Furosemide assays and disposition in healthy volunteers and renal transplant patients.

A variety of furosemide assays and their application to biological samples have been described over the past 2 decades. We describe development of a specific, sensitive assay appropriate for efficient measurement of large numbers of biological samples. With development of this methodology we have demonstrated that the putative 2-amino-4-chloro-5-sulfamoylanthranilic acid metabolite is an artifact of sample preparation. In addition, some preparative techniques overestimate furosemide because they hydrolyze the glucuronide conjugate back to the parent compound. Applications of this assay to studies of normal subjects and patients who have received kidney transplants demonstrate a relatively constant amount of glucuronide formation and evidence that this conjugation occurs in the kidney. We also show that patients with renal transplants who are resistant to the effects of furosemide appear to have decreased secretion of the drug into the urinary site of action, the mechanism of which is unknown.