1257P - Blood-based TMB (bTMB) correlates with tissue-based TMB (tTMB) in a multi-cancer phase I IO cohort

High tissue-tumor mutation burden (tTMB) is a predictor of response to immunotherapy (IO). Tissue availability and tumor heterogeneity are barriers to tTMB use in clinical practice. Plasma-based blood TMB (bTMB) is a convenient alternative that strongly correlates with tTMB in non-small cell lung cancer. Whether this correlation holds true in other cancers is unknown. Here, we examined the correlation between bTMB and tTMB as well as the clinical utility of TMB as a predictive marker of response in a heterogeneous Phase I IO cohort. Advanced cancer patients (pts) treated with mono- or combination IO therapy at the Princess Margaret phase I unit were enrolled. Pre-treatment plasma ctDNA and matched normal blood controls were collected via an institutional liquid biopsy program (LIBERATE, NCT03702309). Available archival tissue FFPE samples were analyzed. The GeneseeqPrime 425 gene panel was used to sequence both ctDNA and FFPE samples. From December 2017 to July 2018, 39 pts with 19 tumor types were accrued from 25 different trials, 87% of which involved a PD-1/PD-L1 inhibitor. The median age was 59y (21 – 77) and 52% were female. The most frequent cancers were colorectal, head and neck and breast, each with 5 cases. Thirty-one patients (79%) had detectable mutations in plasma ctDNA. The median bTMB was 5 (1 - 53) mut/Mb. Twenty-one pts had available FFPE samples. Of those, mutations were detected in 20 (95%) samples. The median tTMB was 6 (2 - 124) mut/Mb. Among the 16 pts with detectable mutations in both FFPE and plasma samples, a significant correlation between bTMB and tTMB was observed (r=+0.67; p