Blood levels of tacrolimus at 1 week, and at 1, 2, 4, and 6 months after transplantation were compared for the study group and the 19 patients in the comparison group who received tacrolimus (Fig. 1A). These were not significantly different at any time point, although levels tended to be higher in the comparison group. Sirolimus levels were assessed for the study group at the same time points (Fig. 1B). Despite the fact that all patients initially received daily 5 mg oral doses of sirolimus, levels were quite variable 1 week after transplantation (range 2.0–40.9 ng/mL). After dose adjustments, later levels tended to be at or below the target range of 10 to 15 ng/mL (median at 4 weeks posttransplant=5.9, range=2.0–16 ng/mL). The oral sirolimus doses required to achieve these levels ranged from 1 to 13 mg per day (median=5 mg/day).
A total of 15 subjects were enrolled in the sirolimus group, with a median duration in the study of 4 (range 0.6–8.6) months. Four of the study patients died during the study; their clinical features and courses are summarized in Table 2. The cause of death was directly related to an anastomotic dehiscence in three of the four study-group patients that died; the fourth patient died of fungal sepsis and multi-organ failure without anastomotic failure. In contrast, only one of the six deaths in the comparison group was caused by anastomotic complications. Four of nine (44%) male study subjects developed dehiscence, compared with zero of six female study subjects (P =0.06). Study patients had significantly worse survival compared with the comparison group; survival at 6 months was 0.73 for the study group versus 0.90 for the comparison group (P =0.02) (Table 3).
The most severe morbidity observed in the study group was dehiscence of the airway anastomosis. To evaluate the evolution of this complication, we reviewed and graded bronchoscopy reports for all patients, (score range 0–3) as described in the Methods (Table 3). The maximum (most abnormal) score for each patient was noted and compared for study patients and the comparison group. The median bronchoscopic airway scores did not differ for the two groups. Despite this lack of difference, patients receiving sirolimus had a significantly higher rate of anastomotic dehiscence compared with the comparison group (freedom from dehiscence at 6 months, 0.59 for study patients vs. 0.99 comparison group, P =<0.001). Although all patients with airway dehiscence had bronchoscopically apparent anastomotic abnormalities, only two of the four airway dehiscences could actually be visualized, with the other two being diagnosed at postmortem examination. All of the dehiscences occurred in patients who received bilateral single-lung transplants. In two patients, there was also evidence of poor skin-wound healing. Two patients with dehiscences had cystic fibrosis and two had alpha-1 antitrypsin deficiency; two other study patients with alpha-1 antitrypsin deficiency and three with cystic fibrosis did not have airway healing problems. There was no apparent relationship between sirolimus levels at 1, 4, and 8 weeks after transplantation and the occurrence of bronchial dehiscence (Fig. 2). In fact, the three patients with sirolimus levels greater than 30 ng/mL at 1 week after transplantation did not experience bronchial healing problems.
Patients from the study and comparison groups had similar biopsy frequencies, with a median of 1.2 (0–6.8) biopsies per month for the study group versus 1.1 (0.5–10) for the comparison group (P =0.46). Study patients had significantly less Grade A1 acute rejection at 6 months (P =0.04) but significantly more Grade B1 acute rejection (P =0.01). However, there were no significant differences between the groups for acute rejection grades A2 or higher or B2 or higher (Table 3). In fact, of 71 biopsies of study patients, no biopsies showed acute-rejection grade A2 or higher or B2 or higher.
In view of the known association between infections and impaired airway healing, we compared the rates of infection with bacterial and fungal pathogens between sirolimus patients and the comparison group during the first 6 months after transplantation (Table 4). The sirolimus-treated patients demonstrated a trend toward a lower incidence of infection with bacterial pathogens (Pseudomonas, Klebsiella, Serratia, Staph aureus, and Enterococcus) and Aspergillus species than did the comparison group. When each bacterial pathogen was evaluated individually, the study group had a slightly higher rate of Staphylococcus aureus infection; this did not reach statistical significance.
Sirolimus has been successfully used in organ-transplant recipients, most commonly in renal transplantation (7–9,13–15). The most common adverse effects noted include leukopenia, thrombocytopenia, and hyperlipidemia. Organ-specific adverse events have included hepatic artery thrombosis after liver transplantation and lymphocele formation after kidney transplantation (26). This study evaluated the use of sirolimus as a component of immune suppression after lung transplantation, beginning in the immediate postoperative period. During the study, we observed an unexpectedly high incidence of poor skin and airway-wound healing, which contributed directly to the cause of death in three patients. As a result, patients receiving the study regimen demonstrated worse survival compared with a comparison group that had been treated without sirolimus. Our interpretation of these events is that sirolimus contributed to poor airway and wound healing in these patients.
Poor airway healing has long been identified as a limitation to successful lung transplantation. Proposed risk factors for airway complications include bronchial ischemia, perioperative use of steroids, acute allograft rejection, bacterial and fungal infections, and anastomotic technique. The donor bronchus receives its blood supply postoperatively by way of a low-pressure, retrograde perfusion from pulmonary artery collaterals and consequently is at significant risk of ischemia until neovascularization occurs, a process estimated to take 3 to 4 weeks (16). Despite these problems, recent studies suggest that the incidence of lethal complications is now only 2% to 3% (17,18). This is consistent with the experience in our comparison group, in which 1 of 83 patients died as a result of airway dehiscence. It is unlikely that the increased rate of airway complications in the study group resulted from changes in surgical technique because airway anastomoses were all performed in a standard end-to-end fashion, identical to that used in the comparison group. Perioperative steroid use, other medical therapies, and the incidence of acute rejection and bronchial infections also were similar for both groups. Therefore, our results suggest that sirolimus may have impaired healing of the already precarious airway anastomosis, leading to dehiscence in an increased proportion of patients. Although unexplained local or technical factors may have contributed to the dehiscence in these patients, the additional finding of poor skin healing in two patients suggests that systemic effects from sirolimus were the major cause of this complication.
It is notable that 11 of 15 sirolimus-treated patients had successful healing of their bronchial anastomoses. Also, there was not a significant difference in blood sirolimus levels in patients with and without dehiscence, and the levels were within or near the targeted range of 10 to 15 ng/mL in the subjects with dehiscence. These findings suggest that sirolimus may not be the only factor limiting healing in these patients. It is unclear whether the healing problems we observed resulted from differences among individuals in their wound-healing processes, or whether sirolimus-induced inhibition of wound healing only causes nonhealing when the drug is used in individuals with other healing problems, such as ischemia or airway infections. Therefore, sirolimus-treated patients with bronchial infections should be treated aggressively, as would other lung transplant recipients treated with other immune-suppressive regimens.
The mechanism of the immune-suppressive effect of sirolimus is different from that of other immune-suppressive medication. Sirolimus acts by binding to a specific cell-cycle regulatory protein, mTOR, resulting in inhibition of mTOR activation with subsequent blockade of the progression of the cell cycle in the G1 to S phase (19). This contrasts with cyclosporine and tacrolimus, which act primarily through inhibition of calcineurin-induced production of cytokines, particularly interleukin-2, and the early activation of T cells. In addition, whereas other immune suppressives primarily affect lymphocytes, sirolimus inhibits growth–factor-induced proliferation of many cell types, including fibroblasts, endothelial cells, and smooth muscle cells; these are essential for successful granulation tissue formation and wound healing. In this regard, sirolimus prevents arterial neointimal growth in transplanted vessels and prevents restenosis after coronary revascularization (20–22). These characteristics may be responsible for the impaired wound healing observed in this study; specifically, if the sirolimus treated patients were unable to fully revascularize the airway anastomosis, an increased incidence of ischemia and dehiscence could be expected.
In addition to sirolimus, all study patients were treated with pravastatin or other HMG-CoA reductase inhibitors, which are known to act synergistically with sirolimus in experimental systems to increase fibroblast apoptosis (23). The clinical significance of this effect is presently unknown, but it is conceivable that the use of HMG-CoA reductase inhibitors and sirolimus concurrently may have contributed to the wound-healing problems observed in the study patients.
Our study is limited by the fact that a retrospective, historic comparison group was used, and it was performed at a single center with a small number of patients enrolled. However, we present these findings so that other centers will exercise caution in the use of sirolimus immediately after lung transplantation. The bronchoscopic airway healing scores were not different between the study and the comparison groups, and thus the dehiscence could not be reliably detected or predicted by bronchoscopic appearance, a finding that has been previously noted by other centers (18). Therefore, a potentially serious anantomosic healing problem should be suspected in any sirolimus-treated lung-transplant recipient with any degree of impaired wound healing.
Sirolimus offers many potential advantages for lung-transplant recipients, including reduced nephrotoxicity and the possibility of improved long-term results because of a lower incidence of chronic rejection–bronchiolitis obliterans syndrome. Sirolimus and a related compound, everolimus, have been used safely in other lung-transplant studies. In those studies, the drug was not begun immediately postoperatively but was given 90 days or more after transplantation (24,25). Thus, it appears to be safe to use sirolimus in lung-transplant recipients with well-healed anastomoses; further studies are needed to clarify the efficacy of this approach.
The authors thank Debra A. Dykhuis, Lila M. Schoelkoph, and the clinical lung transplant coordinators at Fairview-University Medical Center, without whom this study would not have been possible.
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© 2003 Lippincott Williams & Wilkins, Inc.
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