The IDO inhibitor coptisine ameliorates cognitive impairment in a mouse model of Alzheimer's disease

Indoleamine 2,3-dioxygenase (IDO), the first and rate-limiting enzyme in the kynurenine pathway (KP) of tryptophan catabolism, was recently established as one of the potential players involved in the pathogenesis of Alzheimer's disease (AD). Coptisine is a main pharmacological active constituent of the traditional Chinese medicinal prescription Oren-gedoku-to (OGT) which has therapeutic potential for the treatment of AD. Our recent studies have demonstrated that OGT significantly inhibited recombinant human IDO activity, which shed light on the possible mechanism of OGT's action on AD. Here, we characterized the effects of coptisine in an AD mouse model on the basis of its IDO inhibitory ability. Coptisine was found to be an efficient uncompetitive IDO inhibitor with a Ki value of 5.8 μM and an IC50 value of 6.3 μM. In AβPP/PS1 transgenic mice, oral administration of coptisine inhibited IDO in the blood and decreased the activation of microglia and astrocytes, consequently prevented neuron loss, reduced amyloid plaque formation, and ameliorated impaired cognition. Neuronal pheochromocytoma (PC12) cells induced with amyloid-β peptide 1-42 and interferon-γ showed reduction of cell viability and enhancement of IDO activity, while coptisine treatment increased cell viability based on its reversal effect on the enhanced activity of IDO. In conclusion, our present findings provide further evidence supporting the critical links between IDO, KP, and AD, and demonstrate coptisine, a novel IDO inhibitor, as a potential new class of drugs for AD treatment.