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Tiotropium, a new potent anticholinergic bronchodilator, is excreted mainly by the kidney. To investigate the pharmacokinetics of tiotropium in renal impairment, the authors evaluated the pharmacokinetics and safety after administration of a single dose of intravenous tiotropium 4.8 μg, given as an infusion over 15 minutes in subjects with normal renal function and a wide range of renal impairment based on measured creatinine clearance (normal: > 80 mL/min, n = 6; mild impairment: > 50–80 mL/min, n = 5; moderate impairment: 30–50 mL/min, n = 7; severe impairment: < 30 mL/min, n = 6). As expected for a drug excreted predominantly in unchanged form by the kidneys, tiotropium plasma concentrations increased as renal impairment worsened, with mean values of 55.5 (16.2 percent geometric coefficient of variation [%gCV]), 77.1 (20.1 %gCV), 101 (29.8 %gCV), and 108 (27.3 %gCV) pg·h/mL for AUC0–4h in the normal renal function and the mild, moderate, and severe renal impairment groups, respectively. The percentage of tiotropium dose excreted unchanged in the urine decreased from 60.1% of dose (17.7 %gCV) to 59.3% (14.4 %gCV), 39.9% (34.5 %gCV), and 37.4% (10.2 %gCV) in the normal renal function and the mild, moderate, and severe renal impairment groups, respectively. Plasma protein binding of tiotropium did not significantly change in the renal-impaired subjects. Two subjects with normal renal function experienced headache 10 hours after the infusion, which was mild and transient. No adverse events occurred in subjects with renal impairment. There were no clinically relevant changes in blood pressure, pulse rate, 12-lead ECG, physical examination, hematology, or clinical chemistry, compared with baseline values, in any subject after intravenous administration of tiotropium. Tiotropium should only be used in patients with moderate to severe renal insufficiency if the potential benefit outweighs the potential risks.