Background: KEYNOTE-799 (NCT03631784) is an ongoing study of the anti‒PD-1 antibody pembrolizumab (pembro) plus concurrent chemoradiation therapy (cCRT) in patients (pts) with unresectable, locally advanced stage III NSCLC. Prior results from this study in a subset of pts (primary efficacy population) showed an ORR of 69.6% in cohort A (squamous and nonsquamous, n = 112) and 70.5% in cohort B (nonsquamous, n = 61), and grade ≥3 pneumonitis in 8.0% and 7.9% of pts, respectively. Here, we present results for all pts enrolled in KEYNOTE-799. Methods: This nonrandomized, multisite, open-label phase 2 trial enrolled pts aged ≥18 y with previously untreated, unresectable, pathologically confirmed, stage IIIA‒C NSCLC with measurable disease per RECIST v1.1. Pts in cohort A (squamous and nonsquamous) received 1 cycle of carboplatin AUC 6 and paclitaxel 200 mg/m² and pembro 200 mg. After 3 wks, pts received carboplatin AUC 2 and paclitaxel 45 mg/m² QW for 6 wks and 2 cycles of pembro 200 mg Q3W plus standard thoracic radiotherapy (TRT). Pts in cohort B (nonsquamous only) received 3 cycles of cisplatin 75 mg/m², pemetrexed 500 mg/m²,and pembro 200 mg Q3W, and TRT in cycles 2 and 3. All pts received an additional 14 cycles of pembro 200 mg Q3W. Primary endpoints were ORR per RECIST v1.1 by blinded independent central review (BICR) and the incidence of grade ≥3 pneumonitis (per NCI CTCAE v4.0). Efficacy and safety were assessed in all pts as-treated. Results: Of 216 pts enrolled in KEYNOTE-799 (cohort A, n = 112; cohort B, n = 104), 112 in cohort A and 102 in cohort B received treatment. As of October 28, 2020, the median (range) time from first dose to database cutoff was 18.5 (13.6–23.8) mo in cohort A and 13.7 (2.9–23.5) mo in cohort B. ORR (95% CI) was 70.5% (61.2%‒78.8%) in cohort A and 70.6% (60.7%‒79.2%) in cohort B. Median DOR was not reached in either cohort (Table). ORR was similar regardless of PD-L1 status ([TPS <1% and TPS ≥1%]; Cohort A, 66.7% and 75.8%; Cohort B, 71.4% and 72.5%) and tumor histology (Cohort A, squamous, 71.2% and nonsquamous, 69.2%). Grade ≥3 pneumonitis occurred in 9 pts (8.0%) in cohort A and 7 (6.9%) in cohort B. Grade 3‒5 treatment-related AEs occurred in 72 pts (64.3%) in cohort A and 51 (50.0%) in cohort B. Conclusions: Pembro plus cCRT continues to demonstrate promising antitumor activity, regardless of PD-L1 TPS and tumor histology, and manageable safety in pts with previously untreated, locally advanced, stage III NSCLC with longer follow-up. Clinical trial information: NCT03631784

NR, not reached. “+” indicates no PD since the time of last disease assessment.