MELPIDA is proposed for the treatment of subjects with SPG50 and targets neuronal cells to deliver a fully functional human AP4M1 cDNA copy via intrathecal injection to counter the associated neuronal loss. Outcomes will evaluate the safety and tolerability of a single dose of MELPIDA, which will be measured by the treatment-associated adverse events (AEs) and serious adverse events (SAEs). Secondarily, the trial will explore efficacy in terms of disease burden assessments.