Abstract GS3-01: Additional efficacy endpoints from the phase 3 KEYNOTE-355 study of pembrolizumab plus chemotherapy vs placebo plus chemotherapy as first-line therapy for locally recurrent inoperable or metastatic triple-negative breast cancer

Abstract Background: Pembrolizumab (pembro) monotherapy showed durable antitumor activity and manageable safety in patients (pts) with metastatic triple-negative breast cancer (TNBC) in the KEYNOTE-012, -086, and -119 studies. In a prespecified interim analysis of KEYNOTE-355 (NCT02819518), pembro combined with chemotherapy (chemo) showed a statistically significant improvement in PFS versus chemo alone in pts with previously untreated locally recurrent inoperable or metastatic TNBC whose tumors expressed PD-L1 CPS ≥10 (HR for progression or death, 0.65, 95% CI, 0.49-0.86; one-sided P=0.0012 [prespecified statistical criterion was alpha=0.00411 at this interim analysis]). Additionally, pembro + chemo was generally well tolerated, with no new safety signals. Here, we examine PFS outcomes for each chemo partner and present key secondary endpoints from KEYNOTE-355. Methods: 847 pts with measurable disease per RECIST v1.1, ECOG PS 0-1, and ≥6 mo DFI were randomized 2:1 to pembro + chemo (nab-paclitaxel 100 mg/m2 days 1, 8, and 15 every 28 days; paclitaxel 90 mg/m2 days 1, 8, and 15 every 28 days; or gemcitabine 1000 mg/m2 + carboplatin AUC 2 days 1 and 8 every 21 days) or pbo + chemo for up to 35 administrations of pembro/pbo or until progression/intolerable toxicity. Steroid premedication for paclitaxel was given according to local guidelines and practices, and was not restricted by the protocol. Crossover between arms was not allowed. Pts were stratified by chemo type (taxane vs gemcitabine/carboplatin), PD-L1 status (CPS ≥1 vs <1), and prior (neo)adjuvant treatment with same-class chemo (yes vs no). Dual primary endpoints are PFS (RECIST v1.1, blinded independent central review) and OS in pts with PD-L1 positive tumors (CPS ≥10 and ≥1) and in all pts. Secondary endpoints include ORR, DCR (CR + PR + SD ≥24 weeks), and DOR. The PFS treatment effect was assessed in subgroups descriptively using HRs and 95% CIs; although subgroup analysis by on-study chemo were pre-specified, the trial was not powered to compare efficacy among treatment groups by different chemo regimens. Results: As of Dec 11 2019, median follow-up was 25.9 mo for pembro + chemo (n=566) and 26.3 mo for pbo + chemo (n=281). The HR for PFS favored pembro regardless of choice of chemo or CPS population (Table). Results for the key secondary endpoints of ORR, DCR, and DOR favored pembro + chemo, with the treatment effect increasing as CPS increased (Table). Conclusion: In subgroup analysis, PFS with pembro + chemo compared to pbo + chemo in pts with metastatic TNBC was improved regardless of chemo partner. A trend toward improved efficacy with PD-L1 enrichment with pembro + chemo was observed for ORR, DCR and DOR endpoints. These data further support the potential of pembro + chemo as a first-line treatment option for metastatic TNBC. CPS ≥ 10CPS ≥ 1All ptsPembro + ChemoPbo + ChemoPembro + ChemoPbo + ChemoPembro + Chemo|Pbo + ChemoN = 220N = 103N = 425N = 211N = 566N = 281PFS, mo median (95% CI)9.7 (7.6 - 11.3)5.6 (5.3 - 7.5)7.6 (6.6 - 8.0)5.6 (5.4 - 7.4)7.5 (6.3 - 7.7)5.6 (5.4 - 7.3)HR (95% CI)0.65 (0.49 - 0.86)0.74 (0.61 - 0.90)0.82 (0.69 - 0.97)PFS by on-study chemo, (n [%)) mo median(95% CI)Nab-Paclitaxel(63 [28.6%]) 9.9(36 [35.0%]) 5.5(130 [30.6%]) 6.3(74 [35.1%)) 5.3(173 [30.6%]) 7.5(95 [33.8%]) 5.4HR (95% CI)0.57 (0.34 - 0.95)0.66 (0.26 - 0.82)0.69 (0.51 - 0.93)Paclitaxel(33 [15.0%]) 9.6(11 [10.7%]) 3.6(62 [14.6%]) 9.4(22 [10.4%]) 3.8(82 [14.5%]) 8.0(32 [11.4%]) 3.8HR (95% CI)0.33 (0.14 - 0.76)0.46 (0.26 - 0.82)0.57 (0.35 - 0.93)Gemcitabine-Carboplatin(124 [56.4%]) 8.0(56 [54.4%]) 7.2(233 [54.8%]) 7.5(115 [54.5%]) 7.5(311 [54.9%]) 7.4(154 [54.8%]) 7.4HR (95% CI)0.77 (0.53 - 1.11)0.86 (0.66 - 1.11)0.93 (0.74 - 1.16)ORR, % (95% CI)53.2 (46.4 - 59.9)39.8 (30.3 - 49.9)45.2 (40.4 - 50.0)37.9 (31.3 - 44.8)41.0 (36.9 - 45.2)35.9 (30.3 - 41.9)DCR, % (95% CI)65.0 (58.3 - 71.3)54.4 (44.3 - 64.2)58.6 (53.7 - 63.3)53.6 (46.6 - 60.4)56.0 (51.8 - 60.1)51.6 (45.6 - 57.6)DOR, mo, median (range)19.3 (1.6+ - 29.8)7.3 (1.5 - 32.5+)10.1 (1.0+ - 29.8)6.5 (1.5 - 32.5+)10.1 (1.0+ - 29.8)6.4 ( 1.5 - 32.5+)"+" indicates there is no progressive disease by the time of last disease assessment. Citation Format: Hope S. Rugo, Peter Schmid, David W. Cescon, Zbigniew Nowecki, Seock-Ah Im, Mastura Md Yusof, Carlos Gallardo, Oleg Lipatov, Carlos Henrique Barrios, Jose Perez-Garcia, Hiroji Iwata, Norikazu Masuda, Marco Torregroza Otero, Erhan Gokmen, Sherene Loi, Zifang Guo, Jing Zhao, Vassiliki Karantza, Gursel Aktan, Javier Cortes. Additional efficacy endpoints from the phase 3 KEYNOTE-355 study of pembrolizumab plus chemotherapy vs placebo plus chemotherapy as first-line therapy for locally recurrent inoperable or metastatic triple-negative breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr GS3-01.