Effect of Methylxanthines on Urate Crystallization: In Vitro Models of Gout and Renal Calculi

Background: Common forms of pathological crystals are uric acid or urates, which are responsible for gout, urolithiasis, and other conditions. Methods: We used a kinetic–turbidimetric crystallization assay to evaluate the effect of ten specific methylxanthines on the crystallization of monosodium urate, potassium urate, and ammonium urate in conditions that mimicked urine. We also studied the effect of different levels of 7-methylxanthine in the presence of other biological compounds (albumin and hyaluronic acid) on the solubility of monosodium urate in conditions that mimicked synovial fluid. Results: The results showed that 7-methylxanthine in the range of 16.61–49.84 mg/L inhibited the crystallization of each urate when the initial urate concentration was 3 × 10−3 M (500 mg/L) and the conditions mimicked urine, and that the greatest inhibitory effect was for monosodium urate. In addition, 7-methylxanthine at a concentration of 25 mg/L totally prevented the crystallization of monosodium urate at an initial urate concentration of 2.38 × 10−3 M (400 mg/L) in conditions that mimicked synovial fluid. Moreover, at a low concentration of 7-methylxanthine, albumin and hyaluronic acid increased this inhibitory effect. Conclusions: Our in vitro results demonstrate that 7-methylxanthine inhibits the crystallization of urates in conditions that mimic synovial fluid and urine.