The Highly Potent Bromodomain (BRD) Inhibitor FV-281 Displays Preclinical Efficacy in Acute Myeloid Leukemia (AML)

Recent studies have demonstrated the therapeutic potential of targeting BRD2, BRD3 and BRD4 in hematological malignancies including AML. Here, we report a novel orally bioavailable BRD inhibitor, FV-281, that displays preclinical efficacy in AML in vitro and in vivo.

In a binding screen assay of 32 bromodomains (BromoScan^TM), FV-281 selectively bound BRD2, BRD3 and BRD4 with K_d values of 2.7 to 7.3 nM and was 2-5 fold more potent than the leading bromodomain inhibitor, OTX015. In AML cell lines (OCI-AML2, Tex, HL-60 and MV4-11), FV-281 reduced cell growth and viability after 72 hour incubation with IC₅₀ values < 50 nM and was 6-fold more potent than OTX015. FV-281 was not cytotoxic to normal hematopoietic cells (n=4) with all IC₅₀ values > 10 µM. In contrast, FV-281 induced cell death as measured by Annexin V/PI staining in 3 of 4 primary AML samples with IC₅₀values < 850 nM.

Within 4 hours of incubation with FV-281, c-Myc mRNA and protein level in OCI-AML2 cells were reduced by >95% and >80%, respectively. Following FV-281 washout, c-Myc protein expression returned to baseline within 20 hours with no loss of cell viability observed. Thus, the FV-281 is a reversible bromodomain inhibitor and sustained target inhibition is required to induce cell death. Likewise, after 6 and 48 hours of incubation with OCI-AML2 cells, FV-281 reduced Bcl-2 mRNA and protein level by >60% and >90%, respectively.

To assess in vivo efficacy and toxicity, MV4-11 AML cells were engrafted subcutaneously in NOD-SCID mice (n=10). Once tumors were palpable, mice were treated with FV-281 orally (30mg/kg/d x 2 weeks) or vehicle control. FV-281 suppressed tumor growth without evidence of overt toxicity. In addition, primary AML cells were engrafted into the femurs of NOD-SCID mice (n=10). Three weeks after implantation, mice were treated with FV-281 orally (30mg/kg, 5 of 7 days, x 4 weeks) or vehicle control. FV-281 decreased AML engraftment in the injected and non-injected femur without evidence of overt toxicity (% human CD45⁺/CD19^-/CD33⁺ in non-injected femur treated vs control: 20% vs 60%, p<0.0001).

Thus, FV-281 is a novel oral reversible bromodomain inhibitor with significant in vivo activity in murine models of AML. These data support evaluation of this agent in upcoming phase I clinical trials.