Background
Patients with relapsed/refractory acute lymphoblastic leukemia (R/R ALL) usually have a very poor prognosis with an expected median survival of less than 6 months, and the overall survival (OS) at 5 years is only 5-10% [
1]. Complete remission (CR) rates after the first salvage chemotherapy are approximately 30-46%, and these rates drop sharply to 18-25% after the second salvage chemotherapy. Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a potentially curative option for hematological malignancies and has improved the prognosis of R/R ALL over the past two decades. However, for patients who failed to achieve minimal residual disease (MRD) negativity before allo-HSCT, their 3-year OS and leukemia-free survival (LFS) were only 23.5% and 20.6%, respectively [
2]. Thus, novel therapeutic strategies to improve prognosis of these patients are urgently needed.
Recently, chimeric antigen receptor T cells (CAR-Ts) targeting CD19 or CD22 have been reported to successfully improve treatment outcomes for R/R ALL [
3,
4]. In our previous clinical trial of CD19-targeted CAR-T therapy against R/R ALL, a CR rate of 92.3% was achieved [
5]. However, data from long-term follow-ups in CAR-T trials show that relapse after CAR-T treatment still remains a predominant obstacle. Relapse rates of 20–70% were described when the follow-up period was sufficiently long [
6]. The median LFS in patients with a low disease burden was 10.6 months, whereas that in patients with a high disease burden was 5.3 months [
3]. With respect to the high relapse rates and the potentially unique relapse mechanisms after CAR-T treatment, consolidation therapy following CAR-T treatment may be considered as a necessary strategy, to reduce the risk of relapse and to maintain the status quo of CR.
Notably, consolidative allo-HSCT after CAR-T therapy is still a controversial option for improving long-term LFS. Park et al. reported that of seventeen patients who underwent allo-HSCT after CAR-T therapy [
3]. Relapse and transplantation-associated complications were the main causes of death for those who received CAR-T therapy before allo-HSCT, and the patients seemed not to benefit from allo-HSCT after CAR-T treatment [
3].
Hay et al. found that allo-HSCT after CD19 CAR-T cell therapy was associated with a better LFS; however, they also reported a better LFS in MRD-negative CR patients who proceeded to allo-HSCT than those who did not [
7]. In addition to allo-HSCT from conventional donors, considerable progress has been made regarding haploidentical HSCT (haplo-HSCT) in recent years, and clinical outcomes of patients receiving haplo-HSCT have been reported to be similar to those receiving HLA-matched HSCT [
8]. To date, the efficacy and safety profiles of haplo-HSCT after CAR-T treatment have not been assessed. Regarding the efficacy and transplant-associated complications, whether patients would benefit from allo-HSCT after CAR-T treatment remains controversial. And, no pre-transplant biomarker has been recommended to predict outcomes after transplantation. Further researches are needed to determine effectiveness of allo-HSCT consolidation therapy and to assess factors affecting long-term clinical outcomes.
Therefore, we designed a multicenter retrospective study to assess the efficacy and safety profiles of CAR-T therapy alone or CAR-T therapy followed by haplo-HSCT in patients with R/R ALL. We also determined potential prognostic factors associated with clinical outcomes in these patients.
Discussion
At present, patients with R/R ALL have a low likelihood of being cured with salvage regimens or available investigational agents. CAR-T therapy has emerged as a rescue for induction therapy in patients with R/R ALL, and clinical studies have reported favorable outcomes in those cases [
15‐
17]. However, relapse after CAR-T treatment remarkably decreases the long-term LFS and OS. As reported, allo-HSCT after CAR-T therapy has the potential to reduce the risk of relapse and improve long-term LFS and OS. Nevertheless, up to date, limited data are available on the clinical outcomes of this combination strategy. In this study, for the first time to our knowledge, we retrospectively reviewed data from CAR-T therapy alone or CAR-T therapy followed by haplo-HSCT for R/R B-ALL in 11 domestic centers in China. Our results showed that patients could greatly benefit from haplo-HSCT after CAR-T therapy. Further analysis indicated that the OS and LFS of patients who received CAR-T treatment followed by haplo-HSCT and achieved pre-transplant MRD negativity tend to be higher than those treated with CAR-T therapy without allo-HSCT, or those who received CAR-T treatment followed by haplo-HSCT and showed pre-transplant MRD positivity. For patients receiving CAR-T treatment followed by haplo-HSCT, MRD negativity at the time of haplo-HSCT was a significant prognostic factor associated with higher LFS and OS. In addition to the beneficial long-term outcomes, risks of treatment-related toxicities were not increased.
Patients with R/R ALL who achieved MRD negativity after chemotherapy and subsequently underwent allo-HSCT are reported to have the best clinical outcomes [
18,
19]. Pavlů et al. reported that compared with pre-transplant MRD negativity, pre-transplant MRD positivity was associated with significantly lower OS (61% versus 67%) and LFS (50% versus 58%), and with a higher cumulative incidence of relapse (32% versus 24%) at 2 years post transplantation [
20]. In this study, all patients were in the second or third complete remission (CR2 or CR3) or were primary refractory; the 2-year OS, LFS, and cumulative incidence of relapse for patients with pre-transplant MRD negativity and MRD positivity were 83.3% versus 62.7%, 76.1% versus 27.6%, and 17.3% versus 65.8%, respectively. The significantly high OS, LFS, and low incidence of relapse suggest that MRD negativity after CAR-T therapy followed by haplo-HSCT is an effective option for patients with R/R ALL. Several studies have reported that intermediate or high risk identified through risk stratification at diagnosis acts as a significant risk factor for OS and NRM in patients receiving allo-HSCT [
21,
22]. Remarkably, multivariate analysis of our data showed that not only risk stratification at diagnosis but also several classic risk factors (such as adverse cytogenetics, total number of relapses, or high leukocyte counts) were no longer associated with poor OS or LFS, which may be related to patient selection and therapy decisions.
Notably, to the best of our knowledge, the present study is the first report of a combined CAR-T and haplo-HSCT strategy against high tumor burden of R/R ALL. CAR-T therapy contributing to pre-transplant MRD negativity plays an important role in favorable clinical outcomes. Haplo-HSCT as consolidation also has a positive effect. In general, haplo-HSCT could have the following advantages for patients: (1) a theoretically high donor availability of almost 100%, (2) a less time-consuming process of finding a donor, and (3) a superior graft-versus-leukemia (GVL) effect [
23,
24]. In our previous prospective study, we developed a protocol for T cell-replete haplo-HSCT with low-dose anti-T-lymphocyte globulin and showed that high-risk patients receiving haplo-HSCT experienced protection against relapse. Our protocol provided clinical evidence supporting haplo-identical donors as first-line alternative donors, especially for high-risk patients [
25], in line with studies at other centers [
26,
27]. Because all patients undergoing CAR-T treatment are relapsed/refractory with high-risk cytogenetics and/or molecular abnormalities, in the present study, we developed a novel strategy using CAR-T cells for re-induction followed by haplo-HSCT for consolidation in patients with R/R ALL. Theoretically, CD19-targeted CAR-T cells would eradicate all CD19-positive leukemia cells; however, we cannot exclude the possibility that certain CD19-negative leukemia sub-clones exist. Considering the results of this study, CAR-T cytotoxicity and GVL effect could be attenuated by conditioning therapy. The synergistic effects of CAR-T and conditioning therapy would potentially eradicate leukemia cells. Hay et al. reported that for patients who received CAR-T therapy to achieve MRD negativity before allo-HSCT, the 2-year LFS and OS were 61% and 72%, respectively. The 2-year cumulative incidence of relapse was 17%
7, implying a prolonged LFS and OS. The present study showed similar results supporting a combined efficacy of CAR-T therapy and haplo-HSCT. Our results also support that haploidentical immune cells exert a potent GVL effect in such a combination modality.
Recently, studies in adult patients with Ph-negative ALL have established that the initial MRD response is a strong prognostic factor. The German Multicenter Study Group for Adult ALL analyzed the largest cohort of adult ALL data to assess MRD in Ph-negative patients and reported that molecular response was the only parameter with a significant prognostic effect [
28]. Other studies confirmed a strong and independent prognostic effect of MRD after induction and early consolidation treatment [
29‐
31]. In the present study, we clearly identified pre-transplant MRD negativity as an important independent predictor of high LFS and OS. Our data support a beneficial modality of haplo-HSCT following CAR-T treatment in patients with R/R ALL.
In addition, we showed that the experience of relapses before CAR-T therapy was another adverse factor for high LFS. In patients with more relapse scenarios, leukemia cells exhibited stronger chemotherapy resistance, more genetic mutations, and immune escape [
32,
33]. Moreover, consistent with previous studies in patients who received haplo-HSCT after chemotherapy [
34,
35], our study found that age over 40 years was an independent risk factor associated with poor OS [
36].
Safety issues are major concerns associated with combination therapy including immunotherapy and HSCT. In the present study, we observed no increased risk of treatment-related complications or immune toxicities. Moreover, no patient died of treatment-related complications. The 100-day cumulative incidence of grade III-IV aGVHD was less than 10% in our study, similar to previously reported values [
8,
17,
37]. Allo-HSCT after CAR-T treatment does not seem to increase the risk of therapy-associated complications.
Infection was another severe complication despite CAR-T treatment or allo-HSCT. Park et al. reported that 22 of 53 adult patients experienced 26 infections within the first 30 days after CAR-T infusion, and three patients died of an infection-related cause [
38]. Infection is a primary or contributing cause of death in more than half of patients who die in the follow-up period after allo-HSCT [
39]. Slade et al. reported that 62% and 6% of patients experienced at least one bacterial and one invasive fungal infection, respectively [
39], whereas in the present study, the rates were 14.0% and 5.3%, respectively. Moreover, CMV viremia was detected in the present study and in another study [
40]. Our results show that despite the severe CRS and long-term duration of pancytopenia, the novel protocol of CAR-T treatment combined with haplo-HSCT did not increase the risk of infection.
The present study has several limitations, including the retrospective nature, lack of common prospective transplant protocols among the reporting transplant centers, and limited sample size, which may affect the reliability of the statistical analysis. The choice of covariates for the multivariate analysis was constrained by the small number of observed events.
Acknowledgements
We are indebted to colleagues from the Bone Marrow Transplantation Center, The First Affiliated Hospital, School of Medicine, Zhejiang University, Zhejiang Province Engineering Laboratory for Stem Cell and Immunity Therapy, Institute of Hematology, Zhejiang University, Department of Hematology, Beijing Tiantan Hospital, Department of Hematology, Tianjin First Central Hospital, Department of Hematology, Shanghai Tongji Hospital, Department of Hematology, Guangdong Second Provincial General Hospital, Department of Hematology, Changhai Hospital of Shanghai, Department of Hematology, Xiangya Third Hospital, Department of Hematology, Shanghai Children’s Medical Center, Department of Hematology, Shanghai General Hospital, Department of Hematology, Peking University Third Hospital, Department of Hematology, Zhujiang Hospital of Southern Medical University, Department of Hematology, Xinhua Hospital of Shanghai, Innovative Cellular Therapeutics Co, Ltd, and Shanghai YaKe Biotechnology Ltd. We especially thank all the patients who participated in the study, without whom this study would never have been accomplished.
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