In this issue of Liver Transplantation, Levy et al.1 describes a randomized, placebo-controlled trial evaluating the safety and tolerability of everolimus, a semisynthetic derivative of sirolimus, given with cyclosporine in recipients of liver transplantation. All patients received cyclosporine and 4 groups were compared, those taking either 0.5 mg twice daily (bid) (N = 28) everolimus, 1.0 mg bid (N = 30) everolimus, 2 mg bid (N = 31) everolimus, or a placebo (N = 30). The results showed no significant differences in rejection rates, although patients taking 1 or 2 mg bid had the lowest numerical rate of rejection. Freedom from rejection correlated with trough blood levels of everolimus; patients with trough levels of 3 ng/mL or less had rejection rates 3-fold higher than patients with trough levels exceeding 3 ng/mL. There were no statistically significant differences in side effects or adverse events between the 4 groups. Renal function declined to a similar extent in all 4 groups and there was no difference in renal function at any time point between the everolimus-treated and placebo arms. Rates of hepatic artery thrombosis were 0%, 3.3%, 3.2%, and 3.3% for the 0.5 mg bid, 1.0 mg bid, 2.0 mg bid, and placebo groups, respectively. There was no increase in infectious complications, including cytomegalovirus (CMV), serious wound infection or dehiscence, in the everolimus-treated patients and overall graft and patient survival was similar among the 4 groups of patients. Interpretation of the results of this trial is hampered by the small sample sizes of patient groups and the high dropout rates. Given this caveat, the study suggests that everolimus, like its parent molecule, sirolimus, is an effective immunosuppressive agent with an acceptable patient tolerance and safety profile. Certainly, larger, randomized controlled studies are required to fully address both efficacy and safety of everolimus in recipients of liver transplantation.
Prior to the current study, nearly all the experience using this class of immunosuppressive agents in liver transplant recipients was related to use of sirolimus, also known as rapamycin. The latter compound was isolated from a soil fungus (Streptomyces hygroscopicus) from Easter Island (Rapa Nui) in 1969. Everolimus, also designated RAD, is the 40-O-[2-hydroxyethyl] derivative of rapamycin.2 Sirolimus and everolimus represent a new class of immunosuppressive compounds designated mammalian target of rapamycin (mTOR) inhibitors. Immunosuppressive activity is primarily related to the blockade of interleukin-2 and interleukin-15 induction of proliferation of T and B cells via inhibition of p70S6 kinase through high-affinity binding of sirolimus or everolimus to the FK506 binding protein. Sirolimus also has demonstrated potent antifungal, and antiproliferative effects, independent of immunosuppressive activity, and has been approved by the U.S. Food and Drug Administration for use in recipients of renal transplantation. Compared to calcineurin inhibitors (tacrolimus and cyclosporine), mTOR inhibitors may offer a favorable side-effect profile. They are not independently associated with nephrotoxicity, neurotoxicity, renal dysfunction, hypertension, or diabetes, the 5 most important long-term metabolic consequences of calcineurin inhibitors. The most common side effects of mTOR inhibitors are dose-related cytopenia and hyperlipidemia. Some studies have suggested that mTOR inhibitors may carry additional unique and potentially graft- or life-threatening complications, such as hepatic artery thrombosis and wound dehiscence.
As noted above, the mechanism of action of mTOR inhibitors is fundamentally different than calcineurin-inhibitors. Sirolimus and everolimus bind intracellularly to the same binding protein as tacrolimus (the FK binding protein). However, their immunosuppressive effects are mediated through a novel signal transduction pathway, the mTOR, which inhibits proliferation of lymphocytes.3–5 The half-life of sirolimus is approximately 60 hours and the half-life of everolimus is 40 hours. As a result, oral administration is required only once daily. Both compounds are cleared through the liver via the hepatic cytochrome P450-3A4 microsomal system, which is the same metabolic pathway used by cyclosporine and tacrolimus. Drugs which inhibit or compete with the activity of the cytochrome P450 system may significantly impair clearance of both sirolimus and everolimus and lead to significant increase in systemic levels. Common drugs that may cause clinically significant elevations in blood concentrations through inhibition of metabolism include fluconazole, azithromycin, and protease inhibitors.
Experience with mTOR inhibitors in liver transplantation is limited compared to the breadth of experience with these drugs in renal transplantation. In liver transplant recipients, sirolimus has been used primarily to convert from a calcineurin-inhibitor (cyclosporine or tacrolimus) months or years after transplantation. The goal of this strategy is to prevent progression of renal insufficiency caused by calcineurin inhibitors.6, 7
The first report of sirolimus used as primary immunosuppression in recipients of liver transplantation was published in 1999 by Watson et al.8 Subsequently, McAlister et al.9 reported their experience with sirolimus plus tacrolimus as primary immunosuppression in 32 solid organ transplants including 23 liver recipients. Only 1 of the 32 patients (3%) experienced acute cellular rejection and this occurred in a patient who discontinued sirolimus. Pridohl et al.10 reported their experience in 22 patients who received tacrolimus, sirolimus, and corticosteroids. Patient and graft survival at 1 yr were 91% and 78%, respectively. The rate of acute cellular rejection and steroid-resistant rejection was 14% and 0%, respectively. Additional encouraging studies of larger numbers of patients have shown excellent patient and graft survivals with low rates of acute rejection or steroid-refractory rejection.11–13 We compared our experience with sirolimus to historical controls under conditions of steroid avoidance, and observed a 30% reduction in rate of acute cellular rejection (48% vs. 70%; P < 0.05), and 65% reduction in steroid-refractory rejection (8% vs. 37%; P < 0.05).
Immunosuppressive regimens containing mTOR inhibitors may be less nephrotoxic due to sparing of the dosage of calcineurin inhibitors. However, the current study failed to demonstrated a renal-sparing effect, since the degree of renal dysfunction was nearly identical among the 4 treatment arms of the trial. Lack of renal sparing in this trial may have been related to small sample size, high dropout rate, and use of cyclosporine as the base calcineurin inhibitor. In renal transplant recipients, sirolimus has been associated with higher incidence of nephrotoxicity when used with cyclosporine.14 Animal studies have suggested that sirolimus enhances the nephrotoxic effects of sirolimus primarily by increasing renal partitioning of cyclosporine into the renal tubular cell.15 It is possible that a renal-sparing effect would have been observed in the current trial if tacrolimus had been used in place of cyclosporine in combination with everolimus.
Dyslipidemia is perhaps the most common side effect of sirolimus. Studies in renal transplantation have demonstrated that up to 80% of patients on sirolimus have hypercholesterolemia with mean cholesterol levels of 240 mg/dL.14, 16, 17 Risk may be lower in liver recipients because of lower doses of calcineurin inhibitors, steroid withdrawal, and steroid avoidance. In our study in which steroid were given for only 3 days, mean serum cholesterol in sirolimus-treated patients (176 mg/dL) was only 9% higher than controls (162 mg/dL; P < 0.0001).18 We and others have observed a greater tendency toward dyslipidemia when sirolimus is used with cyclosporine, compared to its use with tacrolimus.
Some have suggested that mTOR inhibitors may be especially useful in patients with chronic hepatitis C, due to their antifibrotic properties. Others have suggested that increased immunosuppression by mTOR inhibitors may enhance viral replication and worsen recurrence of hepatitis C. In our own experience in Colorado, there has been no clear indication of either worsening or improvement in either rates of recurrence or progression of disease. Resolution of the impact of mTOR inhibition on hepatitis C virus recurrence will require long-term follow-up of patients in properly controlled trials.
Enthusiasm for use of mTOR inhibitors in liver recipients has been dampened by concern for possible increase in risk of hepatic artery and wound complications.19 In the current study by Levy et al.,1 incidences of hepatic artery thrombosis, infectious complications, and wound dehiscence in everolimus treatment arms were no greater than incidences in the placebo arm. In a large single-center experience, there was no excessive risk for either of these complications in sirolimus treated liver recipients.20, 21 In contrast, in the international, multicenter trial of sirolimus in liver transplantation, there was a statistically significant increase in development of hepatic artery thrombosis and infectious complications (unpublished results). Nonetheless, the results in the international trial prompted temporary discontinuation of patient enrollment and a “black-box warning” from the U.S. Food and Drug Administration22: “…The use of sirolimus in combination with tacrolimus was associated with excess mortality and graft loss in a study in de novo liver transplant recipients. Many of these patients had evidence of infection at or near the time of death. In this and another study in de novo liver transplant recipients, the use of sirolimus in combination with cyclosporine and tacrolimus was associated with an increase in hepatic artery thrombosis; most cases of hepatic artery thrombosis occurred within 30 days post-transplantation and most led to graft loss or death. The safety and efficacy of Rapamune (sirolimus) as immunosuppressive therapy have not been established in liver transplant patients, and therefore is not recommended.” As a result, sirolimus has not yet been approved by the U.S. Food and Drug Administration for use in liver recipients and the future role of sirolimus and perhaps other mTOR inhibitors in liver transplantation remains uncertain. It is my opinion that the increasing experience with mTOR inhibitors from single centers performing liver transplantation, and the results of trials, such as the one by Levy et al.1 published in this issue of Liver Transplantation, should encourage the U.S. Food and Drug Administration to “open-the-box” on this class of compounds under the guidance of properly conducted clinical trials.23
In summary, the study by Levy et al.1 strengthens the argument for safety and tolerance of mTOR inhibitors in immunosuppressive regimens for recipients of liver transplantation. However, safety, efficacy, and utility of these compounds will require additional randomized trials with inclusion of large numbers of patients. Ongoing advances in immunosuppressive and antiviral medications will allow tailoring of the immunosuppressive prescription, which undoubtedly will benefit current and future liver recipients.