Circulating tumor DNA profiling and serial analysis in salivary gland carcinomas reveal unique mutational subsets and actionable alterations.

Abstract

6097 Background: Salivary gland carcinoma is a rare head and neck malignancy without any FDA approved treatment options. ctDNA profiling has been shown to be a valuable tool in identifying novel targets and expanding potential therapeutic options. Here, we report the landscape of ctDNA in a large dataset of salivary gland carcinomas. Methods: Analysis of genomic results from blood samples with a diagnosis of salivary gland carcinoma that were prospectively collected between January 2017 to December 2021 for clinical Guardant360 testing. Histologic subtype was documented by ordering providers as available. Serial tests were defined as having Guardant360 analysis performed on more than one sample collected at more than one timepoint. Results: Among 222 patients tested with Guardant360, 59.5% were male and the median age was 64 years (range 22-93 years). Of reported diagnoses, 137 (61.7%) patients had salivary gland carcinoma NOS. The remaining samples were designated by specific salivary histology: 13 (5.9%) salivary gland mucoepidermoid carcinoma (SGMC), 19 (8.6%) salivary gland adenocarcinoma (SGA), 37 (16.7%) salivary gland adenoid cystic carcinoma (SGACC), and 16 (72%) salivary duct carcinoma (SDC). ctDNA genomic alterations (GAs) were identified in 205 (81.3%) of 252 samples. The median variant allele fraction was 0.6% (range 0.01%-46.4%). TMB was evaluated in 45 samples of which 33 were evaluable and the median and 80th percentile of TMB was 8.44 and 11.81 mut/Mb. MSI-H was not detected in any of the 181 samples tested for MSI. The most commonly altered genes were TP53, PIK3CA, ERBB2, ATM, EGFR and HRAS across all collected salivary samples. Further analysis stratified genetic alterations by tissue subtype. Other than TP53, common mutations by salivary gland carcinoma subtype included: PI3KCA (SGACC/SDC), ERBB2 (SGA), and EGFR (SGMC). In regards to serial analysis, 16 patients had at least 2 serial tests. When comparing the subsequent testing within the same patient, 3 patients had potentially new actionable GAs (including BRAF, KRAS mutations and EGFR amplification) on the subsequent test that were not identified on the original test. Conclusions: Blood-based liquid biopsy can be applied in salivary gland tumors to detect genomic alterations in ctDNA which may provide opportunities for therapeutic intervention in a cancer with limited treatment options. Additionally, ctDNA testing may be used to identify resistant alterations to potential therapy. Finally, longitudinal assessment of ctDNA may shed light on tumor evolution and additional therapeutic targets may be found. Further assessment of serial ctDNA analysis and the potential impact on patient clinical outcomes needs to be elucidated.