**3.2 QUIN and excitotoxicity**

QUIN is an excitotoxin and can be linked to excitotoxicity in ALS in two ways: (1) through the activation of the NMDA receptor; and (2) its effect on glutamate levels. The heteromeric NMDA receptor (NR) has three families of subunits: NR1 (A and B), NR2 (A to D) and NR3 (A and B). In the ventral and dorsal horns of ALS spinal cord, up to 78% loss of NR2A has been detected (Samarasinghe *et al.* 1996). Interestingly, QUIN acts on the NR subtypes, NR1+NR2A and NR1+NR2B (Priestley *et al.* 1995), and the loss of NR2A in ALS patients may possibly reflect an excitotoxic mechanism involving QUIN.

Glutamate induced toxicity has been implicated in the selective neuronal damage seen in ALS and counteracting glutamatergic toxicity, thus far, is the only treatment available for ALS. QUIN can potentiate its own toxicity and that of other excitatory amino acids, such as glutamate, under energy deprived conditions (Schurr and Rigor 1993). Moreover, QUIN

The Kynurenine Pathway 363

In ALS, apoptosis is evident from the increased expression of pro-apoptotic protooncogenes, BCl-2 and c-jun, and caspases 1 and 3 in tissue, and from the morphological features of apoptosis displayed by dying motor neurons. QUIN has been demonstrated to induce neuronal and astrocytic apoptosis involving the activation of caspase 3 (Macaya *et al.* 1994; Jeon *et al.* 1999; Guillemin *et al.* 2005). Astrocytes are essential for the homeostasis of the CNS and so, the well-being of neurons. Hence, the loss of normal astrocytes in ALS would be detrimental to motor neurons and could exacerbate disease progression in ALS

**4. Potential therapeutics targeted at the kynurenine pawthay for ALS** 

In 1995, riluzole became the first drug, and remains the only drug, approved by the FDA (USA) for treatment of ALS. The approval was based on two large placebo controlled clinical studies where riluzole decreased the rate of muscle deterioration and modestly improved the survival rate of ALS patients (Bensimon *et al.* 1994; Lacomblez *et al.* 1996). Though the precise mechanism of riluzole remains unclear, it appears to interfere with excitatory amino acid signalling, perhaps through the inhibition of glutamate release (Mizoule *et al.* 1985; Cheramy *et al.* 1992; Martin *et al.* 1993), blockade of inactivated sodium channels (Benoit and Escande 1991) and interaction with guanosine triphosphate (GTP)-binding proteins (Doble *et al.* 1992). 16 years on, there is still a lack of effective treatment available and an intense search is on going to discover better treatments for

In developing therapeutic agents aimed at modulating the kynurenine pathway, two approaches may be taken: (1) to develop analogues of the neuroprotective kynurenines; (2) to inhibit the synthesis of the neurotoxic QUIN. Figure 4 summarizes the drugs targeting the

As the first enzyme in the kynurenine pathway, suppression of IDO would lead to decrease QUIN production. Although it has not been specifically tested in neurodegenerative disorders, it is a novel therapeutic target in cancer research and the results have been positive. Using transgenic mouse model of breast cancer, IDO-1 inhibitors, 1-MT and methyl-thiohydantoin-tryptophan, were able to potentiate the efficacy of chemotherapy drugs, promoting tumour regression without increasing the side

QUIN neurotoxicity can be prevented by blocking the glycine modulatory site of the NMDA receptor (Foster *et al.* 1990; Hartley *et al.* 1990). 7-chlorokynurenate, a synthetic derivative of KYNA, is such an NMDA receptor antagonist (Kemp *et al.* 1988) but has difficulty crossing the BBB (Rao *et al.* 1993). On the other hand, its precursor, 4-chlorokynurenine, is rapidly transported across the BBB (Hokari *et al.* 1996). Intracerebral and intraperitoneal administration of 4-chlorokynurenine with QUIN showed successful enzymatic transamination of 4-chlorokynurenine into the neuroprotective 7-chlorokynurenate (Wu *et* 

kynurenine pathway that could be potential candidates for ALS.

**3.6 QUIN and apoptosis** 

(Yamanaka *et al.* 2008).

ALS.

**4.1 IDO inhibitors** 

effects (Muller *et al.* 2005).

**4.2 4-chlorokynurenine** 

*al.* 1997; Wu *et al.* 2000).

contributes to excessive microenvironment glutamate concentrations and neurotoxicity via at least three mechanisms: (1) stimulation of synaptosomal glutamate release by neurons (Tavares *et al.* 2002); (2) inhibition of glutamate uptake into synaptic vesicle by astrocytes (Tavares *et al.* 2000); and (3) limiting glutamate to glutamine recycling in astrocytes by decreasing glutamine synthetase activity (Baverel *et al.* 1990).
