Imatinib TDM in GIST

Inclusion Criteria: * Histologically or cytologically confirmed GIST with KIT mutation or PDGFRA mutation (non-D842V) * Currently receiving imatinib initiated within the last 3 months or to be started in the next 1 month * Prior systemic chemotherapy for other malignancy is allowed as long as it was completed within the past 6 months and there is no evidence of disease * Age ≥18 years * ECOG performance status of 0 or 1 * Normal organ function Exclusion Criteria: * Presence of PDGFRA D842V mutation * Known allergy to imatinib or allergic reactions to compounds of similar chemical or biologic composition to the study drug * Concomitant anticoagulation with oral warfarin. * Use of strong inhibitors or inducers of CYP3A or CYP3A4, drugs metabolized by CYP3A4 or CYP2D6 * Uncontrolled intercurrent illness * Concurrent malignancy