INTRODUCTION AND RATIONALE Gastrointestinal stromal tumors (GIST) are thought to develop from the interstitial cells of Cajal or their stem cell precursors. They are the most common mesenchymal tumors occurring in the gastrointestinal (GI) tract. The annual incidence of GIST is about 1/100,000 \~ 2/100,000 globally. Biological behavior can range from benign to malignant, and immunohistochemical staining is usually positive for CDs 117 and DOG-1, showing Cajal cell differentiation. Eighty-five percent of GISTs is caused by activating mutations in the receptor tyrosine kinase KIT or platelet-derived growth factor receptor alpha (PDGFRA) gene. Targeted therapy for KIT and PDGFRA alters metastatic or unresectable GISTs without effective drug therapy, allowing tumor control and significantly prolonging patient survival.

Following the marketing application approved in March 2021 for Avapritinib and Repaitinib (the indication for the former is GIST with PDGFRA exon18 mutation and the latter is the standard four-line therapy for GIST), five targeted drugs are currently available for treatment of unresectable or recurrent metastatic GIST. The first, second and third-line treatments were imatinib, sunitinib and regorafenib, respectively. Different targeting drugs can inhibit the activation of GIST- or PDGFRA--encoded transmembrane tyrosine kinases by binding to GIST- driven genes to induce apoptosis. There were differences in molecular structure and mechanism of action among the five targeted drugs, which led to different mutations in different exons of GIST, and the sensitivity to targeted drugs was different. KIT protein is a common type III receptor tyrosine kinase (RTK), which is a transmembrane receptor protein. PDGFRA protein is highly homologous and has similar functions. KIT proteins are predominantly extracellular (encoded by exon 9), transmembrane, near-membrane (encoded by exon 11), kinase domains (further divided into kinase binding domains encoded by exon 13/14, and activation loops encoded by exon 17/18). KIT protein has two kinds of three-dimensional spatial conformation (type I-activated conformation and type II-non-activated conformation), which can be transformed and dynamically balanced under normal conditions. The first-line targeted drug imatinib can competitively bind to ATP in the kinase-binding domain of RTK, and the mechanism of action of imatinib is that after the ATP-binding site of the kinase domain is occupied by it, it blocks substrate phosphorylation and signal transduction, inhibits the proliferation of tumor cells and promotes apoptosis. More than 80% of all GISTs in the advanced stage of treatment present as KIT single-gene driven types, and therefore, the clinical benefit rate of first-line imatinib is more than 84%. One of the reasons for primary resistance to first-line therapy is that tumors derive from primary mutations in the activated loop, such as PDGFRA exon 18 D842V mutations, which cause the kinase to tend to an activated conformation for most of the time, whereas imatinib, sunitinib, and regorafenib belong to the same type II TKIs (tyrosine kinase inhibitors), which theoretically can only bind to kinase in a non-activated conformation and are therefore primary resistant to this mutant GIST. In addition, secondary resistance occurs after first-line treatment is effective, with a median of PFS 24 months. The most common cause of resistance is mutations in the KIT gene at the secondary site. The most common secondary mutations are almost concentrated in the exon 13/14 and exon 17/18 regions. Mutations encoding binding domains (exon 13 and 14) prevented the binding of imatinib to RTKs; Mutations that encode activated rings (exon 17 and 18) cause KIT to tend to activate the conformation, preventing most TKIs, including imatinib, from binding to it. Avapritinib is the only type I TKI that has been approved for the treatment of GIST compared to the other 4 targeted agents. Previous studies have found that this compound has a strong inhibitory effect on KIT exon17 D816V mutants that cause mast cell hyperplasia. The mutant is located in the activation ring. Mutations that are highly similar in structure and function are multidrug resistant PDGFRA exon18 D842V mutations accounting for around GIST 5%. Preclinical studies showed broad-spectrum inhibition of KIT or PDGFRA, including KIT exon 11,17,18, and PDGFRA exon 18 mutations, weak affinity for wild-type KIT and PDGFRA, PDGFRB, CSF1R, and FLT3. In the Phase 1 clinical study in patients with metastatic or unresectable PDGFRA D842V-mutant GISTs, the objective response rate (ORR) for Avapritinib was 91%, with a complete response rate of 13%, median duration of efficacy (DOR) of 27.6 m (95% CI 17.6-N%), and median progression-free survival (PFS) of 34 m (95% CI 22.9-N%), with an estimated overall survival (OS) of 61% at 36months. In patients with KIT or non-D842V PDGFRA mutations treated more than three lines, atorvastatin ORRs were 17%, all in partial response (PRs), median DORs were 10.2 m (95% CI 7.% -10.2), clinical benefit rate (CBRs) were 38%, median PFSs were 3.7 m (95% CI 2.% -4.6), and median OSs were 11.6 m (95% CI 9.% -12.6). These studies indicate that Avapritinib is effective in PDGFRA exon 18, especially in the D842V mutant GISTs, and that some of the remaining KIT mutations can also be treated with Avapritinib for tumor control. In vitro studies suggest that KIT activating ring mutations GISTs are sensitive to Avapritinib, but there are no clinical studies evaluating the exact efficacy of Avapritinib in this type of patient population. In addition, on the surface of the preliminary study, secondary resistance to the PDGFRA exon 18 mutation GIST following the benefit of Avapritinib was due to secondary mutations in the PDGFRA gene; However, the causes of secondary resistance to KIT-mutant GISTs that have been shown to be effective in the treatment of Avapritinib and changes in the driver gene are unknown.

Therefore, it is necessary to conduct a multicenter observational study in patients with unresectable or relapsed/metastatic GISTs other than PDGFRA exon 18 mutations in order to explore the benefit of Avapritinib in addition to PDGFRA exon 18 mutations, and to investigate changes in the status of primary and secondary resistance mutations of Avapritinib in order to provide evidence for individualized treatment of advanced GISTs.