Final overall survival for the phase III KN177 study: Pembrolizumab versus chemotherapy in microsatellite instability-high/mismatch repair deficient (MSI-H/dMMR) metastatic colorectal cancer (mCRC)

3500 Background: In the phase III, randomized open-label KEYNOTE-177 (NCT02563002) study 1L pembrolizumab (pembro) versus chemotherapy (chemo) provided superior progression-free survival (PFS) at second interim analysis (IA2) in patients (pts) with MSI-H/dMMR mCRC. The study continued to final analysis of overall survival (OS), planned after 190 OS events or 12 months after IA2, whichever occurred first. We present results of the final analysis of OS, 12 months after IA2. Methods: A total of 307 pts with MSI-H/dMMR mCRC and ECOG PS 0 or 1 were randomized 1:1 to 1L pembro 200 mg Q3W for up to 2y or investigator’s choice of mFOLFOX6 or FOLFIRI Q2W ± bevacizumab or cetuximab. Treatment continued until PD, unacceptable toxicity, pt/investigator decision to withdraw, or completion of 35 cycles (pembro only). Pts receiving chemo could crossover to pembro for up to 35 cycles after confirmed PD. Primary end points were OS and PFS (RECIST v1.1, central review). Secondary end points included ORR, duration of response (DOR) (RECIST v1.1, central review), and safety. For OS significance, the p-value had to meet a prespecified α of 0.0246 (one-sided). Sensitivity analyses to adjust for crossover effect were performed. Data cut-off for final analysis was Feb 19, 2021. Results: Median (range) study follow-up was 44.5 mo (36.0-60.3) with pembro vs 44.4 mo (36.2-58.6) with chemo. 56 (36%) pts crossed over from chemo to pembro, with 37 more receiving anti-PD-1/PD-L1 therapies off study (60% effective crossover rate in the ITT). The HR for OS favored pembro vs chemo with a trend toward reduction in the risk of death (HR 0.74; 95% CI, 0.53-1.03; P=0.0359; median not reached [NR] vs 36.7 mo); this difference did not reach statistical significance. Sensitivity analysis by the rank-preserving structure failure time model and inverse probability of censoring weighting showed OS HRs of 0.66 (95% CI 0.42-1.04) and 0.77 (95% CI 0.44-1.38), respectively. Pembro vs chemo met the prespecified criteria for PFS superiority at IA2. At final analysis, median PFS was 16.5 mo vs 8.2 mo (HR 0.59; 95% CI, 0.45-0.79), but was not formally tested per analysis plan. Confirmed ORR was 45.1% (20 CR, 49 PR) vs 33.1% (6 CR, 45 PR). Median (range) DOR was NR (2.3+ to 53.5+) vs 10.6 mo (2.8 to 48.3+), respectively. Treatment-related adverse events (TRAEs) occurred in 79.7% vs 98.6% of pts; 21.6% vs 66.4%, respectively, had grade ≥3 TRAEs. Conclusions: As 1L therapy for pts with MSI-H/dMMR mCRC, pembro vs chemo provides statistically superior PFS with fewer TRAEs, and is associated with a trend toward reduced mortality that did not meet statistical significance likely due to the high crossover rate from chemo to anti-PD1/PD-L1 therapies. Together these data confirm pembro as a new standard-of-care in the 1L for pts with MSI-H/dMMR mCRC. Clinical trial information: NCT02563002.