Abstract P6-10-08: AKT1 alterations following exposure to endocrine-based therapy in patients with hormone-receptor positive (HR+) metastatic breast cancer (MBC): Clinical and functional implications

Background: The AKT1 oncogene represents a central node in the well-characterized PI3K/AKT/mTOR signal transduction pathway involved in the regulation of cellular proliferation as well as therapeutic resistance. With the emergence of multiple targeted therapies for HR+ MBC, we sought to explore the prevalence of AKT1 alterations, the spectrum of co-occurring mutations, and any potential impact on response to endocrine-based therapy, including CDK 4/6 and PIK3CA inhibitors. Methods: Presence or absence of AKT1 mutation was interrogated via cell-free DNA (cfDNA) analysis (Guardant assay) among patients with HR+/HER2- MBC receiving treatment at the Massachusetts General Hospital who underwent routine clinical cfDNA testing. Genomic sequencing results were then compared to detailed clinical annotation based upon retrospective chart review. In the laboratory, HR+/HER2- T47D breast cancer cells were transfected with AKT1, under the control of a doxycycline-inducible promoter, to induce AKT1 overexpression. Sensitivity to escalating doses of fulvestrant and palbociclib were interrogated via cell-titer-glo viability assay in vitro. Results: Among 272 patients with HR+/HER2- MBC, we identified 13 patients with AKT1 mutation (4.7%). 12/13 (92%) patients had a canonical AKT1E17K alteration, one patient (8%) had an AKT1 E49K alteration. AKT1 mutations occurred concurrently with alterations in ESR1 (31%), PIK3CA (23%), TP53 (23%), ERBB2 (15%), and RB1 (8%). All patients had been exposed to some form of anti-estrogen prior to sequencing, either via (neo)adjuvant therapy and/or in the metastatic setting; 5/13 (38%) had exposure to a CDK4/6 inhibitor prior to cfDNA sequencing. Following the identification of an AKT1 alteration, examples of rapid clinical progression on fulvestrant, CDK4/6i, and/or everolimus were identified, however the presence of an AKT1 alteration was not sufficient to predict pan-resistance to these agents, as counter-examples of clinical benefit were also identified. In an index patient with an AKT1 mutation and prior progression on letrozole and palbociclib, subsequent treatment with fulvestrant and abemaciclib did not yield clinical benefit, highlighting the potential role of AKT1 in mediating clinical resistance to endocrine therapy and CDK4/6i. In the pre-clinical model, HR+/HER2- breast cancer cells demonstrated resistance to both fulvestrant and palbociclib following upregulation of exogenous AKT1 expression. Updated clinical data with additional patients and response to regimens incorporating an anti-estrogen, a CDK4/6 inhibitor, an mTOR inhibitor, and/or a PIK3CA inhibitor will be presented at the meeting. Conclusions: Activating AKT1 mutations can be identified via targeted sequencing of cfDNA in patients with HR+/HER2- MBC, and may play an important role in mediating resistance to anti-estrogens, CDK4/6 inhibitors, and other emerging targeted therapies. Heterogeneity in response to endocrine-based therapy following the identification of an AKT1 alteration may be related to tumor-extrinsic factors, the AKT1 allelic fraction, or cooperativity between multiple resistance drivers. Further research is needed to confirm these findings and guide optimal therapeutic sequencing strategies for patients with HR+/HER2- AKT1 mutant MBC. Citation Format: Seth Wander, Pingping Mao, Maxwell Lloyd, Kailey Kowalski, Gabriela N. Johnson, Giuliana Malvarosa, Beverly Moy, Leif W. Ellisen, John Iafrate, Nikhil Wagle, Aditya Bardia. AKT1 alterations following exposure to endocrine-based therapy in patients with hormone-receptor positive (HR+) metastatic breast cancer (MBC): Clinical and functional implications [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-10-08.