*4.3.1.2. Limited sampling strategies for estimation of MPA AUC*

The dose interval MPA AUC0–12 h is generally regarded as the most reliable pharmacokinetic parameter index of risk for acute rejection but is impractical to measure in routine clinical practice. Single time-point samples such as the trough concentration or others, do not correlate well with the MPA AUC, especially in the early posttransplantation period renal transplant patients and for regimens that include MMF plus CsA, TRL, or SRL [104]. Therefore, assess‐ ment of whether C0 concentrations or other single time points correlate well with the AUC is important for establishing routine monitoring of the drug. Apart of the C0 level other single time points after MMF dosing are examined for their ability to predict full AUC values. A full MPA AUC typically requires at least eight blood samples during 12 hour dose interval. In clinical practice this is impractical; therefore, abbreviated sampling schemes involving the collection of there to five plasma samples have been investigated. The abbreviated sampling approach has provided estimations of MPA AUC with high correlations (r2 >0.8). Several models have been developed all of them in renal transplant patients [105-108].

#### **4.4. Sirolimus (SRL)**

dynamics and gastrointestinal motility) as well as more gradual changes (reflecting change in bodyweight, plasma proteins and organ function) [100]. The greatest variability in MPA pharmacokinetic is noted in the initial 2 months following transplantation, when adequate immunosuppression is critical to graft function and survival. It has also become apparent from longer term pharmacokinetic studies that exposure to MPA increases over time due to reduced clearance of the drug. A possible additional factor that could con‐ tribute to the higher oral clearance of MPA early after transplantation is corticosteroid therapy, which is significantly higher in that period but then is tapered to low dose lev‐ els or completely withdrawn. Based upon the marked pharmacokinetic variability ob‐ served with MPA and the pharmacodynamic relationship of pharmacokinetic parameters to rejection outcome, several scientific societies and consensus conferences have advocat‐ ed the use of concentration monitoring for patients undergoing treatment with MMF or

The incorporation of MMF into immunosuppressive regimens has been associated with decrease rates of acute rejection and decreased chronic allograft loss. Indications for TDM of mycophenolates were reviewed in a consensus meeting [101]. They included high-risk patients, patients with delayed graft function, or patients with immunosuppressive protocols excluding induction therapy or steroids or calcineurin inhibitor or patients with calcineurin minimization. Most of these patients (especially high-risk patients) are often excluded from the clinical trials. In fact, MPA TDM is currently only used in a few transplant centers on a routine basis, whereas a few others only checked MPA exposure in case of unexpected acute rejection or adverse event or drug interaction. Most of the centers never measure MPA. It is clear that the use of MPA TDM is conditioned by the faith of the physicians in its use, local

Although a relationship between AUC and outcome exists, the clinical utility of concentration monitoring, particularly C0 monitoring for MMF, has been questioned. Over the past decade, several studies were conducted to evaluate the clinical utility of prospective concentration controlled MMF therapy. While these studies were anticipated to fully clarify the utility of monitored MMF therapy, the outcomes from these studies are conflicting and have done little to settle the controversies surrounding this area of therapeutic drug monitoring [100]. With trough concentration, plasma concentration of MPA is measured immediately before a dose, it is easy to measure because only -ask patient to return to give sample, it is immediately before a dose, and only requires single simple possible association between C0 and decreased rejection noted in transplant recipients. However this method represents some disadvantages. Timing may not be accurate (depends on remembering time of last dose). Timing may vary from the "ideal" (12 h after last dose) by several hours. There is no high-level evidence of a strong association between C0 and outcome, or between C0 and AUC0–12, C0 is not a very informative time point for estimation of individual pharmacokinetic parameters. Single time-point samples

availability of MPA measurements, and organization of the nursing staff [102].

enteric-coated MPA [101].

328 Current Issues and Future Direction in Kidney Transplantation

*4.3.1. Therapeutic monitoring*

*4.3.1.1. Trough concentration (C0) monitoring*

SRL (formerly known as rapamycin) is a macrolide antibiotic with immunosuppressive properties that was introduced relatively recently (September 1999) into clinical practice for maintenance therapy in organ transplantation [109]. Pharmacokinetics studies of SRL in renal transplant patients have been shown great variability between patients. Several features contribute to the interpatient pharmacokinetic variability observed with SRL and can include any combination of the following: absorption, distribution, metabolism and/or excretion [110].

This drug presents a rapid gastrointestinal absorption (tmax from 0.33 to 5 hours) as well as a low (mean value 14%) and variable bioavailability. It has been reported that SRL is a substrate for the multidrug P-glycoprotein transporter and that the biotransformation of SRL is medi‐ ated by CYP3A enzymes. Accordingly, considerable variability in its pharmacokinetic parameters may be expected (apparent blood clearance rates after oral administration from 87 to 416 mL/h/kg). In addition, the disposition of SRL in humans includes a large volume of distribution, a long half-life (35 to 95 hours) and dose proportionality for Cmax and AUC. Also, some interracial variability and an influence of hepatic dysfunction have been noted with SRL [111]. Although structurally similar to TRL, SRL has a novel mechanism of action, which leads to synergy with CsA. The long half-life of the drug necessitates a loading dose to achieve therapeutic concentrations quickly, and also allows for once daily administration. Highly variable absorption and metabolism of the drug result in large differences in blood concen‐ trations among patients receiving the same dose. Efficacy for the prevention of acute rejection episodes, and the rate of common adverse effects (thrombocytopenia, leucopenia and hyper‐ triglyceridemia), are concentration-dependent [112].

#### *4.4.1. Therapeutic monitoring*

Clinical data suggest that the immunosuppressive efficacy and the occurrence and severity of adverse effects of SRL correlate with blood concentrations [112]. Drug interactions with concomitant immunosuppressant medications will alter SRL whole blood concentrations. The appropriate SRL through concentration at steady state (Cmin,ss) for acute rejection episode prophylaxis is a function of the concomitant immunosuppressive regimen. When it is used as base therapy with azathioprine and prednisone, a regimen stipulating initial Cmin,ss values equal to 30 μg/L during the first 2 months, and 15 μg/L(LC/UV assay) thereafter, led to a 41% rate of acute rejection episodes among 41 cadaveric kidney transplant recipients [113].

kinetics of EVL, were characterized over the first 6 months post-transplant in 731 patients receiving either 0.75 or 1.5mg bid EVL in addition to CsA and corticosteroids. The within- and between-patient variability of dose interval AUC was 27% and 31% respectively. There was no detectable influence of sex, age (16–66 years), or weight (42-132 kg) on AUC, but EVL exposure was significantly lower by an average of 20 % in blacks. In a study of 659 AUC profiles the correlation between trough concentration and overall exposure (AUC) there was a significant linear correlation with a regression coefficient of 0.89 and corresponding coefficient

Clinical Pharmacology and Therapeutic Drug Monitoring of Immunosuppressive Agents

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

331

For example, see [121] reported that multiple daily dosing of EVL, in doses up to 5 mg/day, is adequately well tolerated as add-on therapy in stable renal transplant patients receiving maintenance Neoral® immunosuppression. Similar degrees of correlation between EVL trough concentration and thrombocytopenia, leukopenia, hypertriglyceridemia, or hypercho‐

EVL is a drug with a narrow therapeutic index. The limited and variable bioavailability, intrinsic interindividual pharmacokinetic variability, the number of factors affecting the pharmacokinetics, and the number of drug interactions limits the use of fixed doses of this drug. The EVL Cmin is a good surrogate marker of EVL exposure (AUC), and correlates with pharmacological response and clinical outcomes. Therefore, prospective dose adjustments to obtain and maintain a therapeutic EVL Cmin have the potential to improve efficacy and reduce

A role for EVL drug monitoring has been suggested because of the potential for improving efficacy and reducing adverse effects, the EVL Cmin is a good surrogate marker of EVL exposure (AUC), and correlates with pharmacological response and clinical outcomes. Therefore, prospective dose adjustments to obtain and maintain a therapeutic EVL Cmin have the potential to improve efficacy and reduce toxicity [123]. Mere clinical monitoring of efficacy is insufficient because clinical presentations of graft rejection vary for each patient and are nonspecific. Thus, some authors have used a previously published 9-step decision-making algorithm to evaluate the utility of TDM for EVL. The recommended therapeutic range for EVL is a trough concentration of 3 to 8 ng/mL, as concentrations over 3 ng/mL have been associated with a decreased incidence of rejection, and concentrations >8 ng/mL with increased toxicity. Patients on EVL who have problems with absorption, who take concurrent cyto‐ chrome P450 inhibitors or inducers, or are noncompliant will attain the greatest benefit from

Maintenance immunosuppressive therapy over the past decade has become more diversified. Until the mid-1990s, CsA and azathioprine were the cornerstones in maintenance immuno‐ suppressive therapy. Today, these agents have been largely replaced by the newer agents TRL

lesterolemia in 54 stable renal transplant patients (18-68 years) were found.

of determination of 0.79 [120].

*4.5.1. Therapeutic drug monitoring*

therapeutic drug monitoring [124].

**5. Advances in immunosuppression – Future trends**

toxicity [122].

When combined with MMF and prednisone, this SRL regimen was associated with a 27.5% rate of acute rejection episodes among 40 cadaveric renal transplant recipients. Indeed, the combination of SRL (Cmin,ss of 10 to 20 μg/L; LC/UV assay) and basiliximab with late introduction of low dosage CsA has provided excellent prophylaxis of acute rejection episodes and renal function for primary, non-African-American recipients of cadaveric kidney trans‐ plants that displayed delayed graft function [112,114,115].

In purely Caucasian low-risk liver and kidney-pancreas transplant recipients, Cmin,ss of 6 to 12 μg/L (IMx® assay) in combination with low dosage TRL has been reported to yield low rates of acute rejection episodes and toxicity [116]. Because of the long half-life and extensive tissue distribution of the drug, steady-state concentrations are not reached before day 6 after initiation of therapy or after a dosage change. Thus, daily concentration monitoring is not necessary; the first SRL measurements should not be obtained before day 4 after inception of, or change in therapy. Thereafter, recommend monitoring Cmin,ss weekly for the first month and bi-weekly for the next month, targeting a 5 to 15 μg/L range if CsA is being used concom‐ itantly at Cmin,ss concentrations of 75 to 150 μg/L. If the patient fails to attain these values despite a dosage of 20mg/day, a full pharmacokinetic study should be performed to assess whether the defect is due to limited absorption or rapid clearance rates [112]. Modest correla‐ tion (r = 0.59) exists between SRL dose and peak plasma concentration (Cmax) or AUC, but a good correlation (r = 0.85) exists between trough concentration prior to the dose (minimum Cmin,ss and AUC. For this reason, Cmin,ss is a simple and useful index for therapeutic monitoring of SRL [112,117,118].

#### **4.5. Everolimus (EVL)**

In April 2010, EVL, a more water-soluble analog of SRL was approved for use in CsA-sparing regimens, including the requirement for adjusting EVL doses using target trough blood concentrations in renal transplant patients [51]. EVL, which has greater polarity than SRL, was developed in an attempt to improve the pharmacokinetic characteristics of SRL, particularly to increase its oral bioavailability. After a single oral dose of EVL 4mg in 12 healthy volunteers, it was absorbed rapidly (within 30 minutes after drug intake). The Cmax of EVL amounted to 44.2 ± 13.3 μg/L and was reached (tmax) after 30 minutes (range 0.5–1 hour). The AUC was 219 ± 69 μg\* h/L. The overall absorption of EVL, like that of SRL, is probably affected by the activity of P-gp. It is recommended that patients should take the drug consistently with or without food to reduce fluctuations in drug exposure [119]. In an international study, the pharmaco‐ kinetics of EVL, were characterized over the first 6 months post-transplant in 731 patients receiving either 0.75 or 1.5mg bid EVL in addition to CsA and corticosteroids. The within- and between-patient variability of dose interval AUC was 27% and 31% respectively. There was no detectable influence of sex, age (16–66 years), or weight (42-132 kg) on AUC, but EVL exposure was significantly lower by an average of 20 % in blacks. In a study of 659 AUC profiles the correlation between trough concentration and overall exposure (AUC) there was a significant linear correlation with a regression coefficient of 0.89 and corresponding coefficient of determination of 0.79 [120].

For example, see [121] reported that multiple daily dosing of EVL, in doses up to 5 mg/day, is adequately well tolerated as add-on therapy in stable renal transplant patients receiving maintenance Neoral® immunosuppression. Similar degrees of correlation between EVL trough concentration and thrombocytopenia, leukopenia, hypertriglyceridemia, or hypercho‐ lesterolemia in 54 stable renal transplant patients (18-68 years) were found.

### *4.5.1. Therapeutic drug monitoring*

*4.4.1. Therapeutic monitoring*

330 Current Issues and Future Direction in Kidney Transplantation

monitoring of SRL [112,117,118].

**4.5. Everolimus (EVL)**

Clinical data suggest that the immunosuppressive efficacy and the occurrence and severity of adverse effects of SRL correlate with blood concentrations [112]. Drug interactions with concomitant immunosuppressant medications will alter SRL whole blood concentrations. The appropriate SRL through concentration at steady state (Cmin,ss) for acute rejection episode prophylaxis is a function of the concomitant immunosuppressive regimen. When it is used as base therapy with azathioprine and prednisone, a regimen stipulating initial Cmin,ss values equal to 30 μg/L during the first 2 months, and 15 μg/L(LC/UV assay) thereafter, led to a 41% rate of acute rejection episodes among 41 cadaveric kidney transplant recipients [113].

When combined with MMF and prednisone, this SRL regimen was associated with a 27.5% rate of acute rejection episodes among 40 cadaveric renal transplant recipients. Indeed, the combination of SRL (Cmin,ss of 10 to 20 μg/L; LC/UV assay) and basiliximab with late introduction of low dosage CsA has provided excellent prophylaxis of acute rejection episodes and renal function for primary, non-African-American recipients of cadaveric kidney trans‐

In purely Caucasian low-risk liver and kidney-pancreas transplant recipients, Cmin,ss of 6 to 12 μg/L (IMx® assay) in combination with low dosage TRL has been reported to yield low rates of acute rejection episodes and toxicity [116]. Because of the long half-life and extensive tissue distribution of the drug, steady-state concentrations are not reached before day 6 after initiation of therapy or after a dosage change. Thus, daily concentration monitoring is not necessary; the first SRL measurements should not be obtained before day 4 after inception of, or change in therapy. Thereafter, recommend monitoring Cmin,ss weekly for the first month and bi-weekly for the next month, targeting a 5 to 15 μg/L range if CsA is being used concom‐ itantly at Cmin,ss concentrations of 75 to 150 μg/L. If the patient fails to attain these values despite a dosage of 20mg/day, a full pharmacokinetic study should be performed to assess whether the defect is due to limited absorption or rapid clearance rates [112]. Modest correla‐ tion (r = 0.59) exists between SRL dose and peak plasma concentration (Cmax) or AUC, but a good correlation (r = 0.85) exists between trough concentration prior to the dose (minimum Cmin,ss and AUC. For this reason, Cmin,ss is a simple and useful index for therapeutic

In April 2010, EVL, a more water-soluble analog of SRL was approved for use in CsA-sparing regimens, including the requirement for adjusting EVL doses using target trough blood concentrations in renal transplant patients [51]. EVL, which has greater polarity than SRL, was developed in an attempt to improve the pharmacokinetic characteristics of SRL, particularly to increase its oral bioavailability. After a single oral dose of EVL 4mg in 12 healthy volunteers, it was absorbed rapidly (within 30 minutes after drug intake). The Cmax of EVL amounted to 44.2 ± 13.3 μg/L and was reached (tmax) after 30 minutes (range 0.5–1 hour). The AUC was 219 ± 69 μg\* h/L. The overall absorption of EVL, like that of SRL, is probably affected by the activity of P-gp. It is recommended that patients should take the drug consistently with or without food to reduce fluctuations in drug exposure [119]. In an international study, the pharmaco‐

plants that displayed delayed graft function [112,114,115].

EVL is a drug with a narrow therapeutic index. The limited and variable bioavailability, intrinsic interindividual pharmacokinetic variability, the number of factors affecting the pharmacokinetics, and the number of drug interactions limits the use of fixed doses of this drug. The EVL Cmin is a good surrogate marker of EVL exposure (AUC), and correlates with pharmacological response and clinical outcomes. Therefore, prospective dose adjustments to obtain and maintain a therapeutic EVL Cmin have the potential to improve efficacy and reduce toxicity [122].

A role for EVL drug monitoring has been suggested because of the potential for improving efficacy and reducing adverse effects, the EVL Cmin is a good surrogate marker of EVL exposure (AUC), and correlates with pharmacological response and clinical outcomes. Therefore, prospective dose adjustments to obtain and maintain a therapeutic EVL Cmin have the potential to improve efficacy and reduce toxicity [123]. Mere clinical monitoring of efficacy is insufficient because clinical presentations of graft rejection vary for each patient and are nonspecific. Thus, some authors have used a previously published 9-step decision-making algorithm to evaluate the utility of TDM for EVL. The recommended therapeutic range for EVL is a trough concentration of 3 to 8 ng/mL, as concentrations over 3 ng/mL have been associated with a decreased incidence of rejection, and concentrations >8 ng/mL with increased toxicity. Patients on EVL who have problems with absorption, who take concurrent cyto‐ chrome P450 inhibitors or inducers, or are noncompliant will attain the greatest benefit from therapeutic drug monitoring [124].
