GENE202 is an in vivo lentiviral gene therapy for methylmalonic acidemia developed by SR-Tiget and Genespire and now moving towards clinical testing.
GENE202 is the gene therapy that SR-Tiget and Genespire are advancing towards clinical testing for methylmalonic acidemia (MMA), a rare and severe metabolic disorder for which there are currently no approved therapies that address the underlying cause. The gene therapy involves a single systemic administration of engineered lentiviral vectors designed to deliver a functional copy of the MMUT gene to hepatocytes.
Preclinical results recently published in the Journal of Hepatology by the group led by Alessio Cantore, group leader of the Liver Gene Therapy Unit at the San Raffaele-Telethon Institute for Gene Therapy (SR-Tiget), represent part of the efficacy and safety evidence supporting the move towards testing the therapy in the first patients.
Behind this programme lies a scientific journey that began more than twenty years ago in the laboratory of Luigi Naldini, Director of SR-Tiget, and was subsequently carried forward by Cantore’s group.
A major part of this work has involved extending the use of lentiviral vectors to an in vivo approach. This technology is well established primarily in ex vivo applications, in which cells are collected from the patient, modified in the laboratory and then reinfused. Their direct use within the body, by contrast, had been far less explored: it required the vectors to be adapted for systemic administration and engineered so that they could efficiently reach liver cells.
The rationale behind this choice is the aim of intervening early in genetic diseases that begin to cause damage within the first years of life. “We had the ambition to design a gene therapy suitable for treating paediatric patients at an early stage,” explains Cantore. To achieve this, however, the available technology had to be rethought.
To understand the path that led to GENE202, it is necessary to start with the liver. Methylmalonic acidemia belongs to the group of monogenic diseases that can potentially be targeted by the work of Cantore’s group: disorders caused by a defect in a single gene, in which the aim of gene therapy is to provide a functional copy of that gene or correct the defective one.
In metabolic diseases, the liver is a natural target because of the central role it plays in metabolism. It is also a filtering organ, through which a substantial proportion of circulating blood passes, and can therefore be reached by a vector administered intravenously. Over the years, this feature has also been exploited in the study of other genetic diseases, including haemophilia, in which the liver can produce and release therapeutic proteins such as clotting factors into the bloodstream.
Choosing the target organ, however, raised an important question: which technology should be used to deliver the gene? Historically, in vivo gene therapy has developed mainly through the use of adeno-associated viral vectors, or AAVs, which do not integrate genetic material into the genome of target cells. Lentiviral vectors, by contrast, have become established primarily in ex vivo applications, particularly for the modification of cells of the haematopoietic system, such as lymphocytes and haematopoietic stem and progenitor cells. In these rapidly proliferating cells, the ability of lentiviral vectors to integrate genetic material into the genome allows it to be retained even after cell division.
In diseases that manifest within the first years of life, early intervention can be crucial. In children, however, the liver is still growing and hepatocytes continue to divide. If the genetic material remains outside the genome in episomal form, it is progressively diluted with each cell division: it may persist for a long time in tissues with low proliferative activity, but much less so in a rapidly growing organ. The ability of lentiviral vectors to integrate the therapeutic gene into the genome addresses precisely this need, allowing the genetic correction to be maintained as the liver grows.
For years, SR-Tiget has worked to adapt for systemic administration a technology originally developed to work outside the body, while preserving the stability of the genetic modification in a growing liver. One major question nevertheless remained: what happens to the vector as soon as it enters the bloodstream and encounters the immune system?
On its journey towards liver cells, the vector encounters macrophages and other components of the immune system, which may recognise and phagocytose it before it reaches its target cells. A major part of SR-Tiget’s work has therefore focused on engineering the vectors to minimise this recognition as much as possible. “During the development of the technology, we worked to make the vectors as invisible to the immune system as possible: these are what we call immune-shielded lentiviral vectors,” explains Cantore. The platform developed by the group combines three layers of protection from the immune system.
The first modification concerns MHC molecules, or major histocompatibility complex molecules, proteins found on the surface of cells that are involved in immune recognition. Lentiviral vectors are produced in cells grown in the laboratory and, as they form, they incorporate part of the cell membrane, including the MHC molecules present on its surface. In the recipient’s body, these molecules may be recognised as foreign. To avoid this, SR-Tiget modified the cells used to produce the vectors so that they no longer express MHC molecules.
The second modification involves CD47, a protein that inhibits phagocytosis, the process by which macrophages and other immune cells capture and eliminate particles and microorganisms. By increasing the amount of CD47 on the surface of the vector, researchers aim to reduce its clearance from the circulation and increase the proportion of particles that can reach the liver.
The third level of control concerns transgene expression. A liver-specific promoter and microRNAs ensure that the therapeutic gene is expressed primarily in hepatocytes and as little as possible in immune cells. This limits the production and presentation of the therapeutic product by immune cells, helping to maintain its expression over time.
Together, these modifications define the lentiviral vector platform developed at SR-Tiget for in vivo targeting of the liver. Vectors based on the same proprietary immune-shielded technology developed at SR-Tiget, but engineered to target T cells, have already entered clinical testing for blood cancers and form the basis of the in vivo CAR-T therapies developed by EsoBiotec, which was acquired by AstraZeneca in 2025. For liver applications, however, clinical testing has yet to begin.
In 2020, following investment from Sofinnova Partners, a leading European venture capital firm specialising in the life sciences, Genespire was founded by Fondazione Telethon and Ospedale San Raffaele together with Alessio Cantore and SR-Tiget Director Luigi Naldini. The biotech company licensed a number of patents developed at SR-Tiget and invested in further development of the platform. This work also led to GENE202 for methylmalonic acidemia.
GENE202 is part of a preclinical development pathway that began before the approach was extended to metabolic diseases. For years, Cantore’s group worked on the in vivo use of liver-targeted lentiviral vectors in haemophilia, harnessing hepatocytes to produce the missing clotting factor. Cantore identifies this experience as one of the foundations for the subsequent development of in vivo lentiviral vector approaches for metabolic diseases.
Extending the approach required identifying a suitable indication for the first clinical application. Methylmalonic acidemia (MMA) was prioritised given the high unmet medical need: a disease that manifests very early in life, causes progressive damage and for which therapeutic options remain limited. In addition to a low-protein diet and supportive treatments, liver transplantation can improve metabolic control but requires a suitable donor organ and subsequent lifelong immunosuppression.
In the form caused by mutations in the MMUT gene, the disease results from a deficiency of the enzyme methylmalonyl-CoA mutase. Disruption of this metabolic pathway leads to the accumulation of methylmalonic acid and to abnormalities affecting not only the liver, but also the kidneys, muscles and central nervous system. GENE202 is designed to enable hepatocytes to produce the functional enzyme, increasing the liver’s capacity to clear toxic metabolites.
The preclinical evidence package assembled in preparation for translation to patients included several layers of evidence: demonstration of efficacy in a disease model, studies in mice with human hepatocytes, long-term assessments in animals treated during the neonatal period and studies in non-human primates. The study published by Cantore’s group in the Journal of Hepatology assessed efficacy and safety in a mouse model of MMA. In mice treated at a young age, the therapy produced a rapid effect that was maintained for more than a year, a period approaching the average lifespan of a mouse. The extended follow-up made it possible to assess both the durability of efficacy and the absence of detectable signs of toxicity over time. The study also revealed a selective advantage of corrected hepatocytes, which progressively become more abundant as they regain metabolic function.
“We have a package of efficacy and safety data in animal models that gives us a strong basis for advancing the therapy for young patients” explains Cantore.
On the basis of the preclinical data, clinical testing will begin in paediatric patients, as the possibility of intervening early in children was one of the key reasons that shaped the development of lentiviral vectors for in vivo use from the outset. In the most severe metabolic diseases, damage begins to accumulate within the first years of life and waiting until adulthood may mean intervening too late. Moreover, Genespire has announced the commencement of patient enrolment in a prospective, multicenter observational study in children with MMA caused by mutations in the MMUT gene.
Clinical translation will also help guide the future development of the platform. The group is already working on new generations of vectors that are better able to evade phagocytosis and other antiviral defence mechanisms, with the aim of achieving the same efficacy at lower doses. Reducing the amount of vector required could lower certain toxicity risks and also ease the burden of large-scale manufacturing, which remains one of the challenges for the clinical use of these therapies.
Alongside lentiviral vectors, the group is also investigating targeted gene correction approaches, in which the transgene is inserted, or the genetic defect corrected, at specific sites in the genome. These technologies are still less mature and require further preclinical development. The most appropriate approach will depend on the disease, the degree of control over gene expression that is required and the level of efficiency each technology is able to achieve.
With GENE202, however, one milestone has already been reached: a technology originally developed primarily to modify cells outside the body has been redesigned to work in vivo, adapted for systemic administration and brought close to clinical testing in the liver. It represents the culmination of around twenty years of work on lentiviral vectors and, at the same time, the platform’s first real test in patients. The starting point is methylmalonic acidemia in young children, but the potential scope is broader. If safety and efficacy are confirmed in humans, the growing body of evidence could progressively support the application of the platform to other metabolic liver diseases and, in the future, potentially to other conditions as well.
