Anticancer Activity of a Highly Potent Small Molecule Tubulin Polymerization Inhibitor, AB8939

We have identified the small chemical molecule AB8939 as being a structurally novel, synthesized tubulin inhibitor that can circumvent resistance mechanisms known to limit the effectiveness of existing tubulin inhibitors; e.g., P-glycoprotein (Pgp) and myeloperoxidase (MPO) mediated resistance. A series of in vitro preclinical studies provide proof-of-concept that AB8939 has broad applicability as a potent anticancer drug, particularly in tumors of hematopoietic and lymphoid tissues, including acute myeloid leukemia (AML).

Regarding mechanism of action, x-ray crystallography demonstrated that AB8939 binds to the colchicine-binding site on the beta-subunit of tubulin. Cell cycle arrest in the G2/M phase was evaluated using HCT116 cells (a human colorectal tumor), treated at various concentrations of AB8939 for 24 hours. It was seen that AB8939 produced a strong mitotic arrest at the sub-micromolar concentration range (90% of cells in G2/M phase at 10 nM), which was of comparable strength to that of established microtubule targeting agents, each at a concentration of 100 nM. Additional assays using cytarabine (Ara-C) resistant MOLM14 AML cells confirmed this activity, also demonstrating dose dependent (2 to 20 nM) G2/M phase cell cycle arrest in patient-derived AML blasts and that G2/M cell cycle arrest lead to cellular death by apoptosis at nanomolar concentrations.

The effect of AB8939 (100 nM) on the integrity of the microtubule and actin networks was tested in 3T3NIH cells (murine embryonic fibroblast cell line). AB8939 induced a rapid (within 1 hour) and radical destabilization of the microtubule network but did not affect the actin network. Similarly, destabilization of the microtubule network was observed in human primary cardiomyocytes and primary human lung fibroblast cells treated for 24 hours at 10 to 1000 nM AB8939. Further in vitro analysis showed that AB8939 produces a direct and potent, dose-dependent depolymerization effect (50% inhibition of in vitro microtubule polymerization at around 1 µM, with 100% inhibition at >5 µM).

The potential of AB8939 to overcome resistance to chemotherapeutic agents in Pgp-dependent multidrug-resistant cell lines was assessed using the drug-sensitive human sarcoma cell line MES-SA (parental) and its multidrug-resistant counterparts MES-SA/MX2 and MES-SA/Dx5 in a 6-day proliferation/survival assay. AB8939 efficiently inhibited each of these cells with an IC₅₀ ≤10 nM. By comparison, the MES-SA/MX2 and MES-SA/Dx5 cell lines were highly resistant to the chemotherapeutic agents of doxorubicin and vincristine, as compared with the effect on parental cells (IC₅₀ <1.5 - 2.0 µM versus 20 nM, respectively). Additional tests showed that AB8939 is a very poor substrate of Pgp efflux pump, comparable with combretastatin-4, and therefore has the potential to overcome multidrug resistance in cancer patients.

The anti-proliferative activity of AB8939 in various hematopoietic tumors and solid tumors was evaluated using a colorimetric cell proliferation and viability assay. AB8939 produced good anti-tumor activity after 72 hours (IC₅₀ of ≤50 nM) in 19 hematopoietic tumor cell lines tested, including AML (3 cell lines), B cell lymphoma (8 cell lines), T cell lymphoma (6 cell lines), and multiple myeloma (2 cell lines). AB8939 also showed good anti-tumor activity after 6 days (IC₅₀ of ≤10 nM) in several solid tumor cell lines, including breast, colon, glioblastoma, head and neck, lung, kidney, melanoma neuroblastoma, ovary, pancreas and prostate cell lines.

The therapeutic potential of AB8939 in refractory/resistant AML was investigated further on doxorubicin-resistant AML cell lines (HL60 and U937), doxorubicin being a commonly used AML induction drug and Pgp substrate. AB8939 produced a strong anti-proliferative effect in both cell lines whereas both were resistant to doxorubicin, thus demonstrating AB8939's potential to overcome refractory/resistant AML. Notably, HL60 and U937 are respectively MPO-positive and MPO-negative, indicating that unlike vinca alkaloids (e.g. vincristine or vinblastine) AB8939 it is not deactivated by this myeloid enzyme.

These data show that AB8939 is a prolific and highly potent (nanomolar concentrations) Pgp-independent, next-generation microtubule-destabilizer drug for cancer therapy; in particular, difficult to treat hematopoietic tumors such as relapsed/refractory AML.