An interesting discovery for research into multiple sclerosis

Research into multiple sclerosis has made enormous progress over the last thirty years, to the extent that many patients can now live with the condition for decades, significantly reducing the number of relapses and the risk of disability. However, one problem remains that medicine has not yet resolved satisfactorily: what should be done when the nervous system has already suffered damage and neurological functions begin to be lost? Reducing the activity of the immune system helps to slow down the destruction of nerve tissue, but restoring what has been compromised is far more difficult. This is why a study published on 23 September in Science Translational Medicine deserves attention: in it, a team of American researchers, led by Jayakrishna Ambati of the University of Virginia, achieved a surprising result. By administering an experimental molecule to mice that were already paralysed and had lost their sight, the animals showed a recovery of neurological functions. The substance used is derived from a family of drugs developed many years ago to combat HIV, the virus responsible for AIDS.

To understand why this discovery is significant, it is important to bear in mind that multiple sclerosis is a chronic inflammatory disease of the central nervous system, in which an abnormal immune response primarily attacks myelin, the lipid-rich sheath that surrounds axons – the extensions through which neurons transmit electrical signals. Myelin enables nerve impulses to travel rapidly and efficiently; consequently, its destruction causes disturbances in nerve conduction, which can manifest as vision problems, changes in sensation, muscle weakness and difficulty coordinating movements. When the inflammatory process persists, the axon itself may also be damaged, with potentially permanent consequences.

The therapies currently available target different components of the immune response, and some have proved extraordinarily effective in preventing new episodes of inflammation. However, once significant neurological damage has occurred, suppressing pathological immune activity may not be sufficient to restore lost functions. This distinction explains why a molecule capable of improving the condition of animals that are already symptomatic is of particular interest, especially when the clinical effect is accompanied by evidence of better preservation of myelin and axons.

However, the scientific journey that led to the new drug began far removed from multiple sclerosis, in studies on age-related macular degeneration, a retinal condition that can severely impair central vision. In 2014, Ambati and his colleagues published a study in Science demonstrating that certain drugs already used to treat HIV possessed a previously unexpected property. These substances, known as nucleoside reverse transcriptase inhibitors, or NRTIs, had been designed to interfere with an enzyme essential for retrovirus replication. Yet, in experiments conducted by the American team, they were found to be capable of preventing the activation of a specific molecular system responsible for certain inflammatory responses, independently of their antiviral activity.

The system in question is called the inflammasome. It is a protein complex that certain cells assemble when they recognise danger signals, such as those associated with cellular damage or infections. Once activated, the inflammasome promotes the production of potent inflammatory mediators and can trigger a form of cell death known as pyroptosis, which helps to amplify the immune response. One of its main components is the NLRP3 protein, which is involved in numerous conditions characterised by persistent inflammation.

In the 2014 experiments, NRTIs were able to block the activation of the NLRP3 inflammasome induced by specific RNA molecules, by interfering with the response associated with a cell receptor called P2X7. The result suggested a much broader therapeutic potential than that for which these drugs had originally been developed: molecules designed to block a viral enzyme could be used to modulate certain inflammatory processes in the body.

This observation gave rise to the idea of chemically modifying the NRTIs to preserve their anti-inflammatory activity, whilst seeking to reduce the side effects associated with some of the original molecules. The resulting family of compounds was named kamuvudine. Among these, kamuvudine-9, abbreviated to K-9, was selected to investigate whether blocking the inflammasome might also have therapeutic effects in a neuroinflammatory disease such as multiple sclerosis.

To test this hypothesis, the researchers used an experimental model known as experimental autoimmune encephalomyelitis, or EAE, in which an immune response against components of myelin is induced in mice. The animals thus develop an inflammatory disease that replicates certain key aspects of human multiple sclerosis, including motor deficits and visual impairment. The model naturally has its limitations, as the human disease is far more complex and can progress through mechanisms that are not fully replicated in the animal. It remains, however, one of the most widely used tools for studying the consequences of autoimmune inflammation in the nervous system.

The most interesting aspect of the new study concerns the point at which treatment began. The researchers also administered K-9 to animals in which neurological symptoms had already appeared, continuing the treatment for around two weeks. In the treated mice, disease progression was halted and a recovery in motor and visual abilities was observed; this recovery was not achieved to the same extent in the animals in the control groups, including the group receiving conventional treatment for multiple sclerosis.

This observation alone could have been due to a reduction in inflammation, allowing the nerve structures that were still intact to resume functioning. For this reason, the authors also looked for signs of nerve tissue protection by examining the animals’ spinal cords. In mice treated with K-9, they found better preservation of myelin and axons, accompanied by a particularly significant finding: the treatment prevented an increase in the blood concentration of the neurofilament light chain, denoted by the abbreviation NfL. Neurofilaments are structural proteins found mainly in axons and are released when these structures are damaged. Their concentration in the blood can therefore provide an indirect measure of the activity of neurodegenerative processes.

The correlation between improved neurological function and these indicators of nerve tissue protection makes the result more significant than a mere reduction in symptoms. It remains to be determined how much of the observed recovery stems from the survival of previously compromised nerve structures and how much from any repair processes. In particular, the study does not demonstrate that K-9 is capable of regenerating destroyed axons or of rebuilding myelin that has already been permanently lost. The possibility of regaining neurological function after the onset of a deficit is, however, an important finding, precisely because it suggests that some of the functional damage may be reversible when the appropriate inflammatory mechanisms are targeted.

Molecular experiments have also yielded a result worthy of attention. K-9 interferes with the interaction between NLRP3 and a protein called NEK7, which is necessary for inflammasome activation under certain conditions. It also hinders the association of NLRP3 with NLRC4, another protein involved in cellular danger recognition systems. Accordingly, the authors propose that the molecule acts on two pathways of inflammasome activation, thereby limiting an inflammatory response capable of contributing to the degeneration of nerve tissue.

This mechanism allows the results to be interpreted within a broader context. In multiple sclerosis, in fact, the adaptive immune system – which is responsible for the specific recognition of antigens via lymphocytes – plays a central role in triggering the autoimmune response. However, a significant part of the damage to the nervous system also depends on the activity of innate immune cells, including microglial cells, which reside in the brain and spinal cord and are involved in the response to damage. The persistent activation of these systems may contribute to the persistence of inflammation and axonal damage, even when some of the immune processes that initially triggered the disease are controlled by medication.

The potential to influence these mechanisms is particularly significant in progressive forms of multiple sclerosis, where disability can continue to worsen even in the absence of obvious clinical relapses. It would be premature to conclude that K-9 can alter the course of these forms of the disease, as the model used in the study reproduces only part of their complexity. The research does, however, point to a possibility that warrants further investigation: targeting the inflammatory processes that contribute directly to neurological damage could complement therapeutic strategies aimed at the autoimmune response.

The authors sought to bolster this interpretation by also using data from human patients. They examined three distinct populations using large US health and insurance databases, covering a total of over three million people. The analysis included individuals treated with NRTIs for HIV infection, people taking these drugs to prevent infection, and patients undergoing antiviral therapy for hepatitis B. In these populations, exposure to NRTIs was associated with a lower incidence of multiple sclerosis compared with the control groups. Among individuals who had already been diagnosed with multiple sclerosis, taking NRTIs was also associated with a lower frequency of subsequent relapses.

This is a striking finding, as it stems from human observations independent of experiments conducted on mice and concerns a family of molecules that, like K-9, have the ability to interfere with certain inflammatory mechanisms. It must be borne in mind, however, that these analyses are observational. People who take antivirals differ from those who do not in terms of clinical conditions, exposure to infection and patterns of healthcare, all of which are factors that can influence the risk of diagnosis and the course of the disease. Even a statistically robust association, after appropriate adjustments, cannot replace experimental proof of a treatment’s efficacy in patients.

There is also an important biological distinction. The NRTIs used against HIV and hepatitis B interfere with enzymes necessary for viral replication, whilst K-9 has been developed specifically to separate its anti-inflammatory activity from its antiviral activity. The epidemiological data collected on traditional NRTIs therefore support the plausibility of the proposed mechanism, without proving that the new molecule possesses the same efficacy in humans as that observed in animals.

The link between viral infections and multiple sclerosis adds a further element of interest. In 2022, a study published in Science, based on the monitoring of millions of US military personnel, showed that infection with the Epstein-Barr virus almost invariably preceded the onset of the disease and was associated with an approximately 32-fold increase in the risk of developing it. Other research has identified possible mechanisms through which the immune response to the virus might also recognise proteins in the nervous system. This phenomenon, known as molecular mimicry, occurs when different antigens share sufficient similarities to trigger cross-reactive immune responses.

These findings have reinforced the hypothesis that Epstein-Barr virus infection represents a key step in the development of multiple sclerosis, although further conditions are required for the disease to manifest itself. The new study on kamuvudine, on the other hand, focuses on the processes that contribute to neurological damage once the pathological response has taken hold. The two lines of research are compatible and could contribute to a more comprehensive understanding of the disease, in which the factors that promote its onset and the mechanisms responsible for the subsequent nerve degeneration play different roles.

However, there are still crucial steps to be taken before considering a therapeutic application. It will be necessary to verify the safety of K-9 when administered over prolonged periods and to establish whether the concentrations achievable in the body are sufficient to modify inflammatory activity in the human nervous system. The molecule has already entered clinical trials for certain eye conditions, including diabetic macular oedema, and this may provide useful information on its behaviour in humans. However, its efficacy in multiple sclerosis remains to be demonstrated through controlled clinical trials capable of measuring both disease activity and any recovery of neurological function.

Meanwhile, the story of the nucleoside analogues demonstrates just how fruitful it can be to study the biological effects of drugs beyond the target for which they were designed. A compound developed to inhibit viral replication has revealed a second mode of action, from which it has been possible to derive a new family of molecules with different characteristics. The journey from AIDS research to age-related macular degeneration and finally to multiple sclerosis shows how an understanding of molecular mechanisms can suggest applications that nobody would have imagined when the original substances were first introduced.

If the results obtained in animals are confirmed in humans, the most significant benefit could be for precisely those patients for whom preventing new lesions is no longer sufficient. For those who have lost the ability to walk unaided, or have seen their vision progressively deteriorate, even partial recovery can represent a huge improvement in quality of life. Proof that this is possible with K-9 is yet to be provided, and it will be clinical trials that determine whether the promise observed in mice can be translated into a treatment. Today’s result, however, justifies a question that until a few years ago would have seemed particularly ambitious: to what extent can we restore the functions of the nervous system when we learn to control the causes of its deterioration?