Abstract 2957: A novel circulating tumor DNA (ctDNA) assay enables monitoring of disease progression and treatment response in disseminated preclinical hematologic cancer models
Authors: Christie Amanda L. , Labrousse Paul , Andersen Courtney L. , Roderick Justine E. , Gordon Jacob , Lawson Deborah , Hughes Denise , Maratea Kimberly , Stetson Daniel , Willis Brandon , Bloecher Andrew , Reimer Corinne , Dougherty Brian 
Published: July 2021

Abstract
We have established a novel assay to assess circulating tumor DNA (ctDNA) in mice engrafted with disseminated cell line and patient-derived xenografts (PDX) of hematologic malignancies. Disseminated models recapitulate many features of human disease, but engraftment in multiple tissues makes monitoring of disease and treatment response difficult. Existing assays also lack the sensitivity required to assess minimal residual disease (MRD). This novel ddPCR assay targets highly conserved human-specific regions of LINE-1 and HERV-K repeat elements resulting in exceptionally sensitive detection of shed human ctDNA. Initial data shows sensitivity of 0.8 haploid genome equivalents (one haploid genome is ~3.3pg of human DNA), whereas ctDNA monitoring assays currently on the market require 1,000 times more input DNA to reach a sensitivity of just 2-9 genome equivalents. Serial dilution experiments confirm this assay is suitable to detect increases in ctDNA over several orders of magnitude, while also establishing a minimum plasma input of 40uL. To validate the assay we assessed the ability of ctDNA to detect disease alongside traditional histology, bioluminescence imaging, and flow cytometry assays in 10 leukemia and lymphoma models. Levels of ctDNA correlated well with disease progression across models engrafting in bone marrow, spleen, liver, blood and other tissues. In all cases where early stage disease was analyzed, the ctDNA assay was able to detect disease earlier relative to other methods due to its increased sensitivity, and these data can be effectively used for randomization into treatment groups. We also assessed the utility of ctDNA for determining drug treatment response in mice engrafted with a disseminated mantle cell lymphoma PDX. Baseline ctDNA assessment was completed on day 11 post-engraftment confirming ctDNA levels above baseline, at which point mice were divided into 7 treatment groups: Vehicle, Acalabrutinib, and 5 combination arms of Acalabrutinib plus clinically used agents. After 4 weeks of treatment disease burden was assessed by flow cytometry of the bone marrow (femur) compared to ctDNA detected in plasma. Efficacy readouts from both assays agreed for all groups, while the ctDNA assay was able to identify a significant difference between the two best responding arms which appeared to have near complete responses when assessed by flow cytometry alone. The fold difference between the two best responding arms was 1.1 fold by flow cytometry and 3.6 fold by ctDNA, illuminating a greater disparity in MRD and relapse potential between these 2 groups. This novel ctDNA assay allows for robust experimental designs to answer questions about the depth and durability of treatment responses, as well as to identify early stage disease. Work is ongoing to explore extended monitoring for relapse and application to solid tumor models.
Citation Format: Amanda L. Christie, Paul Labrousse, Courtney L. Andersen, Justine E. Roderick, Jacob Gordon, Deborah Lawson, Denise Hughes, Kimberly Maratea, Daniel Stetson, Brandon Willis, Andrew Bloecher, Corinne Reimer, Brian Dougherty. A novel circulating tumor DNA (ctDNA) assay enables monitoring of disease progression and treatment response in disseminated preclinical hematologic cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2957.