Rifaximin Ameliorates Urea Cycle Disorder in a Murine Model of humanOrnithine Transcarbamylase Deficiency

Introduction: Urea cycle disorder (UCD) is one of the major groups of inherited rare diseases that cause hyperammonaemia requiring early detection and appropriate clinical management to avoid neurological damage and death. Besides protein intake reduction, ammonia scavenger pharmacological therapy is used to prevent or treat acute hyperammonaemia episodes and related damage. Among recent treatments, the Ravicti prescription has been shown to reduce plasma ammonia, hyperammonaemia episodes, and hospital admissions. Another product, potentially able to counteract hyperammonemia in urea cycle disorder via modulation of the intestinal microbiota, is rifaximin, also approved by the FDA for prevention and treatment of hepatic encephalopathy. This study aims to investigate possible pharmacological interactions between Ravicti and rifaximin in the translational murine model of UCD induced by ornithine transcarbamylase (OTC) deficiency Methods: We characterized B6EiC3Sn a/A-Otcspf-ash/J (disease model, which is OTC-deficient, hereafter referred to as Otcspf-ash mouse/mice) as a valid UCD murine model to evaluate the tolerability and efficacy of rifaximin with and without ammonia scavenger Ravicti. 15-week old Otcspf-ash mice expressing the variant (c.386G>A, p.Arg129His) in the OTC enzyme of liver, also found in patients, were given by oral route for 2 weeks rifaximin and 4 weeks Ravicti or the combination. After sacrifice, liver, small intestine, colon, hippocampus, and brain motor cortex were collected for whole-mount immunostaining. Results: Analysis of OTC-deficient mice showed increased plasma ammonia, liver damage, and colonic inflammation, as well as brain alterations in terms of astrocyte swelling, microglia activation, and neuronal apoptosis, in addition to abnormal mouse behavior with impaired motor coordination. Administration of rifaximin and Ravicti alone was shown to improve mice behavior and significantly hinder hepatic impairment, intestinal inflammation, and neuronal dysfunction. The combination of the two drugs showed no additional improvement over their individual efficacy. Discussion: Interestingly, rifaximin enhanced bacteria with protective functions towards the host, SCFAs producers, or involved in ureagenesis or stimulation of FX receptors. showing a consistent ability to reduce bacteria able to lyse urea or positively correlated to urea cycle or bile salts pathway disorders Conclusion: Otcspf-ash was a good model to study UCD and investigate novel therapeutic strategies. Rifaximin seems promising regarding its capability to protect patients with UCD and warrants further investigation