Abstract 5412: Xenograft based detection of rare leukemic clones in minimal residual disease negative diagnostic specimens from pediatric patients

Despite achieving complete remission with negative evaluation for minimal residual disease (MRD-) in the bone marrow, a significant proportion of pediatric leukemia patients experience relapse of disease. This conundrum highlights the limitations of standard methods in detecting extremely rare residual blasts. The heterogenous nature of leukemia also complicates the detection of minor residual subclones that may differ from the dominant clone and thus escape molecular and/or immunophenotypic detection during post-therapy assessments. Novel methods to detect, expand and characterize post-therapy residual disease would potentially allow new prognostic and diagnostic insights into leukemia relapse and its prevention. In the current study, we have engrafted MRD- samples into immune deficient mice in an attempt to use the patient-derived xenograft (PDX) assay to provide a functional readout for clinically undetected residual leukemia-initiating cell (LIC) activity. To date, we have successfully detected residual LICs in clinically MRD- specimens taken from 10 pediatric cases (5 B-ALL and 5 AML). The cases with positive functional LIC detection followed two general scenarios - outgrowth of undetectable marrow disease in the setting of extramedullary relapse, and outgrowth of undetectable marrow disease following either stem cell transplant or chimeric antigen receptor T-cells (CAR-T) therapy. Six bone marrow samples from patients with isolated CNS or extra-medullary relapse were able to establish engraftment despite MRD negativity on clinical testing. Four of the six patients ultimately succumbed to disease progression, and remission was successfully salvaged in the remaining two. Additionally, another four sets of samples acquired after bone marrow transplant or CAR-T therapy established PDX grafts, thereby potentially allowing for prediction of relapse in some patients. 1 AML and 1 ALL patient relapsed with similar kinetics as seen in the mouse. A second PDX-predicted relapse ALL patient succumbed to transplant complications before overt clinical relapse and a second AML patient subsequently relapsed many months after the positive PDX assay. These results highlight the remarkable sensitivity of immune deficient mice to engraft human LICs. Detection of LICs in MRD- BM specimens via PDX formation could aid in teasing apart the various mechanisms for relapse in individual patients. Additionally, these results provide proof of principle for the use of PDX as a surveillance tool for prediction of leukemic relapse and may identify a subset of patients who could benefit from earlier or additional therapy. Citation Format: Mark Wunderlich, Nicole Manning, Eric O9Brien, Christina Sexton, Luke Byerly, John P. Perentesis, Benjamin Mizukawa, James C. Mulloy. Xenograft based detection of rare leukemic clones in minimal residual disease negative diagnostic specimens from pediatric patients [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5412.