Conference Coverage

Inhibitor outperforms rivals in leukemia, lymphoma


Photo by Rhoda Baer

Researchers in the lab

CHICAGO—Preclinical research suggests the pan-FLT3/pan-BTK inhibitor CG’806 is more effective than other kinase inhibitors in fighting certain hematologic malignancies.

In one study, CG’806 proved more potent than comparator drugs in primary samples of acute myeloid leukemia (AML) and chronic lymphocytic leukemia (CLL).

In another study, CG’806 demonstrated greater cytotoxicity than comparators in a range of malignant B cell lines.

Data from both studies were presented at the AACR Annual Meeting 2018 (abstracts 791 and 794).

The research was supported by Aptose Biosciences, Inc., the company developing CG’806.

CG’806 is a small molecule that inhibits wild-type (WT) FLT3, as well as FLT3 housing the ITD mutation or with point mutations in the tyrosine kinase domain (TKD, including D835G, D835Y, D835H) or in the gatekeeper region (F691L). CG’806 also inhibits BTK-WT and BTK-C481S.

Abstract 791

Stephen E. Kurtz, PhD, of Oregon Health & Science University in Portland, and his colleagues presented results with CG’806 in primary patient samples.

The team found that CG’806 demonstrated greater potency against AML samples relative to other FLT3 inhibitors.

Median IC50 values in 188 AML patient samples were 0.0765 µM for CG’806, 0.125 µM for gilteritinib, 0.199 µM for quizartinib, 0.551 µM for dovitinib, 2.25 µM for midostaurin, 2.93 µM for sorafenib, and 5.01 µM for crenolanib.

The researchers said CG’806 sensitivity was enhanced in FLT3-ITD and FLT3-TKD positive cases.

In CLL patient samples, CG’806 exhibited greater potency and a greater range of activity than the BTK inhibitor ibrutinib. Across 95 CLL samples, the median IC50 values were 0.114 µM for CG’806 and 4.09 µM for ibrutinib.

The researchers said this greater potency of CG’806 may be due to the activity of CG’806 on CSF1R, which has been identified as a therapeutic target in CLL.

“The clinical benefit of current FLT3 inhibitors in AML is transient, as resistance develops after several months of treatment,” Dr Kurtz noted. “Similarly, ibrutinib . . . is limited by acquired resistance as well as refractory disease and tolerance challenges. As a pan-FLT3/pan-BTK inhibitor . . ., CG’806 offers important potential to address these limitations.”

Abstract 794

Hongying Zhang, MD, PhD, of Aptose Biosciences, and her colleagues presented results with CG’806 in malignant B-cell and AML cell lines.

The researchers found that CG’806 inhibited FLT3-ITD signaling and induced apoptosis more effectively than quizartinib in FLT3-ITD AML cells (MV4-11). The team noted that CG’806 caused G0/G1 cell-cycle arrest in the cells.

CG’806 also exhibited greater cytotoxic activity than quizartinib in FLT3-WT AML cell lines (KG-1 and NOMO-1).

In addition, CG’806 was more potent than quizartinib, gilteritinib, and crenolanib in Ba/F3 cells transfected with FLT3-WT, ITD, D835Y, and ITD-F691. CG’806 was more potent than quizartinib and crenolanib—but not gilteritinib—in Ba/F3 cells transfected with FLT3-ITD-D835Y.

The researchers said they found that CG’806 inhibits BTK, AURK, and downstream signals in FLT3-WT AML cells.

The team also found that CG’806 decreased BTK phosphorylation in all tested cell lines of B-cell malignancies. This included acute lymphoblastic leukemia, mantle cell lymphoma, Burkitt lymphoma, diffuse large B-cell lymphoma, and follicular lymphoma cell lines.

Across all cell lines, CG’806 killed malignant B cells more effectively than ibrutinib. And CG’806 was “equally potent” against WT and C481S-mutant BTK, according to the researchers.

The team also said CG’806 inhibited AURK and induced polyploidy in B-cell malignancies.

“[C]G’806 has demonstrated the ability to kill a broad range of AML and B-cell malignancies through inhibition of multiple oncogenic pathways,” said William G. Rice, PhD, chairman and chief executive officer of Aptose.

“These studies are critical for understanding how to develop and position CG’806 as we prepare for clinical development in these challenging hematologic malignancies.”

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