Imatinib Therapeutic Drug Monitoring for the Treatment of Gastrointestinal Stromal Tumors

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 (only those patients on the standard intramuscular [IM] dose as noted in the protocol imatinib dosing). * NOTE: Patients receiving neoadjuvant prior to surgery should have at least 3 months of planned neoadjuvant imatinib prior to surgery remaining. Patients on imatinib initiated within the last 3 months who have had a dose reduction are not eligible for the protocol
• Prior systemic chemotherapy for other malignancy is allowed as long as it was completed at least 6 months prior to screening and there is no evidence of disease
• Age ≥ 18 years
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
• Life expectancy of at least 6 months
• Women of childbearing potential and male participants who are sexually active must agree to use a medically effective means of birth control
• Creatinine ≤ institutional upper limit normal (ULN) OR glomerular filtration rate (GFR) ≥ 60 mL/min/1.73 m^2, unless data exists supporting safe use at lower kidney function values, no lower than 30 mL/min/1.73 m^2
• White blood count (WBC) ≥ 3,000/mcL
• Absolute neutrophil count (ANC) ≥ 1,500/mcL
• Platelets ≥ 100,000/mcL
• Total bilirubin ≤ institutional ULN
• Aspartate aminotransferase (AST)/alanine aminotransferase (ALT) ≤ 3 × institutional ULN
• Ability to comprehend study rationale and willing to sign a written informed consent document

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. The following anticoagulants are allowed: Low-dose aspirin for cardioprotection (per local applicable guidelines) and low-dose low molecular weight heparins (LMWH)
• Use of strong inhibitors or inducers of CYP3A4, drugs metabolized by CYP3A4 or CYP2D6 which cannot be discontinued at least 7 days prior to randomization
• Pregnant or lactating women. Women of child-bearing potential will be excluded if they have a positive serum/urine pregnancy test
• Uncontrolled intercurrent illness
• Psychiatric illness/social situations that would limit compliance with study requirements
• Concurrent malignancy or history of malignancy whose natural history or treatment has the potential to interfere with the safety or efficacy assessment of the investigational regimen should be excluded. (Patients with a malignancy or history of malignancy which, as documented by treating investigator and approved by the principal investigator, does not have the potential to interfere with the safety or efficacy assessment of the investigational regime, are eligible to participate in the study)