GvHD prophylaxis using posttransplant cyclophosphamide and antithymocyte globulin after HSCT

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a curative treatment option in many hematological malignancies, but it remains challenging due to the associated high morbidity and mortality rates. Posttransplant cyclophosphamide (PTCy) and antithymocyte globulin (ATG) as individual treatment regimens have been found to reduce graft-versus-host-disease (GvHD) following allo-HSCT. Recently, two studies have assessed the combined effect of PTCy plus ATG as GvHD prophylaxis in patients with hematological malignancies receiving reduced intensity conditioning (RIC) before allo-HSCT.^{1, 2}

Maria Queralt Salas, Shruti Prem, and colleagues aimed to assess the safety and efficacy of PTCy + ATG for patients who underwent allo-HSCT using peripheral blood from different donor types. They published their findings in Bone Marrow Transplantation.¹

Dhon Roméo Makanga et al. previously reported on GvHD rates when using PTCy in RIC haploidentical (h)-HSCT.³ Their more recent publication in The Journal of Immunology aimed to assess combination treatment with PTCy + ATG as GvHD prophylaxis in a new cohort of patients who had received h-HSCT, and compare the findings with the outcomes of the previously reported cohort.²

Study design

• GvHD prophylaxis for patients in the Salas and Prem study¹ consisted of ATG, PTCy, and cyclosporine. Cyclosporine tapering began between Day +45 and +60 for all patients without GvHD.
  • RIC contained fludarabine, busulfan, and 200 cGy of total body irradiation.
• All patients in the Makanga study² received GvHD prophylaxis with cyclosporine plus mycophenolate mofetil, followed by either PTCy (n = 32), or PTCy + ATG (n = 26).
  • RIC regimen: PTCy arm had Baltimore-based RIC with fludarabine or clofarabine, while the PTCy + ATG combination treatment arm received the CloB2A1 regimen (clofarabine, busulfan, ATG).
• Other study and patient characteristics are detailed in Table 1.

Table 1. Patient and donor characteristics

• The main outcomes assessed by Salas and Prem et al. were overall survival (OS), relapse-free survival (RFS), GvHD-free RFS (GRFS), non-relapse mortality (NRM), cumulative incidence of relapse, and cumulative incidence of GvHD.¹
• The primary outcome of the Makanga study was to compare recovery of T and natural killer (NK) cells between the two groups. They also aimed to compare similar outcomes to the Salas and Prem study (OS, disease free survival [DFS], GRFS, relapse, NRM, and GvHD).²

Results

• Both studies found an increased graft failure rate in the PTCy + ATG combination treatment arm when using either mismatched or haploidentical donors.
  • Salas and Prem et al.¹ reported a graft failure rate for mismatched unrelated donor and haploidentical donor HSCT of 21% and 17%, respectively, compared to matched related and matched unrelated donor transplant (2% and 3%, respectively).
  • Makanga et al.² found a higher graft failure rate of 19.3% when using h-HSCT in the combination treatment cohort, compared to a historical cohort using PTCy only (0%).
• In terms of GvHD incidence:
  • Makanga et al. reported a significantly lower incidence of Grade 2–4 acute (a)GvHD in the PTCy + ATG arm when compared with PTCy alone (23.8% vs 59.3%; p = 0.004) in patients with successful engraftment, but no significance when comparing incidence of Grade 3–4 aGvHD (4.7% vs 18.7%; p = 0.29). Salas et al. found that there was no significant difference in cumulative incidence of Grade 2–4 aGvHD across the different donor types (p = 0.440).
  • Moderate/severe chronic GvHD did not different differ significantly between the PTCy + ATG and PTCy alone treatment arms (11.1% vs 20.6%; p = 0.65)² nor across donor types (p = 0.738).¹
• Salas and Prem et al. found that donor type had a significant impact on OS, RFS, NRM, and GRFS, and that a Karnofsky performance status of 70–80% (HR, 1.92; 95% CI; 1.25–2.9; p = 0.003) and high/very high disease risk index prior to allo-HSCT (HR, 1.79; 95% CI, 1.21–2.66; p = 0.003) were significant predictors of a worse RFS.
• Makanga and team reported that OS, DFS, GRFS, relapse, NRM, and death were similar between the PTCy and PTCy + ATG arms at a median follow-up of 43.5 and 24.2 months, respectively, for patients who were alive, and that relapse incidence when comparing patients infused with a similar number of CD3+ T cells did not differ significantly across the treatment groups (PTCy: 38.7%, n = 12/31 vs PTCy + ATG: 31.8%, n = 7/22; p = 0.82).
• Outcomes between the different groups are detailed in Table 2.

Table 2. Outcomes between the different groups included in the studies

• When assessing T-cell population in patients, Makanga et al.² found that, in patients who received PTCy and ATG, the median percentage of T cells among lymphocytes was significantly reduced (37.9%) versus patients in the PTCy-only cohort (75.7%; p = 0.0001) at Day 5 posttransplant. This reduction was maintained at Day 20 (27.7% vs 54.8%; p = 0.0003), and at Day 30 (24.8% vs 48.1%; p = 0.002).
• In contrast, the median percentage of NK cells among lymphocytes was higher at Day 20 (24.5% vs 14.0%; p = 0.01), at Day 25 (41.4% vs 18.4%; p = 0.006), and at Day 30 (52.5% vs 27.9%; p = 0.001).
  • Different subsets of NK cells were present in different quantities, with PTCy prophylaxis being associated with a higher frequency of the immature NK cells (NKp46^- 2B4⁺) at both Day 20 and Day 25.
  • PTCy + ATG prophylaxis resulted in a higher number of mature NK cells (NKp46⁺ 2B4^-) at Day 20 and Day 25.

Conclusions

The use of combined PTCy and ATG for GvHD prophylaxis in both studies resulted in low rates of acute GvHD, without apparent increase in relapse rates. However, this did not translate into improved survival for patients who had received mismatched donor stem cells (haploidentical or mismatched unrelated donor), possibly due to very high rates of graft failure in both studies which may be linked to delayed T-cell engraftment. The best survival using the combination treatment was seen in patients who had received a matched unrelated donor transpant.¹ Salas and Prem et al.¹ also commented on the high infection rates in their PTCy + ATG cohort and felt that reducing the ATG dose could reduce the infectious toxicity of this regimen.

Combined treatment with PTCy + ATG caused longer T-cell depletion,¹ as supported by the findings of Makanga et al. demonstrating a significantly lower percentage of T cells at Day 30 compared to PTCy alone.² However, there are differential subsets of NK cells present with the different prophylactic treatments, and Makanga et al.² postulate that better knowledge of these subsets may enable prognostication of patients.

The authors acknowledge that the studies had limitations, namely their retrospective nature, the heterogeneity of the groups (i.e., AML and ALL being combined as a subset)² and sample sizes,¹ and in the Makanga et al. study, the low number of participants.