Society: SSAT

Introduction:
Pancreatic Adenocarcinoma (PDAC) is highly lethal due to overwhelming metastatic burden. The mechanisms underpinning metastasis in PDAC are lacking. The TMEM doorway is the sole portal of intravasation in breast cancer (BC). Importantly, intravasation and dissemination via TMEM doorways is inhibited by Tie2 blockade in BC. We evaluated whether TMEM doorways are a common targetable mechanism of dissemination in PDAC.
Methods:
Tumor samples from treatment-naïve and neoadjuvant-treated patients who underwent curative-intent resection of PDAC were stained for TMEM doorways using a triple immunohistochemistry (IHC) stain (tumor cells expressing the actin regulatory protein Mena, macrophages expressing CD68, and endothelial cells expressing CD31). TMEM doorways were manually counted in the highest TMEM doorway-bearing 400x field (“hotspot”) identified at 100x scanning magnification for each case. TMEM doorway function was assessed in a murine model of PDAC treated with or without rebastinib (an investigational oral Tie2 inhibitor) by analyzing; [1] TMEM doorway associated vascular opening (TAVO) using high-molecular weight rhodamine dextran (155kD-TMR dextran), [2] circulating tumor cells (CTCs) and [3] disseminated tumor cells (DTCs) in murine livers. The Wilcoxon Test was used for the human TMEM doorway scoring comparisons. A student t-test was used for the murine analyses.
Results:
TMEM doorways were observed in all PDAC tumors from 50 unique treatment-naïve patients and 14 of 15 neoadjuvant-treated patients. The TMEM doorway score was ≈6x higher than observed in published BC patient samples. Mean TMEM doorway score in “hotspots” was 19 and 34 from low- and high- grade treatment-naïve tumors, respectively (p < 0.01). The mean TMEM doorway score was 3.1 in PDAC from neoadjuvant-treated patients. TAVO was decreased by rebastinib treatment (159.8 µm² ± sd26.5 vs. 22.6 µm² ± sd3.5, p < 0.01). PDAC tumor bearing mice treated with rebastinib had a 6.5-fold decrease in the number of CTCs compared to control (5554 ± sd1017 vs. 852 ± sd412 cells/mL blood, p < 0.01). PDAC tumor bearing mice treated with rebastinib had fewer DTCs in the liver compared to untreated mice (11.6 vs. 7.0 DTCs/mm²).
Conclusions:
TMEM doorways appear to be a common mechanism of dissemination in PDAC and BC. TMEM doorway concentration is significantly associated with PDAC tumor grade and decreases with neoadjuvant therapy. Inhibition of Tie2 with the selective inhibitor, rebastinib, preclinically leads to decreased TMEM doorway function and dissemination in PDAC. Tie2 is a promising therapeutic target for decreasing dissemination and potential metastasis in PDAC that warrants further clinical development.

John Condeelis

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