Impact of Immune Reconstitution (IR) and Graft-Versus-Host Disease (GvHD) on Clinical Outcomes after Treatment with Donor T Cells Transduced to Express the Herpes Simplex Virus Thymidine-Kinase Suicide Gene (TK cells) in Acute Leukemia Patients Undergoing Haploidentical Hematopoietic Stem Cell Transplantation (HSCT)

Background: Haploidentical HSCT is a readily available platform for most patients lacking an HLA-matched donor, but the T-cell depleted (TCD) approach without any post-transplant donor cell therapy is impaired by high non-relapse mortality (NRM) due to slow IR, while the T-cell replete (TCR) approach followed by cyclophosphamide and immunosuppressive therapy is hampered by high chronic GvHD and relapse incidence (RI)

Methods: The impactof IR and TK-cell dose on outcomes was assessed in 45 patients with several hematological malignancies treated with 1 to 4 monthly TK infusions (0.1-1.0x10⁷/Kg, 21-49 days after haploidentical HSCT) in a phase 2 trial (n=30; Lancet Oncol 2009; 10: 489) and in the experimental arm of an ongoing phase 3 trial (n=15; NCT00914628). Median follow-up time for TK-treated patients was 3.7 years (interquartile range [IQR], 1.5-8.5). Outcome measures included 1-year NRM, overall survival (OS), leukemia-free survival (LFS), RI and GvHD. Hazard ratios (HR) and odds ratios (OR) were derived by regression models adjusted for baseline risk factors. IR (T cells ≥ 100/µL) was modelled as a time-dependent covariate. These pooled data were the basis for the positive opinion for conditional marketing authorization recently granted for TK cells (www.ema.europe.eu/EPAR/Zalmoxis)

Results: IR was achieved by 34 patients (76%) after a median of two TK infusions (IQR, 1-2), a median cumulative cell dose of 1.3x10⁷/kg (1.0-2.4) and a median time from HSCT of 83 days (65-108). IR was not influenced by baseline risk factors or TK-cell doses and was associated with decreased NRM (12% vs 75%; p<0.0001) and improved LFS (HR=0.20; p=0.005) and OS (HR=0.08; p<0.0001). Grade II-IV acute GvHD (35%; grade III-IV: 7%) was unrelated to TK-cell dose and occurred nearly exclusively in IR (OR=7.0; p=0.07) and female patients (OR=9.7; p=0.007). Only one patient had de novo chronic GvHD. All GvHD events fully resolved (median, 14 days; 10-27) after suicide-gene induction with ganciclovir (median, 15 days; 13-16). Cumulative RI (31%) did not differ according to IR, while it was inversely related to TK-cell dose, with decreased relapse rates (60%, 33% and 0%) observed with increasing cell doses (<1.0, 1.0-2.4 and >2.4x10⁷, respectively; p=0.004). TK patients had significant outcomes in OS (51%), NRM (20%) and chronic GvHD (6%). Importantly, TK patients with grade II-IV acute GvHD treated with ganciclovir experienced high OS rates (67%), thus confirming the ability of TK machinery to selectively targeting GvHD effector cells, while sparing a wide repertoire protective against infections and leukemia recurrence

Conclusions: TK-cell treatment is characterized by early IR and full GvHD control that translate in improved NRM and OS and by dose-related antileukemic effects, with overall outcomes comparing very favorably with those of current approaches to haploidentical HSCT