PROJECT SUMMARY Antiviral nucleoside analogues are a type of broad-spectrum medication used to prevent viral replication. Only one FDA approved treatment for COVID-19 is a nucleoside analogue and was used under FDA emergency directive to reduce hospitalization times to treat patients infected with the SARS-CoV-2. However, in the past, FDA approved antiviral ribonucleoside analogues used to control infection during the US HIV/AIDS epidemic were shown years later to cause mitochondrial DNA mutations, mitochondrial dysfunction, myopathies, and cause chronic side effects to treated patients. This proposal addresses whether these novel antiviral ribonucleoside analogues (Remdesivir) currently the only FDA approved mediation or (N4-Hydroxycytidine) in Phase II/III clinical trials for COVID-19 affect mitochondrial DNA and mitochondrial function causing cellular and tissue dysfunction. This proposal will use NextGen sequencing, biochemical approaches, mitochondrial assays, and preclinical rodent models of different strains, sexes, and ages to address the following aims. Aim 1: Characterize mtDNA alterations and consequences to OXPHOS function after exposure to a panel of antiviral ribonucleoside analogues. Aim 2: Determine if antiviral ribonucleoside analogues differentially affect mitochondrial function in aged physiology. Even though vaccines are now available for COVID-19, vaccine hesitancy and the appearance of more transmissible SARS-CoV-2 strains are an emerging threat. Also, antiviral nucleoside analogues are often recycled for new viral infections as in the case of Remdesivir which was initially developed against Hepatitis C. This means that these medications may be reused in future viral infections. The research and medical community needs to know whether these antiviral ribonucleoside analogues have off- target side effects, so physicians will be able to make better informed decisions on the costs and benefits of these type of medications for their patients.