Reduced Intensity Conditioning Before Allogeneic Hematopoietic Stem Cell Transplantation for Acute Myeloid Leukemia in Complete Remission and Myelodysplastic Syndrome: A Meta-Analysis of Randomized Controlled Trials

Abstract

Background

Reduced intensity conditioning (RIC) before allogeneic hematopoietic stem cell transplantation (allo-HSCT) was reported had the same overall survival (OS) as myeloablative conditioning (MAC) for acute myeloid leukemia (AML) in complete remission (CR) and myelodysplastic syndrome (MDS) but results in different studies are contradictory. Therefore, we conducted a meta-analysis according to the PRISMA 2009 guidelines to confirm the efficacy and safety of RIC vs. MAC for AML in CR and for MDS.

Methods

PubMed, Web of Science, Embase, Cochrane central, related websites, major conference proceedings were searched, and related journals were hand-searched from Jan 1, 1980 to July 1, 2020 for studies comparing RIC with MAC before the first allo-HSCT in patients with AML in CR or MDS. Only RCTs were included. OS was the primary endpoint and generic inverse variance method was used to combine hazard ratio (HR) and 95% CI.

Results

We retrieved 7770 records. Six RCTs with 1413 participants (711 in RIC, 702 in MAC) were included. RIC had the same OS (HR = 0·95, 95% CI 0·64–1·4, P = 0·80) and cumulative incidence of relapse as MAC (HR = 1·18, 95% CI 0·88–1·59, P = 0·28). RIC reduced non-relapse mortality more than total body irradiation/busulfan based MAC (HR = 0·53, 95% CI 0·36–0·80, P = 0·002).

Conclusion

RIC also had similar long-term OS and graft failure as MAC. RIC is also a good choice for patients with AML in CR or MDS.

Background

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) has the lowest risk of relapse than any other treatment for acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS).¹ However, allo-HSCT, like traditional myeloablative conditioning (MAC) regimens, has been associated with a high risk of serious adverse events and high non-relapse mortality (NRM).² Over the past three decades, the development of less toxic and more tolerable pre-transplantation regimens—the reduced intensity conditioning (RIC) regimen—has thus become the focus of clinical research.³ Specifically, the RIC regimens consisted of less than 8 Gray (Gy) of total body irradiation (TBI), less than 8 mg/kg PO of busulfan (Bu), or intravenous equivalent dose or other medications with high powered immuno-suppressive effect but with less tissue toxicity to replace TBI or Bu along with fludarabine (Flu) to replace cyclophosphamide (Cy).³ RIC reduces tissue injury and consequently reduces the incidences of acute graft versus host disease (aGVHD) and other complications but maintains graft versus leukemia effect to prevent leukemia relapse.³ Some non-randomized controlled studies reported that RIC reduced NRM but increased disease relapse, generally resulting in the same overall survival (OS) as MAC.^4–6 However, these observational studies lack the benefit of random allocation, which is important to balance the baseline characteristics of patients among different treatment arms, especially to control for confounding by indication bias. Recently, several high-quality randomized controlled trials (RCTs) compared RIC with MAC for fit patients with AML in complete remission (CR) and MDS, but the results were not consistent.^7–12 The number of patients receiving RIC is rapidly increasing. In the United States, RIC accounts for more than 50% of all allo-HSCTs.¹³ Except for AML and MDS, there have been no prospective studies comparing RIC with MAC for other hematologic malignancies. Therefore, we undertook this meta-analysis to clarify the efficacy and safety of RIC vs. MAC for AML in CR and for MDS.

Methods

This meta-analysis was guided by PRISMA 2009 guidelines (Supplement 1). The protocol of the meta-analysis is registered on PROSPERO with the ID of CRD42020185436.

We included only RCTs compared RIC with MAC before first allo-HSCT in patients with AML in CR or MDS defined with 2008 World Health Organization¹⁴ (recruitment began after 2008) and French-American-British criteria (recruitment began before 2008). We did not restrict age, sex, race, recruitment period, complicated diseases or languages. Any aGVHD prophylaxis regimen except T cell depletion in vitro was allowed. Median follow-up time was more than one year. There were no other exclusion criteria.

Primary endpoint was OS. Secondary endpoints were leukemia-free survival (LFS), cumulative incidences of relapse (CIR), NRM, aGVHD and chronic (c) GVHD. Survival data were evaluated from the first day after stem cell transfusion until the event first occurred and used the longest follow-up data. Glucksberg,¹⁵ International Bone Marrow Transplant Registry grading systems¹⁶ and Seattle criteria¹⁷ were used to grade aGVHD and cGVHD. Incidences of III-IV aGVHD, extensive cGVHD, graft failure (GF), overall organ toxicity, oral mucositis, specific organ toxicities and reported infection were safety outcomes.

We electronically searched databases and hand-searched related articles between Jan 1, 1980 and July 1, 2020. Supplement 2 showed the detailed searching strategy. Cochrane highly sensitive search filters were used for identifying RCTs in Medline and Embase.¹⁸

Yanzhi Song (YS) and Zhichao Yin (ZY) independently screened retrieved records, extracted data of the characteristics of included studies according to Table 1 and Supplement 3 and used Cochrane Collaboration-recommended tool to assess quality of included studies (Table 2 and Supplement 3).¹⁹ Only studies in low-risk group were included. Any disagreement was resolved by discussion through YS, ZY and Jie Ding. We contacted authors if data was not enough.

Revman software (Version 5.3; Copenhagen: The Nordic Cochrane Centre, The Cochrane Collaboration, 2012) was used. We combined hazard ratio (HR) and its 95% confidence interval (CI) for OS, CIR, LFS, NRM, aGVHD and cGVHD with generic inverse variance method.²⁰ Log HR and variance statistics were calculated according to Parmar et al.²¹ We used Mantel-Haenszel²² and DerSimonian–Laird²³ method with relative risk (RR) or odds ratio (OR) and 95% CIs to combine dichotomous data. Two-sided P < 0·05 was statistically significant. Heterogeneity was calculated with Q test and I² statistics. Fixed effect model was used if heterogeneity was not significant (P > 0·10 and I² < 50%). If heterogeneity was significant (P ≤ 0·10 and/or I² ≥ 50%) we used random effects model. Because treosulfan was less toxic than TBI/Bu,^8,24 we predefined three subgroups named RIC vs. TBI/Bu based MAC, RIC vs. treosulfan 30 g/m² based MAC and RIC vs. treosulfan 42 g/m² based MAC. In NRM and aGVHD meta-analyses, we only combined HR of every subgroup but did not combine the total HR of all included studies. Except for NRM and aGVHD, we combined both the HR in the subgroups and all included studies. Sensitivity analyses removing included studies were used to evaluate whether quality of studies and clinical characteristics influenced results. We had planned to use funnel plot and meta-regression to detect publication bias.

Quality of evidence on main endpoints were evaluated with “GRADE evidence profiles” table.²⁵

Results

Our search retrieved 7770 references. We first screened the titles and abstracts and excluded 7751 records that were not relevant to RIC for AML in CR and MDS or not RCTs. After further examined fulltexts of the remained 19 records, we excluded 10 references that were not RCT studies, not relevant to RIC or not compared with MAC regimens and the duplicated reports. In the end, we included 6 RCTs reported in 9 references into meta-analyses. All authors agreed to include the six studies (Bornhäuser et al.,⁹ Kröger et al.,¹⁰ Ringdén et al.,¹¹ Scott et al.,¹² Beelen et al.⁸ and MC-FludT·14/L Trial I;⁷ for flow diagram see Fig. 1). Studies Bornhäuser et al.,⁹ Kröger et al.,¹⁰ Ringdén et al.¹¹ and Scott et al.¹² reported the long-term follow up data.^11,26−28

The six included studies with 1413 participants (711 in the RIC group and 702 in the MAC group) all focused on the efficacy and safety of RIC compared with MAC followed by allo-HSCT for AML in CR and MDS. Four studies focused on RIC vs. TBI/Bu based MAC while two studies focused on RIC vs. treosulfan based MAC regimens. All the studies used peripheral stem cell and bone marrow as stem cell sources. Donors included matched related, mismatched related and matched unrelated donors. The demographic characteristics of the two treatment arms were similar in the included studies and are shown in Table 1. All included studies displayed low risk of bias. Details of quality assessment of the included studies are shown in Table 2 and Supplement 3. All the studies used the intention-to-treat method to analyze OS, CIR and LFS. There was no selective reporting in all the included studies. Because funnel plots and meta-regression should only be used with more than 10 studies, we did not use them to detect publication bias in our analysis.²⁹

OS was not statistically different between RIC and MAC (HR = 0·95, 95% CI 0·64–1·4, P = 0·80). Heterogeneity of the meta-analysis was significant (P = 0·003, I² = 72%) (Fig. 2A). The result was also similar in the RIC vs. TBI/Bu based MAC subgroup analysis (HR = 0·84, 95% CI 0·5–1·4, P = 0·50) with significant heterogeneity (P = 0·04, I² = 65%), but in the RIC vs. treosulfan 30 g/m² based MAC subgroup analysis, RIC was significantly inferior to the treosulfan based MAC conditioning regimen (HR = 1·63, 95% CI 1·17 − 2·28, P = 0·004). The combined long-term follow-up data also showed there was no difference between RIC and MAC (HR = 0·86, 95% CI 0·53–1·41, P = 0·56) with significant heterogeneity (P = 0·01, I² = 73%) (Fig. 4).

There was no difference in CIR (HR = 1·18, 95% CI 0·88–1·59, P = 0·28) between RIC and MAC (Fig. 2B). There was also no difference in CIR in the three subgroup analyses. Heterogeneity in the meta-analysis and in the RIC vs. TBI/Bu-based MAC subgroup was significant. Bornhäuser et al.,⁹ Kröger et al.¹⁰ and Scott et al.¹² reported LFS, the combined result showed RIC had similar LFS to MAC (HR = 1·09, 95% CI 0·69–1·74, P = 0·71) with significant heterogeneity (P = 0·05, I² = 66%) (Fig. 2C).

RIC significantly reduced NRM compared to TBI/Bu based MAC (HR = 0·53, 95% CI 0·36–0·8, P = 0·002) with no heterogeneity (P = 0·40, I² = 0%) (Fig. 3A). However, the treosulfan 30 g/m² based MAC⁸ significantly reduced NRM compared to RIC (HR = 1·67, 95% CI 1·02–2·72, P = 0·04). RIC did not show a significant difference compared with treosulfan 42 g/m² based MAC (MC-FludT·14/L Trial I;⁷ HR = 0·76, 95% CI 0·45–1·30, P = 0·32).

RIC showed a tendency to reduce aGVHD (Fig. 3B) and III-IV aGVHD (Supplement 4) compared to TBI/Bu based MAC (HR = 0·79, 95% CI 0·60–1·03, P = 0·08) and (RR = 0·61, 95% CI 0·36–1·04, P = 0·07) with no significant heterogeneity (P = 0·15, I² = 43%) and (P = 0·19, I² = 39%). In the Beelen et al.⁸ and MC-FludT·14/L Trial I⁷ studies, RIC still did not show a significant difference from treosulfan based MAC (either 30 g/m² or 42 g/m²).

There was no difference between RIC and MAC in cGVHD (Fig. 3C) and extensive cGVHD (Supplement 4) (HR = 1·01, 95% CI 0·79–1·28, P = 0·96 and RR = 1·03, 95% CI 0·77–1·37, P = 0·84, respectively) with significant heterogeneity (P = 0·08, I² = 49% and P = 0·09, I² = 51%, respectively). There was also no difference between RIC and MAC in the subgroup analyses.

RIC showed a trend of increasing GF (OR 2·19, 95% CI 0·96–5·03, P = 0·06) without heterogeneity (P = 0·34, I² = 12%). The incidence of GF in the RIC and MAC arms were both rare, 2.57% (18 events in 701 participants) and 1.16% (8 events in 690 participants), respectively. RIC did not show significant difference from MAC in terms of overall organ toxicity and oral mucositis, with significant heterogeneity. On the other hand, RIC significantly reduced renal and urinary disorders (RR 0.61, 95% CI 0·39–0.97, P = 0·04) and infection (RR 0.87, 95% CI 0·78–0.97, P = 0·01) without heterogeneity (Supplement 4).

We repeated the meta-analyses for the OS, CIR and long-term OS with the fixed-effect model because of their significant heterogeneity and the results did not change the overall conclusions of these endpoints (Supplement 5).

We removed one study at a time and then repeated the meta-analysis in the sensitivity analysis. The pooled HRs ranged from 0.84 to 1.05 for OS and from 1.02 to 1.26 for CIR. The results after removing any study (including Beelen et al.⁸ and Scott et al.¹² studies) were overall stable. After we removed the Scott et al.¹² study, the heterogeneity of CIR disappeared (Supplement 6) and the results of CIR did not change. Eight CML patients were included in the Ringdén et al.¹¹ study. When we removed it in the sensitivity analysis there were no significant changes were observed in the results of OS, CIR, and NRM (Supplement 7).

The quality of evidence for the OS, CIR, LFS and cGVHD endpoints was moderate. The quality of the NRM and aGVHD endpoints was high (Supplements 8 and 9).

Discussion

Retrospective studies and their meta-analyses cannot balance the baseline characteristics of patients among different treatment arms. Most patients in the RIC arm in these studies were older or had higher comorbidity burden, which might underestimate the efficacy and safety of RIC. Half of all finished RCTs (Bornhäuser et al.,⁹ Scott et al.¹² and Kröger et al.¹⁰) did not enroll enough participants as the studies had planned which limited their power to demonstrate the difference between RIC and MAC. All the finished studies cannot provide reliable evidence to evaluate RIC for AML in CR and MDS, so we need higher level of evidence on this question. Our meta-analysis included six high quality RCTs with 1413 participants, furthermore, we included both published and unpublished data which limit the risk of publication bias. Therefore, it was more powerful and covered more patients than previous studies. As far as we know, our study is the first comprehensive meta-analysis of RCTs combined HR value to clarify the efficacy and safety of RIC vs. MAC and provides the highest current level of evidence for this question.

Worrying that RIC may increase CIR is the main concern for physicians to prescribe these conditioning regimens. The Scott et al.¹² study demonstrated that RIC increased relapse significantly and suggested physicians should choose MAC first for fit patients. However, when we combined data from all available RCTs, we failed to show any difference in CIR between RIC and MAC. The heterogeneity was caused by the Scott et al.¹² study. After we removed it in the sensitivity analysis, there was no heterogeneity between the remaining five studies and the results did not change (Appendix F). The relapse rate is affected by many factors; for example, the cytogenetic and molecular biologic characteristics of diseases, minimal residual disease (MRD) before HSCT and immunosuppressant ajustment protocol, etc.^30–33 It was impossible that all factors before transplantation were similar in every study, hence the CIR was expected to be heterogeneous between studies. In a large observational analysis by the EBMT included 2974 middle-aged AML patients, relapse incidence was higher in intermediate- or high-risk patients but not in low-risk patients in the RIC group.^32,33 Most of our included studies did not examine MRD before HSCT to stratify participants, which might have substantially influenced the results as patients who were MRD-positive would have higher CIR after RIC more than after MAC.^34,35 In the Scott et al. study, nearly two-thirds of the AML participants were found to have commonly mutated genes in AML using next generation sequencing techniques, and in these patients RIC significantly increased CIR compared to MAC, whereas in the remaining third of participants in whom these genes were not detected, RIC had the same CIR as MAC.³⁶ In addition, all of the six included studies used the same GVHD prophylaxis in RIC and MAC, but the dose-adjustment protocol of immunosuppressant that was appropriate for MAC might have increased CIR for RIC. Therefore, it was possible there was heterogeneity between the included studies. Three RCTs demonstrated that RIC still did not increase CIR in the long-term follow-up data.^11,26,28 As there were insufficient long-term data reported in all the included studies, we could not combine the long-term CIR. However, as most of relapses after HSCT occur within two years;³⁵ we conclude that RIC conditioning regimens do not increase CIR more than MAC for AML in CR and MDS.

A more intensive conditioning regimen causes more serious tissue damage, which may result in more severe aGVHD.³⁶ Therefore, RIC is expected to not only decrease organ toxicity and tissue damage but also will cause less aGVHD and NRM than TBI/Bu based MAC. Our meta-analysis showed a trend for RIC to decrease aGVHD and III-IV aGVHD compared to TBI/Bu based MAC, but it was not statistically significant. We are still in need of more high-quality studies to confirm whether there is difference between RIC and MAC on the incidences of aGVHD and III-IV aGVHD. Our results indicated that there was no difference in cGVHD between RIC and MAC and confirmed the incidence of cGVHD was not related to conditioning intensity.³⁷ In the retrospective studies, RIC reduced NRM^4–6 but RCTs failed to demonstrate the reduction. Our meta-analysis confirmed that RIC significantly reduced NRM compared with TBI/Bu based MAC. There was no heterogeneity, and the quality of evidence was high (Appendix I). The RCTs were relatively small sample size, especially some RCTs did not include enough participants as planned so they might be not powerful enough to demonstrate the difference. We included all the RCTs which expanded the sample size and provided more powerful evidence to clarify the difference. Additionally, the four included studies in the RIC vs. TBI/Bu based MAC subgroup analysis involved relatively young and fit patients but not old patients and in the subgroup analysis RIC also caused less NRM. Consequently, RIC significantly reduces NRM more than TBI/Bu based MAC for both young and old patients.

Moreover, our results showed RIC significantly reduced some organ toxicity and infections compared to MAC, which indicated that RIC was more tolerable than MAC. On the other hand, our result did not show the difference on mucositis between RIC and MAC as generally expected. The heterogeneity of the meta-analysis was significant so more studies are needed to clarify the problem. RIC had a trend to increase GF compared to MAC, but it was not significant. There were only 18 GFs out of 701 patients and 8 GFs out of 690 patients reported in the RIC and MAC groups, respectively. The incidence of GF in the two groups was rare. According to the evidence available we can conclude RIC also caused little GF.

According to our results RIC had the same OS as MAC, but heterogeneity was significant. In the HSCT procedure, the individualized prescriptions of different physicians will inevitably interfere with the results. Therefore, heterogeneity is common in clinical studies on HSCT, even when all the included studies are RCTs. And then we used fixed-effect model to verify the results and there was still no difference between RIC and MAC on OS (Appendix E). In study Beelen et al.⁸ treosulfan 30 g/m² based MAC which caused less NRM than RIC was used. Even it was included in the meta-analysis RIC did not increase OS than MAC. Just the same as it, RIC was still not different to MAC in OS after we excluded it in the sensitivity analysis (Appendix J). A report from the Acute Leukemia Working Party of the EBMT retrospectively included 883 RIC compared with 1041 MAC demonstrated RIC increased OS for ≥ 50 years patients than MAC and had the same OS for ≤ 50 years patients as MAC.³⁸ A large sample retrospective study also showed there was no significant difference in long-term survival between RIC and MAC.³⁹ Both of the two studies also showed RIC did not increase relapse. Our meta-analysis could not divide participants according to age, but our results also showed RIC at least did not decrease OS than MAC. The RIC vs. TBI/Bu based MAC subgroup analysis included more young patients, RIC also showed no difference from MAC on OS. Additionally, our long-term follow-up OS data meta-analysis showed RIC did not decrease long-term OS compared with TBI/Bu based MAC. Consequently, we concluded RIC did not increase cumulative incidence of relapse but decreased NRM compared with traditional MAC regimens, furthermore, it at least did not increase aGVHD and had the same cGVHD as MAC, as a result, RIC did not decrease OS. Therefore, we confirmed there was no difference between RIC and MAC in OS for AML in CR and MDS.

In the RIC vs. treosulfan 30 g/m² based MAC subgroup analysis, treosulfan caused less NRM than RIC and furthermore increased OS.⁸ Treosulfan is a novel myeloablative agents with less toxicity than Bu.²⁴ Treosulfan based MAC was named reduced-toxicity conditioning regimen.²⁴ The subgroup analysis confirmed treosulfan was less toxic than Bu and suggested treosulfan 30 g/m² based MAC was better than Bu or TBI based RIC. It was a promising result and provided a new myeloablative agents that was superior to the traditional Bu or TBI. However, only one RCT finished until recently and the RIC vs. treosulfan 42 g/m² based MAC subgroup analysis did not show treosulfan caused less NRM or OS than RIC;⁷ hence, we need more high-quality studies to confirm the result.

There are some limitations of our meta-analysis. First, a relatively small number of clinical trials were included. Second, in OS, CIR, and LFS meta-analyses, there was significant heterogeneity between included studies. We suggested the reason for the heterogeneity was the difference in treatment details available from the different transplantation centers and the inevitable patient heterogeneity between included studies. Third, not all the included studies used blinding to personnel and patients. Allo-HSCT is a treatment with high NRM⁴⁰ and the treatment details should be individualized to every patient; therefore, blinding to patients and personnel could not be maintained. Forth, because we used data extracted from published reports but not individual patient data, we could not perform subgroup analysis based on diseases (AML in CR and MDS) and age. MDS patients may have less relapse than AML and young patients tolerate MAC better than old patients, so RIC may demonstrate better results in MDS patients and elderly patients. Even given these limitations, our meta-analysis is still reliable and can be used to guide physicians’ clinical decisions.

Conclusion

RIC had the same OS and CIR as MAC for AML in CR and MDS and significantly decreased NRM more than TBI/Bu based MAC. Furthermore, RIC was more tolerable and comfortable and caused little GF. RIC is equally effective as MAC. Therefore, RIC is also a good choice of conditioning regimen before allo-HSCT for patients with AML in CR and MDS and not just an alternative treatment to MAC for unfit patients. On the other hand, more high-quality studies should continue to focus on the OS and LFS comparing RIC with MAC. MRD, disease (AML or MDS), cytogenetic and molecular biologic characteristics and age should be considered as classification factors in future studies to identify the factors from which patients will derive more benefit from RIC. Additionally, future studies should try to improve GVHD prophylaxis that would be more appropriate for RIC. We also need more studies to compare treosulfan-based MAC with RIC.

Abbreviations

Declarations

Ethics approval and consent to participate：Not applicable.

Consent for publication: Written informed consent for publication was obtained from all participants.

Availability of data and materials: No additional unpublished data are available.

Competinginterests: The authors have no conflict of interest to declare.

Funding: We received no funding.

Authors’ contributions: Yanzhi Song conceived and designed the study, searched and selected trials for inclusion, assessed methodological quality of included trials, extracted data, performed the statistical analysis and wrote the article. Zhichao Yin searched trials, selected trials for inclusion, assessed methodological quality of included trials and extracted data. Jie Ding wrote and revised the review. Tong Wu written and revised the manuscript.

Acknowledgements: Thanks to Dr. Hans-Juergen Kuehnel, the Medical Director of Medac, who provided the GVHD and relapse data of the MC-FludT·14/L Trial I study.

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