Epothilones as lead structures for new anticancer drugs--pharmacology, fermentation, and structure-activity-relationships

The Progress of the Anticancer Agents Related to the Microtubules Target

Anticancer drugs based on the microtubules target are potent mitotic spindle poison agents, which interact directly with the microtubules, and were classified as microtubule-stabilizing agents and microtubule-destabilizing agents. Researchers have worked tremendously towards the improvements of anticancer drugs, in terms of improving the efficacy, solubility and reducing the side effects, which brought about advancement in chemotherapy. In this review, we focused on describing the discovery, structures and functions of the microtubules as well as the progress of anticancer agents related to the microtubules, which will provide adequate references for researchers.

Quantitative method for the determination of iso-fludelone (KOS-1803) in human plasma by LC-MS/MS

Epothilones are relatively new tubulin-poison anticancer drugs. Iso-fludelone (KOS-1803) is a synthetic third generation epothilone drug discovered at Memorial Sloan Kettering Cancer Center, and currently in phase I clinical trials. We report an LC-MS/MS assay for the sensitive, accurate and precise quantitation of iso-fludelone in 0.2mL of human plasma. Validation was performed according to FDA guidance. The assay comprised of KOS-1724 as the internal standard and an MTBE liquid-liquid extraction with a water wash step. Separation was achieved with an YMC-Pack ODS-AQ column and an isocratic mobile phase of 0.1% formic acid in acetonitrile and water (70:30, v/v) at 0.3mL/min for 4min. Chromatographic separation was followed by electrospray, positive-mode ionization tandem mass spectrometric detection in the multiple reaction monitoring (MRM) mode. The assay was linear from 0.1 to 300ng/mL and was accurate (-9.41 to -7.07%) and precise (1.03-13.7%) which fulfilled FDA criteria for validation. Recovery from plasma was 73.9-79.7% and ion suppression was negligible (-22.8 to -31.3%). Plasma freeze-thaw stability (99.97-105.7%), stability for 11 months at -80°C (94.93-107.9%), and stability for 6h at room temperature (94.75-105.5%) were all acceptable. This assay is currently being applied to quantitate iso-fludelone in clinical samples.

Pretubulysin: a new option for the treatment of metastatic cancer

Tubulin-binding agents such as taxol, vincristine or vinblastine are well-established drugs in clinical treatment of metastatic cancer. However, because of their highly complex chemical structures, the synthesis and hence the supply issues are still quite challenging. Here we set on stage pretubulysin, a chemically accessible precursor of tubulysin that was identified as a potent microtubule-binding agent produced by myxobacteria. Although much simpler in chemical structure, pretubulysin abrogates proliferation and long-term survival as well as anchorage-independent growth, and also induces anoikis and apoptosis in invasive tumor cells equally potent to tubulysin. Moreover, pretubulysin posseses in vivo efficacy shown in a chicken chorioallantoic membrane (CAM) model with T24 bladder tumor cells, in a mouse xenograft model using MDA-MB-231 mammary cancer cells and finally in a model of lung metastasis induced by 4T1 mouse breast cancer cells. Pretubulysin induces cell death via the intrinsic apoptosis pathway by abrogating the expression of pivotal antiapoptotic proteins, namely Mcl-1 and Bcl-x_L, and shows distinct chemosensitizing properties in combination with TRAIL in two- and three-dimensional cell culture models. Unraveling the underlying signaling pathways provides novel information: pretubulysin induces proteasomal degradation of Mcl-1 by activation of mitogen-activated protein kinase (especially JNK (c-Jun N-terminal kinase)) and phosphorylation of Mcl-1, which is then targeted by the SCF^Fbw7 E3 ubiquitin ligase complex for ubiquitination and degradation. In sum, we designate the microtubule-destabilizing compound pretubulysin as a highly promising novel agent for mono treatment and combinatory treatment of invasive cancer.

The myxocoumarins A and B from Stigmatella aurantiaca strain MYX-030

The myxobacterial strain Stigmatella aurantiaca MYX-030 was selected as promising source for the discovery of new biologically active natural products by our screening methodology. The isolation, structure elucidation and initial biological evaluation of the myxocoumarins derived from this strain are described in this work. These compounds comprise an unusual structural framework and exhibit remarkable antifungal properties.

Novel microtubule-targeting agents - the epothilones

Epothilones are a new class of antimicrotubule agents currently in clinical trials. Their chemical structures are distinct from taxanes and are more amenable to synthetic modification. Six epothilones have been studied in preclinical and clinical trials: patupilone (epothilone B), ixabepilone (BMS247550), BMS 310705, sagopilone (ZK-EPO), KOS-862 (epothilone D), and KOS-1584. In vitro data have shown increased potency in taxane-sensitive and taxane-resistant cancer cell lines. This enhanced cytotoxic effect has been attributed to epothilone being a poor substrate for p-glycoprotein drug resistance protein and having high affinity to the various beta tubulin isoforms. Phase I clinical data have shown different dose-limiting toxicities for each of the epothilones. These effects are drug specific, dose specific, and schedule of administration specific. While diarrhea and myelosuppression are the dose-limiting toxicities for patupilone and BMS 310705, respectively, neurologic toxicity, as seen with taxanes, is the dose-limiting toxicity of ixabepilone, sagopilone, and KOS-862. In an effort to decrease neurologic toxicity, investigators have modified dosing schedules with limited success. Ixabepilone has the most mature clinical results with published phase II and III data, and regulatory approval for clinical use in the treatment of breast cancer. Ixabepilone has also been combined with other anticancer agents and has regulatory approval in combination with capecitabine for heavily treated breast cancer.