Imatinib and other tyrosine kinase inhibitors (TKIs) can induce long-term remissions in leukemia patients whose tumors are driven by the BCR-ABL tyrosine kinase gene. However, the drugs often fail to kill leukemia-initiating cells, leaving patients vulnerable to relapse if and when their tumor cells develop resistance.
Now, researchers have uncovered a potent mechanism that leukemia cells use to escape death in the presence of TKIs. Additionally, the team reported in this week’s Nature that they can overcome TKI resistance by blocking a single protein called BCL6 in mouse models of Philadelphia-chromosome positive acute lymphoblastic leukemia (Ph+ ALL).
The team is currently optimizing their small molecule inhibitor of BCL6. When that’s done, they aim to test it in leukemia patients whose tumors have become resistant to TKIs.
Remarkably, there are hints that the BCL6-dependent resistance mechanism may be important in other kinase-driven tumors, including some breast and lung cancers.
A Strong Blockade
To uncover potential mechanisms of resistance, the investigators, led by
Markus Müschen, MD, PhD, Professor of Laboratory Medicine at the University of California San Francisco, examined gene expression profiles from BCR-ABL-driven leukemias, including TKI-treated tumors. BCL6 expression was found to have ncreased dramatically with TKI exposure.
Putting together clues from a variety of sources, the team hypothesized that the increased expression of BCL6, which is a transcription factor, might hamper p53 gene expression.
A series of in vitro experiments showed that was indeed the case. Therefore, while the TKI blocks proliferation through the BCR-ABL oncogene, the cells remain alive in a dormant state because they lack p53, which would normally trigger cell death.
To demonstrate that this was indeed the mechanism protecting TKI-resistant leukemia cells, the team grew Ph+ ALL cells in the presence of imatinib.
Initially the clones were sensitive to the drug, but subsequently became resistant and started proliferating again. When the team treated the cells with both the TKI and a peptide inhibitor of BCL6, the leukemia cells died. The peptide inhibitor, however, had little effect on the cells by itself.
“When we reinstate the p53 pathway with the BCL6 inhibitor, it allows the cells to undergo cell death in the presence of a TKI,” Dr. Müschen said. “The cells do not enter a dormancy mode.”
Similarly, TKI-resistant leukemia cell lines isolated from patients showed a similar response to the combination in vitro. And when primary Ph+ ALL cells were injected into mice, as xenograft models, eight animals treated with nilotinib alone died within 99 days, whereas seven out of eight animals treated with the combination remained alive at 140 days.
The data reported in the Nature paper focus on BCL6 inhibition in ALL, however Dr. Müschen says his team has similar results in chronic myelogenous leukemia, which is also driven by BCR-ABL.
“The results are quite similar, so it seems that this is a general mechanism,” he said. The chronic myelogenous leukemia data will be published in the Journal of Experimental Medicine.
The team has also seen up regulation of BCL6 in other leukemias driven by tyrosine kinases, including FLT3 and PDGFR. They have functional studies on-going to determine whether BCL6 up regulation works in these tumors as it does in Ph+ ALL.
When asked whether BCL6 may play a role in drug resistance in solid tumors, Dr. Müschen acknowledged that it may. Again, the team does not yet have functional data, but they have seen that BCL6 is upregulated in HER2-positive breast cancer treated with trastuzumab and lung cancers with mutant or overexpressed epidermal growth factor receptor treated with EGFR inhibitors.
Working on a Drug
The BCL6 inhibitor used in the current study is a large peptide and not suitable for patient use. Therefore the team has developed a small molecule inhibitor of BCL6, which blocks the protein’s function by getting in the way of its binding to critical co-factors.
The first-generation small molecule inhibitor, though, is substantially less potent than the peptide inhibitor. The team is working on a more potent second-generation small molecule inhibitor.