Post-Transplant Cyclophosphamide (PTCY) Vs Anti-Thymoglobulin (ATG) As Part of the Gvhd Prophylaxis for Fludarabine/Clofarabine/Busulfan Reduced Intensity Conditioning (RIC) in Allogeneic Stem Cell Transplantation (allo-SCT): Influence on Early Immune Reconstitution

Introduction: The influence of PTCY on early immune reconstitution after allo-SCT has been poorly studied so far, especially in comparison to standard use of ATG as GVHD prophylaxis.

Patients and Methods: A prospective study was conducted at the CHU of Nantes with the aim to compare early immune recovery between patients receiving PTCY or ATG as GVHD prophylaxis after a RIC allo-SCT. Secondary objectives were to study the impact of 1 vs 2 days of PTCY (CY1 vs CY2) or ATG (A1 vs A2), and of fludarabine (Flu) vs clofarabine (Clo) as part of the RIC regimen. As such, 3 RIC regimens were considered in both groups: FluCY2 (Baltimore regimen, Luznic, BBMT 2008), FluCY1, CloCY2 (where Clo replaces Flu), on one hand, and FluB2A2, CloB2A2, CloB2A1 (Chevallier, Haematologica, 2014), on the other hand. FluCY2 and FluB2A2 are currently standard of care RIC regimens for patients with haplo-identical and matched donors, respectively. Five patients had to be included in each RIC subgroup, depending on the type of disease and donor (/): FB2A2 (lymphoid/matched); FluCY2 (lymphoid/haplo); CloB2A2 or CloB2A1 (myeloid/matched); CloCY2 (myeloid/haplo), FluCY1 (myeloid or lymphoid/matched). The source of graft was peripheral blood stem cells for all cases. Blood samples were collected before starting the conditioning regimen then 3 times per week from day +0 until day+30, and at days +60 and +90. The following cell subsets were studied: a/b and g/d CD3+, CD8+ and regulatory (Tregs) T cells, B and NK cells and monocytes. The median percentage (%) compared to total lymphocytes was considered for all lymphocytes subsets between days 0-30. Thereafter, median absolute numbers (AN)/mm3 were considered for samples collected at days +30, +60 and +90. The study was approved by the IRB of the CHU of Nantes and all patients provided informed consent.

Results: Between August 2014 and May 2015, thirty patients were included, including 15 in both groups and 5 in each RIC subgroups. Median age was 61 years. There were more patients with active disease at transplant (47% vs 7%) and more haplo-identical donors (67% vs 0%) in the PTCY group. All patients engrafted and were alive at day +90. However, 1 PTCY patient with T-ALL relapsed before day+100.

Within the first month post-transplant, PTCY group had a significantly higher median % of a/b T cells (69.1 vs 18.9, p<0.0001) and Tregs (3.46 vs 0.45, p<0.0001) while ATG group had higher median % of NK (23 vs 2.57, p<0.0001) and B-cells (0.88 vs 0.43, p=0.0002).

Between day+30 and day+90, ATG group had significant higher median counts of a/b T cells at days +60 (1316 vs 79.6, p=0.0001) and +90 (795.8 vs 151.6, p=0.03); g/d T cells at day+60 (27.6 vs 1.26, p=0.002); CD8 T cells at day+60 (735 vs 29.6, p=0.008); NK cells at day+30 (203.7 vs 89, p=0.04) and monocytes at days +30 (455.5 vs 221.7, p=0.009) and +60 (832.5 vs 247.2, p=0.004).

Compared to the standard FluCY2 regimen, although not significant, FluCY1 was associated with higher median %, between days 0-30 of g/d T cells (2.32 vs 0.8) and higher median AN of g/d T cells at days +30 (9.2 vs 1.02) and +60 (9.22 vs 1.05), of B cells at days +30 (0.4 vs 0.14) and +60 (1.6 vs 0.39) and of NK cells at day+30 (213.9 vs 82.7).

Compared to the standard FB2A2 regimen, although not always significant, CloB2A1 was associated with higher median % between days 0-30 of Tregs (0.97 vs 0.25, p=0.002) and higher median AN of g/d T cells at day+30 (6.8 vs 2), B cells at days +30 (2.35 vs 0) and +60 (43.7 vs 0.19), NK cells at day +30 (288 vs 62.1) and Tregs at days +30 (4.3 vs 0.2), +60 (8.8 vs 1.14) and +90 (8.96 vs 3.45).

Compared to the standard FB2A2 regimen, although not always significant, CloB2A2 was associated with higher median % between days 0-30 of a/b T cells (28.87 vs 3.78, p=0.01) and B cells (0.76 vs 0.5, p=0.02) and higher median AN of a/b T cells at days +30 (324.3 vs 125.8) and +90 (1594 vs 604.3), of g/d T cells at day +30 (7.35 vs 2), of CD8 + T cells at days +30 (209.7 vs 38.9) and +90 (1211.7 vs 504.6), of B cells at day +30 ( 1.41 vs 0), of NK cells at days +30 (303 vs 62.1), +60 (514.5 vs 225.7) and +90 (647 vs 102.8, p=0.03) and of monocytes at days +30 (688.9 vs 257.4, p=0.03) and +90 (2157.4 vs 611.2).

Conclusion: Strong differences exist in term of early immune recovery when using PTCY or ATG as part of the GHVD prophylaxis for RIC allo-SCT. Dose or drug modifications within the standard RIC regimen in both groups may be envisaged to favor some cell population recoveries after allo-SCT in order to increase outcome in patients.