Free mycophenolic acid determination in human plasma ultrafiltrate by a validated liquid chromatography-tandem mass spectrometry method

Therapeutic drug monitoring of mycophenolic acid (MPA) using volumetric absorptive microsampling (VAMS) in pediatric renal transplant recipients: ultra-high-performance liquid chromatography-tandem mass spectrometry analytical method development, cross-validation, and clinical application

BACKGROUND

Mycophenolic acid (MPA) is widely used in posttransplant pharmacotherapy for pediatric patients after renal transplantation. Volumetric absorptive microsampling (VAMS) is a recent approach for sample collection, particularly during therapeutic drug monitoring (TDM). The recommended matrix for MPA determination is plasma (PL), and conversion between capillary-blood VAMS samples and PL concentrations is required for the appropriate interpretation of the results.

METHODS

This study aimed to validate and develop a UHPLC-MS/MS method for MPA quantification in whole blood (WB), PL, and VAMS samples, with cross and clinical validation based on regression calculations. Methods were validated in the 0.10-15 µg/mL range for trough MPA concentration measurement according to the European Medicines Agency (EMA) guidelines. Fifty pediatric patients treated with MPA after renal transplantation were included in this study. PL and WB samples were obtained via venipuncture, whereas VAMS samples were collected after the fingerstick. The conversion from VAMS_MPA to PL_MPA concentration was performed using formulas based on hematocrit values and a regression model.

RESULTS

LC-MS/MS methods were successfully developed and validated according to EMA guidelines. The cross-correlation between the methods was evaluated using Passing-Bablok regression, Bland-Altman bias plots, and predictive performance calculations. Clinical validation of the developed method was successfully performed, and the formula based on regression was successfully validated for VAMS_MPA to PL_MPA concentration and confirmed on an independent group of samples.

CONCLUSIONS

This study is the first development of a triple matrix-based LC-MS/MS method for MPA determination in the pediatric population after renal transplantation. For the first time, the developed methods were cross-validated with routinely used HPLC-DAD protocol.

Recombinant Peptide Mimetic NanoLuc Tracer for Sensitive Immunodetection of Mycophenolic Acid

Mycophenolic acid (MPA) is an immunosuppressant drug commonly used to prevent organ rejection in transplanted patients. MPA monitoring is of great interest due to its small therapeutic window. In this work, a phage-displayed peptide library was used to select cyclic peptides that bind to the MPA-specific recombinant antibody fragment (Fab) and mimic the behavior of MPA. After biopanning, several phage-displayed peptides were isolated and tested to confirm their epitope-mimicking nature in phage-based competitive immunoassays. After identifying the best MPA mimetic (ACEGLYAHWC with a disulfide constrained loop), several immunoassay approaches were tested, and a recombinant fusion protein containing the peptide sequence with a bioluminescent enzyme, NanoLuc, was developed. The recombinant fusion enabled its direct use as the tracer in competitive immunoassays without the need for secondary antibodies or further labeling. A bioluminescent sensor, using streptavidin-coupled magnetic beads for the immobilization of the biotinylated Fab antibody, enabled the detection of MPA with a detection limit of 0.26 ng mL^-1 and an IC₅₀ of 2.9 ± 0.5 ng mL^-1. The biosensor showed good selectivity toward MPA and was applied to the analysis of the immunosuppressive drug in clinical samples, of both healthy and MPA-treated patients, followed by validation by liquid chromatography coupled to diode array detection.

Validated LC-MS/MS method for quantitation of total and free mycophenolic acid concentration and its application to a pharmacokinetic study in pediatric renal transplant recipients

A simple and sensitive LC-MS/MS method was established to quantify total and free mycophenolic acid (MPA) plasma concentrations during immunosuppressive medication for pediatric renal transplantation. The chromatographic separation was performed with the Hypersil GOLD C₁₈ column, using a mobile phase consisting of 0.1% formic acid in water and acetonitrile (60:40, v/v) at an isocratic flow rate of 0.4 ml/min. An Agilent 6420 triple quadrupole mass spectrometer was operated via a positive electrospray ionization interface using the transitions m/z 321.14 → 206.9 for MPA and m/z 324.15 → 209.9 for MPA-d3 (internal standard). The linearity was 0.1-50 μg/ml for total MPA and 0.0025-0.5 μg/ml for free MPA. The within-run and between-run precisions were all <5% and accuracy was within 96.23-107.63%. The validated method was successfully aspplied to a pharmacokinetic study in 28 pediatric renal recipients. The mean free fraction of MPA in our patients was 0.89% (ranging from 0.62 to 1.25%) and albumin level played a major role in the variability of free fraction of MPA, thus, in pediatric patients with hypoproteinemia, close free drug monitoring and dose adjustments should be considered to prevent toxicity.

Pharmacokinetics of free and total mycophenolic acid in adult lupus nephritis patients-implications for therapeutic drug monitoring

PURPOSE

To evaluate the relationship between total and free MPA pharmacokinetic (PK) parameters and renal outcome markers, and to verify whether conducting therapeutic drug monitoring (TDM) in lupus nephritis (LN) patients would be of value in routine clinical practice.

METHODS

Eighty-four samples were collected from sixteen LN patients. Total and free MPA concentrations were measured at predose, 0.5 and 2 h after mycophenolate mofetil (MMF) intake. Area under the concentration time curve from 0 to 2 h (AUC_0-2) and free fraction were calculated.

RESULTS

High between-patient variability was observed (CV% of 53.5% for dose-normalized total MPA AUC_0-2). A significant but weak correlation between dose-normalized total C₀ and AUC_0-2 was noted (r = 0.5699). Dose-normalized total C₀ above 2.76 μg/mL·g may indicate patients with eGFR < 81 mL/min with sensitivity of 83.3% and specificity of 75.0%. Hypoalbuminemic LN patients demonstrated significantly elevated MPA free fraction when compared with patients with serum albumin concentration ≥ 3.5 g/dL (1.49 ± 0.64% vs 1.08 ± 0.75%).

CONCLUSION

This study examined relationship between free and total pharmacokinetic MPA parameters as well as the effect of hypoalbuminemia on MPA plasma protein binding in adult LN patients. The study results suggest that TDM of MPA in LN seems to be a more reasonable approach than the fixed-dose protocol. Moreover, predose total MPA concentration may be a possible estimation of MPA exposure, while monitoring free rather than total MPA may be more beneficial in hypoalbuminemic patients.