Aims: The aims of this study were to describe the pharmacokinetics of tacrolimus immediately after kidney transplantation, and to develop a clinical tool for selecting the best starting dose for each patient.Methods: Data on tacrolimus exposure were collected for the first 3 months following renal transplantation. A population pharmacokinetic analysis was conducted using nonlinear mixed‐effects modelling. Demographic, clinical and genetic parameters were evaluated as covariates.Results: A total of 4527 tacrolimus blood samples collected from 337 kidney transplant recipients were available. Data were best described using a two‐compartment model. The mean absorption rate was 3.6 h⁻¹, clearance was 23.0 l h^–1 (39% interindividual variability, IIV), central volume of distribution was 692 l (49% IIV) and the peripheral volume of distribution 5340 l (53% IIV). Interoccasion variability was added to clearance (14%). Higher body surface area (BSA), lower serum creatinine, younger age, higher albumin and lower haematocrit levels were identified as covariates enhancing tacrolimus clearance. Cytochrome P450 (CYP) 3A5 expressers had a significantly higher tacrolimus clearance (160%), whereas CYP3A4*22 carriers had a significantly lower clearance (80%). From these significant covariates, age, BSA, CYP3A4 and CYP3A5 genotype were incorporated in a second model to individualize the tacrolimus starting dose: urn:x-wiley:03065251:media:bcp13838:bcp13838-math-0001 Both models were successfully internally and externally validated. A clinical trial was simulated to demonstrate the added value of the starting dose model.Conclusions: For a good prediction of tacrolimus pharmacokinetics, age, BSA, CYP3A4 and CYP3A5 genotype are important covariates. These covariates explained 30% of the variability in CL/F. The model proved effective in calculating the optimal tacrolimus dose based on these parameters and can be used to individualize the tacrolimus dose in the early period after transplantation.

Tacrolimus is metabolized by CYP3A4 and CYP3A5 enzymes. Patients expressing CYP3A5 (in Caucasian patients about 15% of the population but more frequent in African Americans and Asians) have a dose requirement that is around 50% higher than non-expressers to reach the target concentration. CYP3A5 expressers can be considered fast metabolizers. The trough concentration/dose (C₀/D) ratio of tacrolimus has recently been proposed as a prognostic marker for poor outcome after kidney transplantation. Patients with a low C₀/D ratio (also referred to as fast metabolizers) seem to have more tacrolimus-related nephrotoxicity, more BK-viremia, and a lower graft survival. At first sight, the expression of CYP3A5 and a low C₀/D ratio seem to be overlapping factors, both pointing towards patients in whom a higher tacrolimus dose is needed to reach the tacrolimus target concentration. However, there are important differences, and these differences may explain why the impact of the C₀/D ratio on long term outcome is stronger than for CYP3A5 genotype status. Patients with a low C₀/D ratio require a high tacrolimus dose and are exposed to high tacrolimus peak concentrations. The higher peak exposure to tacrolimus (and/or its metabolites) may explain the higher incidence of nephrotoxicity, BK-viremia and graft loss. A potential confounder is the concurrent maintenance treatment of corticosteroids, as steroids are sometimes continued in patients at high immunological risk. Steroids induce the metabolism of tacrolimus via pregnane X receptor mediated increased CYP3A4 expression, resulting in lower tacrolimus C₀/D ratio in high risk patients. Also non-adherence may result in lower C₀/D ratio which is also associated with poor outcome. The C₀/D ratio of tacrolimus does seem to identify a group of patients with increased risk of poor outcome after kidney transplantation. Our recommendation is to monitor tacrolimus peak concentrations in these patients, and if these are high then target slightly lower pre-dose concentrations. Another possibility would be to switch to a prolonged release formulation or to dose the drug more frequently, in smaller doses, to avoid high peak concentrations.

Personalized medicine is currently hampered by the lack of flexible drug formulations. Especially for pediatric patients, manual compounding of personalized drug formulations by pharmacists is required. Three‐Dimensional (3D) printing of medicines, which enables small‐scale manufacturing at the point‐of‐care, can fulfill this unmet clinical need. This study investigates the feasibility of developing a 3D‐printed tablet formulation at the point‐of‐care which complies to quality requirements for clinical practice, including bioequivalence. Development, manufacturing, and quality control of the 3D‐printed tablets was performed at the manufacturing facility and laboratory of the department of Clinical Pharmacy and Toxicology at Leiden University Medical Center. Sildenafil was used as a model drug for the tablet formulation. Along with the 3D‐printed tablets a randomized, an open‐label, 2‐period, crossover, single‐dose clinical trial to assess bioequivalence was performed in healthy adults. Bioequivalence was established if areas under the plasma concentration curve from administration to the time of the last quantifiable concentration (AUC_0‐t) and maximum plasma concentration (C_max) ratios were within the limits of 80.00–125.00%. The manufacturing process provided reproducible 3D‐printed tablets that adhered to quality control requirements and were consequently used in the clinical trial. The clinical trial was conducted in 12 healthy volunteers. The 90% confidence intervals (CIs) of both AUC_0‐t and C_max ratios were within bioequivalence limits (AUC_0‐t 90% CI: 87.28–104.14; C_max 90% CI: 80.23–109.58). For the first time, we demonstrate the development of a 3D‐printed tablet formulation at the point‐of‐care that is bioequivalent to its marketed originator. The 3D printing of personalized formulations is a disruptive technology for compounding, bridging the gap toward personalized medicine.

The current coronavirus disease 2019 (COVID-19) pandemic requires extra attention for immunocompromised patients, including solid organ transplant recipients. We report on a case of a 35-year-old renal transplant recipient who suffered from a severe COVID-19 pneumonia. The clinical course was complicated by extreme overexposure to the mammalian target of rapamycin inhibitor everolimus, following coadministration of chloroquine and lopinavir/ritonavir therapy. The case is illustrative for dilemmas that transplant professionals may face in the absence of evidence-based COVID-19 therapy and concurrent pressure for exploration of experimental pharmacological treatment options. However, the risk-benefit balance of experimental or off-label therapy may be weighed differently in organ transplant recipients than in otherwise healthy COVID-19 patients, owing to their immunocompromised status and potential drug interactions with immunosuppressive therapy. With this case report, we aimed to achieve increased awareness and improved management of drug-drug interactions associated with the various treatment options for COVID-19 in renal transplant patients.

Measurement of pre‐dose tacrolimus concentrations, also referred to as trough concentrations or C0 (in this paper the term C0 will be used), is the most frequently used parameter for therapeutic drug monitoring in patients after solid organ transplantation. C0 is relatively easy to obtain, and can be combined with other lab tests. C0 monitoring is convenient for patient and hospital staff. Adjusting the dose based on C0 assumes that the C0 has a good correlation with the overall exposure to the drug, as reflected in the area under concentration—time curve (AUC). However, C0 may not be the panacea it is suggested to be, and there are patients who may benefit from additional measurements to more precisely assess drug exposure. Especially in patients with a low C0/dose ratio, the peak tacrolimus concentrations after oral administration may be unexpectedly high, resulting in toxicity and (as has been shown already) in poor long‐term graft survival. At the other extreme, patients who only need a very low dose to reach target C0 may have a low peak and also a low AUC and may be underexposed. In this paper, the limitations of C0 will be discussed, and the type of studies needed to provide the evidence for implementation of more sophisticated therapeutic drug monitoring. The paper focuses on treatment of adult kidney transplant recipients.

The most frequently used immunosuppressive treatment in kidney transplant recipients is the combination therapy of a calcineurin inhibitor (CNI) and mycophenolate mofetil, with or without corticosteroids. Cyclosporine and tacrolimus are the 2 CNIs registered for this indication. Also, in the treatment of glomerular diseases, CNIs and mycophenolate are being used on a worldwide scale, either alone or as combined treatment. In January 2021, the US Food and Drug Administration approved voclosporin, a novel CNI, for the treatment of adult patients with active lupus nephritis. There is a clinically relevant drug-drug interaction between cyclosporine and mycophenolate. As a result of cyclosporine-induced inhibition of the enterohepatic recirculation of mycophenolate, the mycophenolic acid area under the curve is significantly lower (40%) in case of cyclosporine coadministration compared with cotreatment with either tacrolimus or voclosporin (or no CNI cotreatment). The aim of this mini review is to summarize this potential drug-drug interaction and explain how cyclosporine affects the pharmacokinetics of mycophenolate. The optimal dose of mycophenolate mofetil is likely to depend on the CNI with which it is coadministered. Furthermore, clinical implications are discussed, including the potential emergence of mycophenolic acid-related adverse effects after discontinuation of cyclosporine cotreatment.