Vinca alkaloid-induced tubulin spiral formation correlates with cytotoxicity in the leukemic L1210 cell line

Anticancer mechanism of 7-α-hydroxyfrullanolide on microtubules and computational prediction of its target binding in triple-negative breast cancer cells

Background

Triple-negative breast cancer (TNBC) responds poorly to the available drugs; thus, the mortality rate associated with TNBC remains high. 7-α-Hydroxyfrullanolide (7HF) possesses anticancer properties and arrests cells in the G2/M-phase via modulation of several proteins involved in the G2/M-phase transition, as well as the mitotic checkpoint in MDA-MB-468 (TNBC) cells. Microtubules (MTs) dynamically regulate cell division in the G2/M phase and are related to cancer cell stress response. However, antimitotic drug cytotoxicity to multiple cancer resistance developed in response to drugs are obstacles faced to date. Here, the activity and mechanism via which 7HF controls MTs dynamics was investigated in MDA-MB-468 cells.

Methods

7HF uptake by MDA-MB-468 cells was assessed using spectrophotometry. The drug-like properties of 7HF were predicted using the Swiss-absorption, distribution, metabolism, and excretion (ADME) webtool. Then, the effect of 7HF treatment (6, 12, and 24 µM) on the dynamic arrangement of MTs was assessed for 1, 12, and 24 h using indirect immunofluorescence. Polymerization of α- and β-tubulin was assessed using different 7HF concentrations in a cell-free system for 1 h. Cell proliferation assay with bromodeoxyuridine plus propidium iodide staining and flow cytometry was performed at different 7HF concentrations and time points. The mechanism of action was assessed by detecting the expression of proteins, including Bub3, cyclin B1, p-Cdk1 (Tyr15), Rb, p-Rb (Ser780), Chk1, p-Chk1 (Ser345), Chk2, p-Chk2 (Ser516), and p-H2AX (Ser139), using western blotting. Molecular docking was used to predict the molecular interactions between 7HF and tubulins in MTs.

Results

We observed that 7HF was able to enter the MDA-MB-468 cells. The ADME webtool analysis predicted that it possesses the high passive permeation and gastrointestinal absorption properties of drugs. Various concentrations of 7HF disrupted the dynamic arrangement of spindle MTs by causing radial spindle array shrinkage and expansion of fibrous spindle density and radial array lengths in a time-dependent manner. 7HF reduced polymerization of α-, β-tubulin in dose-dependent manner. 7HF also triggered DNA damage response by inducing G2/M and G1 phase arrests in a concentration and time-dependent manner, which occurred due to the upregulation of Bub3, Chk1, p-Chk1 (Ser345), p-Cdk1 (Tyr15), and cyclin B1. According to molecular docking analysis, 7HF preferred to bind to β-tubulin over α-tubulin. The lactone, ketone, and hydroxyl groups of 7HF supported the 7HF-tubulin interactions. Hydrogen bonding with a hydrocarbon ring and salt bridge attractive forces were responsible for the binding versatility of 7HF.

Conclusions

This is the first study to investigate the molecular mechanism, MTs interacting sites, and the internalization and drug-like properties of 7HF in TNBC cells. The findings will be useful for developing 7HF-based treatment for patients with TNBC.

Synthesis, Characterization and Anticancer Efficacy Evaluation of Benzoxanthone Compounds toward Gastric Cancer SGC-7901

Three benzoxanthone derivatives were synthesized through a new photochemical strategy. The in vitro cytotoxic activity of these compounds was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, and their partition coefficients (logP) were measured by shake flask method. The pK_a values of the compounds were detected by potentionmetric titration. The interaction of the compounds with calf thymus DNA (CT-DNA) was investigated by electronic absorption, luminescence spectra and viscosity. A molecular docking analysis was performed. The antitumor efficacy of the compounds was evaluated by cell apoptosis, cell cycle arrest, intracellular Ca²⁺ concentrations and reactive oxygen species (ROS) levels. The mitochondrial membrane potential was assayed using JC-1 (5,5′,6,6′-tetrachloro-1,1,3′,3′-tetraethyl-imidacarbocyanine iodide) as the fluorescence probe. The expression of Bcl-2 family protein, caspase 3 and poly ADP-ribose polymerase (PARP) was explored by western blot. The results showed that the compounds induced apoptosis through a ROS-mediated mitochondrial dysfunction pathway. This work provides an efficient approach to synthesize benzoxanthone derivatives, and is helpful for understanding the apoptotic mechanism.

Development of Natural Bioactive Alkaloids: Anticancer perspective

Cancer is a frightful disease that still poses a 'nightmare' worldwide, causing millions of casualties annually due to one of the human race's most significant healthcare challenges that requires a pragmatic treatment strategy. However, plants and plant-derived products revolutionize the field as they are quick, cleaner, eco-friendly, low-cost, effective, and less toxic than conventional treatment methods. Plants are repositories for new chemical entities and have a promising cancer research path, supplying 60% of the anticancer agents currently used. Alkaloids are important chemical compounds that serve as a rich reservoir for drug discovery and development. However, some alkaloids derived from natural herbs display anti-proliferation and antimetastatic activity on different forms of cancer, both in vitro and in vivo. Alkaloids have also been widely formulated as anticancer medications, such as camptothecin and vinblastine. Still, more research and clinical trials are required before final recommendations can be made on specific alkaloids. This review focuses on the naturally-derived bioactive alkaloids with prospective anticancer properties based on the information in the literature.

Vincristine-Induced Peripheral Neuropathy (VIPN) in Pediatric Tumors: Mechanisms, Risk Factors, Strategies of Prevention and Treatment

Vincristine-induced peripheral neurotoxicity (VIPN) is a very common side effect of vincristine chemotherapy among pediatric patients with cancer. Neuropathy may be sensory, motor and/or autonomic, with consequent reduction, delay or discontinuation of vincristine-chemotherapy, but also pain, disability, reduced quality of life of patients and an increase in medical costs. Vincristine acts out its antineoplastic function by altering the normal assembly and disassembly of microtubules, with their consequent mitosis block and death. Vincristine leads to VIPN through a complex mechanism of damage, which occurs not only on the microtubules, but also on the endothelium and the mitochondria of nerve cells. Furthermore, both patient-related risk factors (age, race, ethnicity and genetic polymorphisms) and treatment-related risk factors (dose, time of infusion and drug–drug interactions) are involved in the pathogenesis of VIPN. There is a lack of consensus about the prophylaxis and treatment of VIPN among pediatric oncologic patients, despite several molecules (such as gabapentin, pyridoxine and pyridostigmine, glutamic acid and glutamine) having been already investigated in clinical trials. This review describes the molecular mechanisms of VIPN and analyzes the risk factors and the principal drugs adopted for the prophylaxis and treatment of VIPN in pediatric patients with cancer.

Vinca alkaloids and analogues as anti-cancer agents: Looking back, peering ahead

Cancer still represents a "nightmare" worldwide, causing annually millions of victims. Several antiproliferative molecules are currently used as drugs market and offer a pharmaceutical opportunity for attenuating and treating tumor manifestations. In this context, natural sources have a relevant role, since they provide the 60% of currently-used anticancer agents. Among the numerous natural products, acting via different mechanisms of action, microtubule-targeting agents (MTAs) have a high therapeutic potential, since they disrupt the abnormal cancer cell growth, interfering with the continuous mitotic division. Vinca alkaloids (VAs) are the earliest developed MTAs and approved for clinical use (Vincristine, Vinblastine, Vinorelbine, Vindesine, and Vinflunine) as agents in the treatment of hematological and lymphatic neoplasms. Here, we review the state-of-art of VAs, discussing their mechanism of action and pharmacokinetic properties and highlighting their therapeutic relevance and toxicological profile. Additionally, we briefly disclosed the technological approaches faced so far to ameliorate the pharmacological properties, as well as to avoid the drug resistance. Lastly, we introduced the recent advances in the discovery of new derivatives.

Lipid-Based Drug Delivery Systems in Cancer Therapy: What Is Available and What Is Yet to Come

Cancer is a leading cause of death in many countries around the world. However, the efficacy of current standard treatments for a variety of cancers is suboptimal. First, most cancer treatments lack specificity, meaning that these treatments affect both cancer cells and their normal counterparts. Second, many anticancer agents are highly toxic, and thus, limit their use in treatment. Third, a number of cytotoxic chemotherapeutics are highly hydrophobic, which limits their utility in cancer therapy. Finally, many chemotherapeutic agents exhibit short half-lives that curtail their efficacy. As a result of these deficiencies, many current treatments lead to side effects, noncompliance, and patient inconvenience due to difficulties in administration. However, the application of nanotechnology has led to the development of effective nanosized drug delivery systems known commonly as nanoparticles. Among these delivery systems, lipid-based nanoparticles, particularly liposomes, have shown to be quite effective at exhibiting the ability to: 1) improve the selectivity of cancer chemotherapeutic agents; 2) lower the cytotoxicity of anticancer drugs to normal tissues, and thus, reduce their toxic side effects; 3) increase the solubility of hydrophobic drugs; and 4) offer a prolonged and controlled release of agents. This review will discuss the current state of lipid-based nanoparticle research, including the development of liposomes for cancer therapy, different strategies for tumor targeting, liposomal formulation of various anticancer drugs that are commercially available, recent progress in liposome technology for the treatment of cancer, and the next generation of lipid-based nanoparticles.

Synthesis and SAR study of novel anticancer and antimicrobial naphthoquinone amide derivatives

A series of novel naphthoquinone amide derivatives of the bioactive quinones, plumbagin, juglone, menadione and lawsone, with various amino acids were synthesized. The compounds were characterized by (1)H NMR, (13)C NMR, Mass, IR and elemental analysis. All the compounds were evaluated for their anticancer activity against HeLa and SAS cancer cell lines and 3D-QSAR indicated the presence of electron donating group near sulphur enhanced the activity against HeLa cells. Among the derivatives synthesized, compounds 11f, 10a, 10b and 10g were the most active with IC50 values of 16, 12, 14 and 24.5 μM, respectively. The analogues were also screened for antimicrobial activity against two human bacterial pathogens, the Gram-positive Methicillin resistant Staphylococcus aureus (MRSA) and the Gram-negative Pseudomonas aeruginosa and a human yeast pathogen, Fluconazole resistant Candida albicans (FRCA). Among the synthesized compounds, 8g, 10g and 11g exhibited maximum antibacterial activity towards MRSA and antifungal activity against FRCA in well diffusion method.

George M, Freedman H, Barakat KH, Damaraju S, Hameroff S, Tuszynski JA. Computational predictions of volatile anesthetic interactions with the microtubule cytoskeleton: implications for side effects of general anesthesia

The cytoskeleton is essential to cell morphology, cargo trafficking, and cell division. As the neuronal cytoskeleton is extremely complex, it is no wonder that a startling number of neurodegenerative disorders (including but not limited to Alzheimer's disease, Parkinson's disease and Huntington's disease) share the common feature of a dysfunctional neuronal cytoskeleton. Recently, concern has been raised about a possible link between anesthesia, post-operative cognitive dysfunction, and the exacerbation of neurodegenerative disorders. Experimental investigations suggest that anesthetics bind to and affect cytoskeletal microtubules, and that anesthesia-related cognitive dysfunction involves microtubule instability, hyper-phosphorylation of the microtubule-associated protein tau, and tau separation from microtubules. However, exact mechanisms are yet to be identified. In this paper the interaction of anesthetics with the microtubule subunit protein tubulin is investigated using computer-modeling methods. Homology modeling, molecular dynamics simulations and surface geometry techniques were used to determine putative binding sites for volatile anesthetics on tubulin. This was followed by free energy based docking calculations for halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) on the tubulin body, and C-terminal regions for specific tubulin isotypes. Locations of the putative binding sites, halothane binding energies and the relation to cytoskeleton function are reported in this paper.

Spectroscopic studies of dactinomycin and vinorelbine binding to deoxyribonucleic acid and chromatin

Dactinomycin and vinorelbine are anticancer drugs, widely used as chemotherapeutic agents for the treatment of various cancers. This study represents an attempt to compare the effect of these drugs on DNA and soluble chromatin employing UV/vis, fluorescence and circular dichroism spectroscopy techniques. The results demonstrated that the absorbance at 260 and 210 nm was decreased and quenching of drugs with DNA and chromatin chromospheres induced reduction of fluorescence emission intensity. Circular dichroism profiles showed that the binding of drugs induced structural changes in both positive and negative extremes of DNA and chromatin revealing release or displacement of histone proteins from chromatin upon dactinomycin binding whereas vinorelbine preceded the chromatin into compaction. The results suggest that dactinomycin shows higher affinity to DNA compared to chromatin, whereas, vinorelbine recognizes the chromatin structure with higher affinity than free DNA. Also chromatin proteins play a fundamental role in drug-chromatin complex process.

Vinflunine in the treatment of advanced bladder cancer

Accounting for 14,000 deaths in the USA last year, research informs us that advanced bladder cancer is a lethal disease with a median survival that has remained a little over 1 year for the past two decades. For the majority of patients with metastatic disease, chemotherapy with cisplatin-based combinations is the standard first-line treatment. Although initial response rates are high, disease progression is common, creating a growing number of patients in need of effective second-line chemotherapy. For this population, no standard of care currently exists. Salvage chemotherapy is associated with low response rates and studies exploring potential clinical benefit over supportive care alone are limited to nonrandomized Phase II trials. Vinflunine, a novel anti-mitotic drug from the Vinca alkaloid class, is the first and only agent that has been compared with supportive care in the second-line setting. In Europe, vinflunine is approved as a treatment option for patients with advanced urothelial cancer who have failed a prior platinum-containing regimen. To date, in the USA, there is no FDA-approved second-line chemotherapy for patients with metastatic bladder cancer and treatment continues to emphasize patient enrollment into a clinical trial.

Rapid and sensitive determination of vinorelbine in human plasma by liquid chromatography-tandem mass spectrometry and its pharmacokinetic application

Vinorelbine is a semi-synthetic vinca alkaloid with demonstrated high activities against various types of advanced cancer. To support a clinical pharmacokinetic study, a simple, rapid and sensitive method to determine vinorelbine in human plasma was developed using reversed phase liquid chromatography (LC) coupled with electrospray ionization mass spectrometry/mass spectrometry (ESI-MS/MS). Vinorelbine and vinblastine (the internal standard) were extracted from human plasma by one-step liquid-liquid extraction (LLE) with methyl-t-butyl ether. The chromatographic separation was achieved on a Spursil polar-modified C(18) column (50 mm×2.1 mm, 3 μm, Dikma Technologies) with an isocratic mobile phase of a 75:25 (v/v) acetonitrile-4 mmol/L ammonium formate (pH 3.0) mixture at a flow-rate of 0.4 mL/min. The MS/MS detection was performed in the positive ion multiple reaction monitoring (MRM) mode by monitoring the precursor→product ion transitions at m/z 779.4→122.0 and m/z 811.3→224.2 for vinorelbine and the internal standard, respectively. The assay was validated in the range 0.1-200 ng/mL (r>0.997), the lowest level of this range being the lower limit of quantification (LLOQ) based on 50 μL of plasma. The intra- and inter-day precisions were within 6.0%, while the accuracy was within ±4.7% of nominal values. Detection and quantification of both analytes within 2 min make this method suitable for high-throughput analyses. The method was successfully applied to evaluate the systemic pharmacokinetics of vinorelbine after a 20-min intravenous infusion of 25 mg/m(2) of vinorelbine to patients with metastatic breast cancer.

Analysis of tubulin oligomers by analytical ultracentrifugation

This chapter describes the use of analytical ultracentrifugation in a Beckman XLA to study the self-association properties of tubulin and the interaction of tubulin with antimitotic drugs. Procedures for sample preparation, operation of the ultracentrifuge, and collection of data conform to standard modern methods. Analysis of sedimentation velocity data initially includes generation of g(s) sedimentation coefficient distributions with DCDT(+2) and determination of weight average sedimentation coefficients S(w). S(w) versus concentration data are then fit to isodesmic or indefinite assembly models to extract K(iso) values, the association constant for each successive assembly step. Alternatively the raw data can also be analyzed by direct boundary analysis methods using the program Sedanal. Direct boundary analysis also extracts the K(iso) value by fitting to the shape of the sedimentation boundary as a function of total concentration. While the fitting of weight average data as a function of protein or drug concentration to indefinite assembly models has been shown to be equivalent to direct boundary fitting of multiple data sets with Sedanal, direct boundary fitting is preferred because it robustly identifies the presence of irreversible aggregation or mechanisms that are more complex.

Vinflunine: review of a new vinca alkaloid and its potential role in oncology

Objective: To review the pharmacology, pharmacokinetics, in vitro and in vivo efficacy, and safety profile of vinflunine in the treatment of various solid tumors.

Data sources: A literature search was conducted using keywords included vinflunine, vinca alkaloid, Javlor, and solid tumor in PubMed/MEDLINE (1950—January 2009) and International Pharmaceutical Abstracts (1950—January 2009). Study selection and data extraction: Published studies, posters, and meeting abstracts evaluating the in vitro and in vivo efficacy of vinflunine were reviewed.

Data synthesis: Vinflunine is the newest member of the vinca alkaloid family. It has the weakest affinity to tubulins, but is shown to have unique receptor-independent antiangiogenesis, and antimetastasis properties. After administration, it is distributed extensively into tissues, metabolized via the CYP3A4 system, and eventually excreted in urine and feces. Phase II/III trials reported activities of vinflunine in advanced stage nonsmall-cell lung cancer, metastatic breast cancer, metastatic renal cell carcinoma, transitional cell carcinomas of the urothelium, small-cell lung cancer, and malignant pleural mesothelioma as monotherapy and in combination with other chemotherapy agents. More ongoing trials are evaluating its use in other solid tumors and in combination regimens. The most common adverse events in these trials were hematological (anemia and neutropenia), constipation, fatigue, abdominal pain, and myalgia.

Conclusions: Vinflunine is a new vinca alkaloid for the treatment of advanced staged solid tumors. Available data showed promising activities in various malignancies. Further studies are needed to further define vinflunine’s role in oncology.

Microtubule targeting agents: from biophysics to proteomics

This review explores various aspects of the interaction between microtubule targeting agents and tubulin, including binding site, affinity, and drug resistance. Starting with the basics of tubulin polymerization and microtubule targeting agent binding, we then highlight how the three-dimensional structures of drug-tubulin complexes obtained on stabilized tubulin are seeded by precise biological and biophysical data. New avenues opened by thermodynamics analysis, high throughput screening, and proteomics for the molecular pharmacology of these drugs are presented. The amount of data generated by biophysical, proteomic and cellular techniques shed more light onto the microtubule-tubulin equilibrium and tubulin-drug interaction. Combining these approaches provides new insight into the mechanism of action of known microtubule interacting agents and rapid in-depth characterization of next generation molecules targeting the interaction between microtubules and associated modulators of their dynamics. This will facilitate the design of improved and/or alternative chemotherapies targeting the microtubule cytoskeleton.

Investigation of the micellar effect of pluronic P85 on P-glycoprotein inhibition: cell accumulation and equilibrium dialysis studies

The objective of this study was: (1) to characterize the P-gp inhibitory effect of different concentrations of Pluronic P85 on anti-HIV-1 drug cellular accumulation, and (2) to investigate the relationship between cellular accumulation and free fraction of drug. Cellular accumulation studies in MDCKII-WT and MDCKII-MDR1 cell monolayers showed a biphasic dose response characterized by decline in accumulation at Pluronic concentrations greater than the CMC. This phenomenon was independent of the inhibition of P-gp efflux by Pluronic. Cell-free equilibrium dialysis was used to determine the effect of Pluronic P85 on drug free fraction and the affinity of Pluronic micelles for drug was modeled. Nelfinavir and saquinavir associated extensively with micelles and equilibrium free fractions were low at P85 concentrations above the CMC, with association constants being in the order nelfinavir > saquinavir >>> abacavir. Abacavir, a P-gp substrate, showed no association with micelles yet showed a biphasic response in cellular accumulation. These data suggest that, above the CMC, inhibition of P-gp is not affected but rather factors such as micellar trapping could contribute to decreased accumulation. Therefore, the in vitro evaluation of the effect of Pluronic formulations on active transport should take into account both the physicochemical properties of drug and the composition of Pluronic.

Macromolecular interaction of halichondrin B analogues eribulin (E7389) and ER-076349 with tubulin by analytical ultracentrifugation

Halichondrin B is an antimitotic drug that inhibits microtubule assembly. To understand the molecular details of its interaction with tubulin, we investigated the binding of two halichondrin B analogues, eribulin (previously, ER-086526, E7389) and ER-076349, to tubulin by quantitative analytical ultracentrifugation. Eribulin is currently undergoing phase III clinical trials for cancer; ER-076349 is a closely related analogue with C.35 hydroxyl instead of C.35 primary amine [Towle, M. J., et al. (2001) Cancer Res. 61, 1013]. Below the critical concentration for microtubule assembly and in the presence of GDP, tubulin undergoes weak self-association into short curved oligomers. Eribulin inhibits this oligomer formation 4-6-fold, while ER-076349 slightly stimulates oligomer formation by 2-fold. This is in contrast to vinblastine which strongly stimulates large spiral polymers by 1000-fold under these same conditions. Vinblastine-induced spiral formation is strongly inhibited by both eribulin and ER-076349. Colchicine binding to the intradimer interface has no significant effect on small oligomer formation or the inhibitory activity of eribulin on this process. These results suggest that halichondrin B analogues bind to the interdimer interface or to the beta-subunit alone, disrupt polymer stability, and compete with vinblastine-induced spiral formation. Stathmin is known to form a tight 1:2 complex with tubulin. Eribulin strongly inhibits formation of the 1:2 stathmin-tubulin complex (>3.3 kcal/mol), while ER-076349 weakens formation of the 1:2 complex by approximately 1.9 kcal/mol. These results suggest that eribulin is a global inhibitor of tubulin polymer formation, disrupting tubulin-tubulin contacts at the interdimer interface. ER-076349 also perturbs tubulin-tubulin contacts, but in a more polymer specific manner, reflecting adaptability of the interdimer interface to drug and polymer polymorphism. These results suggest halichondrin B analogues exhibit unique tubulin-based activities that may underlie the clinical utility of these compounds.

Characterization of highly stable liposomal and immunoliposomal formulations of vincristine and vinblastine

Purpose

Liposome and immunoliposome formulations of two vinca alkaloids, vincristine and vinblastine, were prepared using intraliposomal triethylammonium sucroseoctasulfate and examined for their ability to stabilize the drug for targeted drug delivery in vivo.

Methods

The pharmacokinetics of both the encapsulated drug (vincristine or vinblastine) and liposomal carrier were examined in Sprague Dawley rats, and the in vivo drug release rates determined. Anti-HER2 immunoliposomal vincristine was prepared from a human anti-HER2/neu scFv and studied for targeted cytotoxic activity in cell culture, and antitumor efficacy in vivo.

Results

Nanoliposome formulations of vincristine and vinblastine demonstrated similar pharmacokinetic profiles for the liposomal carrier, but increased clearance for liposome encapsulated vinblastine (t_1/2 = 9.7 h) relative to vincristine (t_1/2 = 18.5 h). Immunoliposome formulations of vincristine targeted to HER2 using an anti-HER2 scFv antibody fragment displayed a marked enhancement in cytotoxicity when compared to non-targeted liposomal vincristine control; 63- or 253-fold for BT474 and SKBR3 breast cancer cells, respectively. Target-specific activity was also demonstrated in HER2-overexpressing human tumor xenografts, where the HER2-targeted formulation was significantly more efficacious than either free vincristine or non-targeted liposomal vincristine.

Conclusions

These results demonstrate that active targeting of solid tumors with liposomal formulations of vincristine is possible when the resulting immunoliposomes are sufficiently stabilized.

Exploring the mechanisms of action of the novel microtubule inhibitor vinflunine

Microtubules have been identified as a suitable target for anticancer therapy, primarily based on their biological importance in coordinating chromosomal segregation at mitosis. Two main classes of microtubule-targeted agents, the taxanes and vinca alkaloids, suppress the dynamic behavior of spindle microtubules, inducing mitotic arrest and subsequent apoptotic cell death. Clinical activity of taxanes and first-generation vinca alkaloids in the treatment of solid tumors and hematologic malignancies, respectively, has prompted further research for novel analogs with improved clinical efficacy and safety. Such efforts have led to the development of vinflunine, a bifluorinated vinca alkaloid endowed with unique antitumor properties. Highlighted in this review are the key features of vinflunine that lead to effective suppression of microtubule dynamics and induction of cell death in cancer cells.

Comparative study of the radiosensitizing and cell cycle effects of vinflunine and vinorelbine, in vitro

Background

Vinca alkaloids are an important class of anticancer agents and semisynthetic vinca alkaloids are developed to improve the therapeutic index of this class of drugs. In the present study, a direct comparison was made between vinflunine and vinorelbine regarding their radiosensitising and cell cycle effects.

Methods

Four human tumour cell lines were tested under identical experimental conditions, using equitoxic concentrations of vinflunine and vinorelbine.

Results

Vinflunine and vinorelbine induced a comparable radiosensitising effect (p-value never below 0.01) when cells were incubated for 24 h immediately prior to radiation. Regarding the cell cycle effects, a statistically significant concentration-dependent G2/M block was seen after 24 h incubation with vinorelbine in all tested cell lines. Similar results, with small cell line-related differences, were observed with vinflunine.

Conclusion

The radiosensitising effects of both semisynthetic vinca alkaloids were comparable (not statistically different) and nearly always cell line-specific and concentration-dependent. The cell cycle effects could be related to the observed radiosensitising effects. Considering the more favourable toxicity profile of vinflunine, this agent might be more promising than vinorelbine for chemoradiation studies in the clinic.

Further mechanistic unravelling of the influence of the cell cycle effects on the radiosensitising mechanism of vinflunine, in vitro

PURPOSE

Vinflunine is an innovative microtubule inhibitor belonging to the vinca alkaloid class that possesses radiosensitising properties, which could lead to promising activity in chemoradiation studies in the clinic.

METHOD

In the current study, different incubation times with vinflunine, immediately before radiation and different time intervals between vinflunine treatment and radiation were investigated, in vitro, using four different human tumour cell lines differing in cell type and p53 status. Results were correlated with the cell cycle distribution at the moment of radiation, in order to elucidate the role of cell cycle perturbations caused by vinflunine on its radiosensitising effect.

RESULTS

Radiosensitisation was observed in all cell lines, and maximal radiosensitisation was both cell line- and schedule-dependent. The cell cycle distributions were cell line-dependent also, and when correlated with the observed radiosensitising effects could explain many (but not all) of the radiosensitising properties of vinflunine.

CONCLUSION

The cell cycle perturbations caused by vinflunine may definitely have an impact on its radiosensitising potential, but other factors must play a role because of some unaccountable differences between cell cycle distribution and the radiosensitising potential.

Peloruside A synergizes with other microtubule stabilizing agents in cultured cancer cell lines

The microtubule stabilizing agent peloruside A binds to a unique site on the tubulin alpha,beta-heterodimer compared to taxoid site drugs such as paclitaxel (Taxol), docetaxel (Taxotere), epothilone A, and discodermolide. Because the binding sites differ, peloruside A may be able to synergize with these taxoid site drugs when added in combination to cultured cells. Ovarian carcinoma cells (1A9) and myeloid leukemic cells (HL-60) were treated with different concentrations of peloruside A and taxoid site drugs, both compounds given singly and in combination in the nanomolar range, and the antiproliferative activity, G2/M blocking potency, and microtubule stabilizing activity of the treatments assessed. Cell proliferation was monitored using the MTT cell proliferation assay, cell cycle block was determined by flow cytometry, and stabilization of the tubulin polymer was assessed by Western blotting for beta-tubulin distributions in supernatant and pellet fractions of cell lysates. A combination index (CI) was calculated from the equation CI = D1/Dx1 + D2/Dx2 in which D1 and D2 are the concentrations of drug 1 and drug 2 that in combination give the same response as drug 1 alone (Dx1) or drug 2 alone (Dx2). A CI of less than 1 indicates synergy, equal to 1, additivity, and greater than 1, antagonism. Confidence intervals for each CI value were obtained using a bootstrapping procedure. In cell proliferation assays, statistically significant synergy was found between peloruside A and paclitaxel and epothilone A. Combinations of these two taxoid site drugs, however, also showed synergy in their effects on cell proliferation. These results confirm that peloruside A, when added in combination with other microtubule stabilizing agents, acts synergistically to enhance the antimitotic action of the drugs, but also highlight the complexity of drug interactions in intact cells.

Drug target interaction of tubulin-binding drugs in cancer therapy

Microtubules and their component protein, tubulin, constitute a popular target for the treatment of cancer. Many drugs that are presently used in clinics or in clinical trials and drugs that show promise as anticancer drugs bind to tubulin and microtubules. There are three conventional binding sites on beta-tubulin where many of these drugs bind. The binding properties, conformational changes upon binding, association constants and thermodynamic parameters for the drug-tubulin interaction on these three sites are discussed. The antiproliferative activities of these drugs and the possible correlation with the binding properties are also described.

The role of the anticancer drug vinorelbine in lipid bilayers using differential scanning calorimetry and molecular modeling

Differential scanning calorimetry (DSC) has been employed to investigate the thermal changes caused by the anticancer alkaloid drug vinorelbine in dipalmytoylphosphatidylcholine (DPPC) bilayers. The total enthalpy change was increased by the presence of the drug molecule, indicating a partial interdigitation of the lipid alkyl chains. The presence of cholesterol in DPPC bilayers including vinorelbine induced an obstruction of the interdigitation, since cholesterol interrupts the upraise of enthalpy change. Vinorelbine's interdigitation ability and stabilizing properties with the active site of the receptor have been compared with those of similar in structure amphipathic and bulky alkaloid vinblastine. The obtained results may in part explain their similar mechanism of action but different bioactivity.

The thermodynamics of vinca alkaloid-induced tubulin spirals formation

Vinca alkaloids are antimitotic, anticancer agents that induce tubulin to form spiral polymers at physiological protein concentrations. We used sedimentation velocity to investigate the effects of six vinca alkaloids on tubulin spiraling. Fitting to a Wyman linkage model reveals a drug dependent change of over two orders of magnitude in spiraling potential, K(1)K(2). Thermodynamic analysis of LnK(1)K(2) data demonstrates large and positive DeltaS values, indicating that tubulin spiral formation is entropically-driven. From the curvature in van't Hoff plots of vinblastine data, we estimate DeltaC(p) for GTP and GDP conditions to be -439 and -396 cal/mol K. Partitioning of DeltaS into the hydrophobic effect, DeltaS(HE), change in rotational/translational freedom, DeltaS(RT) and change in protein conformation, DeltaS(other), demonstrates that the major driving force for tubulin spiral formation is burial of hydrophobic surfaces and that protein conformational changes do not make a significant contribution. Spiraling potential is an indicator of antimitotic activity in vivo, although turbidity studies indicate that there is no correlation between spiraling potential and microtubule inhibition in vitro. Mechanisms that explain this discrepancy are discussed.

Cell cycle effects of vinflunine, the most recent promising Vinca alkaloid, and its interaction with radiation, in vitro

PURPOSE

Vinflunine (VFL) is a novel third generation Vinca alkaloid with superior antitumour activity in preclinical models and an anticipated more favourable toxicity profile compared to the other Vinca alkaloids.

METHOD

We investigate the radiosensitising properties of VFL and its cell cycle effects in four human tumour cell lines (ECV304, MCF-7, H292, and CAL-27). The sulforhodamine B test was used to determine cell survival, and cell cycle analysis was performed by flow cytometry. Radiosensitisation (RS) was represented by dose enhancement factors (DEFs).

RESULTS

Twenty-four hours treatment with VFL before radiation caused dose-dependent RS in all cell lines. This was most pronounced in ECV304 cells with RS already at VFL concentrations that reduced cell survival by 10% (IC10). DEFs ranged from 1.57 to 2.29 in the different cell lines. A concentration-dependent G2/M block was observed (starting at 4 h of incubation). After maximal G2/M blockade cells started cycling again, mainly by mitosis, while a small portion of cells started a polyploid cell cycle. Also drug removal immediately caused recycling of cells and induction of a polyploid cell population. The polyploid cell population was most impressively noticeable after prolonged incubation with VFL (48 h), in particular in CAL-27 and ECV304. This was never observed in a tested normal fibroblast cell line (Fi 360). The fate of these cells is of particular interest, but yet uncertain.

CONCLUSION

VFL has radiosensitising potential. The exact role of the cell cycle effects of VFL in its radiosensitising mechanism is still not fully elucidated and requires further study.

Structural basis for the regulation of tubulin by vinblastine

Vinblastine is one of several tubulin-targeting Vinca alkaloids that have been responsible for many chemotherapeutic successes since their introduction in the clinic as antitumour drugs. In contrast with the two other classes of small tubulin-binding molecules (Taxol and colchicine), the binding site of vinblastine is largely unknown and the molecular mechanism of this drug has remained elusive. Here we report the X-ray structure of vinblastine bound to tubulin in a complex with the RB3 protein stathmin-like domain (RB3-SLD). Vinblastine introduces a wedge at the interface of two tubulin molecules and thus interferes with tubulin assembly. Together with electron microscopical and biochemical data, the structure explains vinblastine-induced tubulin self-association into spiral aggregates at the expense of microtubule growth. It also shows that vinblastine and the amino-terminal part of RB3-SLD binding sites share a hydrophobic groove on the alpha-tubulin surface that is located at an intermolecular contact in microtubules. This is an attractive target for drugs designed to perturb microtubule dynamics by interfacial interference, for which tubulin seems ideally suited because of its propensity to self-associate.

Liposome-encapsulated vincristine, vinblastine and vinorelbine: A comparative study of drug loading and retention

A comparative study of the loading and retention properties of three structurally very closely related vinca alkaloids (vincristine, vinorelbine and vinblastine) in liposomal formulations has been performed. All three vinca alkaloids showed high levels of encapsulation when accumulated into egg sphingomyelin/cholesterol vesicles in response to a transmembrane pH gradient generated by the use of the ionophore A23187 and encapsulated MgSO4. However, despite the close similarities of their structures the different vinca drugs exhibited very different release behavior, with vinblastine and vinorelbine being released faster than vincristine both in vitro and in vivo. The differences in loading and retention can be related to the lipophilicity of the drugs tested, where the more hydrophobic drugs are released more rapidly. It was also found that increasing the drug-to-lipid ratio significantly enhanced the retention of vinca alkaloids when the ionophore-based method was used for drug loading. In contrast, drug retention was not dependent on the initial drug-to-lipid ratio for vinca drugs loaded into liposomes containing an acidic citrate buffer. The differences in retention can be explained on the basis of differences in the physical state of the drug inside the liposomes. The drug-to-lipid ratio dependence of retention observed for liposomes loaded with the ionophore technique may provide a way to improve the retention characteristics of liposomal formulations of vinca drugs.

Mechanism of mitotic block and inhibition of cell proliferation by the semisynthetic Vinca alkaloids vinorelbine and its newer derivative vinflunine

The two second-generation Vinca alkaloids, vinorelbine and vinflunine, affect microtubule dynamics very differently from vinblastine, a first generation Vinca alkaloid. For example, vinblastine strongly suppresses the rate and extent of microtubule shortening in vitro, whereas vinorelbine and vinflunine suppress the rate and extent of microtubule growing events. We asked whether these differences result in differences in mitotic spindle organization that might be responsible for the superior antitumor activities of the two second-generation Vinca alkaloids. IC(50) values for inhibition of HeLa cell proliferation for vinflunine, vinorelbine, and vinblastine were 18, 1.25, and 0.45 nM, respectively, similar to the concentrations that induced mitotic block at the metaphase/anaphase transition (38, 3.8, and 1.1 nM, respectively), indicating that mitotic block is a major contributor to antiproliferative action for all three drugs. Mitotically blocked cells exhibited aberrant spindles, consistent with induction of block by suppression of microtubule dynamics. Despite differences in their actions on individual dynamic instability parameters, morphologically detectable differences in spindle effects among the three drugs were minimal, indicating that overall suppression of dynamics may be more important in blocking mitosis than specific effects on growth or shortening. We also found that the peak intracellular drug concentration at the mitotic IC(50) value was highest for vinflunine (4.2 +/- 0.2 microM), intermediate for vinorelbine (1.3 +/- 0.1 microM), and more than 10-fold lower for vinblastine (130 +/- 7 nM), suggesting that intracellular binding reservoir(s) may be partially responsible for vinflunine's high efficacy and minimal side effects.