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Minimization of immunosuppressionTransplant immunology
Jignesh Patel, Jon A. Kobashigawa ⁎Division of Cardiology, Department of Medicine, David Geffen School of Medicine at UCLA, Los Angeles, CA, United States
⁎ Corresponding author. 100 UCLA Medical Plaza, SuiUnited States. Tel.: +1 310 794 1200; fax: +1 310 794 121
Induction of tolerance, wh Received 9 October 2008Accepted 9 October 2008
Keywords:Heart transplantationImmunosuppressionCalcineurin inhibitorsProliferation signal inhibitorsCorticosteroidsRejection
ich obviates the need for maintenance immunosuppression following organtransplantation remains elusive. In cardiac transplantation, ongoing immunosuppressive therapy is essentialto ensure long-term graft survival. Although drug regimens have substantially improved in recent years, theiradverse effects continue to cause significant morbidity and affect quality of life. Newer immunosuppressivetherapies have been effective at reducing allograft rejection rates in the short term but long term outcomeshave changed little in the last two decades. Minimization of immunosuppression requires appreciation of thepotential consequence. High risk patients in particular need to be identified and excluded from low intensityimmunosuppressive regimens. A variety options exist for lowering of immunosuppression and steroidweaning has now become common practice with about 40% of all cardiac transplant recipients remainingsteroid free in the long term. Minimization of calcineurin inhibitor exposure may be achieved withconcurrent use of the more potent anti-proliferative agents mycophenolate mofetil and sirolimus. Patientsrequire close monitoring for rejection during weaning. In addition to the conventional clinical parameterswhich include therapeutic drug monitoring, endomyocardial biopsy and echocardiography, newertechniques for monitoring hold future promise. These include detection of circulating alloantibodies andquantitative measurement of the net state of immunosuppression (Cylex®). However, the efficacy of thesemodalities requires further investigation.
The principal goal of immunosuppressive therapy in cardiactransplantation is to maintain a fine balance of a quiescent state whichminimizes the risk of allograft rejectionorgraft dysfunctionbyachievingadequate immunosuppression, yet also ensures that this level ofimmunosuppression does not contribute to long term morbidity(Fig. 1). The early years of cardiac transplantation were characterizedby suboptimal immunosuppression regimens which led to eitheroverwhelming infection or allograft rejection. Short-term survivalrates were therefore compromised [1]. Immunosuppression regimensconsisted of equine polyclonal antithymocyte globulin (ATG), predni-soneand azathiprine. These agents hadpoor specificityandawide rangeof immune responses were suppressed, leading to an increase in theincidence of opportunistic infections which included fungi, protozoaand viruses. Limiting the use of ATG resulted in fairly high rejection rateswith prednisone and azathioprine (AZA) combination alone unlesshigher doses were used.
te 630, Los Angeles, CA 90095,1.gawa).
l rights reserved.
The introduction of calcineurin inhibitors (CNI) substantiallychanged the picture. Infection rates decreased significantly as hostresponses to bacterial and fungal infection were relatively preserved.Rejection rates also sharply declined and improvement in survival ratespermitted cardiac transplantation as a practical therapeutic option forend-stage heart disease. Subsequent improvements in donor selection,organpreservation, surgical technique and immunosuppression have allled to significantly improved outcomes such that one year survival ratesare now in excess of 85% and 5 year survival is above 70% [2]. Despitethese improvements, the need for post-transplant ongoing immuno-suppression remains paramount as the holy grail of tolerance inductionremains elusive. Drugs have certainly evolved to minimize long termadverse effects — nonetheless complications from long-term immuno-suppression continue to have a significant impact on post-transplantmorbidity andmortality (Fig. 2). There has been a gradual improvementin the half-life of graft survival, but causes of long-term morbidity andmortality after cardiac transplantation have not changed in a decade [2].International Society of Heart and Lung Transplant (ISHLT) Registry datashow that by 8 years after transplantation, virtually all patients havehypertension, 40% have diabetes, and 40% have angiographic cardiacallograft vasculopathy (CAV); renal insufficiency is common, with long-term dialysis required in 10% of patients. The three leading causes ofmortality and morbidity remain unchanged. CAV and unexplained graft
Fig. 1. The principal goal of immunosuppressive therapy in cardiac transplantation is tomaintain a quiescent state so as to minimize the risk of allograft rejection or graftdysfunction by achieving adequate immunosuppression, yet also ensuring that thislevel of immunosuppression does not contribute to long term morbidity.
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failure (probably due to CAV) account for 30% of mortality; malignancyaccounts for 22% ofmortality; and renal failure is the third cause of long-term morbidity. Infectious complications, acute rejection, and cytome-galovirus infections aremost common in thefirst year after transplanta-tion but less trouble-some in the long-term, and non-cytomegalovirusinfection accounts for only 10% of the late deaths.
Cyclosporine is associated with a number of common side effectsincluding hypertension, dyslipidemia, hirsuitism and chronic renalinsufficiency, with upto 10% of cardiac transplant recipients developingend-stage renal disease. The introduction of micro-emulsion formula-tion improved bioavailability allowing more predictable monitoring tominimize toxicityand enhanceefficacy. The drugwas shown tobebettertolerated and resulted in a reduced number of rejection episodes [3–5].
Tacrolimus has provided an alternative to cyclosporine especiallyin patients experiencing steroid-resistant allograft rejection [6,7]. Itsuse is also associated with a number of adverse effects many of whichare in commonwith cyclosporine but also with a profile portending to
Fig. 2. Common complications of chronic immunosuppression by class (CNI calcineurininhibitors; anti-proliferatives MMF and sirolimus).
less gingival hyperplasia, hypertension, dyslipidemia and perhaps lessrenal insufficiency [8].
Mycophenolate mofetil (MMF), an inhibitor of de novo purinesynthesis, inhibits both T and B cell proliferation. In a randomized trial,myocophenolate mofettil was shown to improve rejection rates andone-year survival over AZA [9] but gastrointestinal intolerance andleucopenia can limit its use.
These improvements in immunosuppression have significantlycontributed to improved short-term outcomes with lower rates ofallograft rejection and infection episodes. However, chronic immuno-suppression also impacts host immune surveillance against malignan-cies. Cancer-related mortality in cardiac transplant recipients remainshigh [10] especially in older recipients [11]. Given that the average age ofcardiac transplant recipients has continued to increase in recent years,morbidity andmortality from post-transplantmalignancies nowcloselychallenges transplant vasculopathy for limiting long-term survival. Incontrast, the proliferation signal inhibitors (sirolimus and everolimus)may be beneficial in this respect, leading to less malignancy and CAV.However, higher rates of bacterial and fungal infections are observedwith these agents. Chronic immunosuppression has also impacted theburdenof post-transplant chronic infectious diseases suchaspulmonaryaspergillosis and coccidiodomycosis which remain difficult to treat.Maintaining a low level of immunosuppression to ensure minimizationof adverse effects, while at the same time avoiding allograft rejectioncontinues to remain the predominant challenge of post-transplant care.
2. High risk recipients
The risk of allograft rejection generally determines the immunosup-pression strategy used in any individual patient. The increasingemphasis on minimizing immunosuppression in general necessitatesan astute appreciation for identifying patients who may be particularlyat risk of allograft rejection. Patients with circulating HLA antibodies(panel-reactive antibodies (PRA)),multiparous female recipients, cardiactransplant recipients bridged with an assist device, recipients with ahistory of multiple blood transfusions and young recipients generallypose the greatest risk for allograft rejection [12] (Fig. 3). A number ofstrategies may be deployed in minimizing the risk in this population.
A prospective crossmatch will avoid the use of donor hearts at risk ofexposure to circulating cytotoxic antibodies. A ‘virtual’ cross-matchsystem is increasingly being deployed bymany centers to circumvent thelogistical difficulties of having to perform a prospective cross-matchwhen an organ becomes available. Recipient blood is required for aprospective cross-match to be performed. The test is time-consumingand requires local expertise. In a virtual crossmatch, the recipientantibody profile is determined at the time of listing and cytotoxicantibodies to HLA antigens identified. These antigens are then docu-mented as unacceptable on the transplant list. This strategy will helpidentify patientswhoare potentially at elevated risk of rejection inwhomimmunosuppression may need to be augmented after transplantation.
Patients with high PRAs may undergo ‘desensitization’ with the useof a number of treatments including plasmaphoresis, intravenousimmunoglobulin, rituximab or cyclophosphamide therapy, although
Fig. 3. Certain patients may be identified prior to transplant at particular risk for rejection.These patients generally require a higher level of chronic immunosuppression.
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the clinical efficacy of these regimens has yet to be confirmed in clinicaltrials.
High risk patients may be considered for induction therapy at thetime of transplantwith cytolytic agents such as anti-thymocyte globulinor OKT3 antibody although efficacy has not been established. Polyclonalandmonoclonal antibodies have longbeen used for induction therapyatthe time of transplant and in combinationwith conventional therapy tosuppress acute rejection. Their use, however, has been limited byadverse effects including infection and lymphoproliferative disease dueto over-immunosuppression, serum sickness and thrombocytopenia,leucopenia or anemia.Monoclonal agents include the anti-CD3 antibodyOKT3 and the IL-2 receptor antagonists daclizumab and basiliximab.OKT3 is a murine monoclonal antibody directed against the CD3molecule of the T cell receptor complex, blocking function of both naïveand established cytotoxic T cells. It is highly effective in arresting T-cellmediated rejection and suppressing T-cell dependent allo-antibodyresponses. Its use is howeverassociatedwith anumberof adverse effectsincluding cytokine release syndromewhich is common andmay be life-threatening. It frequently occurs as afirst dose effect and is characterizedby fever, chills, generalized weakness, non-cardiac pulmonary edemaand hypotension. Bronchospasm, vomiting and diarrhea may also beseen. Associated morbidity also includes increased risk of viralinfections, particularly cytomegalovirus, and lymphoproliferative dis-ease which may be abrogated by reduction of concomitant immuno-suppressive agents.
To assess the impact of cytolytic induction therapy in hearttransplantation, a multi-institutional study of over 6000 cardiactransplant recipients [13] suggested by multi-variate analysis thatamong patients with a 1-year risk of rejection death at N5%, inductiontherapy provided a survival advantage, but survival with inductionwasdecreased when the risk of rejection death was b2%. Specific patientsub-sets that received a survival benefit with induction includedyounger patients of black race with N/=4 HLA mismatches and long-term (N6 months) support on a ventricular assist device. Cytolyticinduction therapyappeared to bebeneficial in select patients at high riskfor rejection death, but likely detrimental in patients who are at low riskfor rejection death.
Because the IL-2 receptor antagonists affect primarily activated T-cells, the immunosuppression they provide may be effective inpreventing allograft rejection without depressing global immuno-competence. These antibodies are directed against the CD25 antigenwhich is only expressed on activated T cells. Basiliximab (Simulect®) isa chimeric antibody retaining the murine elements of the variableportion of the immunoglobulin chain. Daclizumab (Zenapax®) is morecompletely humanized. Due to their lower immunogenicity, IL-2receptor blockade is achieved for several weeks after the last dose, andtherefore, these agents are putative candidates for induction therapy.In a randomized study of daclizumab as induction therapy comparedwith standard immunosuppressive regimen, the antibody preparationwas noted to safely reduce the frequency and severity of cardiacallograft rejection [14].
Cardiac transplant recipients at a high risk for rejection are typicallytreatedwith a triple-drug regimen after transplantationwhichgenerallyincludes a CNI, an anti-proliferative agent and corticosteroids. Due totheir higher overall risk for allograft rejection these patients will begenerally continued long-term on a triple-drug regimen even in theabsence of allograft rejection.
3. Temporal risk of rejection
Rejection rates are typically highest in the first 6–12 monthsfollowing cardiac transplantation. The endomyocardial biopsy remainsthe standard for the detection of both cellular and humoral rejection.Insidious onset with few symptoms necessitates routine surveillancebiopsies which are generally performed most frequently in the firstthree months, with the biopsy schedule becoming less frequent there
after. Atour institution, very lowrates of rejectionobservedafter thefirstyear has obviated the need to perform routine annual endomyocardialbiopsies. With improved post-transplant care early rejection rates havefallen and many centers are now performing fewer biopsies, although arecent study suggests that it may still have a useful role in the detectionof persistent asymptomatic humoral rejection [15] which may promptchanges in immunosuppression to avoid adverse outcomes. Patientswhodevelop rejection requiring treatment are also at subsequent risk ofanother rejection episode [16] and therefore require closer surveillance.
4. Strategies for minimization of immunosuppression
4.1. Steroid wean
Consequences of chronic steroid therapy have been of concern sincecardiac transplantation became a therapeutic option. Steroid therapy isassociated with glucose intolerance, dyslipidemia, hypertension, osteo-porosis and infection. Steroid-free immunosuppression regimens havepreviously been associatedwith adverse long-term outcomes. Five-yearsurvival was reported to be similar according to Keogh et al., being 82%with prednisone and 85% without prednisone [17]. However, rejectionrates were significantly higher in the steroid-free group and almost halfthe patients were converted to maintenance steroids. In a recent studyhowever, 32 low-risk cardiac transplant patients were randomized toreceive either an anti-thymocyte globulin -based corticosteroid-avoid-ance regimen or a long-term corticosteroid-based regimen with noantibody induction as a control group [18]. Pulse steroid therapy wasused for the treatment of acute cellular rejection in both groups. At oneyear, there was no significant difference in the mean incidence of acutecellular rejection (ISHLTN or =3A) episodes between two groups.Importantly, the steroid-avoidance patients had significant improve-ment in muscle strength and less bone loss compared with the controlpatients during the first six months post-transplant. The authorsreported that the steroid-avoidance regimen with anti-thymocyteglobulin induction appeared to be safe and effective in cardiactransplantation. However, further studies are required to demonstratethe long-term safety and benefits of such a regimen.
Yacoub initially described the utility of a low-dose steroid regimenand complete withdrawal of corticosteroids in patients treated withanti-thymocyte globulin [19]. Patients at low risk of rejection, includingthose without circulating antibodies, women without a history ofmultiple pregnancies, those without a history of rejection and oldertransplant recipients may be considered for steroid withdrawal.Generally, two strategies have been utilized. Early withdrawal ofprednisone has been achieved during the first month of transplantationin conjunctionwith cytolytic induction. Long-term outcomes have beenvariable, with success in 49% to 70% of patients [20–22].
However, given that the majority of acute rejection episodes occurduring the first 6 months post-transplant, steroid withdrawal afterthis period has generally been more successful with steroid with-drawal being achieved in upto 80%, even without the use of inductiontherapy [23,24].
To determine the safety of gradual weaning from prednisonemaintenance, 68 patients (more than 6 months from transplantation)were weaned from 5 mg/day by decreasing the daily dose by 1 mgeach month, with monthly biopsies in a study by Kobashigawa [23].Asymptomatic moderate rejection occurred in 13 compliant patients.Rejection with hemodynamic compromise occurred in two patientswith documented medication noncompliance, who were excludedfrom further analysis. Successful weaning without rejection waspossible in 53 of 66 (80%) compliant patients. All moderate rejectionepisodes responded to oral steroid pulse therapy. The two seriousrejections after noncompliance responded to OKT3. There were nosymptoms from steroid withdrawal that required taper alteration.Steroid weaning after cardiac transplantation is now common place,with the most recent registry data shows that over 40% of heart
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transplant recipients are successfully maintained off steroids at5 years [2].
4.2. Calcineurin inhibitor minimization
The cumulative incidence of chronic renal failure has been reportedat 10.9% over 5 years [25] and has been chiefly attributed to the use ofCNI use. A number of trials have addressed the feasibility of CNIreduction or elimination in cardiac transplantation [26–37].
4.2.1. CNI minimizationLower rejection rates and improved outcomes with MMF over AZA
haspermitted successful loweringof CNI exposure. In a recent sequentialstudy [38], 240cardiac transplantpatientswere treatedwitheitherMMF(n=119) or AZA (n=121) both in combination with cyclosporine andcorticosteroids after rabbit antithymocyte globulin induction. Byprotocol lower cyclosporine levels were targeted in the MMF groupduring the first year (e.g. 203+/−52 ng/mL MMF vs. 236+/−59 ng/mLAZA, P=0.0006 at 6months). Patient survival at 1 year and at 3 yearswassimilar in both groups. The cumulative probability of receivingantirejection treatment within 1 year was lower in the MMF group, aswas biopsy-proven acute rejection with ISHLT gradeN or =3A (24% vs.35%, P=0.03). The MMF group also had fewer episodes requiringcytolytic therapy (6% vs. 13%, P=0.04) and more patients had steroidswithdrawn by 1 year (66% vs. 32%, Pb0.001). Renal function wassignificantly better in the MMF group with lower creatinine levels andcreatinine clearance at 1 year, presumably reflecting the lower levels ofCNI used. These results suggest that immunosuppression with MMFrather than AZA following induction therapy allow lower cyclosporinelevels, better renal function, and increased steroid weaning at 1 yearwhile also achieving better control of acute rejection.
In another prospective multicenter study [27], MMFwas substitutedfor AZA prior to cyclosporine reduction in 109 patients and compared toa control group of 52 patientswhose treatment remained unchanged. Ateight months there was a significant decrease (Pb0.0001) in SCr in theintervention arm (23.3±50.7 μmol/L) compared to a modest increase inthe control group (7.3±46.9 μmol/L). In the intervention arm, thecyclosporine level was 57±24 vs. 116±36 ng/mL in controls. Thepresence of diabetes did not affect the change in renal function in theintervention arm. Cyclosporine reduction was associated with asignificant decrease in blood pressure. Systolic/diastolic 24 h bloodpressure values at study end had improved by −5±15/−5±14 mm Hg(P=0.005/0.001) for day averages and by −5±18/−5±14 mm Hg(P=0.008/0.010) for night averages in the intervention group comparedwith the control group.
A number of smaller single-center studies have also suggested acomponent of reversible renal function with cyclosporine dose reduc-tion in conjunction with a change from AZA to MMF [28,29,35].
Late cyclosporine reduction in heart transplant recipients withoutadjunctive therapy however appears not to improve renal function. In astudy of pediatric recipients with a creatinine clearance (CrCl) of~70 mL/min, this was essentially unchanged at more than five-yearfollow-up [34]. The substitution of sirolimus for AZA also seems to be oflimited benefit in the setting of CNI minimization, given that agents ofthis class (proliferation signal inhibitors) tend to exacerbate CNInephrotoxicity. The use of proliferation signal inhibitors with CNIsgenerally requires CNI dose-reduction to prevent nephrotoxicity. In onestudy, CrCl improved in thefirst threemonths in theminimization groupbut was similar to baseline values by six months [36]. In another non-randomized study of 38 patients [39] whowere a median of 82monthsfrom transplant, the only predictors of benefit at 6 months in CrCl fromthe introduction of sirolimuswith concomitant CNI dose reductionwerebetter renal function 3 months before conversion and a shorter timefrom transplant to conversion. A recent pilot studywith everolimus andreduced-dose cyclosporine in recipients more than 1 year fromtransplant [40] was associated with no significant decline in renal
function, and no indication of increased rejection at three months post-conversion.
Monitoring of cyclosporine levels at 2 h after dosing (C2) may be abetter indicator of immunosuppression efficacy than trough levels andmay be associated with lower CNI exposure. In a longitudinal study of114 stable cardiac allograft recipients more than a year out fromtransplantation, patientswere initially followed for about a yearwith C2monitoring (target range 300–600 μg/L) [41]. The patients were thenswitched to trough level (C0) monitoring (target range 100–200 μg/L).During C2monitoring, cyclosporine dosage, C0 levels, C2 and creatininedecreased by 26%, 56%, 45% and 2.3% respectively compared to baseline.When changed to C0monitoring, these variables increased by 24%, 56%,38% and 10% respectively (Pb0.0001). In a more recent randomizedcontrolled trial of 125 patients, C2 monitoring compared to C0monitoring allowed a significant cyclosporine dose reduction withoutcompromising patient outcome in stable heart transplant patients [42].Catarovich recently reported that C2 monitoring could be safely used inde novo heart transplant recipients and a low C2 range in combinationwith basiliximab induction resulted in preserved renal functionwithoutincreased risk of acute rejection [43]. These studies suggest that C2monitoring allows a safe reduction in cyclosporine exposurewithout therisk of acute rejection. However, whether this translates into long-termbenefit remains to be determined.
4.2.2. CNI avoidanceDespite the pivotal role of CNIs in the success of cardiac transplanta-
tion, efforts have been made to avoid this class of agents all together.Initial small studies suggested that CNI avoidance may be possible. Apilot study evaluated the efficacy of sirolimus, MMF, and corticosteroidsin 11 de novo patients. With a follow-up of three to 12 months, patientsurvival (n=8) was 100% and freedom from rejection 75%. Meancreatinine levels initially decreased and remained stable thereafter.Adverse events included moderate myelosuppression, elevated choles-terol and triglyceride levels, pericardial effusion, and peripheral edema[33]. Larger randomized studies however have yet to be performed andthe true safety of avoiding CNI use in cardiac transplantation remains tobe established.
4.2.3. CNI withdrawalWithdrawal of CNI using sirolimus [37] or sirolimus with MMF
[30,32,44] has been evaluated in a number of studies. Groetzner [30]studied cardiac transplant patients with late post-transplant renalimpairmentwith conversion fromCNI toMMFand sirolimus. 31 patientswith CNI-based immunosuppression and SCrN1.9 mg/dL were startedwith sirolimus and the dosewas adjusted to achieve target trough levelsbetween8 and 14ng/mL.MMFwas continuedwith trough level adjustedbetween1.5 and4 μg/mL. Subsequently, the CNIswere tapereddownandstopped. Survival was 90% after a mean follow-up of 13±95 months. Noacute rejection episode was detected during the study period. Renalfunction improved significantly after conversion: creatinine pre-conver-sion vs. post-conversion was 3.14±0.76 mg/dL vs. 2.14±0.83 mg/dL,P=0.001. In threepatients, hemodialysis therapywas stopped completelyafter conversion.
Similarly, in 2005, 80 heart transplant patients (range 1 to 14 yearspost-transplant) with renal dysfunction had their CNIs abruptly discon-tinued and were switched to sirolimus at 5 mg twice daily for two daysand then2mgdaily ina studybyKushwaha [44]. The treatmentgoalwas atrough level of 6–12ng/mL, and all patientswere alsomanagedwithMMFat 1000mg twice daily. At a mean of 304 days post-conversion, themeanSCr decreased from 2.04±0.57mg/dL pre-conversion to 1.64±0.48mg/dL(Pb0.001). In patients whose SCr was b2.5 mg/dL before conversion, themean SCr level decreased from 1.81±0.39 to 1.62±0.5 mg/dL post-conversion (P=0.01), with no patient developing end-stage renal disease.In the patients with SCr≥2.5 mg/dL before conversion, four patientsdeveloped end-stage renal disease requiring dialysis despite conversion.For all patients with SCr≥2.5 mg/dL before conversion, the SCr decreased
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from2.85±0.29 to1.73±0.43mg/dL (P=0.001) at ameanof 304dayspost-conversion.
The substitution of CNIs with sirolimus also appears to have noadverse effect on cardiac function and rejection in cardiac transplantrecipients with renal dysfunction [32]. Thirty-four stable cardiactransplant recipients with CNI-induced nephrotoxicity (creatinineclearance 25–50 mL/min) or cardiac allograft vasculopathy wereenrolled. Twelve patients were prospectively enrolled for renaldysfunction, and the remaining patients were converted to sirolimuson clinical grounds because of poor renal function or the presence ofcardiac allograft vasculopathy. A further 24 patients were retrospectivecontrols, stable (range two to 10-year post-transplant), and maintainedon a standard CNI-based immunosuppressant regimen. CNI was with-drawn gradually over 12 weeks, adjunct immunosuppression was leftunchanged, and sirolimuswas started at 1mg/d, with titration over twoweeks to achieve levels of 10–15 ng/mL. Creatinine clearance improvedsignificantly in both sirolimus-treated groups without exacerbatingrejection or compromising cardiac function. In the control grouphowever, the baseline renal clearance declined from significantly overthe course of one year.
In a non-randomized study [45] in heart transplant recipients withrenal dysfunction (GFR≤50 mL/min) and/or CAV detected by annualangiography, 78 patients were changed to a sirolimus-based immuno-suppression with complete CNI withdrawal a mean of 3.8 years aftertransplantation. The study demonstrated an improvement in CAVprogression by intravascular ultrasound with sirolimus treatmentwithout an increase in rejection at 12 months. Although there was asignificant decrease in creatinine at 12months, calculatedGFR remainedunchanged in the sirolimus group. Additionally there was a significantincrease in proteinuria in the sirolimus arm at 12 months. In the CNIgroup, proteinuria did not change at 12 months. The difference inproteinuria between the sirolimus andCNI groupswas significant. Bloodalbumin level did not change in patients treated with sirolimus, and nopatients developed edema or hematuria.
In contrast to the above studies, in five patients with chronic renaldisease, replacement of cyclosporine with sirolimus resulted in a 50%increase in SCr and universal dialysis requirement [37]. However, fromall studies mentioned, it appears that CNI withdrawal is successful,particularly when withdrawal is initiated at a time when SCr is notexcessively elevated and withdrawal is not initiated too early aftertransplantation.
Data of greatest concern relating to sirolimus as a primaryimmunosuppressant however comes from the Heart Save the Nephron(STN) clinical trial that prompted premature termination of the study[46]. This study aimed to assess the potential renal-sparing impact ofsirolimus in heart transplantation as early as 12weeks after transplanta-tion. Patients were initially treatedwith CNIs but then at 12weekswererandomized to either CNIs or sirolimus; MMF and prednisone wereadjunctive immunosuppressants. The study was terminated prema-turely because 4 of 7 patients randomized to sirolimus experiencedISHLT grade IIIA rejection episode. Three of 4 episodes occurred at 1transplant center, and 1 patient experienced hemodynamic compro-mise. Low blood levels of sirolimus were recorded in 2 patients. Itappears therefore that the introduction of sirolimus may not be safe inthe absence of CNI use in the acute phases of heart transplantation, at atimewhen the risk of rejection is highest and therapeutic immunosup-pression is of greatest importance, an area where CNIs have been ofproven efficacy.
4.3. CNI monotherapy
Although minimization of CNI and corticosteroid use has been thefocus of a number of clinical trials, very few studies have investigatedthe safety and efficacy of CNI monotherapy in cardiac transplantrecipients. An initial retrospective study [47] reported in 2001revealed that of 43 patients initiated with tacrolimus and prednisone
alone, 32 patients were subsequently weaned off steroids at a mean of246 days post transplant (range 106 to 730 days). The freedom fromtreated rejection (ISHLT 3A/3B and Grade 2 rejection in the first90 days) was 69% at 90 days and 52% at 1 year. In a subsequent follow-up report [48] 90 of 124 patients initially treated with tacrolimus andcorticosteroids were weaned to tacrolimus monotherapy without theuse of an anti-proliferative agent, resulting in an overall success rate of75% at an average of 271+/−18 days after transplant. The remaining 69recipients were treated with other tacrolimus- or cyclosporine-basedregimens. Survival was significantly greater in the tacrolimus mono-therapy group. The prevalence of high-grade rejection within the firstyear and incidence of cardiac allograft vasculopathy were similarbetween groups. Ten patients (11%) in the monotherapy grouprequired recommencement of combination immunosuppression atan average of 768+/−772 days. The investigators concluded that CNImonotherapy is achievable in the majority of cardiac transplantrecipients. Patients who tolerated reduced immunosuppressionenjoyed greater survival than those treated with other regimens,without additional high-grade rejection or vasculopathy.
These promising results prompted a randomized study— TacrolimusIn Combination, Tacrolimus Alone Compared (The TICTAC Trial) [49]. 58adult heart transplant patients were studied. All received oraltacrolimus, MMF, and corticosteroids. Induction therapy was notroutinely used. Patients were then randomized to a group where MMFwas maintained or to a group where it was discontinued 14 days post-transplant. Tacrolimus levels were maintained between 9 and 11 ng/dL.Corticosteroids were rapidly withdrawn in both groups between 8 and12 weeks. The primary end point (mean 6-month ISHLT biopsy score)was 0.44±0.04 in the monotherapy group and 0.60±0.05 in thecombination therapy group (P=0.013). The freedom from rejectiongrade of 2R or higher at 6 months was 93.3% with monotherapy and92.9% with combination treatment (P=NS) with no further rejectionepisodes ≥2R occurring at 12 months. Tacrolimus levels and renalfunctionwereunchangedbetween the twogroups at3, 6 and12months.CAV at 12 months was comparable in both groups. Interestingly, whileone patient crossed over from themonotherapy to combination therapygroup following rejection, 9 patients crossed over from the combinationtherapy group to the monotherapy group due to MMF intolerance(gastrointestinal side-effects or leucopenia). Infections requiring hospi-talization were more common in the combination group (12 vs. 5patients). These data support initial safety and efficacy of tacrolimusmonotherapy but longer term resultswill be important to determine theimpact of this strategy on mortality, rejection, renal function, andmalignancy.
5. Monitoring
Minimization of immunosuppression requires close vigilance toavoid the risk of rejection. Therapeutic drug monitoring becomesincreasingly important as the number of maintenance drugs decreases,especially when weaning occurs relatively early post-transplant. In theTICTAC trial, tacrolimus levels were carefully maintained at between 9and 11 ng/mL [49]. Clinical status, endomyocardial biopsy andechocardiography remain the principal tools for rejection monitoringduring drug weaning. With prednisone tapering at our institution,patients are weaned 1 mg per month with monthly endomyocardialbiopsies and echocardiograms [50]. Although gene expression profilingfrom peripheral blood (AlloMap) has recently become available as analternative to endomyocardial biopsy for the detection of acute cellularrejection [51], its utility and safety has yet to be demonstrated for thepurpose of immunosuppression weaning.
Another peripheralwhole blood assaywhich assesses thenet state ofimmunosuppression in transplant recipients is also clinically available.In ameta-analysis of 504 solid organ transplant recipients (heart, kidney,kidney–pancreas, liver and small bowel) from 10 U.S. centers, the CylexImmuKnow® assay was performed at various times posttransplant and
Fig. 4. Strategies for minimizing immunosuppression. Assessment of risk allowsformulation of treatment plan and the degree of immunosuppression required. Followingtransplantation, monitoring for rejection and adverse outcomes of immunosuppressionremains important in reducing morbidity and mortality. Monitoring of antibodies (PRA)and immune cell function may be useful but clinical efficacy has yet to be determined.
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compared with clinical course (stable, rejection, infection) [52]. In thisanalysis, 39 biopsy-proven cellular rejections and 66 diagnosedinfections occurred. Odds ratios of infection or rejectionwere calculatedbased on measured immune response values. A recipient with animmune response value of 25 ng/mL adenosine triphosphate (ATP) was12 times (95% confidence of 4 to 36) more likely to develop an infectionthan a recipient with a stronger immune response.
Similarly, a recipient with an immune response of 700 ng/mL ATPwas 30 times (95% confidence of 8 to 112) more likely to develop acellular rejection than a recipient with a lower immune responsevalue. The intersection of odds ratio curves for infection and rejectionin the moderate immune response zone was 280 ng/mL ATP. Thisintersection of risk curves provides an immunological target ofimmune function for solid organ recipients. These data suggest thatthe Cylex ImmuKnow® assay has a high negative predictive value andprovides a target immunological response zone for minimizing riskand managing patients to stability. However, in a recently reportedretrospective analysis of 111 cardiac transplant recipients mostly morethan a year from transplant, this assay showed wide variability. Nocorrelation was observed between the baseline Cylex® response andsubsequent risk of either infection or rejection within 6 months,although the number of rejection episodes (3) and infection episodes(8) were low in the study. Lower white blood cell count and AfricanAmerican ethnicity were correlated with a lower Cylex® response. Theassay shows some promise in monitoring the immune status incardiac transplant recipients with post-transplant lymphoprolifera-tive disorder in whom minimization of immunosuppression is thepredominant treatment strategy [53]. Cylex® has not been utilized inpatients being routinely weaned off immunosuppressive agents andits clinical utility in this regard remains unclear.
The level of immunosuppression utilized post-transplant is generallydetermined by the pre- and post-transplant risk for rejection. A highernumber of HLAmismatches have been shown to be amarker for adverseoutcomes with steroid weaning in cardiac transplantation [50]. Pre-transplant panel reactive antibodies are also correlated with post-transplant adverse outcomes in heart transplant recipients [54,55].Detection of anti-HLA antibodies by flow-cytometry both pre- and posttransplant has been shown to be more predictive of rejection comparedwith complement-dependent cytotoxicity assay [55]. Interestingly, inthis study, the majority of pretransplant HLA antibodies were notdirected against donor antigens. These non-donor-directed antibodiesare likely surrogatemarkers that correspond to previous Tcell activationand thus, the rejection episodes that occur in these patients may be inresponse to donor-derived MHC peptides that share cryptic determi-nantswith theHLA antigens that initially sensitized the patient. Routineperiodic monitoring of flow-panel reactive antibodies (flow-PRAs) maytherefore provide a useful marker in a strategy to minimize immuno-suppression. Quantitation of flow-PRAs is also now possible withmeasurement of mean fluorescence intensities which may further helpstratify risk inpatientswith circulating antibodies. The presence of post-transplant antibodies is also a marker for the subsequent developmentof cardiac allograft vasculopathy [56]. Detection of post-transplant allo-antibodies may therefore determine the type and strength of ongoingimmunosuppression utilized.
6. Risks of minimization of immunosuppression
The most profound risk of weaning immunosuppression is allograftrejection. This risk is greatest in the first few months after transplanta-tion and therefore most protocols opt for lowering immunosuppression3–6 months after transplantation. When rejection does occur in thissetting, aside from the morbidity and mortality associated with thecondition, there are further consequences of the treatment itself. Theseinclude infection, metabolic derangements due to pulse-dose steroidtherapy and renal dysfunction. Patients who are successfully treatedgenerally have to be maintained on higher levels of immunosuppres-
sion. In the long-term, there is a potential increased risk of malignancyand transplant vasculopathy.
7. Concluding remarks
Induction of tolerance, which obviates the need for maintenanceimmunosuppression following organ transplantation remains the holygrail in the field. However, in cardiac transplantation, ongoingimmunosuppressive therapy is essential to ensure long-term graftsurvival. Although drug regimens have substantially improved in recentyears, adverse effects cause significant morbidity and affect quality oflife. Minimization of immunosuppression (Fig. 4) requires appreciationof the potential consequences and high risk patients need to beidentified and excluded from low intensity immunosuppressive regi-mens. A variety options exist for lowering of immunosuppression andsteroid weaning has become common practice. Patients require closemonitoring for rejectionduringweaning. In addition to the conventionalclinical parameters which include therapeutic drug monitoring, endo-myocardial biopsy and echocardiography, newer techniques for mon-itoring, including detection of circulating alloantibodies andquantitative measurement of the net state of immunosuppression(Cylex®) hold some promise. However, the efficacy of these modalitiesrequires further investigation. Future development of more effectiveimmunosuppressive regimens will need to focus on minimizing long-term morbidity which currently limits long-term outcomes in cardiactransplantation.
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