The Effect of ATG Exposure Based on Recipient Absolute Lymphocyte Counts in Patients Undergoing Allo-HCT with ATG and Ptcy As Gvhd Prophylaxis

Background Recent studies have shown that using absolute lymphocyte count (ALC) based dosing of ATG (anti-thymoglobulin) can improve survival after allogeneic hematopoietic cell transplantation (Allo–HCT). At our center we routinely use a combination of ATG + post-transplant cyclophosphamide (PTCy) for GVHD prophylaxis. In this analysis we explored the effect of ALC- based ATG dose on patients’ outcomes. Method Between Oct 2015 and Mar 2017, 100 consecutive adult patients who underwent allo-HCT in our center were studied, with last follow up till March 2018. All received reduced intensity conditioning with fludarabine (30mg/m2 D-5 to D-2) / busulfan (3.2mg/kg D-3, D-2) and total body irradiation (200 cGy). GVHD prophylaxis included ATG (4.5 mg/kg) on day -3 to -1, PTCy (50mg/kg/day on day +3, +4), and CsA from day+5 onwards. The actual weight-based ATG doses were collected and the ALC predicted doses was calculated by the formula 400 + [350x ALC on day -3] (Admiraal R. Lancet Haematol 2017). We analyzed patients who received ATG doses that were less than or greater than 50% predicted by their ALC. Overall survival (OS) and progression free survival (PFS) were calculated with Kaplan-Meier product method and compared using the Log-rank test. Non-relapse mortality (NRM) rates were estimated using cumulative incidence method suggested by Pepe & Mori (stat in med, 1993) considering relapse as competing risk. Results Total patients were 100 (54 males), median age 59 (range 26-73). Diagnosis (number): AML (52), MDS (21), Myelofibrosis (12), ALL (7) and others (8). Donors were MRD (16), MUD 10/10 (50), MUD 9/10 (15) and haploidentical (19). All received peripheral blood stem cells. Only 2 patients received ATG doses higher than ALC based doses. ALC and ATG doses along with OS, PFS and NRM results are shown in Table. Conclusion Our study is unable to provide any opinion on higher doses of ATG as only 2% of our cases received a dose higher than predicted by ALC. There was an inferior PFS and a trend towards inferior OS in those who received