Post-Transplant Cyclophosphamide or ATG As Gvhd Prophylaxis in Mismatched Unrelated Stem Cell Transplantation? – a Registry Analysis By the EBMT Transplant Complications Working Party

There is an increased risk of GVHD and non-relapse mortality (NRM) after allogeneic stem cell transplantations (alloSCT) when mismatched unrelated donors (MMUD) are used. Prophylaxis with either rabbit anti-thymocyte globulin (rATG, also termed anti-T-lymphocyte globulin) or post-transplantation Cyclophosphamide (PTCy) is often used to reduce the risk of NRM and GVHD. However, it is currently impossible to give general recommendations regarding preference for rATG or PTCy since comparative evidence from larger data sets is lacking.

To better understand the outcome of these GHVD prophylactic regimens, we analyzed outcomes of rATG vs. PTCy prophylaxis in adult patients with hematologic malignancies, who underwent peripheral blood alloSCT from 9/10 antigen MMUD between Jan 2018 and June 2021 in the EBMT database. We performed multivariate analyses using Cox cause-specific hazard models, adjusting for known risk factors, and variables that showed significant difference between the two comparison groups.

Overall 2123 patient were included, treatment- and patient characteristics are shown in Table 1. Patients receiving PTCy (n=583) were younger and had undergone alloSCT more recently compared to patients receiving rATG (n=1540). Median follow up was 2.0 years (PTCy) and 2.4 years (rATG). Outcome graphs are shown in figure 1. All outcomes are given starting from AlloSCT (with 95% CI).

NRM (fig 1A) was higher in the rATG arm (2-years NRM: PTCy 18% [14.6-21.6] vs. rATG 24.9% [22.7-27.3]; HR 0.74, p=0.03), Relapse incidence was slightly higher but not significant in the rATG arm (2-years RI: PTCy 22.9% [19.2 - 26.9] vs. rATG 26.2% [23.9 - 28.7]; HR 0.82, p=0.07), Overall survival (fig 1B) was better in the PTCy arm (2-years OS: PTCy 65.7% [61.2 - 69.9] vs. rATG 55.7% [52.9 - 58.3]; HR 0.77, p<0.001), same for Progression free survival (fig 1C) (2-years PFS: PTCy 59.1% [54.4 - 63.5] vs. rATG 48.8% [46.1 - 51.5]; HR 0.78, p=0.001). The incidences of chronic and acute GVHD were not significantly different between the groups; (2-years cGVHD: PTCy 31.7% [27.4 – 36] vs. rATG 30.3% [27.8 - 32.8]; HR 0.95, p=0.67. And 100-days aGVHD grades II-IV: PTCy 29.9% [25.9-34.1] vs. rATG 32.5% [30-34.9]; HR 0.83, p=0.11).

In summary, we found significantly higher survival and lower NRM in recipients of peripheral blood alloSCTs from MMUD receiving PTCy as compared to rATG. The current analysis improves the evidence base for decision making on GVHD prophylaxis in MMUD alloSCT.