Abstract 1778: A clinical genomic biomarker study of the CHK1 inhibitor prexasertib in advanced head and neck squamous cancer and squamous cell carcinoma of the anus
Authors: Martinez Ricardo , Wijayawardana Sameera R. , Hong David , Bendell Johanna , Russell Anna Maria , Beckmann Richard P. , Lin Aimee Bence 
Published: July 2017

Abstract
CHK1 plays a critical role in DNA damage repair (DDR) pathways as well as in coordinating DNA replication. Selective CHK1/CHK2 compounds are being tested in clinical trials but predictive biomarkers of patient response are lacking. A phase 1b expansion cohort study (I4D-MC-JTJA, NCT01115790) with the CHK 1 inhibitor, prexasertib, included patients with advanced, metastatic head and neck squamous cell carcinoma (HNSCC) or squamous cell carcinoma of the anus (SCCA). To identify genomic biomarkers associated with single-agent drug response, pre-treatment tissues (archived or biopsy) from 71 consented patients (HNSCC=47, SCCA=24) were subjected to next-generation sequencing (NGS) using the FoundationOne gene panel. In this subset of patients, the disease control rate (DCR) (Complete Response (CR) + Partial Response (PR) + Stable Disease (SD) based on RECIST Criteria (v 1.1)) was 60% (28/47) and 75% (18/24), respectively. We present here the observed genetic alterations corresponding to three pathways, Cell Cycle, DNA Damage Repair (DDR) and PI3K. In addition, patients’ human papillomavirus (HPV) carrier status was inferred from DNA sequencing using HPV-specific capture probes. HPV+ was 47% for HNSCC and 87% for SCCA. HPV+ and TP53 mutations were mutually exclusive across the two patient cohorts. In HNSCC patients with evaluable progression-free survival (PFS) data, greater clinical efficacy was observed in the HPV+ cohort (median PFS: 4.5 vs 1.4 months, log-rank p = 0.0008). Known or likely loss-of-function (LOF) mutations in FBXW7 and PARK2, two genes implicated in Cyclin E1 proteolysis, were noted in patients with favorable response in both tumor types. Across both HNSCC and SCCA cohorts, mutations and/or germline variants in the DDR genes BRCA1, BRCA2, MRE11A and ATR but not in Fanconi (FANC) pathway genes, were found in patients with treatment benefit. Whereas PIK3CA mutations were infrequent in the HNSCC cohort, in SCCA, mutations occurred in 5/8 (63%) patients with disease control vs 1/6 (17%) with PD. All 7 PI3KCA mutations observed in HPV+ HNSCC and SCCA patients mapped to the helical domain suggestive of Apobec-induced mutagenesis as their source of origin. The enhanced clinical benefit to prexasertib associated with HPV+ in HNSCC may reflect a prognostic effect. Alternatively, the clinical biomarker findings may support the hypothesis that oncogene-induced replication stress (RS) (i.e. arising from HPV E6/E7 and/or FBXW7 loss-dependent Cyclin E1 dysregulation) in the context of attenuated DDR (i.e. BRCA1/2, MRE11A mutations) may sensitize patients to prexasertib monotherapy.
Citation Format: Ricardo Martinez, Sameera R. Wijayawardana, David Hong, Johanna Bendell, Anna Maria Russell, Richard P. Beckmann, Aimee Bence Lin. A clinical genomic biomarker study of the CHK1 inhibitor prexasertib in advanced head and neck squamous cancer and squamous cell carcinoma of the anus [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1778. doi:10.1158/1538-7445.AM2017-1778