Comparison of outcomes of immunosuppressive therapy with rabbit versus horse antithymocyte globulin and cyclosporine a in children with acquired severe aplastic anemia
Authors: Ayami Yoshimi, Peter Noellke, Jan Starý, Krisztián Kállay, Owen Smith, Franco Locatelli, Jochen Buechner, Ivana Bodova, Julian Sevilla, Markus Schmugge, Marc Bierings, Tania Masmas, Michael Dworzak, Veerle Labarque, Katarzyna Pawelec, Kirsi Jahnukainen, Sophia Polychronopoulou, Paula Kjollerstrom, Marko Kavcic, Miriam Erlacher, Charlotte M. Niemeyer, Brigitte Strahm
Journal: American Journal of Hematology (2024)

Immunosuppressive therapy (IST) combining antithymocyte globulin (ATG) and cyclosporine (CSA) is a consolidated therapy for aplastic anemia (AA). Currently, there are two animal sources of ATG available, namely horse (h-ATG) and rabbit (r-ATG) ATG. While the h-ATG Atgam® (Pfizer) continues to be the standard ATG for IST in the US, the h-ATG traditionally used in Europe and Asia, Lymphoglobulin® (Genzyme), was withdrawn from the market in 2007. Since then, first-line therapy in many European and Asian countries consisted of Thymoglobulin® (r-ATG, Sanofi). Several groups reported inferior results of IST with r-ATG compared to those with h-ATG, while comparable results of both ATG were reported by others group.1-3 Importantly, a randomized controlled trial published in 2011 showed an inferior response rate at 6 months (37% vs. 68%, p < .001) and a decreased overall survival (OS) at 2 years (76% vs. 96%, p = .04) in patients in the United States treated with r-ATG (Thymoglobulin®) compared with h-ATG (Atgam®).2 Based on these results, most European centers introduced Atgam® as first line IST, while Thymoglobulin® continues to be applied in countries where h-ATG is not available. Recently, Atgam® has been approved for the treatment of AA in most European countries. Here, we report the outcome of IST with ATG and CSA in 150 children with AA from 19 European countries, who were registered to the European Working Group of Severe Aplastic Anemia (EWOG-SAA) 2010 study between 2011 and 2020 (German Clinical Trials Register: DRKS00000610), comparing h-ATG Atgam® and r-ATG Thymoblobulin®. Our results show that patients treated with h-ATG have better early response, which may be clinically beneficial for patients with AA, while there was no difference in survival between the two ATG groups. One hundred-fifty consecutive patients with AA less than 18 years of age, who received first-line IST with either h-ATG (Atgam®, 40 mg/kg × 4 days, n = 110) or r-ATG (Thymoglobulin®, 3.5 mg/kg × 5 days, n = 40) with at least 180 days follow-up after start of treatment were included in this study. Patients with an HLA compatible sibling at diagnosis received a first-line hematopoietic stem cell transplantation (HSCT). A diagnosis of Fanconi anemia was excluded by a chromosomal breakage test. Patients with other inherited bone marrow failure syndromes were excluded from the study. h-ATG was the first choice of ATG, but r-ATG was used if h-ATG was not available. CSA was started on the first day of ATG administration and was slowly tapered (10% per month) at 12 months regardless of response status. Granulocyte-colony stimulating factor (5 μg/kg/day) was administered in the first 28 days to patients with <0.5 G/L neutrophils and was tapered and discontinued by day 60. Complete remission (CR), good partial remission (GPR), poor partial remission (PPR), no response (NR), and relapse were defined according to the criteria described in Supplementary Table 1.4 HSCT was recommended as a second-line therapy in all patients with non-response at day 180, relapse, or evidence of clonal evolution, while early HSCT (<day 180) was considered in cases with persistent severe neutropenia (<0.2 G/L) at day 90 due to high risk of severe infections. The median age at diagnosis was 10 years (ranged 1.1–18 years) (Supplementary Table 2). Remarkably, 15% of patients had hepatitis-associated aplastic anemia. Of note, 85% of patients had less than 0.2 G/L neutrophils prior to IST, and 41% of patients had 0 G/L neutrophils. The median time from diagnosis of AA to start of IST was 36 days (range 7–605 days). There were no differences in patient characteristics between the h-ATG and r-ATG groups. The median follow-up after IST was slightly but not significantly longer in the r-ATG group (4.2 vs. 2.9 years, p = .1). The overall response rate (CR + GPR + PPR) at 6 months was significantly higher in the h-ATG group than in the r-ATG group (42% vs. 22%, p = .03) (Supplementary Figure 1). In the univariate and multivariate analyses, a lower absolute neutrophil count (ANC) prior to IST and use of r-ATG were associated with an inferior response to IST at 6 months (Supplementary Table 3). For the h-ATG group, the response rate (42%) was similar to that of the control arm (h-ATG, Atgam® + CSA only) of the recent randomized trial comparing h-ATG (Atgam®) + CSA ± eltrombopag (EPAC) in adults with AA in Europe (41% at 6 months).5 However, the response rates at 6 months for both h-ATG and r-ATG in the current study were lower than those of our previous report comparing h-ATG, Lymphoglobulin® and r-ATG, Thymoglobulin® (65% and 34%, respectively).3 There are several possible reasons for the lower response rates in the current study. First, this cohort included 41% of patients with ANC of 0 G/L before IST. A Japanese study reported that such patients with a "fulminant" AA had a significantly lower response rate at 6 months and showed slower responses to IST.6 Indeed, there were many late responses after day 180 in both ATG groups in our study (r-ATG: 5/18, 28% vs. h-ATG: 8/41, 20% of non-responder at day 180, p = n.s.). Moreover, 22% (n = 33) of patients (h-ATG: 20% vs. r-ATG: 28%) received early second-line HSCT (< day 180) which resulted in an underestimation of the response rate. The main reason of early HSCT was persistent severe neutropenia, the ANC prior to HSCT was <0.1 G/L in 16 of 28 patients with available data. Excluding patients with early HSCT from the analysis, the response rate at 6 months were 53% and 31% in the h-ATG and r-ATG groups, respectively (p = .05). Moreover, the interval between diagnosis and start of IST in this study was considerably longer than that in our previous report (36 vs. 25 days), this potentially contributing to an exhausted stem cell reserve and lower response rate.3 Finally, it should be also noted that there is no evidence that the efficacy of the two h-ATGs, Atgam® and Lymphoglobulin®, is comparable, and preparations, schedules, and doses of the two products are different. Relapse was observed in 16% of day 180 responders at the median time of 667 days (range 552–1767 days). Interestingly all relapses were noted in the h-ATG group: h-ATG: 9/46 (20%) vs. r-ATG: 0/9 responders (p = n.s.). Of note, there was no difference in the overall response rates at the last follow-up between the two groups (h-ATG: 39% vs. r-ATG: 35%). Marsh et al. reported similar results with the lower response rate at 6 month in the r-ATG group, but comparable best response rates of h-ATG and r-ATG groups due to the higher incidence of late responses in the r-ATG group.1 In our cohort, most responders had a good hematological condition at the last follow-up, either CR or GPR, and only a few patients had PPR (supplementary Figure 1). There were only two patients (1%), who developed clonal evolution (monosomy 7 and trisomy 8 each). Eighty-two patients (55% in the total cohort; 55% in the h-ATG group, 53% in the r-ATG group) treated with IST received second-line HSCT for lack of response (n = 72), relapse (n = 8), or clonal evolution (n = 2) at a median of 203 days (range 45–1030 days) after IST initiation. Six patients (h-ATG: n = 2, r-ATG: n = 4 for the 1st IST) were given a second course of IST (2 for relapse, 3 for NR or 1 PPR, h-ATG: n = 2, r-ATG: n = 4 for the 2nd IST), and four of them showed response. Although recent studies have shown the benefit of adding eltrombopag to standard IST in adults with AA, currently available data do not support the addition of eltrombopag to IST in children with treatment-naïve AA.7 None of the patients in this study received eltrombopag initially, but the drug was added in 5 patients at the median of 192 days (ranged 122–671 days) after IST start, leading to hematological improvement in two patients. Although inferior OS due to lower response rate has been reported in adults patients with AA treated with r-ATG,1, 2 our study demonstrates a favorable OS for r-ATG (88%) and h-ATG (93%, p = n.s.) due to successful second-line HSCT (Figure 1A,B). Causes of deaths (n = 11) were mainly due to transplant-related complications (n = 10). One patient died at day 45 after IST due to bacterial infection (Supplementary Table 4). In light of the recently improved results of unrelated HSCT, search for a matched unrelated donor should be initiated at diagnosis and HSCT should be offered in all nonresponders at 6 months. There was also no difference in failure-free survival (FFS, failure was defined by death, relapse, clonal disease, second-line HSCT or second course of IST) (h-ATG: 33% vs. r-ATG: 38%, p = n.s.) between the two groups despite of superior response at 6 months in the h-ATG group. Responders at 6 months (n = 55) showed an excellent outcome with 100% OS and 73% FFS. Importantly, patients with persistent severe neutropenia (<0.2 G/L ANC) at day 90 had an inferior OS (75%), while nonresponders with at least 0.2 G/L ANC showed an excellent OS of 95% (p < .01) (Figure 1C). Therefore, early second-line HSCT should be considered in patients with persistent severe neutropenia. There was no difference in OS between patients who receive HSCT before and after day 180 after IST (Figure 1D). Upfront HSCT from an unrelated donor can be an option in patients with 0 neutrophil at diagnosis, if a 10/10 HLA matched donor is available and HSCT can be performed in a reasonable time frame (e.g., within 2–3 months)." In conclusion, the overall response rate at 6 months was significantly higher in the h-ATG group than in the r-ATG group, while there was no difference in response rate and survival at time of last follow-up between both ATG groups. Since prolonged severe neutropenia increases the risk for severe infections and their associated morbidity, the earlier response rate noted in the h-ATG group is clinically beneficial for patients with AA. Ayami Yoshimi and Brigitte Strahm designed the study. Ayami Yoshimi wrote the manuscript. Peter Noellke performed the statistical analysis. Jan Starý, Krisztián Kállay, Owen Smith, Franco Locatelli, Jochen Buechner, Ivana Bodova, Julian Sevilla, Markus Schmugge, Marc Bierings, Tania Masmas, Michael Dworzak, Veerle Labarque, Katarzyna Pawelec, Kirsi Jahnukainen, Sophia Polychronopoulou, Paula Kjollerstrom, Marko Kavcic, Miriam Erlacher, and Charlotte M Niemeyer confirmed the diagnosis and gathered clinical information of patients. All authors have reviewed and approved the manuscript before submission. The authors acknowledge all the patients, their families, treating physicians from participating centers and reference pathologists and cytogeneticists of the EWOG-SAA. The authors thank Ms. Alexandra Fisher and Ms. Annemarie Breier for the data management. This study is supported by the Parent Association in Freiburg, Germany (Förderverein für Krebskranke Kinder Freiburg e.V.). MB is a member of a Pfizer advisory board on Atgam. All other authors declare no competing financial interests. The data that support the findings of this study are available from the corresponding author upon reasonable request. Data S1. Supporting information. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.