Abstract PD1-03: A phase Ib trial of fulvestrant + CDK4/6 inhibitor (CDK4/6i) palbociclib + pan-FGFR tyrosine kinase inhibitor (TKI) erdafitinib inFGFR-amplified/ ER+/ HER2-negative metastatic breast cancer (MBC)

Background: Somatic alterations in the FGFR pathway (mainly FGFR1, amplified in about 15% of ER+ BC) have been implicated in resistance to endocrine therapy (ET), and more recently, to CDK4/6i as well. Based on our preclinical data showing that the FGFR pan-inhibitor erdafitinib when added to fulvestrant/palbociclib resulted in marked PDX regressions, we initiated a phase Ib trial combining erdafitinib with fulvestrant/palbociclib in patients with ER+/HER2-/FGFR-amplified MBC (NCT03238196) to determine safety, tolerability and anti-tumor activity of this combination. Methods: Patients with evaluable ER+/HER2- MBC with FGFR1-4 amplification (detected on tumor next generation sequencing or plasma ctDNA) exposed to at least one ET regimen (but no more than 2 lines of chemotherapy) in the metastatic setting, were treated with fulvestrant, palbociclib (standard of care dosing/ schedule) and oral erdafitinib, tested in 4 doses ranging from 4 mg to 8 mg daily. Once MTD reached, we planned to enroll 20 patients in the expansion portion of the trial. Tumor blocks were collected for FGFR1 FISH amplification analysis, and plasma ctDNA was collected at baseline, 4 weeks and at treatment discontinuation. Tumor assessments were performed every 8 weeks. Results: Since August 2017, 26 eligible patients with evaluable ER+/HER2-/FGFR-amplified MBC were enrolled across 4 institutions. Patient characteristics are summarized in Table 1. Main grade 1 and 2 adverse events (AE) were consistent with on-target toxicities of erdafitinib and/or palbociclib: mucositis (67%), hyperphosphatemia (61%), dysgeusia (52%), diarrhea (48%), fatigue (48%), neutropenia (47%), hand-foot syndrome (38%), anemia (29%), and onycholysis (14%). Febrile neutropenia occurred in 5% patients, no cases of central serous retinopathy were seen. Serious AE were rare: one grade 4 elevation of transaminases (DLT; attributed to fulvestrant), one grade 3 colitis (attributed to erdafitinib), and one thromboembolic event (attributed to palbociclib). In combination with fulvestrant/ palbociclib, the MTD of erdafitinib was 6 mg. No drug-drug interaction was seen. 8 patients were deemed non-evaluable for anti-tumor effect as treatment discontinuation (mainly due to AE) occurred prior to first tumor assessment. Of the 18 evaluable patients: 7 had disease progression, 8 had stable disease (4 of which discontinued treatment due to AE), 3 have not completed their first tumor assessment, 4 are still on treatment; median PFS was 3 months and CBR at 6 months was 28%. However, higher PFS (6 months) was seen in 6/8 patients with high levels of FGFR1 amplification (FISH FGFR1:CEP8 ratio >5; gene copy number >10) and in both patients with FGFR3 amplification. Conclusion: To our knowledge, this is the first time an FGFR inhibitor has been tested in combination with ET and CDK4/6i in patients with MBC harboring FGFR alterations. Erdafitinib-related side effects appeared to be on target, leading to treatment discontinuation in several patients despite optimal medical treatment. Clinical activity was seen in heavily pre-treated patients with molecular evidence of high FGFR amplification despite 100% prior exposure to ET and CDK4/6i. Full clinical and correlative work will be presented at the meeting, and a future phase II trial is being planned. Citation Format: Ingrid A. Mayer, Barbara B. Haley, Vandana G. Abramson, Adam Brufsky, Brent Rexer, Erica Stringer-Reasor, Komal L. Jhaveri, Melinda Sanders, Paula I. Ericsson-Gonzalez, Fei Ye, Carlos L. Arteaga. A phase Ib trial of fulvestrant + CDK4/6 inhibitor (CDK4/6i) palbociclib + pan-FGFR tyrosine kinase inhibitor (TKI) erdafitinib in FGFR-amplified/ ER+/ HER2-negative metastatic breast cancer (MBC) [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 PD1-03.