Paediatric immunosuppression following solid organ transplantation

Each year in the United Kingdom approximately 240 children benefit from solid organ transplantation.¹ Eleven paediatric centres variously perform kidney, heart, lung, liver and small bowel transplant operations. With current survival data approaching 90% or better at one year for most paediatric transplants, it is probable that at some point, these children will present to their local hospital or general practitioner (GP) for management of non-transplant issues or for shared care. Long term health and survival is dependent on careful adjustment of life long immunosuppressant medication. All of these children will be discharged from hospital on a combination of immunosuppression medications. The aim of immunosuppression is to prevent acute rejection, using a combination of drugs, while avoiding infection and toxicity. The combination of drugs (type and number) will depend on the organ transplanted, the time post-transplant, the patient’s rejection history, and side effects. There is no “one size fits all” immunosuppression protocol. Drugs and doses are adjusted according to clinical need. Children are seen regularly by the transplant centre during the first three months post-transplant, during which time most will be stabilised on their medication. This article focuses on maintenance immunosuppression that paediatricians, GPs, nurses, and pharmacists may encounter while caring for children in hospital or in the community.

EVIDENCE IN SOLID ORGAN TRANSPLANTATION

In adult transplantation, there are many more randomised controlled trials (RCTs) published in the field of renal transplantation than there are in the other fields of transplantation. This is probably because enough kidney transplants are performed to recruit a sufficient number of patients to the trials. In heart transplantation, there are only four randomised controlled studies despite the first operations being done back in 1967. Drug immunosuppression protocols have evolved based on results seen in renal transplantation, and empirical observation. Generally, immunosuppression protocols include a calcineurin inhibitor, an anti-metabolite, and steroids. Whether to use induction immunosuppression with ATG (anti-thymocyte globulin) or interleukin-2 receptor antagonists, such as basiliximab or daclizumab, is disputed territory—it is standard protocol in some institutions but not in others.^2–4 The combinations of drugs and variations in the doses, formulations, timing, and weaning schedules are almost endless, which makes comparisons between trials difficult. The primary efficacy end-point in immunosuppression clinical trials is the incidence of clinical or biopsy-proven acute rejection in the first six months or one year. Graft survival and patient survival are also reported. Secondary end points usually include the incidence of side-effects, such as infection, renal dysfunction, diabetes, and lymphoproliferative disease. Paediatric randomised controlled studies in renal and liver transplantation have been conducted.^5–7 However, there are no paediatric randomised controlled studies published in heart, lung or small bowel transplantation. This article discusses the paediatric trial literature, with some references from adult studies when paediatric trials are absent. The main emphasis is on the immunosuppressant drugs.

DRUGS

Calcineurin inhibitors

Ciclosporin was the first calcineurin inhibitor (CNI) to become available in the early 1980s. Its use as part of a triple therapy regimen with azathioprine and steroids was a vast improvement from previous immunosuppression protocols, and represented a major advance in immunosuppression therapy. However, most paediatric transplant centres in the UK have changed to using tacrolimus as first line CNI following results from adult and paediatric studies showing superiority in reducing the incidence of acute rejection, and a more favourable side effect profile. In paediatric renal transplantation, the incidence of acute rejection was significantly lower in the tacrolimus group compared to the ciclosporin group at six months (36.9% v 59.1%; p  =  0.003).⁵ At one year and four years, patient survival was similar between groups. However, graft survival was significantly higher for the tacrolimus group at four years (86% v 69%; p  =  0.025).⁶ Similarly in liver transplantation, children randomised to receive tacrolimus were significantly more likely to be free from rejection at 12 months compared to those in the ciclosporin group (55.5% v 40.2%; p  =  0.0288), though there was no significant difference in patient survival (93.4% v 92.2%; p  =  0.77) or graft survival (92.3% v 85.4%; p  =  0.16).⁷ In both studies, the overall incidence of side effects was comparable between groups. Infection, hypertension, hypomagnesaemia, diabetes, nephrotoxicity, neurotoxicity and gastrointestinal side effects are common to both, with some differences in incidence (table 1). Hirsutism and gum hyperplasia are cosmetic side effects exclusive to ciclosporin^8,9 that are troubling for children and adolescents who may be teased at school, and which may lead to poor compliance.

An RCT comparing tacrolimus to ciclosporin in adult heart transplant recipients showed tacrolimus-based treatment was comparable with ciclosporin for incidence of acute rejection, and patient and graft survival at 12 months, but was associated with a lower incidence of hypertension and hyperlipidaemia.¹⁰ In a three-arm study that randomised heart transplant recipients to receive either tacrolimus/sirolimus, tacrolimus/mycophenolate mofetil (MMF) or ciclosporin/MMF combinations with steroids, there was no statistically significant difference in the incidence of rejection at six months between the three groups (tacrolimus/sirolimus 24.3%, tacrolimus/MMF 22.4%, ciclosporin/MMF 31.6%; p = 0.271). However, at one year there was significantly less rejection in the tacrolimus/MMF group compared to the ciclosporin/MMF group (23.4% v 36.8%; p = 0.029). The incidence of adverse effects was broadly similar across groups, except median serum creatinine and triglyceridaemia, which were both statistically significantly lower in the tacrolimus/MMF group (p = 0.032 and p = 0.028), respectively.¹¹ In a retrospective analysis of paediatric heart transplantation, freedom from acute rejection was lower in children treated with ciclosporin compared with tacrolimus although the difference did not reach significance (40% v 56%; p  =  NS). More severe rejection episodes occurred in ciclosporin treated children, some of which were successfully managed by switching to tacrolimus. The incidence of infection and serum creatinine at follow up were not significantly different between groups. This study is limited by the small number of enrolments and comparison between immunosuppression era.¹²

Other paediatric reports show that tacrolimus allowed earlier weaning of steroids without an increase in rejection,¹³ and lower risk of hyperlipidaemia.¹⁴ In adult lung transplantation, freedom from acute rejection was significantly higher in tacrolimus treated patients compared to ciclosporin when combined with azathioprine (10% v 0%; p < 0.01).¹⁵ A later lung transplant study¹⁶ did not show a significant difference in incidence of acute rejection in tacrolimus/MMF treated adults compared to ciclosporin/MMF combination, possibly because of the small number of enrolments. However, rejection, expressed as the mean number of acute rejection episodes per 100 patient-days, was lower than that observed in the previous study using azathioprine,¹⁵ suggesting that MMF combinations are more potent. In both studies, overall infection rates were not significantly different between groups, although there was a trend towards a significant higher infection rate in the ciclosporin groups. Tacrolimus is used in paediatric lung¹⁷ and small bowel transplantation.¹⁸

Tacrolimus is a large macrolide molecule produced by Streptomyces tsukubaensis. Its immunosuppressant effect occurs through blocking synthesis of interleukin-2, a cytokine necessary for T cell activation and proliferation. It binds to intracellular protein FKBP-12, which in turn blocks activation of calcineurin, an enzyme necessary for interleukin-2 transcription. Ciclosporin is structurally different to tacrolimus, and does not bind to FKBP-12, but inhibits calcineurin through an alternative pathway. Both drugs are characterised by incomplete and variable absorption, have large volumes of distribution,¹⁹ and undergo extensive metabolism at the gut wall and in the liver. The cytochrome P450 isozyme 3A4 is the most important for metabolism and drug interactions (table 2). Oral bioavailability may be improved by taking medication on an empty stomach. However, this can be problematic for patients and families, and in the cardiothoracic transplant programme at our institution, we advise them to take medication the same way with respect to food, and the dose will be adjusted according to the blood concentration. Other transplant centres may insist on administration on an empty stomach.

Both tacrolimus and ciclosporin have a narrow therapeutic range. Blood tests are done frequently early post-transplant when drug whole blood concentrations may be erratic because of changes in gut function, liver function and diet, and interacting medications are added or stopped. The target concentration range will depend on the organ transplanted, the time post-transplant, the patient’s rejection history, concomitant immunosuppression drugs, and side effects. Generally for tacrolimus, the therapeutic range is 5–20 ng/ml, and for ciclosporin 100–450 ng/ml. Doses are given twice daily. High inter- and intra-patient variability in absorption and metabolism means that doses are adjusted according to blood concentration. The dose range for tacrolimus could be 0.25–6 mg twice a day, and for ciclosporin 6.25–350 mg twice a day. Children clear drugs more rapidly than adults. Where therapeutic concentrations cannot be achieved with high doses, it may be necessary to give a dose three times a day (ciclosporin) or add in an interacting drug that will inhibit hepatic metabolism, such as erythromycin or itraconazole (tacrolimus).

Ciclosporin is available as soft gel capsules and an oral liquid. Tacrolimus is available as capsules. There is no licensed oral liquid for tacrolimus, though some “specials” manufacturers now offer a tacrolimus suspension. The formulation for tacrolimus suspension may differ from manufacturer to manufacturer, and thus absorption profiles may not be comparable. Because of the serious issues associated with toxicity or subtherapeutic concentrations with different absorption profiles, not all transplant programmes use tacrolimus oral liquid for infants and children who cannot swallow capsules. In this setting, parents use the technique used before tacrolimus liquid was manufactured, and are shown how to open capsules, disperse the contents in a volume of water, and give the correct proportion for the dose.

Anti-metabolites

Azathioprine has been used in transplantation medicine since the 1960s and is still the preferred agent in combination with CNIs in UK renal transplant programmes. A paediatric renal transplant study compared a ciclosporin/MMF combination with historical controls taking ciclosporin/azathioprine. This study showed no significant advantage for MMF over azathioprine in terms of incidence of acute rejection, number of rejection episodes or glomerular filtration rate at six and 12 months.²¹ There was, however, a trend towards significance for the MMF group for freedom from rejection. The study was limited by the small number of enrolments, and was insufficiently powered to detect significant differences. Adult studies comparing MMF with azathioprine in renal transplantation are conflicting,²² but there is a trend towards superiority for MMF.^23,24 In UK paediatric renal transplant programmes, MMF is second line, and is commenced if there is rejection while on azathioprine, or side effects intervene. In paediatric liver transplantation, MMF may be started if rejection occurs on tacrolimus and prednisolone, or to minimise renal toxicity associated with CNIs.²⁵

In heart transplantation, adult studies indicate that MMF is superior to azathioprine for prevention of acute rejection and improved survival after 12 months.^26,27 Furthermore, at 36 months, significantly fewer patients in the MMF group had died or needed a retransplant compared to azathioprine (11.8% v 18.3%; p < 0.01).²⁸ Leucopenia was observed with both drugs but was more common with azathioprine. Diarrhoea and opportunistic infections were more common with MMF.^22,26 In a retrospective adult lung transplant study, tacrolimus/MMF combination was superior to tacrolimus/azathioprine in preventing acute rejection. The incidence of infection was not different between groups (table 3).²⁹

Azathioprine is converted to 6-mercaptopurine in T and B cells where it inhibits synthesis of purine analogues required for DNA production. The dose is 1–3 mg/kg or 45 mg/m² orally once a day. Some individuals produce defective thiopurine methyltransferase, the key enzyme responsible for metabolising azathioprine. Such patients show an exaggerated myelotoxic response to azathioprine at normal doses. Genotype screening to identify at risk individuals is not routinely performed, but can be used to guide treatment in patients with severe leucopenia. MMF inhibits purine synthesis by blocking inosine monophosphate dehydrogenase, an enzyme in the predominant pathway for T and B cell DNA synthesis.³⁰ Oral absorption is > 90%. MMF is converted to its active form mycophenolic acid (MPA) in the liver. MPA is further metabolised by glucuronidation and undergoes hepatic recirculation and conversion back to MPA in the gut, where it is reabsorbed, and finally eliminated in the kidney.

MMF is given orally or intravenously twice a day. Food delays the onset of absorption but does not affect the extent of absorption. When gastrointestinal side effects are problematic, symptoms may improve when the total daily dose is given three times a day, and after food. There is some evidence in renal and heart transplantation that low MPA blood concentrations are associated with a higher incidence of rejection.^31–33 It is our experience that very high doses are needed in children to achieve therapeutic levels, and side effects may prevent achieving this target. The starting dose is 300–600 mg/m² twice a day, rounded up or down to the nearest whole tablet or capsule for children and adolescents. For babies and small children an oral liquid is commercially available.

Glucocorticoids

Methylprednisolone and prednisolone have been part of immunosuppression protocols since the first transplants. Steroids have multiple mechanisms of action. They bind intracellular steroid receptors that influence intracellular biochemical pathways by inducing or inhibiting protein synthesis. In B and T cells, reduced synthesis of cytokines, down regulation of adhesion molecules, up regulation of inhibitory growth factor, and interference with signal pathways are mechanisms for reducing lymphocyte proliferation and migration.³⁴ The side effects of steroids (table 4) have prompted clinicians to try steroid-free, steroid sparing and fast wean protocols.^35–38 In UK paediatric kidney and lung transplant programmes, oral prednisolone is tapered to a low maintenance dose. In liver transplantation, steroids can be stopped when there is no concern about autoimmune hepatitis. In heart transplantation, prednisolone is tapered and stopped. There is a significant survival advantage in paediatric heart transplantation for those children who are not taking steroids at 12 months,³⁹ and it is the practice in our heart transplant programme to wean and stop steroids by six months if no rejection has been detected on routine biopsy.

TOR inhibitors

Despite similar sounding names, sirolimus and everolimus differ from tacrolimus in their mechanism of action and their side effect profiles. Where tacrolimus inhibits interleukin-2 synthesis, sirolimus and everolimus inhibit TOR (target of rapamycin) inside T and B cells which in turns stops the interleukin- 2 signal, and cell cycle progression is halted. In addition to immunosuppression activity, sirolimus and everolimus inhibit proliferation of vascular smooth muscle.⁴⁰ This property is of interest in heart transplantation for the prevention of coronary allograft vasculopathy, a form of chronic rejection and the leading cause of late deaths.^41,42 In kidney, liver, lung and heart transplantation, sirolimus has been used in uncontrolled studies in children who have CNI nephrotoxicity.⁴³ Nephrotoxicity, neurotoxicity, hypertension and glucose intolerance are not usually observed with TOR inhibitors. The main side effects are hypercholesterolaemia, infection, acne, mouth ulcers, pneumonitis, gastrointestinal side effects and wound dehiscence.

Sirolimus has a licence for the prevention of rejection in renal transplantation in addition to ciclosporin and steroids. It is available in the UK as tablets and oral liquid. It is metabolised via cytochrome P450 3A4 and potential drug interactions are similar for tacrolimus and ciclosporin. It is given once daily. Doses could be between 0.5–6 mg once a day, and are adjusted according to blood concentration. Target whole blood concentrations for sirolimus differ depending on the organ, concomitant immunosuppressant drug treatment and the time post-transplant, but could be between 5–20 ng/ml. Everolimus differs from sirolimus by the addition of an –O—OH functional group, which confers a higher clearance than sirolimus. Everolimus has been studied in heart transplantation,⁴² but does not yet have a licence in the UK.

LATE DRUG-INDUCED PROBLEMS

Renal failure, osteoporosis, diabetes and post-transplant lymphoproliferative disease are serious late complications of immunosuppression therapy.^44–47 Drug doses are reduced, and drug combinations are individualised, to minimise toxicities.

CONCLUSIONS

Tacrolimus is superior to ciclosporin for reducing the incidence of acute rejection following solid organ transplantation, and has a more favourable side effect profile. MMF is being used more frequently in paediatric heart transplantation, and is used second line to manage rejection in renal, lung and liver transplantation. CNI nephrotoxicity is a major long term problem. Immunosuppressant combinations are adjusted to minimise nephrotoxicity without compromising rejection—for example, the use of TOR inhibitors. Immunosuppression is tailored to individual and clinical need.