Orphan drugs in development for urea cycle disorders: current perspectives

Abstract:

The urea cycle disorders are caused by deficiency of one of the six hepatic enzymes or two transporters involved in detoxification of ammonia. The resulting hyperammonemia causes severe brain injury unless aggressive steps are taken to reduce the accumulation of ammonia, which is thought to be the most toxic metabolite. This review describes the current state of chronic management of urea cycle disorders, focusing on new and emerging therapies. Management strategies include the mainstay of treatment, namely dietary protein restriction and supplementation with l-arginine or l-citrulline. Several currently approved medications utilize and enhance alternative pathways of waste nitrogen excretion (sodium benzoate, sodium phenylacetate, sodium phenylbutyrate in several formulations, and glycerol phenylbutyrate), working through conjugation of the drug to either glycine (in the case of benzoate) or glutamine, the products of which are excreted in the urine. Carglumic acid activates the first committed step of conversion of ammonia to urea, carbamoylphosphate synthetase, and thus effectively treats defective synthesis of the endogenous activator, N-acetylglutamate, whether due to genetic defects or biochemical inhibition of the N-acetylglutamate synthase enzyme. Approaches to neuroprotection during episodes of hyperammonemia are discussed, including the use of controlled hypothermia (brain cooling), as well as proposed, but as yet untested, pharmacologic therapies. Finally, cell-based therapies, including liver transplantation, infusion of fresh or cryopreserved hepatocytes, use of stem cells, and new approaches to gene therapy, are reviewed.

Keywords::

• urea cycle disorders
• inherited hyperammonemias
• orphan drugs
• phenylbutyrate
• N-carbamyl-l-glutamate

Acknowledgments

This review was stimulated by a recent workshop held during the fourth International Symposium on Urea Cycle Disorders in Barcelona, Spain. This workshop was jointly organized by the “Urea Cycle Disorders Consortium”, UCDC, an NIH-sponsored network and registry in the US, and by the “European registry and network for intoxication type metabolic diseases” (E-IMD, EAHC no 2010 12 01), which has received funding from the European Union, in the framework of the Health Programme. The work on urea cycle disorders is supported by the Swiss National Science Foundation (grants 310030_127184 and 310030_153196 to JH).

Disclosure

The authors report no conflicts of interest in this work.