Abstract

Agents that interact with cytoskeletal elements such as tubulin include synthetic spiroketal pyrans (SPIKET), targeting the spongistatin binding site of β- tubulin, and monotetrahydrofuran compounds (COBRA compounds), targeting a unique binding cavity on α-tubulin. At nanomolar concentrations, the SPIKET compound SPIKET-P caused tubulin depolymerization and demonstrated potent cytotoxic activity against cancer cells. COBRA-1 inhibited GTP-induced tubulin polymerization. Treatment of human breast cancer and brain tumor cells with COBRA-1 caused destruction of microtubule organization and apoptosis. Other agents that have shown promise for cancer treatment include phorboxazoles, natural products that are extremely cytostatic towards the National Cancer Institutes panel of 60 tumor cell lines. In standard MTT assays, synthetic phorboxazole A exhibited potent cytotoxicity against NALM-6 acute lymphoblastic leukemia cells (IC50 = 1.7 nM), BT- 20 breast cancer cells (IC50 = 3.4 nM), and U373 glioblastoma cells (IC50 = 6.7 nM). Structure-activity studies were reported for seven synthetic analogs of phorboxazole A. Out of these, two showed potent anti-cancer activity. Phorboxazole analog 2 was active against NALM-6 cells (IC50 = 4.8 nM), BT-20 cells (IC50 = 12.6 nM) and U373 cells (IC50 = 27.4 nM), as was analog 3 (NALM-6 IC50 = 5.2 nM, BT-20 IC50 = 11.3 nM, and U373 IC50 = 29.2 nM). Anticancer activity of the phorboxazole analogs was correlated to the presence of certain structural moieties such as portions of the macrolide group, the central oxazole group, and the polyene side chain. The requirement of more than one structural element for activity suggested that at least bimodal interactions of the natural product with key cellular components may occur. Promising anti-mitotic agents with pro-apoptotic activity include inhibitors of the tyrosine kinase BTK. The leflunomide metabolite analog LFM-A13 inhibited BTK in leukemia and lymphoma cells (IC50 = 17 μM). Consistent with the anti-apoptotic function of BTK, treatment of leukemic cells with LFM-A13 enhanced their sensitivity to chemotherapy-induced apoptosis.