Abstract 3840: Plocabulin, a tubulin inhibitor, presents antitumor activity in patient-derived xenograft (PDX) models of gastrointestinal stromal tumor (GIST)

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-GIST3 sens (KIT: exon 11 p.W557_V559delinsF; IMA-sensitive) or -GIST9 res (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 Results: PLO treatment resulted in a reduction of tumor volume to 59% in GIST3 sens and to 70% of the baseline volume in the GIST9 res model. Good HR (grade 3, 4) was observed in 70% (GIST3 sens ) and 50% (GIST9 res ) of tumors. HR obtained with PLO was mainly characterized by necrosis, while IMA produced mainly myxoid degeneration in the sensitive model. In addition, in GIST3 sens PLO decreased the microvessel area and increased apoptosis as assessed by immunohistochemistry, which was not observed in GIST9 res . In the latter model PLO showed better activity than IMA in terms of tumor volume reduction (59% vs. 146%, p 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 Guillen, Carlos M. Galmarini, Pablo M. Aviles, Maria Debiec-Rychter, Raf Sciot, Patrick Schoffski. 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.