SUMMARY The incidence of heart failure (HF), a global health threat, is growing. Current therapies for HF are largely directed at maladaptive extra-cardiac neurohormonal circuits in a “one size fits all” approach. Evidence has emerged that myocardial fuel and energy metabolic disturbances contribute to the early stages of HF leading to a vicious cycle of energy starvation and contractile dysfunction. We have conducted myocardial metabolomic and proteomic profiling in well-defined mouse models of early stage HF and in the end-stage failing human heart. The results of these profiling studies have identified protein and metabolite signatures in HF that are indicative of bottlenecks in cardiac fatty acid oxidation (FAO), the chief source of acetyl-CoA for the TCA cycle along with evidence for increased ketone body oxidation in the hypertrophied and failing heart. More recently, we have found that increasing delivery of the ketone body, 3-hydroxybutyrate (3OHB) to heart, retards the development of HF in mice and in a canine tachypacing model. However, the biological mechanisms accounting for the cardioprotective effect of 3OHB are unknown. For example, are these beneficial effects cardiac autonomous? Is 3OHB providing a fuel or does it act via other mechanisms? This MPI proposal is designed to test the hypothesis that increasing myocardial ketones reduces pathological cardiac remodeling by providing a more readily oxidizable fuel for mitochondrial ATP production. To address this hypothesis: 1) we will probe the cardiac-specific beneficial actions of 3OHB during the development of HF. The efficacy of a panel of orally administered ketone esters on cardiac function and pathological remodeling will be assessed in wild-type and cardiac-specific Bdh1-deficient mice (unable to oxidize 3OHB in heart) during development of HF. In addition, the cell-autonomous impact of 3OHB on contractility and calcium transients will be assessed in cardiac myocytes isolated from humans with HF; 2) we will investigate the role of 3OHB as a myocardial fuel in its beneficial actions on pathologic cardiac remodeling by assessing the efficacy of R-3OHB (oxidized) vs S-3OHB (unoxidized) ketone esters on the development of HF in mice; and 3) we will develop and validate strategies to increase myocardial 3OHB delivery as a therapeutic strategy for HF in a large animal model by assessing the effects of R-3OHB and S-3OHB on cardiac hemodynamics and substrate metabolism in a canine tachypacing model of progressive HF. Lastly, the potential of 3OHB as a therapeutic agent will be explored by comparing the impact of administration before and after the onset of HF as well as testing oral 3OHB esters. The planned studies will provide important new insight into the mechanisms whereby 3OHB ameliorates HF and will provide in-depth pre- clinical assessment of increasing delivery of ketone bodies to heart as a novel therapeutic.