Abstract RF02-01: A Multicenter, Phase I/II Trial of Anastrozole, Palbociclib, Trastuzumab, and Pertuzumab in Hormone Receptor (HR)-Positive, HER2-Positive Metastatic Breast Cancer (ASPIRE)

Abstract Background: Among patients (pts) with HR-positive and HER2-positive breast cancer, crosstalk between HER2 and estrogen receptor (ER) signaling pathways may contribute to endocrine resistance but anti-HER2 agents in combination with endocrine therapy can restore endocrine sensitivity. Mechanistically, HER2/HER3 signaling promotes survival by way of PI3K-AKT activation whereas ER-CDK4/6-Rb signaling promotes cell cycle progression. The combination of anti-HER2 therapy with an aromatase inhibitor (AI) and a CDK 4/6 inhibitor would allow for blockade of both pathways and provide a novel, all biologic, and chemotherapy-free approach to the treatment of HR-positive, HER2-positive metastatic breast cancer (MBC). Methods: We conducted a phase I/II multi-institution trial in pts with previously untreated, HR-positive and HER2-positive MBC. In the Phase I portion, pts received escalating doses of palbociclib (100mg, 125mg) in conjunction with trastuzumab, pertuzumab and an AI anastrozole, using a 3+3 dose escalation trial design. In the phase II portion, pts received palbociclib at the maximum tolerated dose (MTD), anastrozole, trastuzumab, and pertuzumab. The primary endpoints of the Phase I and II portions were MTD and clinical benefit rate (CBR) defined as the sum of complete response, partial response, and stable disease for /= 6 months, respectively. Secondary endpoints included progression free survival (PFS), objective response rate (ORR) and safety. The Phase II portion of this study was powered with 30 pts to show efficacy of palbociclib administered at the MTD in combination with anastrozole, trastuzumab and pertuzumab if the CBR achieved at 6 months exceeded 58%. The Clopper-Pearson method was used to calculate confidence intervals for ORR and CBR. The PFS distribution was estimated using the Kaplan-Meier method. Results: In the Phase I portion, a total of 9 pts were enrolled. No DLTs were observed at the 100mg dose (N=3) or the 125mg dose (N=6) level, and thus, 125mg was established as the MTD. An additional 24 pts were enrolled to the Phase II portion at the MTD, with a total of 30 pts in the modified intention-to-treat population included in the efficacy analysis. The median age of the population was 57.6 years (range 50.5-63.8) and 27% of pts were premenopausal and received ovarian function suppression as part of treatment. As shown in Table 1, the primary endpoint, CBR, was 97% (95% CI: 0.83-1.0, p0.0001). ORR was 70% (95% CI: 0.51-0.85). Median time to objective response was 2.8 months with earliest response at 3 months. Median duration of response was not reached with range of 5.1-42.2 months. Median PFS was also not reached with range of 8-44.8 months. Safety data were consistent with known toxicity profiles of agents. Most common adverse events included diarrhea (80%), neutropenia (77%), leukopenia (70%), anemia (67%), and fatigue (60%). Grade 3-4 events occurred in 63% (19/30) of pts and included neutropenia (68%), leukopenia (32%), decrease in absolute neutrophil count (21%), and anemia (21%). Conclusions: The combination of anastrozole, palbociclib, trastuzumab, and pertuzumab was well tolerated and effective with a clinical benefit rate of 97% in pts with previously untreated HR-positive, HER2-positive MBC. The combination provides a chemotherapy-free alternative for pts with triple positive breast cancers. Further follow up will determine impact on PFS. Table 1. Results in Patients on Anastrozole, Palbociclib, Trastuzumab, and Pertuzumab Citation Format: Rima Patel, Krystal Cascetta, Paula Klein, Erin Moshier, Maryann Kwa, Julie Fasano, Anupama Goel, Melissa Accordino, Charles Shapiro, Rita Vaccaro, Gargi Atul Joshi, Joseph Sparano, Amy Tiersten. A Multicenter, Phase I/II Trial of Anastrozole, Palbociclib, Trastuzumab, and Pertuzumab in Hormone Receptor (HR)-Positive, HER2-Positive Metastatic Breast Cancer (ASPIRE) 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 RF02-01.