Efficacy and Safety of Bosutinib (SKI-606) in Patients with Chronic Phase (CP) Ph+ Chronic Myelogenous Leukemia (CML) with Resistance or Intolerance to Imatinib.
Authors: Cortes Jorge , Kantarjian Hagop M , Kim Dong- Wook , Khoury H. Jean , Turkina Anna G. , Shen Zhi-Xiang , Brummendorf Tim H , Chandy Mammen , Arkin Steven , Gambacorti-Passerini Carlo 
Published: April 2020

Abstract
Bosutinib (SKI-606) is an orally bioavailable dual Src/Abl inhibitor demonstrating inhibitory activity against BCR-Abl phosphorylation, and is 200 times more potent than imatinib but with minimal inhibition of platelet-derived growth factor receptor (PDGFR) or c-kit. The phase I portion of this study identified a treatment dose of 500 mg daily and showed evidence of clinical efficacy. The phase II portion of the study to investigate the efficacy and safety of bosutinib in patients (pts) with CP Ph+ CML who have failed imatinib therapy is ongoing. Preliminary data for 283 treated pts, median age 54 yrs (range 18 – 91 yrs) and 52% male are reported. A subset of pts received treatment in addition to imatinib, including interferon (91 pts), dasatinib (71 pts), nilotinib (7 pts) and stem cell transplant (13 pts). Among pts who failed imatinib (and received no other tyrosine kinase inhibitor treatment), 137 were imatinib-resistant (all received imatinib ≥600mg) and 64 pts were imatinib-intolerant; median duration of bosutinib treatment to date is 7.7 mos (range 0.2 – 28.2 mos) and 4.5 mos (range 0.5 – 21.5 mos), respectively. Among 67 imatinibresistant pts evaluable for hematological response, 53 (79%) had complete hematological response (CHR). Of 84 imatinib-resistant pts evaluable for cytogenetic response, 34 (40%), achieved a major cytogenetic response (MCyR), including 24 (29%) with a complete cytogenetic response (CCyR). Of 34 pts with MCyR, 31 have maintained their response to date. Of 60 evaluable imatinib-resistant pts, 20 (33%) achieved major molecular response, 10 (17%) of which were complete. Among imatinib-intolerant pts, 22 of 29 evaluable (76%) achieved CHR, and 13 of 22 evaluable (59%) achieved MCyR, including 11 (50%) with CCyR. Of 25 evaluable imatinib-intolerant pts, 7 (28%) achieved major molecular response, 5 (20%) of which were complete. Of 105 pts with baseline samples tested for mutations, 17 different mutations were found in 45 pts (43%). CHR occurred in 5/6 pts (83%) with P-loop mutations and 13/17 (76%) with non-P-loop mutations; MCyR occurred in 3/6 pts (50%) and 11/24 pts (46%), with P-loop and non-P-loop mutations, respectively. Treatment was generally well tolerated. The most common adverse events among treated pts (n=283) were gastrointestinal (nausea, vomiting, diarrhea), these were usually grade 1 – 2, manageable and transient, diminishing in frequency and severity after the first 3 – 4 weeks of treatment. Grade 3 – 4 non-hematologic toxicity occurring in ≥5% of pts were diarrhea (8%), rash (8%) and increased ALT (5%). 27 pts (10%) reported grade 1/2 fluid retention adverse events, including 21 pts with edema, and 6 pts with effusions: 4 pleural, 1 pericardial, and 1 pleural and pericardial. A single patient experienced grade 3 pleural effusion possibly related to bosutinib with concomitant pneumonia and a pre-treatment history of recurrent pleural effusions. Grade 3 – 4 hematologic laboratory abnormalities included thrombocytopenia in 65 pts (23%), neutropenia in 37 pts (13%) and anemia in 17 pts (6%). 124 pts (44%) had at least 1 temporary treatment interruption and 85 pts (30%) had at least 1 dose reduction due to toxicity. 37 pts (13%) have permanently discontinued treatment due to adverse event. Bosutinib is effective in pts with CP CML with resistance or intolerance to imatinib across a range of mutations. Unlike other tyrosine kinase inhibitors, bosutinib does not significantly inhibit PDGFR or c-kit, and this may be responsible for the relatively favorable toxicity profile with few pts experiencing hematologic toxicity or fluid retention.