Sirolimus pharmacokinetics in pediatric renal transplant recipients receiving calcineurin inhibitor co-therapy

Treatment optimization of maintenance immunosuppressive agents in pediatric renal transplant recipients

Introduction: Graft survival in pediatric kidney transplant patients has increased significantly within the last three decades, correlating with the discovery and utilization of new immunosuppressants as well as improvements in patient care. Despite these developments in graft survival for patients, there is still improvement needed, particularly in long-term care in pediatric patients receiving grafts from deceased donor patients. Maintenance immunosuppressive therapies have narrow therapeutic indices and are associated with high inter-individual and intra-individual variability.Areas covered: In this review, we examine the impact of pharmacokinetic variability on renal transplantation and its association with age, genetic polymorphisms, drug-drug interactions, drug-disease interactions, renal insufficiency, route of administration, and branded versus generic drug formulation. Pharmacodynamics are outlined in terms of the mechanism of action for each immunosuppressant, potential adverse effects, and the utility of pharmacodynamic biomarkers.Expert opinion: Acquiring abetter quantitative understanding of immunosuppressant pharmacokinetics and pharmacodynamic components should help clinicians implement treatment regimens to maintain the balance between therapeutic efficacy and drug-related toxicity.

Sirolimus Pharmacokinetics Variability Points to the Relevance of Therapeutic Drug Monitoring in Pediatric Oncology

Sirolimus is widely used in transplantation, where its therapeutic drug monitoring (TDM) is well established. Evidence of a crucial role for sirolimus in the PI3K/AkT/mTor pathway has stimulated interest in its involvement in neoplasia, either as monotherapy or in combination with other antineoplastic agents. However, in cancer, there is no consensus on sirolimus TDM. In the RAPIRI phase I trial, the combination sirolimus + irinotecan was evaluated as a new treatment for refractory pediatric cancers. Blood sampling at first sirolimus intake (D1) and at steady state (D8), followed by LC/MS² analysis, was used to develop a population pharmacokinetic model (Monolix^® software). A mono-compartmental model with first-order absorption and elimination best fit the data. The only covariate retained for the final model was “body surface area” (D1 and D8). The model also demonstrated that 1.5 mg/m² would be the recommended sirolimus dose for further studies and that steady-state TDM is necessary to adjust the dosing regimen in atypical profiles (36.4% of the population). No correlation was found between sirolimus trough concentrations and efficacy and/or observed toxicities. The study reveals the relevance of sirolimus TDM in pediatric oncology as it is needed in organ transplantation.

Sirolimus Therapy and Follow-up in a Patient with Severe Congenital Hyperinsulinism Following Subtotal Pancreatectomy

Congenital hyperinsulinism (CHI) is the most common cause of severe, persistent hypoglycemia in neonates and infants. If the patient does not respond to medical treatment the currently available treatment is subtotal pancreatectomy, but some patients still experience severe hypoglycemia after surgery. Sirolimus, a mammalian target of rapamycin inhibitor has recently been reported to be effective in the treatment of insulinoma and CHI patients. Here we report a patient with CHI who had prolonged hypoglycemia after subtotal pancreatectomy. The patient had a heterozygous mutation in ABCC8 but was unresponsive to an optimal dose of diazoxide (15 mg/ kg/day) and octreotide (30 μg/kg/day). The patient subsequently had subtotal pancreatectomy but severe and persistent hypoglycemia continued post-operatively. Sirolimus was commenced. There was a remarkable improvement in glycemic control without major adverse events, although he required a small dose of octreotide to maintain euglycemia. Sirolimus therapy was discontinued when the patient was 15 months old. At the time of this report, at an age of three years and eight months, the patient continues to maintain good glycemic control. This report suggests that sirolimus may be an effective treatment option in patients with CHI resistant to established medical therapy or failure of ubtotal pancreatectomy. However, the long-term safety requires study in larger groups of very young patients.

Pharmacokinetics of nanotechnology-based formulations in pediatric populations

The development of therapeutics for pediatric use has advanced in the last few decades. However, off-label use of adult medications in pediatrics remains a significant clinical problem. Furthermore, the development of therapeutics for pediatrics is challenged by the lack of pharmacokinetic (PK) data in the pediatric population. To promote the development of therapeutics for pediatrics, the United States Pediatric Formulation Initiative recommended the investigation of nanotechnology-based delivery systems. Therefore, in this review, we provided comprehensive information on the PK of nanotechnology-based formulations from preclinical and clinical studies in pediatrics. Specifically, we discuss the relationship between formulation parameters of nanoformulations and PK of the encapsulated drug in the context of pediatrics. We review nanoformulations that include dendrimers, liposomes, polymeric long-acting injectables (LAIs), nanocrystals, inorganic nanoparticles, polymeric micelles, and protein nanoparticles. In addition, we describe the importance and need of PK modeling and simulation approaches used in predicting PK of nanoformulations for pediatric applications.

Sirolimus: Efficacy and Complications in Children With Hyperinsulinemic Hypoglycemia: A 5-Year Follow-Up Study

Introduction

Sirolimus, a mammalian target of rapamycin inhibitor, has been used in congenital hyperinsulinism (CHI) unresponsive to diazoxide and octreotide. Reported response to sirolimus is variable, with high incidence of adverse effects. To the best of our knowledge, we report the largest group of CHI patients treated with sirolimus followed for the longest period to date.

Methods

Retrospective study of CHI patients treated with sirolimus in a tertiary service and review of the 15 publications reporting CHI patients treated with mammalian target of rapamycin inhibitors. Comparison was made between the findings of this study with those previously published.

Results

Twenty-two CHI patients treated with sirolimus were included in this study. Twenty showed partial response, one showed complete response, and one was unresponsive. Five of the partially/fully responsive patients had compound heterozygous ABCC8 mutations and five had heterozygous ABCC8 mutations. A total of 86.4% (19/22) developed complications, with infection being the most frequent (17/22), of which 11 were of bacterial etiology, followed by persistent diarrhea (3/22) and hyperglycemia (2/22). Seventeen patients stopped sirolimus: 13 from infections; 2 from hyperglycemia; and 2 from alternative treatment (lanreotide) response. Compared with data previously published, our study identified a higher number of partially sirolimus-responsive CHI cases, although the high rate of complications while on this medication limited its potential usefulness.

Conclusion

Sirolimus candidates must be carefully selected given its frequent and potentially life-threatening side effects. Its use as a short-term, last-resort therapy until normoglycemia is achieved with other agents such as lanreotide could avoid pancreatectomy. Further studies evaluating the use of sirolimus in patients with CHI are required.

Sirolimus-Induced Hepatitis in Two Patients with Hyperinsulinemic Hypoglycemia

Sirolimus has been reported to be effective in the treatment of the diffuse form of congenital hyperinsulinism (CHI), unresponsive to diazoxide and octreotide, without causing severe side effects. Two newborns with CHI due to homozygous ABCC8 gene mutations were started on sirolimus aged 21 and 17 days, due to lack of response to medical treatment. A good response to sirolimus was observed. At follow-up after ten and two months of treatment, liver enzymes were found to be increased [serum sirolimus level 1.4 ng/mL (normal range: 5-15), aspartate aminotransferase (AST): 298U/L, alanine aminotransferase (ALT): 302U/L and serum sirolimus level: 9.9 ng/mL, AST: 261U/L, ALT: 275U/L, respectively]. In Case 1, discontinuation of the drug resulted in normalization of liver enzymes within three days. Two days after normalization, sirolimus was restarted at a lower dose, which resulted in a repeated increase in transferases. In Case 2, a reduction of sirolimus dose caused normalization of liver enzymes within ten days. When the dose was increased, enzymes increased within three days. Sirolimus was discontinued in both cases. The rapid normalization of liver enzyme levels after sirolimus withdrawal or dose reduction; elevation of transaminases after restart or dose increase and rapid normalization after sirolimus withdrawal were findings strongly suggestive of sirolimus-induced hepatitis. To the best of our knowledge, this is the first report of sirolimus-induced hepatitis in CHI. Sirolimus is a promising drug for CHI patients who are unresponsive to medical treatment, but physicians should be vigilant for adverse effects on liver function.

Sirolimus and tacrolimus coefficient of variation is associated with rejection, donor-specific antibodies, and nonadherence

BACKGROUND

Immunosuppression medication nonadherence has been associated with donor-specific antibodies and treatment-refractory rejection. Drug-level monitoring is a practical direct marker for nonadherence, as variations indicate erratic ingestion of medication. We previously reported that high variability in tacrolimus trough levels determined by the percent coefficient of variation (CV %) and standard deviation (SD) were associated with biopsy-proven rejection. We hypothesized that the CV % and SD in patients on a sirolimus/low-dose tacrolimus regimen may associate with self-reported medication nonadherence, rejection and donor-specific antibodies.

METHODS

In this pilot feasibility study, we studied 37 biopsies in 23 pediatric renal transplant patients on both sirolimus and tacrolimus immunosuppression; CV %, SD, de novo donor-specific antibodies, rejection, and self-reported adherence were examined.

RESULTS

A cut-off sirolimus CV % of 25 maximized the percentage of biopsies correctly classified as rejection (32 of 37, or 86 %, p = 0.001). A cut-off tacrolimus CV % of 31 maximized the percentage of correctly classified biopsies (25 of 37, or 68 %, p = 0.09). Among patients with both high sirolimus and tacrolimus CV %, 67 % developed de novo donor-specific antibodies (p = 0.002) with a DQ predominance and 71 % reported nonadherence (p = 0.05).

CONCLUSIONS

In pediatric renal transplantation, sirolimus and tacrolimus CV % is a potential tool for monitoring patients at risk for allograft rejection and donor-specific antibodies secondary to medication nonadherence.

Therapeutic drug monitoring in pediatric renal transplantation

Finding the balance between clinical efficacy and toxicity of immunosuppressive drugs is a challenge in renal transplantation (RTx), but especially in pediatric RTx patients. Due to the expected longer life-span of pediatric transplant patients and the long-term consequences of drug-induced infectious, malignant and cardiovascular adverse effects, protocols which minimize immunosuppressive therapy make conceptual sense. In this context, therapeutic drug monitoring is a tool which provides support for the individualization of therapy. It has, however, limitations, and specific data in the pediatric cohort are comparatively sparse. There is large heterogeneity among the studies conducted to date in terms of methods, follow-up, endpoints, immunosuppressive regimens and patients. In addition, data from adult studies are not readily transferrable to the pediatric situation. This educational review gives a concise overview on aspects of therapeutic drug monitoring in pediatric RTx.

mTOR inhibitors in pediatric kidney transplantation

The mammalian target of the rapamycin (mTOR) inhibitors sirolimus and everolimus are increasingly being used in pediatric kidney transplantation in different combinations and doses. Several studies have shown beneficial effects of using mTOR inhibitors in children after pediatric renal transplantation. A switch to a low-dose calcineurin inhibitor (CNI) and mTOR inhibitor has been proven to stabilize the glomerular filtration rate. Additionally, de novo studies using a low-dose CNI and an mTOR inhibitor have shown good graft survival and a low number of rejections. Side effects of mTOR inhibitors, such as hyperlipidemia, wound healing problems, and proteinuria, mainly occur if high doses are given and if treatment is not combined with a CNI. Lower doses of mTOR inhibitors do not result in growth impairment or reduced testosterone levels. Treatment with mTOR inhibitors is also associated with a lower number of viral infections, especially cytomegalovirus. Due to their antiproliferative effect, mTOR inhibitors could theoretically reduce the risk of post-transplant lymphoproliferative disease. mTOR inhibitors, especially in combination with low-dose CNIs, can safely be used in children after kidney transplantation as de novo therapy or for conversion from CNI- and mycophenolate mofetil-based regimens.

Population pharmacokinetics of sirolimus in pediatric patients with neurofibromatosis type 1

PURPOSE

The narrow therapeutic index and large interpatient variability in sirolimus pharmacokinetics (PK) make therapeutic drug monitoring necessary. Factors responsible for PK variability are not well understood, and published PK studies do not include pediatric patients with neurofibromatosis type 1 (NF1). The objectives of this study were to estimate sirolimus clearance in a cohort of children with NF1 using data collected in a concentration-guided trial, to evaluate the effect of treatment duration on clearance and dose requirements, and to evaluate the association of sirolimus clearance with patient-specific factors, including age, weight, body surface area (BSA), race, and sex.

METHODS

Sirolimus concentration-time data were collected from an ongoing prospective trial in children with NF1. An iterative 2-stage Bayesian method was used for the PK parameter analyses.

RESULTS

Data from 44 patients with NF1 were included in the analyses. Mean age was 8.4 years (SD 4.5, range 3-18), and mean weight was 29.8 kg (SD 16.7, range 12-85.8). Mean sirolimus clearance was 11.8 L/h (SD 4.6, range 2.2-24.1), and the mean dose to obtain a target trough concentration of 10-15 ng/mL was 2.0 mg/m administered twice daily (SD 0.72, range 0.77-3.85). A nonlinear relationship between age and clearance was observed. Total body weight and BSA were strong predictors of sirolimus clearance (r = 0.67 and 0.65, respectively).

CONCLUSIONS

Sirolimus clearance in children with NF1 is comparable with that in pediatric transplant patients. Clearance was most associated with body size parameters (BSA and total body weight) in children with NF1. When normalized for size, an age effect on clearance was observed in the youngest patients, most likely because of the maturational changes in drug absorption and metabolism. A mean dose of 2.0 mg/m twice a day was required for attainment of target trough concentrations of 10-15 ng/mL in children greater than 3 years of age who have NF1. The updated model will allow PK-guided individualized dosing of sirolimus in patients with NF1.

Sirolimus pharmacokinetics in early postmyeloablative pediatric blood and marrow transplantation

This study examined the pharmacokinetics of sirolimus in pediatric allogeneic blood and marrow transplantation (BMT) recipients in the presence and absence of concomitant fluconazole. Forty pediatric BMT recipients received a daily oral dose of sirolimus and a continuous i.v. infusion of tacrolimus for graft-versus-host disease prophylaxis. Fluconazole was administered i.v. to 19 patients and orally to 6 patients. Full pharmacokinetic profiles of sirolimus within a single dosing interval were collected. Whole-blood sirolimus concentrations were measured by HPLC/mass spectrometry. Noncompartmental analysis was performed using WinNonlin. Nonlinear mixed-effects pharmacokinetic models were developed using NONMEM following standard procedures. The mean ± SD sirolimus trough level before the dose (C0) was 8.0 ± 4.6 ng/mL (range, 1.8-21.6 ng/mL). The peak concentration was 19.9 ± 11.8 ng/mL (range, 3.9-46.1 ng/mL), and the trough level 24 hours later (C24) was 9.1 ± 5.3 ng/mL (range, 1.0-19.1 ng/mL). The terminal disposition half-life (T1/2) was 24.5 ± 11.2 hours (range, 5.8-53.2 hours), and the area under the concentration-versus-time curve (AUC0-24) was 401.1 ± 316.3 ng·h/mL (range, 20.7-1332.3 ng·h/mL). In patients at steady state, C0 and C24 were closely correlated (R(2) = 0.77) with a slope of 0.99, indicating the achievement of steady state. C24 was 1.7-fold greater (P = .036) and AUC0-24 was 2-fold greater (P = .012) in Caucasian patients (n = 22) compared with Hispanic patients (n = 9). The average apparent oral clearance was 3-fold greater (P = .001) and the apparent oral volume of distribution was 2-fold greater (P = .018) in patients age ≤12 years compared with those age >12 years. C24 was significantly lower in patients (n = 10) who developed grade III-IV aGVHD (n = 10) than in those with grade 0-II aGVHD (n = 22) (6.1 ± 2.9 ng/mL versus 9.4 ± 5.5 ng/mL; P = .044). Dose-normalized sirolimus trough concentrations were significantly higher in patients receiving concomitant fluconazole therapy compared with those not receiving fluconazole (C0: 3.9 ± 2.5 versus 2.4 ± 1.5 ng/mL/mg, P = .030; C24: 4.8 ± 3.3 versus 2.5 ± 1.7 ng/mL/mg, P = .018). This pharmacokinetic study of sirolimus in pediatric patients documents a large interindividual variability in the exposure of sirolimus. Steady-state trough blood concentrations were correlated with drug exposure. Trough concentrations were higher with a concomitant use of fluconazole and were higher in Caucasian patients than in Hispanic patients. Oral clearance was greater in children age ≤12 years than in older children and adolescents. With therapeutic drug monitoring, the majority (79%) of sirolimus trough levels could be maintained within the target range (3-12 ng/mL). This study provides a rationale and support for dose adjustments of sirolimus based on steady-state blood concentrations aimed at achieving a target concentration to minimize toxicity and maximize therapeutic benefits in pediatric BMT recipients.

Sirolimus for the treatment of complicated vascular anomalies in children

BACKGROUND

Vascular anomalies comprise a diverse group of diagnoses. While infantile hemangiomas are common, the majority of these conditions are quite rare and have not been widely studied. Some of these lesions, though benign, can impair vital structures, be deforming, or even become life-threatening. Vascular tumors such as kaposiform hemangioendotheliomas (KHE) and complicated vascular malformations have proven particularly difficult to treat.

PROCEDURE

Here we retrospectively evaluate a series of six patients with complicated, life-threatening vascular anomalies who were treated with the mTOR inhibitor sirolimus for compassionate use at two centers after failing multiple other therapies.

RESULTS

These patients showed significant improvement in clinical status with tolerable side effects.

CONCLUSIONS

Sirolimus appears to be effective and safe in patients with life-threatening vascular anomalies and represents an important tool in treating these diseases. These findings are currently being further evaluated in a Phase II safety and efficacy trial.

Sirolimus in pediatric renal transplantation

SRL, an mTOR inhibitor that inhibits cell cycle progression, represents an important alternative to CNIs, which are still the cornerstones of pediatric solid organ tx. Because there are still limited data on SRL use among pediatric solid organ recipients, further studies are needed to verify the efficacy and safety of SRL. It has unique pharmacokinetic characteristics concerning dosing intervals and reduction of the dose in combination with other immunosuppressants. SRL also has antineoplastic, antiviral, and antiatherogenic advantages over other immunosuppressive agents. The adverse effects of SRL including thrombocytopenia, hyperlipidemia, proteinuria, impaired wound healing, mouth ulcers, edema, male hypogonadism, TMA, and interstitial pneumonitis must be considered carefully in pediatric population. This article reviews the most recent data on SRL application in the field of pediatric renal tx.

Developmental pharmacogenetics of immunosuppressants in pediatric organ transplantation

Cyclosporine, tacrolimus, sirolimus, and mycophenolate mofetil are the primary immunosuppressants used on pediatric organ transplantation. Therapeutic drug monitoring is used in daily practice, because their clinical use is hampered by a narrow therapeutic index and large variability. Tailoring immunosuppressive therapy to the individual patient to optimize efficacy and minimize toxicity is therefore essential. Because research in pharmacogenetics already identified polymorphisms impacting their pharmacokinetic parameters in adults, developmental pharmacogenetics of immunosuppressants holds promises for optimizing dosage regimens and improving clinical outcome in children. In this review, we focus on the impact of age and pharmacogenetics on these immunosuppressants in children undergoing organ transplantation.

Randomizing research participants: promoting balance and concealment in small samples

Randomization is central to rigorous scientific trials. An effective but underutilized approach is urn randomization. To test the ability of urn randomization versus simple randomization to produce balanced groups with small sample sizes, we conducted simulated randomizations: 10 times under sample size scenarios of 20, 40, 60 (group sizes of 10, 20, and 30, respectively), for 30 trials in total. For groups of 20-30, urn surpassed simple randomization in the equal distribution of confounding variables between groups, leading to effects of these variables that were both smaller on average and more consistently close to zero over multiple trials. The urn method is easy to implement and has the advantages of unpredictability of assignment and decreased susceptibility to investigator bias.

Immunosuppression armamentarium in 2010: mechanistic and clinical considerations

Effective immunosuppression is the key to successful organ transplantation, with success being defined as minimal rejection risk with concomitant minimal drug toxicities. Despite the general recognition of this fact, a paucity of appropriate clinical trials in children has contributed to lack of standardization of clinical management regimens, resulting in an extensive diversity of favored approaches. Nonetheless, although consensus has not been reached on the ideal approach to immunosuppression in pediatric transplantation, new drug therapies have contributed to a continuing improvement in graft and patient survival. Future clinical research must focus on diminishing the extensive burden of toxicities of these therapeutic agents in children.

Limited sampling strategies for sirolimus after pediatric renal transplantation

SRL has been increasingly used in renal transplantation, but limited sampling approaches for estimation of AUC remain elusive. A post-hoc analysis of 94 PK profiles in 75 patients from four previous studies was performed to generate limited sampling approaches for approximation of AUC based on two to four time points for both BID and OD SRL dosing. AUC was calculated using the trapezoid rule. Stepwise linear regression was performed to generate an abbreviated AUC from the limited sampling approaches. For BID dosing, complete AUC had a strong correlation with the trough levels (r(2) = 0.882, p < 0.0001) and with C2 level (r(2) = 0.9025, p < 0.0001). A three-point and a four-point limited sampling approach showed improved agreement with complete AUC compared with single-point sampling. A convenient and accurate (r(2) = 0.992) four-point limited sampling approach reads: AUC = 10;(1.085 + 0.117 x log C0 + 0.164 x log C1-0.131 x log C2 + 0.823 x log C4). Similarly, complete AUC had a statistically significant correlation with the trough levels (r(2) = 0.549, p < 0.0001) and with C2 level (r(2) = 0.716, p < 0.0001) for OD dosing. The estimation of AUC for OD dosing was improved over single-point sampling (r(2) = 0.951) using the formula: AUC = 10;(1.100 + 0.115 x log C0 + 0.803 x log C4). This study represented the first limited sampling approach for SRL. Further studies are required to determine the optimal SRL target AUC.

Rapamycin-eluting stents in the arterial duct: experimental observations in the pig model

BACKGROUND

Maintaining arterial duct patency by stent implantation may be advantageous in congenital heart disease management algorithms. Rapamycin, an immunosuppressant drug that demonstrates antiproliferative properties and inhibits smooth muscle cell migration, may deter the intimal hyperplasia that occurs during spontaneous closure and after-stent implantation of the arterial duct.

METHODS AND RESULTS

Twenty-eight Yorkshire piglets (7 to 11 days old; weight, 2.2 to 4.9 kg) underwent stent implantation of the arterial duct (rapamycin-eluting (n=14) or bare metal (n=14) stents, 3.5-mm diameter) and were euthanized at 2, 4, and 6 weeks. Dissected arterial ducts were analyzed for lumen diameter, smooth muscle cell, and extracellular matrix components. Isolated arterial duct-derived smooth muscle cells were cultured in the presence or absence of rapamycin. Cellular proliferation rates were assessed by Ki-67 detection and [(3)H]-thymidine incorporation. No significant neointimal proliferation was present in either stent type at 2 weeks. At 4 weeks, the median luminal diameters of the bare metal stents were 87% (P=0.009), 54% (P=0.004), and 77% (P=0.004) that of the drug-eluting stents at the middle and aortic and pulmonary artery ends, respectively. At 6 weeks, the median luminal diameters of the bare metal stents were 0% (P=0.18), 5% (P=0.25), and 61% (P=0.13) that of the drug-eluting stents at the same respective levels. Complete histological occlusion was found in at least 1 level of the lumen in 9 pigs: 1 (17%) in the BMS group at 4 weeks, 5 (83%) in the BMS group at 6 weeks, and 3 (50%) in the DES group at 6 weeks. In vitro studies demonstrated 50%-lower proliferation rates in rapamycin-treated cultures of duct-derived smooth muscle cell cultures (P<0.001).

CONCLUSIONS

Rapamycin has antiproliferative actions on the arterial duct. Drug-eluting stents may be a more efficient tool than current palliative options for maintaining patency in critically duct-dependent states, but there may be a finite time-related benefit.

Calcineurin inhibitor sparing in paediatric solid organ transplantation : managing the efficacy/toxicity conundrum

Despite their efficacy, the calcineurin inhibitors (CNIs) ciclosporin and tacrolimus carry a risk of debilitating adverse effects, especially nephrotoxicity, that affect the long-term outcome and survival of children who are given organ transplants. Simple reduction in dosage of CNI has little or no long-term benefit on their adverse effects, and complete withdrawal without threatening graft outcome may only be possible after liver transplantation. Until the last decade, the only option was to increase corticosteroid and/or azathioprine doses, which imposed additional long-term hazards. Considered here are the emerging generation of new agents offering an opportunity for improving long-term graft survival, minimizing CNI-related adverse events and ensuring patient well-being.A holistic, multifaceted strategy may need to be considered - initial selection and optimized use and monitoring of immunosuppressant regimens, early recognition of indicators of patient and graft dysfunction, and, where applicable, early introduction of CNI-sparing regimens facilitating CNI withdrawal. The evidence reviewed here supports these approaches but remains far from definitive in paediatric solid organ transplantation. Because de novo immunosuppression uses CNI in more than 93% of patients, reduction of CNI-related adverse effects has focused on CNI sparing or withdrawal.A recurring theme where sirolimus and mycophenolate mofetil have been used for this purpose is the importance of their early introduction to limit CNI damage and provide long-term benefit: for example, long-term renal function critically reflects that at 1 year post-transplant. While mycophenolic acid shows advantages over sirolimus in preserving renal function because the latter is associated with proteinuria, sirolimus appears the more potent immunosuppressant but also impairs early wound healing. The use of CNI-free immunosuppressant regimens with depleting or non-depleting antibodies plus sirolimus and mycophenolic acid needs much wider investigation to achieve acceptable rejection rates and conserve renal function. The adverse effects of the alternative immunosuppressants, particularly the dyslipidaemia associated with sirolimus, needs to be minimized to avoid replacing one set of adverse effects (from CNIs) with another. While we can only conjecture that judicious combinations with the second generation of novel immunosuppressants currently in development will provide these solutions, a rationale of low-dose therapy with multiple immunosuppressants acting by complementary mechanisms seems to hold the promise for efficacy with minimal toxicity until the vision of tolerance achieves reality.

Pharmacokinetics of mycophenolate mofetil and sirolimus in children

This review summarizes the pharmacokinetics in children and youths of 2 commonly used immunosuppressive drugs, mycophenolate mofetil (MMF) and sirolimus (Sir), as presented at the IATDMCT 2007 conference. The review focuses on the developmental changes of drug disposition during childhood and adolescence. Regarding mycophenolate mofetil, the authors were unable to demonstrate age dependency of MMF in combination with cyclosporine. By contrast, there was an inverse relationship between age and the dose-normalized mycophenolate (MPA) area-under-the-time-concentration curve (AUC) in children who received concomitant tacrolimus (Tac). Dose-normalized MPA AUCs were higher than commonly observed in adult patients. It can be hypothesized that the age dependency is related to developmental changes in the expression of the UDP-glucuronosyltransferases. Sirolimus half-life and mean residence time (MRT) are shorter than in adults. Similar to that in adults, there is a profound drug-drug interaction between cyclosporine and Sir. In our own experience, Sir was started at 0.13 +/- 0.05 mg/kg/day. The average Sir AUC was 64.9 +/- 29.7 ng*h/mL. The median (range) AUC for each metabolite was as follows: 12-hydroxy-Sir, 7.6 (0.2-18.8); 46-hydroxy-Sir, 3.1 (0.0-12.4); 24-hydroxy-Sir, 4.3 (0.0-12.6); piperidine-hydroxy-Sir, 3.5 (0.0-8.3); 39-desmethyl-Sir, 3.6 (0.0-11.3); 16-desmethyl-Sir, 5.0 (0.1-9.9); and di-hydroxy-Sir, 4.3 (0.0-32.5) ng*h/mL. Of the total metabolite AUC, 77.5% was due to hydroxylated metabolites, while 39-O-desmethyl Sir (the main metabolite in adults) comprised only 8.4% of the metabolites. This is clinically relevant, as 39-O-desmethyl Sir shows 86% to 127% cross-reactivity with the Sir immunoassay. Metabolites reached a median AUC of 60% of that of Sir, but the range was 2.6% to 136%. The age dependency of Sir metabolite formation was confirmed in a human liver microsome model. On the basis of the age dependency of piperidine-hydroxy Sir, the authors postulate that the ontogeny of the drug disposition can be largely explained by developmental changes of the CYP2C8 expression. In conclusion, both Sir and MMF drug disposition vary in children and adolescents from adult patients, most likely because of developmental changes of biliary transporters and metabolic enzymes.

Posttransplant monitoring of pediatric lung transplant recipients

PURPOSE OF REVIEW

This article reviews current trends in pediatric lung posttransplant management, reveals pitfalls that exist, and introduces additional parameters that may have an impact on long-term survival.

RECENT FINDINGS

A number of parameters are monitored after transplantation to prevent or identify early complications related to lung transplantation in hope of reducing morbidity and mortality. These include routine laboratory studies, imaging, and monitoring of drug levels and lung function. Drug monitoring allows individualization of a patient's immunosuppressive therapy; however, drug levels alone may not reflect the patient's immune status. ImmuKnow is a general immune-monitoring assay that may help guide therapy. Two major complications are rejection and infection, and bronchoscopy is used to differentiate these two entities. Silent rejection may occur and increase the chance of developing bronchiolitis obliterans; therefore, many centers perform surveillance bronchoscopies. Recently, de-novo anti-histocompatibility locus antigen antibodies and gastroesophageal reflux have been associated with poor outcomes, and many centers are monitoring these entities as part of care following lung transplant.

SUMMARY

There has been little improvement in long-term outcomes of lung transplantation. Current monitoring methods are utilized to maintain or improve outcomes and recently additional monitoring parameters have been identified which hopefully will improve long-term outcomes.

Optimization of Immunosuppressive Drug Monitoring in Children

BACKGROUND

The metabolism of drugs in children differs from adults, although pediatric pharmacokinetic (PK) studies remain scarce. Many of the drugs are metabolized by polymorphically expressed enzymes (cytochrome P450 [CYP450]; glucuronyl transferase [GT]) and/or transported by drug transporters (ABC and SLC families). In children, there is added complexity because of the age dependency of drug metabolism. This review addresses the age dependency of drug metabolism in childhood on the basis of routine PK monitoring.

METHODS

Standard pharmacokinetic studies in pediatric renal transplant recipients were analyzed to study drug-drug interactions between mycophenolic acid and cyclosporine on the one hand, and tacrolimus and sirolimus on the other hand. The exposure was compared with age. We also studied sirolimus metabolites, both by mass spectrometry as well as using human liver microsomes.

RESULTS

We demonstrated age dependency for MPA exposure. Independent of the concomitant medication, infants required approximately twice as much drug for the same exposure. The drug-drug interaction between sirolimus and tacrolimus demonstrated age dependency. Sirolimus metabolites showed a remarkably different pattern in children. Whereas 39-O-desmethyl sirolimus is the most prevalent metabolite in adults, we found 77.5% hydroxylated metabolites in children. Similarly, pediatric human liver microsomes produced 86.1% hydroxylated metabolites.

CONCLUSIONS

Our long-term objective is to develop evidence-based guidelines for age-appropriate drug dosing of all drugs commonly used during childhood and adolescence, based on pharmacokinetically/pharmacogenetically determined drug exposure to maximize therapeutic yield while minimizing toxicity. The potential need for lifelong medications warrants efforts to minimize toxicity in chronically ill pediatric patients.