Abstract PS14-05: Safety evaluation from the KEYNOTE-522 study of neoadjuvant pembrolizumab (or placebo) plus chemotherapy followed by adjuvant pembrolizumab (or placebo) in patients with early triple-negative breast cancer (TNBC)

Abstract Background: In KEYNOTE-522 (NCT03036488), neoadjuvant (neoadj) pembrolizumab (pembro) + chemotherapy (chemo) followed by adjuvant (adj) pembro led to statistically significant and clinically meaningful improvements in the primary endpoints, pCR and EFS, vs neoadj placebo (pbo) + chemo followed by adj pbo in patients (pts) with newly diagnosed, high-risk, early TNBC. In the safety population at preplanned interim analysis 4 (IA4), treatment-related AEs in the combined phases (neoadj + adj) occurred in 98.9% of pts with pembro + chemo/pembro and 99.7% of pts with pbo + chemo/pbo; immune-mediated AEs of any grade occurred in 33.5% and 11.3% of pts, respectively. We report additional safety findings, beyond the already reported safety results, on immune-mediated AEs and management in the combined phases from IA4 of KEYNOTE-522. Methods: Eligible pts were randomized 2:1 to receive neoadj pembro 200 mg or pbo Q3W + paclitaxel-carboplatin for 4 cycles and then doxorubicin or epirubicin + cyclophosphamide for 4 cycles. After definitive surgery, pts received radiation therapy as indicated + adj pembro 200 mg or pbo Q3W for up to 9 cycles. Safety was assessed in all randomized pts who received ≥1 dose, underwent surgery, or both. AEs were monitored throughout the study and for 30 d post-treatment (90 d for serious AEs). Results: At IA4 (data cut-off: March 23, 2021), median treatment duration was 13.3 (range, 0-21.9) mo with pembro + chemo/pembro (n = 783) and 13.6 (range, 0-19.8) mo with pbo + chemo/pbo (n = 389). Of 341 pts with immune-mediated AEs and infusion reactions in the pembro + chemo/pembro arm (most events occurred in neoadj phase), 224 had grade 1-2 events and 117 had grade 3–5 events. The most common immune-mediated AEs with pembro + chemo/pembro were hypothyroidism (15.1%) and severe skin reactions (5.7%); infusion reactions occurred in 18.0% of pts (table). Of 118 pts with hypothyroidism, median time to onset was 105 d (range, 7–510 d) and 106 were treated with thyroid replacement, suggesting an endocrine abnormality and need for continued thyroid replacement. Of 45 pts with severe skin reactions, median time to onset was 64 d (range, 4–479 d) and 28 were treated with corticosteroids. Of 141 pts with infusion reactions, median time to onset was 16 d (range, 1–458 d) and 85 were treated with corticosteroids. Other immune-mediated AEs of interest with pembro + chemo/pembro were adrenal insufficiency (2.6%), pneumonitis (2.2%), and hypophysitis (1.9%); most of these events were grade 2–3. All 20 pts with adrenal insufficiency were treated with hormone replacement. Of 17 pts with pneumonitis, median time to onset was 167 d (range, 22-537 d) and 12 were treated with corticosteroids; median episode duration was 92 d. Of 15 pts with hypophysitis, 14 were treated with corticosteroids. Conclusion: In pts with newly diagnosed, high-risk, early TNBC, neoadj pembro + chemo followed by adj pembro had a manageable safety profile that was generally consistent with the known safety profiles of pembro and the chemo regimens. Most immune-mediated AEs and infusion reactions were grade 1-2, manageable with treatment interruption, corticosteroids, and/or hormone replacement therapy, and did not result in treatment discontinuation. Together with the efficacy findings, our results support neoadj pembro + chemo followed by adj pembro as a standard of care regimen for these pts. Table Citation Format: Javier Cortés, Rebecca Dent, Lajos Pusztai, Heather McArthur, Sherko Kuemmel, Carsten Denkert, Yeon Hee Park, Rina Hui, Masato Takahashi, Carlos Barrios, Yalin Zhu, Xiaoli Zhang, Wilbur Pan, Vassiliki Karantza, Joyce O'Shaughnessy, Peter Schmid. Safety evaluation from the KEYNOTE-522 study of neoadjuvant pembrolizumab (or placebo) plus chemotherapy followed by adjuvant pembrolizumab (or placebo) in patients with early triple-negative breast cancer (TNBC) [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS14-05.