Peripheralized sepiapterin reductase inhibition as a safe analgesic therapy

Abstract

The development of novel analgesics for chronic pain in the last 2 decades has proven virtually intractable, typically failing due to lack of efficacy and dose-limiting side effects. Identified through unbiased gene expression profiling experiments in rats and confirmed by human genome-wide association studies, the role of excessive tetrahydrobiopterin (BH4) in chronic pain has been validated by numerous clinical and preclinical studies. BH4 is an essential cofactor for aromatic amino acid hydroxylases, nitric oxide synthases, and alkylglycerol monooxygenase so a lack of BH4 leads to a range of symptoms in the periphery and central nervous system (CNS). An ideal therapeutic goal therefore would be to block excessive BH4 production, while preventing potential BH4 rundown. In this review, we make the case that sepiapterin reductase (SPR) inhibition restricted to the periphery (i.e., excluded from the spinal cord and brain), is an efficacious and safe target to alleviate chronic pain. First, we describe how different cell types that engage in BH4 overproduction and contribute to pain hypersensitivity, are themselves restricted to peripheral tissues and show their blockade is sufficient to alleviate pain. We discuss the likely safety profile of peripherally restricted SPR inhibition based on human genetic data, the biochemical alternate routes of BH4 production in various tissues and species, and the potential pitfalls to predictive translation when using rodents. Finally, we propose and discuss possible formulation and molecular strategies to achieve peripherally restricted, potent SPR inhibition to treat not only chronic pain but other conditions where excessive BH4 has been demonstrated to be pathological.

Keywords: PROTAC; Sepiapterin Reductase; aldoketoreductase; carbonyl reductase; hyaluronic acid; immune cells; nociceptors; salvage pathways.

FIGURE 1

Tetrahydrobiopterin and pain. (A) Tetrahydrobiopterin de novo production pathway. BH4 is a cofactor for phenylalanine hydroxylase (PAH) (produces tyrosine), tyrosine hydroxylase (TH) (produces dopamine), tryptophan hydroxylase (TPH) (produces serotonin), alkyl glycerol monooxygenase (AGMO) (involved in lipid metabolism) and nitric oxide synthases (NOS) (produces NO). (B) A GCH1 haplotype is associated with reduced BH4 production and pain ratings (dashed line). (C) The ‘pain protective’ haplotype has been characterized in several pain conditions, most of them involving nerve injury. Note that in sickle cell disease, the pain protective haplotype is pain-aggravating. (D) Reporter and tissue-specific gain- or loss-of-function experiments have identified sensory neurons, macrophages, mast cells and T Cells to utilize BH4 in promoting pain. (E) Several agents that target GCH1 or SPR have analgesic properties in preclinical models. (F), Graphical depiction showing the therapeutic window to normalize/reduce BH4 levels for pain relief. Red color indicates undesirable outcomes (pain sensitivity and side effects from BH4 deficiency) Green color and double-arrow area indicate the pain-free, side-effect-free therapeutic zone potentially offered by peripheral SPR inhibition. BSL, baseline.

FIGURE 2

Enzymatic pathways to produce BH4 from 6-pyruvoyl-tetrahydropterin. In bold blue, SPR can produce 2′ox-PH4, isomerize it into 1′ox-PH4 and reduce it to BH4, which represents the main production route of the BH4 de novo synthesis pathway. SPR can also transform sepiapterin into BH2. Aldoketoreductases (AKR; purple) and carbonyl reductases (CR; pink) can also carry some of the reactions that lead to BH4. It is estimated that ∼35% of 6-pyruvoyl-tetrahydropterin will be reduced into 1′ox-PH4 by CRs, before SPR reduces it into BH4. In absence of SPR, the production of BH4 from 1′ox-PH4 represents the ‘salvage pathway I’, and the production of BH4 from 2′ox-PH4 represents the ‘salvage pathway II’. DHFR: dihydrofolate reductase (in yellow). White arrows represent pathways only possible in absence of SPR.

FIGURE 3

Enzymes involved in the ‘salvage pathways’ in brain and peripheral tissues of SRD. (A) SRD patients have limited salvage pathways in the brain, which causes side effects associated with neurotransmitter defects. In the CSF, there is an accumulation of sepiapterin and an increase in total biopterin levels, caused by the accumulation of BH2. (B) In contrast, organs in the periphery (liver, blood vessels and heart) express most enzymes of the ‘salvage pathways’ I and II, allowing enough BH4 production for its cofactor activities. Green arrows and text indicate BH4 production mediated by salvage pathways. Blue arrows indicate increased production of metabolite that does not lead to BH4 production. Note: high levels of 1′ox-PH4 cannot be all processed by CRs and DHFR in the liver, resulting in accumulation of sepiapterin. +, indicates an increase in levels; = , indicates unchanged levels.

FIGURE 4

Peripherally-restricted SPR inhibition could have analgesic properties for several chronic pain conditions, without major side effects. Salvage pathways can produce minimal BH4 levels for physiological function in the liver, heart, blood vessels and kidney. SPR inhibition in the CNS is not required to modulate pain sensitivity and could cause undesirable side effects (motor problems such as dystonia, sleep disturbances). Patients with sepiapterin reductase deficiency (SRD) mostly display CNS developmental defects, which are well treated with L-DOPA and 5-HTP. Green represents a good therapeutic profile (analgesia and no side effects) and red undesirable effects.

FIGURE 5

Possible strategies to produce peripherally-restricted SPR inhibitors. Left, modification of the physico-chemical properties of SPR inhibitors to promote CNS exclusion, selective tissue targeting via hyaluronic acid (HA) encapsulation, PROteolysis TArgeting Chimeras (PROTACs) and transcriptional regulators of key enzymes responsible for BH4 production. Injured peripheral sensory neurons, macrophages, mast cells and T Cells engage the BH4 production pathway in the periphery to increase pain sensitivity. Right, peripherally-restricted SPR inhibition (in blue) will reduce BH4 overproduction and pain sensitivity, while salvage pathways will allow alternate routes of BH4 production to avoid side effects associated with BH4 rundown. Bottom: urine sepiapterin levels can be used to monitor SPR target engagement to titer doses.