e15140 Background: Small subsets of pts treated with targeted therapies (TT) present exceptional and unexpected response (1-10%), which could be driven and correlated by a low level of genomic alterations in genes identified as causally implicated in cancer. Conversely, the multiplication of the genomic alterations in such genes would decrease the oncogenic de-addiction process. Methods: EXPRESS is an exploratory, multicenter, prospective trial conducted in pts with locally advanced or metastatic solid tumors, who presented an exceptional and unexpected response to an antineoplastic TT. This research aimed to explore whether tumors with a low level genomic alteration (mutation, amplification, deletion) as assessed by whole exome sequencing (WES) are associated with exceptional and unexpected response. Secondary endpoints including the identification of novel candidate somatic molecular profiles and host immunologic characteristics susceptible to drive exceptional and unexpected response will be reported later. Results: 439 pts were identified over 32 centres from December 2016 to April 2021. Out of 270 pts confirmed with an exceptional response by our review committee of cases, 183 were included in the EXPRESS study. Exceptional responder population is summarised (Table). Out of 183, WES analysis was performed for 103 pts (56.3%). Genomic analysis failure was predominant in the breast and lung cohort (60.9% and 52.5%). The number of genomic alterations in the EXPRESS cohort was lower than in the TCGA control cohort (median[Q1-Q3]: 11[7-17] vs 19 [11-32], p<0.001). A lower number of genomic alterations was also reported in the melanoma (8[5-13] vs 33[17-51], p<0.001) and lung cancer cohort (8[5-17] vs 22[13-34], p<0.001) but not in the breast and kidney cohorts (13[10-21] vs 17[10-29], p=0.41 and 12[9-14] vs 11[7-17], p=0.89). Conclusions: Compared to the TCGA cohort, a low level of genomic alteration is associated with exceptional response in pts with lung cancer and melanoma. A comparison with a matched population based on genomic criteria in the TCGA is undergoing and will be presented at the conference. Clinical trial information: NCT02701907 . [Table: see text]
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