CBX–663, a novel TCR-mimetic T-cell engager targeting a telomerase reverse transcriptase (TERT) peptide-HLA complex, that exhibits potent efficacy against AML cells Free

Telomerase Reverse Transcriptase (TERT) is an oncogenic driver highly expressed in ~90% of all tumors, including hematologic malignancies such as acute myelogenous leukemia (AML). However, therapeutic targeting of TERT has remained challenging. The presentation of HLA-A*02:01-restricted TERT₅₄₀ peptide by tumor cells represents an opportunity to target TERT-expressing malignancies using TCR-mimetic (TCRm) antibodies that recognize peptide-Human Leukocyte Antigen (pHLA) complexes. The feasibility of this strategy is supported by previous studies demonstrating that TERT-specific cytotoxic lymphocytes (CTLs) can target and kill leukemia cell lines in vitro as well as in mouse xenograft models in vivo.

Using our T-Bolt® platform, we have developed CBX-663, a potent and specific TCRm-based T cell engager (TCE) that targets the TERT₅₄₀ / HLA-A*02:01 pHLA complex on the surface of tumor cells through bivalent interaction, while simultaneously engaging CD3 on T cells. CBX-663 binds to TERT₅₄₀ pHLA with sub-nM affinity and low-pM avidity, inducing potent, target-dependentT-cell activation and T-cell-mediated cytotoxicity across multiple TERT- and HLA-A*02:01-positive leukemia cell lines in vitro, with EC₅₀ values ranging from 4.4 to 30.9 pM. Furthermore, CBX-663 shows potent cytotoxicity against primary AML blasts ex vivo, at clinically relevant effector-to-target (E:T) ratios (~1:20 and lower). Notably, CBX-663 demonstrates activity against primary AML blasts with a complex karyotype and TP53 mutation, highlighting the promise of targeting TERT in AML patient subpopulations with poor prognosis. Furthermore, leukemic stem cells (LSC), defined as CD45low CD33- CD34+ CD38-, are even more potently targeted by CBX-663 ex vivo, consistent with known high levels of telomerase activity in these cells. Effective targeting of LSC is highly desirable given their role in initiating and maintaining malignant proliferation. In vivo, treatment with CBX-663 leads to tumor growth control in a Cell-Derived Xenograft (CDX) AML model in PBMC-humanized mice.

To assess its safety profile, CBX-663 was evaluated in multiple studies. On-target activity of CBX-663 against CD34+ bone marrow progenitor cells was studied in an in vitro colony-forming unit (CFU) assay. Consistent with known transient expression of TERT in this cell population, CBX-663 treatment resulted in a dose-dependent decrease in colony counts, but to a lesser extent than a CD123-targeting TCE which was included as a comparator. Comparison of EC₅₀ values achieved with CBX-663 in cytotoxicity assays against AML cell lines versus normal progenitors in the CFU assay suggests the presence of an acceptable therapeutic index. To further investigate the impact of CBX-663 on human bone marrow progenitors and peripheral blood cells, repeated administration of CBX-663 (at dose levels demonstrated to be efficacious in vivo) was performed in non-tumor-bearing humanized CD34 (huCD34) mice which contain all human hematopoietic lineages. The treatment was well tolerated, with no deleterious effects on body weight or general health over vehicle control. An initial decrease in white blood cells and several immune cell subsets was observed in peripheral blood, but this phenomenon was reversible, with the exception of a moderate decrease in neutrophils that persisted during the period of study observation (2 weeks after the last dose). Red blood cells and platelets were minimally affected, indicating an overall manageable bone marrow toxicity profile. To further study its safety profile, CBX-663 was tested in vitro in a panel of 27 primary human cell types from multiple HLA-A*02:01-positive vital organs and tissues. Co-culturing primary human cells with PBMCs and CBX-663 did not induce T cell activation, even at concentrations far exceeding efficacious doses in vitro. Additional studies confirmed the specificity and developability of CBX-663, including a pharmacokinetic (PK) study in humanized FcRn mice, which demonstrated a conventional IgG-like PK profile with a half-life ≥12 days.Collectively, these data strongly support the therapeutic potential of CBX-663 in myeloid malignancies, including AML. IND enabling studies are currently underway with the goal of advancing CBX-663 into clinical development.