Long-Term Outcomes of Subjects with Epstein-Barr Virus-Driven Post-Transplant Lymphoproliferative Disorder (EBV⁺PTLD) Following Solid Organ (SOT) or Allogeneic Hematopoietic Cell Transplants (HCT) Treated with Tabelecleucel on an Expanded Access Program

Background: Patients (pts) undergoing solid organ (SOT) or allogeneic hematopoietic cell transplants (HCT) are at risk for developing Epstein-Barr (EBV) virus-driven post-transplant lymphoproliferative disorder (PTLD). Pts with EBV⁺PTLD not responding to rituximab ± chemotherapy after SOT or HCT have poor outcomes. Many SOT and HCT recipients are also not good candidates for aggressive chemotherapy regimens. There is an unmet need for effective and well tolerated therapies in this patient population. Tabelecleucel (tab-cel^®) is an investigational, off-the-shelf, allogeneic, EBV-specific, T-cell immunotherapy generated from healthy donors, which functions through native, endogenous T-cell receptors (TCRs) and HLA restrictions. Tabelecleucel is selected for an individual patient from an existing library, based on HLA restriction and matching. Here, we report long-term study results from US centers using tabelecleucel for subjects with EBV⁺PTLD following HCT or SOT.

Methods: Subjects with EBV⁺PTLD after HCT (n=14) or SOT (n=12) were treated with tabelecleucel on Atara's expanded access program (EAP; ATA129-EBV-201, NCT02822495, ongoing). Subjects received tabelecleucel matched by ≥ 2/10 HLA alleles, including ≥1 HLA allele through which tabelecleucel exerts cytotoxicity (HLA restriction). Key inclusion criteria were: presence of biopsy-proven EBV⁺PTLD, adequate organ function (ANC ≥ 500/µL +/- cytokine support; platelets ≥ 20,000/µL +/- transfusion support if no ≥ grade 2 bleed in prior 6 months; ALT, AST, T. Bili < 3X ULN; Creatinine < 3X ULN) and performance status (ECOG ≤ 4 or Lansky ≥ 20), and lack of approved alternative therapies. Non-PTLD-related vasopressor or ventilatory support, pregnancy, concomitant need for T-cell inhibiting medications were exclusionary. Tabelecleucel was given at 1.6-2 x 10⁶ cells/kg/dose on days 1, 8, and 15 of every 5-week cycle with imaging-based response assessment at ~d28 of each cycle. Subjects were treated to maximal response with up to 4 tabelecleucel products (cell lines) with different HLA restrictions, occurrence of an adverse event leading to tabelecleucel discontinuation, or withdrawal of consent. The results presented herein reflect a data snapshot from 3^th June 2019.

Results: All subjects had received prior rituximab and 7/12 SOT subjects received prior chemotherapy. Intermediate/high risk PTLD-prognostic index (PTLD-IPI; Choquet et al, Ann Hematol 2007) was noted in 79% and 42% of HCT and SOT subjects, respectively. The results are presented in table 1.

While the median follow-up time in HCT subjects is short, 3 subjects were followed for over 12 months including 2 who were followed for more than 24 months.

In subjects responding to tabelecleucel, 1-year OS was 85.7% in HCT and 100% in SOT, and no deaths were attributable to PTLD progression. In a subset of study subjects (HCT: n=11; SOT: n=11) with adequate ECOG, no CNS disease, and no PTLD-related ventilatory support, who would have likely been eligible for Atara's ongoing phase-3 trials, the ORR was 55% (HCT) and 82% (SOT), with a 2-yr OS of 79% (HCT) and 81% (SOT). The safety profile of tabelecleucel was consistent with previously published data. At the data snapshot for this abstract, no tabelecleucel-related adverse events led to treatment discontinuation or death. In addition, no cytokine release syndrome, organ rejection or tumor flare adverse events were reported in the PTLD subjects treated with tabelecleucel on this EAP.

Conclusions: The data demonstrate a high response rate for tabelecleucel in PTLD in both HCT and SOT after initial treatment failure. Longer term follow-up shows a favorable 2-year OS in this predominantly high-risk population for whom there are no approved alternative therapies. Similar outcomes were observed in the subset of subjects potentially eligible for ongoing phase 3 studies of tabelecleucel in relapsed/refractory EBV⁺PTLD following SOT (NCT03394365) or HCT (NCT03392142). Tabelecleucel appears to be an effective and well-tolerated option in the subset of subjects with EBV⁺PTLD treated on this EAP.

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