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

Background and ObjectivesMycophenolic 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.MethodsUsing 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.ResultsForty 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.ConclusionWe 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.