S. Webber

Pediatric Transplantation in the United States, 1995–2004

Solid organ transplantation is accepted as a standard lifesaving therapy for end‐stage organ failure in children. This article reviews trends in pediatric transplantation from 1996 to 2005 using OPTN data analyzed by the Scientific Registry of Transplant Recipients. Over this period, children have contributed significantly to the donor pool, and although the number of pediatric donors has fallen from 1062 to 900, this still accounts for 12% of all deceased donors. In 2005, 2% of 89 884 candidates listed for transplantation were less than 18 years old; in 2005, 1955 children, or 7% of 28 105 recipients, received a transplant. Improvement in waiting list mortality is documented for most organs, but pretransplant mortality, especially among the youngest children, remains a concern. Posttransplant survival for both patients and allografts similarly has shown improvement throughout the period; in most cases, survival is as good as or better than that seen in adults. Examination of immunosuppressive practices shows an increasing tendency across organs toward tacrolimus‐based regimens. In addition, use of induction immunotherapy in the form of anti‐lymphocyte antibody preparations, especially the interleukin‐2 receptor antagonists, has increased steadily. Despite documented advances in care and outcomes for children undergoing transplantation, several considerations remain that require attention as we attempt to optimize transplant management.

Epstein–Barr virus load monitoring: its role in the prevention and management of post-transplant lymphoproliferative disease

Abstract: The Epstein–Barr virus load in the peripheral blood at the time of diagnosis of post‐transplant lymphoproliferative disease (PTLD) is elevated 1000‐ to 10,000‐fold compared to the level detected in normal latency. With the use of quantitative polymerase chain reaction (PCR), changes in the viral load over time can be measured with a two‐ to fourfold accuracy. This has allowed early detection of first‐time infections and reactivations that may lead to PTLD and has provided an opportunity to intervene before symptomatic disease has occured. Viral load monitoring has also been used to follow patients with PTLD and, along with other parameters, provided an assessment of the effectiveness of therapeutic protocols. Viral load monitoring has led to the discovery that at least two‐thirds of transplant recipients become persistent viral load carriers. While the persistent load appears to be largely carried in latently infected memory B cells, more work is needed to clearly define this type of persistent infection and determine the risks associated with it. New diagnostic tests need to be developed to distinguish the persistent latent viral loads from viral loads that are likely to become symptomatic PTLD.

Chronic High Epstein-Barr Viral Load State and Risk for Late-Onset Posttransplant Lymphoproliferative Disease/Lymphoma in Children

Increased use of serial EBV‐PCR monitoring after pediatric transplantation has led to the identification of asymptomatic patients who carry very high viral loads over prolonged periods. The significance of this high‐load state is unknown. We speculated that this state may identify patients at high risk for development of late PTLD/lymphoma. We reviewed data on 71 pediatric heart recipients who had serial viral load monitoring since 1997. Chronic high‐load state was defined as the presence of >16 000 genome copies/mL whole blood on ≥50% of samples over at least 6 months. Among 20 high‐load carriers (eight following prior PTLD, seven with prior symptomatic EBV infection, five without previous EBV disease), 9 (45%) developed late‐onset PTLD 2.5–8.4 years posttransplant (including with four Burkitts lymphoma). Among 51 controls with low (n = 39) or absent (n = 12) loads, only 2 (4%; p < 0.001 absent/low vs. high load) developed late PTLD/lymphoma. By multivariable analysis, high‐load carrier state (OR = 12.4, 95% CI 2.1–74.4) and prior history of PTLD (OR = 10.7, 95% CI 1.9–60.6) independently predicted late PTLD. A chronic high EBV‐load state is not benign and is a predictor of de novo or recurrent PTLD.

IMPDH1 Gene Polymorphisms and Association With Acute Rejection in Renal Transplant Patients

Inosine 5′‐monophosphate dehydrogenase 1 (IMPDH1) catalyzes the rate‐limiting step of the de novo pathway for purine synthesis and is a major target of the immunosuppressive drug mycophenolic acid (MPA). Few variants of the IMPDH1 gene have been reported. The objective of this study was to identify and characterize IMPDH1 variants to determine whether genetic variation contributes to differences in MPA response and toxicity in transplant patients. Seventeen genetic variants were identified in the IMPDH1 gene with allele frequencies ranging from 0.2 to 42.7%. In this study, 191 kidney transplant patients who received mycophenolate mofetil were genotyped for IMPDH1. Two single‐nucleotide polymorphisms, rs2278293 and rs2278294, were significantly associated with the incidence of biopsy‐proven acute rejection in the first year post‐transplantation. Future studies of the multifactorial nature of acute rejection must consider IMPDH1 polymorphisms in MPA‐treated patients.

Pharmacokinetics of Sirolimus and Tacrolimus in Pediatric Transplant Patients

To evaluate the pharmacokinetics of Sirolimus (SRL) and Tacrolimus (TAC) in pediatric transplant recipients. Fifty‐one SRL and 55 TAC pharmacokinetic profiles were obtained from 20 male and 14 female recipients of liver alone (LTx, n = 23), small bowel alone, and with liver (SBTx, n = 11). The median age was 13.3 years (range 0.9–23.9). Whole blood concentrations of SRL and TAC were determined by liquid chromatography‐mass spectrometry (LC‐MS) and microparticulate enzyme immunoassay (MEIA‐Abbott Laboratories, IL), respectively. Pharmacokinetic (PK) parameters were derived from noncompartmental model analysis. The half‐life was estimated using the data points during the terminal disposition phase and area under the concentration (AUC) time curve was calculated within a dosing interval using the trapezoidal method. The dose of SRL or TAC did not correlate with the corresponding AUCs. Trough concentrations of SRL and TAC correlated well with AUC (r = 0.8544 and 0.8603, respectively). Half‐life (h) did not differ significantly between different transplant groups for SRL and TAC (SRL: LTx: 21.2 h, SBTx: 19.3 h; TAC: LTx: 14.1 h, SBTx: 12.7 h). Serial PK analysis with the first measurement within 14 days after transplantation revealed no difference in AUC/dose/BSA for SRL, with time. During this period, SRL half‐life increased significantly, from 11.2 ± 1.0–20.1 ± 3.1 h (n = 4; p = 0.02). After introduction of SRL, TAC half‐life did not change (11.6 ± 3.9–14.0 ± 10.4, n = 10, P = 0.52) in all the groups analyzed together. TAC AUC/dose/BSA decreased significantly in LTx and SBTx patients (90.9 ± 55.3 vs. 48.8 ± 27.3). Trough concentration measurements provide good estimates of SRL and TAC exposure in pediatric transplant recipients. The short half‐life of SRL in children may support twice‐daily administration early after liver and small intestinal transplantation. During conversion of subjects from TAC to combined TAC and SRL, aggressive therapeutic drug monitoring must be used to individualize therapy and avoid serous adverse events.

A novel variant L263F in human inosine 5'-monophosphate dehydrogenase 2 is associated with diminished enzyme activity.

Background and objective Inosine 5′-monophosphate dehydrogenase 2 is required for purine synthesis in activated lymphocytes. Variants in the IMPDH2 gene may account for the large inter-individual variability in baseline enzyme activity, immunosuppressive efficacy and side effects in transplant recipients receiving mycophenolic acid. Therefore, the objective of this study was to identify and functionally characterize IMPDH2 variants. Methods DNA samples from 152 solid organ transplant patients were screened at exons and exon/intron junctions of the IMPDH2 genes by PCR amplification followed by bidirectional direct DNA sequencing. Genetic variant was constructed by site-directed mutagenesis and transformed to an inosine 5′-monophosphate dehydrogenase-deficient strain of Escherichia coli h712. Proteins were purified to homogeneity and the enzymatic activity was measured by reduced nicotinamide adenine dinucleotide production. Results Nine genetic variants were identified in the IMPDH2 gene, with frequencies of the rarer alleles ranging from 0.5 to 10.2%. A novel nonsynonymous variant L263F was identified, and the kinetic assay demonstrated that the inosine 5′-monophosphate dehydrogenase activity of L263F variant was decreased to 10% of the wild-type. The Ki for mycophenolic acid inhibition of the L263F variant was comparable with the wild-type, and the variant Km for inosine 5′-monophosphate and nicotinamide adenine dinucleotide did not change significantly. Conclusions IMPDH2 has low genetic diversity, but the nonsynonymous variant L263F has a significant impact on inosine 5′-monophosphate dehydrogenase activity. This novel functional variant may be one of the factors contributing to the inter-individual difference of baseline inosine 5′-monophosphate dehydrogenase activity as well as drug efficacy and adverse events in transplant patients.

Heart transplantation in children: Indications*

Abstract: This review details the indications for heart transplantation in children. Contraindications have evolved from absolute to relative. Controversial issues remain and this paper represents a consensus of more than a dozen centers that have programs that remain active performing pediatric heart transplants.

A 16-Year Multi-Institutional Study of the Role of Age and EBV Status on PTLD Incidence Among Pediatric Heart Transplant Recipients

The objective was to determine the incidence and hazard for posttransplant lymphoproliferative disease (PTLD) in a study of 3170 pediatric primary heart transplants between 1993 and 2009 at 35 institutions in the Pediatric Heart Transplant Study. 147 of 151 reported malignancy events were classified as PTLD. Overall freedom from PTLD was 98.5% at 1 year, 94% at 5 years and 90% at 10 years. Freedom from PTLD was lowest in children (ages 1 to < 10 years) versus infants (<1 year) and adolescents (10 to < 18 years) with children at highest risk for PTLD with a relative risk of 2.4 compared to infants and 1.7 compared to adolescents. Positive donor EBV status was a strong risk factor for PTLD in the seronegative recipient, but risk magnitude was dependent on recipient age at the time of transplantation. Nearly 25% of EBV seronegative recipients of EBV+ donors at ages 4–7 at transplantation developed some form of PTLD. The overall risk for PTLD declined in the most recent transplant era (2001–2009, p = 0.003). These findings indicate that EBV status and the age of the recipient at the time of transplantation are important variables in the development of PTLD in the pediatric heart transplant recipient.

Cytokine gene polymorphisms in children successfully withdrawn from immunosuppression after liver transplantation

BACKGROUND Cytokine genetic polymorphisms have been associated with transplant outcome in some experimental and clinical studies, but the cytokine profile of patients who are clinically tolerant has not been investigated. AIM Allelic variations in tumor necrosis factor (TNF)-alpha, interferon (INF)-gamma, transforming growth factor (TGF)-beta1, interleukin (IL)-6, and IL-10 were evaluated in patients successfully withdrawn from immunosuppression. METHODS Pediatric liver transplant recipients who were successfully withdrawn from immunosuppression (n=12) or who are on minimal immunosuppression (n=7) were genotyped. A control group of liver recipients who required maintenance immunosuppression served as a control group (n=37). RESULTS Compared to the control group, low TNF- alpha and high/intermediate IL-10 profiles were seen in all 12 children maintained off immunosuppression and in 6 of 7 children requiring minimal immunosuppression. CONCLUSION Children successfully maintained off immunosuppression are more likely to have a genetic predisposition toward low TNF-alpha and high/intermediate IL-10 production. Children maintained on minimal immunosuppression exhibit a similar cytokine profile to those successfully weaned.

Post-transplant Burkitt lymphoma is a more aggressive and distinct form of post-transplant lymphoproliferative disorder†

Although the literature reports a low incidence of Burkitt lymphoma (BL) as a post‐transplant lymphoproliferative disorder (PTLD), this entity appears to be different from other monomorphic PTLDs (M‐PTLDs), both in its aggressive clinical presentation and its distinct pathologic profile.