SUMMARY Thymidine Kinase 2 (TK2) is a mitochondrial enzyme that performs the first of the sequential phosphorylation steps that produce deoxythymidine triphosphates and deoxycytidine triphosphates, which are required for mitochondrial DNA synthesis. Genetic deficiency of TK2 results in depletion of mitochondrial DNA and loss of functional mitochondria. These events result in TK2 deficiency, a disease that manifests as progressive myopathy, primarily of skeletal muscle and the diaphragm, and occasional neurological disorders such as seizures. The most aggressive form of the disease has its onset in infancy, and most of these toddlers die within a year of diagnosis. Elucidation of the pathogenesis of TK2 deficiency has led to two potential treatment options that were evaluated under a compassionate use program: (1) substrate enhancement therapy and (2) molecular bypass therapy. Substrate enhancement therapy consists of administering combinations of deoxycytidine and deoxythymidine (dC+dT) and molecular bypass therapy consists of administering combinations of dC monophosphate and dT monophosphate (dCMP+dTMP). Either combination pair was shown to reverse disease progression and lengthen the lives of patients with TK2 deficiency. The combination of dC+dT is currently in clinical development and is administered at very high doses (ranging from approximately 8 to >50 grams daily) in patients who have difficulty swallowing, a subset of whom require feeding tubes to eat. Furthermore, dC+dT offers the benefit of substrate enhancement therapy, but not molecular bypass therapy. MitoRainbow is pursuing preclinical development of a MTR1, a single-agent dinucleotide that is metabolized to all four therapeutic compounds (dC, dT, dCMP and dTMP) in vivo, thus providing both substrate enhancement and molecular bypass therapies. MTR1 is administered by parenteral administration at <1% the dose of oral dT+dC. This approach is anticipated to provide superior bioavailability and ease of use to reach the threshold of effectiveness to treat TK2 deficiency. In this Direct-to-Phase II SBIR we will (1) manufacture MTR1 drug substance (Aim 1), (2) perform a preclinical, non-GLP PK, and acute tolerability study in minipigs (Aim 2), and (3) conduct IND-enabling bioanalytical, PK, and safety studies (Aim 3). Completion of these Aims will provide sufficient data for MitoRainbow to file an IND with the FDA, and to initiate clinical investigations of MTR1 drug product in patients with TK2 deficiency.