[Summary of the recommendations of the International Association of Therapeutic Drug Monitoring and Clinical Toxicology (IATDMCT) on the therapeutic drug monitoring of tacrolimus]

Interaction Between Tacrolimus and Proton Pump Inhibitor in a Kidney Transplant Recipient

We aimed to present a drug monitoring profile of tacrolimus and proton pump inhibitor coadministration in a 23-year-old male patient with a history of high blood pressure who underwent kidney transplant. The patient's serum trough levels of tacrolimus were in the therapeutic range until omeprazole 20 mg daily was prescribed. Tacrolimus trough serum level increased to 29.5 ng/mL under the same daily dose and to 13.9 ng/mL after tacrolimus daily dose was decreased to 6 mg/day. This increase in tacrolimus serum level was behind a renal function alteration. After withdrawal of omeprazole, tacrolimus trough serum level returned to the therapeutic range. Because interactions between tacrolimus and omeprazole could result in toxicities, careful monitoring of tacrolimus serum levels should be considered to adjust the dosage.

Drug-Drug Interaction between Tacrolimus and Fluconazole in a Kidney Transplant Recipient

We report a case of tacrolimus and fluconazole drug-drug interaction in a 20-year-old female kidney transplant recipient with stable kidney function. The patient's tacrolimus blood concentrations were in the therapeutic range until fluconazole was administrated for Candida albicans infection, on day 58 posttransplant. Tacrolimus blood concentration increased by 125% (18.4 ng/mL) on day 79 and by 212% (25.4 ng/mL) on day 84 posttransplant. On day 92, tacrolimus trough blood concentration returned to the therapeutic range (5.6 ng/mL), with decrease of tacrolimus daily dose by 50% (to 4 mg). After fluconazole withdrawal, the patient was returned to the initial tacrolimus daily dose (8 mg) to maintain a tacrolimus trough blood concentration in the therapeutic range. Fluconazole coadministration with tacrolimus shows a significant clinical effect on tacrolimus trough blood concentration in kidney transplant patients. Maintaining a tacrolimus trough blood concentration in the therapeutic range is crucial for these patients; therefore, physicians should be aware of fluconazole prescriptions.

Insights into the Pharmacogenetics of Tacrolimus Pharmacokinetics and Pharmacodynamics

The influence of pharmacogenetics in tacrolimus pharmacokinetics and pharmacodynamics needs further investigation, considering its potential in assisting clinicians to predict the optimal starting dosage and the need for a personalized adjustment of the dose, as well as to identify patients at a high risk of rejection, drug-related adverse effects, or poor outcomes. In the past decade, new pharmacokinetic strategies have been developed to improve personalized tacrolimus treatment. Several studies have shown that patients with tacrolimus doses C0/D < 1 ng/mL/mg may demonstrate a greater incidence of drug-related adverse events and infections. In addition, C0 tacrolimus intrapatient variability (IPV) has been identified as a potential biomarker to predict poor outcomes related to drug over- and under-exposure. With regard to tacrolimus pharmacodynamics, inconsistent genotype-phenotype relationships have been identified. The aim of this review is to provide a concise summary of currently available data regarding the influence of pharmacogenetics on the clinical outcome of patients with high intrapatient variability and/or a fast metabolizer phenotype. Moreover, the role of membrane transporters in the interindividual variability of responses to tacrolimus is critically discussed from a transporter scientist’s perspective. Indeed, the relationship between transporter polymorphisms and intracellular tacrolimus concentrations will help to elucidate the interplay between the biological mechanisms underlying genetic variations impacting drug concentrations and clinical effects.

Utility, promise, and limitations of liquid chromatography-mass spectrometry-based therapeutic drug monitoring in precision medicine

Therapeutic drug monitoring (TDM) is typically referred to as the measurement of the concentration of drugs in patient blood. Although in the past, TDM was restricted to drugs with a narrow therapeutic range in order to avoid drug toxicity, TDM has recently become a major tool for precision medicine being applied to many more drugs. Through compensating for interindividual differences in a drug's pharmacokinetics, improved dosing of individual patients based on TDM ensures maximum drug effectiveness while minimizing side effects. This is especially relevant for individuals that present a particularly high intervariability in pharmacokinetics, such as newborns, or for critically/severely ill patients. In this article, we will review the applications for and limitations of TDM, discuss for which patients TDM is most beneficial and why, examine which techniques are being used for TDM, and demonstrate how mass spectrometry is increasingly becoming a reliable and convenient alternative for the TDM of different classes of drugs. We will also highlight the advances, challenges, and limitations of the existing repertoire of TDM methods and discuss future opportunities for TDM-based precision medicine.