Mycophenolate Mofetil in Pediatric Renal Transplantation

Abstract

Mycophenolate mofetil (MMF) has become a popular immunosuppressive agent in pediatric renal transplantation and has helped to address the unique challenges of transplantation in this population. This paper reviews the early studies that proved MMF's efficacy in reducing the risk of acute renal rejection, as well as its safety and tolerability in comparison to azathioprine. Key conclusions about the pharmacokinetics of MMF from studies of MMF alone and in combination with other immunosuppressive therapies are outlined. The importance of therapeutic drug monitoring (TDM) in this population is also explored in this review. Finally, recent studies showing that newer agents used in combination with MMF can further increase efficacy and reduce the risk of adverse events such as posttransplant lymphoproliferative disease are discussed.

Mycophenolate mofetil (MMF) is the 2-morpholinoethyl ester prodrug of mycophenolic acid (MPA), which was first isolated from a Penicillium culture more than 100 years ago. It was not until the early 1990s, however, that Allison and Eugui (¹) suggested that MPA could be a useful immunosuppressive agent in transplantation because of its ability to block proliferation of both T and B lymphocytes. The basis for the development of this natural product arose from observations that children who have an inherited lack of adenosine deaminase, a key enzyme in maintaining guanosine pools, have a dramatic immunodeficiency involving both lymphocyte types while those who have a deficiency of hypoxanthine-guanine phosphoribosyl transferase—a major purine synthesis salvage pathway—do not. These findings suggest that lymphocytes are particularly dependent on de novo purine synthesis and that an inhibitor of inosine-5′-monophosphate dehydrogenase (IMPDH), a key enzyme in de novo synthesis of guanosine nucleotides, should preferentially inhibit these cells. In fact, this has been borne out with the development of MPA (administered as MMF to improve oral bioavailability), which is an effective IMPDH inhibitor that has proven to be an effective and safe immunosuppressive agent (^{1, 2}).

As in adults, MMF has become a popular immunosuppressive agent in pediatric renal transplantation with a rapid increase in its utilization from 20% of all pediatric recipients in 1996 to approximately 60% in 2003 (³). The special interest in MMF for renal transplantation in children reflects its efficacy in adults in reducing the occurrence of acute rejection episodes in comparison to azathioprine (AZA) when used for maintenance immunosuppression, and the lack of any clinically significant impact on nephrotoxicity. These considerations are relevant not only for renal transplants but for other allografted organs as well (⁴).

Transplantation in children carries unique challenges. While issues such as controlling rejection and minimizing side effects are similar between adults and children, maintenance immunosuppressant regimens that affect developmental processes have a disproportionate impact on children. This is particularly true in the case of corticosteroids which have many side effects, including infection, hypertension, hyperlipidemia, growth suppression during crucial growth years, glucose intolerance and diabetes mellitus, bone loss, cataracts, acne, Cushingoid appearance, weight gain, and changes in mood and behavior (⁵). Some of these effects can be quite devastating in pediatric patients. In addition, the calcineurin inhibitors (CNIs), most commonly cyclosporine A (CsA), induce cosmetic side effects, such as gingival hyperplasia and hirsutism (^{6, 7}). These side effects especially affect adolescents, who have been shown to be up to four times more nonadherent with their medications than adults or younger patients, and who have the least successful long-term graft survival of all age groups (⁸).

In this paper, we will review the initial pediatric MMF experience as reported by various transplant centers and the applicability of immunosuppressive sparing regimens using MMF in this population. The pharmacokinetics of MMF in pediatric renal transplant recipients and the importance of therapeutic drug monitoring (TDM) in this population will be discussed.

CLINICAL TRIALS OF MYCOPHENOLATE MOFETIL WITH CYCLOSPORINE AND STEROIDS

Early Multicenter Dose-escalation Study

The introduction of MMF into pediatric renal transplantation came with the promise of addressing some of the problems with its predecessor azathioprine (AZA), a drug which appeared to have limited efficacy in preventing acute rejection and which bore the significant side effect of bone marrow suppression in many patients. Given the specific antiproliferative effects of MMF on lymphocytes, as well as its benefits in reducing gene transcription of fibrogenic factors with the potential for preventing chronic injury (⁹), MMF was anticipated to be a more specific, less toxic, and more powerful immunosuppressive agent.

The first study of MMF in pediatric renal allograft recipients was initiated in 1994 by the US Pediatric Mycophenolate Study Group, composed of investigators from the University of California, Los Angeles (UCLA), the University of California, San Francisco, the Egelston Children's Hospital in Atlanta, Georgia, and the Columbus Children's Hospital in Columbus, Ohio. Early results were presented in 1996 (¹⁰) with follow-up data presented in 1999 (¹¹). This open-label, dose-escalation trial examined the safety, tolerance, and pharmacokinetics of oral MMF in 40 pediatric renal transplant recipients (11; data on file). Twice daily oral doses, 15, 23 or 30 mg/kg, were combined with CsA and prednisone with an option of antilymphocyte globulin preparations for induction. Six months posttransplant, the occurrence of biopsy-proven rejection was 22.5%, with the composite of clinical rejection and biopsy-proven rejection of 35% overall, increasing to 47% after 3 years of follow-up. Three-year patient survival was 100% while graft survival was 95%. In comparison, contemporaneous rates of rejection at 3 years drawn from the North American Pediatric Renal Transplant Cooperative Study (NAPRTCS) database were 51% (for living donor kidneys) and 62% (for deceased donor kidneys) (³).

Important pharmacokinetic data were obtained from this study as well. Doses of 23 and 30 mg/kg yielded MPA exposure (area under the concentration-time curve [AUC_0-12]) in the desired range, derived from studies of adult patients, of approximately 30 μg·h/mL (¹⁰). Closer analysis of the pharmacokinetics indicated that pediatric MMF dosage could be determined more accurately when calculated on a body-surface-area basis, and that under the immunosuppressive regimen employed this target could be achieved using a starting dose of 600 mg/m²/dose (1200 mg/m²/day). It is important to recognize that this dose was derived in patients who were receiving CsA and steroids as their other immunosuppressive agents.

MMF was generally well tolerated. Through the first 6 months after transplantation, dose reductions were most frequently instituted for diarrhea (37%) and leukopenia (30%). At 3 years posttransplant, 68% (27/40) of the pediatric renal transplant patients still were receiving MMF (¹¹). Principal reasons for withdrawal included gastrointestinal side effects (n=4) and rejection episodes requiring alternate immunosuppressive therapy (n=3). There was also one case of posttransplant lymphoproliferative disease (PTLD) that was successfully treated by chemotherapy and withdrawal of immunosuppression.

First Studies Evaluating Mycophenolate Mofetil Efficacy and Safety in Pediatric Patients

Following the success of the early dose-escalation study, investigators began to publish results of other studies in which pediatric renal transplant recipients received MMF in combination with CsA and steroids. These generally were open-label studies with historical controls, since randomized, controlled trials were quite difficult to carry out in light of the relatively small numbers of pediatric kidney transplants performed each year. Since studies in adult renal transplant recipients had previously established the superiority of MMF over AZA or placebo in reducing the risk of acute rejection, it was important for the pediatric transplant community to have prompt access to open-label studies (^12–14). For a summary of selected published studies of MMF used in combination with CsA and steroids in pediatric renal transplantation, see Table 1 (^15–22).

TABLE 1: Selected published studies of mycophenolate mofetil in combination with cyclosporine and steroids in pediatric renal transplantation

Results of the UCLA experience involving 37 patients (mean age, 12.2 years) were published in 1997 (¹⁵) (Table 1). Maintenance CsA and prednisone and equine antithymocyte globulin induction were administered with MMF at doses ranging from 8–30 mg/kg. At a mean of 11 months follow-up, 97% of the transplants were functioning; one patient lost a graft to vascular thrombosis 5 days posttransplant (this patient was found to have a previously undiagnosed deficiency of Protein C). The incidence of clinically significant acute rejection (demonstrated by a significant rise in serum creatinine) was 13%, which was lower than we had previously observed in our institutional transplant program. Cytomegalovirus (CMV) infection was observed in 19% (7/37) of patients; in response, the CMV prophylaxis regimen was augmented, leading to reductions in incidence of significant CMV in more recent recipients. The most frequent side effects necessitating dosage reduction were diarrhea and/or nausea in six (16%) and neutropenia in three (8%) patients (¹⁵).

At 1 year posttransplant, protocol biopsies performed in a subset of 12 patients found that 75% had no or only borderline acute rejection (²³). Mild acute rejection accompanied by a rise in serum creatinine was diagnosed in one patient while two additional patients had mild mononuclear infiltrates unaccompanied by clinical changes. Ten of the 12 patients were free from chronic allograft nephropathy (CAN) while two had Grade II changes by Banff classification.

Renal function was evaluated in 11 patients, using both single-injection radionuclide glomerular filtration rate (GFR) studies and estimated creatinine clearance determinations using the Schwartz formula. There were no differences in either mean GFR or estimated CrCl values at both 3 months and 1 year posttransplant in these patients when compared to a contemporaneous control group (matched for age, sex, and donor source) who received AZA. Both groups received the same doses of CsA as assessed by trough blood levels. However, patients treated with MMF received a significantly lower dose of prednisone. Thus, although the renal function between the two groups was not different, the patients on MMF could be maintained on a significantly lower dose of prednisone.

In 2000, we reexamined our experience with this triple immunosuppressive maintenance regimen (using induction therapy with either antithymocyte globulin or anti-interleukin-2 receptor humanized monoclonal antibody). Data were evaluated from 80 patients who had been followed for a mean of 34 months (R.E., unpublished data). Forty-one percent of recipients received allografts from living related donors. The low incidence of acute rejection demonstrated in the earlier patients also was seen in this larger population, with 17% of patients experiencing acute rejection within the first year. Patient and graft survival at follow-up were 97.5%, and 91.0%, respectively. CMV and Epstein-Barr virus (EBV) infections occurred in 11% and 6% of patients, respectively, while PTLD occurred in 2.5% of patients.

Another study from the same era was conducted at the University of Alabama at Birmingham and at the Children's Hospital of Boston, in which the first 31 enrolled patients received AZA 2 mg/kg/d from the first postoperative day while the next 36 patients received 1 g/m²/d of MMF (¹⁶). Induction was with either intravenous CsA or OKT3; both treatment groups received additional maintenance immunosuppression with oral CsA and steroid. Although fewer patients in the MMF group experienced an episode of acute rejection versus the AZA group (25% versus 39%, respectively), the study was not sufficiently powered to assign statistical significance to a difference of this magnitude. There were also no differences in mean GFR or in graft and patient survival between treatment groups. The authors concluded that the addition of 1 g/m²/d MMF to a maintenance immunosuppressive regimen of CsA and prednisone had no impact on short-term allograft outcome in pediatric patients (¹⁶).

Multicenter Trials Examining Mycophenolate Mofetil's Efficacy and Safety

Data from three large multicenter studies (^17–21) and one smaller study (²²) provided further support for the safety and efficacy of MMF in the pediatric renal transplant population when used with CsA and prednisone (Table 1). Induction therapy was optional in one study (¹⁷) and not used in the other two studies (^{19, 21}). The incidence of acute rejection within the first 6 months to 1 year for patients receiving MMF in these studies ranged from 28% to 37% (^{17, 19, 21}). Those studies comparing MMF patient groups to historical controls reported significant reductions in the incidence of acute rejection with MMF versus AZA (^19,21). There was also a significant improvement in the incidence of acute rejection between patients receiving MMF and those receiving AZA at 3 years in a follow-up report to one study (²⁰).

In the study by Bunchman et al. (¹⁷) there were no differences in the incidence of acute rejection when the results were stratified by age. At 1 year posttransplant, 72 patients still were receiving MMF. Reasons for withdrawal (Table 2) included the need for a prohibited medication (10/28), adverse events (10/28), or other (8/18). In general, the risk of developing side effects declined with increasing age (Table 3). Long-term (3-year) graft and patient survival were excellent, with a 30% incidence of acute rejection (Table 1) (¹⁸).

TABLE 2: Reasons for withdrawal of mycophenolate mofetil (MMF) in the first year after transplantation in the Multicenter MMF Suspension Trial

TABLE 3: Summary of adverse events that led to dosage reduction or interruption in the Mycophenolate Mofetil Multicenter Suspension trial

Effects of Mycophenolate Mofetil on Long-Term Renal Allograft Function

Although the above studies support the use of MMF in pediatric renal transplantation, with reduced risks of acute rejection and excellent patient and graft survival when used in combination with CsA and steroids, they provide little information on the impact of MMF on long term allograft function. Ferraris et al. (²²) evaluated renal allograft function at 5 years after transplantation in 29 patients. Although there was no significant difference in the incidence of chronic rejection (10.3% versus 25% for MMF versus AZA), patients receiving MMF were significantly more likely to have event-free survival (events included a 20% or greater decline in 1/serum creatinine, death, or graft loss) than those receiving AZA (69% versus 17%, respectively; P<0.0001). The use of MMF in these patients allowed a lower dose of steroids to be used at 5 years posttransplant, resulting in significant growth benefits in patients receiving MMF when compared with those receiving AZA (²²).

MMF also has been evaluated in 36 pediatric renal transplant patients with CAN in a study by Henne et al. (²⁴). MMF was either added to a regimen consisting of CsA or tacrolimus (TAC) and steroids, or was substituted for AZA in patients receiving a triple-therapy regimen. A year after conversion, 61% had an improvement in GFR, with 22% remaining stable and only 17% having a further decline in renal function. Taken together, these studies suggest that MMF may improve both early and late allograft outcome in children and adolescents.

PHARMACOKINETICS OF MYCOPHENOLATE MOFETIL IN COMBINATION WITH CYCLOSPORINE AND STEROIDS

Key Conclusions from Pharmacokinetic Studies

The earliest studies of MMF in pediatric patients used fixed doses based on patient weight while later studies calculated doses based on body surface area. Subsequently, pharmacokinetic data from clinical trials of patients receiving MMF in combination with CsA and steroids have aided our understanding of MMF dosing in pediatric renal transplant recipients (see the article by van Gelder and Shaw in this supplement for a more comprehensive overview of MMF pharmacokinetics). The work of the German Study Group on Mycophenolate Mofetil Therapy (^25–29) as well as that of others (^{10, 17}) contributed greatly to our understanding of MMF pharmacokinetics within the context of CsA/steroid-based regimens, suggesting that fixed-dose regimens are not ideal for many patients. Key conclusions drawn from these studies are discussed below, with a summary provided in Table 4.

TABLE 4: Key pharmacokinetic (PK) findings in pediatric patients receiving mycophenolate mofetil with cyclosporine and steroids

AUC_0-12 values for total and free MPA generally are comparable between pediatric (²⁵) and adult patients (³⁰). As in adults, there is substantial variation in the pharmacokinetics of MMF both within and between pediatric renal transplant patients (^{10, 26–29}). Over time, the intra-individual variation decreases to some degree.

There does not appear to be a significant correlation between patient age and MMF pharmacokinetic parameters overall (^{25, 28}). Although low MPA-AUC_0-12 values were associated with younger age at 1 week posttransplant, this association did not extend to later time points (3 weeks, 3 months, and 6 months) (²⁸).

Data support the validity of MMF dosing regimens based on body surface area in pediatric patients. Doses of 600 mg/m² given twice daily in children between 6–15 years of ages receiving CsA and steroids yielded comparable free and total MPA-AUC_0-12 values at 3 weeks posttransplant to 1 g MMF twice daily in adults (²⁵). When used in conjunction with CsA, target MPA-AUC_0-12 values of 30–60 μg·h/mL, considered desirable in adults, were achieved in pediatric patients at doses of 600 mg/m² twice daily (²⁵).

There is a progressive rise in mean MPA exposure over the first several months posttransplant in pediatric patients. Bunchman et al. (¹⁷) demonstrated that mean dose-adjusted AUC_0-12 values rose between 7 days, 3 months, and 9 months posttransplant, with no significant differences in values between age groups (Fig. 1). In contrast to the increases observed in the first 9 months posttransplant, a subgroup analysis of 25 patients determined that mean dose-adjusted AUC_0-12 values at 24 and 36 months were stable in all age groups (²⁹).

FIGURE 1.: Mean dose-adjusted mycophenolic acid-area under the time-concentration curve_0-12 values (μg·h/mL) in patients of the three age groups in the Mycophenolate Mofetil Suspension Trial. From Bunchman et al. (¹⁷) with permission.

Early renal dysfunction has an important impact on MMF pharmacokinetics in pediatric patients. This may be related to elevated MPA glucuronide (MPAG) levels. MPAG is the primary MPA metabolite excreted by the kidney. Patients with renal allograft dysfunction have MPAG AUC_0-12 values that are significantly higher than in patients with good renal function. MPAG-AUC_0-12 values were found to be 3-fold higher (P<0.001) in pediatric patients with primary renal allograft dysfunction than in those with normally functioning allografts (²⁵). Since MPAG and MPA compete for binding to serum albumin, elevated levels of MPAG can increase the free MPA fraction (²⁵). With elevated free MPA levels, there is a consequent enhanced metabolism of MPA, since it is the free fraction that is primarily available for metabolism and excretion. This increased metabolism and excretion would result in a reduction in total MPA AUC_0-12. Since free MPA AUC_0-12 values have been shown to be inversely correlated with the patients' GFRs (²⁵), elevated free MPA levels can be expected in pediatric patients with allograft dysfunction.

There is an inverse correlation between free MPA AUC_0-12 and serum albumin levels (²⁵). As noted above, a larger free MPA fraction may increase excretion/metabolism of MPA, resulting in a decrease in total MPA AUC_0-12 values. Also, some adverse events of MMF appear to be correlated with free MPA AUC_0-12 (²⁸). The consequences of these findings may have particular clinical relevance in pediatric renal transplant recipients since children are more likely than adults to develop posttransplant nephrotic syndrome due to recurrence of glomerulonephritis and development of focal segmental glomerulosclerosis (FSGS). A study of adult renal transplant recipients by Atcheson et al. (³¹) determined that a serum albumin level of ≤ 31g/L can be used to identify patients who are likely to have an abnormally elevated free MPA fraction and thereby require therapeutic monitoring of free MPA levels.

To summarize, there is high variability of MPA exposure at fixed dosages, particularly in the early posttransplant period and in those patients with early allograft dysfunction. Together these findings support a role for TDM in pediatric patients receiving MMF.

Therapeutic Drug Monitoring

Therapeutic drug monitoring of MPA exposure is an important management tool in pediatric patients receiving MMF despite the methodological issues that complicate its application (^{32, 33}). First, since studies defining the previously discussed pharmacokinetic parameters involved patients receiving MMF in combination with CsA, these findings are largely outdated with newer regimens based on TAC. Perhaps more importantly, there are conflicting reports on the value of pharmacokinetic parameters in predicting outcomes in pediatric patients.

In an analysis of data from the International Pediatric Study, Bunchman et al. (¹⁷) found no association between low MPA or MPAG plasma concentrations and the incidence of acute rejection. However, Weber et al. (²⁸) reported that a number of parameters had predictive value in distinguishing between patients with or without acute rejection, including MPA evening predose concentration (i.e., predose trough level) (P=0.0006), MPA AUC_0-12 (P=0.005), and MPA AUC_{0min, 75min, 4hour} (i.e., abbreviated pharmacokinetic sampling)(P=0.009). Ten of 14 patients (71%) with acute rejections in the early posttransplant period had MPA-AUC_0-12 values below 33.8 mg·h/L prior to experiencing an episode of acute rejection for a diagnostic sensitivity of 75% and a specificity of 64% (²⁸). The sensitivity and specificity of having an evening predose MPA level of 1.2 mg/L was 83% and 64%, respectively (²⁸).

Abbreviated MPA-AUC_0-12 determinations have been advocated as a substitute for the more cumbersome AUC_0-12 (^34–36), although even these limited sampling procedures are difficult in children, since they require more than one, and often three or four venipunctures. Time points that have been advocated as most valuable for limited sampling include C_1hour and C_4hour (³⁷), C₀, C_1hour, and C_4hour (³⁵), C_75min (³⁵), and C₀, C_30min, and C_2hour (²⁹).

It should be noted these parameters were all defined using data from patients receiving MMF in combination with CsA and steroids. It is likely that these measurements are not generalizable to patients on other regimens, since both CsA and prednisone appear to decrease MPA exposure following MMF administration (see below for further discussion of the mechanisms involved). Limited data are available for pediatric patients receiving MMF with TAC. Filler (³⁷) reported that a formula derived for an abbreviated AUC in adults receiving MMF in combination with TAC could not be applied to pediatric patients. The author concluded that, while good prediction of MPA AUC_0-12 could be obtained by sampling at C₀, C_30min, and C_2hour, a more reliable approximation of AUC_0-12 could be derived using a C₀, C_1hour, C_2hour, and C_4hour sampling protocol (³⁶).

An additional consideration in implementing TDM in patients receiving MMF is the difference in values obtained with high-performance liquid chromatography (HPLC) versus the enzyme multiplied immunoassay technique (EMIT) assay. In general, the EMIT assay overestimates the results obtained with HPLC by approximately 10% due to its detection of both MPA and the MPA acylglucuronide metabolite (³⁸) (for a comprehensive discussion about the differences between these assays, see the article by van Gelder and Shaw in this supplement). From a functional standpoint, the two measurements may be generally equivalent since the acylglucuronide metabolite is immunologically active.

Even with these challenges, cogent arguments can be made for the use of TDM to guide MMF dosing (so-called “concentration-control” techniques). Of a cohort of pediatric allograft recipients from UCLA who were prescribed a posttransplant MMF dose of 600 mg/m² twice daily (or 1200 mg/m² /day), only 14% actually were receiving this dose >6 months later (Fig. 2, R.E. unpublished data). To the same point, David-Neto et al. (³⁵) performed full pharmacokinetic profiles in 20 children who were receiving MMF for a mean of 46 months. Despite the fact that these patients were receiving a relatively low dose of MMF (785±183 mg/m²/day), only three of the 20 had a total MPA AUC_0-12 of <36 μg·h/mL, revealing that generally adequate MMF exposure was achieved with less than the “standard dose.” In yet a different illustration of the potential importance of monitoring MPA exposure, Gajarski et al. (³³) determined after careful evaluation of a cohort of pediatric and young adult heart transplant recipients that “standard” MMF dosing failed to consistently achieve therapeutic MPA concentrations. An MPA trough level of <2.5 μg/mL was most frequently associated with high-grade rejection as determined by endomyocardial biopsy (³³).

FIGURE 2.: Latest stable mycophenolate mofetil doses per m² in pediatric recipients of kidney allografts >6 months posttransplant (n=86).

On a final note, there is very little information to assess the long-term impact of MPA monitoring in pediatric renal transplant patients. A recent study by Pape et al. (³⁴) examined the relationship between long-term renal function and 12-hour MPA trough levels in a cohort of 42 children with biopsy-proven CAN at a minimum of 1 year posttransplant. MMF was added to a regimen of CsA and steroids at this time without an adjustment of the CsA dose. After a 2-year follow-up period, there was no correlation between the mean MPA levels and change in GFR, although mean GFR levels stabilized (³⁴). However, the relevance of these findings in assessing the value of MPA monitoring in the pediatric transplant population is unclear, since most patients are begun on MMF from the outset of transplantation. Clearly, there is a critical need for data with which to validate the usefulness of TDM in maintaining long-term efficacy and safety in pediatric renal transplant patients receiving MMF.

MYCOPHENOLATE MOFETIL IN ALTERNATE IMMUNOSUPPRESSIVE REGIMENS

Current and Evolving Practice

Significant changes in the immunosuppressive regimens used in US pediatric renal transplant recipients have occurred since 1997 (Fig. 3) (³). MMF largely has replaced AZA and is now part of the initial maintenance immunosuppressive regimen in about two thirds of US pediatric renal transplant recipients. The use of CsA also is diminishing, with concurrent dramatic increases in the use of TAC and sirolimus (SRL) with steroids and also in steroid-sparing regimens.

FIGURE 3.: Recent trends in the use of maintenance immunosuppressive agents in pediatric renal transplantation (³). MMF, mycophenolate mofetil.

As discussed previously, optimal MMF dosing was established from studies of patients receiving immunosuppressive regimens based on CsA and steroid maintenance. Newer regimens using TAC or SRL, and those involving reduction or withdrawal of CNIs or steroids are becoming increasingly common.

Pharmacokinetics of Mycophenolate Mofetil with Alternate Immunosuppressive Agents

Both CsA and steroids have an impact on MMF pharmacokinetics; thus, the relevance of findings based on regimens containing these agents to alternate regimens has yet to be established. CsA decreases plasma MPA levels by 20% to 40% compared to levels observed alone or with the coadministration of TAC or SRL (^39–41). The mechanism likely involves inhibition of MPAG excretion into the bile, interfering with enterohepatic recirculation of MPA (i.e., absence of the pharmacokinetic "second peak") (⁴²). In general, for pediatric patients not receiving CsA (including those in which CsA is replaced with TAC), the starting MMF dose should be 300 to 450 twice daily (600-900 mg/m²/day).

It has been suggested that, when used with TAC, proper MMF dosing may be age dependent. Filler reported that a significant negative correlation between age and MMF dose exists in pediatric renal transplant recipients receiving TAC and steroids. He further noted that the negative correlation was more pronounced when the dose was based on body weight than by body surface area (⁴³). To maintain an MPA AUC_0-12 of 60 μg·h/mL, a twice-daily dose of 500 mg/m² (1000 mg/m²/day) was required in a 2-year-old patient while 250 mg/m² twice-daily (500 mg/m² /day) was needed in adolescents (⁴³). However, optimal dosing guidelines for combinations of MMF with immunosuppressive agents such as TAC and SRL are still not well defined for pediatric patients.

Steroids induce hepatic glucuronyltransferase expression and enhance the activity of uridine diphosphate glucuronlytransferase, the enzyme responsible for MPA metabolism to the inactive metabolite MPAG (⁴⁴). As a consequence, the bioavailability of MPA is reduced when MMF is administered with steroids; discontinuation of steroids results in higher MPA exposure (⁴⁴). Thus, it may be possible to reduce the dose of MMF in pediatric patients on steroid-free regimens.

Clinical Outcomes of Newer Regimens in Pediatric Renal Transplantation

Immunosuppressive regimens in pediatric transplantation are evolving, with clinical data from newer regimens beginning to emerge. Schwartz et al. (⁴⁵) employed a combination of TAC, MMF, and prednisone as maintenance immunosuppression following thymoglobulin induction. In the first posttransplant year, an episode of acute rejection was observed in only three of the 34 pediatric patients (8.8%). Hoecker et al. (⁴⁶) followed 19 patients who had been either switched from CsA to SRL (n=10) or underwent CsA dose reduction (n=9) following diagnosis of moderate or severe CsA toxicity. Over 1 year, seven of 10 in the former group and six of nine in the latter experienced stabilized or improved graft function; of those receiving SRL, 70% developed transient hyperlipidemia and 40% required erythropoietin to treat anemia. Other studies have demonstrated successful CsA reduction or withdrawal following introduction of MMF in pediatric renal transplant patients with CAN (⁴⁷) or CsA nephrotoxicity (⁴⁸). In each case, there were marked improvements in graft function following the introduction of MMF (^{47, 48}).

As mentioned above, steroids are seriously detrimental to pediatric patients. The goal of all pediatric nephrologists is to utilize immunosuppressive regimens that withdraw or avoid steroids altogether; the possibility of complete elimination of steroids with the concomitant use of MMF holds particular appeal for the pediatric population. A recent retrospective analysis of pediatric patients withdrawn from steroids (due to Cushingoid habitus, hypertension, growth retardation, patient demand, osteopathia, hyperlipidemia, and cataract) while on an MMF-CsA regimen reported no episodes of acute rejection and stable graft function following prednisone withdrawal (⁴⁹). Prepuberal patients had significant improvements in growth and body-mass index following withdrawal from prednisone (⁴⁹).

Steroid avoidance has been successful in the short term when MMF is combined with TAC and either extended-dose daclizumab or antithymocyte globulin is added (^9,50–53). With the goal of eliminating steroids, the combination of MMF and TAC may strike the correct balance between adequate and overimmunosuppression.

Results of a study evaluating a novel CsA- and steroid-free protocol support the validity of the hypothesis that reducing or eliminating CsA and steroids should permit the use of lower doses of MMF in pediatric renal transplant recipients (⁵⁰). In a single-center pilot study, 57 pediatric renal transplant recipients received extended (6-month) induction with daclizumab in place of steroids (⁵⁰). Maintenance therapy consisted of low-dose MMF (starting doses of 450 mg/m²/dose twice daily reduced to 300 mg/m²/dose after 2 weeks) and TAC (target trough levels: 3 to 5 ng/mL). With the use of these low doses of MMF, side effects (bone marrow suppression and gastrointestinal toxicity) were limited without sacrificing graft outcomes (8% acute rejection at 1 year). Additionally, there were improvements in graft function, hypertension, and growth with this protocol (⁵⁰).

A recent analysis of data from this study found that patient survival was 97.4% and death-censored graft survival was 100% at a mean follow-up of 32.9±13.8 months (⁹). Graft function was significantly better in steroid-free patients at 1, 2, and 3 years when compared with a historical cohort of patients receiving steroids in combination with TAC and matched according to age, sex and race. As the proportion of patients receiving MMF at the time of transplantation was significantly higher in steroid-free group than the steroid-based cohort (100% versus 47%; P<0.0001) (⁵⁰), this may represent a beneficial effect of MMF on chronic graft injury. Patients receiving the steroid-sparing regimen also had significantly less hypertension, hypercholesterolemia, glucose intolerance and linear growth (⁹). We currently are conducting a multicenter trial to confirm and extend these findings.

The use of SRL in a steroid-withdrawal regimen recently was described in a report from the Study of the Cooperative Clinical Trials in Pediatric Transplantation (⁵⁴). Immunosuppressive therapy consisted of basiliximab induction and maintenance with prednisone, CNIs (CsA or TAC), and SRL, followed by gradual steroid withdrawal in patients without biopsy-proven acute rejection at 6 months (59 patients were randomized to continue steroids and 73 were randomized to steroid withdrawal). Prednisone withdrawal was successful, with a 5% incidence of acute rejection in the withdrawal group versus 7% for those remaining on prednisone at 1 year (⁵⁴). However, there was an excess risk of PTLD in the trial population (5.8%) that led to early termination of the trial (⁵⁵). EBV-negative patients were significantly more likely to develop PTLD, with EBV-negative recipients of allografts from EBV-positive donors at especially high risk (⁵⁵).

The impact of immunosuppressive regimens on PTLD is an important issue in pediatric renal transplantation in that children are at increased risk for developing this life-threatening complication. An analysis of United Network for Organ Sharing and the Organ Procurement and Transplant Network (UNOS/OPTN) data determined that pediatric renal transplant recipients had a 427% increased risk of developing PTLD compared to adults (⁵⁶). Based on analysis of data from NAPRTCS (⁵⁷), the incidence of PTLD among all pediatric recipients was 1.2%, or 298/100,000 posttransplantation years (⁵⁷). There is evidence that the incidence of PTLD has increased in recent years, from 254/100,000 years between 1987 and 1991 to 395/100,000 years after 1992. The time to onset of PTLD also decreased during this period, from a median of 356 days (range, 64 to 3048 days) to 190 days (range, 42 to 944 days) (⁵⁷).

Despite the possibility that this apparent increase in the incidence of PTLD might be a consequence of more intense immunosuppressive regimens, analysis of the NAPRTCS database (⁵⁷) was unable to establish a correlation between dosage of CsA, AZA, or prednisone and the risk of PTLD. Still, patients receiving TAC were reported to have a higher risk of developing PTLD than those receiving CsA (P<0.0001). A follow-up analysis confirmed these findings for the 1987-95 era but found no correlation between TAC use and PTLD in the 1996-2000 era (⁵⁸). The authors suggested the shorter follow-up period for the recent era, and current use of lower doses of TAC may explain these differences. Alternately, it was suggested that newer immunosuppressive regimens that combine TAC with other agents might be protective. It was noted that there was a very low risk (0.8%) of PTLD in those patients receiving both MMF and TAC (⁵⁸).

Similarly, analysis of UNOS/OPTN data, which includes data from both adults and children, found no correlation between CNI use (TAC versus CsA) and PTLD. The use of MMF, though, decreased the risk of developing PTLD when compared with AZA (risk ratio: 0.64; P=0.005). The use of monoclonal antilymphocyte induction (but not induction with other agents) increased the risk of developing PTLD (risk ratio: 1.72; P=0.03) (⁵⁶).

Clearly, much additional work is needed to define optimal immunosuppressive regimens in pediatric renal transplant patients, particularly with respect to newer and evolving regimens. The safety and efficacy of these protocols with special emphasis on long-term graft survival and PTLD need to be established. Trials currently are under way that should improve our understanding of these important issues.

CONCLUSIONS

Studies in the late 1990s demonstrated the safety and efficacy of MMF in pediatric renal transplant recipients (^{10, 15}), leading to its rapid acceptance in the pediatric transplant community. Data from the NAPRTCS indicate that the probability of acute rejection among pediatric patients transplanted during this era has been markedly improved (Fig. 4) (³). Improvements in immunosuppressive therapy, including the use of MMF, no doubt play an important role in this success.

FIGURE 4.: Prevalence of first acute rejection, stratified by era (³).

Original protocols used MMF in triple-therapy regimens with CsA and steroids. Pharmacokinetic analyses demonstrated that dosing strategies based on body surface area were the most appropriate for pediatric patients and that TDM of MPA exposure may be indicated for pediatric patients, especially those with poor renal function and/or low serum albumin levels. Recently, TAC has surpassed CsA as the CNI agent of choice in pediatric renal transplant patients. Furthermore, CNI-sparing protocols as well as those limiting exposure to steroids are becoming increasingly important in pediatric renal transplantation. Data from these newer protocols are now emerging and suggest that MMF will play an integral role in the success of these immunosuppressive regimens.