Background: KIT and PDGFRA mutations (mut) are the crucial transforming events in most GISTs, and tyrosine kinase inhibitors (TKIs) with activity against KIT and PDGFRA, such as imatinib (IM) (front-line therapy) and sunitinib (SU) (second-line therapy), are effective treatments in GIST patients (pts). Resistance to IM and SU is commonly associated with evolution of secondary kinase mut. REGO is a multi-targeted TKI that inhibits KIT, PDGFR, and other oncologic targets and has recently shown benefit in pts with metastatic GIST after progression on standard treatments. We evaluated the in vitro and in vivo activity of REGO compared with IM, SU, and sorafenib (SOR) (a multi-TKI structurally related to REGO). Methods: REGO, IM, SU, and SOR inhibition of viability and KIT phosphorylation was assessed in human GIST cell lines and in Ba/F3 cells transformed by KIT oncoproteins with IM-resistant ATP binding pocket or activation-loop mut. KIT/PDGFRA genotyping was performed in GISTs responding or progressing on REGO in the academic phase II clinical trial. Results: In GISTs with KIT exon 11 mutant oncoproteins, REGO potently inhibited viability, KIT phosphorylation, and downstream effector phosphorylation (AKT, MAPK, S6). IM-resistant activation loop mut were more potently inhibited by REGO than SU, whereas the gatekeeper IM-resistant mut T670I was inhibited by both REGO and SU, and the common ATP-binding pocket mutant V654A was more potently inhibited by SU than REGO. Two GIST metastases progressing in one pt after initial response to REGO contained KIT V654A mut. SOR and REGO demonstrated comparable in vitro overall activity. Representative GIST cell line viability IC50s are shown in the Table (values in bold indicate expected clinical relevance). Conclusions: In vitro studies confirm REGO is a potent inhibitor of KIT exon 11 mut in GIST and appears to have stronger activity than SU against the most common KIT activation-loop mut observed in GIST. Ongoing clinical correlative analyses from REGO-treated study patients will be presented.