Discussion
This study is a real-world cohort study that compared the effectiveness and safety of sirolimus with tacrolimus for clinically active SLE. Sirolimus showed similar overall effectiveness to tacrolimus but better effectiveness in serological improvement and steroid tapering. More adverse events were observed in the sirolimus group than in the tacrolimus group, but none was severe or resulted in discontinuation of sirolimus.
Sirolimus, which is an mTOR inhibitor, has been successfully used for graft-versus-host disease prevention and for treatment of several autoimmune disorders. Its effectiveness and safety in SLE treatment have been reported in uncontrolled studies.13–16 However, a comparison of sirolimus with other classic immunosuppressants has not been reported. In our study, tacrolimus was chosen as the positive control due to its structural similarities with sirolimus and its proven effectiveness as a medication recommended for SLE treatment according to several guidelines.8 24 25 We believe that comparison with tacrolimus could help to properly determine the effectiveness of sirolimus. Propensity score matching was used to ensure that patients had similar baseline conditions.
SLEDAI and PhGA are well-accepted indices for evaluating SLE disease activity. In this study, there were remarkable and equivalent reductions in SLEDAI and PhGA scores in both groups at all follow-up timepoints. The Systemic Lupus Erythematosus Responder Index 4 (SRI4) is a novel index for evaluating treatment response in patients with SLE.26 SRI4 response is defined as the following: ≥4-point reduction in the Safety of Estrogens in Lupus Erythematosus National Assessment-SLEDAI, <0.3-point increase in PhGA, no new British Isles Lupus Assessment Group (BILAG) score,27 and no BILAG organ domain score or two new BILAG B organ domain scores. Lai et al13 reported a 66% response rate at 12 months in patients receiving sirolimus. Referring to SRI4, we analysed the proportions of patients who achieved ≥4-point reduction in SLEDAI-2K with <0.3-point increase in PhGA (although the BILAG Disease Activity Index was not used in this study). Similar good responses were observed in both groups.
While clinical remission is a strict treatment goal in SLE, no patient in either group achieved remission at 3 or 6 months, and only a few patients achieved remission at 9 and 12 months. Thus, as an alternative treatment target, LLDAS was more achievable. The proportions of patients who achieved remission or LLDAS were similar in the two groups. Higher percentages in the sirolimus group were seen at 3, 6 and 12 months, but were not significant. Hence, larger sample size and longer follow-up period may be necessary to see larger percentages of LLDAS or clinical remission.
Anti-dsDNA antibody and complements are important activity-related markers. The percentages of patients whose anti-dsDNA turned negative during follow-up were not different between the two groups. Nevertheless, elevations in C3 and C4 (at 3 and 6 months) in the sirolimus group were significantly higher than those in the tacrolimus group, which indicated better effectiveness for serological improvement in the early stage of treatment.
In addition to disease activity, steroid dose is an important index for measuring the effectiveness of an immunosuppressant. At all timepoints, the percentages of patients whose prednisone doses (or equivalent) were ≤7.5 mg/day were significantly higher in the sirolimus group. This result suggests that sirolimus may be more effective in steroid tapering.
Yap et al14 reported the long-term effectiveness of sirolimus for treatment of lupus nephritis. In a long-term follow-up study, significant improvements in proteinuria and haematuria were observed in patients with SLE who received sirolimus.28 Tacrolimus is an effective medication for treatment of lupus nephritis, especially for reduction of urine protein.17–19 25 In our study, the renal effectiveness of sirolimus was compared with that of tacrolimus. Our results showed their similar effectiveness in achieving complete and partial renal remission and in reducing haematuria. Tacrolimus was found to be significantly more effective in reducing 24hUP at 6 months, and the same tendency was observed at other timepoints. It is probable that sirolimus is not as effective in reducing urine protein as calcineurin inhibitors. The number of patients with renal involvement was relatively small. Hence, a larger sample size would be necessary to fully investigate the effects on patients with renal involvement.
A recent systematic review and meta-analysis analysed the data of 145 patients from 9 studies.29 In 111 clinically active patients, the pooled decrease in SLEDAI and prednisone dosages was 4.85 and 13.17 mg/day, respectively. The results indicated the good effectiveness of sirolimus in disease activity control and steroid tapering. In our study, similar results were observed, and comparison with tacrolimus provided more credible and quantifiable results.
Sirolimus was well tolerated in this study. Although more adverse events were reported in the sirolimus group, all were mild or moderate and did not result in drug discontinuation. The safety of sirolimus as an immunosuppressant has been proven in other immunity-related diseases. More data are required in the future to assess its safety in SLE treatment.
The limitations of this study include the following. First, the sample size was not large enough. Some differences, especially the advantages of sirolimus over tacrolimus, may be obscured due to sample size. Nevertheless, since sirolimus is a novel SLE medication, we believe that our data have provided valuable information for specialists. Hence, further study with a larger sample size will be conducted. Second, as previously mentioned, the BILAG Disease Activity Index was not included; thus, accurate SRI4 could not be recorded. We used ≥4-point reduction in SLEDAI-2K with <0.3-point increase in PhGA to measure treatment response and there might be slight deviations. Third, serum sirolimus concentration was not recorded, and the relationships between serum concentration and effectiveness or adverse effects were not analysed. Fourth, cholesterol and triglyceride levels were not recorded in this study; thus, we could not assess the incidence of dyslipidaemia, which is a commonly reported adverse effect of sirolimus. Fifth, antiphospholipid antibodies were not tested during the follow-up period. Previous studies with sirolimus showed improvement of antiphospholipid antibody levels,13 and sirolimus was reported to be effective in antiphospholipid nephropathy in patients with renal transplantation.30 It is worthy to comprehensively collect data regarding antiphospholipid antibodies in further follow-up to show the effect of sirolimus on antiphospholipid antibodies. Sixth, in this real-world study, there might have been some bias in the recording of adverse events. Since sirolimus is a relatively new drug for SLE, rheumatologists might not have been very familiar with its adverse effects. Distinguishing adverse effects from SLE disease activity was a challenge. Physicians might have recorded all abnormal situations as adverse events to avoid missing any adverse effects of sirolimus. Therefore, there might have been an overestimation of the adverse effects of sirolimus. Rigorous judgement and assessment of adverse events should be conducted in future randomised controlled trials.
In conclusion, we conducted the first study comparing the effectiveness and safety of sirolimus in the treatment of SLE with that of a classic immunosuppressant. Sirolimus and tacrolimus showed equivalent effectiveness in disease activity control, and sirolimus showed better effectiveness in terms of serological improvement and glucocorticoid tapering. Sirolimus was well tolerated in patients with SLE. We believe that sirolimus is effective and safe for SLE treatment.