Abstract 3840: Plocabulin, a tubulin inhibitor, presents antitumor activity in patient-derived xenograft (PDX) models of gastrointestinal stromal tumor (GIST)
Authors: Wozniak Agnieszka , Wang Yannick , Wellens Jasmien , Gebreyohannes Yemarshet K. , Guillén Maria Jose , Galmarini Carlos M. , Avilés Pablo M. , Debiec-Rychter Maria , Sciot Raf , Schöffski Patrick 
Published: August 2018

Abstract
Introduction: Advanced GIST is commonly treated with tyrosine kinase inhibitors (TKI) [e.g. imatinib (IMA)]. With time the vast majority of patients develops TKI-resistance. GIST is generally believed to be resistant to chemotherapy with cytotoxic agents. The aim of our study was to test plocabulin (PLO; PM060184, PharmaMar), a potent cytotoxic tubulin-dynamics modifier, in two PDX models of GIST, characterized by different sensitivity to IMA.
Experimental set-up: NMRI nu/nu mice (n=34) were transplanted bilaterally with human xenografts UZLX-GIST3sens (KIT: exon 11 p.W557_V559delinsF; IMA-sensitive) or -GIST9res (KIT: exon 11+17: p.P577del;W557LfsX5;D820G; IMA-resistant). Xenografted animals were randomly assigned to three treatment groups: control [vehicle, 5ml/kg/QW intravenously (i.v.)], IMA (50mg/kg/BID orally) and PLO (16mg/kg/QW, i.v.). Treatment lasted 22 days and the antitumor activity was assessed by tumor volume measurement, histopathology and KIT signaling pathway by Western blotting. Histological response (HR) was evaluated as previously described by Antonescu et al. 2005. Mann Whitney U test was used for statistical analysis with p &lt;0.05 considered as significant.
Results: PLO treatment resulted in a reduction of tumor volume to 59% in GIST3sens and to 70% of the baseline volume in the GIST9res model. Good HR (grade 3, 4) was observed in 70% (GIST3sens) and 50% (GIST9res) of tumors. HR obtained with PLO was mainly characterized by necrosis, while IMA produced mainly myxoid degeneration in the sensitive model. In addition, in GIST3sens PLO decreased the microvessel area and increased apoptosis as assessed by immunohistochemistry, which was not observed in GIST9res. In the latter model PLO showed better activity than IMA in terms of tumor volume reduction (59% vs. 146%, p&lt;0.01) and HR (grade 3, 4 in 50% vs. 0%). KIT signaling was not affected by PLO. The experimental drug was well tolerated throughout the experiment at the dose administered.
Conclusions: PLO is the first anti-tubulin agent showing antitumor activity in GIST PDX, both in models sensitive or resistant to IMA. The drug causes cytotoxicity in GIST, mainly through necrosis, without affecting KIT signaling. Due to the different modes of action of PLO and established TKI our work provides a scientific rationale to combine PLO and IMA to overcome resistance to small molecule TKI.
Citation Format: Agnieszka Wozniak, Yannick Wang, Jasmien Wellens, Yemarshet K. Gebreyohannes, Maria Jose Guillén, Carlos M. Galmarini, Pablo M. Avilés, Maria Debiec-Rychter, Raf Sciot, Patrick Schöffski. Plocabulin, a tubulin inhibitor, presents antitumor activity in patient-derived xenograft (PDX) models of gastrointestinal stromal tumor (GIST) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3840.