Development and validation of a sensitive liquid-chromatography tandem mass spectrometry assay for mycophenolic acid and metabolites in HepaRG cell culture: Characterization of metabolism interactions between p-cresol and mycophenolic acid

Liquid Chromatography-Mass Spectrometry Analytical Methods for the Quantitation of p-Cresol Sulfate and Indoxyl Sulfate in Human Matrices: Biological Applications and Diagnostic Potentials

Indoxyl sulfate (IxS) and p-cresyl sulfate (pCS) are toxic uremic compounds with documented pathological outcomes. This review critically and comprehensively analyzes the available liquid chromatography-mass spectrometry methods quantifying IxS and pCS in human matrices and the biological applications of these validated assays. Embase, Medline, PubMed, Scopus, and Web of Science were searched until December 2023 to identify assays with complete analytical and validation data (N = 23). Subsequently, citation analysis with PubMed and Scopus was utilized to identify the biological applications for these assays (N = 45). The extraction methods, mobile phase compositions, chromatography, and ionization methods were evaluated with respect to overall assay performance (e.g., sensitivity, separation, interference). Most of the assays focused on human serum/plasma, utilizing acetonitrile or methanol (with ammonium acetate/formate or formic/acetic acid), liquid-liquid extraction, reverse phase (e.g., C18) chromatography, and gradient elution for analyte separation. Mass spectrometry conditions were also consistent in the identified papers, with negative electrospray ionization, select multiple reaction monitoring transitions and deuterated internal standards being the most common approaches. The validated biological applications indicated IxS and/or pCS were correlated with renal disease progression and cardiovascular outcomes, with limited data on central nervous system disorders. Methods for reducing IxS and/or pCS concentrations were also identified (e.g., drugs, natural products, diet, dialysis, transplantation) where inconsistent findings have been reported. The clinical monitoring of IxS and pCS is gaining significant interest, and this review will serve as a useful compendium for scientists and clinicians.

Development and application of a thin-film molecularly imprinted polymer for the measurement of mycophenolic acid in human plasma

BACKGROUND

Mycophenolic acid (MPA) is used to suppress the immune response following organ transplantation; however, complex pharmacokinetic behavior and a large interpersonal variability necessitate therapeutic drug monitoring. To overcome the limitations of current sample preparation techniques, we present a novel thin-film molecularly imprinted polymer (TF-MIP) extraction device as part of a simple, sensitive, and fast method for analysis of MPA from human plasma.

METHODS

Mycophenolic acid is extracted from plasma using a tailor-made TF-MIP that is subsequently desorbed into an organic solvent system compatible with mass spectrometry. The MIP yielded higher recovery of MPA relative to a corresponding non-imprinted polymer. The method allows for the determination of MPA in 45 min including analysis time and can be scaled for high throughput to process as many as 96 samples per hour.

RESULTS

The method gave an LOD of 0.3 ng mL^-1 and was linear from 5 to 250 ng mL^-1 . Patient plasma samples (35 μL) were diluted using charcoal-stripped pooled plasma to a final extraction volume of 700 μL; when MPA in patient plasma is high, this ratio can easily be adjusted to ensure samples are within the method linear range. Intra- and inter-day variability were 13.8% and 4.3% (at 15 ng mL^-1 ) and 13.5% and 11.0% (at 85 ng mL^-1 ), respectively (n = 3); inter-device variability was 9.6% (n = 10).

CONCLUSIONS

Low inter-device variability makes these devices suitable for single use in a clinical setting, and the fast and robust method is suitable for therapeutic drug monitoring, where throughput and time-to-result are critical.

Significant Correlations between p-Cresol Sulfate and Mycophenolic Acid Plasma Concentrations in Adult Kidney Transplant Recipients

BACKGROUND AND OBJECTIVES

Mycophenolic acid (MPA) is a commonly prescribed life-long immunosuppressant for kidney transplant recipients. The frequently observed large variations in MPA plasma exposure may lead to severe adverse outcomes; therefore, characterizations of contributing factors can potentially improve the precision dosing of MPA. Our group recently reported the potent inhibitory effects of p-cresol (a protein-bound uremic toxin that can be accumulated in kidney transplant patients) on the hepatic metabolism of MPA in human in vitro models. Based on these data, the hypothesis for this clinical investigation was that a direct correlation between p-cresol and MPA plasma exposure should be evident in adult kidney transplant recipients.

METHODS

Using a prospective and observational approach, adult kidney transplant recipients within the first year after transplant on oral mycophenolate mofetil (with tacrolimus ± prednisone) were screened for recruitment. The exclusion criteria were cold ischemia time > 30 h, malignancy, pregnancy, severe renal dysfunction (i.e., estimated glomerular filtration rate, eGFR, < 10 mL/min/1.73 m²), active graft rejection, or MPA intolerance. Patients' demographic and biochemistry data were collected. Total and free plasma concentrations of MPA, MPA glucuronide (MPAG), and total p-cresol sulfate (the predominant, quantifiable form of p-cresol in the plasma) were quantified using validated assays. Correlational and categorical analyses were performed using GraphPad Prism.

RESULTS

Forty patients (11 females) were included: donor type (living/deceased: 20/20), induction regimen (basiliximab/thymoglobulin/basiliximab followed by thymoglobulin: 35/3/2), post-transplant time (74 ± 60 days, mean ± standard deviation), age (53.7 ± 12.4 years), bodyweight (79.8 ± 18.5 kg), eGFR (51.9 ± 18.0 mL/min/1.73 m²), serum albumin (3.6 ± 0.5 g/dL), prednisone dose (18.5 ± 13.2 mg, n = 33), and tacrolimus trough concentration (9.4 ± 2.4 µg/L). Based on Spearman analysis, significant control correlations supporting the validity of our dataset were observed between total MPA trough concentration (C₀) and total MPAG C₀ (correlation coefficient [R] = 0.39), ratio of total MPAG C₀-to-total MPA C₀ and post-transplant time (R = - 0.56), total MPAG C₀ and eGFR (R = - 0.35), and p-cresol sulfate concentration and eGFR (R = - 0.70). Our primary analysis indicated the novel observation that total MPA C₀ (R = 0.39), daily dose-normalized total MPA C₀ (R = 0.32), and bodyweight-normalized total MPA C₀ (R = 0.32) were significantly correlated with plasma p-cresol sulfate concentrations. Consistently, patients categorized with elevated p-cresol sulfate concentrations (i.e., ≥ median of 3.2 µg/mL) also exhibited increased total MPA C₀ (by 57 % vs those below median), daily dose-normalized total MPA C₀ (by 89 %), and bodyweight-normalized total MPA C₀ (by 62 %). Our secondary analyses with MPA metabolites, unbound concentrations, free fractions, and MPA metabolite ratios supported additional potential interacting mechanisms.

CONCLUSION

We have identified a novel, positive association between p-cresol sulfate exposure and total MPA C₀ in adult kidney transplant recipients, which is supported by published mechanistic in vitro data. Our findings confirm a potential role of p-cresol as a significant clinical variable affecting the pharmacokinetics of MPA. These data also provide the justifications for conducting subsequent full-scale pharmacokinetic-pharmacodynamic studies to further characterize the cause-effect relationships of this interaction, which could also rule out potential confounding variables not adequately controlled in this correlational study.

Recent lessons learned from population pharmacokinetic studies of mycophenolic acid: physiological, genomic, and drug interactions leading to the prediction of drug effects

INTRODUCTION

Mycophenolic acid (MPA) is a widely used immunosuppressant in transplantation and autoimmune disease. Highly variable pharmacokinetics have been observed with MPA, but the exact mechanisms remain largely unknown.

AREAS COVERED

The current review provided a critical, comprehensive update of recently published population pharmacokinetic/dynamic models of MPA (n=16 papers identified from PubMed and Embase, inclusive from January 2017 to August 2021), with specific emphases on the intrinsic and extrinsic factors influencing the pharmacology of MPA. The significance of the identified covariates, potential mechanisms, and comparisons to historical literature have been provided.

EXPERT OPINION

While select covariates affecting the population pharmacokinetics of MPA are consistently observed and mechanistically supported, some variables have not been regularly reported and/or lacked mechanistic explanation. Very few pharmacodynamic models were available, pointing to the need to extrapolate pharmacokinetic findings. Ideal models of MPA should consist of: i) utilizing optimal sampling points to allow the characterizations of absorption, re-absorption, and elimination phases; ii) characterizing unbound/total MPA, MPA metabolites, plasma/urinary concentrations, and genetic polymorphisms to facilitate mechanistic interpretations; and iii) incorporating actual outcomes and pharmacodynamic data to establish clinical relevance. We anticipate the field will continue to expand in the next 5 to 10 years.

Effects of p-Cresol on Oxidative Stress, Glutathione Depletion, and Necrosis in HepaRG Cells: Comparisons to Other Uremic Toxins and the Role of p-Cresol Glucuronide Formation

The toxicological effects of p-cresol have primarily been attributed to its metabolism products; however, very little human data are available in the key organ (i.e., liver) responsible for the generation of these metabolites. Experiments were conducted in HepaRG cells utilizing the following markers of cellular toxicity: 2′-7′-dichlorofluorescein (DCF; oxidative stress) formation, total cellular glutathione (GSH) concentration, and lactate dehydrogenase (LDH; cellular necrosis) release. Concentrations of p-cresol, p-cresol sulfate, and p-cresol glucuronide were determined using validated assays. p-Cresol exposure resulted in concentration- and time-dependent changes in DCF (EC₅₀ = 0.64 ± 0.37 mM at 24 h of exposure) formation, GSH (EC₅₀ = 1.00 ± 0.07 mM) concentration, and LDH (EC₅₀ = 0.85 ± 0.14 mM) release at toxicologically relevant conditions. p-Cresol was also relatively more toxic than 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indole-3-acetic acid, indoxyl sulfate, kynurenic acid, and hippuric acid on all markers. Although the exogenous administration of p-cresol sulfate and p-cresol glucuronide generated high intracellular concentrations of these metabolites, both metabolites were less toxic compared to p-cresol at equal-molar conditions. Moreover, p-cresol glucuronide was the predominant metabolite generated in situ from p-cresol exposure. Selective attenuation of glucuronidation (without affecting p-cresol sulfate formation, while increasing p-cresol accumulation) using independent chemical inhibitors (i.e., 0.75 mM l-borneol, 75 µM amentoflavone, or 100 µM diclofenac) consistently resulted in further increases in LDH release associated with p-cresol exposure (by 28.3 ± 5.3%, 30.0 ± 8.2% or 27.3 ± 6.8%, respectively, compared to p-cresol treatment). These novel data indicated that p-cresol was a relatively potent toxicant, and that glucuronidation was unlikely to be associated with the manifestation of its toxic effects in HepaRG cells.

Mechanisms of Metabolism Interaction Between p-Cresol and Mycophenolic Acid

Mycophenolic acid (MPA) is commonly prescribed for preventing graft rejection after kidney transplantation. The primary metabolic pathways of MPA are hepatic glucuronidation through UDP-glucuronosyltransferase (UGT) enzymes in the formation of MPA-glucuronide (MPAG, major pathway) and MPA-acyl glucuronide (AcMPAG). p-Cresol, a potent uremic toxin known to accumulate in patients with renal dysfunction, can potentially interact with MPA via the inhibition of glucuronidation. We hypothesized that the interaction between MPA and p-cresol is clinically relevant and that the estimated exposure changes in the clinic are of toxicological significance. Using in vitro approaches (ie, human liver microsomes and recombinant enzymes), the potency and mechanisms of inhibition by p-cresol towards MPA glucuronidation were characterized. Inter-individual variabilities, effects of clinical co-variates, in vitro-in vivo prediction of likely changes in MPA exposure, and comparison to other toxins were determined for clinical relevance. p-Cresol inhibited MPAG formation in a potent and competitive manner (Ki=5.2 µM in pooled human liver microsomes) and the interaction was primarily mediated by UGT1A9. This interaction was estimated to increase plasma MPA exposure in patients by approximately 1.8-fold, which may result in MPA toxicity. The mechanism of inhibition for AcMPAG formation was noncompetitive (Ki=127.5 µM) and less likely to be clinically significant. p-Cresol was the most potent inhibitor of MPA-glucuronidation compared with other commonly studied uremic toxins (eg, indole-3-acetic acid, indoxyl sulfate, hippuric acid, kynurenic acid, and 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid) and its metabolites (ie, p-cresol sulfate and p-cresol glucuronide). Our findings indicate that the interaction between p-cresol and MPA is of toxicological significance and warrants clinical investigation.