(P 276) REVISITING IN- AND OFF LABEL DRUGS FREQUENTLY USED IN METASTATIC GIST USING A BROAD IN VITRO PROFILING PANEL OF PRIMARY AND SECONDARY KIT AND PDGFRA-MUTANT GIST CELLS – CLINICAL IMPLICATIONS

Objective: Resistance to imatinib in KIT-mutant and avapritinib in PDGFRA-mutant GIST is mostly driven by secondary mutations of KIT or PDGFRA in metastatic gastrointestinal stromal tumors (GIST). Despite five approved kinase inhibitors for the treatment of GIST, most patients eventually progress and tyrosine-kinase inhibitors not approved for GIST are frequently used in an off-label setting based on small trials in the pre-regorafenib and/or ripretinib era. Here, we used our novel and comprehensive panel of cell lines covering primary and secondary resistance mutations to identify genotypes that would benefit from off-label treatment.

Methods: Cas9-based gene editing was used to generate a panel of isogenic cell lines comprising 16 KIT- and 5 PDGFRA-mutant sublines. Sulforhodamine B (SRB) assays were used to study cytotoxicity/proliferation. Apart from approved drugs (imatinib, sunitinib, regorafenib, ripretinib, and avapritinib, we evaluated cabozantinib, pazopanib, dasatinib, and sorafenib. Immunoblotting was used to evaluate effects on KIT and KIT dependent signaling.

Results: Secondary mutations within the ATP-binding pocket (AP) and the activation loop (AL) confer high levels of resistance to IM (GR50: 30- to >500-fold higher than exon 11 primary mutation) with the exception of the D820Y mutation (5-fold). Pazopanib exhibits no activity (GR50: >5uM) in any resistant subline with the exception of the C809G secondary resistance mutation, which is effectively inhibited by ripretinib and sunitinib. Cabozantinib is highly effective against primary exon 11 and exon 9 mutations and the T670I secondary mutation but shows a 100-fold higher GR50 against the common AP mutation V654 and >50-fold higher GR50 against most AL mutations (except D820Y). Dasatinib is highly potent against primary mutations (GR50: 1-6nM) and AL-mutations C809G (GR50: 1nM) and D820Y (GR50: 3nM) but shows only modest activity against other AL-mutations (33-94nM). Sorafenib exhibits a highly similar inhibitory profile compared to regorafenib. PDGFRA-D842V mutant GIST are potently inhibited by avapritinib only. No activity is seen with any drug against D842V-sublines harboring secondary resistance mutations.

Conclusion: Preclinical testing suggests that dose-escalation of or retreatment with imatinib is not effective in exon 11 mutant GIST with secondary mutations. Pazopanib appears highly ineffective against any secondary mutation but shows notable activity in exon 9 mutant GIST. Cabozantinib is the only drug with strong activity against the gatekeeper T670I and the AL D820Y mutation. Avapritinib but no other drug shows activity in PDGFRA-D842V mutant GIST. Comprehensive preclinical profiling does not identify a particular genomic niche for off-label drugs. However, in an era of sensitive ct-DNA testing this data could be useful in the context of un-manageable toxicity or in health-systems with lack of access to some of the approved drugs.