A Phase 1/2 clinical trial (LibraT1, NCT03590574) published in Nature Medicine has demonstrated promising results for AUTO4, an autologous CAR T cell therapy targeting the T cell receptor beta-chain constant region 1 (TRBC1), in patients with relapsed or refractory TRBC1-positive peripheral T cell lymphoma (PTCL). This multicenter, single-arm study evaluated the safety and efficacy of AUTO4 in patients who had failed at least one prior line of therapy. The study employed a rigorous screening process to identify patients with TRBC1-positive tumors, using both immunohistochemistry (IHC) and a commercially available in vitro diagnostic assay (LymphoTrack Dx TRB Assay).
AUTO4 Design and Manufacturing
AUTO4 is engineered to co-express a humanized second-generation CAR targeting TRBC1 and the RQR8 safety switch. The CAR is constructed from a single-chain variable fragment derived from a humanized form of the JOVI-1 antibody, fused to the CD8α stalk and the endodomains of 41BB and CD3ζ. The RQR8 safety switch allows for selective depletion of transgenic T cells with rituximab in the event of unmanageable toxicity and enables tracking and selection of CAR T cells. CAR T cell production was performed on the Miltenyi CliniMACS Prodigy, using autologous leukapheresate as starting material. The manufacturing process includes TRBC1 depletion, cytokine stimulation, and transduction with a γ-retroviral vector.
Study Design and Patient Population
The LibraT1 trial included a phase 1 dose escalation and a phase 2 dose expansion. Key inclusion criteria were age ≥18 years, confirmed diagnosis of selected T-NHLs including PTCL-NOS, AITL, or ALCL, confirmed TRBC1-positive tumor, relapsed or refractory disease after ≥1 prior lines of therapy, and PET-positive measurable disease. Exclusion criteria included T cell leukemia, prior gene therapy, clinically relevant CNS pathology, and active infections. Patients underwent leukapheresis, followed by preconditioning with fludarabine (30 mg/m² on days -6 to -3) and cyclophosphamide (500 mg/m² on days -6 and -5) before AUTO4 infusion. AUTO4 was administered intravenously as a single infusion on day 0.
Efficacy and Safety Results
In the phase 1 dose escalation, four dose levels were explored: 25 × 10^6, 75 × 10^6, 225 × 10^6, and 450 × 10^6 AUTO4 T cells. The primary endpoints in phase 1 were the incidence of ≥grade 3 toxicity within 60 days of AUTO4 infusion and the frequency of dose-limiting toxicities (DLTs) within 28 days of AUTO4 infusion. Overall response rate (ORR) by Lugano PET-CT criteria was a secondary endpoint. Adverse events were graded according to CTCAE version 5.0, and cytokine release syndrome (CRS) and neurotoxicity were graded by ASTCT/CTCAE v5.0 and ASBMT criteria.
Translational Analysis
Whole-blood flow cytometry was used to determine CAR positivity. CAR T cell persistence was assessed by ddPCR. Lymph node FFPE IHC and multiplexed IF were performed to assess CAR T cell persistence and T cell infiltration. Serum cytokine levels were measured using Meso Scale Discovery multiplex sandwich immunoassay panels. In vitro CAR T cytotoxicity was assessed using standard cytotoxicity assays.
Clinical Implications
The results of this Phase 1/2 trial suggest that TRBC1-CAR T cell therapy is a promising treatment option for patients with relapsed or refractory PTCL. The ability to selectively target TRBC1-positive T cells while sparing TRBC2-positive T cells may reduce off-target toxicities. The inclusion of the RQR8 safety switch provides an additional layer of safety, allowing for the depletion of CAR T cells if needed. Further studies are warranted to confirm these findings and to optimize the use of TRBC1-CAR T cell therapy in PTCL.
