The epothilones: new therapeutic agents for castration-resistant prostate cancer

The management of castration-resistant prostate cancer (CRPC) presents a clinical challenge because of limitations in efficacy and durability with currently available therapeutics. The epothilones represent a novel class of anticancer therapy that stabilizes microtubules, causing cell death and tumor regression in preclinical models. The structure of the tubulin-binding site for epothilones is distinct from that of the taxanes. Moreover, preclinical studies suggest nonoverlapping mechanisms of resistance between epothilones and taxanes. In early-phase studies in patients with CRPC, treatment with ixabepilone, a semisynthetic analog of epothilone B, induced objective responses and prostate-specific antigen declines in men previously progressing on docetaxel-based regimens. Clinical activity has been observed in nonrandomized trials for patients with CRPC using ixabepilone in the first- and second-line settings as a single agent and in combination with estramustine. Patupilone and sagopilone were also shown to have promising efficacy in phase II clinical trials of patients with CRPC. All three epothilones appear to be well tolerated, with modest rates of neutropenia and peripheral neuropathy. The lack of crossresistance between epothilones and taxanes may allow sequencing of these agents. Evaluating epothilones in phase III comparative trials would provide much-needed insight into their potential place in the management of patients with CRPC.

The epothilones: how pharmacology relates to clinical utility

OBJECTIVE

To review the pharmacologic properties of a novel class of chemotherapeutic agents, the epothilones, and to summarize findings from recent clinical trials investigating the various epothilones in cancer therapy.

DATA SOURCES

Literature searches were conducted using MEDLINE, PubMed, and the abstract search engines for the American Society of Clinical Oncology and American Association for Cancer Research annual meetings (all searches through November 2008). Primary search terms included epothilone, BMS-247550, ixabepilone, EPO906, patupilone, sagopilone, and ZK-EPO.

STUDY SELECTION AND DATA EXTRACTION

Publications were given priority for inclusion if they discussed structural or pharmacologic properties of the epothilones as a class or if they included preclinical or clinical data for epothilones currently in clinical development.

DATA SYNTHESIS

The epothilones are a novel class of microtubule-stabilizing agents (MSAs). Epothilones are structurally and pharmacologically distinct from taxanes, but the exact ways in which the pharmacophores of the 2 classes differ has not been firmly established. A number of natural, semisynthetic, and fully synthetic epothilones are in various stages of clinical development. These agents differ from each other and from existing MSAs; these differences influence potency, stability, and solubility. Ixabepilone is currently approved to treat multidrug-resistant metastatic breast cancer and has demonstrated efficacy in earlier stages of breast cancer and in several other tumor types. Patupilone and sagopilone are currently under clinical investigation and have each shown promise in a number of treatment settings and tumor types. All 3 agents appear to be associated with manageable toxicities, but no class-wide toxicity profile exists for the epothilones and dose-limiting toxicities differ among the agents.

CONCLUSIONS

The epothilones have demonstrated significant potential for addressing the growing therapeutic challenge of taxane resistance, and the ever-increasing pool of information regarding structure-activity relationships of these MSAs will help to optimize microtubule-targeted chemotherapy.