Ethnic Differences in Hematologic Toxicity from Imatinib in Patients with Gastrointestinal Stromal Cell Tumor (GIST): Coincidence or a Real Phenomenon

OBJECTIVES: There is a wide variability in the pharmacokinetics, pharmacodynamics and tolerance of anticancer drugs based on ethnicity. GIST is a rare cancer, (~1% of GI cancers). Imatinib is used in the neo-adjuvant, adjuvant and metastatic setting. The purpose of this study was to report the difference in hematologic toxicities to imatinib among different ethnicities when treated for GIST either in the adjuvant or metastatic setting. METHODS: We performed a retrospective study to collect data on patients with GIST (in any stage), who were on imatinib and presenting with grade 2 or more anemia, neutropenia and/or thrombocytopenia from July 1, 2005 to January 31, 2018. The degree of cytopenia was graded as per National Cancer Institute Common Toxicity criteria; version 4.0. We collected included age, gender, ethnicity, pathology, adverse effects-hematologic and non-hematologic, management of toxicities including dose modifications and administration of pegfilgrastim. RESULTS: Among 57 patients (median age 61 years, M: F=41:16 (F); ethnicities: White 65%, African-American (AA 19%, Asian 12% and Hispanic 4%), neutropenia (Grade 3 & 4) was seen in 6 patients (10%): 5 AA and 1 Asian. 45% of all AA patients developed neutropenia. Median absolute neutrophil count (ANC) nadir was 700/μL, median duration on drug prior to onset of neutropenia was 4.5 weeks and median duration of neutropenia was 4 weeks. One patient developed febrile neutropenia. Dose interruptions were needed in 3, dose-reductions in all patients, and 3 patients required pegfilgrastim. One patient had to discontinue imatinib, while one patient was escalated back to 400mg daily dose. CONCLUSION: This is the first study to examine ethnic variations in myelosuppression following imatinib in patients with GIST.