks Khrapko. ‘But the success of clinical trials implies that such kind of negative effect, if it exists, is outweighed by the advantages,’ he concludes.
Yeast treated with deuterium-modified linoleic acid was fully rescued from the harmful effects of lipid oxidation, even though only 1/5th of the linoleic acid had been deuterated at bis-allylic sites.
Most current large-scale methods to synthesise site-selectively deuterated PUFAs are rather laborious and expensive. Retrotope is currently investigating more effective synthetic approaches that could lower d-PUFAs prices so they may potentially be useful as a food supplement to slow down the ageing process. In collaboration with researchers from Serbia and Australia, the company discovered an inexpensive catalytic process for site-specific deuteriation of PUFAs using heavy water (D2O), the cheapest source of deuterium.10 If successfully scaled up, this process would substantially reduce the cost of most d-PUFAs.
Applications of d-PUFAs could be extended to a wide range of other non-neurological diseases, Retrotope believes. The company recently reported positive pre-clinical results using d-PUFAs in cell and animal models of non-neurological ROS-related disorders, such as vascular atherosclerosis and macular degeneration of eyes.11-13
In addition, Retrotope is expanding its proprietary drug platform to other site-specifically deuterated essential natural biomolecules, including amino acids and DNA nucleobases involved in pathogenesis of certain cancers and inflammation.13 Thus, even non-oxidative diseases could possibly be treated by using the deuterium KIE.
In spring 2018, when patient ‘Z’ was two and a half years old, she was diagnosed with infantile neuroaxonal dystrophy (INAD) – an inherited rare neurological deadly disorder. With no available cure and no treatment to stop the progression of symptoms, children with INAD typically die between the ages of five and ten.
Caused by a genetic defect in a vital gene that removes damaged lipids from cell membranes, INAD is often referred to as Parkinson’s mixed with Alzheimer’s for kids. It affects nerve cells that carry messages from the brain to other parts of the body, resulting in progressive loss of vision, muscular control and mental skills.
Soon after being diagnosed, ‘Z’ had partly lost her mobility and speech. But fortunately, six months later, she was enrolled into an expanded access trial of a promising new drug RT001 developed by US company Retrotope. Sometimes called ‘compassionate use’, expanded access is a way to provide an investigational therapy to a patient with a serious or life-threatening illness for which other treatments are not available. RT001 – a modified version of a nutritionally essential fatty acid that must be obtained through the diet – is currently being tested in a more formal clinical trial on 19 children with INAD at Goryeb Children’s Hospital in New Jersey and Benioff Children’s Hospital in San Francisco.
‘Z’ and all other children in these trials, responded well to the treatment, which helped halt some of her INAD-caused deterioration. She recently celebrated her fourth birthday and is now at preschool full time. A potential genetic therapy fix for INAD is expected to be ready for clinical trials in a couple years at Great Ormond Street Hospital in London, UK. It’s hoped that the experimental drug RT001 will keep her from further decline until a gene therapy cure becomes available.
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13 V.V. Demidov, Drug Discov Today (in press), 2020 https://doi.org/10.1016/j.drudis.2020.03.014