Summary: An adeno-associated viral (AAV) vector-mediated gene addition approach to treat paediatric patients with OTC deficiency has been optimised for highly efficient gene expression and delivery to human hepatocytes. Pre-clinical biodistribution, toxicology and vector stability studies have been carried out and funding obtained for a Phase I/II open label multicentre clinical trial.
Background: We have a long-standing interest in developing AAV vector-based therapies for paediatric metabolic liver disease with a focus on X-linked inherited OTC deficiency. OTC is a key enzyme of the urea cycle which functions primarily to detoxify ammonia, a by-product of protein catabolism. Excess ammonia is highly neurotoxic and failure to maintain normal levels leads to recurrent life-threatening hyperammonaemic decompensations and adverse neurological development. Severe presentation during the neonatal period can be fatal if treatment is delayed. Life-long medical supervision is required, with conventional interventions rarely achieving a normal quality of life. While liver transplantation is curative, clinical demand outweighs supply and patients risk disease progression while waiting for a donor liver. Furthermore, transplantation necessitates life-long immunosuppressive therapy accompanied by increased risk of malignancy. Alternative treatment options are desperately required.
Methods and Results: An AAV vector expressing human codon-optimised OTC successfully normalised urinary orotic acid (a sensitive marker) and conferred supraphysiological activity in a mouse model of OTC deficiency when pseudo-serotyped with the murine-tropic AAV8 capsid. The vector was re-packaged with a novel human-specific liver-tropic capsid, LK03, and tested in OTC patient hepatocytes engrafted in immune-deficient FRG mice. At ~30% human liver chimerism, urinary orotic acid is elevated. Following treatment with AAV-LK03.hOTC, orotates were normalised, remaining elevated in mock injected and control vector groups. Long-term toxicology studies in wildtype mice show no significant levels of tumorigenesis. Results from safety and biodistribution studies in NHPs are currently being analysed and stability studies on vector stored long-term are ongoing.