Development of novel chemotherapeutic agents to evade the mechanisms of multidrug resistance (MDR).

A number of novel microtubule-targeting agents are currently under investigation. These agents can potentially evade the mechanisms underlying the development of the multidrug resistance (MDR) phenotype commonly associated with recurrent breast cancer. Epothilones are among the most advanced of the new agents in clinical development. Structurally unrelated to taxanes, epothilones may be poor substrates for MDR, and the expression of MDR proteins is not altered in epothilone-resistant in vitro models. Cross resistance between epothilones and taxanes is not observed in vitro or in vivo. Ixabepilone (BMS-247550) is a semisynthetic analog of epothilone B that has shown antitumor activity both in vitro and in vivo, including taxane-resistant human tumor xenograft models. Ixabepilone is currently being studied in phase III trials in patients with metastatic breast cancer as monotherapy and in combination with capecitabine. Activity has also been observed in other solid tumors. Patupilone (EPO906, epothilone B) and epothilone D (KOS-862) are in early phase I/II clinical studies in patients with a variety of solid tumors. The development of these novel agents may evade MDR and may improve the outcome of patients with breast cancer.