Ixabepilone: a novel antineoplastic agent with low susceptibility to multiple tumor resistance mechanisms.

Tumor resistance to chemotherapeutic agents ultimately leads to treatment failure in the majority of cancer patients. The identification of new agents that are less susceptible to mechanisms of tumor resistance could, therefore, bring significant clinical benefits to patients with advanced cancer. One new drug class of great interest in this respect is the epothilones and their analogues, which are microtubule inhibitors with low susceptibility to several mechanisms of drug resistance. Ixabepilone is an analogue of natural epothilone B with activity against a wide range of tumor types, including drug-resistant tumors. This is consistent with the preclinical activity of ixabepilone against human cancer cell lines resistant to taxanes and other agents. Taxane resistance in these cells may be acquired or primary and results from several mechanisms, such as overexpression of multidrug-resistance proteins and the betaIII-tubulin isoform. Ixabepilone has demonstrated efficacy as monotherapy or in combination with capecitabine in anthracycline- and taxane-pretreated/resistant metastatic breast cancer (MBC), and has recently been approved for use in resistant/refractory MBC. Other epothilones, such as patupilone, KOS-1584, and ZK-EPO, are also being evaluated in drug-resistant cancers. Ixabepilone represents a new treatment option for MBC patients with cancers resistant to available chemotherapeutic agents.