The pharmacokinetics of LCP-tacrolimus (Envarsus^®) has been sparsely investigated [1], and clinical trials [2–5] have left some uncertainty on the exact starting dose, dose ratio with regards to other prolonged-release formulations, and blood levels to be expected in kidney (KTR) and liver transplant recipients (LTR). Previous experience with Advagraf^® showed that absorption could be almost nil in the first days post-transplantation, and that in stable patients, the 1:1 dose ratio resulted in lower C₀ but comparable AUC_0–24 h [6].

The relationship between tacrolimus exposure and effects renders individual dose adjustment essential to avoid under- or overexposure [7]. The exposure index best associated with clinical effects is the area under the concentration–time curve (AUC) [7]. To overcome the inconveniences of collecting 10–12 blood samples over the dose interval, Bayesian estimators based on sparse sampling strategies have been developed for the AUC estimation of all tacrolimus formulations [1, 8–12] and are routinely used through the ISBA expert system ¹ [13]. However, the collection of several blood samples by venipuncture in a medical environment induces costs and logistical constraints. Therefore, dried blood spot (DBS) sampling, which can easily be performed at home, has been proposed for the therapeutic drug monitoring (TDM) of tacrolimus [14–20]. After a fingerprick, blood is applied onto a special filter paper, which is subsequently mailed to the laboratory. Good acceptability by the patients [21] and reliability of measured drug levels [16–20] are arguments in favor of DBS for the TDM of tacrolimus in transplantation. Furthermore, DBS are particularly suited to LCP-tacrolimus for which the optimal sampling times for AUC_0–24 h estimation are 0, 8, and 12 h post-dose [1].

In this context, we hypothesized that implementing DBS home sampling for the Bayesian estimation of tacrolimus AUC_0–24 h before and after a conversion, and considering the pre-switch AUC_0–24 h as a reference for LCP-tacrolimus dose adjustment after the switch, would allow maintaining of tacrolimus AUC_0–24 h. Therefore, the aims of the ENVARSWITCH study were to verify, in KTR and LTR, the equivalence of the AUC_0–24 h values before and after a switch from IR-tacrolimus to LCP-tacrolimus at a 1:0.7 dose, followed by individual dose adjustment targeting the pre-switch AUC_0–24 h. The study also aimed to compare tacrolimus exposure indices (AUC_0–24 h, C_max and C₀) before vs. after the switch, before and after dose adjustment.

ENVARSWITCH confirms bioequivalent exposure to tacrolimus in terms of AUC_0–24 h in 101 stable KTR or LTR converted from IR-tacrolimus to LCP-tacrolimus using a 1:0.7 dose ratio. It is the first clinical study proposing the combination of DBS and Bayesian estimation for tacrolimus AUC_0–24 h determination and dose adjustment. The Westlake interval in the entire population fell within the bioequivalence criteria for narrow therapeutic index drugs [26–28], and no significant difference was found between the mean AUC_0–24 h before the switch and after the switch followed by individual dose adjustment. Despite the removal of 30/131 (23%) patients from the FAS, the study remained sufficiently powered to validate its primary objective (N > 96 patients). The higher than expected proportion of drop-outs was compensated by the lower than expected proportion of patients not meeting the inclusion criteria at V2 (hence not eligible for formulation switching). Also, although the analysis was less powered, the Westlake interval calculated from data of the PPS still fulfilled the bioequivalence criteria. These results suggest that the 1:0.7 dose conversion ratio combined with individual dose adjustment is overall adapted. Importantly, as patients’ ethnicity was not collected for regulatory reasons, and because the 1:0.7 conversion factor is not recommended for patients of African origin, we hypothesize that the patients to whom this study was proposed by their treating physician were of other origins, and mostly white Europeans.

The Westlake interval between the AUC_0–24 h measured before and right after the switch also fell within the bioequivalence criteria in the entire population. This suggests that before any individual dose adjustment, the 1:0.7 dose conversion ratio is adapted, as proposed from the conversion studies [2–4]. Nevertheless, while subgroup analyses found no difference between AUC_0–24 h at V2 and at V3 and V4 in KTR recipients, the Westlake interval was close to, but did not fall within, the bioequivalence criteria in LTR. This might partly be due by a lack of power in the subgroup analysis. Additionally, the correlation between the theoretical and actual doses in both contexts of the conversion and dose adjustment proposal was better in KTR than in LTR (Figure 5). More precisely, the LTR group tended to receive lower doses than those they were supposed to receive, especially for theoretical doses above 5 mg/day, while KTR overall received the theoretical doses. This lack of compliance may partly explain why the Westlake interval did not meet the bioequivalence criteria for narrow therapeutic index drugs in LTR. Still, the correlation between AUC_0–24 h at V4 and at V2 in LTR who did not need and did not have dose adjustment after the conversion was poorer than that observed in KTR in the same situation (Figure 6). This observation clearly suggests that the 1:0.7 dose conversion ratio is adequate for KTR patients overall but may need to be slightly decreased and followed by dose adjustment in LTR patients. Still, the Westlake interval fell within the larger acceptance interval [0.8–1.25] recommended by the FDA for bioequivalence studies [29].

The poor correlation between AUC_0–24 h at V2 and at the subsequent visits confirms the wide intra-individual variability in tacrolimus exposure [7], which was unfortunately not compensated for by individual dose adjustment. Given the short time of participation in the study, this variability cannot be attributed to long-term tacrolimus clearance variation observed mainly in patients in the late vs early period after transplantation (after M12 vs. before M1). Correlations between C₀ values at V2 and the subsequent visits were even poorer. Although a decrease in the intra-individual variability of tacrolimus exposure on LCP-tacrolimus vs. IR-tacrolimus could be expected, studies in solid organ transplantation have reported comparable intra-patient variability on C₀ [30, 31]. Only one study has reported a significantly lower intra-patient variability of the AUC on LCP-tacrolimus (10.9%) vs. IR-tacrolimus (14.1%) [32]. In any case, the poorer correlation between C₀ values is in favour of considering the AUC_0–24 h rather than C₀, at least when patients are converted from IR-to LCP-tacrolimus, then at regular time points during follow-up.

The poor correlation between AUC_0–24 h values before the switch and afterwards may also be due to the relatively short time period between the switch and the subsequent AUC_0–24 h measurement. It was ≤3 days in 85/101 patients, so that V3 AUC_0–24 h may not reflect steady-state. This may have led to imprecise or even wrong dose recommendations. Furthermore, steady state may not even have been reached at V4 in all patients, as suggested by the poor correlation between AUC_0–24 h at V2 and V4 in LTR who did not need and did not have tacrolimus dose adjustment (Figure 6).

Variability may have also come from the use of DBS collected using non-volumetric devices and from the study design, where nurses collected the morning AUC at V2 while the other AUCs were collected by the patients. A comparison of AUC at V3 vs. V4, all sampled by the patients themselves, confirmed the high intra-individual variability (data not shown), dwarfing inter-operator differences as a source of variability. At the time the study was launched, analytical validation data for the measurement of tacrolimus concentrations were available only for the Whatman™ 903 protein saver cards [14, 17, 33]. In the meantime, experience has shown that the insufficient standardization of the volume of blood drops contributes to a relative imprecision of concentration measurements [22]. Another potential source of imprecision could have been the hematocrit (varying between 26.2% and 47.0% among patients at V2), as no correction of the analytical results was performed based on the hematocrit. Various patient-centered volumetric micro-sampling devices are now favored for the TDM of immunosuppressants [15, 16, 22, 34].

Twenty-two patients (17%) were withdrawn from the FAS because of unexploitable AUCs, mostly due to non-compliance with sampling times or poor quality of the DBS samples. Yet, training and a user manual had been provided to the healthcare teams and patients. This suggests that using home-based collection of microsamples requires may require even more training for certain patients, in order for them to understand the importance of respecting the sampling schedule, rigorously collect sampling information, and proceed to proper sample collection.

Interestingly, significantly lower exposure was observed in LTR compared to KTR. This may be related to the large C₀ target window at inclusion, allowing liver transplant physicians to target lower C₀ than kidney transplant doctors. This hypothesis is confirmed by the lower daily doses received by LTR compared to KTR (Table 2; p < 0.001). As the individual AUC_0–24 h at V2 was used as a target for LCP-tacrolimus dose adjustment after V3, the lower exposure in LTR compared to KTR was carried forward throughout the study. Of note, no AUC target has been validated so far for either kidney or liver transplant patients in late periods after transplantation; the only proposed AUC_0–12 h target of 150 h.μg/L [7], was derived from a study performed in 100 kidney transplant patients in the early post-transplantation period [35].

The ENVARSWITCH study used an original approach, where the Westlake interval served to compare the mean exposure obtained with the twice daily IR-tacrolimus vs the once daily LCP-tacrolimus formulation at a 0.7 dose ratio and after dose adjustment. The Westlake interval is generally used in bioavailability studies comparing generic to reference formulations, or newer to reference formulations of brand name drugs for instance. The results obtained here allow for the conclusion that LCP-tacrolimus and IR-tacrolimus had bioequivalent AUCs (since C_max and T_max were not studied), despite the above-mentioned sources of intra-individual variability. This is consistent with previous studies showing that respecting the 1:0.7 dose ratio obviated the need for dose adjustment in the majority of patients [30]. This means that the IR-tacrolimus to LCP-tacrolimus 1:0.7 conversion dose ratio is appropriate on average, in particular in KTR, but may deserve to be refined in LTR. We calculated that the dose ratio that would zero-in the Westlake interval within the acceptance limits for LTR should be 5% lower, i.e., 1:0.665. However, given the small difference and the impossibility of giving each patient a very precise dose, this option was not considered. The best recommendation would therefore be that tacrolimus exposure should be closely monitored in LTR, preferably based on the AUC_0–24 h, in order to adjust their dose individually, i.e., to compensate for the largest individual exposure differences.

Finally, a relatively low incidence of adverse events was reported in the ENVARSWITCH study. This is related to the short duration of patient participation in the study, especially while on IR-tacrolimus (mean of 6 days between inclusion at V1 and the switch to LCP-tacrolimus).

In conclusion, while the design of the ENVARSWITCH study does not allow comparing therapeutic drug monitoring strategies (AUC or C₀ monitoring from venous blood or DBS samples), its results suggest that the combination of DBS and Bayesian estimation for tacrolimus dose adjustment elicits reaching rapidly appropriate exposure to tacrolimus after the switch from IR-tacrolimus to LCP-tacrolimus. The use of volumetric microsampling devices should further improve the reliability of AUC_0–24 h estimation and individual dose adjustment.