Abstract A065: High throughput drug screening to decipher the mechanisms of trametinib adaptive resistance in low-grade serous ovarian cancer cell lines
Authors: Wong Kwong-Kwok , Ng Chun Wai , Tsang Yvonne , Gershenson David M. 
Published: March 2024

Abstract
Low Grade Serous Ovarian Cancer (LGSOC) is a rare disease but accounts for a significant percentage of epithelial ovarian cancers in young women. Targeted therapy with trametinib, a MEK inhibitor (MEKi), has emerged as a new treatment option for women with recurrent LGSOC from a recent clinical trial. Initial tumor shrinkage was observed in approximately 26% patients treated with trametinib but most patient eventually died of the disease because the tumor became trametinib adaptive resistance. Thus, further investigation to overcome trametinib resistance is necessary to improve the survival of patients with LGSOC. To understand how LGSOC patients develop adaptive resistance to trametinib, we generated several trametinib adaptive resistant LGSOC cell lines. By comparing the expression of over 400 signaling proteins with reverse phase protein array (RPPA) between an adaptive resistant cell (HOC-7R) to the parental cells (HOC-7), we discovered several proteins that were up-regulated in the adaptive resistant cell lines such as plasminogen activator inhibitor-1 (PAI-1 or SERPINE1), p-Rb, p-ERK1/2, MEK1, p-S6, p-Yb1, p-YAP, p-Chk1, and histone H3. By over-expressing SERPINE1 in cell line HOC-7, we were able to show that HOC-7-SERPINE1 (a stable clone of HOC-7 overexpressing SERPINE1) became more resistant to trametinib. By high-throughput drug screening with 555 drugs from libraries with targeted molecules, approved drugs, investigational agents, and bioactive molecules, HOC-7-SERPINE1 cells were confirmed to have increased resistance to other MEK inhibitors in comparison to the parental cell HOC-7. The fold increase in IC50 for MEKi for HOC-7-SERPINE1 were 3.46 (18.65 nM/5.39 nM), 3.74 (28.74 nM/7.68 nM), 2.65 (52.2 nM/19.68 nM) and 1.66 (22.79 nM/13.73 nM) for trametinib, binimetinib, selumetinib and cobimetinib respectively. In addition, overexpression of SERPINE1 in HOC-7 cells also enhanced their resistance to an SRC inhibitor (saracatinib; about 1.7 folds increase in IC50), histone deacetylases inhibitor (belinostat; about 1.2 folds increase in IC50) and Bcl-2 inhibitor (obatoclax; about 2.38 folds increase in IC50). On the other hand, HOC-7-SERPINE1 became more sensitive to farnesyltransferase inhibitor (lonafarnib; about 36.8 folds decrease in IC50), PARP (Poly ADP Ribose Polymerase) inhibitor (talazoparib; about 2.69 folds decrease in IC50). These results suggest that HOC-7-SERPINE1 cells become less dependent on SRC, histone deacetylases, and Bcl-2 than the parental HOC-7 cells but become more dependent on signaling through farnesyltransferase and PARP. The combination of MEKi and PARPi (PARP inhibitor) has previously been shown to be a promising combination and is in clinical trial. It is possible that combinations of trametinib and lonafarnib treatment could overcome trametinib adaptive resistance. In conclusion, high-throughput drug screening allows us to discover potential mechanisms of trametinib adaptive resistance and potential drugs for treatment of LGSOC patients who have developed trametinib resistance.
Citation Format: Kwong-Kwok Wong, Chun Wai Ng, Yvonne Tsang, David M. Gershenson. High throughput drug screening to decipher the mechanisms of trametinib adaptive resistance in low-grade serous ovarian cancer cell lines [abstract]. In: Proceedings of the AACR Special Conference on Ovarian Cancer; 2023 Oct 5-7; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84(5 Suppl_2):Abstract nr A065.