Signaling Adaptation to TKI Treatment Reactivates ERK Signaling in FLT3/ITD Leukemia
Authors: Bruner Joshua Kyle , Li Li , Ma Hayley S , Qin Alice Can Ran , Levis Mark J. , Pratz Keith W. , Pratilas Christine A. , Small Donald 
Published: October 2019

Abstract
FMS-like tyrosine kinase-3 (FLT3) is one of the most commonly mutated genes in acute myeloid leukemia (AML). The internal tandem duplication (FLT3/ITD) is an established driver mutation in AML and results in constitutive activation of the receptor, activating downstream signaling pathways including STAT5, PI3K/AKT, and RAF/MEK/ERK. The use of FLT3 tyrosine kinase inhibitors (TKIs) for the treatment of FLT3/ITD AML has been explored as a promising strategy for over a decade, but clinical responses have remained limited.
In some cancers driven by mutated oncogenes, small molecule inhibition of the target is known to result in reactivation of downstream signaling pathways. This response has been observed in the context of PI3K/AKT/mTOR and BRAF inhibition, but it remains unclear whether this phenomenon extends to cancers driven by receptor tyrosine kinase (RTK) activation, such as FLT3/ITD AML. We hypothesized that FLT3/ITD leukemia cells exhibit mechanisms of intrinsic signaling adaptation to FLT3 TKI treatment that are associated with an incomplete biologic response.
To test this, we treated FLT3/ITD AML cell lines (Molm14 and MV4;11) with FLT3 TKIs for up to 48 hours. We observed a rebound in ERK phosphorylation beginning as early as 6 hours and continuing for the duration of treatment, while no such rebound was observed in the phosphorylation of downstream targets AKT or STAT5. When these cells were treated with inhibitors of both FLT3 and MEK in combination, little to no pERK rebound was observed, and the anti-leukemia effects were more pronounced compared to either drug alone, both in vitro and in vivo. In vitro, the addition of low dose MEK inhibitor to FLT3 TKI treatment synergistically increased cell death and decreased cell proliferation. In a xenograft transplant model of leukemia, the addition of low-dose MEK inhibitor to FLT3 TKI treatment resulted in a significant reduction of both peripheral blood and bone marrow blasts (p < 0.05).
We next sought to determine whether rebound in ERK signaling following FLT3 inhibition was likely to occur in patients as well. We first examined whether primary leukemic cells from patients with FLT3/ITD AML also exhibit the same phenomenon observed in FLT3/ITD cell lines. After 24 hours of sorafenib treatment, the majority of samples revealed a similar rebound in ERK phosphorylation, despite sustained inhibition of phospho-STAT5. As observed with the cell lines, the addition of a MEK inhibitor to sorafenib treatment resulted in sustained pERK inhibition after 24 hours. Next, we explored whether signaling adaptation observed in culture conditions would be recapitulated by the concentrations of sorafenib present in human plasma from patients being treated with this FLT3 TKI. To replicate these conditions, we cultured Molm14 cells in plasma samples obtained from AML patients treated with sorafenib for a prolonged period. Using a variant of the plasma inhibitory activity assay, Molm14 cells were treated in either normal human plasma for one hour or patient plasma for either one or 24 hours. For all plasma samples, profound pERK rebound was observed by 24 hours after treatment, despite maximal inhibition at one hour and persistent inhibition of FLT3 and STAT5 phosphorylation over the duration.
Finally, we sought to determine whether TKI-mediated rebound in ERK activity was restricted to FLT3/ITD AML or may also occur in the context of other RTK-driven cancers, such as those driven by EGFR mutation or HER2 amplification. We treated an EGFR mutant lung cancer (PC-9) and a HER2 amplified breast cancer (SKBR3) cell line with the EGFR/HER2 inhibitor lapatinib. As was seen for FLT3/ITD driven AML, pERK rebound was observed following 24 hours of TKI treatment despite persistent inhibition of the receptor and downstream target AKT. Again, the rebound could be overcome with the addition of low concentrations of MEK inhibitor.
Together, these studies reveal that FLT3/ITD and other RTK driven cancer cells demonstrate an adaptive feedback mechanism capable of reactivating ERK signaling in response to upstream target inhibition. This adaptation limits the efficacy of TKI treatment and can be abrogated by the addition of a MEK inhibitor. Our data suggest that the addition of low-dose MEK inhibitor to TKI treatment as a means to overcome signaling adaptation may improve outcomes for patients with FLT3/ITD AML and possibly other RTK-driven cancer types.

Disclosures
Levis: Novartis: Consultancy, Honoraria, Research Funding; Millennium: Consultancy, Research Funding; Astellas: Consultancy, Honoraria, Research Funding; Daiichi-Sankyo: Consultancy, Honoraria.