1558P Preoperative pembrolizumab plus chemotherapy for resectable esophageal squamous cell carcinoma (ESCC): The phase II Keystone-001 trial

The Keystone-001 study was designed to prospectively evaluate the efficacy and safety of preoperative treatment with pembrolizumab plus paclitaxel and cisplatin followed by Da Vinci robotic radical surgery in patients with locally advanced (stage III) ESCC. The study also explored the potential mechanism of this treatment strategy (NCT 04389177, 5/15/2020). The full study methods for this prospective, single-arm, single-center, open-label, phase-II study have been published previously. For further investigating the immune cell subpopulations that determine the efficacy, we performed single-cell sequencing on peripheral blood and tumor tissues after neoadjuvant therapy, and validated using flow cytometry. Of the 49 patients enrolled between July 8, 2020, and February 10, 2022, 47 received neoadjuvant therapy and were included in the ITT analysis. Among all patients who received neoadjuvant therapy, no grade ≥3 AEs were reported. Improvements from baseline nutritional status were observed with increases in patients’ median body weight and NRI. QoL evaluations showed improvements from baseline to post-adjuvant therapy in all functional scales and symptom scales of EORTC QLQ-C30 and in all items of the EORTC QLQ-OES18 questionnaire. Among all 45 patients in the efficacy analysis, the MPR rate was 73.3%, the pCR rate was 42.2%, the ORR was 95.6%, and the DCR was 100%. After a median follow-up of 23.3 months (95% CI 20.6, 24.9; maximum follow-up 34 months), the 1- and 2-year OS and DFS rates were 95.6 and 90.5% and 95.6 and 86.3%, respectively. The single-cell sequencing results showed that the main cell populations that affect the response of this treatment strategy were NKT and NK cell subpopulations in peripheral blood. Further analysis results showed that the TRGC2+NKT and FGFBP2+NK cell subpopulation in peripheral blood were more significantly increased in peripheral blood from patients with a good response. The preliminary findings of this trial were safe and effective, and further phase III studies (Keystone-002) have begun to recruit participants.