Ixabepilone, first in a new class of antineoplastic agents: The natural epothilones and their analogues

Although targeted therapies are becoming increasingly important in oncology, cytotoxic agents are likely to remain a valuable element in the treatment paradigm of cancer. However, resistance to chemotherapy is a major obstacle to the successful treatment of cancer. Therefore, there is a need for novel antineoplastic agents that are able to overcome mechanisms of tumor resistance. Drugs that target microtubules, including paclitaxel and docetaxel, are among the most commonly prescribed anticancer therapies. However, the utility of taxane-based therapies is limited by difficulties with formulation, administration, and resistance induced by P-glycoprotein. The epothilones and their analogues are a novel class of antimicrotubule agents that has demonstrated antitumor activity in the setting of resistance. These antimicrotubule agents are structurally unrelated to the taxanes, with a distinct P-tubulin-binding mode. Ixabepilone is a rationally designed, semisynthetic analogue of natural epothilone B, which displays reduced susceptibility to a range of common tumor resistance mechanisms. Promising phase II activity and a manageable safety profile with ixabepilone have been seen in a wide range of tumor types, including heavily pretreated/resistant and early-stage breast cancer. Moreover, encouraging phase III results with ixabepilone and capecitabine for patients with breast cancer have recently been presented. Clinical trials are also planned for ixabepilone in combination with targeted agents, such as trastuzumab and bevacizumab. Ixabepilone is likely to be the first available drug in its class, with the potential to bring clinical benefit to patients with a wide range of malignancies.