Xanthine Oxidase Inhibition by 1,3-dipropyl-8-sulfophenyl-xanthine (DPSPX), an Antagonist of Adenosine Receptors

The Role of Oxidative Stress in Hyperuricemia and Xanthine Oxidoreductase (XOR) Inhibitors

Uric acid is the end product of purine metabolism in humans. Hyperuricemia is a metabolic disease caused by the increased formation or reduced excretion of serum uric acid (SUA). Alterations in SUA homeostasis have been linked to a number of diseases, and hyperuricemia is the major etiologic factor of gout and has been correlated with metabolic syndrome, cardiovascular disease, diabetes, hypertension, and renal disease. Oxidative stress is usually defined as an imbalance between free radicals and antioxidants in our body and is considered to be one of the main causes of cell damage and the development of disease. Studies have demonstrated that hyperuricemia is closely related to the generation of reactive oxygen species (ROS). In the human body, xanthine oxidoreductase (XOR) catalyzes the oxidative hydroxylation of hypoxanthine to xanthine to uric acid, with the accompanying production of ROS. Therefore, XOR is considered a drug target for the treatment of hyperuricemia and gout. In this review, we discuss the mechanisms of uric acid transport and the development of hyperuricemia, emphasizing the role of oxidative stress in the occurrence and development of hyperuricemia. We also summarize recent advances and new discoveries in XOR inhibitors.

Inhibitors of Xanthine Oxidase: Scaffold Diversity and Structure-Based Drug Design

Xanthine oxidase (XO) is the enzyme responsible for the catabolism of purines and their conversion into uric acid. XO is thus the target for the treatment of hyperuricemia and gout. For more than 50 years the only XO inhibitor drug available on the market was the purine analogue allopurinol. In the last decade there has been a resurgence in the search for new inhibitors of XO, as the activity of XO and hyperuricemia have also been associated with a variety of conditions such as diabetes, hypertension, and other cardiovascular diseases. In recent years the non-purine inhibitor febuxostat was approved in Europe and the USA for the treatment of hyperuricemia. This drug was followed by another XO inhibitor called topiroxostat. This review discusses the molecular structures and activities of the multiple classes of inhibitors that have been developed since the discovery of allopurinol, with a brief review of the molecular interactions between inhibitors and XO active site residues for the most important molecules. The challenges ahead for the discovery of new inhibitors of XO with novel chemical structures are discussed.

Scavenging of nitric oxide by an antagonist of adenosine receptors

Chronic treatment of rats with 1,3-dipropyl-8-sulfophenylxanthine (DPSPX), an antagonist of adenosine receptors, causes hypertension, cardiovascular hypertrophy and hyperplasia and impaired endothelium-dependent vasodilatation. An accelerated degradation of nitric oxide (NO) by scavenging molecules could account for endothelial dysfunction and trophic changes in this hypertension. Our aim was to determine whether DPSPX is a scavenger of NO and if this putative effect is shared by caffeine (1,3,7-trimethylxanthine) and DPCPX (1,3-dipropyl-8-ciclopentylxanthine), which are also adenosine receptor antagonists but do not induce hypertension in rats. This effect was evaluated by electrochemical and spectrofluorometric assays. Urinary NOx (nitrate + nitrite) excretion was also evaluated in controls and DPSPX-treated rats as a marker for NO bioavailability. DPSPX behaved as a scavenger of NO in a concentration-dependent manner in the electrochemical and spectrofluorometric assays. Caffeine and DPCPX had no scavenging effect. DPSPX-treated rats had decreased excretion of urinary nitrites. We can conclude that: DPSPX has NO scavenging properties that may be involved in the alterations described for DPSPX-hypertensive rats; this NO-scavenging effect is not shared by caffeine and DPCPX, which are also xanthine derivatives and adenosine antagonists.

Role of superoxide and hydrogen peroxide in hypertension induced by an antagonist of adenosine receptors

Treatment of Wistar rats for 7 days with 1,3-dipropyl-8-sulfophenylxanthine (DPSPX), an antagonist of adenosine receptors, induces long-lasting hypertension associated with marked changes in vascular structure and reactivity and renin-angiotensin system activation. This study aimed at evaluating the role of oxidative stress in the development of DPSPX-induced hypertension and also at identifying the relative contribution of superoxide radical (O2.-) vs hydrogen peroxide (H2O2). Vascular and systemic prooxidant/antioxidant status was evaluated in sham (saline, i.p., 7 days) and DPSPX (90 microg/kg/h, i.p., 7 days)-treated rats. Systolic blood pressure was determined by invasive and non-invasive methods. The activity of vascular NADPH oxidase, superoxide dismutase (SOD), catalase and glutathione peroxidase was assayed by fluorometric/spectrophotometric methods. H2O2 levels were measured using an Amplex Red Hydrogen Peroxide kit. Plasma thiobarbituric acid reactive substances and plasma antioxidant capacity were also measured. In addition we tested the effects of antioxidants or inhibitors of reactive oxygen species generation on blood pressure, vascular hyperplasia and oxidative stress parameters. DPSPX-hypertensive rats showed increased activity of vascular NADPH oxidase, SOD, catalase and glutathione peroxidase, as well as increased H2O2 generation. DPSPX-hypertensive rats also had increased plasma lipid peroxidation and decreased plasma antioxidant capacity. Treatment with apocynin (1.5 mmol/l, per os, 14 days), or with polyethylene glycol (PEG)-catalase (10,000 U/kg/day, i.p., 8 days), prevented the DPSPX-induced effects on blood pressure, vascular structure and H2O2 levels. Tempol (3 mmol/l, per os, 14 days) failed to inhibit these changes, unless PEG-catalase was co-administered. It is concluded that O2.- generation with subsequent formation of H2O2 plays a major role in the development of DPSPX-induced hypertension.

Lesion of the caudal ventrolateral medulla prevents the induction of hypertension by adenosine receptor blockade in rats

The continuous infusion for 7 days of the adenosine receptor antagonist 1,3-dipropyl-8-sulfophenylxanthine (DPSPX) causes a sustained hypertension in rats, with an enhancement of sympathetic neurotransmission and activation of the renin-angiotensin system. We studied the involvement of the caudal ventrolateral medulla in the establishment of this hypertensive model by evaluating the effect of local lesioning in blood pressure (BP). Male adult Wistar rats received stereotaxic injections of 0.3 mul of saline or quinolinic acid (QA; 180 mM) in the caudal ventrolateral medulla followed by abdominal implant of minipump for infusion of saline or DPSPX (90 microg(-1) kg(-1) h(-1)). BP was measured in conscious animals every 2 days for 12 days. The sustained increase of BP (22.1 mm Hg; P < 0.001) detected in rats infused with DPSPX was reverted (6.7 mm Hg; P > 0.05) from day six onwards in animals with lesion of the lateralmost part of caudal ventrolateral medulla (VLMlat). The present results suggest that the development of hypertension induced by adenosine receptor antagonist involves the participation of the VLMlat. They further add new data as to the functional complexity of this medullary area involved in a variety of functions such as cardiovascular, respiratory, motor and pain control.