Favorable T Cell Reconstitution In Reduced Intensity Conditioned Allogeneic Stem Cell Transplantation with Low-Dose Rabbit Anti-Thymocyte Globulin and Total Body Irradiation
Authors: Thorn Radhika , Meier Jeremy , Wang Elaine , Sabo Roy T , Scalora Allison F , Roberts Catherine H , Song Shiyu , Manjili Masoud H , Clark William B , McCarty John M , Chung Harold M , Toor Amir A 
Published: October 2019

Severe graft versus host disease (GVHD) continues to compromise transplant outcomes following reduced intensity (RI) allogeneic stem cell transplantation (SCT). ATG reduces the risk of GVHD in unrelated donor (URD) stem cell transplant (SCT) recipients. Here we report the results of a randomized phase II clinical trial evaluating two doses of rabbit ATG (Thymoglobulin, Sanofi Aventis) 5.1 (ATG5) and 7.5 mg/kg (ATG7.5) administered along with 450 cGy total body irradiation (TBI). This study followed an earlier trial which examined the optimal schedule of ATG administration in this regimen (Toor et al, BMT 2008). ATG was administered from day -9 to day -7, and fractionated TBI on day -1 and 0. Tacrolimus and mycophenolate mofetil were administered for GVHD prophylaxis.
Forty-one patients with advanced hematologic malignancies were randomized and conditioned with ATG-TBI, stratified according to donor type, HLA-matched related (MRD) or unrelated (URD) donor. ATG5 had 10/19 URD recipients compared to 12/22 ATG7.5. Median age of the patients was 57; 9 patients had MM, 15 NHL, 12 CLL/PLL and 6 MDS/AML. Twenty-six were in CR at SCT (11 ATG5, 15 ATG7.5), 13 were in PR/SD (7 and 6 in the two arms) and 2 (1 in each arm) were in untreated relapse. Median number of prior therapies was 4 in each arm; 6 (ATG5) and 9 (ATG7.5) patients had a prior autologous SCT.
Myeloid engraftment occurred at a median of 12 days after stem cell infusion in both arms; median neutrophil chimerism at 4, 8, 12 and 24 weeks was 0% recipient in both arms. Persistent mixed T cell chimerism was observed more frequently in the ATG7.5 arm with a trend toward full donor chimerism over time in the ATG5 patients (Figure 1); mean T cell chimerism was 14% in ATG5 and 25% in ATG7.5 at 4 weeks; 8 and 26 (repeated measures ANOVA p=0.058) at 8 weeks; 9 and 31 (RMANOVA p=0.016) at 12 weeks; and 6 and 27 (p=0.029) at 24 weeks respectively. Mean donor derived T cell counts (ddCD3; product of absolute CD3+ cell count and donor T cell chimerism) were 182/microliter, 1080, 872, 876 in ATG5 and 182, 290, 296, 445 in ATG7.5 patients at the corresponding time points, being significantly larger in the ATG5 arm compared to ATG7.5 at 8 (p = 0.008) and 12 weeks (p = 0.052). The changes in ddCD3 between successive months were significantly larger in the ATG5 cohort between 1 and 2 months (p = 0.004). Further, 2 patients in the ATG7.5 arm developed secondary graft loss indicating more robust engraftment in the ATG5 cohort, likely due to superior immune reconstitution.
There was no significant difference in the survival between arms (Log-Rank P = 0.8). The 2-year survival rates were 64% (95%CI: 61%, 67%) ATG5 and 54% (95%CI: 50%, 58%) ATG7.5. Day 100 TRM was 0% in both arms; classical onset grade III-IV acute GVHD was not observed in either arm, 2 patients in the ATG7.5 arm developed grade II acute GVHD. Delayed onset acute GVHD occurred in 5 patients in the ATG5 cohort, (3 Grade III-IV) and 4 ATG7.5 (2 Grade III-IV) following early tapering of immunosuppression. Chronic GVHD occurred in 2 patients in ATG5 (1 mild, 1 severe) compared to 7 in ATG7.5 cohort (2 mild, 1 moderate, 4 severe). Overlap syndrome occurred in 2 in ATG5 patients and none in ATG7.5 cohort. GVHD was observed more frequently in patients with higher ddCD3 counts at 8 weeks. Relapse occurred in 26% vs 27%, ATG5 and ATG7.5 (Grey's test P=0.9). Two patients in the ATG5 cohort needed donor lymphocyte infusions (DLI) compared to 8 in ATG7.5 (Grey's test P = 0.07); 6 of these patients received prophylactic DLI for declining donor chimerism (5 in the ATG 7.5 cohort). A composite index (relapse, DLI, non-relapse mortality) was not significantly different between the cohorts (Log Rank P = 0.9). High-dose valacyclovir was used for CMV prophylaxis; CMV reactivation (>1000 copies/microl) developed in 2 ATG5 patients and 3 in ATG7.5 arm; EBV reactivation (>10 copies/microl) developed in 1 and 2 patients respectively in the two cohorts.
In summary our clinical trial demonstrates that ATG5 in combination with low dose TBI yields optimal T cell engraftment, with an excellent safety profile and good clinical outcomes, and may serve as a platform for adoptive immunotherapy. Phase III clinical trial comparing this regimen with more established regimens for RI-SCT is being planned.

Disclosures:
Toor: Sanofi Avnetis: Research Funding.