Effects of high altitude exposure on the pharmacokinetics of furosemide in healthy volunteers

Introduction: A cascade of pathophysiological events occurs with the ascension to high altitude (H). We have performed studies on the effects of exposure to H on the pharmacokinetics of drugs. The hypothesis behind these studies has been that the exposure to H, which produces marked physiological changes in the body, may alter pharmacokineties, and consequently, pharmacodynamics. Our previous studies suggest that drugs highly bound to plasma proteins are most likely to exhibit altered disposition. Objective: In continuation of our research, we selected furosemide which is about 98% bound to plasma proteins, renally excreted and has low binding to red blood cells. Subjects, materials and methods: Furosemide (40 mg) was administered orally to 3 groups of young healthy volunteers. One group who had been residing at sea level (group L), the same group after 15 hours of exposure to high altitude (3,600 m, group HA) and a group of volunteers living at H for at least 6 months (group HC). Results: Our results are in accordance with the most recent pharmacokinetic studies on furosemide in which a terminal half-life of approximately 20 - 30 h was reported. Total proteins were 9.3% and 12.7% higher in groups HA and HC, respectively, than in group L. Albumin in group HC was 8.2% higher than group L. Bilirubin increased 17.7% and 41.2% in groups HA and HC, respectively, in comparison with group L. A rapid disposition rate constant in groups HA and HC was the only pharmacokinetic parameter that was significantly different from those in group L. Concentration of furosemide in plasma water increased significantly after H exposure, thus, the binding diminished from 97.2% in group L to 95.1% and 91.1% in groups HA and HC, respectively. Conclusion: Exposure to H produces an increase in the free fraction of furosemide in humans, which could be of therapeutic importance