– Cancer-related fatigue is one of the most common and disruptive symptoms experienced by patients. It is often present at the time of diagnosis, worsens throughout treatment, and persists well after the cessation of treatment in a significant proportion of patients. The specific mechanisms responsible for fatigue remain largely unknown. Consequently, there are no mechanism-guided therapies for fatigue and the primary approach to patients reporting severe fatigue is education and counseling in the self-management of fatigue. Although conservation of energy is an important strategy in the management of fatigue, the possibility that cancer-related fatigue originates from alterations in energy metabolism has not been examined. The present project fills this void. Our working hypothesis is that cancer-related fatigue is the behavioral consequence of the excess metabolic demand imposed on the organism by the tumor and the inflammation it is possibly associated with. The relative metabolic inefficiency that results from this condition is worsened by the mitochondrial impairment that develops in peripheral tissues and the brain in response to chemotherapy and radiotherapy. To test our hypothesis, we will use two syngeneic murine models of cancer that both respond to a combination of cisplatin and local irradiation, a non-inflammatory model mimicking human papilloma virus-related head and neck cancer, and an inflammatory model represented by Lewis lung carcinoma. We will measure behavioral fatigue in both conditions by decreased voluntary wheel running and alterations in motivated behavior to account for the motivational component of fatigue. In Aim 1, we will determine whether inflammation associated with the tumor and its treatment needs to propagate to the brain for fatigue to develop. This will be done by comparing the time course of inflammation at the periphery and in the brain to that of fatigue before intervening to either block immune signaling molecules by passive immunization or deplete the innate immune cells that mediate the inflammatory process at the periphery and in the brain. In Aim 2, we will test the hypothesis that metabolic reprogramming by cancer and inflammation leads to a condition of relative energy metabolism deficiency that is exacerbated by cancer therapy-induced mitochondrial dysfunction. This will be done by determining the association between metabolic reprogramming and behavioral fatigue before assessing whether intensifying mitochondrial damage exacerbates the behavioral and metabolic phenotypes of fatigue while preventing mitochondrial damage has the reverse effect. In Aim 3, we will test the hypothesis that activation of cytosol DNA sensors by self DNA leaking from mitochondria and cell nuclei triggers this whole process. This research should help understand and treat cancer-related fatigue.