Mitochondrial DNA depletion syndromes (MDDS) are severe primary mitochondrial disorders (PMDs) caused by marked reductions in mitochondrial DNA (mtDNA) copy number, leading to impaired oxidative phosphorylation and multi-organ dysfunction. Mutations in DNA polymerase γ (POLG), the sole polymerase for mtDNA replication, are most prevalent, with over 300 variants identified. These conditions result in profound morbidity and early mortality, and this genetic diversity underscores the urgent need for mutation-agnostic therapies. Clinically, MDDS manifests heterogeneously, but neurological symptoms such as seizures, neuropathy and ataxia predominate, often accompanied by hepatic and muscular involvement.
PX578 represents a first-in-class, orally bioavailable, brain-penetrant small-molecule activator of Polγ designed to restore mtDNA replication and mitochondrial function independent of MDDS patient genotype. PX578 binds an allosteric site at the interface between the catalytic subunit POLGA and the proximal POLGB subunit, enhancing enzyme activity and processivity. In patient-derived fibroblasts and POLG mutant neural stem cells, PX578 drives mtDNA synthesis, increases mtDNA copy number, and improves mitochondrial function. Preclinical studies in two MDDS mouse models demonstrate robust efficacy and favorable pharmacology, supporting its potential as a disease-modifying therapy.
PX578 is currently in Phase 1 trials in healthy volunteers, with MDDS patient studies planned for 2026. By directly addressing the fundamental defect of mtDNA depletion, PX578 represents a novel, mechanism-based therapeutic strategy for a broad spectrum of MDDS. This approach represents a transformative advance in mitochondrial medicine with the potential to reverse or stop disease progression.