BCL-2 is highly expressed in a large subset of neuroblastoma patients and plays an important oncogenic role. Previous studies with the small-molecule BCL-2 inhibitor navitoclax showed favorable antitumor activity for BCL-2-dependent neuroblastoma cells but unfortunately, navitoclax was associated with dose-limiting thrombocytopenia in patients due to BCL-XL inhibition. Therefore, the BCL-2 specific inhibitor, venetoclax was developed. In this study, we explored the preclinical therapeutic potential of venetoclax for neuroblastoma treatment and strategies to prevent venetoclax resistance.
We show that venetoclax induces strong apoptotic responses in cell lines with high BCL-2 expression levels, indicated by dose-dependent cytochrome c release from the mitochondria into the cytoplasm and PARP cleavage. The in vitro efficacy of the compound was ascertained by BIM displacement from BCL-2. Venetoclax significantly inhibited the growth of high BCL-2 expressing neuroblastoma xenografts in mice. Despite complete displacement of BIM from BCL-2 and increased cleaved caspase 3 levels, complete tumour regression was not observed. We showed that the in vitro and in vivo resistance to venetoclax results from upregulation of the anti-apoptotic BCL-2 family protein MCL-1 and BIM sequestration by MCL-1. Knockdown of MCL-1 re-sensitized neuroblastoma cell lines to venetoclax, confirming a pivotal role of MCL-1 in venetoclax resistance. To identify potential synergistic combination treatments we are currently performing a large compound combination screen using Venetoclax resistant cell lines.
Taken together, the results presented in this study strongly suggest that children with neuroblastoma tumors expressing high levels of BCL-2 and BIM/BCL-2 complex might benefit from combined treatment with venetoclax and targeted compounds that interfere with MCL1 function.