Pembrolizumab in microsatellite instability high (MSI-H)/mismatch repair deficient (dMMR) cancers: Updated analysis from phase 2 KEYNOTE-158 study

2565 Background: Approval of pembrolizumab for the treatment of unresectable or metastatic MSI-H/dMMR solid tumors that have progressed on prior therapy was supported by data from KEYNOTE-158 (NCT02628067). At the data cutoff of Dec 6, 2018, the ORR was 34.3% among 233 patients (pts) with MSI-H/dMMR solid tumors enrolled in all cohorts of KEYNOTE-158, 77.6% had duration of response (DOR) ≥24 mo, median PFS was 4.1 mo, and median OS was 23.5 mo. We present results from 351 pts enrolled in KEYNOTE-158 cohort K at the data cutoff of Oct 5, 2020. Methods: Cohort K of this phase 2, open-label study enrolled adults with any previously treated advanced noncolorectal MSI-H solid tumor, measurable disease per RECIST v1.1, and ECOG PS of 0–1. MSI-H/dMMR status was assessed locally from a tumor tissue sample and defined as ≥1 of 4 MMR proteins absent by immunohistochemistry or as ≥2 allelic loci size shifts of 5 microsatellite markers by PCR. Pts received pembrolizumab 200 mg Q3W for up to 35 cycles or until PD, unacceptable toxicity, investigator decision, or withdrawal of consent. The primary endpoint was ORR per RECIST v1.1 by blinded independent central review (BICR). Secondary endpoints were DOR and PFS per RECIST v1.1 by BICR, OS, and safety. Efficacy was assessed in all pts who received ≥1 dose of treatment with ≥6 mo follow-up; safety was assessed in all treated pts. Results: 351 pts were enrolled in KEYNOTE-158 cohort K across multiple tumor types, including endometrial (22.5%), gastric (14.5%), small intestine (7.4%), ovarian (7.1%), cholangiocarcinoma (6.3%), and pancreatic (6.3%). 41.0% had 1 prior line of therapy; 55.6% had ≥2 prior lines. Median time from first dose to database cutoff (Oct 5, 2020) was 37.5 (range, 0.2–55.6) mo; 16.0% were continuing treatment. The ORR among the 321 eligble pts was 30.8% (CR, 27; PR, 72); median DOR was 47.5 mo (Table). Treatment-related AEs occurred in 64.7% of pts (grade 3–5, 12.0%), led to discontinuation in 6.6%, and led to death in 3 pts (myocarditis, pneumonia, and Guillain-Barre syndrome). Immune-mediated AEs and infusion reactions occurred in 20.2% of pts (grade 3–4, 4.3%) and led to death in 2 pts with no other contributing factors (myocarditis [AE start, day 26; death, day 33] and Guillain-Barre syndrome [AE start, day 22; death, day 41]). Conclusions: Pembrolizumab demonstrated a high ORR (30.8%), durable clinical benefit, and a manageable safety profile in this heavily pretreated advanced MSI-H/dMMR noncolorectal pan-tumor population. Clinical trial information: NCT02628067. [Table: see text]