Hepatotoxicity as dose-limiting toxicity of the combination of bosutinib and atezolizumab in patients with chronic myeloid leukemia. Results of the ZEROLMC study

In the field of chronic myeloid leukemia (CML), new strategies are needed to increase the rate of successful treatment discontinuations, a crucial goal in this disease. Anti-PD-L1 checkpoint inhibitors are a promising therapeutic approach in CML after the demonstration of an increase of these inhibitory molecules in patients with CML. A phase Ib/II (NCT04793399, registration date March 11, 2021) open-label exploratory trial has been conducted to evaluate the safety of atezolizumab, a humanized anti-PD-L1 antibody, in combination with bosutinib in patients with newly diagnosed chronic phase CML. A total of 36 patients were planned to be enrolled, but the study had to be prematurely terminated due to safety concerns. Nine patients were included in the study, and only 8 went on to receive the combination with atezolizumab. There were a total of 44 adverse events (AEs) during the study period. The most frequent were gastrointestinal (50%), mostly mild (86% grade 1-2). The most serious AEs were hepatic. There were 17 hepatic AEs in 5 patients. Of the hepatic AEs 5 were during the bosutinib monotherapy phase and 12 during the combination phase (AST increase x4, ALT increase x4, blood bilirubin increase x1, alkaline phosphatase elevation x2, GGT increase x2), most of them grade 3-4. There were 2 patients who presented a dose-limiting toxicity; a grade 3 elevation of transaminases, that led to premature termination of the study. The combination of atezolizumab with bosutinib presents hepatotoxicity as a dose-limiting effect and therefore we do not recommend to explore this combination in future studies.