Short-term high-dose nicotinamide treatment across glaucoma subtypes reveals increased mtDNA content and minimal metabolomic change in blood

Nicotinamide (NAM) supplementation has emerged as a potential treatment for the retinal ganglion cell (RGC) degeneration of the eye disorder glaucoma. In glaucoma, RGC degeneration has been linked to declining mitochondrial metabolic capacity. NAM is a precursor for the synthesis of nicotinamide adenine dinucleotide (NAD) through the salvage pathway. NAD is a REDOX cofactor and an essential metabolite for cellular functions and the production of energy through mitochondria. In this study, we assessed the effects of a 2- week NAM supplementation treatment on blood NAM levels, NAM metabolism, and wider blood metabolome in 90 glaucoma subjects from 3 different glaucoma subtypes (high tension glaucoma (HTG), normal tension glaucoma (NTG), and pseudoexfoliative glaucoma (PEXG), n = 30 per group), and 30 age and sex-matched healthy controls. We performed small-molecular-weight high-resolution mass spectrometry to assess the metabolome and qPCR to assess the mtDNA amount per cell. At baseline (pre-NAM), only ethylmalonic acid, a compound related to defects in β-oxidation and mitochondrial dysfunction, was found to be modestly increased in the 3 glaucoma subtypes in comparison to controls. All groups showed a similar metabolome response to treatment with a specific increase in NAM and related species (1-methylnicotinamide, 6-hydroxynicotinamide, N1-methyl-2-pyridone-5- carboxamide), an increase in 5-methylcytosine, and a decrease in 4-pyridoxic acid. A linear model corrected by age and sex demonstrated that between groups, only Sarcosine had a different response, with a small reduction in HTG and NTG post-treatment. NAM treatment resulted in a significant but slight within-group increase in blood mtDNA amount in controls and HTG (∼12% and ∼17%, respectively), as determined by a generalized linear mixed effects model adjusted for age and sex. This study suggests that NAM treatment leads to similar plasma metabolome changes between glaucoma groups and controls, which predominantly reflect increased NAM metabolites and intermediates, with minimal effects on the wider metabolome, and a modest increase in mtDNA amount in HTG and controls. As this was observed in a short-term accelerated dosing context, long-term studies will be required to provide more information on the long-term effects of oral NAM supplementation.