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ASCO Webinar Summary: Breast Cancer

KEYNOTE-355: A Randomized, double-blind, phase III study of pembrolizumab plus chemotherapy vs. placebo plus chemotherapy for previously untreated locally recurrent inoperable or metastatic triple-negative breast cancer


Author, Company: Javier Cortes, MD, PhD

Abstract Number: 1000

Indications: Triple Negative Breast Cancer

Interventions: Pembrolizumab plus chemotherapy

Background and objectives:

  • KEYNOTE-355, a randomized, double blind, Phase III trial, studied the comparison of pembro + chemo vs. placebo + chemo for previously untreated locally recurrent inoperable or metastatic TNBC.
  • Dual primary outcome measures were PFS and OS by tumor PD-L1 expression (CPS ≥10 and ≥1) as well as ITT. Secondary end points were ORR, disease control rate, duration of response, and safety.

Results and observations:

Regimen n Size mFollow up mPFS Grade 3-5 AEs
CPS ≥10 CPS ≥1 ITT
Pembro +Chemo vs Pbo+Chemo 566



26.1 9.7 vs 5.6

(HR: 0.65)

7.6 vs 5.6

(HR: 0.74)

7.5 vs 5.6

(HR: 0.82)

68.1% vs 66.9%


  • Pembro + chemo was not able to show any meaningful results in mPFS in

patients with CPS ≥ 1, and therefore, formal testing in ITT was not performed

  • IMPassion130 which was a randomized, placebo, phase III trial that assessed efficacy and safety of atezolizumab + nab-paclitaxel in patients with mTNBC.
IMPassion130 KN – 355
Reduction in risk progression 40% 35%
Median PFS 7.5 mo. vs. 5.0 mo. (atezo + chemo) vs. pbo+chemo);

(HR: 0.62)

9.7 mo vs 5.6 mo.  (pembro + chemo vs. pbo+chemo)



  • According to KN-355, pembro + chemo showed a significant relevant improvement in PFS vs. placebo + chemo in patients with metastatic Triple Negative Breast Cancer whose tumors expressed PD-L1 (CPS >=10). The combination was well tolerated and had no new safety concerns.

Key take-away:

  • The study highlighted that addition of Immunotherapy could significantly lead to a paradigm shift in the overall treatment of metastatic triple-negative breast cancer.
  • These results also indicate routine testing for PD-L1+ status in newly diagnosed patients with metastatic triple-negative breast cancer to determine if they could benefit from these combinations

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