CG’806 is a highly potent, non-covalent small molecule therapeutic agent, exhibiting a picomolar IC50 toward the FMS-like tyrosine kinase 3 with the Internal Tandem Duplication (FLT3-ITD and single-digit nanomolar IC50’s against Bruton’s tyrosine kinase (BTK) and its C481S mutant (BTK-C481S)). Further, CG’806 is a multi-targeted BTK / FLT3-ITD inhibitor as it impacts other relevant oncogenic targets, including the Aurora kinases (AURK), RET, MET, DDR2, and SRC kinases.
As a potent inhibitor of FLT3-ITD and other mutant forms of FLT3, CG’806 may become an effective therapy in this subset of acute myeloid leukemia (AML) patients. FLT3-ITD occurs in approximately 30% of patients with AML. Importantly, CG’806 targets other oncogenic kinases which may also be operative in FLT3-ITD AML, thereby potentially allowing the agent to become an important therapeutic option for a difficult-to-treat patient population.
The C481S mutation of BTK arises from therapy with covalent, irreversible BTK inhibitors that target the active site cysteine residue of BTK, thereby conferring resistance to other covalent BTK inhibitors. As a non-covalent, reversible inhibitor, CG’806 does not rely on the cysteine 481 residue (Cys481) for binding in the active site of BTK. Consequently, patients relapsed or refractory to other commercially approved or development stage BTK inhibitors with chronic lymphocytic leukemia (CLL) or mantle cell lymphoma (MCL) may continue to be sensitive to CG’806 therapy.
Role of BTK in B-cell signaling
BTK, a member of the TEC family kinase, is an essential element of B-cell receptor (BCR) signaling, which is required for B-cell maturation, survival and proliferation. It is an upstream activator of multiple pro-survival / anti-apoptotic pathways, including the NF-KB, mTOR-AKT and ERK pathways. BTK is overexpressed in malignant cells from patients with various B-cell malignancies, such as CLL, MCL, AML, and diffuse large b-cell lymphoma (DLBCL). Disruption of BCR signaling via inhibition of BTK, has been shown to lead to clinical remissions in these patients.
CG’806 as a non-covalent, reversible kinase inhibitor
Binding studies of CG’806 have confirmed non-covalent, reversible inhibition of BTK, FLT3-ITD and Aurora Kinase A. Commercially-approved, covalent BTK inhibitors possess a Michael acceptor to react with Cys481 in BTK and irreversibly inactivate the BTK enzyme. In contrast, CG’806 does not require reactivity with the Cys481 residue for inhibition of the BTK enzyme, thereby allowing CG’806 to inhibit the wild type and Cys481 mutant form of the BTK enzyme.
Preclinical In Vitro Evaluation of CG’806
CG’806 is a potent inhibitor of BTK and FLT3 wild types, as well as the BTK C481S and FLT3-ITD mutants, which represent major sources of therapy relapse or are negative prognostic signals in patients. In enzymatic assays, CG’806 has demonstrated best-in class potency against the BTK C481S mutant with an IC50 of 2.5 nM. CG’806 also has potent activity against the FLT-ITD mutation, occurring in 30-35% of AML patients, with an IC50 against the purified enzyme of 0.8 nM (800pM). Similarly, CG’806 demonstrated picomolar potency against Aurora A (IC50 0.4 nM). Notably, CG’806 is a potent inhibitor of interleukin-2-inducible T-cell kinase (ITK), at approximately 4 nM. ITK is speculated to play a role in suppressing activated T-cell function, hence inhibition of ITK alleviates this suppression, and provide for a potential immunomodulatory anti-tumor mechanism. Finally, CG’806 does not exhibit any inhibition of EGFR. EGFR inhibition has been speculated to contribute to the toxicity observed from the commercially approved BTK inhibitor.
BTK is overexpressed in the blast cells of approximately 80% of AML patients as compared to normal PBMCs in healthy subjects. Researchers have shown that BTK inhibition attenuates the proliferation and survival of FLT3-ITD primary AML blasts and AML cell lines, as well as inhibits the downstream activation FLT3-ITD-dependent Myc and STAT5 kinases. CG’806 is the only drug in development that inhibits both FLT3-ITD and BTK pathways reported to synergize to drive the proliferation and survival of AML.
In vivo subcutaneous AML tumor models of anti-cancer efficacy revealed CG’806 induced rapid and sustained tumor eradication (Figure 1). CG’806 was administered orally once daily, for 14 days. Moreover, CG’806 exhibited the sustained tumor elimination post therapy, while demonstrating no impact to murine body weight, no impacts to hematology cell counts or visible organ toxicities – necropsy and clinical pathology findings did not reveal any abnormal observations. A maximum tolerated dose has not yet been identified with murine xenograft studies, having been performed up to 450 mg/kg orally for 14 days (CrystalGenomics preliminary toxicity data).
Figure 1. Efficacy of CG’806 in MV4-11 xenograft model.
MV4-11 tumor bearing mice were administered an oral suspension once daily for 14 days of CG’806 at 2 mg/kg (blue line), 10 mg/kg (green line) or 100 mg/kg (red line), Ibrutinib, 12 mg/kg (turquoise line), or vehicle (Control; black line) with 7-day post-treatment follow-up. Tumor volumes and body weights were measured 3 times weekly.
A PCT application providing composition of matter and use protection for CG’806 was filed in late-2013, with a potential expiry in 2033 before extension opportunities, across all major geographies.
CG’806 is currently in IND-enabling studies and Aptose expects to file an IND for a first-in-human clinical trial by late-2017. CG’806 is being developed as a once-daily, oral therapeutic.
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