Personalising Immunosuppression:Is Tacrolimus C/D Ratio the New frontier in predicting allograft outcomes

Written by  Priyadarshini John DM

Reviewed by Sophia Ambruso, DO

Infographics by @Krithikamohan , @Natalia Nombera Aznaran

Kidney transplantation remains the most effective form of treatment for end-stage renal disease, with a substantial improvement in patient survival as well as quality of life. A cornerstone of post-transplant maintenance therapy among kidney transplant recipients (KTRs) is the calcineurin inhibitor (CNI) tacrolimus. Although extremely effective against allograft rejection, tacrolimus has a narrow therapeutic window and shows considerable pharmacokinetic inter-individual as well as intra-individual variability. Maintaining optimal tacrolimus levels is essential; underexposure will elevate the risk of acute allograft rejection, while overexposure may result in severe nephrotoxicity and other side effects, both of which are detrimental to long-term allograft survival.

Tacrolimus metabolism is multifaceted, with the cytochrome CYP3A5 enzyme system playing the main role, but also with influences from genetic polymorphisms and other extrinsic factors like drug interaction and patient distribution volume. Although the role of genetic factors has become better understood, optimizing tacrolimus dosing in relation to CYP3A5 expression alone has not led to consistent improvements in allograft function in the clinical environment. This highlights the requirement for more practical and consistent instruments of individualized tacrolimus care.

Tacrolimus Metabolism Explained: Factors Shaping Drug Response

Recently, the tacrolimus concentration-dose ratio C0/D, being the tacrolimus trough level (C0 in ng/ml) divided by the daily dose (D in mg), has risen to prominence as a straightforward and promising measure of tacrolimus metabolic status. Those patients who have a low C0/D ratio (<1.05) are usually described as "fast metabolizers" and have been reported anecdotally with worse allograft function and shorter allograft survival, as demonstrated in the TOMATO study. Yet, the clinical usefulness of C0/D in clinical practice is still plagued by essential outstanding issues, such as its reproducibility within the critical first few months after transplant and the best time point for reliably categorizing patients according to this ratio.

The current study is centered to determine whether C0/D can serve as a clinically useful instrument to identify early kidney transplant recipients at risk for poor outcomes, thus allowing more personalized immunosuppressive treatment.The aim of this study was to scientifically examine the reproducibility of the C0/D ratio in the first few months after kidney transplant and to identify the earliest reliable time point for classifying KTRs as fast or slow metabolizers. In addition, the present research also examined the long-term allograft performance, including allograft function, survival, and the incidence of rejection and CNI-induced damage, in patient cohorts stratified by their tacrolimus metabolic status derived from this earliest reliable point.

The study included a retrospective cohort of 1979 KTRs transplanted between the years 2000 and 2019 who had a functional allograft at 1 month and were on either immediate-release or extended-release tacrolimus.The C0/D ratio represents how efficiently a patient's body utilizes a given daily dose of tacrolimus. It is calculated by dividing the tacrolimus trough level (C0), measured in nanograms per milliliter (ng/mL), by the total daily tacrolimus dose (D) in milligrams (mg). The timing of the C0 measurement depends on the formulation: it's taken 12 hours after the dose for immediate-release tacrolimus and 24 hours after the dose for extended-release tacrolimus.Patients were categorised as "High" metabolisers (tacrolimus C0​/D<1.05 on >75 of prior measurements) or "Low" metabolizers (C0​/D≥1.05 on >25 of prior measurements) at different times following transplantation. The earliest time point to accurately categorize patients was identified using variability analysis. Multivariate cause-specific Cox models were subsequently employed to evaluate allograft outcomes.

The study established that 2 months post-transplantation was the earliest and most accurate time point to identify "High" tacrolimus metabolisers.  85% of patients were classified as "High" at this 2-month time point and this cohort always remained in this metabolic category for long term (Brier score = 0.06), reflecting the early stability of such classification.The Brier Score checks how far the predicted odds vary from the real yes-or-no results, squaring that difference each time. Across a group of forecasts, a lower score means chances match the outcomes more closely and that the odds offered reflect what is truly unfolded. Brier score usually range from 0 to 1.Larger numbers signal more error, so the smaller the penalty, the stronger and clearer the probability of forecast turns out to be.The Brier Score of 0.06 is a measurement of how accurate that "staying in the category" prediction was.

Most importantly, "High" metabolisers (n=499), defined at 2 months, had significantly poorer long-term outcomes:

They had a two-fold increased risk of allograft loss (cause-specific hazard ratio [CS-HR] = 2.00, 95% CI: 1.48–2.69) over "Low" metabolisers, even after controlling for confounding variables.Risk of allograft rejection was also significantly increased (CS-HR = 1.71, 95% CI: 1.15–2.54) in the "High" metaboliser group. In addition, their allograft function was significantly lower (46.7 ml/min vs. 52.9 ml/min at 3 years, P<0.0001), with a greater incidence of chronic vascular lesions at 1 year, suggestive of higher nephrotoxicity.Multivariate analysis revealed that the development of de novo donor-specific antibodies (dnDSA) was not significantly increased in the "High" metaboliser group (CS-HR = 1.40, 95% CI: 0.96–2.05).Increased rates of serious infectious complications were also noted in "High" metabolisers.

The current study significantly strengthens the evidence for the early predictive value of the tacrolimus C0/D ratio. It aligns with The TOMATO Study, which similarly found the C0​/D<1.05 ratio to be an independent and early predictor of poorer death-censored graft survival (HR ~2.25). Both studies emphasise the early stability of this metabolic classification and its strong link to long-term allograft outcomes. A key strength of the Masset et al. paper is its detailed analysis of C0​/D reproducibility and the identification of a specific early time point (2 months) for reliable classification, which directly addresses a clinical ambiguity. Another significant strength of the study is in providing data-driven insights into pharmacokinetics of extended release vs intermittent tacrolimus as reflected by the C0/D ratio, which is a practical tool for guiding therapy in transplantation.

This study is further corroborated by others: Tholking et al have shown that an unfavorable C0​/D ratio is associated with higher nephrotoxicity, lower renal function, and higher rates of acute rejection. Likewise, study by Egeland et al observed  high tacrolimus clearance is associated with acute rejection. Although Masset et al. discovered no significant relationship between high metabolizer status and de novo DSA, mechanistic studies by Van Gelder et al indicate higher doses of tacrolimus and possible peak exposures in fast metabolizers may be responsible for nephrotoxicity and other adverse effects.

The increasing consensus from these large groups establishes the C0​/D ratio as a useful clinical discriminator for early risk stratification, promoting personalized immunosuppression. Prospective interventional trials in the future, possibly investigating novel tacrolimus formulations such as LCP-tacrolimus (once-daily formulation) or other immunosuppressants, are necessary to validate whether therapy changes according to C0​/D status can alter outcomes

Nevertheless, some limitations of the current study include

Group definition: Although most studies employ the C0​/D<1.05 classification criterion for fast metabolizers, specific methodologies for classifying (e.g., time-varying vs. fixed-point, proportion of measurements below threshold) can vary, as Masset et al. themselves acknowledge for their pragmatic definition.

Underlying mechanisms: Although greater peak concentrations and metabolite exposure are surmised, absolute mechanistic studies (e.g., direct measurement of tacrolimus AUC or metabolites) are required to completely understand why fast metabolizers do worse.

Genetic vs. non-genetic determinants: Although C0​/D is itself a phenotypic indicator of metabolism, adding genetic data (e.g., CYP3A5 genotype) would allow for a more detailed explanation of the etiology of rapid metabolism and improve risk stratification, an aspect Masset et al. recognize as a drawback due to data availability.

In conclusion, in keeping with a mounting evidence base including the landmark TOMATO study, the C0​/D ratio could be considered as a useful early allograft loss and risk of rejection predictor. This reinforces the call for personalized immunosuppression beyond single reliance on trough levels to recognize patient-dependent tacrolimus metabolism.

Final thoughts in a nutshell:The current study proposes C0/D ratio (tacrolimus trough level ÷ daily dose) as a simple, cost-effective surrogate for tacrolimus metabolism.It categorizes patients into:

• High metabolisers (C0/D <1.05) → faster drug clearance

• Low metabolisers (C0/D ≥1.05) → slower clearance

From Trough to Tailored Care: Understanding the C/D Ratio

The earliest and most accurate time to classify metabolic status was found to be two months after transplantation. High stability and predictive accuracy of classification are confirmed by the Brier score of 0.06.A high metaboliser status is linked to a 1.7-fold increased risk of acute rejection and a 2-fold increased risk of allograft loss. greater rates of nephrotoxicity and severe infections, more chronic vascular lesions, and a lower eGFR at three years. The C0/D ratio provides a platform for customised immunosuppression and permits early risk stratification. Thus this study marks a shift in post-transplant care from relying solely on fixed tacrolimus trough levels to embracing a more personalised, metabolism-guided immunosuppressive strategy.