**1. Introduction**

144 Liver Transplantation – Basic Issues

Wiese, M.; Grungreiff, K.; Guthoff, W.; Lafrenz, M.; Oesen, U.; Porst, H. & East German

Wise, M.; Bialek, S.; Finelli, L.; Bell, B.P. & Sorvillo, F. (2008). Changing trends in hepatitis

Vol. 43, No. 4, (Oct), pp.590-8, ISSN 0168-8278.

pp. 1128-35, ISSN 0270-9139.

Hepatitis C Study Group. (2005). Outcome in a hepatitis C (genotype 1b) single source outbreak in a Germany - a 25- year multicenter study. *Journal of Hepatology*,

C-related mortality in the United States, 1995-2004. *Hepatology*, Vol 47, No. 4, (Apr),

Ammonia plays a major role in the pathogenesis of hepatic encephalopathy (HE). Systemic hyperammonemia has been largely found in patients with HE with underlying cirrhosis and acute liver failure. HE is associated with a poor prognosis of both acute and chronic liver disease (Bustamante, Rimola et al. 1999; Hui, Chan et al. 2002). Cirrhosis is the major cause of chronic liver dysfunction and affects 6.5 million people worldwide (Eurostat. 2010, as cited in Albrecht, 2010). HE is the hallmark for acute liver failure (ALF), a syndrome with very high mortality in which liver transplant is the often the only effective treatment.

The HE pathophysiological mechanisms include alterations of blood brain barrier function cytokine production, ammonia-induced changes in neurotransmitter synthesis and release, neuronal oxidative stress, impaired mitochondrial function and osmotic disturbances resulting from astrocytic metabolism. However, none of the metabolic discoveries have yet lead to a therapy which improves prognosis. In the clinic, liver transplantation remains the only curative therapeutic option. Cytotoxic brain edema and intracranial hypertension occurring in encephalopathic ALF patients account for a large number of deaths owing to cerebral herniation. It has been shown that in chronic liver failure there is a low grade brain edema (Andrade, Lucena et al. 2005) that is resolved after transplantation. In comatose patients, moderate hypothermia using cooling blankets to depress energy consumption in the brain seems to be the only relatively effective palliative therapy, but it is expensive, difficult to implement, and not routinely available (Stravitz, Lee et al. 2008; Albrecht 2010).

Studies focusing on inter-organ ammonia metabolism in patients with cirrhosis indicate that the liver, muscles, kidney and the small bowel are important in regulating the circulating levels of ammonia. Historically it was thought that the majority of ammonia was produced by gut bacteria, and treatment regimens including non-absorbable antibiotics and enemas have been extensively used. Contrary to popular belief, it has now been shown that at least 50-60% of total gut ammonia is derived from uptake of glutamine, which is metabolized to glutamate and ammonia by the enzyme glutaminase (GA) (Olde Damink, Jalan et al. 2002; Romero-Gomez, Ramos-Guerrero et al. 2004). Ammonia that would normally be converted to urea by a healthy liver increases to toxic levels. In this situation, the enzyme glutamine synthetase (GS) plays a pivotal role in ammonia detoxification, effectively removing ammonia during the conversion of glutamate to glutamine (Rose, Michalak et al. 1999). Glutamine deamidation by intestinal GA seems to be the main source of ammonia in patients with cirrhosis (Olde Damink, Jalan et al. 2002), and hyperammonaemia and hepatic encephalopathy can appear without the participation of gut bacteria (Weber and Veach 1979).

The following data support the hypothesis that a genetic factor is implicated in the development of overt hepatic encephalopathy: Glutaminase activity has been linked to hepatic encephalopathy and ammonia production; 40% of persons with cirrhosis and minimal hepatic encephalopathy do not develop overt hepatic encephalopathy in long-term follow-up (Romero-Gomez, Boza et al. 2001); patients with cirrhosis who have the same degree of liver dysfunction and the same precipitating factor (for example, variceal bleeding) may or may not develop overt hepatic encephalopathy; and at least 2 different polymorphisms in the promoter region of the glutaminase gene influence protein activity by increasing or decreasing glutaminase activity (Taylor L 2001).

In this chapter we analyze the studies supporting the inhibition of glutaminase based in the identification of mutations in the glutaminase gene to facilitate selection of patients for close monitoring and evaluation for expedited transplantation. Firstly, we have identified a variant in the promoter region of the glutaminase gene that increases glutaminase activity and is associated with the development of HE. Following a simple blood test to identify these patients with the variant, it would be possible to offer a treatment with glutaminase inhibitors. Based on these and another studies we have developed a new molecule, THDP17 that inhibits glutaminase in CACO-2 cell cultures (intestinal cells).

An alternative treatment to HE is currently being investigated: Ornithine phenylacetate (OP). OP is a novel drug that is targeted at reducing ammonia concentration in patients with liver disease and therefore a potential treatment for HE (Jalan, Wright et al. 2007). The mechanism by which OP directly reduces ammonia levels in cirrhosis is by normalization of gut glutaminase activity and concomitant increasing muscle glutamine synthesis activity, subsequently trapping the increased glutamine with phenylacetate,and increasing ammonia excretion as phenylacetylglutamine in the urine.

These studies support glutaminase such as focus for new treatments of HE and the identification of a genetic variation greatly facilitates the selection of patients for close monitoring and evaluation for expedited transplantation.
