Targeting the Tetrahydrobiopterin Pathway for Identification of Novel Chronic Pain Therapeutics

Tetrahydrobiopterin (BH4) is a cofactor in the conversion of amino acids such as phenylalanine, tryptophan and tyrosine to precursors of neurotransmitters and signalling molecules including dopamine, adrenaline, noradrenaline and nitric oxide. The involvement of BH4 is vital to a range of biochemical processes associated with the cardiovascular system, inflammation, mood, and neurotransmission. Growing evidence implicates high BH4 levels in chronic pain states. Therefore, enzymes that are involved upstream of BH4 biosynthesis within the de novo pathway offer an exciting pharmacological target for small molecule inhibitors to modulate BH4 production and possibly relieve chronic pain.

Pharmacological targeting of both GTP cyclohydrolase 1 (GCH1), the rate-limiting enzyme in the pathway, and sepiapterin reductase (SPR), the terminal enzyme in BH4 production, were both investigated. A combinational approach of various in silico and in vitro techniques was used to generate compound libraries and demonstrate that inhibition of either GCH1 or SPR activity reduces BH4, revealing novel inhibitory interactions for a number of compounds. In particular, tranilast, licensed for use since 1982 in bronchial asthma treatment, inhibited SPR activity (IC50 3.85 ± 1.9 μM), reducing sepiapterin reduction, BH4 production, and downstream reactive oxygen species (ROS) and nitrite production in vitro. Computational ligand docking and molecular dynamics (MD) simulations suggest that tranilast interacts with human SPR at the same spatial position in the active site as sepiapterin, including one of the catalytic triad residues of SPR, S157.

This thesis demonstrates that pharmacological targeting of both GCH1 and SPR in the BH4 pathway are attractive options for chronic pain treatment, and reports novel inhibitory properties of the approved drug tranilast on SPR, a viable prospect for drug repurposing in chronic pain treatment.