Bosutinib (SKI-606) Demonstrates Clinical Activity and Is Well Tolerated among Patients with AP and BP CML and Ph+ ALL.
Authors: Gambacorti-Passerini C. , Kantarjian H. , Bruemmendorf T. , Martinelli G. , Baccarani M. , Fischer T. , Pogliani E.M. , Hewes B. , Volkert A.D.G. , Bardy-Bouxin N. , Cortes J. 
Published: October 2019

Abstract
Bosutinib (SKI-606) is an orally bioavailable dual Src/Abl inhibitor. Biochemical assays have shown it to be up to 200-fold more potent than imatinib as an inhibitor of Bcr-Abl phosphorylation. Unlike imatinib, bosutinib does not exhibit significant inhibition of c-kit or platelet-derived growth factor receptor (PDGFR), which may result in a relatively favorable safety profile. This is an ongoing open-label study in patients (pts) with Philadelphia chromosome positive (Ph+) accelerated phase (AP) and blast phase chronic myelogenous leukemia and (Ph+) ALL who failed prior imatinib therapy or other TKIs. Objectives are to assess safety and clinical activity of bosutinib. Pts receive bosutinib 500 mg/day. We report preliminary data for 57 pts, median age 54 yrs (range 22–83 yrs), 54% male. 23 pts (40%) were in AP, 15 (26%) in blast crisis (BC), 14 (25%) had Ph+ALL, and 5 (9%) were unclassified. Prior therapy included interferon (22 pts), imatinib (55 pts; data missing for 2 pts), dasatinib (17 pts), nilotinib (10 pts), stem cell transplant (5 pts). Overall median duration of bosutinib treatment was 2.7 mos (range 0.03–10.8 mos). Complete hematological response (CHR) was obtained in 7/25 evaluable pts (28%), including 4/14 (29%) pts with AP-CML, 2/8 (25%) pts with BC-CML, and 1/3 (33%) pts with Ph+ ALL. Among pts with no other TKI exposure, major cytogenetic responses (MCyR) were observed in 5/14 evaluable pts (36%), including 3/6 (50%) pts with AP-CML, 2/5 (40%) pts with BC-CML. Among pts with prior TKI exposure, 3/10 (30%) had MCyR, including 0/3 AP, 1/4 BP, and 2/3 ALL pts. Median time to MCyR was 8.9 weeks for pts previously exposed and 12 wks for unexposed to other TKIs. Duration of MCyR was 18 wks. 19 previously unexposed patients were evaluable for major molecular response. 4 (21%) had major molecular response, 3 (16%) of which were complete. Of 42 pts with samples tested for mutations, 13 different mutations were found in 20 pts (48%), including 5 cases of T315I. CHR occurred in 2/3 pts with P-loop mutations and 5/17 with non-P-loop mutations; MCyR occurred in 2/2 pts and 4/9 pts, respectively. Treatment was generally well tolerated in this cohort of heavily pretreated patients. The most common adverse events were gastrointestinal (diarrhea [56%], nausea [37%], vomiting [35%]) but these were usually grade 1–2, manageable and transient, reducing in frequency and severity after the first 3–4 weeks of therapy. Grade 3–4 hematologic laboratory abnormalities reported included thrombocytopenia in 31 pts (59%), neutropenia in 20 pts (38%), and anemia in 13 pt (25%). Grade 3–4 non-hematologic toxicities were diarrhea (9%) and vomiting (9%). Fluid retention was reported in 8 pts (14%), including 2 cases (3%) of pleural effusion (grade 2 and 3). Both were considered unrelated to treatment. Bosutinib is effective in imatinib-resistant pts with advanced CML. Responses were observed across a wide range of Bcr/Abl mutations. Bosutinib has a favorable toxicity profile with a small number of pts experiencing hematologic toxicity and fluid retention, possibly due to the lack of c-kit inhibition and PDGFR inhibition, respectively.