Establishment of a Novel Canine Mastocytoma Cell Line, NI-1: a Model for Studying Resistance Against KIT Tyrosine Kinase Inhibitors In Canine Neoplastic Mast Cells
Authors: Hadzijusufovic Emir , Peter Barbara , Herrmann Harald , Schuch Karina , Thaiwong Tuddow , Yuzbasiyan-Gurkan Vilma , Rülicke Thomas , Pickl Winfried , Willmann Michael , Valent Peter 
Published: October 2019

Abstract
Abstract 4936
Advanced mast cell (MC) disorders are characterized by uncontrolled growth of neoplastic MC in various organ systems, resistance to conventional cytoreductive drugs, and a poor prognosis. In most patients, transforming mutations in the KIT proto-oncogene are detectable and are considered to contribute to resistance. MC lines are an important model for analyzing drug resistance in neoplastic MC. We have established a novel canine mastocytoma cell line, NI-1 from a canine patient suffering from mast cell leukemia. NI-1 cells were found to harbour several homozygous KIT mutations including two single nucleotide mutations, one at nucleotide 107 (C to T, leading to a missense mutation, P36L) and one at nucleotide 1187 (A to G, leading to a missense mutation, Q396R), a 12 bp duplication at nucleotide 1263, and a 12 bp deletion at nucleotide 1550, the latter reflecting a transcriptional variant. NI-1 cells contained histamine and formed tumors in NOD-SCID IL-2Rgammanull (NSG) mice. As assessed by 3H-thymidine uptake, a number of targeted drugs were found to inhibit the proliferation of NI-1 cells at pharmacologically relevant concentrations. Among these drugs were the KIT kinase blockers midostaurin (IC50 <0.1 μM), dasatinib (IC50 <0.1 μM), sunitinib (IC50 <0.1 μM), tozasertib (0.1-0.5 μM), imatinib (IC50 0.25–0.37 μM), sorafenib (IC50 0.25–0.5 μM), masatinib (0.1-1.0 μM) as well as drugs targeting important KIT-downstream signaling molecules (PI3 kinase, mTOR), i.e. RAD001 (IC50 <0.1 μM) and NVP-BEZ235 (IC50 0.01–0.05 μM). In most instances, drug-induced growth inhibition was found to be accompanied by apoptosis. No clear effects were seen with drugs that are not capable of blocking KIT or KIT-downstream kinases in MC, i.e. bosutinib, erlotinib, gefitinib, and lapatinib (IC50 >2 μM). However, the HDAC inhibitor vorinostat was found to block growth of NI-1 cells (0.1-0.5 μM). Drug response profiling also revealed that NI-1 cells differ markedly from canine C2 mastocytoma cells harbouring an exon 11 KIT mutation, and the human mast cell lines HMC-1.1 (exon 11 mutation in KIT) and HMC-1.2 (exon 11 and exon 17 KIT mutations). In contrast to C2 cells, NI-1 cells were found to be largely resistant against the growth-inhibitory effects of masatinib, sorafenib, and sunitinib. By contrast, NI-1 cells were found to be even more sensitive against the growth-inhibitory effects of the mTOR blocker RAD001 and the PI3-kinase/mTOR inhibitor NVP-BEZ235. Together, our data suggest that certain KIT mutations may be associated with relative resistance of neoplastic MC against KIT-targeting drugs, a phenomenon that has also been described for human MC and the KIT mutant D816V that mediates resistance against masatinib and imatinib. The novel MC line NI-1 may serve as a novel tool to investigate the mechanisms of resistance against TKI in neoplastic MC.

Disclosures:
Valent: Novartis: Research Funding; Bristol-Myers Squibb: Research Funding.