Effects of Uremic Toxins and Fatty Acids on Serum Protein Binding of Furosemide: Possible Mechanism of the Binding Defect in Uremia

To elucidate the mechanism of impaired serum binding of furosemide observed in patients with renal dysfunction, we examined in vitro the serum protein binding of furosemide in the absence and presence of uremic toxins that are endogenously retained solutes in uremic serum and act as inhibitors of drug binding. Analysis of the binding data of furosemide at its therapeutic concentration (6.6 mg/L) indicated that, among the four uremic toxins studied, 3-carboxy-4-methyl-5-propyl-2-furanpropionate (CMPF) showed the greatest inhibitory potency for the binding of furosemide to serum; moreover, the inhibition was competitive. CMPF thus most likely represents the primary determinant for the serum binding defect of furosemide in uremia. However, CMPF and oleate appear to exert a synergistic effect on the inhibition of furosemide serum binding--perhaps through a cascade effect on furosemide-binding inhibition in the oleate-CMPF-furosemide system, in which the binding of oleate to its low-affinity sites indirectly displaces furosemide from albumin and thus increases the transiently liberated CMPF molecules. Similar cascade effects on furosemide binding in the presence of CMPF were also originated by other long-chain (C18) fatty acids, linoleate and stearate, although to a lesser extent. Because CMPF is not effectively removed by ordinary hemodialysis treatment, the combined direct and cascade effects of CMPF and fatty acids appear to contribute to the increase in the free fraction of furosemide during hemodialysis.