Abstract P5-16-12: Neoadjuvant letrozole plus palbociclib in patients (pts) with hormone receptor (HR)-positive/HER2-negative early breast cancer (EBC) with baseline Ki67 ≥20% and an Oncotype DX Breast Recurrence Score® result (RS) ≥18: DxCARTES

Abstract Background: Cyclin-dependent kinases 4 and 6 inhibitors in combination with endocrine therapy frequently lead to a complete cell-cycle arrest (CCCA) in luminal EBC. However, the rates of pathological complete response (pCR) or Residual Cancer Burden (RCB) 0-I are modest. The effect of this treatment in terms of molecular downstaging as assessed by a genomic signature more refined than Ki67 remains undetermined. We aimed to assess the biological and clinical activity of letrozole plus palbociclib as neoadjuvant treatment in HR-positive/HER2-negative EBC pts with an Oncotype DX RS ≥18. Methods: DxCARTES was a multicenter, open-label, non-comparative, phase 2 trial across 16 hospitals in Spain. Participants were pre- and post-menopausal women aged ≥18 years with centrally confirmed HR-positive/HER2-negative, Ki67≥20%, stage II-IIIB EBC with a RS ≥18. Eligible pts with baseline RS 18-25 (cohort A) and RS 26-100 (cohort B) received six 28-days cycles of letrozole (2.5 mg QD), ± goserelin if premenopausal, plus palbociclib (125 mg QD, 3/1 schedule) before surgery. Tumor samples were prospectively collected at baseline and surgery for Ki67 and RS assessments. The coprimary endpoint was the proportion of pts in either cohort who achieved RS ≤25 or a pCR (ypT0/is ypN0) at surgery. Secondary endpoints included analysis of RCB, preoperative endocrine prognostic index (PEPI), CCCA (Ki67<2.7%), overall response rate (ORR) as per RECIST v.1.1, and safety as per CTCAE v.5.0. A Simon’s 2-stage design was planned to recruit 33 pts in each cohort. Interim futility analyses were planned with 12 and 18 pts included in cohort A and B, respectively. The analyses were designed to attain an 80% power, with a 10% drop out rate assumption, at a nominal 1-sided α level of 2.5%. The response probabilities for null (H0) and alternative hypotheses (H1) in cohort A were H0: ≤ 72% and H1: ≥ 93%, and in cohort B were H0: ≤ 13% and H1: ≥ 35%, respectively. Results: Between May 5, 2019 to Dec 30, 2019, 67 pts were enrolled and received the study treatment (33 pts allocated to cohort A and 34 pts to cohort B). Pts’ characteristics at diagnosis were well balanced between the two cohorts and of the 67 pts included, 32.8% were premenopausal, 49.3% had axillary node involvement, and median Ki67 was 27% (24-37.5%). A total of 32 pts from cohort A and 33 pts from cohort B were evaluable for the coprimary endpoint. Median duration of treatment was 5.6 months (IQR 5.5-5.8) and median time from the last dose of palbociclib and surgery was 10.5 days (IQR 5.5-14). At surgery, 22 of 32 (68.8%) pts in cohort A and 19 of 33 (57.6%) pts in cohort B had RS ≤25. No pts in cohort A and 2 of 33 (6.1%) pts in cohort B achieved a pCR, meeting the coprimary endpoint in cohort B, but not in cohort A. In cohort A, the proportion of pts with RCB 0-1 was 5.3%, PEPI 0 was 11.1%, and CCCA was 58.1%. In cohort B, the proportion of pts with RCB 0-1 was 23.1%, PEPI 0 was 33.3%, and CCCA was 59.4%. The ORR by breast MRI was 78.1% (25 of 32 pts) in cohort A and 63.6% (21 of 33 pts) in cohort B. The most common grade 3-4 adverse event in both cohorts was neutropenia (23 of 33 [69.7%] pts in cohort A vs 20 of 34 [58.8%] pts in cohort B). No deaths were observed during the study in either cohort. Conclusions: Our results suggest that the neoadjuvant palbociclib plus letrozole activity is independent of the molecular aggressiveness of the disease. Although the prognostic value of molecular downstaging is unknown, a significant proportion of HR-positive/HER2-negative EBC pts with high RS at baseline achieve a pathological or molecular downstaging with this regimen. Further investigation is needed to determine if this strategy could avoid the use of chemotherapy in this population. Citation Format: Antonio Llombart-Cussac, Ángel Guerrero-Zotano, Manuel Ruiz, Begoña Bermejo, Miguel Gil-Gil, Juan de la Haba, Emilio Alba, Vanesa Quiroga, Vicente Carañana, Ander Urruticoechea, Serafín Morales, Meritxell Bellet, Antonio Antón, José Manuel Pérez-García, María Fernández-Abad, Sonia Servitja, Pedro Sánchez-Rovira, Sofia Braga, Miguel Sampayo-Cordero, Andrea Malfettone, Javier Cortes. Neoadjuvant letrozole plus palbociclib in patients (pts) with hormone receptor (HR)-positive/HER2-negative early breast cancer (EBC) with baseline Ki67 ≥20% and an Oncotype DX Breast Recurrence Score® result (RS) ≥18: DxCARTES [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-16-12.