Study of ASP2957 in Male Participants With X-linked Myotubular Myopathy Who Need Ventilators

X-linked myotubular myopathy (XLMTM) is a rare and serious condition present at birth where the muscles do not work properly. There are currently no approved therapies for XLMTM. The protein myotubularin is needed for muscle development, movement and breathing. A gene called MTM1 tells the body to make myotubularin. XLMTM is caused by changes, or mutations, in the MTM1 gene. Changes in the MTM1 gene cause low or no levels of myotubularin to be made, so the muscles do not work properly. Gene therapy is a way of getting a healthy copy of a gene into the body. This allows the body's cells to make a normal protein that may reduce disease symptoms. Researchers have developed ASP2957 to get a healthy MTM1 gene into the body. This could help improve muscle development and function in young children with XLMTM. In this study, ASP2957 will be given to humans for the first time. ASP2957 has the healthy MTM1 gene inside a type of empty (killed) virus. The virus delivers the healthy MTM1 gene directly into cells in the body. It's possible that some boys may have antibodies to the virus if they have previously been infected with a similar virus. The antibodies could stop ASP2957 from working properly and cause an immune reaction to ASP2957. To prevent this, the boys will also be given medicines to lower the immune system. The main aims of this study are to check the safety of ASP2957, how well it is tolerated, and to find a suitable dose of ASP2957. The study was designed in 2 phases. In Phase 1, different small groups of boys will receive lower to higher doses of ASP2957. Each boy will receive a single infusion of ASP2957. Any medical problems will be recorded for each dose. This is done to find a suitable dose of ASP2957 to use in Phase 2. In Phase 2, another small group of young boys will receive a single infusion of ASP2957. The most suitable dose of ASP2957 worked out from Phase 1 will be used. The boys will be checked for up to 1 year after their single infusion of ASP2957. After this, there will be the option for the boys to join another study so they will continue to be checked longer term.