Overview of the pharmacology and toxicology of immunosuppressant agents that require therapeutic drug monitoring

Michael C. Milone,    Hospital of the University of Pennsylvania, Philadelphia, PA, USA

This chapter provides an overview of the mechanisms of action, pharmacological parameters, reference range, and toxicological aspects of various immunosuppressants that require therapeutic drug monitoring. These immunosuppressants include cyclosporine, tacrolimus, sirolimus, everolimus, and mycophenolic acid. Chemical structures of these immunosuppressants are also provided.

Keywords

Cyclosporine; tacrolimus; sirolimus; everolimus

1.1 Introduction

More than 100,000 solid organ and 50,000 allogeneic bone marrow transplants are currently performed worldwide each year. Outcomes vary widely depending on the transplant type and underlying disease; however, solid organ allograft survival has improved significantly during the past quarter century coinciding with the introduction of new immunosuppressive drugs (ISDs). ISDs are critical to transplantation success due to the potent cellular and humoral immune mechanisms that restrict allogeneic transplantation. Whereas early ISDs consisted primarily of glucocorticoids and antimetabolite drugs to block lymphocyte proliferation, several ISDs with differing mechanisms of action have been introduced, including the recent introduction of the first biologic agent, belatacept (Nulojix, a CTLA4–Ig fusion protein), which interferes with a critical step in the initiation of T cell-mediated immunity. Although these agents have significantly improved outcomes, their benefits often come at a cost of increased risk of infection as well as toxicity. The use of ISDs to control allograft rejection and graft-versus-host disease is very difficult because errors can lead to serious and sometimes fatal consequences for the transplant recipient.

One of the greatest challenges to effectively using ISDs is their widely variable pharmacokinetic behavior across individuals. This pharmacokinetic variability makes it difficult to predict a priori an individual’s response to a drug following administration of a particular dose. Applying knowledge of a drug’s concentration and pharmacokinetic behavior within an individual to the clinical use of a drug, often termed therapeutic drug monitoring (TDM), has therefore become a standard approach to ISD therapy aimed at mitigating the risks associated with the use of these drugs. An important prerequisite to successful TDM is the ability to measure a drug of interest. Using modern technologies that are available within most analytical chemistry laboratories, the measurement of drugs, including ISDs and their metabolites, is readily achieved as described in the following chapters of this book.

Unfortunately, having the plasma or whole blood concentration of a drug is not enough for proper patient management. Effective use of drug concentration data also requires a thorough understanding of the pharmacodynamics relationship between drug exposure and important clinical outcomes of toxicity or efficacy. Like pharmacokinetics, the pharmacodynamics of ISDs also vary greatly across individuals [1], but a measured drug concentration does not provide insight into this variability. Biomarkers of organ function, tissue injury, and immune function provide some insight into the pharmacodynamics of ISDs. In the broadest sense, TDM may be considered to encompass an array of testing modalities beyond traditional concentration monitoring, such as the use of serum creatinine to monitor the nephrotoxic effects of drugs such as the calcineurin inhibitors, tacrolimus, or cyclosporine. Several of the subsequent chapters are devoted to exploring these biomarker-based testing approaches. In this chapter, the ISDs currently approved for use in solid organ and bone marrow transplantation are discussed with a focus on their pharmacology and clinical use.

1.2 Calcineurin Inhibitors

Currently, two calcineurin inhibitors (CNIs), cyclosporine and tacrolimus, are commonly used clinically as immunosuppressants. This section provides an overview of these two drugs.

1.2.1 Cyclosporine A

Introduced in the 1980s, cyclosporine A (CsA) revolutionized the care of transplant patients through its potent inhibition of acute cellular transplant rejection. Although its use has gradually been replaced by tacrolimus, it is currently used in approximately 10% of transplants. It is typically used in combination with other immunosuppressive drugs such as mycophenolic acid, azathioprine, and glucocorticoids. Originally isolated in 1969 from the soil fungus Tolypocladium inflatum by Hans Peter Frey, a biologist working at Sandoz Pharmaceuticals, CsA is a lipophilic, cyclic endecapeptide composed of N-methylated amino acids, making it resistant to intestinal digestion as shown in Figure 1.1A. It is highly lipophilic and only slightly water-soluble. It derives its primary immunosuppressive activity by selectively binding to cyclophilin A, a peptidylprolyl isomerase present within the cytoplasm of cells. Once bound, the CsA/cyclophilin complex inhibits the enzymatic activity of the calcineurin (CN), a heterodimeric, calcium-dependent serine/threonine phosphatase composed of CNA and CNB subunits that is activated by the rapid rise in intracellular calcium following T cell receptor engagement. CN removes a critical regulatory phosphorylation on nuclear factor of activated T cells (NFATc) triggering its translocation to the nucleus of T cells, where it synergizes with other factors to mediate the transcription of a large number of genes, including interleukin-2 (IL-2), an important cytokine for T cell proliferation, and CD40 ligand (CD40L), an important costimulatory ligand for B cells, as schematically diagrammed in Figure 1.1B. Although the calcineurin–NFATc pathway is critical to T cell activation, this pathway plays a role in diverse cell types, including neurons [2,3], skeletal and cardiac myocytes [4,5], and endothelium [6]. These non-immune roles of calcineurin–NFATc signaling may contribute to the toxicity observed with the clinical use of cyclosporine, which includes nephrotoxicity, neurologic toxicity (e.g., tremors and headaches), and diabetes.

Due to the highly lipophilic nature of CsA, the original therapeutic formulation of CsA (Sandimmune) was an oral solution of the drug dissolved in oil. This solution was then mixed with a liquid such as juice prior to consumption. Early pharmacokinetic studies revealed that CsA absorption with this formulation was slow and erratic with poor bioavailability, leading to significant intra- and interindividual variability in CsA exposure. Studies of CsA given to healthy volunteers by intravenous (IV) and oral routes demonstrated a median oral bioavailability of 21.2% [7]. CsA is highly protein bound and exhibits a large volume of distribution at steady state that ranges from 3 to 5 L/kg due to the high affinity for cyclophilins within tissues including red blood cells (RBCs) [8]. As a result of the extensive binding to RBCs, whole blood concentrations of CsA are commonly used for most pharmacokinetic (PK) studies. In addition to highly variable bioavailability, CsA also displays significant variability in clearance that spans greater than an order of magnitude (0.63–23.9 ml/min/kg) in healthy individuals [7].

Due to the poor oral bioavailability observed with these early preparations of CsA, formulations based on an oil-based microemulsion (Neoral or Gengraf) were developed in an effort to improve absorption [9]. The oral bioavailability of the microemulsion formulations was significantly improved. Bioavailability is still lower in liver transplant recipients compared to kidney transplant recipients [8]. Biliary flow and the presence of bile is a major factor affecting intestinal absorption of CsA, as illustrated by the greater than fourfold increase in bioavailability observed in liver transplant patients following T-tube clamping [10]. The improved bioavailability of CsA microemulsion is paralleled by improvements in absorption kinetics leading to a more consistent time to peak concentration and superior dose linearity with exposure. Despite these improvements in formulation, significant pharmacokinetic variability remains, with the dose-adjusted area under the concentration curve (AUC) of microemulsion-formulated cyclosporine demonstrating a greater than 20% coefficient of variation (CV) across individuals [11].

In addition to the wide variability in absorption, the variability in CsA metabolism and elimination is also clinically important. CsA is extensively metabolized to more than 25 different metabolites primarily via the cytochrome P450 3A (CYP3A) system [8,12–14]. Excretion is mostly biliary, with greater than 90% of the parent drug eliminated by this route. Renal excretion in urine accounts for only approximately 6% of drug elimination, with the vast majority excreted as CsA metabolites. As a result, renal failure has minimal effect on the clearance of CsA compared with the dramatic alterations in CsA absorption and...