Efficacy and Safety of Eltrombopag Added to Rabbit Anti-Human Thymocyte Globulin+Cyclosporine in Aplastic Anemia: Results from a Prospective Multicenter Phase II Study

Abstract

Introduction

Combination therapy with horse anti-human thymocyte globulin (hATG), cyclosporine (CsA), and eltrombopag (EPAG) is now recommended as standard of care for patients with severe aplastic anemia (SAA) who are ineligible for allogeneic bone marrow transplantation, after a randomized prospective trial led by the European Society for Blood and Marrow Transplantation (EBMT) demonstrated the efficacy of adding EPAG to hATG+CsA. However, the efficacy of adding EPAG to CsA+rabbit ATG (rATG), which is considered less effective than hATG, has not been evaluated in a prospective study. Although the EBMT study also demonstrated that the addition of EPAG does not induce new karyotypic abnormalities or proliferation of hematopoietic stem cells (HSCs) with somatic mutations in SAA patients, it remains unknown whether the addition of EPAG affects clonal hematopoiesis in Asian SAA patients, including those in Japan, where the incidence of secondary myeloid neoplasms is lower than in Western countries. Additionally, while some immune markers, such as paroxysmal nocturnal hemoglobinuria (PNH)-type cells and HLA-class I allele-lacking leukocytes (HLA-LLs) have been shown to be useful in predicting response to ATG+CsA, whether their predictive role holds in patients receiving ATG+CsA+EPAG remains unclear. To address these issues, we conducted a prospective multicenter study using rATG+CsA+EPAG in Japanese patients with SAA and transfusion-dependent non-severe AA (NSAA).

Methods

Adult AA patients were treated with rATG 2.5 mg/kg/day+CsA 5 mg/kg/day+EPAG 75 mg/day commencing on day 6 post-rATG. The primary endpoint was hematologic response after 12 weeks of treatment, and the secondary endpoints included responses at weeks 8, 26, and 52 after study initiation, correlation between hematologic response and immune markers, and the incidence of grade 3 or higher adverse events and death related to study treatment. Peripheral blood collected at diagnosis, 26 weeks and 52 weeks after treatment was subjected to panel sequencing of genes associated with myeloid malignancies.

Results

Sixty patients (48 with SAA and 12 with NSAA), comprising 33 males, were recruited from 47 centers between October 2019 and September 2022. The median age was 61.5 years. Five patients had karyotypic abnormalities, including del(13q) (n=2), del(20q) (n=1) and -Y (n=2), and somatic gene mutations were detected in 23 (38%) patients at enrollment. Of the 59 patients evaluable for response, the response rate (PR+CR) was 53% (50% in SAA and 64% in NSAA) at 12 weeks and 68% at 26 weeks (67% in SAA and 73% in NSAA), which was than the 44% in SAA and 64% in NSAA patients at 26 weeks reported in a post-marketing study based on 1982 patients in Japan. PNH-type cells and HLA-LLs, detected in 79% and 43%, respectively, did not correlate with the response. Eleven patients developed new karyotypic abnormalities (13q- in six, +Y in two [one of whom also had -Y], +15, inv(3)(p21q27), t(8;9)(q24;p24), -Y in one each), while 13q- detected at baseline (n=1) or after treatment (n=2) disappeared at 52 weeks, and none was linked to myelodysplastic syndrome (MDS). New karyotypic abnormality (7q loss) appeared in one patient, and new somatic mutant clones appeared during the course of the treatment in (36%) of the 39 patients with no somatic mutant clones at enrollment, including ASXL1 in four, BCOR in two, DNMT3A in two, ASXL2, CEBPA, RAD21, SRSF2, STAT3, SUZ12, TET2, U2AF1 and 7q loss in one each, and an increase in clone size was observed in 9 (43%) of the 21 patients with mutant clones detected at enrollment. grade 3 or higher adverse events were reported in 36 patients, with the most frequent being febrile neutropenia (n=12), of which one patient died of sepsis. One patient with TP53 mutation at baseline progressed to acute myeloid leukemia at week 11 and one patient with U2AF1 mutation, 1p loss and 15q UPD detected by deep sequencing at baseline progressed to MDS at week 30.

Conclusion

The addition of EPAG, 1) increased the response rate to rATG+CsA in patients with SAA and NSAA with manageable adverse events, 2) induced new karyotypic abnormalities and somatic mutations without a consistent increase in the abnormal clone, 3) did not induce clonal evolution except in patients with a TP53 mutation or two different cytogenetic abnormalities at enrollment, 4) negated the beneficial effects of possessing PNH-type cells and HLA-LLs.