Liechty, Kirstin B.

TPS2676 Background: Solid tumors comprise > 90% of cancers. Metastatic colorectal cancer, non-small cell lung cancer, and pancreatic cancer are among the leading causes of cancer-related mortality (5-year overall survival: 14%, 6%, and 3%, respectively) (ACS. 2021). Chimeric antigen receptor (CAR) T-cell therapy has demonstrated clinical efficacy in hematologic malignancies (Neelapu S. et al. N Engl J Med. 2017). Translating engineered T-cell therapies to solid tumors has proven to be challenging due to a lack of tumor-specific targets that can discriminate cancer cells from normal cells. Previous studies using carcinoembryonic antigen (CEA) T-cell receptors and mesothelin (MSLN) CARs resulted in dose-limiting on-target, off-tumor toxicities (Parkhurst M, et al. Mol Ther. 2011; Tanyi J. Cellicon Valley '21). To create a therapeutic safety window, Tmod CAR T-cell therapy utilizes dual-signaling receptors to create a robust NOT logic gate capable of killing tumor cells, while leaving healthy cells intact (Hamburger A, et al. Mol Immunol. 2020). The 2 receptors in Tmod CAR T-cell therapy comprise an activator that recognizes an antigen on the surface of tumor cells that may also be present on normal cells, such as CEA and MSLN, and a blocker that recognizes a second surface antigen from an allele lost only in tumor cells. The frequency of HLA LOH among advanced GI solid tumor cancers in the Tempus real-world dataset is 16.3% with a range of 15.6%-20.8% between colorectal, pancreatic, and gastroesophageal tumors (Hecht R. et al. ASCO-GI 2022. Abstract #190). As such, HLA LOH offers a definitive tumor versus normal discriminator target for CAR T-cell therapy. Different activator/blocker combinations can be engineered with the Tmod platform technology and may be applied to T cells and natural killer cells in autologous and allogeneic settings. BASECAMP-1 is a currently enrolling observational study with key objectives of 1) To identify patients with somatic HLA LOH eligible for Tmod CAR T-cell therapy, and 2) To obtain leukapheresis and feasibility for the future EVEREST Tmod CAR T-cell trial. Methods: BASECAMP-1 (NCT04981119) patient eligibility has 2 parts: 1) Patients will be initially screened to identify germline HLA-A*02 heterozygosity by central NGS. If HLA-A*02 heterozygosity is confirmed, primary archival tumor tissue will be analyzed for somatic mutations by xT-Onco NGS testing. 2) If the tumor demonstrates HLA-A*02 LOH and the patient is eligible after screening, the patient will undergo leukapheresis. Banked T cells will be available for the autologous EVEREST Tmod CAR T-cell therapy interventional study to reduce waiting time at relapse. Clinical trial information: NCT04981119.

190 Background: Metastatic colorectal (CRC), pancreatic (PANC), and gastroesophageal (GE) cancers are the leading causes of GI cancer–related mortality (5-yr survival rate, 14%, 3% and ̃5-6%, respectively). T-cell immunotherapy targeting GI-associated tumor antigens has been attempted, but efficacy has been constrained by on-target off-tumor toxicity, limiting the therapeutic window. The Tmod (TM) platform is an AND-NOT logic-gated CAR T modular system, versions of which have a CEA- or MSLN-targeting CAR activator and a separate HLA-A*02-targeting blocker receptor to protect normal cells. Tmod CAR T exploits HLA LOH, common in GI malignancies (10-33% in primary solid tumors [TCGA]) and can kill tumor cells without harming healthy cells in vitro and in vivo. However, the prevalence of HLA LOH across GI tumors is unknown in the real-world setting. We utilized the Tempus xT oncology NGS database of patients with multiple GI tumors. From a standard-of-care NGS assay, GI cancer patients can be readily identified for HLA LOH and future treatment with Tmod CAR T therapy. Methods: The occurrence of HLA LOH in GI tumors of 1439 patients was assessed using paired germline and somatic DNA sequencing using a research assay [6]. CRC, PANC and GE patients with ≥ stage 3 were then extracted, and rates of HLA LOH were identified (ie, whether loss occurred across high-frequency HLA-A alleles). In addition, mutations in KRAS and BRAF, as well as MSI status were stratified to determine any association with HLA-A LOH. Results: HLA-A LOH was detected in 830 (17.3%) of all solid tumor records, and a similar proportion when all GI cancer records were analyzed (17.0%). For GI subtypes, these values ranged from 13.5% to 23.1% (Table). No high-frequency HLA-A allele (A*01, A*02, A*03, A*11) was more likely to be lost. Clinical biomarkers ( KRAS, BRAF and MSI status) were not associated with HLA-LOH. Conclusions: The frequency of HLA LOH among advanced solid tumor cancers in this dataset is 17.3%, with a range of 13.5-23% between CRC, PANC and GE. The HLA LOH frequency observed in these GI tumors is consistent with that in primary tumors from TCGA, which also used germline-matched and tumor samples. Clinical biomarkers were not associated with HLA LOH. Tempus NGS was able to identify HLA LOH, which can be used for Tmod CAR T therapy to an enhanced therapeutic window. Identification of these patients in BASECAMP-1 (NCT04981119) will enable novel Tmod CAR T therapy. [Table: see text]