Restoring ornithine transcarbamylase (OTC) activity in an OTC‐deficient mouse model using LUNAR‐OTC mRNA

Abstract Ornithine transcarbamylase (OTC) catalyses the reaction from ornithine to citrulline in the urea cycle. Ornithine transcarbamylase deficiency (OTCD) results in episodes of hyperammonemia. Arcturus Therapeutics developed a lipid nanoparticle (LNP)‐encapsulated OTC‐mRNA (LUNAR‐OTC) that results in a replacement enzyme and is currently undergoing clinical trials. In this study, the efficacy of LUNAR OTC‐mRNA drug in the spfash mouse model was examined by measuring the OTC enzyme activity and protein expression in the liver and plasma of OTC‐mRNA‐treated mice. Using purified citrulline‐D4 as the substrate improved the sensitivity of the OTC activity assay and allowed us to quantify the ornithine‐D4 product from the mouse plasma samples. OTC activity in the liver showed a clear dose response: The lowest dose, 0.3 mg/kg, resulted in higher activity than that of the untreated group, and the highest dose, 3 mg/kg, resulted in completely restored OTC activity in the liver. OTC activity in plasma was also dose‐dependent. A clear positive correlation between the OTC activity in the liver and that in the plasma suggests that the plasma OTC activity assay may serve as a surrogate for measuring OTC activity in liver biopsy samples. In addition, the OTC protein expression levels correlated well with the OTC activity in liver samples, but there was no quantifiable OTC protein in the plasma samples. This finding suggests that the sensitivity of the OTC activity assay is superior to that of the protein expression assay. Overall, the results of this study suggest that the OTC activity assay described here can be used as a clinical pharmacodynamic endpoint to measure the effectiveness of OTCD treatment.