# FGFR1 overexpression promotes resistance to PI3K inhibitor alpelisib in luminal breast cancer cells through receptor tyrosine kinase signaling-mediated activation of the estrogen receptor

## TL;DR

Overexpression of FGFR1 helps breast cancer cells resist the drug alpelisib, but combining alpelisib with FGFR1 inhibitors can overcome this resistance.

## Contribution

This study identifies FGFR1 as a novel driver of resistance to alpelisib in luminal breast cancer and proposes a triple therapy approach to counteract it.

## Key findings

- FGFR1 overexpression leads to alpelisib resistance in MCF-7 and T47D cells through Akt and Erk pathway activation.
- Combining alpelisib with AZD4547 and fulvestrant synergistically inhibits tumor growth and RTK-ER crosstalk.
- Knockdown of FGFR1 reverses resistance, confirming its role in drug resistance mechanisms.

## Abstract

Aim: Resistance to PI3K inhibitor alpelisib is an emerging challenge in breast cancer treatment. FGFR1 is frequently amplified in breast cancer. We investigated FGFR1 overexpression-mediated alpelisib resistance and its mechanism.

Methods: CCK-8, colony formation, and cell cycle assays assessed FGFR1 overexpression-induced alpelisib resistance in MCF-7 and T47D cells. FGFR1 siRNA knockdown validated FGFR1’s role. Akt, Erk, and ER signaling were analyzed by Western blot. Synergistic effects of alpelisib with AZD4547 and fulvestrant were evaluated using the combination index.

Results: FGFR1 overexpression conferred alpelisib resistance in MCF-7 and T47D cells, evidenced by increased viability, colony formation, and S-phase accumulation post alpelisib treatment. Knockdown of FGFR1 reverse alpelisib resistance in FGFR1 overexpressing MCF-7 and T47D cells. Resistance correlated with sustained activation of Akt and Erk1/2 pathways (p-Akt, p-Erk1/2, p-S6K, p-Rb) and attenuated suppression of ERα phosphorylation (S118/S167), highlighting RTK-ER crosstalk. Combining alpelisib with AZD4547 synergistically inhibited growth and suppressed both RTK signaling and ERα phosphorylation. While alpelisib-fulvestrant was effective, adding AZD4547 further enhanced inhibition, supporting triple therapy to overcome resistance.

Conclusion: Our findings establish FGFR1 as a key mediator of alpelisib resistance in ER+ breast cancer. Combining FGFR1 inhibitors with alpelisib-based therapies offers a viable approach for FGFR1-overexpressing tumors.