Clinical outcomes of patients with gastric cancer according to pre and post-neoadjuvant chemotherapy PD-L1 immunohistochemistry (IHC) expression.

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e16569 Background: The incidence, prognostic and predictive impacts of PD-L1 expression in locally advanced gastric cancer is unknown. We aimed to determine the expression of PD-L1 by CPS in the pre-treatment biopsy and surgical specimens of patients (pts) with gastric cancer who received neoadjuvant therapy and its association with pathological response and survival outcomes. Methods: Retrospective cohort of pts treated at a cancer center from 2007 to 2017. Pts with confirmed gastric or GEJ adenocarcinoma who received neoadjuvant treatment and curative-intent D2 surgery were included. Gastric stump tumors and those who had a total esophagectomy were excluded. Clinical data were obtained from medical charts. Biopsy samples and a tissue microarray with the most representative areas of the surgical specimen were used to detect PD-L1 IHC expression with 22C3 phamDx antibody. Results were analyzed using the CPS score. Overall and DFS survival included the Kaplan-Meier product-limit estimator in an ITT analysis and a Cox regression was used to obtain crude and adjusted HR for prognostic factors. Results: 270 pts were included: median age was 58.9 years, most (51.5%) had cT3-T4N+ stages, 45% had diffuse histology and 87.8% completed the preoperative regimen. 13% had a pCR, while 53% had minimal tumor regression. With a median follow-up of 60.3 months (CI 95% 54.7 – 65.8), the median OS and DFS were not reached. 11.4% of biopsies and 18.6% of surgical specimens had positive CPS, with a median score of 3 (IQR 2,0 – 7,5) and 9 (IQR 5.0 – 20.0) respectively. In 18.9% of paired samples the PD-L1 expression was found to be negative in the biopsy sample and positive in the surgical specimen. PD-L1 expression was neither associated with pathologic response after neoadjuvant chemotherapy, nor with survival outcomes. Conclusions: PD-L1 expression on the setting of locally advanced gastric cancer was low and it was different when biopsy and surgical specimens were compared. No impact on survival results could be detected. [Table: see text]