This study, for the first time, evaluated the feasibility and effectiveness of haplo-SCT with PTCy/ATG retrospectively in the 55 older patients with hematological malignancies (AL 72.7%). The patients received a novel RIC regimen of fludarabine, Cytarabine, busulfan, and low dose of cyclophosphamide. We showed that patients up to the age of 71 years can achieve an acceptable TRM, GVHD, relapse rate, GRFS, and survival using this novel approach.
GVHD historically has been considered the most common and potentially life-threatening transplant-related complication. Several studies have confirmed that the risk of developing GVHD is higher in older patients [7, 8, 31, 32] and to address this problem, various strategies for GVHD prophylaxis have been developed. ATG has been used widely to reduce GVHD in an HLA-mismatched allo-HSCT in elderly patients [5, 33–35]. Sun et al. [5] reported the results for ATG (10 mg/kg) combined with CsA as GVHD prophylaxis in 50 older patients with hematological diseases, with 80% receiving a combination of bone marrow and peripheral blood as grafts. The results showed a lower CI for grades II − IV aGVHD and cGVHD. However, there are reports that using PBSC might lead to higher rates of GVHD compared to the use of bone marrow stem cells [23, 36, 37]. This led us to speculate that the use of bone marrow in 80% of patients in Sun et al.’s study may have contributed to the low incidence of GVHD. Most studies have reported that ATG-based regimens for preventing GVHD in HID transplantation in older patients were associated with relatively high risks of aGVHD, with the incidence of grades II–IV ranging between 29.5–45.6% [33–35], Taken together, these results highlight the importance of optimizing ATG-based regimens in elderly patients receiving GVHD prophylaxis using haplo-SCT, especially in HID-PBSCT. Many studies have investigated the use of PTCy-based regimens including HID for allo-HSCT to decrease the risk of GVHD in older patients. However, the results of these studies are unsatisfactory with an incidence of II − IV aGVHD and cGVHD of 29 − 35% and 11 − 33%, respectively [38–40]. Another retrospective report of a variety of donors who underwent an allo-HSCT using ATG (4.5mg/kg) combined with PTCy (50mg/kg, on days + 3, +4 ) for GVHD prophylaxis, showed that the CIs for grade II − IV aGVHD at day 100 was 11 (5 − 21)%, and for 2 − year cGVHD was 26 (16 − 38)% [41]. While this encouraging result appears to offer a promising method for GVHD prophylaxis only 12 (19%) cases of haplo-SCT were included in the report. To date, no studies have investigated older patients who underwent a haplo-SCT and received lower doses of PTCy combined with ATG for GVHD prophylaxis. Despite the fact that up to 81.8% of the patients in our study received PBSC as a graft, we showed that the CIs for grades II − IV aGVHD and grades III − IV aGVHD at 100 days post-transplantation were 25.0% (95%CI,15%−38%) and 9.1% (95%CI,3.3%−19%), respectively using haplo-SCT and low-dose PTCy and ATG regimen for GVHD prophylaxis. We also showed a lower incidence of moderate-severe cGVHD at 1 year in older individuals who received the PTCy-ATG combination. Moreover, the incidence of CMV reactivation and EBV reactivation was acceptable. These results suggested that low-dose PTCy-ATG effectively and safely prevents aGVHD and cGVHD in elderly patients undergoing a haplo-SCT.
Traditionally, haplo-SCT in older individuals has a higher TRM because of the patient's accumulation of co-morbidities, poor performance status, and acute or chronic GVHD. In a previous study Raiola et al. [39] investigated 634 patients who underwent unmanipulated haploidentical bone marrow transplantation and showed a correlation between age and TRM, with those older than 60 years having a 4.5-fold increased risk of TRM compared to that of younger patients. Three retrospective studies in older patients who received RIC or nonmyeloablative HLA-haploidentical transplantation also demonstrated that the incidence of non-relapse mortality was 6 − 21% at 100 days and 12 − 30% at 1-year [4, 5, 19, 31, 42]. The results of the current study also showed relatively low CIs for TRM in patients of 7.5% at 100 days, and 19.0% at 1-year. In contrast, a report from the Fred Hutchinson Cancer Research Center [43] showed the rate of TRM appeared to increase with higher HCT-CI scores. The reason our study had relatively lower rates of TRM might be because the patients had relatively lower HCT-CI scores, with only 18.4% of patients having a score ≥ 3. Most reports of allo-HSCT in older patients have considered fludarabine-based conditioning regimens historically have lower transplant-related mortality compared to that of busulfan plus cyclophosphamide [5, 44]. We also used a novel RIC regimen consisting of low doses of Cy (1,200 mg/m2/day, on day − 3) and fludarabine, and this may be another reason for the lower TRM we observed. Furthermore, GVHD represents the main source of NRM in transplant patients. For example, only one patient in our study died from GVHD, whereas seven (43.8%) died from infection. Of note, nearly half of the HSCT recipients in the group died of COVID-19 pneumonia in the early post-transplant phase due to the absence of a pathogen-specific adaptive immune response. These data imply that immune recovery was slower in our study, with the results showing there is room for further improvement in our novel conditioning regimen, such as reducing the total dosage of ATG.
To our knowledge, a relapse of malignancy is the predominant cause of treatment failure and death in patients receiving the RIC regimen. At our center, we used fludarabine -intensified RIC regimens and a combination of ATG and PTCy for GVHD prophylaxis. Interestingly, despite one-third of patients in this series not being in complete remission at the time of transplant, we observed a low and encouraging 1-year CIR after transplantation of only 9.0% that compared favorably with the incidence reported in previous studies [4, 5, 34, 45]. A reasonable interpretation of this relatively lower CIR is that the combination of fludarabine with high dosage Ara-C in the preconditioning regimen for AML and MDS may have enhanced the intensity of the pretreatment regimen and reduced later recurrence. This possibility agrees with the findings of Cao et al[26]. Several studies have also shown that HID with PBSC as the stem cell source rather than bone marrow grafts has stronger graft-versus-leukemia (GVL) and results in lower relapse rates [19, 46–48]. The association between the level of GVHD prevention and the risk of relapse has been debated previously. A report on 133 older patients undergoing allo-HSCT showed that the risk of relapse was like that observed between the combination of PTCy/ATG for GVHD prophylaxis and other regimens[41]. These data suggest that potent suppression of GVHD can be accomplished with dual in vivo TCD without a corresponding increase in the risk of relapse. The patient cohort in our study differed from other studies in that it was limited to patients older than 55 years and included more recently treated patients in whom our novel conditioning regimen became more focused on PBSC as the graft type. However, the heterogeneity and relatively small number of patients, the short follow-up time, and possible selection bias when choosing candidates could not be excluded as potential confounding factors in the study and may have contributed to the low CIR we observed.
The current study had some limitations as it was carried out at a single center on a small number of patients with heterogeneous characteristics, making it difficult to perform a multivariable analysis. In addition, post-transplant immune reconstitution data was not monitored routinely in the 55 patients and therefore we could not assess whether administration of PTCy/ATG as the GVHD prophylaxis regimen affected immune reestablishment.
In conclusion, the findings of this study suggest that a low dose PTCy/ATG regimen following a RIC regimen is an effective therapeutic strategy for appropriately selected patients aged 55 years and older with hematologic malignancies.