Non-invasive biomarkers for diagnosis and monitoring of primary mitochondrial diseases.

Primary mitochondrial diseases (PMDs) represent a clinically and genetically heterogeneous group of disorders characterized by impaired oxidative phosphorylation and multisystem involvement, commonly affecting the nervous system. As therapeutic development accelerates, there is a growing need for robust biomarkers capable of supporting diagnosis, stratifying patient subgroups, monitoring disease progression, and providing sensitive pharmacodynamic readouts for clinical trials. This review summarizes recent advances in three major non-invasive biomarker domains relevant to PMDs: circulating serum and molecular biomarkers, functional and digital endpoints, and neuroimaging modalities. Circulating markers, such as FGF21, GDF15, NfL, and NAD⁺-related signatures, have each been proposed for diagnosis and to follow disease progression, while multi-omics approaches are paving the way toward integrated molecular phenotyping. Digital health technologies, including accelerometry and gait analytics, enable objective quantification of real-world functional impairment, although disease-specific validation remains an unmet need. Neuroimaging offers mechanistic insights through metabolic (MRS, CEST), perfusion (ASL), and molecular modalities (mitochondrial PET tracers). Cutting-edge tools, such as Multi-Spectral Optoacoustic Tomography (MSOT), Raman spectroscopy, and Near-Infrared Spectroscopy (NIRS), promise real-time or spatially resolved assessment of mitochondrial function. Together, these developments outline multidimensional biomarker approaches for PMDs, with the potential to directly measure target engagement and clinically meaningful phenotypes in future therapeutic trials. Future progress will depend on longitudinal validation, harmonized acquisition protocols, and the integration of multimodal platforms to support upcoming therapeutic trials and precision medicine strategies.