Introduction and Aims: Topiroxostat, a xanthine oxidoreductase (XOR) inhibitor, approved as a medicine for hyperuricemia and gout in Japan. It has been shown the decrease in urinary albumin to creatinine ratio compared with placebo in the chronic kidney disease (stage3) in the clinical trial as well as the dose-dependent suppression in urinary albumin excretion (UAE) relative to control in diabetic mice, but febuxostat did not show the dose-dependent effect. There are some reports that vessel and adipose tissues XOR activity in metabolic syndrome is high. XOR is localized in the intracellular and extracellular matrix and produces a reactive oxygen species via enzymatic reaction. The aim of this study is to investigate the involvement of the XOR activity in various tissues of type 2 diabetic mice by means of topiroxostat with febuxostat, and to discuss the association of the UAE.
Methods: Male db/db mice were obtained at 8 weeks of age. After acclimatization for a week, mice were divided into 8 groups; control, topiroxostat treatment groups (0.1, 0.3, 1 and 3 mg/kg/day), and febuxostat treatment groups (0.1, 0.3 and 1 mg/kg/day), and were received either drug-containing diet or control diet for 4 weeks. 24 hours urinary albumin excretion and XOR activity and purines in the liver, kidney and plasma were evaluated. The liver and kidney were homogenized in phosphate buffer (pH7.4) and centrifuged at 100000g for 60 min. The supernatant was assayed for the XOR activity. The enzyme activity was started by the addition of the enzyme to the mixture including 15N2-xanthine as substrate, NAD+ and oxonic acid in Tris buffer (pH8.5), and the level of 15N2-uric acid yielded was measured by LC/MS. The concentrations of the uric acid and purines (xanthine and hypoxanthine) in the liver, kidney and plasma were also determined by LC/MS.
Results: Plasma and kidney XOR activity in db/db control group were significantly higher than those in db/m group. Topiroxostat and febuxostat showed the dose dependent inhibition of XOR activity in the liver, kidney and plasma. The inhibition of the XOR activity in the liver and the kidney in the group of treated with febuxostat was slightly potent than those in the group of treated with topiroxostat. As to the plasma XOR activity, the groups treated with topiroxostat showed a strong inhibition compared to the group treated with febuxostat. The levels of plasma uric acid in the group of treated with topiroxostat showed the decrease relative to that of febuxostat groups, in response to the dose-dependent increase in the plasma purine levels, but the group of treated with febuxostat didn’t show the same tendency.
Conclusions: It was suggested that the elevated plasma and kidney XOR activity in db/db mice might contribute to the increase in the UAE. Topiroxostat showed a strong inhibition in the extracellular XOR activity. Thus, the inhibition of the XOR activity localized in the intracellular or extracellular of the kidney and plasma might be associated with the suppression of the UAE.
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