Abstract 6161: PARP inhibition enhances radiotherapy of SMAD4 deficient human head and neck squamous cell carcinomas in experimental models

Purpose: SMAD4 loss causes genomic instability and the initiation/progression of head and neck squamous cell carcinoma (HNSCC). Here, we study if SMAD4 loss sensitizes HNSCCs to olaparib (PARP inhibitor) in combination with radiotherapy (RT). Experimental Design: We analyzed HNSCC TCGA data for SMAD4 expression in association with FANC/BRCA family gene expression. Human HNSCC cell lines were screened for sensitivity to olaparib. Isogenic HNSCC cell lines were generated to restore or reduce SMAD4 expression and treated with olaparib, radiation, or the combination. HNSCC pretreatment specimens from a Phase I trial investigating olaparib were analyzed. Results: SMAD4 levels correlated with levels of FANC/BRCA genes in HNSCC. HNSCC cell lines with SMAD4 homozygous deletion were sensitive to olaparib. In vivo, olaparib or RT monotherapy reduced tumor volumes in SMAD4 mutant but not SMAD4 positive tumors. Olaparib with RT dual therapy sustained tumor volume reduction in SMAD4 deficient (mutant or knockdown) xenografts, which exhibited increased DNA damage and cell death compared to vehicle treated tumors. In vitro, olaparib or in combination with radiation caused lower clonogenic survival, more DNA damage-associated cell death and less proliferation in SMAD4 deficient cells than in SMAD4-positive (endogenous SMAD4 or transduced SMAD4) cells. Applicable to clinic, 5 out of 6 SMAD4-negative HNSCCs and 4 out of 8 SMAD4-positive HNSCCs responded to a standard treatment plus olaparib in a Phase I clinical trial, and SMAD4 protein levels inversely correlated with DNA damage. Conclusion: SMAD4 levels are causal in determining sensitivity to PARP inhibition in combination with RT in HNSCCs. Citation Format: Christian D. Young, Ariel L. Hernandez, Li Bian, Sana D. Karam, David Raben, Xiao-Jing Wang. PARP inhibition enhances radiotherapy of SMAD4 deficient human head and neck squamous cell carcinomas in experimental models [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 6161.