Abstract 583: Rapid release of ctDNA as a biomarker of treatment response in preclinical models of head and neck squamous cell carcinoma
Authors: Rostami Ariana , Yu Caberry , Grappa Marco A. Di , Bratman Scott V. 
Published: August 2018

Abstract
Purpose: High-dose radiotherapy (RT) is a standard treatment for locally advanced head and neck squamous cell carcinoma (HNSCC). Despite known molecular prognostic biomarkers in HNSCC, such as the presence of human papillomavirus (HPV) within tumor tissues, RT regimens remain one-size-fits-all without any patient-specific individualization. We hypothesize that rapid release of circulating tumor DNA (ctDNA) can act as a biomarker of treatment response in HNSCC by reflecting tumor cell death in response to RT.
Methods: Four HNSCC cell lines, two HPV- (Cal33, FaDu) and two HPV+ (HMS-001, 93-Vu147T), were evaluated for ctDNA release following single dose RT in vitro. Quantification of ctDNA was performed by quantitative polymerase chain reaction (qPCR) with primers amplifying the human long interspersed nuclear element 1 (hLINE1). RT-induced apoptosis was measured using a luminometric caspase 3/7 assay, and RT-induced senescence was evaluated using a fluorometric senescence associated β-galactocidase (SA-β-Gal) assay. The pan-caspase inhibitor, z-vad-fmk, was used to block RT-induced apoptosis. Subcutaneous cell-line xenografts were established in Nod-Scid-Gamma (NSG) mice, where plasma from serial blood draws was purified and quantified for ctDNA release using hLINE1 qPCR. Endpoint tumors were evaluated for mechanisms of cell death by histological staining.
Results: HNSCC cell lines exhibited variable magnitude and timing of ctDNA release and apoptosis. Maximal ctDNA release occurred between 72 and 144 hours post-RT. The release of ctDNA was not correlated with HPV status nor with apoptosis occurring following RT. To further interrogate the contribution of apoptosis to ctDNA release, we treated cells with a pan-caspase inhibitor and evaluated ctDNA release and caspase activity following RT. Although caspase inhibition resulted in a near complete reduction in RT-induced caspase activity (84.0% ±8.1%), a comparatively minor reduction in ctDNA release (28.9%±8.9%) was observed. To evaluate the impact of senescence on RT-induced ctDNA release, we measured SA-β-Gal activity post-RT. The degree of senescence following RT was inversely associated with ctDNA release. In cell line xenografts, maximal ctDNA release into mouse plasma occurred 96 hours following RT. Staining patterns of endpoint tumors for caspase-3, TUNEL, p21, and Ki67 are currently under analysis.
Conclusions: Our results demonstrate a robust and sensitive method for longitudinal evaluation of ctDNA release and cell death post-RT both in vitro and in vivo. Timing of ctDNA release demonstrate peaks around 72-96 hours post-RT, highlighting the potential for ctDNA to depict early response to therapy. Ongoing studies aimed at further investigating ctDNA release in association with mechanisms of cell death in vivo will help further elucidate the potential for ctDNA as a biomarker of treatment response.
Citation Format: Ariana Rostami, Caberry Yu, Marco A. Di Grappa, Scott V. Bratman. Rapid release of ctDNA as a biomarker of treatment response in preclinical models of head and neck squamous cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 583.