Abstract 1171: Estimating sensitivity of exploratory cellular biomarkers in oncology clinical trials

Abstract

Abstract Background: In the prolonged and complex process of assessing immunotherapeutic oncology drug candidates, flow cytometry plays an essential role. In oncology studies, biomarker measurement requires robust and reliable flow cytometric assays. A crucial element of these assays is to understand the lower limits of measurable surface biomarkers. Methods: We incorporated into the development and validation of flow cytometry assays the measurement of the lower limit of quantitation (LLOQ). The LLOQ is the lowest number of a reportable parameter in a blood or bone marrow study sample that can be quantitatively determined with acceptable precision and accuracy. Two steps are necessary to determine the LLOQ. First in these assessments a measurable range of lower limits of a particular cell type or receptor is estimated. That is, in validating an assay LLOQ, there are three options to select cells expressing target surface biomarkers of interest: using cultured or bioengineered cells, using samples of known high expression of a biomarker, or using pre-titered positive samples. Then decreasing concentrations of the target cell or receptor may be used to precisely determine the LLOQ of a specific assay reportable parameter. Results: By acquiring at least ten negative control sample results, the limit of the blank (LOB) and limit of detection (LOD) of a biomarker is determined. LOB is calculated as mean +1.645 SD, and LOD as mean +3 SD. The negative control samples either lack the fluorescent monoclonal antibodies against the markers of interest or lack the target cell population. Different reportable parameters, detected by one or more biomarkers have varying LLOQ. The LLOQ for a reportable parameter is a precise measurement of a parameter above or equal to the LOD. We found that typical LLOQ values are from 4 to 145 positive cells, with a frequency of 0.01 to 0.1% among a parent population. For example, in the case of tetramer assays, flow cytometry can reliably detect as few as 25 CD8+ antigen-specific cells among three million lymphocytes. Conclusions: LLOQ determinations for exploratory flow cytometry biomarkers are required for clinical research in cancer immunotherapy and cancer vaccine development. This valuable information allows for interpretation of data at the low end of a measurable range. Citation Format: Chengsen Xue, Christina D. Swenson, Thomas W. Mc Closkey. Estimating sensitivity of exploratory cellular biomarkers in oncology clinical trials [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1171.