Page 30 - Read Online

Lee et al. Cancer Drug Resist 2020;3:980-91 I http://dx.doi.org/10.20517/cdr.2020.73 Page 981

Keywords: Triple negative breast cancer, CHK1, replication, apoptosis, drug resistance, epidermal growth factor
receptor, mitogenic signaling

INTRODUCTION
Triple negative breast cancer (TNBC) is an aggressive form of breast cancer associated with poor prognosis
-
-
and metastasis. TNBC is characterized by the lack of estrogen receptor (ER), progesterone receptor (PR),
-
and the growth factor receptor HER2/erbB2 (HER2 ). The lack of these targetable molecules for hormone
and growth factor therapy limit current treatment options for TNBC.
Given PARP inhibitors’ success in BRCA mutated breast cancers, there has been a renewed focus on
molecular inhibitors for DNA damage and response (DDR) proteins, such as ATM, ATR, and CHK1, as
[1]
molecular targets in aggressive cancers such as TNBC . Inhibition of these critical cell cycle regulators
enhance replication stress in proliferating cancer cells and promote synthetic lethality with defects in
[1,2]
homologous recombination .
Interestingly, activation of ATR/CHK1 has been observed as a method of stabilizing the replication fork
[3-5]
and promoting resistance to PARP inhibitors . CHK1 plays a critical role in response to DNA damage
and is an essential effector in the regulation of replication. Inhibitors of CHK1 alter DNA damage response
and abrogate S and G -M cell cycle checkpoints to promote replication stress, induce double strand breaks,
2
[6]
and promote cell death through mitotic catastrophe in highly proliferating cells . In the last several years,
CHK1 inhibitors, such as prexasertib (LY2606368), have shown success in promoting cell death as a
monotherapy in high grade serous ovarian cancer, squamous cell carcinoma, and neuroblastoma [5,7-9] .

The lack of molecular targets in TNBC has prompted investigations into CHK1 inhibitors as single-agent
[10]
or combination therapies . Several reports have demonstrated that prexasertib is effective, particularly
as combination therapies in TNBC [11-13] . However, as a monotherapy, prexasertib has only shown modest
[14]
activity in the treatment of TNBC (NCT02203513) . Modest clinical activity may be due to innate or
acquired resistance to prexasertib, which is poorly understood.

[8]
In 2019, Lowery et al. described innate and acquired resistance to prexasertib in sarcoma xenografts.
Prexasertib-resistant tumors showed higher levels of the anti-apoptotic protein BCL-xL and increased
phosphorylation along the PI3K and MAPK signaling pathways. Specifically, highly activated AKT,
[8]
MEK1/2, and ERK1/2 were observed in resistant tumors . Increased RAS/MEK/ERK activity has also been
reported in response to CHK1 inhibition in other cell lines [15,16] . However, combinations of MAPK or PI3K
[8]
inhibitors with prexasertib were insufficient to overcome acquired resistance in sarcoma xenografts .

Innate or acquired resistance to prexasertib may arise from mechanisms that drive cell proliferation and
stabilize the replication fork. An upstream regulator of both MAPK and PI3K pathways is the epidermal
growth factor receptor (EGFR). EGFR overexpression or activation stimulates RAS/MAPK and, to a lesser
extent, PI3K/AKT/mTOR signaling, driving cell proliferation and potentially bypassing prexasertib-induced
replication stress. EGFR overexpression is observed in ~50% of TNBC tumors and is associated with poor
overall survival [17-19] . Therefore, we examined EGFR expression and inhibition in TNBC preclinical cell lines
to determine if EGFR promoted resistance to prexasertib and if inhibition of EGFR could enhance the anti-
tumor activity of prexasertib in TNBC tumors.