Application of fluorine-19 nuclear magnetic resonance to the determination of plasma-protein binding of 5′-deoxy-5-fluorouridine, a new antineoplastic fluoropyrimidine

Two distinct fluorine-19 nuclear magnetic resonance (19F NMR) signals have been observed in human serum for free and plasma-protein bound 5′-deoxy-5-fluorouridine (5′dFUrd). The binding of this drug was studied directly in serum using 19F NMR. To evaluate the validity of this method, a parallel study was conducted with equilibrium dialysis as the reference method. Two assay methods were applied after equilibrium dialysis, UV spectrophotometry and 19F NMR spectrometry, the UV assay being used to validate the 19F NMR assay. A study of the binding of 5′dFUrd to human serum albumin was also reported. The reliability of 19F NMR as a technique to measure directly the binding of the drug and as an assay after equilibrium dialysis was demonstrated. The percentage of 5′dFUrd bound to plasma proteins is low and concentration-dependent in the 0.04–3.5 mmol l−1 range