Improved quantitative method for fludarabine in human plasma by liquid chromatography and tandem mass spectrometry

An improved quantitative assay was developed and validated for fludarabine in human plasma. Fludarabine and its internal standard, cladribine, were separated on a C18 analytical column after sample purification by strong anion-exchange solid-phase extraction. Quantitation was performed by electrospray triple-quadrupole mass spectrometry in positive ionization mode using multiple-reaction monitoring. This assay had excellent inter- and intra-assay precisions within 8%, and accuracies ranging from 100 to 116%. The method was linear within the concentration range of 0.2–250 ng/mL using 100 μL of plasma with mean R2 = 0.9999. The extraction recoveries were 85% for fludarabine and 95% for the internal standard, which represent a significant improvement over the previously published methods. We utilized this method for pharmacokinetic (PK) investigations in 215 patients. Interference peaks constantly observed in each blank plasma sample were well resolved from fludarabine using our optimized LC–MS/MS conditions, demonstrating the reliability of this improved assay. The validated method will be further applied to PK studies within our bone marrow transplant program, which will allow for optimal dose and scheduling of fludarabine in these patients.