Abstract A43: Plasma-derived circulating tumor DNA (ctDNA) as a surrogate biomarker for treatment response with the polo-like kinase 1 (PLK1) inhibitor, onvansertib, in combination with LDAC or decitabine in acute myeloid leukemia (AML)
Authors: Samuelsz Errin , Ridinger Maya , Erlander Mark , Hassaine Latifa , Luebbermann Marion , Ross Brittany 
Published: June 2020

Abstract
Background: Cancer cells from solid tumors release ctDNA into the bloodstream, clinically enabling assessment, monitoring tumor mutational heterogeneity, treatment response, minimal residual disease (MRD), and disease progression. However, the clinical utility of ctDNA in AML has yet to be established. In AML, blast cells carrying a driver mutation originate in the bone marrow (BM) and migrate into circulation in peripheral blood (PB). Here, we aimed to determine the potential of ctDNA as a surrogate for response to treatment in the phase 1b/2 study (NCT03303339) testing the efficacy and safety of the PLK1 inhibitor, onvansertib, in combination with either decitabine or LDAC in relapsed or refractory AML (R/R AML).
Methods: R/R AML patients were dosed for 5 days with onvansertib in combination with either decitabine (5 days) or LDAC (10 days) within 21 to 28-day cycle. BM aspirates and blood samples were collected pre-dose, at the end of cycles 1, 2 and every other cycle after. Genomic DNA (gDNA) was extracted from BM and PB mononuclear cells (BMMCs and PBMCs, respectively), while ctDNA was extracted from plasma. Clinically significant mutations were detected in gDNA from PBMCs and BMMCs using a next-generation sequencing panel targeting variants in 75 genes associated with AML. For each patient, a droplet digital PCR (ddPCR) assay was developed to assess mutant allele frequencies (MAF) of a single variant of interest across timepoints and three sample sources (BMMC, PBMC, and Plasma). MAF changes across all 3 sample sources were compared to the clinical response assessed by immunohistochemistry in bone marrow aspirate (BM-IHC).
Results: As of October 16th, 16 patients were evaluated, totaling 33 timepoints with MAF measured in all 3 sources. MAF in plasma highly correlated with BMMCs MAF (R2 = 0.85), more so than PBMCs with BMMCs (R2 = 0.78) and over plasma with PBMCs (R2 = 0.68). Clinically, 4 patients had a complete response (with or without count recovery, CR+CRi) and showed a corresponding drop in ctDNA MAF at or before clinical diagnosis. Similarly, 3 of 4 evaluable patients with progressive disease showed an increase in ctDNA MAF before or at the timepoint when blast percentages rose in BM, with the fourth patient maintaining a stable MAF across timepoints. In 5 patients with stable disease, 2 showed a divergence between ctDNA MAF and BM-IHC, while 3 maintained congruency.
Conclusions: Serial testing of plasma-derived ctDNA was representative of treatment response and relapse in R/R AML patients. The correlation between ctDNA MAF and BMMCs MAF supports the utility of ctDNA as a surrogate biomarker for disease status, and in some cases provides a more immediate diagnostic of response or nonresponse. More patients need to be evaluated to build an objective classification of clinical significance around ctDNA MAF, and, importantly, the biology of R/R AML subclonal heterogeneity and MRD likely necessitate interrogation and monitoring of multiple variants of interest.
Citation Format: Errin Samuelsz, Maya Ridinger, Mark Erlander, Latifa Hassaine, Marion Luebbermann, Brittany Ross. Plasma-derived circulating tumor DNA (ctDNA) as a surrogate biomarker for treatment response with the polo-like kinase 1 (PLK1) inhibitor, onvansertib, in combination with LDAC or decitabine in acute myeloid leukemia (AML) [abstract]. In: Proceedings of the AACR Special Conference on Advances in Liquid Biopsies; Jan 13-16, 2020; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(11_Suppl):Abstract nr A43.