Fast Atom Bombardment Mass Spectrometric Determination of the Molecular Weight Range of Uremic Compounds that Displace Phenytoin from Protein Binding: Absence of Midmolecular Uremic Toxins

Abstract

Uremic compounds are known to displace phenytoin from protein binding, resulting in a higher concentration of the pharmacologically active free fraction of phenytoin. The true chemical identities and molecular weight range of all these compounds are still unknown. We demonstrated that indoxyl sulfate and hippuric acid, which are found at high concentrations in uremic patients, can only partially explain the elevated free phenytoin concentration. Other known uremic compounds, guanidine, methylguanidine, and guanidinosuccinic acid, do not displace phenytoin from the protein-binding sites. Uremic compounds from sera of patients on maintenance hemodialysis were removed using activated charcoal. These compounds were then backextracted from activated charcoal using methanol and analyzed by fast atom bombardment mass spectroscopy. Mass spectra of uremic sera showed no peak over m/z 450, indicating that midmolecular uremic toxins are not involved in displacing phenytoin from protein binding. We also observed additional peaks in the mass spectrum of uremic compounds when compared with the normal serum extract, indicating the presence of several endogeneous compounds in uremic sera, as expected.

Comparisons of Four Protein-Binding Models Characterizing the Pharmacokinetics of Unbound Phenytoin in Adult Patients Using Non-Linear Mixed-Effects Modeling

“…On the other hand, clinical factors such as renal function and co-medications were not found to be significant covariates in this dataset. Uremic toxins accumulated in end-stage renal disease may alter protein binding [63,64] and reduce the intrinsic clearance of phenytoin [65]; however, renal function (measured by SCr) was not identified as a significant covariate in this study, possibly because only a small number of patients had severe renal disease (i.e., GFR < 30 mL/min/1.73 m 2 ). Furthermore, concomitant low-dose aspirin did not impact phenytoin binding in this study, presumably because aspirin's effect is only evident at relatively higher doses [66].…”

Interference in Immunoassays Used for Monitoring Anticonvulsants and the Usefulness of Monitoring Free Anticonvulsants

“…In uremia, the free fraction of phenytoin can be as high as 30%; in normal volunteers the free fraction is usually 10%. Another study indicated that several endogenous compounds with low molecular weights (less than 500 Da) play signifi cant roles in the displacement of strongly protein -bound drugs, but mid -molecular uremic toxins do not displace drugs (39) . Signifi cantly elevated free fractions of highly protein -bound anticonvulsants in uremia can only be partially explained by hypoalbuminemia.…”