Product Characteristics and Multi-Arm Clinical Trial Design for TSC-100 and TSC-101, TCR-T Cells That Target Leukemia Following Hematopoietic Cell Transplantation
Authors: Chattopadhyay Shrikanta , Malcuit Christopher , Takeuchi Ryo , Hedblom Emmett , Jaworowicz Warren , Boudot Antoine J , Bader Hannah , Murray James , Macbeath Gavin 
Published: November 2021

Abstract
Background:
While adoptive cell therapies such as CAR-T therapies have transformed the treatment of lymphoid malignancies by targeting lineage-specific antigens, they have yet to demonstrate safety and efficacy against myeloid malignancies. T cells expressing T cell Receptors (TCRs) for the HLA-A*02:01-restricted minor histocompatibility antigens HA-1 and HA-2 have been observed to clonally expand after hematopoietic cell transplantation (HCT) in donor-recipient pairs mismatched for these antigens. These expanded T cells clones are associated with significantly lower relapse rates (Marijt et al. Proc. Natl. Acad. Sci. 2003; Spierings et al. Biol. Blood Marrow Transplant. 2013) indicating a specific graft versus leukemia effect. Engineered T cells expressing an HA-1-targeting TCR have demonstrated safety and preliminary anti-leukemic activity in patients with relapsed leukemia after HCT (Krakow et al. ASH 2020). We have developed engineered TCR-T cell products, TSC-100 and TSC-101, that target HA-1 and HA-2 respectively for the treatment of leukemias after HCT and present their product characteristics and the clinical trial design here.
Methods and Results:
To minimize potential safety risks, process variability, and costs associated with lentiviruses, our proprietary T-Integrate manufacturing platform uses a transposon/ transposase system delivered into pan T cells. This enables the introduction of larger vectors with an increased number of functional elements. Our transposon vector encodes both the α and β chains of the TCR under a strong promoter. We find high levels of cell-surface TCR that suppresses endogenous TCRs, thereby minimizing non-specific alloreactivity and potential graft versus host disease (GvHD). In mixed lymphocyte reactions, we find undetectable to minimal alloreactivity of the engineered T cell product compared with non-engineered T cells. The vector also encodes the α and β chains of CD8 ensuring that both CD8+ and CD4+ T cells in the product acquire cytotoxicity and we demonstrate efficient killing of target cells by both T cell types. The vector includes a short peptide tag that is recognized by a GMP-grade antibody, enabling efficient purification of engineered T cells during manufacturing along with the ability to track these T cells in patients. The manufacturing process generates more than 10 billion cells with an estimated vein-to-vein time of ~3 weeks, including product release testing. We routinely find high product purity exceeding 90% engineered T cells, high cytotoxicity in vitro and vector copy numbers &lt;5 copies/cell, ensuring low risks of oncogenicity.
The planned clinical trial design is a multi-arm Phase 1/2 trial that includes a control arm for safety and early efficacy comparisons. Patients with AML, ALL and MDS planned for HCT are eligible. Since disease burden is lowest soon after HCT with lower risks of acquired resistance, TCR-T treatment will begin shortly after HCT to prevent disease relapse. Because relapse rates are far higher with reduced intensity conditioning (RIC), only RIC-eligible patients are included. To ensure that all patient-donor pairs are mismatched for the minor antigens, only patients eligible for haploidentical transplantation will be included which enables HLA mismatches. Assignment to treatment or control arms is determined by HLA type as well as the minor antigen status as determined by a PCR-based central lab assay. Since both HA-1 and HA-2 are presented on HLA-A*02:01, all patients with HLA-A*02:01 are eligible for TCR-T treatment. HA-1-positive patients are assigned to the TSC-100 treatment and HA-2 positive patients are assigned to the TSC-101 arm. Donors for these patients are required to be mismatched at either HLA-A*02:01 or the minor antigens. HLA-A*02:01 negative patients will be assigned to the control arm. Analysis of CIBMTR datasets found that this HLA-based 'biological randomization' did not affect outcomes. Dosing of TCR-T cells begins upon count recovery after RIC-HCT and up to three doses will be administered every 40 days if there is no high-grade toxicity. Early readouts of biological activity include monitoring for minimal residual disease and kinetics of donor chimerism. Recruitment begins in Q1 2022 and after the recommended Phase 2 dose has been identified, the study will transition to a Phase 2 study to assess relapse rates of TSC-100- and TSC-101-treated patients versus the control arm.

Disclosures
Macbeath: TScan Therapeutics: Current Employment, Current equity holder in publicly-traded company.