The synthesis, discovery, and development of a highly promising class of microtubule stabilization agents: curative effects of desoxyepothilones B and F against human tumor xenografts in nude mice

Total and Chemoenzymatic Synthesis of Seongsanamide E

The total and chemoenzymatic synthesis of the depsipeptide natural product seongsanamide E, 3, is described. The synthetic C-terminal N-acetylcysteamine thioester of linear natural product 1 was macrolactonized by the excised recombinant purified seongsanamide thioesterase (Sgd-TE) domain, generating 3. Sgd-TE also effects the ring opening of 3. Chemical synthesis provided 3 through a macrolactamization strategy. This work confirms the biosynthesis of 3 and demonstrates the power of Sgd-TE as a biocatalyst.

Natural Compounds as Modulators of Cell Cycle Arrest: Application for Anticancer Chemotherapies

Natural compounds from various plants, microorganisms and marine species play an important role in the discovery novel components that can be successfully used in numerous biomedical applications, including anticancer therapeutics. Since uncontrolled and rapid cell division is a hallmark of cancer, unraveling the molecular mechanisms underlying mitosis is key to understanding how various natural compounds might function as inhibitors of cell cycle progression. A number of natural compounds that inhibit the cell cycle arrest have proven effective for killing cancer cells in vitro, in vivo and in clinical settings. Significant advances that have been recently made in the understanding of molecular mechanisms underlying the cell cycle regulation using the chemotherapeutic agents is of great importance for improving the efficacy of targeted therapeutics and overcoming resistance to anticancer drugs, especially of natural origin, which inhibit the activities of cyclins and cyclin-dependent kinases, as well as other proteins and enzymes involved in proper regulation of cell cycle leading to controlled cell proliferation.

Stabilized Polymer Micelles for the Development of IT-147, an Epothilone D Drug-Loaded Formulation

Epothilones have demonstrated promising potential for oncology applications but suffer from a narrow therapeutic window. Epothilone D stabilizes microtubules leading to apoptosis, is active against multidrug-resistant cells, and is efficacious in animal tumor models despite lack of stability in rodent plasma. Clinical development was terminated in phase II due to dose limiting toxicities near the efficacious dose. Taken together, this made epothilone D attractive for encapsulation in a stabilized polymer micelle for improved safety and efficacy. We have designed a library of triblock copolymers to develop IT-147, a lead formulation of epothilone D that extends plasma circulation for accumulation in the tumor environment, and potentially decrease systemic exposure to reduce dose limiting toxicities. The drug loading efficiency for IT-147 exceeds 90%, is 75 nm in diameter, and demonstrates pH-dependent release of epothilone D without chemical conjugation or enzymatic activation. Administration of IT-147 at 20 mg/kg increases exposure of epothilone D to the plasma compartment over 6-fold compared to free drug. At the same dose, 20 mg/kg epothilone D from IT-147 is considered the no observed adverse effect level (NOAEL) but is the maximum tolerated dose for free drug. Consequently, IT-147 is positioned to be a safer, more effective means to deliver epothilone D.

Synergistic combination of microtubule targeting anticancer fludelone with cytoprotective panaxytriol derived from panax ginseng against MX-1 cells in vitro: experimental design and data analysis using the combination index method

This brief article focuses on two aims: i) To investigate the in vitro pharmaco-dynamic interactions of combining synthetic potent microtubule targeting anticancer agent, Fludelone (FD) with cyto-protective agent, Panaxytriol (PXT) derived from Panax ginseng, and ii) To illustrate step-by-step operation for conducting two-drug combination in vitro using the combination index method, in terms of experimental design, data acquisition, computerized simulation and data interpretation. The Chou-Talalay method for drug combination is based on the median-effect equation, which provides the theoretical basis for the combination index (CI)-isobologram equation that allows quantitative determination of drug interactions, where CI<1, =1, and >1 indicates synergism, additive effect and antagonism, respectively. Based on these algorithms, computer software, CompySyn, is used for determining synergism and antagonism at all doses or effect levels simulated automatically. The use of Chou-Talalay's CI method in quantifying synergism or antagonism is increasing steadily during the past two decades, however, confusing questions and pitfalls were still frequently raised by insufficient understanding of the theory, especially reflected when researchers trying to use the computerized software to design and conduct experiments. In order to specifically address the confusions and to illustrate the practical features of this method, in this paper, a selected example is given based on our unpublished data regarding the combinational pharmacologic interactions of FD and PXT against the growth of breast cancer cell line MX-1. The step-by-step operation from experimental design to the real data analysis is illustrated. The results indicated that FD and PXT combination in vitro exerted synergistic effect when cell growth inhibition was greater than 45%, with CI ranged 0.836-0.609 for the fractional inhibition of Fa=0.50~0.90, as shown by the Fa-CI plot and by the isobologram. Thus, quantitative conclusion of synergism is obtained using the Chou-Talalay CI method, under the well-defined simple conditions for the FD and PXT combinations in vitro.

A Comparison of Signaling Activities Induced by Taxol and Desoxyepothilone B

Desoxyepothilone B (dEpoB), currently in clinical trials, is a novel microtubule inhibitor with similar mode-of-action to paclitaxel (Taxol). Intriguingly, it is effective in some cell lines and tumor xenografts refractory to Taxol. The purpose of this study is to compare signaling induced by the two drugs and identify a molecular basis for increased efficacy of dEpoB in resistant lines. The importance of ERK signaling, already established for Taxol, was shown for dEpoB and other G2-blocking agents. However, a role in differential sensitivity was not observed. Affymetrix analysis shows similar gene modulation by either agent, alone or in combination with MEK inhibitor. Differential sensitivity in a set of Taxol-resistant lines correlated to the expression of P-glycoprotein (P-gp), and its importance was demonstrated directly. These results suggest that Taxol and dEpoB elicit similar cell death pathways, and the increased efficacy of dEpoB in resistant tumor lines lies in differential susceptibility to P-gp.

Antimitotic agents for the treatment of patients with metastatic castrate-resistant prostate cancer

INTRODUCTION

Metastatic castrate-resistant prostate cancer (mCRPC) is the second deadliest cancer in men. The group of taxanes, which target microtubules of mitotic cells, is currently the only chemotherapy which has proven to increase overall survival in mCRPC patients. Other mitotic inhibitors are being explored for their clinical potential in mCRPC treatment.

AREAS COVERED

In this review, we summarize recent developments in the application of mitotic inhibitors for mCRPC from a clinical perspective. The four main groups of mitotic inhibitors currently being tested in clinical trials are microtubule-inhibitors, polo-like kinase 1 inhibitors, aurora kinase inhibitors and kinesin-spindle protein inhibitors. Compounds of these groups of inhibitors that are in clinical development for mCRPC are discussed. For this extensive overview, relevant literature was searched in PubMed and retrieved from clinicaltrials.gov and presentations at ASCO/AACR meetings.

EXPERT OPINION

In general, mitotic inhibitors are clinically well tolerated but exert limited antitumor activity compared to preclinical study results. However, efficacy of mitotic inhibitors is improving, either by personalizing treatment, by introducing more active compounds, by decreasing resistance of cancer cells against mitotic inhibitors or by using mitotic inhibitors in combination therapies.

Novel antitumor indolizino[6,7-b]indoles with multiple modes of action: DNA cross-linking and topoisomerase I and II inhibition

A series of bis(hydroxymethyl)indolizino[6,7-b]indoles and their bis(alkylcarbamates) were synthesized for antitumor studies. These agents were designed as hybrid molecules of β-carboline (topoisomerase inhibition moiety) and bis(hydroxymethyl)pyrrole (DNA cross-linking moiety). The preliminary antitumor studies indicated that these agents exhibited significant cytotoxicity against a variety of human tumor cells in vitro. Treatment of human breast carcinoma MX-1 xenograft-bearing nude mice with compounds 18b and 28c achieved more than 99% tumor remission. We also observed that 18a displayed potent therapeutic efficacy against human lung adenocarcinoma A549 and colon cancer HT-29 xenografts. These results revealed that compound 18a was more potent than irinotecan against HT-29 cells and was as potent as irinotecan against A549 cells in xenograft models. Furthermore, we demonstrated that these derivatives possess multiple modes of action, such as induction of DNA cross-linking, inhibition of topoisomerase I and II, and cell-cycle arrest at the S-phase.

Phase I clinical, pharmacokinetic, and pharmacodynamic study of KOS-862 (Epothilone D) in patients with advanced solid tumors and lymphoma

PURPOSE

To determine the maximum tolerated dose and safety of the epothilone, KOS-862, in patients with advanced solid tumors or lymphoma.

PATIENTS AND METHODS

Patients were treated weekly for 3 out of 4 weeks (Schedule A) or 2 out of 3 weeks (Schedule B) with KOS-862 (16-120 mg/m(2)). Pharmacokinetic (PK) sampling was performed during cycles 1 and 2; pharmacodynamic (PD) assessment for microtubule bundle formation (MTBF) was performed after the 1st dose, only at or above 100 mg/m(2).

RESULTS

Thirty-two patients were enrolled, and twenty-nine completed ≥1 cycle of therapy. Dose limiting toxicity [DLT] was observed at 120 mg/m(2). PK data were linear from 16 to 100 mg/m(2), with proportional increases in mean C(max) and AUC(tot) as a function of dose. Full PK analysis (mean ± SD) at 100 mg/m(2) revealed the following: half-life (t (½)) = 9.1 ± 2.2 h; volume of distribution (V(z)) = 119 ± 41 L/m(2); clearance (CL) = 9.3 ± 3.2 L/h/m(2). MTBF (n = 9) was seen in 40% of PBMCs within 1 h and in 15% of PBMC at 24-hours post infusion at 100 mg/m(2). Tumor shrinkage (n = 2, lymphoma), stable disease >3 months (n = 5, renal, prostate, oropharynx, cholangiocarcinoma, and Hodgkin lymphoma), and tumor marker reductions (n = 1, colorectal cancer/CEA) were observed.

CONCLUSION

KOS-862 was well tolerated with manageable toxicity, favorable PK profile, and the suggestion of clinical activity. The maximum tolerated dose was determined to be 100 mg/m(2) weekly 3-on/1-off. MTBF can be demonstrated in PBMCs of patients exposed to KOS-862.

Phase I dose escalation study of KOS-1584, a novel epothilone, in patients with advanced solid tumors

PURPOSE

First-in-man study of KOS-1584, a second generation epothilone.

METHODS

Patients with advanced solid malignancies received KOS-1584 every 3 weeks until disease progression. Using a modified Fibonacci dose escalation scheme, one patient was enrolled at each dose level until the first instance of grade 2 toxicity. Thereafter, a standard 3 + 3 design was utilized.

RESULTS

Sixty-six patients in 14 cohorts were dosed from 0.8 to 48 mg/m(2). Diarrhea, arthralgias, and encephalopathy were dose-limiting toxicities (DLTs) at doses ≥36 mg/m(2). At the recommended phase II dose (RP2D), the most common adverse effects were peripheral neuropathy (low grade), fatigue, arthralgias/myalgias, and diarrhea (31, 6%). The incidence of neutropenia was low. The overall clearance, volume of distribution, and half-life of KOS-1584 were 11 ± 6.17 L/h/m(2), 327 ± 161 L/m(2), and 21.9 ± 8.75 h, respectively. The half-life for the seco-metabolite (KOS-1891) was 29.6 ± 13.8 h. KOS-1584 exhibited linear pharmacokinetics. A dose-dependent increase in microtubulin bundle formation was observed at doses ≥27 mg/m(2). Two patients achieved partial responses and 24 patients had stable disease (SD).

CONCLUSIONS

The RP2D of KOS-1584 is 36 mg/m(2). The lack of severe neurologic toxicity, diarrhea, neutropenia, or hypersensitivity reactions; favorable pharmacokinetic profile; and early evidence of activity support further evaluation.

A phase 1 study of KOS-862 (Epothilone D) co-administered with carboplatin (Paraplatin®) in patients with advanced solid tumors

PURPOSE

To determine the maximally tolerated dose (MTD) and pharmacokinetics of carboplatin plus KOS-862 (Epothilone D) a novel cytotoxic macrolide capable of causing mitotic arrest, in patients with advanced solid malignancies.

EXPERIMENTAL DESIGN

Patients who have progressed on standard regimens were treated at four different levels of KOS-862(mg/m(2))/Carboplatin(AUC): 50/5,75/5, 75/6 and 100/6 in a "3 + 3" phase I study study design to determine MTD. Patients received KOS-862 on Days 1 and 8, and carboplatin on day 1, of 3-week cycles. Pharmacokinetics of KOS-862 and Carboplatin were studied.

RESULTS

Twenty-seven patients enrolled in the study. At the top dose level, 2 out of the 9 patients experienced Dose Limiting Toxicity. (grade 3 peripheral motor neuropathy in both patients) Twenty-seven patients had sufficient plasma data points for pharmacokinetic analysis Both the parent drug, KOS-862, and the major inactive metabolite Seco-D KOS-862 (KOS-1965) were quantified in plasma. Kinetics of KOS-862 were the same as seen in monotherapy studies using the same route and time of administration. Two patients had tumor response after study treatment. Ten of 20 evaluable patients had stable disease after 2 cycles of study treatment. The MTD in the present study was KOS-862 100 mg/m(2) + carboplatin AUC = 6.

CONCLUSIONS

The pharmacokinetics of KOS-862 were similar in this combination study to those seen in previous monotherapy studies using the same route and time of administration. We have described the MTD of this schedule. The neurotoxicity seen with this regimen should be considered prior to its administration in unselected populations.

The mass-action law based algorithm for cost-effective approach for cancer drug discovery and development

The mass-action law based system analysis via mathematical induction and deduction lead to the generalized theory and algorithm that allows computerized simulation of dose-effect dynamics with small size experiments using a small number of data points in vitro, in animals, and in humans. The median-effect equation of the mass-action law deduced from over 300 mechanism specific-equations has been shown to be the unified theory that serves as the common-link for complicated biomedical systems. After using the median-effect principle as the common denominator, its applications are mechanism-independent, drug unit-independent, and dynamic order-independent; and can be used generally for single drug analysis or for multiple drug combinations in constant-ratio or non-constant ratios. Since the "median" is the common link and universal reference point in biological systems, these general enabling lead to computerized quantitative bio-informatics for econo-green bio-research in broad disciplines. Specific applications of the theory, especially relevant to drug discovery, drug combination, and clinical trials, have been cited or illustrated in terms of algorithms, experimental design and computerized simulation for data analysis. Lessons learned from cancer research during the past fifty years provide a valuable opportunity to reflect, and to improve the conventional divergent approach and to introduce a new convergent avenue, based on the mass-action law principle, for the efficient cancer drug discovery and the low-cost drug development.

The mass-action law based algorithms for quantitative econo-green bio-research

The relationship between dose and effect is not random, but rather governed by the unified theory based on the median-effect equation (MEE) of the mass-action law. Rearrangement of MEE yields the mathematical form of the Michaelis-Menten, Hill, Henderson-Hasselbalch and Scatchard equations of biochemistry and biophysics, and the median-effect plot allows linearization of all dose-effect curves regardless of potency and shape. The "median" is the universal common-link and reference-point for the 1st-order to higher-order dynamics, and from single-entities to multiple-entities and thus, it allows the all for one and one for all unity theory to "integrate" simple and complex systems. Its applications include the construction of a dose-effect curve with a theoretical minimum of only two data points if they are accurately determined; quantification of synergism or antagonism at all dose and effect levels; the low-dose risk assessment for carcinogens, toxic substances or radiation; and the determination of competitiveness and exclusivity for receptor binding. Since the MEE algorithm allows the reduced requirement of the number of data points for small size experimentation, and yields quantitative bioinformatics, it points to the deterministic, efficient, low-cost biomedical research and drug discovery, and ethical planning for clinical trials. It is concluded that the contemporary biomedical sciences would greatly benefit from the mass-action law based "Green Revolution".

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. 

Design, synthesis and antitumor evaluation of phenyl N-mustard-quinazoline conjugates

A series of N-mustard-quinazoline conjugates was synthesized and subjected to antitumor studies. The N-mustard pharmacophore was attached at the C-6 of the 4-anilinoquinazolines via a urea linker. To study the structure-activity relationships of these conjugates, various substituents were introduced to the C-4 anilino moiety. The preliminary antitumor studies revealed that these agents exhibited significant antitumor activity in inhibiting various human tumor cell growths in vitro. Compounds 21b, 21g, and 21h were selected for further antitumor activity evaluation against human breast carcinoma MX-1 and prostate PC-3 xenograft in animal model. These agents showed 54-75% tumor suppression with low toxicity (5-7% body-weight changes). We also demonstrate that the newly synthesized compounds are able to induce DNA cross-linking through alkaline agarose gel shift assay and inhibited cell cycle arrest at G2/M phase.

Ixabepilone for the treatment of breast cancer

Patients with breast cancer that becomes resistant to taxanes and anthracyclines experience considerable morbidity and mortality. The Food and Drug Administration has approved the use of ixabepilone for the treatment of patients with locally advanced or metastatic breast cancer that is refractory or resistant to taxanes and anthracyclines. The purpose of this review is to summarize the evidence from published studies on the pharmacological data and clinical activity of ixabepilone in patients with breast cancer. The conclusion of this review is that ixabepilone demonstrated a high efficacy in combination with capecitabine and as a single agent in breast cancer refractory to taxanes and anthracyclines. The clinical activity of ixabepilone combined with bevacizumab for advanced breast cancer was very promising.

Design, synthesis, and biological evaluation of novel water-soluble N-mustards as potential anticancer agents

A series of novel water-soluble N-mustard-benzene conjugates bearing a urea linker were synthesized. The benzene moiety contains various hydrophilic side chains are linked to the meta- or para-position of the urea linker via a carboxamide or an ether linkage. The preliminary antitumor studies revealed that these agents exhibited potent cytotoxicity in vitro and therapeutic efficacy against human tumor xenografts in vivo. Remarkably, complete tumor remission in nude mice bearing human breast carcinoma MX-1 xenograft and significant suppression against prostate adenocarcinoma PC3 xenograft were achieved by treating with compound 9aa' at the maximum tolerable dose with relatively low toxicity. We also demonstrate that the newly synthesized compounds are able to induce DNA cross-linking through alkaline agarose gel shift assay. A pharmacokinetic profile of the representative 9aa' in rats was also investigated. The current studies suggest that this agent is a promising candidate for preclinical studies.

A phase II clinical trial of ixabepilone (Ixempra; BMS-247550; NSC 710428), an epothilone B analog, in patients with metastatic renal cell carcinoma

PURPOSE

Ixabepilone (Ixempra; BMS-247550) is an epothilone B analog and nontaxane microtubule-stabilizing compound with clinical activity in a range of solid tumors. This phase II study was conducted to assess the efficacy and safety of ixabepilone in patients with metastatic renal cell carcinoma.

EXPERIMENTAL DESIGN

Patients with metastatic renal cell carcinoma who had measurable disease and had not received previous cytotoxic or targeted therapy were treated with 6 mg/m(2) ixabepilone i.v. daily for 5 days every 3 weeks. Levels of Glu-terminated and acetylated tubulin, markers of microtubule stabilization, were assessed by Western blot. VHL gene mutation status was determined by sequencing.

RESULTS

Eighty-seven patients received a total of 590 cycles, with a median of 5 cycles (range, 1-29). The overall response rate was 13% (Response Evaluation Criteria in Solid Tumor). One patient had a complete response, 10 patients had partial responses, and 59 patients had stable disease. The median duration of response was 5.5 months. The median overall survival of renal cell carcinoma Motzer grade 0 and 1 patients with clear cell histology was 19.25 months. Treatment-related adverse events were primarily alopecia, gastrointestinal toxicity, neuropathy, and fatigue. Biopsies were done at baseline and after five doses of ixabepilone. Microtubule target engagement was achieved in 84.6% to 92.3% of patients evaluated. No correlation was identified between the target engagement, VHL gene mutation status, and clinical response.

CONCLUSION

Ixabepilone can cause tumor regression in some patients with metastatic renal cell carcinoma and could be considered in combination regimens with other therapies.

Ecology-based screen identifies new metabolites from a Cordyceps-colonizing fungus as cancer cell proliferation inhibitors and apoptosis inducers

OBJECTIVES

This study aims to identify new anti-cancer agents from Cordyceps-colonizing fungi, using an ecology-based approach. It also aims to explore their anti-cell proliferative mechanisms, and to evaluate their anti-tumour effects in vivo.

MATERIALS AND METHODS

Extracts from Cordyceps-colonizing fungi were tested on HeLa cells, and active extracts were separated to obtain anti-tumour metabolites; their structures were elucidated by mass and nuclear magnetic resonance spectroscopy. Cell cycle analysis was evaluated using flow cytometry. Tumour formation assays were performed using C57BL/6J mice.

RESULTS

Based on ecological considerations, the selected extracts were subjected to initial anti-tumour screening. Bioassay-guided fractionation of the active extract afforded two new epipolythiodioxopiperazines, named gliocladicillins A (1) and B (2). (A) 1 and B (2) inhibited growth of HeLa, HepG2 and MCF-7 tumour cells. Further study demonstrated that both preparations arrested the cell cycle at G(2)/M phase in a dose-dependent manner, and induced apoptosis through up-regulation of expression of p53, p21, and cyclin B, and activation of caspases-8, -9 and -3. These data imply that gliocladicillins A (1) and B (2) induce tumour cell apoptosis through both extrinsic and intrinsic pathways. In addition, in vivo studies showed that they displayed significant inhibitory effects on cell population growth of melanoma B16 cells implanted into immunodeficient mice.

CONCLUSIONS

Gliocladicillins A (1) and B (2) are effective anti-tumour agents in vitro and in vivo and should be further evaluated for their potential in clinical use.

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.

Epothilones: a novel class of microtubule-stabilizing drugs for the treatment of cancer

Microtubule-targeted anticancer drugs are effective in treating various cancers but are limited in use due to development of resistance and unacceptable toxicities. The epothilones are a novel class of microtubule-stabilizing anticancer drugs and may have a role in treating taxane-resistant cancers. Revised and updated data from several clinical studies for ixabepilone were recently published and subsequently resulted in ixabepilone becoming the first epothilone approved as monotherapy or in combination for treatment of locally advanced or metastatic breast cancer. BMS-310705, patupilone, KOS-862, KOS-1584 and ZK-EPO are epothilones that have been developed. Although peripheral sensory neuropathy and neutropenia are the dose-limiting toxicities for ixabepilone, these dose-limiting toxicities are ixabepilone specific. This review will discuss the current preclinical, clinical pharmacokinetic and pharmacodynamic, efficacy and toxicity data of the epothilones.

Therapeutic effect against human xenograft tumors in nude mice by the third generation microtubule stabilizing epothilones

The epothilones represent a promising class of natural product-based antitumor drug candidates. Although these compounds operate through a microtubule stabilization mechanism similar to that of taxol, the epothilones offer a major potential therapeutic advantage in that they retain their activity against multidrug-resistant cell lines. We have been systematically synthesizing and evaluating synthetic epothilone congeners that are not accessible through modification of the natural product itself. We report herein the results of biological investigations directed at two epothilone congeners: iso-fludelone and iso-dehydelone. Iso-fludelone, in particular, exhibits a number of properties that render it an excellent candidate for preclinical development, including biological stability, excellent solubility in water, and remarkable potency relative to other epothilones. In nude mouse xenograft settings, iso-fludelone was able to achieve therapeutic cures against a number of human cancer cell lines, including mammarian-MX-1, ovarian-SK-OV-3, and the fast-growing, refractory, subcutaneous neuroblastoma-SK-NAS. Strong therapeutic effect was observed against drug-resistant lung-A549/taxol and mammary-MCF-7/Adr xenografts. In addition, iso-fludelone was shown to exhibit a significant therapeutic effect against an intracranially implanted SK-NAS tumor.

Improved cellular pharmacokinetics and pharmacodynamics underlie the wide anticancer activity of sagopilone

Sagopilone (ZK-EPO) is the first fully synthetic epothilone undergoing clinical trials for the treatment of human tumors. Here, we investigate the cellular pathways by which sagopilone blocks tumor cell proliferation and compare the intracellular pharmacokinetics and the in vivo pharmacodynamics of sagopilone with other microtubule-stabilizing (or tubulin-polymerizing) agents. Cellular uptake and fractionation/localization studies revealed that sagopilone enters cells more efficiently, associates more tightly with the cytoskeleton, and polymerizes tubulin more potently than paclitaxel. Moreover, in contrast to paclitaxel and other epothilones [such as the natural product epothilone B (patupilone) or its partially synthetic analogue ixabepilone], sagopilone is not a substrate of the P-glycoprotein efflux pumps. Microtubule stabilization by sagopilone caused mitotic arrest, followed by transient multinucleation and activation of the mitochondrial apoptotic pathway. Profiling of the proapoptotic signal transduction pathway induced by sagopilone with a panel of small interfering RNAs revealed that sagopilone acts similarly to paclitaxel. In HCT 116 colon carcinoma cells, sagopilone-induced apoptosis was partly antagonized by the knockdown of proapoptotic members of the Bcl-2 family, including Bax, Bak, and Puma, whereas knockdown of Bcl-2, Bcl-X(L), or Chk1 sensitized cells to sagopilone-induced cell death. Related to its improved subcellular pharmacokinetics, however, sagopilone is more cytotoxic than other epothilones in a large panel of human cancer cell lines in vitro and in vivo. In particular, sagopilone is highly effective in reducing the growth of paclitaxel-resistant cancer cells. These results underline the processes behind the therapeutic efficacy of sagopilone, which is now evaluated in a broad phase II program.

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

Epothilones (Epo's) A and B are naturally occurring microtubule-stabilizers, which inhibit the growth of human cancer cells in vitro at low nM or sub-nM concentrations. In contrast to taxol (paclitaxel, Taxol) epothilones are also active against different types of multidrug-resistant cancer cell lines in vitro and against multidrug-resistant tumors in vivo. Their attractive preclinical profile has made epothilones important lead structures in the search for improved cytotoxic anticancer drugs and Epo B (EPO906, patupilone) is currently undergoing Phase III clinical trials. Numerous synthetic and semisynthetic analogs have been prepared since the absolute stereochemistry of epothilones was first disclosed in mid-1996 and their in vitro biological activity has been determined. Apart from generating a wealth of SAR information, these efforts have led to the identification of at least six compounds (in addition to Epo B), which are currently at various stages of clinical evaluation in humans. The most advanced of these compounds, Epo B lactam BMS-247550 (ixabepilone), has recently obtained FDA approval for the treatment of metastatic and advanced breast cancer. This chapter will first provide a summary of the basic features of the biological profile of Epo B in vitro and in vivo. This will be followed by a review of the processes that have been developed for the fermentative production of Epo B. The main part of the chapter will focus on the most relevant aspects of the epothilone SAR with regard to effects on tubulin polymerization, in vitro antiproliferative activity, and in vivo antitumor activity. Particular emphasis will be placed on work conducted in the authors' own laboratories, but data from other groups will also be included. In a final section, the current status of those epothilone analogs undergoing clinical development will be briefly discussed.

Cellular mechanisms of resistance to anthracyclines and taxanes in cancer: intrinsic and acquired

Taxanes and anthracyclines are two of the most potent and broadly effective classes of chemotherapeutic agents. However, resistance to these agents is common and significantly limits their potential. As such, there is a great need to understand the mechanisms underlying de novo and acquired resistance to these agents. Beyond the resistance barrier lies even greater potential to significantly alter the natural course of human cancer. This review discusses what we currently understand about the mechanisms of resistance to taxanes and anthracyclines. Preclinical models suggest a role for ATP-binding cassette transporters, tubulin isoforms, microtubule-associated proteins, tubulin gene mutations, and mitotic checkpoint signaling proteins in resistance to taxanes. Preclinical models also suggest that drug transport proteins, antioxidant defenses, apoptotic signaling, and topoisomerase modulation may mediate anthracycline resistance. Many of these hypotheses remain untested in appropriately designed clinical studies, but limited clinical evidence will be reviewed. Epothilones represent a novel class of non-taxane microtubule stabilizing agents with distinct drug-resistance profiles. Potential mechanisms behind these differences and their potential role in the treatment of both taxane- and anthracycline-refractory patients are discussed.

Preclinical investigations with epothilones in breast cancer models

The epothilones constitute a novel class of microtubule inhibitors that act like the taxanes by hyperstabilizing tubulin polymerization, thus disrupting functioning of the mitotic spindle. Natural epothilones produced by myxobacteria, and second- or third-generation partially or fully synthesized analogs, have been explored as cancer chemotherapy agents to replace or follow the taxanes. For those epothilones that have gone on to clinical development (epothilone B, ixabepilone, BMS-310705, ZK-EPO, KOS-862, and KOS-1584), preclinical investigations in breast cancer models are reviewed. All of these epothilones improve upon the cytotoxic activity of paclitaxel in various human breast cancer cell lines in vitro, but are also highly active in lines that are resistant to paclitaxel. Comparable antitumor activity has been demonstrated against nude mouse xenografts of paclitaxel-sensitive and -resistant breast cancer lines. Additionally, some analogs have reduced toxicity or increased water solubility that may permit oral administration, while others with enhanced tissue penetration show promise in animal models of breast cancer brain or bone metastasis and may provide benefits in patients with poor-prognosis advanced breast cancer.

Discovery and development of the epothilones : a novel class of antineoplastic drugs

The epothilones are a novel class of antineoplastic agents possessing antitubulin activity. The compounds were originally identified as secondary metabolites produced by the soil-dwelling myxobacterium Sorangium cellulosum. Two major compounds, epothilone A and epothilone B, were purified from the S. cellulosum strain So ce90 and their structures were identified as 16-member macrolides. Initial screening with these compounds revealed a very narrow and selective antifungal activity against the zygomycete, Mucor hiemalis. In addition, strong cytotoxic activity against eukaryotic cells, mouse L929 fibroblasts and human T-24 bladder carcinoma cells was observed. Subsequent studies revealed that epothilones induce tubulin polymerization and enhance microtubule stability. Epothilone-induced stabilisation of microtubules was shown to cause arrest at the G2/M transition of the cell cycle and apoptosis. The compounds are active against cancer cells that have developed resistance to taxanes as a result of acquisition of beta-tubulin overexpression or mutations and against multidrug-resistant cells that overexpress P-glycoprotein or multidrug resistance-associated protein. Thus, epothilones represent a new class of antimicrotubule agents with low susceptibility to key tumour resistance mechanisms. More recently, a range of synthetic and semisynthetic epothilone analogues have been produced to further improve the adverse effect profile (or therapeutic window) and to maximize pharmacokinetic and antitumour properties. Various epothilone analogues have demonstrated activity against many tumour types in preclinical studies and several compounds have been and still are being evaluated in clinical trials. This article reviews the identification and early molecular characterization of the epothilones, which has provided insight into the mode of action of these novel antitumour agents in vivo.

Preclinical efficacy spectrum and pharmacokinetics of ixabepilone

PURPOSE

Ixabepilone, a semisynthetic analog of natural epothilone B, was developed for use in cancer treatment. This study extends previous findings regarding the efficacy of ixabepilone and its low susceptibility to tumor resistance mechanisms and describes the pharmacokinetics of this new antineoplastic agent.

METHODS

The cytotoxicity of ixabepilone was assessed in vitro in breast, lung, and colon tumor cell lines and in vivo in human xenografts in mice. Antitumor activities of ixabepilone and taxanes were compared in multidrug-resistant models in vivo. Differential drug uptake of ixabepilone and paclitaxel was assessed in a P-glycoprotein (P-gp)-resistant colon cancer model in vitro. The pharmacokinetic profile of ixabepilone was established in mice and humans.

RESULTS

Ixabepilone demonstrated potent cytotoxicity in a broad range of human cancer cell lines in vitro and in a wide range of xenografts in vivo. Ixabepilone was *3-fold more potent than docetaxel in the paclitaxel-resistant Pat-21 xenograft model (resistant due to overexpression of betaIII-tubulin and a lack of betaII-tubulin). Ixabepilone activity against P-gp-overexpressing breast and colon cancer was confirmed in in vivo models. Cellular uptake of ixabepilone, but not paclitaxel, was established in a P-gp-overexpressing model. The pharmacokinetics of ixabepilone was characterized by rapid tissue distribution and extensive tissue binding.

CONCLUSIONS

Cytotoxicity studies against a range of tumor types in vitro and in vivo demonstrate that ixabepilone has potent and broad-spectrum antineoplastic activity. This is accompanied by favorable pharmacokinetics. Ixabepilone has reduced susceptibility to resistance due to P-gp overexpression, tubulin mutations, and alterations in beta-tubulin isotype expression.

Epothilones in the treatment of cancer

Epothilones are cytotoxic macrolides with a similar mechanism of action to paclitaxel but with the potential advantage of activity in taxane-resistant settings in preclinical models. The epothilones ixabepilone, patupilone, BMS-310705, KOS-862 and ZK-EPO are in early clinical trials for cancer treatment. Phase I studies have shown that dose-limiting toxicities of epothilones are generally neurotoxicity and neutropoenia although initial studies with patupilone indicated that diarrhoea was dose limiting. Neuropathy induced by ixabepilone may be schedule dependent. Over 20 Phase II studies of epothilones in cancer treatment have been reported, and significant activity in taxane-sensitive tumour types (such as breast, lung and prostate cancers) has been noted. Response rates in taxane-refractory metastatic breast cancer are relatively modest, but ixabepilone and patupilone have shown promising efficacy in hormone-refractory metastatic prostate cancer and in taxane-refractory ovarian cancer.

Phase II study of KOS-862 in patients with metastatic androgen independent prostate cancer previously treated with docetaxel

Based on the pre-clinical spectrum of activity in taxane-resistant cell lines, we evaluated KOS-862 (epothilone D; 12,13-desoxyepothilone B) as second-line chemotherapy in androgen-independent prostate cancer.Thirty-eight men with metastatic androgen-independent prostate cancer and evidence of progression following docetaxel-based chemotherapy were treated with KOS-862, 100 mg/m(2) (maximum of 240 mg) i.v. weekly for 3 weeks, repeated every 4 weeks. The primary objective for this study was to determine the antitumor activity, measured by PSA decline by more then 50% confirmed 4 weeks later. Two patients (5.3%, 90% CI 1-16%) met criteria for confirmed PSA decline. While both of these patients had previously been treated with docetaxel, neither had confirmed docetaxel-refractory disease. None of the 24 patients with measurable disease had a confirmed partial response. Seventy-three percent of patients had an adverse event leading to dose delay, reduction, or treatment discontinuation. Neurological toxicity and fatigue predominated. Seventeen patients (44.7%) had treatment related grade 3 neurological adverse events including peripheral sensory neuropathy (n = 4, 10.5%), ataxia (n = 3, 7.9%), peripheral motor neuropathy (n = 1, 2.6%), involuntary muscle contractions (n = 1, 2.6%) and neuropathic pain (n = 1, 2.6%). One subject (2.6%) had a grade 4 treatment peripheral motor neuropathy. Further study of this dose and schedule of KOS-862 in this patient population cannot be recommended due to both lack of activity and excessive toxicity.

Targeting the microtubules in breast cancer beyond taxanes: the epothilones

Microtubule-targeting agents such as the taxanes are highly active against breast cancer and have become a cornerstone in the treatment of patients with early and advanced breast cancer. The natural epothilones and their analogs are a novel class of microtubule-stabilizing agents that bind tubulin and result in apoptotic cell death. Among this family of compounds, patupilone, ixabepilone, BMS-310705, ZK-EPO, and KOS-862 are in clinical development. Extensive preclinical studies have shown that epothilones are working through partially nonoverlapping mechanisms of action with taxanes. In the clinic, epothilones have been found in a series of phase I and phase II studies to be active even in patients who had recently progressed to taxanes. The toxicity profile of these agents consists mostly of sensory neuropathy, sometimes reversible. Neoadjuvant studies with epothilones have been conducted and a number of phase III studies in advanced breast cancer are either under way or have been recently completed. The results of these studies are eagerly awaited and it is anticipated that epothilones may become an important treatment option in patients with breast cancer.

Theoretical basis, experimental design, and computerized simulation of synergism and antagonism in drug combination studies

The median-effect equation derived from the mass-action law principle at equilibrium-steady state via mathematical induction and deduction for different reaction sequences and mechanisms and different types of inhibition has been shown to be the unified theory for the Michaelis-Menten equation, Hill equation, Henderson-Hasselbalch equation, and Scatchard equation. It is shown that dose and effect are interchangeable via defined parameters. This general equation for the single drug effect has been extended to the multiple drug effect equation for n drugs. These equations provide the theoretical basis for the combination index (CI)-isobologram equation that allows quantitative determination of drug interactions, where CI < 1, = 1, and > 1 indicate synergism, additive effect, and antagonism, respectively. Based on these algorithms, computer software has been developed to allow automated simulation of synergism and antagonism at all dose or effect levels. It displays the dose-effect curve, median-effect plot, combination index plot, isobologram, dose-reduction index plot, and polygonogram for in vitro or in vivo studies. This theoretical development, experimental design, and computerized data analysis have facilitated dose-effect analysis for single drug evaluation or carcinogen and radiation risk assessment, as well as for drug or other entity combinations in a vast field of disciplines of biomedical sciences. In this review, selected examples of applications are given, and step-by-step examples of experimental designs and real data analysis are also illustrated. The merging of the mass-action law principle with mathematical induction-deduction has been proven to be a unique and effective scientific method for general theory development. The median-effect principle and its mass-action law based computer software are gaining increased applications in biomedical sciences, from how to effectively evaluate a single compound or entity to how to beneficially use multiple drugs or modalities in combination therapies.