Potent Small-Molecule BCL6 Inhibitor Active against ABC-, GCB-DLBCL Cells

Researchers have identified a novel, potent, and specific BCL6 inhibitor that effectively inhibited tumor growth in vitro and in vivo in activated B-cell (ABC) and germinal center B (GCB)-cell–like diffuse large B-cell lymphoma (DLBCL) preclinical models. Mariano G. Cardenas, PhD, a postdoctoral associate in Dr. Ari Melnick’s laboratory at Weill Cornell Medical College, presented details of the new BCL6 inhibitor, FX-1085, in the “Advances in Genetics” oral abstract session at the 2014 ASH Meeting on Lymphoma Biology.

BCL6 is a common oncogene in DLBCLs and inhibition of the interaction between BCL6 and its co-repressors has the potential to kill lymphoma cells. The BTB domain of BCL6 has been identified as a docking site for co-repressor complex proteins. This co-repressor interaction is a critical mediator of BCL6 oncogenic activity, making it a targetable surface.

Using computer-aided drug design, Dr. Melnick’s group and collaborators had previously identified a small molecule, 79-6, that bound to the BCL6-BTB, targeted the interaction of BCL6 and its co-repressors, and had anti-lymphoma activity in mice.

“Our research with 79-6 showed that this interaction could be disrupted with a small molecule inhibitor, but the inhibitor was not extremely potent,” Dr. Cardenas told ASH Clinical News.

To address this issue, Dr. Cardenas and colleagues applied a novel, computer-aided drug design methodologydeveloped by Alexander MacKerell, PhD, and colleaguescalled “site identification by ligand competitive saturation,” or SILCS. SILCS is a method for identifying a molecule’s binding site by simulating its physical movement. In this case, the researchers simulated the physical movement of the 79-6 molecule in an aqueous solution to identify areas on the BCL6-BTB protein’s surface where it would likely bind.

The SILCS process produced a 3-dimensional map of the 79-6 molecule’s probable binding sites, which Dr. Cardenas and colleagues then used to design 35 compounds to test in vitro.

Of these compounds, FX-1085 was identified as the most potent BCL6-inhibitor; analysis showed that it was at least 10 times more effective than 79-6 (the lead compound).

FX-1085 works by disrupting the formation of BCL6-co-repressor complexes, as well as specifically re-activating BCL6 target genes, the investigators explained. They also used RNA sequencing and gene set enrichment analysis to better understand how FX-1085 represses RNA transcription; results showed that treatment with FX-1085 mimicked the gene-expression changes induced by BCL6 depletion.

To confirm the findings of their in vitro tests and assess the specific lymphoma-killing effect of FX-1085, Dr. Cardenas and colleagues added the compound to 12 GCB and ABC-DLBCL cell lines that are BCL6-dependent and four cell lines that are BCL6-independent (as controls). As expected, the FX-1085 drug had a selective inhibitory effect on the BCL6-dependent cell lines, but not in those that do not rely on BCL6 to survive.

Finally, Dr. Cardenas and colleagues conducted an in vivo study of FX-1085 using a xenograft model in SCID mice. The compound was administered at 25 mg/kg and 50 mg/kg per day for 10 days. FX-1085 50 mg/kg induced 95 percent tumor regression in OCI-Ly7 (p=0.0016) and SU-DHL-6 (p=0.0050). In the more aggressive ABC-DLBCL cell line, FX-1085 50 mg/kg induced about 70 percent tumor regression. Notably, there were no treatment-related toxicities to normal tissues.

“FX-1085 is a more potent compound that binds more strongly to the [BCL6-BTB] protein than to the natural ligand and was very potent in vitro, in vivo, and in ex vivo patient samples,” Dr. Cardenas said.


Reference

    Cardenas MG. Identification of targetable functional groups on BCL6 dimer allows rational design of potent and selective inhibitors with activity in GCB- and ABC-DLBCLs. Presented at: ASH Meeting on Lymphoma Biology; August 10-13; Colorado Springs, CO.

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