THU543 Acquired Resistance To CDK4/6 Inhibitors Is Associated With Upregulated IGF1R And IR Signaling
Authors: Gerhardt Summer Louise , Baar Courtney , Johnson Ivy , Sachdev Deepali 
Published: October 2023

Abstract
Disclosure: S.L. Gerhardt: None. C. Baar: None. I. Johnson: None. D. Sachdev: None.
The most common subtype of breast cancer is hormone receptor-positive (HR+). While these patients are successfully treated with endocrine therapies (ET), over 30% of patients develop resistance and require other therapies. Cyclin-dependent kinases (CDKs) 4 and 6 are required for cell cycle progression. Three CDK4/6 inhibitors (CDK4/6i), palbociclib, abemaciclib, and ribociclib, have recently been approved in combination with ET. CDK4/6i block phosphorylation of retinoblastoma (Rb) and cause G1/G0 arrest and are standard of care for treating metastatic HR+ Her2- advanced breast cancer with ET. Patients who respond to the CDK4/6i eventually develop resistance to the drugs. Thus, understanding the mechanisms of resistance to CDK4/6i and the role of growth-factor signaling will allow the identification of other pathways that can be targeted. To understand the mechanisms of resistance, we generated HR+ breast cancer cells (MCF-7) with acquired resistance to palbociclib (MCF-7/PalboR) or abemaciclib (MCF-7/AbemaR) by culturing them in increasing concentrations of the drugs. As shown by others, we found that acquired resistance to CDK4/6i results in loss of Rb and thus Rb phosphorylation. We analyzed signaling pathways and in vitro proliferation in the MCF-7/PalboR and MCF-7/AbemaR compared to the CDK4/6i sensitive matched parent cells (MCF-7L MP). MCF-7/PalboR and MCF-7/AbemaR had increased levels of CDK2 and cyclin E2. Both MCF-7/PalboR and MCF-7/AbemaR cells had upregulated type I IGF receptor (IGF1R) and insulin receptor (IR) protein levels compared MCF-7L MP as assayed by western blotting and flow cytometry. MCF-7/PalboR and MCF-7/AbemaR showed higher transcript levels of the fetal form of IR (IR-A) which is not expressed in normal adult tissues. IR-A plays a role in cancer progression and has been implicated in resistance to ET. MCF-7/PalboR and MCF-7/AbemaR had increased sensitivity to signaling by insulin-like growth factor I (IGF-I) and insulin compared to MCF-7 MP, with much lower concentrations of IGF-I and insulin phosphorylating the receptors and activating downstream MAPK and PI3K/Akt/mTOR pathways. Further, IGF-I and insulin mediated growth was enhanced to a statistically greater extent in MCF-7/PalboR and MCF-7/AbemaR compared to MCF-7 MP, and this enhanced growth was not blocked by 1200 nM of palbociclib or abemaciclib. MCF-7/PalboR cells were cross resistant to abemaciclib and MCF-7/AbemaR to palbociclib. Combined treatment with CDK4/6i and rapamycin, an mTOR pathway inhibitor, was not sufficient to overcome growth-factor mediated proliferation in MCF-7/PalboR and MCF-7/AbemaR but was in MCF-7 MP. These data indicate that acquired resistance to CDK4/6i is mediated through upregulated IGF1R and IR signaling. Our data suggest that combining CDK4/6i with drugs targeting IGF1R, IR-A, and mTOR signaling should be tested to delay resistance to CDK4/6i.
Presentation: Thursday, June 15, 2023