Background: Chemoimmunotherapy with a platinum doublet plus a checkpoint inhibitor (CPI) is a standard of care for pts with advanced NSCLC. While some pts experience prolonged responses to initial CPI therapy, the majority of pts will eventually experience PD. It is unknown if continuing CPI treatment beyond progression has any advantages in this setting. We report the results of a phase 2 trial of chemotherapy plus pembrolizumab in pts with advanced NSCLC previously treated with a PD-1 or PD-L1 inhibitor. Methods: Pts experiencing PD after clinical benefit to CPI (PFS > 3 months) were enrolled. Pts received pembrolizumab 200 mg q3wks plus next-line chemotherapy (gemcitabine 1000 mg/m² IV D1 and D8 q3wks, or docetaxel 60-75 mg/m² IV D1 q3wks, or pemetrexed 500 mg/m² IV D1 q3wks [non-squamous histology only]). The primary endpoint was PFS by RECIST 1.1. Key secondary endpoints included ORR, OS, and toxicity. The null hypothesis was median 3-month PFS with pembrolizumab plus next-line chemotherapy and the alternative hypothesis was median 6-month PFS with pembrolizumab plus chemotherapy. Results: 35 pts were enrolled. Median follow-up was 18.1 months and median age 63 (44-80). 51.4% male and 48.6% female. 82.9% were current or former smokers. Histology included 74.3% with adenocarcinoma, 20% with squamous cell carcinoma, 5.7% with NSCLC NOS. Treatment regimens included pembrolizumab/docetaxel (40%), pembrolizumab/gemcitabine (45.7%), or pembrolizumab/pemetrexed (14.3%). Median number of cycles of pembrolizumab was 6 (1-31). Median PFS using RECIST 1.1 and irRECIST was 5.2 months (95% CI 3.6-11.2, p < 0.05) and 6.9 months (95% CI 3.8-12), respectively. Median OS was 26.8 months (95% CI 13.4-30.9). Best response using RECIST 1.1 was PR (23.5%) and SD (53%). 45.7% of pts experienced G3 or higher treatment-related AEs (TRAEs). Most common TRAEs were fatigue (60%), anemia (51.4%), and nausea (42.9%). There were no treatment related deaths. Conclusions: Pembrolizumab plus next-line chemotherapy in pts with advanced NSCLC who experienced PD after clinical benefit to CPI was associated with prolonged PFS compared with historical controls of single agent chemotherapy. Further investigations into which pts would benefit from continued CPI treatment after progression is warranted. Clinical trial information: NCT03083808