Endometrial cancer is the only cancer with a worse survival rate over the past forty years [1]. Deaths due to endometrial cancer are rising by 1.5 % each year, with 13,860 deaths projected in 2025 [1]. This trend may be attributed to the growing incidence of high-risk histological subtypes, such as uterine serous carcinoma (USC) and high grade endometrioid carcinomas [2]. Of the four molecular classes of endometrial cancer, the most aggressive tumors are noted to be in the copy number-high subgroup, which is characterized by recurrent TP53 mutations and poor prognosis [3,4]. Amplification of the oncogene ERBB2, resulting in overexpression of the receptor tyrosine kinase HER2, is a driver of copy number-high carcinomas [[4], [5], [6]], with 25–30 % of USC shown to overexpress HER2 [7,8]. HER2 overexpression has been correlated with worse prognosis [9].

The current standard of care treatment for high HER2-expressing (HER2-high) uterine cancer consists of platinum-based chemotherapy with the addition of the HER2 blocking antibody trastuzumab or the antibody-drug conjugate trastuzumab deruxtecan (T-DXd). In a prior clinical trial, the addition of trastuzumab to standard chemotherapy in USC led to a 4.6-month improvement in progression-free survival (PFS) and a 8.5-month improvement in overall survival (OS) compared to chemotherapy alone when administered as a first-line treatment [10]. However, in those with recurrent disease, trastuzumab led to an improvement of only 3.2 months in PFS and 2.2 months in OS [10]. In a phase II trial of T-DXd for HER2-overexpressing endometrial cancer (immunohistochemistry [IHC] 2+ and 3+), the PFS was 11.1 months for the overall cohort. The PFS was 8.5 months for the patients with HER2 IHC 2+ expression and not reached for those with HER2 IHC3+ expression [11]. Despite these promising results, there are patients who do not respond or may not be able to tolerate the antibody-drug conjugate. This highlights the need to develop additional therapies for HER2+ endometrial cancer.

Recent studies have highlighted AXL, a member of the TAM family of receptor tyrosine kinases, as contributing to decreased response to HER2 blocking therapies in various solid tumors [12,[13], [61]]. AXL is activated via ligand-dependent and ligand-independent mechanisms, with the former mediated by its only ligand, growth arrest-specific protein 6 (GAS6). AXL activation results in dimerization, auto-phosphorylation, and activation of signaling pathways promoting the epithelial-mesenchymal transition, cell proliferation, migration, invasion, angiogenesis, and chemoresistance [13]. Prior work from our laboratory has shown that high-grade endometrial carcinomas are more likely to exhibit high AXL expression compared to low-grade carcinomas, and that high AXL expression is associated with poor prognosis and chemoresponse in USC [14,15].

While crosstalk between HER2 and AXL has yet to be elucidated in endometrial cancer, preclinical studies in breast, gastric, and esophageal cancer have shown that AXL hetero-dimerizes with HER2 and drives resistance to anti-HER2 therapies [13]. In preclinical models of breast cancer, HER2 was shown to complex with AXL, resulting in transphosphorylation of AXL and promotion of cell invasion. Furthermore, AXL inhibition has been shown to overcome resistance to HER2 blockade via trastuzumab in vitro and in vivo in breast cancer [16]. These results, as well as the finding that AXL and HER2 have similarly low mutational rates and high protein expression levels in endometrial cancer, support studies into their interaction and co-inhibition in endometrial cancer [17,18].

Of the various therapeutic agents targeting AXL, one of the most promising is batiraxcept, a soluble Fc fusion protein that selectivity binds to GAS6 with 200-fold higher affinity than the AXL receptor and prevents the AXL-GAS6 interaction [19]. In clinical trials, it has demonstrated tolerability and a manageable toxicity profile [20,[21], [63]]. Our laboratory has previously shown that batiraxcept improves sensitivity of endometrial cancer to both paclitaxel and bevacizumab in vitro and in vivo [15,22]. In this study, we investigated the interaction between AXL and HER2, and the potential of batiraxcept to potentiate the effect of trastuzumab in endometrial cancer. We show that AXL and HER2 colocalize in high-risk endometrial cancer cells. Furthermore, we found that treatment with batiraxcept and trastuzumab improves the sensitivity of high-risk endometrial cancer to trastuzumab in vitro and in vivo, and we identified proteins that may mediate this improved effect. Thus, our study provides a preclinical rationale for co-targeting HER2 and AXL in aggressive endometrial cancers.