*2.4.3. Mechanism of action of EVL*

reported in 36.1% of renal transplant patients, 45.3% of cardiac transplant patients, and 51.3% of hepatic transplant patients in a clinical study. Vomiting was reported in 33.9% of cardiac transplant patients, and 32.9% of hepatic transplant patients. Nausea was reported in 23.6% of renal transplant patients, 54% of cardiac transplant patients, and 54.5% of hepatic transplant. The incidence of adverse gastrointestinal complications requiring dose reduction. Usually occurs early in therapy and respond to dose reduction or switching from two to three divided

Hypertension was reported in 28.2% of renal transplant patients, 77.5% of cardiac transplant patients, and 62.1% of hepatic transplant patients [34]. Leukopenia was 23.2% and 34.5% in renal transplant, 30.4% in cardiac transplant, and 45.8% in hepatic transplant in a clinical study. Complete blood counts should be performed weekly during the first month, twice monthly for the second and third months of treatment, then monthly through the first year [34].

SRL(also known as Rapamycin and Rapamune®) is a macrocyclic lactone produced by the actinomycete *Streptomyces hygroscopicus*, with immunosuppressive, antitumor, and antifungal properties. SRL appears to be synergistic with CsA in kidney transplantation, but with a different side-effect profile. It is an immunosuppressive agent of potential benefit in clinical liver transplantation. EVL is an analogue of SRL with immunosuppressive and antiprolifera‐ tive activity. It is closely related chemically and clinically to SRL but has distinct pharmaco‐ kinetics. The main difference is a shorter half-life and thus a shorter time to achieve steady

SRL has been demonstrated to block the response of T- and B-cell activation by cytokines, which prevents cell-cycle progression and proliferation. Intracellularly, sirolimus forms a complex with cytosolic FK-binding proteins, primarily FKBP-12, considered essential for functionality; however, the sirolimus–FKBP-12 complex does not affect calcineurin activity. It binds to and inhibits a protein kinase, designated mammalian target of rapamycin (mTOR) (Table 1), which is a key enzyme in cell-cycle progression. Inhibition of mTOR blocks cell-cycle progression at the G1 to S phase transition [37,38]. Specific biochemical steps inhibited by SRL include activation of p70S6 kinase, activation of the cdk2/cyclinE complex, and phosphoryla‐ tion of retinoblastoma protein [37,38]. SRL appears to be less nephrotoxic than CsA and TRL; this may be related to its lack of effect on calcineurin [1,38]. In preclinical studies (*in vitro* and *in vivo*), additive or synergistic immunosuppressive effects were observed when SRL was

The use of SRL in renal transplant patients is associated with a dose-dependent increase in serum cholesterol and triglycerides that may require treatment. Other studies have identified hyperlipidemia and thrombocytopenia as significant SRL side effects [39]. In the other hand,

daily doses [34].

**2.4. Sirolimus (SRL) and Everolimus (EVL)**

314 Current Issues and Future Direction in Kidney Transplantation

state concentrations of the drug [1,36].

combined with TRL, CsA, MMF, and brequinar [37].

*2.4.1. Mechanism of action of SRL*

*2.4.2. Side effects of SRL*

Like SRL, EVL binds to the cytosolic immunophyllin FKBP12; both agents inhibit growth factor-driven cell proliferation, including that of T-cells and vascular smooth muscle cells. After binding to and forming a complex with the cytoplasmic protein FKBP-12, this complex binds to and inhibits the mammalian Target Of Rapamycin (mTOR) and phosphorylates P70 S6 ribosomal protein kinase (a substrate of mTOR) (Table 1). The phosphorylation of P70 S6 ribosomal protein kinase by the EVL complex prevents protein synthesis and cell proliferation. The EVL:FKBP-12 complex does not affect calcineurin activity [40,41]. Binding of EVL to FKBP12 is weaker than that of SRL (about 3-fold), related to 40-O-alkylation, and this correlates with a 2- to 3-fold lower in vitro immunosuppressive activity for EVL. However, the oral *in vivo* activity of EVL has been at least equipotent to oral SRL in several animal allotransplan‐ tation/autoimmune disease models. This appears related to the chemical modification in EVL (2-hydroxyethyl chain), providing more favorable pharmacokinetic properties (eg, absorption, disposition) which compensate for relatively poor *in vitro* activity [40]. EVL and SRL antago‐ nize TRL based calcineurin inhibition via saturation of FKBP12 [1,42].

#### *2.4.4. Side effects of EVL*

Side effects seem to be the same as with SRL [1]. Endocrine abnormalities, including hyperli‐ pidemia and hypertriglyceridemia, have been reported with EVL treatment. Monitoring for hyperlipidemia is recommended in all patients; diet, exercise, and lipid lowering therapy should be initiated if hyperlipidemia occurs. In a clinical study with EVL, the most important causes of discontinuation in 69 patients were severe infections (2.3%), pneumonitis (6.8 %), acute rejection episode (4.1%), proteinuria (4.1%). Although the overall incidence discontinu‐ ation due to side effects was higher in the EVL than SRL group, there was no greater frequency of severe side effects [43,44].

#### **2.5. Azathioprine**

Azathioprine is a purine antimetabolite. It is an imidazolyl derivative of 6-mercaptopurine. Azathioprine was first introduced as an immunosuppressive agent in 1961, helping to make allogeneic kidney transplantation possible. It is indicated as an adjunct for prevention of organ transplant rejection and in severe rheumatoid arthritis. It has long been used as a steroid sparing agent in a variety of clinical scenarios [1,45]. In the United States azathioprine was usually combined with prednisone and CsA. Azathioprine was regarded as an adjunctive agent to CsA and the combination was often called "triple therapy". The term "adjunctive agent" is used to describe the immunosuppressive drugs that are used, or were developed for use, in combination with a calcineurin inhibitor to enhance the potency of the immunosup‐ pressive protocol, as measured by a decreased incidence of acute rejection episodes [46].

imab was 7 days. In one randomized trial, basiliximab was safe and effective when used in a

Clinical Pharmacology and Therapeutic Drug Monitoring of Immunosuppressive Agents

http://dx.doi.org/10.5772/54910

317

No cytokine-release syndrome has been observed with these antibodies, but anaphylactic reactions can occur. Although lymphoproliferative disorders and opportunistic infections may occur, as with the depleting antilymphocyte agents, the incidence ascribed to anti-CD25 treatment appears remarkably low. No significant drug interactions with anti–IL-2-receptor

The antibody alemtuzumab is a recombinant DNA-derived humanized monoclonal antibody that is directed against the cell surface glycoprotein CD52, which is expressed on the surface of normal and malignant B and T lymphocytes, NK cells, monocytes, macrophages, and tissues of the male reproductive system; thus, the drug causes extensive lympholysis by inducing apoptosis of targeted cells. It has achieved some use in renal transplantation because it

Alemtuzumab antibody binds to CD52, it triggers an antibody-dependent lysis of these cells. The depletion of lymphocytes is so marked that it takes several months or up to one year

Alemtuzumab's mechanism of depletion is so profound that its adverse-effect profile occurs frequently and with a high level of severity. Adverse effects associated with alemtuzumab use include neutropenia (70%), thrombocytopenia (52%), anemia (47%), nausea (54%), vomiting (41%), diarrhea (22%), headache (24%), dysesthesias (15%), dizziness (12%), and autoimmune

The introduction of glucocorticoids as immunosuppressive drugs in the 1960s played a key role in making organ transplantation possible. Steroids are a cornerstone of immunosuppres‐ sive therapy in kidney transplantation despite their side effects and morbidity. More than 95% of transplant recipients are treated with steroids as a usual component of clinical immuno‐ suppressive regimens. Prednisone, prednisolone, and other glucocorticoids are used alone and in combination with other immunosuppressive agents for treatment of transplant rejection and autoimmune disorders [1,48]. Transplantation specialists are now moving toward protocols that reduce the incidence of infections and minimize adverse events. Most immunosuppressive regimens are currently based on the combination of calcineurin inhibitors (CsA, TRL) with

antiproliferative agents (azathioprine, MMF) and steroids (prednisone) [49].

produces prolonged T- and B-cell depletion and allows drug minimization [1,47].

postadministration for a patient's immune system to be fully reconstituted ([1,47].

maintenance regimen consisting of CsA, MMF, and prednisone [1,47].

*2.6.2. Side effects*

**2.7. Alemtuzumab**

*2.7.1. Mechanism of action*

hemolytic anemia (<5%) [47].

**2.8. Glucocorticoids**

*2.7.2. Side effects*

antibodies have been described [46,47].
