Hyperuricemia-Related Diseases and Xanthine Oxidoreductase (XOR) Inhibitors: An Overview

Abstract

Uric acid is the final oxidation product of purine metabolism in humans. Xanthine oxidoreductase (XOR) catalyzes oxidative hydroxylation of hypoxanthine to xanthine to uric acid, accompanying the production of reactive oxygen species (ROS). Uric acid usually forms ions and salts known as urates and acid urates in serum. Clinically, overproduction or under-excretion of uric acid results in the elevated level of serum uric acid (SUA), termed hyperuricemia, which has long been established as the major etiologic factor in gout. Accordingly, urate-lowering drugs such as allopurinol, an XOR-inhibitor, are extensively used for the treatment of gout. In recent years, the prevalence of hyperuricemia has significantly increased and more clinical investigations have confirmed that hyperuricemia is an independent risk factor for cardiovascular disease, hypertension, diabetes, and many other diseases. Urate-lowering therapy may also play a critical role in the management of these diseases. However, current XOR-inhibitor drugs such as allopurinol and febuxostat may have significant adverse effects. Therefore, there has been great effort to develop new XOR-inhibitor drugs with less or no toxicity for the long-term treatment or prevention of these hyperuricemia-related diseases. In this review, we discuss the mechanism of uric acid homeostasis and alterations, updated prevalence, therapeutic outcomes, and molecular pathophysiology of hyperuricemia-related diseases. We also summarize current discoveries in the development of new XOR inhibitors.

Figure 1

Uric acid production and removal in humans and mice. Xanthine oxidoreductase (XOR) catalyzes the oxidation of hypoxanthine to xanthine to uric acid, accompanied with the generation of reactive oxygen species, superoxide anion (O₂⁻), and hydrogen peroxide (H₂O₂). Uric acid is the final oxidation product of purine (adenine and guanine) metabolism in humans and higher primates, and is removed from renal and gastrointestinal routes. In lower animals such as rats and mice, the enzyme uricase (urate oxidase) further oxidizes uric acid to allantoin for more efficient removal from the urine. Humans and higher primates lack a functional uricase gene.

Figure 2

Structure of xanthine oxidoreductase (XOR). XOR is a large homodimer enzyme; each subunit contains 1 molybdenum (Mo) cofactor, 2 ferredoxin iron-sulfur clusters (2Fe-2S), and 1 adenine dinucleotide (FAD) cofactor. XOR controls the sequential oxidative hydroxylation of hypoxanthine to xanthine then to uric acid, and generates 2 reactive oxygen species (ROS), superoxide anion (O₂⁻) and hydrogen peroxide (H₂O₂). XOR has 2 forms: xanthine dehydrogenase (XDH) and xanthine oxidase (XO). XDH prefers NAD⁺ as the substrate and XO prefers O₂.

Figure 3

Major causes of hyperuricemia. (A) High dietary intake of high-fructose foods and drinks, which increase the production of inosine and purines. Fructose competes with uric acid for the secretion in the kidney. (B) High dietary intake of rich purine foods and drinks. More purines are metabolized to produce more uric acid. High intake of alcohol (ethanol) may increase lactic acid and ketones and cause dehydration, while decreasing uric acid removal in the kidney. (C) Change of body metabolisms. Starvation may enhance the body metabolism of its own (purine-rich) tissues for energy. Chemotherapy may cause tumor lysis, increasing purine degradation and uric acid production (tumor lysis syndrome). (D) Medications. Some drugs, such as anti-uricosurics and diuretics, may decrease the excretion of uric acid from the kidney. Drugs can also cause renal dysfunction, which decreases the excretion of uric acid from the kidney. (E) Elevated blood lead levels may cause renal dysfunction and decrease uric acid excretion. (F) Many different types of kidney diseases may affect uric acid secretion. (G) Genetic factors. The gene SLC2A9 encodes a protein that helps to transport uric acid in the kidney. Several single-nucleotide polymorphisms of SLC2A9 affect the secretion of uric acid in the kidney

Figure 4

Human serum uric acid (SUA) homeostasis and abnormal changes. Normal SUA levels in humans are higher than in mice because humans lack a functional uricase gene. This evolutionary event may provide survival advantages for humans. However, in the modern era such advantages of high SUA may become risk factors for many human diseases. Several factors could cause overproduction of uric acid or under-excretion of uric acid from the kidney, leading to an abnormally high level of SUA, termed hyperuricemia. Hyperuricemia plays a pathophysiological role in many human diseases. On the other hand, several factors could cause an abnormally low level of SUA, termed hypouricemia, which may also be associated with many diseases.

Figure 5

Hyperuricemia and oxidative stress. Increased XOR activity enhances production of uric acid and reactive oxygen species, O₂⁻ and H₂O_2, which could be converted to more toxic ROS, peroxynitrate (ONOO⁻), hydroxyl anion (OH⁻), and hypochorous acid (HOCl), and damage proteins, lipids, carbohydrates, DNA, RNA, subcellular organelles and cell systems. O₂⁻ readily reacts with NO, reducing NO bioavailability, which is a main cause of endothelial dysfunction. Thus, hyperuricemia-induced oxidative stress, chronic inflammation, and endothelial dysfunction can contribute to the development and progression of many human diseases.

Figure 6

The oxidant-antioxidant paradox mechanisms of uric acid. Circulating uric acid is a major aqueous antioxidant in humans. It scavenges carbon centered radicals and peroxyl radicals such as peroxynitrite (ONOO⁻) in the hydrophilic environment. Uric acid can also act as a chelator of iron in extracellular fluids. However, uric acid becomes a strong pro-oxidant under hydrophobic conditions. Uric acid can accelerate the copper-induced peroxidation of human LDL, induces intracellular and mitochondrial oxidative stress, and stimulates expression of inflammation cytokines. Uric acid inhibits eNOS activation and NO release, induces cellular ER stress, and directly reacts with NO to form 6-aminouracil. Uric acid can block the uptake of the substrate L-arginine and stimulate the degradation of L-arginine, thereby reducing NO bioavailability. Thus, hyperuricemia-induced oxidative stress, eNOS dysfunction, and inflammation may contribute to cardiovascular diseases.

Figure 7

Chemical structure of XOR-inhibitor drugs and DHNB. Allopurinol [4-hydroxypyrazolo(3,4-d) pyrimidine] is a synthetic hypoxanthine analog. It is hydrolyzed by XOR to produce oxypurinol, which binds tightly to the reduced molybdenum ion, Mo (IV), in the enzyme and thus inhibits uric acid synthesis. Febuxostat [2-(3-cyano-4-isobutoxy-phenyl)-4-methyl-1,3-thiazole-5 carboxylic acid] and topiroxostat [4-[5-(4-pyridinyl)-1H-1,2,4-triazol-3-yl]-2-pyridinecarbonitrile] are synthetic non-purine analogs. DHNB [3,4-Dihydroxy-5-nitrobenzaldehyde] is a derivative of natural protocatechuic aldehyde (3,4-Dihydroxybenzyl aldehyde, DHB-CHO).