Exploring Sirolimus Pharmacokinetic Variability Using Data Available from the Routine Clinical Care of Renal Transplant Patients - Population Pharmacokinetic Approach

Dosing Regimen Recommendations for Sirolimus in Adult Liver Transplant Recipients: Insights from a Population Pharmacokinetic Model

Background

Sirolimus is a commonly used immunosuppressant administered after solid organ transplantation. It is characterized by a narrow therapeutic window and highly variable exposure, necessitating the identification of the sources of variability and design of individualized drug therapies.

Aim

This study aimed to perform a population pharmacokinetic (PK) analysis of sirolimus in adult liver transplant recipients and develop dosing regimen recommendations according to patient characteristics.

Methodology

A total of 216 measurements of whole blood sirolimus concentrations in 103 adult patients were obtained for analysis. Covariates influencing the PKs of sirolimus were investigated using a stepwise procedure. Monte Carlo simulations were conducted to recommend dosing regimens for patients with different levels of covariates.

Results

A one-compartment model with first-order elimination provided the best fit of the data. Hematocrit (HCT) significantly influenced the apparent clearance of sirolimus. Monte Carlo simulations showed that for patients with a low HCT level of 28%, dosing regimens of 1.5 mg qd or 1 mg qd alternating with 1.5 mg qd should be recommended. For patients with a normal HCT level, the recommended dosing regimens were 1 mg qd, 2 mg qod, or 0.5 mg qd alternating with 1 mg qd.

Conclusion

Based on our population PK model of sirolimus in adult liver transplant recipients, which has the largest sample size to date, we recommend to tailor dosing regimens to various HCT levels in such patients.

Influence of genetic variants on the pharmacokinetics and pharmacodynamics of sirolimus: a systematic review

Sirolimus is an antiproliferative and immunosuppressive compound inhibiting the mTOR pathway, which is often activated in congenital low-flow vascular malformations. Studies have demonstrated the efficacy of sirolimus for this disease. Studies in kidney transplant patients suggest that genetic variants can influence these pharmacokinetic parameters. Therefore, a systematic literature search was performed to gain insight into pharmacogenetic studies with sirolimus. Most studies investigated CYP3A4 and CYP3A5, with inconsistent results. No pharmacogenetic studies focusing on sirolimus have been performed for low-flow vascular malformations. We analyzed two common variants of CYP3A4 and CYP3A5 (CYP3A4*22 and CYP3A5*3, respectively) in patients (n = 59) with congenital low-flow vascular malformations treated with sirolimus. No association with treatment outcome was identified in this small cohort of patients.

Sirolimus Pharmacokinetics Variability Points to the Relevance of Therapeutic Drug Monitoring in Pediatric Oncology

Sirolimus is widely used in transplantation, where its therapeutic drug monitoring (TDM) is well established. Evidence of a crucial role for sirolimus in the PI3K/AkT/mTor pathway has stimulated interest in its involvement in neoplasia, either as monotherapy or in combination with other antineoplastic agents. However, in cancer, there is no consensus on sirolimus TDM. In the RAPIRI phase I trial, the combination sirolimus + irinotecan was evaluated as a new treatment for refractory pediatric cancers. Blood sampling at first sirolimus intake (D1) and at steady state (D8), followed by LC/MS² analysis, was used to develop a population pharmacokinetic model (Monolix^® software). A mono-compartmental model with first-order absorption and elimination best fit the data. The only covariate retained for the final model was “body surface area” (D1 and D8). The model also demonstrated that 1.5 mg/m² would be the recommended sirolimus dose for further studies and that steady-state TDM is necessary to adjust the dosing regimen in atypical profiles (36.4% of the population). No correlation was found between sirolimus trough concentrations and efficacy and/or observed toxicities. The study reveals the relevance of sirolimus TDM in pediatric oncology as it is needed in organ transplantation.

Concept and utility of population pharmacokinetic and pharmacokinetic/pharmacodynamic models in drug development and clinical practice

Due to frequent clinical trial failures and consequently fewer new drug approvals, the need for improvement in drug development has, to a certain extent, been met using model-based drug development. Pharmacometrics is a part of pharmacology that quantifies drug behaviour, treatment response and disease progression based on different models (pharmacokinetic - PK, pharmacodynamic - PD, PK/PD models, etc.) and simulations. Regulatory bodies (European Medicines Agency, Food and Drug Administration) encourage the use of modelling and simulations to facilitate decision-making throughout all drug development phases. Moreover, the identification of factors that contribute to variability provides a basis for dose individualisation in routine clinical practice. This review summarises current knowledge regarding the application of pharmacometrics in drug development and clinical practice with emphasis on the population modelling approach.

Successful management of sirolimus toxicity in a hematopoietic stem cell transplant patient using automated red blood cell exchange

BACKGROUND

Sirolimus is an immunosuppressive agent used in organ rejection prophylaxis in solid-organ transplantation, graft-vs-host disease prophylaxis in hematopoietic stem cell transplantation, and as an immune modulator for patients with lymphangioleiomyomatosis and vascular malformations. Sirolimus has a narrow therapeutic index with potential severe side effects, including hypertension, hepatotoxicity, nephrotoxicity, and neurotoxicity.

CASE REPORT

We report a case of a 19-year-old woman with severe sickle cell disease who underwent a matched unrelated hematopoietic stem cell transplantation, whose course was complicated by sirolimus toxicity. This case was challenging because sirolimus has no specific antidote, is largely bound to red blood cells (RBCs), has a high distribution volume, and cannot be removed by dialysis or plasmapheresis.

RESULT

Due to the concern for toxicity, we looked into possibilities for rapid sirolimus clearance using automated RBC exchange. The treatment was effective in decreasing blood sirolimus levels within the therapeutic ranges.

CONCLUSION

The use of RBC exchange is potentially safe and effective in the management of a case of sirolimus toxicity.

Prior information for population pharmacokinetic and pharmacokinetic/pharmacodynamic analysis: overview and guidance with a focus on the NONMEM PRIOR subroutine

Abstract

Population pharmacokinetic analysis is used to estimate pharmacokinetic parameters and their variability from concentration data. Due to data sparseness issues, available datasets often do not allow the estimation of all parameters of the suitable model. The PRIOR subroutine in NONMEM supports the estimation of some or all parameters with values from previous models, as an alternative to fixing them or adding data to the dataset. From a literature review, the best practices were compiled to provide a practical guidance for the use of the PRIOR subroutine in NONMEM. Thirty-three articles reported the use of the PRIOR subroutine in NONMEM, mostly in special populations. This approach allowed fast, stable and satisfying modelling. The guidance provides general advice on how to select the most appropriate reference model when there are several previous models available, and to implement and weight the selected parameter values in the PRIOR function. On the model built with PRIOR, the similarity of estimates with the ones of the reference model and the sensitivity of the model to the PRIOR values should be checked. Covariates could be implemented a priori (from the reference model) or a posteriori, only on parameters estimated without prior (search for new covariates).

Graphic abstract

Electronic supplementary material

The online version of this article (10.1007/s10928-020-09695-z) contains supplementary material, which is available to authorized users.

Clinical implementation of pharmacogenetics and model-informed precision dosing to improve patient care

Providing maximal therapeutic efficacy without toxicity is a universal goal of rational drug therapy. However, substantial between-patient variability in drug response often impedes such successful treatments and brings the necessity of tailoring drug dose to individual needs for more precise therapy. In many cases plenty of patient characteristics, such as body size, genetic makeup and environmental factors, need to be taken into consideration to find the optimal dose in clinical practice. A pharmacokinetics and pharmacodynamics (PK/PD) model-informed approach offers integration of various patient information to provide an expectation of drug response and derive practical dose estimates to support clinicians' dosing decisions. Such an approach was pioneered in the late 1970s, but its broad clinical acceptance and implementation have been hampered by the lack of widespread computer technology, including user-friendly software tools. This has significantly changed in recent years. With the advent of electronic health records (EHRs) and the ubiquity of user-friendly software tools, we now experience a convergence of clinical information, pharmacogenetics, systems pharmacology and pharmacometrics, and technology. Advanced pharmacometrics research is now more appliable and implementable to improve health care. This article presents examples of successful development and implementation of pharmacogenetics-guided and PK/PD model-informed decision support to facilitate precision dosing, including the development of an EHR-embedded decision support tool. Through the integration of clinical decision support tools in EHRs, clinical pharmacometrics support can be brought directly to the clinical team and the bedside.