A Phase 1/1b Multicenter Ascending Dose Study to Evaluate the Safety of HA-1 Minor Histocompatibility Antigen-Reactive TCR-Modified T Cells (BSB-1001) in Patients Undergoing HLA-Matched Allogenic Hematopoietic Stem Cell Transplant (alloSCT) for MRD+ AML or ALL or Poor/Very Poor Risk MDS

Background: AlloSCT can cure patients with acute leukemia and high-risk myelodysplastic syndromes (MDS). However, post-transplant relapse remains the greatest single cause of post-transplant death, occurring in up to 80% of high-risk patients. In HLA-matched alloSCT, alloreactive donor T cells target minor histocompatibility antigens (miHAs), which are peptide products of coding polymorphisms that distinguish recipients from donors. These alloreactive T cells can target leukemic cells, mediating the graft-vs-leukemia (GVL) effect; unfortunately, they also can cause graft-vs-host disease (GVHD). Pre-clinical mouse models (Blood, 2011) demonstrate that CD8⁺ T cells that target miHAs with expression limited to hematopoietic cells can mediate GVL without causing GVHD. To translate this concept for how to safely augment GVL to the clinic, BlueSphere Bio (BSB) used its high throughput T cell receptor (TCR) discovery and characterization platforms to clone a panel of TCRs reactive against the HLA-A*02-restricted miHA, HA-1, which is expressed mostly in hematopoietic cells. These receptors were cloned from a multiparous A*02:01 HA-1^- woman who was immunized to HA-1 through pregnancy. A high affinity TCR specific for HA-1 and not reactive against allogeneic HLA molecules was chosen to create a cellular therapeutic designed to mediate GVL and enhance donor engraftment with a lower risk for GVHD, relative to polyclonal donor T cells.

The anti-HA-1 T cell product (BSB-1001) is composed of donor CD8⁺ T cells transduced with a lentivirus vector encoding the anti-HA-1 TCR and the RQR8 tag, which expresses an epitope from CD20, thereby enabling the in vivo killing of BSB-1001 cells with rituximab. CRISPR-cas9 editing of the CD8⁺ T cells deletes the endogenous TCRα and β chains with >95% efficiency. This increases expression of the TCR and reduces risk of GVHD caused by native TCRs, as well as possible off-target reactivities generated by pairing of endogenous and introduced TCR chains. BSB-1001 cells specifically kill HLA-A*02:01, HA-1⁺ targets, with no activity against cells that are HLA-A*02:01⁺ but HA-1^-, or cells that are HA-1⁺ but are HLA-A*02:01^-. BSB-1001 is also not reactive against a panel of B lymphoblastic cell lines (LCL) expressing allogeneic HLA molecules. Importantly, efficient target killing is seen at effector:target ratios as low as 0.1, and maintained after several rounds of ex-vivo expansion following target cell challenge, a likely predictor of potency and persistence in vivo.

Study Design: The first in human Phase 1/1b study is a multicenter, open-label, single dose study with a 3+3 ascending design with the option to expand accrual at a promising dose. Subjects must have ALL, AML or high-risk MDS with evidence of disease after the last therapy, be HLA-A*02:01 HA-1⁺ and have a 10/10 HLA-matched HA-1^- related or unrelated donor. GCSF-mobilized donors will undergo leukapheresis. From this product, a small fraction of cells will be removed for BSB-1001 manufacturing; the remainder will undergo CD34-selection and cryopreservation until SCT. After BSB-1001 manufacturing is complete, patients will undergo ablative conditioning (choice of three regimens), followed by infusion of the CD34-selected cells and then BSB-1001. Patients will not receive anti-T cell antibodies and will receive no prophylactic pharmacologic immunosuppression. Subjects must otherwise be suitable for ablative conditioning using standard criteria. The primary objective is to establish a recommended dose of BSB-1001 based on its dose-limiting toxicities and the magnitude of post-infusion BSB-1001 expansion. The primary endpoint is determining the incidence of treatment-emergent adverse events, including dose-limiting toxicities. Additional endpoints include rates of engraftment, incidences of cytokine release syndrome (CRS) and acute and chronic GVHD, quantitation of malignant hematopoiesis and lineage-specific donor chimerism. CRS and GVHD, should they occur, will be treated per institutional standards. Rituximab can be administered to kill BSB-1001 cells in case of refractory CRS or GVHD.

Enrollment is anticipated to begin in 1Q2023. BSB is also developing panels of TCRs targeting additional relatively hematopoietically-restricted miHAs with prioritization based on the frequencies of the restricting HLAs and the immunogenic/nonimmunogenic miHA alleles.