Saša Dimitrijević

BACKGROUND: Constitutive activation of KIT receptor tyrosine kinase is critical in the pathogenesis of gastrointestinal stromal tumors. Imatinib mesylate, a selective tyrosine kinase inhibitor, has been shown in preclinical models and preliminary clinical studies to have activity against such tumors. METHODS: We conducted an open-label, randomized, multicenter trial to evaluate the activity of imatinib in patients with advanced gastrointestinal stromal tumor. We assessed antitumor response and the safety and tolerability of the drug. Pharmacokinetics were assessed in a subgroup of patients. RESULTS: A total of 147 patients were randomly assigned to receive 400 mg or 600 mg of imatinib daily. Overall, 79 patients (53.7 percent) had a partial response, 41 patients (27.9 percent) had stable disease, and for technical reasons, response could not be evaluated in 7 patients (4.8 percent). No patient had a complete response to the treatment. The median duration of response had not been reached after a median follow-up of 24 weeks after the onset of response. Early resistance to imatinib was noted in 20 patients (13.6 percent). Therapy was well tolerated, although mild-to-moderate edema, diarrhea, and fatigue were common. Gastrointestinal or intraabdominal hemorrhage occurred in approximately 5 percent of patients. There were no significant differences in toxic effects or response between the two doses. Imatinib was well absorbed, with pharmacokinetics similar to those reported in patients with chronic myeloid leukemia. CONCLUSIONS: Imatinib induced a sustained objective response in more than half of patients with an advanced unresectable or metastatic gastrointestinal stromal tumor. Inhibition of the KIT signal-transduction pathway is a promising treatment for advanced gastrointestinal stromal tumors, which resist conventional chemotherapy.

PURPOSE: Most gastrointestinal stromal tumors (GISTs) express constitutively activated mutant isoforms of KIT or kinase platelet-derived growth factor receptor alpha (PDGFRA) that are potential therapeutic targets for imatinib mesylate. The relationship between mutations in these kinases and clinical response to imatinib was examined in a group of patients with advanced GIST. PATIENTS AND METHODS: GISTs from 127 patients enrolled onto a phase II clinical study of imatinib were examined for mutations of KIT or PDGFRA. Mutation types were correlated with clinical outcome. RESULTS: Activating mutations of KIT or PDGFRA were found in 112 (88.2%) and six (4.7%) GISTs, respectively. Most KIT mutations involved exon 9 (n = 23) or exon 11 (n = 85). All KIT mutant isoforms, but only a subset of PDGFRA mutant isoforms, were sensitive to imatinib, in vitro. In patients with GISTs harboring exon 11 KIT mutations, the partial response rate (PR) was 83.5%, whereas patients with tumors containing an exon 9 KIT mutation or no detectable mutation of KIT or PDGFRA had PR rates of 47.8% (P =.0006) and 0.0% (P <.0001), respectively. Patients whose tumors contained exon 11 KIT mutations had a longer event-free and overall survival than those whose tumors expressed either exon 9 KIT mutations or had no detectable kinase mutation. CONCLUSION: Activating mutations of KIT or PDGFRA are found in the vast majority of GISTs, and the mutational status of these oncoproteins is predictive of clinical response to imatinib. PDGFRA mutations can explain response and sensitivity to imatinib in some GISTs lacking KIT mutations.

Gastrointestinal stromal tumors are a group of mesenchymal neoplasms that arise from precursors of the connective-tissue cells of the gastrointestinal tract.1 They occur predominantly in middle-aged and older persons, and approximately 70 percent of the tumors are found in the stomach, 20 to 30 percent are found in the small intestine, and less than 10 percent are found elsewhere in the gastrointestinal tract.1 Recent studies have shown that cells in gastrointestinal stromal tumors express a growth factor receptor with tyrosine kinase activity termed c-kit. This receptor, the product of the proto-oncogene c-kit, can be detected by immunohistochemical staining for . . .

BACKGROUND: A small proportion of patients with chronic myeloproliferative diseases have constitutive activation of the gene for platelet-derived growth factor receptor beta (PDGFRB), which encodes a receptor tyrosine kinase. The gene is located on chromosome 5q33, and the activation is usually caused by a t(5;12)(q33;p13) translocation associated with an ETV6-PDGFRB fusion gene. The tyrosine kinase inhibitor imatinib mesylate specifically inhibits ABL, PDGFR, and KIT kinases and has impressive clinical efficacy in BCR-ABL-positive chronic myeloid leukemia. METHODS: We treated four patients who had chronic myeloproliferative diseases and chromosomal translocations involving 5q33 with imatinib mesylate (400 mg daily). Three of the four patients presented with leukocytosis and eosinophilia; their leukemia cells carried the ETV6-PDGFRB fusion gene. The fourth patient had leukocytosis, eosinophilia, and a t(5;12) translocation involving PDGFRB and an unknown partner gene; he also had extensive raised, ulcerated skin lesions that had been present for a long time. RESULTS: In all four patients, a normal blood count was achieved within four weeks after treatment began. In the patient with skin disease, the lesions began to resolve shortly after treatment began. The t(5;12) translocation was undetectable by 12 weeks in three patients and by 36 weeks in the fourth patient. In the three patients with the ETV6-PDGFRB fusion gene, the transcript level decreased, and in one patient, it became undetectable by 36 weeks. All responses were durable at 9 to 12 months of follow-up. CONCLUSIONS: Imatinib mesylate induces durable responses in patients with chronic myeloproliferative diseases associated with activation of PDGFRB.