Elsevier

Pharmacological Research

Volume 111, September 2016, Pages 610-618
Pharmacological Research

Review
Intra-cellular immunosuppressive drugs monitoring: A step forward towards better therapeutic efficacy after organ transplantation?

https://doi.org/10.1016/j.phrs.2016.07.027Get rights and content

Abstract

Immunosuppressive drugs (IS) used in solid organ transplantation are critical dose drugs with high intra- and inter-subject variability. Therefore, IS therapeutic drug monitoring (TDM), mainly as trough levels analysis, is a major support to patient management, mandatory to optimize clinical outcome. Even though transplant patients undoubtedly benefited by this pre-dose (C0) monitoring, the relationship between these C0 concentrations and the incidence of graft rejections remains hardly predictable. Identification and validation of additional biomarkers of efficacy are therefore very much needed. As the main IS effects are mediated through the inhibition of lymphocyte proliferation pathways, direct drug quantification within this target compartment would appear meaningful, providing hopefully more consistent information on drug efficacy. Due to the analytical performances improvement, these intracellular concentrations became accessible for comprehensive studies regarding clinical benefit of intracellular IS TDM after solid organ transplantation. Over the last ten years, number of studies investigated the potential relationship between IS blood and intracellular pharmacokinetics, genetic variability, and clinical efficacy after solid organ transplantation. A recent literature review suggests that calcineurin inhibitors (tacrolimus and cyclosporine) intracellular concentrations appear more closely related to drug efficacy than blood levels. This closer association has however not been described for the m-TOR inhibitors (sirolimus, everolimus) and the antimetabolite (mycophenolic acid). Additional larger and multicenter clinical trials are needed to confirm these observations.

Introduction

Despite spectacular progress in immunosuppression control, acute cellular rejection (ACR) remains a major concern, especially during the first 2 years following solid organ transplantation (SOT) [1], [2]. Among factors contributing to organ rejection, inadequate immunosuppressive (IS) drug levels and exposure variability ⿿for multiple reasons- increase the risk of therapeutic failure, when drugs are used at fixed doses. Hence, therapeutic drug monitoring (TDM) of IS drugs is highly recommended to optimize patient⿿s outcome, providing useful information on actual drug exposure. Practically, assessment of the trough levels (C0), the two hours post-dose concentrations (C2) as well as the full area under the curve of drug (AUC) has been proposed [1], [3], [4], [5], [6], [7] to individualize dosage regimen, sometimes together with population pharmacokinetics analysis. If such TDM greatly contributed to improve efficacy and reduce toxicity of these drugs, the relationship between blood concentrations and ACR remains unclear. Therefore, better strategies for drug optimization are needed. For this purpose, various complementary approaches emerged with some significant success, such as metabolic enzymes/transport proteins genotyping or pharmacodynamic biomarkers identification and follow-up, improving therapeutic individualization [8], [9], [10], [11], [12], [13]. However, ACR remains an issue even though blood drug levels are within the recommended therapeutic ranges, suggesting that immunosuppressants blood concentrations do not reflect accurately their pharmacological effects. The site of action of calcineurin inhibitors (CNIs), proliferation signal inhibitors (mTORi), and mycophenolic acid (MPA) is inside the lymphocyte. It seems therefore reasonable to assume that drug concentrations at the target sites (intra-lymphocytes) are more relevant than whole blood concentrations, in predicting treatment efficacy.

To reach the intracellular target, it is postulated that most immunosuppressive drugs pass through the cell membrane mainly by passive diffusion (as free fraction), since most of them are apolar and non-ionized, but also partly by active transport. Once in the cytoplasm, part of the drug may be rejected out of the cells by efflux transport proteins, resulting in variable amount of intracellular drugs depending on the efflux activity. As an active process, this efflux is subject of competition, saturation, inhibition/induction phenomena but may also depend on genetic polymorphism of the transport proteins expressed in cells membrane. Most IS drugs are substrates of the highly polymorphic efflux pump P-glycoprotein (P-gp). P-gp is of particular interest because it has been found in lymphocytes membranes [14], [15], [16], [17], [18]. Moreover, IS drugs are substrates of cytochrome P450 enzymes, in particular CYP3A4-5. As cytochromes oxidative activity has been described within the lymphocytes, one could expect some intracellular metabolism modulating the final residual drug concentrations [19]. The intracellular drug may also bind to proteins, resulting in variable unbound concentrations available to the drug receptor, triggering the pharmacological effect. These phenomena may contribute explaining the poor relationship between pharmacological activity and blood and peripheral blood mononuclear cells (PBMC) concentrations. It is obvious that intra-lymphocyte drugs measurement is more technically challenging than routine blood TDM, and requires complex pretreatment and highly sensitive techniques, currently available [20], [21], [22]. For these reasons, since more than a decade, number of centers compared intracellular to whole blood concentrations in the capability to predict the pharmacological effect of IS drugs [23].

The primary aims of this review are to provide an update on the advances in intracellular levels assessment of the main IS drugs, and to explore the clinical added value of such measurements compared to the current TDM, as well as the impact of some pharmacogenetic data, when available.

Section snippets

Tacrolimus

Tacrolimus (TAC) is a cornerstone in IS therapy after SOT. It is characterized by a high inter-individual pharmacokinetics variability, with relatively better relationship between blood concentrations with side effects than with therapeutic efficacy. TDM is mandatory and trough whole blood concentrations have been largely used as a guidance for TAC dose individualization [1].

The first intracellular TAC reports were published from intra-graft tissue concentrations with some evidences of a better

mTOR inhibitors intracellular concentrations

Sirolimus (SRL) and everolimus (EVE) are regularly used for maintenance immunosuppression following SOT, frequently in association with calcineurin inhibitors and steroids. Both drugs display narrow therapeutic ranges and present high inter-individual pharmacokinetic variability, requiring TDM. Current TDM is generally based on whole blood SRL and EVE trough concentrations, considered as good surrogate markers for drug exposure in relationship with pharmacological effect and clinical outcome [3]

Mycophenolic acid

MPA is a selective, reversible inhibitor of IMPDH, a cornerstone enzyme in de novo purine synthesis. As T and B cells are largely dependent of IMPDH for cell proliferation, MPA is selective for the inhibition of lymphocytes proliferation. TDM has been recommended to optimize the treatment efficiency and to avoid toxicity, either by trough plasma concentration or, even better, by AUC0-12h [6]. Little is known about the potential interest to determine MPA intracellular concentration and very few

Discussion and conclusion

Whole blood or plasma concentrations are commonly used for tailoring immunosuppressive therapy. Despite the fact that transplant patients have undoubtedly benefited from this systemic monitoring allowing a more rational and efficient use of the drug, a considerable number of patients still experienced acute graft rejection with blood levels apparently within the ⿿normal ranges⿿. This lack of association between clinical efficacy and blood concentrations may be partly explained by the

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