Sagopilone (ZK-EPO): from a natural product to a fully synthetic clinical development candidate

Cytoskeletal proteins as glioblastoma biomarkers and targets for therapy: A systematic review

Glioblastoma, the most common primary brain malignancy, is an exceptionally fatal cancer. Lack of suitable biomarkers and efficient treatment largely contribute to the therapy failure. Cytoskeletal proteins are crucial proteins in glioblastoma pathogenesis and can potentially serve as biomarkers and therapeutic targets. Among them, GFAP, has gained most attention as potential diagnostic biomarker, while vimentin and microtubules are considered as prospective therapeutic targets. Microtubules represent one of the best anti-cancer targets due to their critical role in cell proliferation. Despite testing in clinical trials, the efficiency of taxanes, epothilones, vinca-domain binding drugs, colchicine-domain binding drugs and γ-tubulin binding drugs remains to be confirmed. Moreover, tumor treating field that disrupts microtubules draw attention because of its high efficiency and is called "the fourth cancer treatment modality". Thereby, because of the involvement of cytoskeleton in key physiological and pathological processes, its therapeutic potential in glioblastoma is currently extensively investigated.

Structural Mechanism of Regioselectivity in an Unusual Bacterial Acyl-CoA Dehydrogenase

Terminal alkenes are easily derivatized, making them desirable functional group targets for polyketide synthase (PKS) engineering. However, they are rarely encountered in natural PKS systems. One mechanism for terminal alkene formation in PKSs is through the activity of an acyl-CoA dehydrogenase (ACAD). Herein, we use biochemical and structural analysis to understand the mechanism of terminal alkene formation catalyzed by an γ,δ-ACAD from the biosynthesis of the polyketide natural product FK506, TcsD. While TcsD is homologous to canonical α,β-ACADs, it acts regioselectively at the γ,δ-position and only on α,β-unsaturated substrates. Furthermore, this regioselectivity is controlled by a combination of bulky residues in the active site and a lateral shift in the positioning of the FAD cofactor within the enzyme. Substrate modeling suggests that TcsD utilizes a novel set of hydrogen bond donors for substrate activation and positioning, preventing dehydrogenation at the α,β position of substrates. From the structural and biochemical characterization of TcsD, key residues that contribute to regioselectivity and are unique to the protein family were determined and used to identify other putative γ,δ-ACADs that belong to diverse natural product biosynthetic gene clusters. These predictions are supported by the demonstration that a phylogenetically distant homologue of TcsD also regioselectively oxidizes α,β-unsaturated substrates. This work exemplifies a powerful approach to understand unique enzymatic reactions and will facilitate future enzyme discovery, inform enzyme engineering, and aid natural product characterization efforts.

Epothilones

Epothilones A and B are naturally occurring microtubule stabilizers with nanomolar or even sub-nanomolar activity against human cancer cells in vitro and potent in vivo antitumor activity against multidrug-resistant tumors. Over the last decade, ten epothilonetype agents have entered clinical trials in humans; of these, the epothilone B lactam ixabepilone (BMS-247550; Ixempra®) was approved by the FDA for breast cancer treatment in 2007. Numerous synthetic and semisynthetic analogs of epothilones have been prepared and their in vitro and (in selected cases) in vivo biological activity has been determined, producing a wealth of SAR information on this compound family. This chapter will provide a brief summary of the in vitro and in vivo biological properties of epothilone B (Epo B). The major part of the discussion will then be organized around those epothilone analogs that have entered clinical development. For each analog the underlying synthetic chemistry and the most important preclinical features will be reviewed, together with the properties of some important related structures.

Microtubule-stabilizing agents as potential therapeutics for neurodegenerative disease

Microtubules (MTs), cytoskeletal elements found in all mammalian cells, play a significant role in cell structure and in cell division. They are especially critical in the proper functioning of post-mitotic central nervous system neurons, where MTs serve as the structures on which key cellular constituents are trafficked in axonal projections. MTs are stabilized in axons by the MT-associated protein tau, and in several neurodegenerative diseases, including Alzheimer's disease, frontotemporal lobar degeneration, and Parkinson's disease, tau function appears to be compromised due to the protein dissociating from MTs and depositing into insoluble inclusions referred to as neurofibrillary tangles. This loss of tau function is believed to result in alterations of MT structure and function, resulting in aberrant axonal transport that likely contributes to the neurodegenerative process. There is also evidence of axonal transport deficiencies in other neurodegenerative diseases, including amyotrophic lateral sclerosis and Huntington's disease, which may result, at least in part, from MT alterations. Accordingly, a possible therapeutic strategy for such neurodegenerative conditions is to treat with MT-stabilizing agents, such as those that have been used in the treatment of cancer. Here, we review evidence of axonal transport and MT deficiencies in a number of neurodegenerative diseases, and summarize the various classes of known MT-stabilizing agents. Finally, we highlight the growing evidence that small molecule MT-stabilizing agents provide benefit in animal models of neurodegenerative disease and discuss the desired features of such molecules for the treatment of these central nervous system disorders.

Microtubule stabilizing agents as potential treatment for Alzheimer's disease and related neurodegenerative tauopathies

The microtubule (MT) associated protein tau, which is highly expressed in the axons of neurons, is an endogenous MT-stabilizing agent that plays an important role in axonal transport. Loss of MT-stabilizing tau function, caused by misfolding, hyperphosphorylation, and sequestration of tau into insoluble aggregates, leads to axonal transport deficits with neuropathological consequences. Several in vitro and preclinical in vivo studies have shown that MT-stabilizing drugs can be utilized to compensate for the loss of tau function and to maintain/restore effective axonal transport. These findings indicate that MT-stabilizing compounds hold considerable promise for the treatment of Alzheimer disease and related tauopathies. The present article provides a synopsis of the key findings demonstrating the therapeutic potential of MT-stabilizing drugs in the context of neurodegenerative tauopathies, as well as an overview of the different classes of MT-stabilizing compounds.

5-alpha-reductase type I (SRD5A1) is up-regulated in non-small cell lung cancer but does not impact proliferation, cell cycle distribution or apoptosis

BACKGROUND

Non-small cell lung cancer (NSCLC) is one of the most frequent malignancies and has a high mortality rate due to late detection and lack of efficient treatments. Identifying novel drug targets for this indication may open the way for new treatment strategies. Comparison of gene expression profiles of NSCLC and normal adjacent tissue (NAT) allowed to determine that 5-alpha-reductase type I (SRD5A1) was up-regulated in NSCLC compared to NAT. This raised the question whether SRD5A1 was involved in sustained proliferation and survival of NSCLC.

METHODS

siRNA-mediated silencing of SRD5A1 was performed in A549 and NCI-H460 lung cancer cell lines in order to determine the impact on proliferation, on distribution during the different phases of the cell cycle, and on apoptosis/necrosis. In addition, lung cancer cell lines were treated with 4-azasteroids, which specifically inhibit SRD5A1 activity, and the effects on proliferation were measured. Statistical analyses using ANOVA and post-hoc Tamhane-T2-test were performed. In the case of non-parametric data, the Kruskal-Wallis test and the post-hoc Mann-Whitney-U-test were used.

RESULTS

The knock-down of SRDA51 expression was very efficient with the SRD5A1 transcripts being reduced to 10% of control levels. Knock-down efficiency was furthermore confirmed at the protein level. However, no effect of SRD5A1 silencing was observed in the proliferation assay, the cell cycle analysis, and the apoptosis/necrosis assay. Treatment of lung cancer cell lines with 4-azasteroids did not significantly inhibit proliferation.

CONCLUSIONS

In summary, the results suggest that SRD5A1 is not a crucial enzyme for the sustained proliferation of NSCLC cell lines.

Phase I/II study of sagopilone (ZK-EPO) plus carboplatin in women with recurrent platinum-sensitive ovarian cancer

BACKGROUND

Sagopilone is the first fully synthetic epothilone in clinical development and has demonstrated promising preclinical activity. This phase I/II, prospective, open-label trial investigated the efficacy and safety of sagopilone plus carboplatin in patients with recurrent platinum-sensitive ovarian cancer (OC).

METHODS

In phase I (dose-escalation stage), patients with OC recurring at least 6 months after platinum-containing chemotherapy received 3-h infusions of sagopilone (initial dose of 12 mg m(-2)) followed by carboplatin every 3 weeks, for 2-6 treatment courses. Patients enrolled in phase II received 3-h infusions of 16 mg m(-2) sagopilone. Efficacy was assessed using modified Response Evaluation Criteria in Solid Tumors (modRECIST) and Gynecologic Cancer InterGroup CA125 criteria. The safety and tolerability of sagopilone were also evaluated.

RESULTS

In all, 45 patients received sagopilone at 12 mg m(-2) or 16 mg m(-2). There were 29 confirmed tumour responses (21 modRECIST and 8 CA125) across both treatment groups, indicating that the primary objective of the study was reached. The main adverse events (AEs) reported were peripheral neuropathy (75.6%), fatigue (71.1%) and nausea (64.4%). Grade ≥3 AEs occurred in 35 patients (77.8%). No deaths related to the study drug were reported.

CONCLUSION

Sagopilone in combination with carboplatin was effective and toxicities were manageable in patients with recurrent platinum-sensitive OC.

C6-C8 bridged epothilones: consequences of installing a conformational lock at the edge of the macrocycle

A series of conformationally restrained epothilone analogues with a short bridge between the methyl groups at C6 and C8 was designed to mimic the binding pose assigned to our recently reported EpoA-microtubule binding model. A versatile synthetic route to these bridged epothilone analogues has been successfully devised and implemented. Biological evaluation of the compounds against A2780 human ovarian cancer and PC3 prostate cancer cell lines suggested that the introduction of a bridge between C6-C8 reduced potency by 25-1000 fold in comparison with natural epothilone D. Tubulin assembly measurements indicate these bridged epothilone analogues to be mildly active, but without significant microtubule stabilization capacity. Molecular mechanics and DFT energy evaluations suggest the mild activity of the bridged epo-analogues may be due to internal conformational strain.

A phase II study of sagopilone (ZK 219477; ZK-EPO) in patients with breast cancer and brain metastases

Treatments for women with recurrent brain metastases from breast cancer are limited. In this phase II study,we administered sagopilone to patients with breast cancer and brain metastases. We observed modest activity with a central nervous system objective response rate of 13.3%; however, median PFS was disappointing. Further studies should focus on other agents to treat this challenging clinical problem.

BACKGROUND

Patients with progressive metastatic breast cancer to the central nervous system (CNS) have limited treatment options.

PATIENTS AND METHODS

We conducted a phase II study of sagopilone, an epothilone B analogue that crosses the blood-brain barrier, in patients with breast cancer brain metastases. Women were treated with 16 mg/m(2) or 22 mg/m(2) intravenously every 21 days. The primary endpoint was CNS objective response rate (ORR). Secondary endpoints included toxicity, progression-free survival (PFS), and overall survival (OS). Using modified, high-resolution magnetic resonance angiography (MRA), we also evaluated changes in vessel tortuosity with treatment.

RESULTS

Fifteen women were enrolled; all had progressive CNS disease despite whole-brain radiotherapy. Two patients achieved a partial response (ORR, 13.3%) and remained in the study for 6 cycles. Responses were not associated with normalization of tumor-associated vessels on correlative imaging studies. Median PFS and OS were 1.4 months and 5.3 months, respectively. The most common grade 3 toxicities were lymphopenia and fatigue. Enrollment was stopped prematurely because of limited observed activity and slow accrual.

CONCLUSIONS

Sagopilone was associated with modest CNS activity in patients with breast cancer; however median PFS was disappointing. Further studies should examine other potentially active agents and/or combinations for this challenging clinical problem.

A simplified synthesis of novel dictyostatin analogues with in vitro activity against epothilone B-resistant cells and antiangiogenic activity in zebrafish embryos

The natural product (--)-dictyostatin is a microtubule-stabilizing agent that potently inhibits the growth of human cancer cells, including paclitaxel-resistant clones. Extensive structure-activity relationship studies have revealed several regions of the molecule that can be altered without loss of activity. The most potent synthetic dictyostatin analogue described to date, 6-epi-dictyostatin, has superior in vivo antitumor activity against human breast cancer xenografts compared with paclitaxel. In spite of their encouraging activities in preclinical studies, the complex chemical structure of the dictyostatins presents a major obstacle for their development into novel antineoplastic therapies. We recently reported a streamlined synthesis of 16-desmethyl-25,26-dihydrodictyostatins and found several agents that, when compared with 6-epi-dictyostatin, retained nanomolar activity in cellular microtubule-bundling assays but had lost activity against paclitaxel-resistant cells with mutations in β-tubulin. Extending these studies, we applied the new, highly convergent synthesis to generate 25,26-dihydrodictyostatin and 6-epi-25,26-dihydrodictyostatin. Both compounds were potent microtubule-perturbing agents that induced mitotic arrest and microtubule assembly in vitro and in intact cells. In vitro radioligand binding studies showed that 25,26-dihydrodictyostatin and its C6-epimer were capable of displacing [3H]paclitaxel and [14C]epothilone B from microtubules with potencies comparable to (--)-dictyostatin and discodermolide. Both compounds inhibited the growth of paclitaxel- and epothilone B-resistant cell lines at low nanomolar concentrations, synergized with paclitaxel in MDA-MB-231 human breast cancer cells, and had antiangiogenic activity in transgenic zebrafish larvae. These data identify 25,26-dihydrodictyostatin and 6-epi-25,26-dihydrodictyostatin as candidates for scale-up synthesis and further preclinical development.

Overcoming tumor multidrug resistance using drugs able to evade P-glycoprotein or to exploit its expression

Multidrug resistance (MDR) is a major obstacle to the effective treatment of cancer. Cellular overproduction of P-glycoprotein (P-gp), which acts as an efflux pump for various anticancer drugs (e.g. anthracyclines, Vinca alkaloids, taxanes, epipodophyllotoxins, and some of the newer antitumor drugs) is one of the more relevant mechanisms underlying MDR. P-gp belongs to the superfamily of ATP-binding cassette transporters and is encoded by the ABCB1 gene. Its overexpression in cancer cells has become a therapeutic target for circumventing MDR. As an alternative to the classical pharmacological strategy of the coadministration of pump inhibitors and cytotoxic substrates of P-gp and to other approaches applied in experimental tumor models (e.g. P-gp-targeting antibodies, ABCB1 gene silencing strategies, and transcriptional modulators) and in the clinical setting (e.g. incapsulation of P-gp substrate anticancer drugs into liposomes or nanoparticles), a more intriguing strategy for circumventing MDR is represented by the development of new anticancer drugs which are not substrates of P-gp (e.g. epothilones, second- and third-generation taxanes and other microtubule modulators, topoisomerase inhibitors). Some of these drugs have already been tested in clinical trials and, in most of cases, show relevant activity in patients previously treated with anticancer agents which are substrates of P-gp. Of these drugs, ixabepilone, an epothilone, was approved in the United States for the treatment of breast cancer patients pretreated with an anthracycline and a taxane. Another innovative approach is the use of molecules whose activity takes advantage of the overexpression of P-gp. The possibility of overcoming MDR using the latter two approaches is reviewed herein. 

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.

Tubulin-interactive natural products as anticancer agents

This review provides an overview of the discovery, structures, and biological activities of anticancer natural products that act by inhibiting or promoting the assembly of tubulin to microtubules. The emphasis is on providing recent information on those compounds in clinical use or in advanced clinical trials. The vinca alkaloids, the combretastatins, NPI-2358, the halichondrin B analogue eribulin, dolastatin 10, noscapine, hemiasterlin, and rhizoxin are discussed as tubulin polymerization inhibitors, while the taxanes and the epothilones are the major classes of tubulin polymerization promoters presented, with brief treatments of discodermolide, eleutherobin, and laulimalide. The challenges and future directions of tubulin-interactive natural products-based drug discovery programs are also discussed briefly.