Methotrexate plasma pharmacokinetics: importance of assay method

Calibrating from Within: Multipoint Internal Calibration of a Quantitative Mass Spectrometric Assay of Serum Methotrexate

BACKGROUND

Clinical LC-MS/MS assays traditionally require that samples be run in batches with calibration curves in each batch. This approach is inefficient and presents a barrier to random access analysis. We developed an alternative approach called multipoint internal calibration (MPIC) that eliminated the need for batch-mode analysis.

METHODS

The new approach used 4 variants of 13C-labeled methotrexate (0.026-10.3 µM) as an internal calibration curve within each sample. One site carried out a comprehensive validation, which included an evaluation of interferences and matrix effects, lower limit of quantification (LLOQ), and 20-day precision. Three sites evaluated assay precision and linearity. MPIC was also compared with traditional LC-MS/MS and an immunoassay.

RESULTS

Recovery of spiked analyte was 93%-102%. The LLOQ was validated to be 0.017 µM. Total variability, determined in a 20-day experiment, was 11.5%CV. In a 5-day variability study performed at each site, total imprecision was 3.4 to 16.8%CV. Linearity was validated throughout the calibrator range (r2 > 0.995, slopes = 0.996-1.01). In comparing 40 samples run in each laboratory, the median interlaboratory imprecision was 6.55%CV. MPIC quantification was comparable to both traditional LC-MS/MS and immunoassay (r2 = 0.96-0.98, slopes = 1.04-1.06). Bland-Altman analysis of all comparisons showed biases rarely exceeding 20% when MTX concentrations were >0.4 µM.

CONCLUSION

The MPIC method for serum methotrexate quantification was validated in a multisite proof-of-concept study and represents a big step toward random-access LC-MS/MS analysis, which could change the paradigm of mass spectrometry in the clinical laboratory.

A novel high-performance liquid chromatography/mass spectrometry method for improved selective and sensitive measurement of methotrexate polyglutamation status in human red blood cells

The folate antagonist methotrexate is commonly used in low dose for treatment of rheumatoid arthritis and juvenile idiopathic arthritis. Therapeutic effects are attributed to intracellular levels of various methotrexate polyglutamates. The present methodology, combining a simple preparation step with ion-pairing reversed-phase liquid chromatography and electrospray ionization mass spectrometry, is suitable for the measurement of methotrexate and its polyglutamates(2-7), in human red blood cells. Sample preparation consists of perchloric acid protein precipitation followed by solid-phase extraction. Baseline separation of all analytes was achieved within 10 min using a Phenomenex Synergy C18 column together with a gradient solvent program obtained from blending acetonitrile with pH 7.5, 5 mM aqueous dimethylhexylamine. Seven methotrexate polyglutamates were detected using multiple reaction monitoring, with the mass spectrometer operating in positive ion mode. Using 20 microL injection volumes, limits of detection were 2.5 nM for individual methotrexate polyglutamates, while large volume (100 microL) injections led to detection limits of 0.5 nM and linear calibration from 0.5 to 100 nM for individual analytes. Finally, the presented methodology was applied for the analysis of methotrexate and its polyglutamates in red blood cells obtained from patients being treated for juvenile idiopathic arthritis with methotrexate. Significantly, the methodology proved suitable for determination of long-chain methotrexate polyglutamates(5-7) and further, appears to be superior with respect to sensitivity, selectivity and speed as compared to all previously described approaches.

Separation methods for methotrexate, its structural analogues and metabolites

Methotrexate (MTX) is the prototype folate antagonist cytotoxic drug, employed in the therapy of solid tumors and leukaemias, and recently also as an immunosuppressive agent in organ transplantation, in the treatment of some autoimmune diseases and in the therapy of severe asthma. MTX is one of the very few antineoplastic drugs the therapeutic concentration monitoring of which is currently employed in clinical practice and can be routinely measured in biological samples by a number of different analytical techniques, among which are immunoenzymatic and chromatographic methods. Each technique has of course its own advantages in terms of sensitivity, specificity, speed, cost and level of expertise required. Along with therapeutic drug concentration monitoring and clinical pharmacology, fundamental research into the mechanism of action of antifolate drugs is still a field which requires the measurement of MTX, of its new analogues and of their metabolites in biological samples. This review summarizes the instrumental conditions and the performance of several published chromatographic methods employed to measure MTX, its metabolites and some analogues in clinical and biological research. More than 70 papers describing chromatographic assays for MTX and its metabolites have been published in the literature between 1975 and 2000. A wide array of experimental conditions for sample preparation, analyte separation and detection have been employed. According to their chemical properties, MTX, its metabolites and analogue drugs present in several biological samples (plasma, serum, saliva, urine, cerebrospinal fluid, tissue specimens) can be extracted, separated and detected under a variety of chromatographic conditions, i.e. on different stationary phases, under a wide choice of mobile phase conditions (acidic or neutral, employing ion-pair or micellar chromatography), followed by several detection techniques (UV-Vis spectrophotometry, pre- or post-column oxidation and fluorimetry, electrochemistry, mass spectrometry). Optimized methods allow simultaneous measurement within a few minutes of the plasma levels of MTX and its main metabolites at concentrations in the low-nM range. One special field which needs sensitive, fast and inexpensive methods for the detection and measurement of MTX is the monitoring of contamination in workplace environments, such as pharmaceutical industries and oncological hospital pharmacies, and in sewage waters. The measurement of the intracellular gamma-oligo-glutamate metabolites of biological folates, of MTX and of some analogue drugs is of great importance in basic pharmacological research. The existence of empirical quantitative relationships between the retention of individual oligomers under different chromatographic conditions and the number of added glutamic acid units allows identification of the metabolites even when authentic standards are not available.

Variability in methotrexate serum and cerebrospinal fluid pharmacokinetics in children with acute lymphocytic leukemia: relation to assay methodology and physiological variables

Methotrexate (MTX) steady state concentrations were evaluated in 42 children who had received high-dose infusions (6-8 g/m2) for acute lymphocytic leukemia. Concentrations in serum and cerebrospinal fluid (CSF) measured by immunoassay were found to be highly variable. Reanalysis by a reference high-pressure liquid chromatography method ruled out analytical factors as a source of this variability. The correlation coefficient between the analytical methods was 0.77 for the serum data and 0.88 for the CSF data. The variability of serum and CSF concentrations was higher in younger patients (serum; P = 0.05 and CSF; P = 0.18), and the CSF concentration decreased with decreasing age and in later courses. Body surface area, body mass index, weight, and gender were not significantly related to MTX variability. We conclude that the pronounced pharmacokinetic variability seen during MTX infusions remains largely unexplained.

Liquid chromatographic analysis and preliminary pharmacokinetics of methotrexate in cancer patients co-treated with docetaxel

A new HPLC method has been developed for the quantitative determination of methotrexate (MTX) and its 7-hydroxyl metabolite in human plasma. Samples were purified by protein precipitation with acetone and methanol, and a sample clean-up with a mixture of n-butanol and diethyl ether. The analytes were separated on an RP Inertsil ODS-80A column and eluted in a solvent system containing 5% (v/v) tetrahydrofuran in water (pH 2.0). UV absorption measurement was performed at 313 nm, and the detector response was linear in a concentration range of 10-10,000 ng/ml. The lower limit of quantitation of MTX was 10 ng/ml using 1 ml sample aliquots. Values for accuracy and (within-run and between-run) precision were between 95.5-111% and 3.69-11.0%, respectively, at four concentrations analyzed in quintuplicate on four separate occasions. The assay was applied to study the effects of docetaxel co-administration on the pharmacokinetics and metabolism of MTX in cancer patients.

Use of recombinant flagellin protein as a tracer antigen in a fluorescence polarization assay for diagnosis of leptospirosis

The objective of the present study was to investigate the usefulness of a recombinant flagellar protein, FlaB, of Leptospira interrogans serovar pomona in the serodiagnosis of leptospirosis by the fluorescence polarization assay (FPA). The recombinant protein FlaB was purified to homogeneity by a combination of nickel-nitriloacetic acid agarose chromatography, electrophoresis, and electroelution. Purified FlaB was labeled with fluorescein isothiocyanate (FITC). Western blotting was performed by using bovine sera with microscopic agglutination test (MAT) titers of antibodies against L. interrogans serovar pomona and L. bergpetersenii serovars hardjo and sejroe to confirm the antigenicity of FlaB. Western blot analysis demonstrated that labeled as well as unlabeled FlaB was recognized by the positive sera tested, indicating the broad serovar cross-reactivity of this protein. It also indicated that labeling with FITC did not affect the antigenicity. By using FITC-labeled FlaB as a tracer antigen, a homogeneous FPA was developed to detect antileptospiral antibodies in bovine sera. A population of 208 MAT-positive and 208 MAT-negative serum samples was tested by FPA. The FPA cutoff was determined by receiver operating characteristic analysis. By FPA, 83. 7% of the MAT-positive serum samples were positive and 81.2% of the MAT-negative serum samples were negative. Compared to the results of MAT, the positive predictive value of FPA was 81.7% and the negative predictive value of FPA was 83.3%. The FPA is a simple and rapid technique for the detection of anti-Leptospira antibodies.