Differential Effects of Natural Product Microtubule Stabilizers on Microtubule Assembly: Single Agent and Combination Studies with Taxol, Epothilone B, and Discodermolide

Microtubule-stabilizing agents: New drug discovery and cancer therapy

Microtubule-stabilizing agents (MSAs) have been highly successful in the treatment of cancer in the past 20years. To date, three classes of MSAs have entered the clinical trial stage or have been approved for clinical anticancer chemotherapy, and more than 10 classes of novel structural MSAs have been derived from natural resources. The microtubule typically contains two MSA-binding sites: the taxoid site and the laulimalide/peloruside site. All defined MSAs are known to bind at either of these sites, with subtle but significant differences. MSAs with different binding sites may produce a synergistic effect. Although having been extensively applied in the clinical setting, paclitaxel and other approved MSAs still pose many challenges such as multidrug resistance, low bioavailability, poor solubility, high toxicity, and low passage through the blood-brain barrier. A variety of studies focus on the structure-activity relationship in order to improve the pharmaceutical properties of these agents. Here, the mechanisms of action, advancements in pharmacological research, and clinical developments of defined MSAs during the past decade are discussed. The latest discovered MSAs are also briefly introduced in this review. The increasing number of natural MSAs indicates the potential discovery of more novel, natural MSAs with different structural bases, which will further promote the development of anticancer chemotherapy.

MT-Stabilizer, Dictyostatin, Exhibits Prolonged Brain Retention and Activity: Potential Therapeutic Implications

Inclusions comprising the microtubule (MT)-stabilizing protein, tau, are found within neurons in the brains of patients with Alzheimer's disease and related neurodegenerative disorders that are broadly referred to as tauopathies. The sequestration of tau into inclusions is believed to cause a loss of tau function, such that MT structure and function are compromised, leading to neuronal damage. Recent data reveal that the brain-penetrant MT-stabilizing agent, epothilone D (EpoD), improves cognitive function and decreases both neuron loss and tau pathology in transgenic mouse models of tauopathy. There is thus a need to identify additional MT-stabilizing compounds with blood-brain barrier (BBB) permeability and slow brain clearance, as observed with EpoD. We report here that the MT-stabilizing natural product, dictyostatin, crosses the BBB in mice and has extended brain retention. Moreover, a single administration of dictyostatin to mice causes prolonged stabilization of MTs in the brain. In contrast, the structurally related MT-stabilizer, discodermolide, shows significantly less brain exposure. Thus, dictyostatin merits further investigation as a potential tauopathy therapeutic.

Modulation of protein-protein interactions as a therapeutic strategy for the treatment of neurodegenerative tauopathies

The recognition that malfunction of the microtubule (MT) associated protein tau is likely to play a defining role in the onset and/or progression of a number of neurodegenerative diseases, including Alzheimer's disease, has resulted in the initiation of drug discovery programs that target this protein. Tau is an endogenous MT-stabilizing agent that is highly expressed in the axons of neurons. The MT-stabilizing function of tau is essential for the axonal transport of proteins, neurotransmitters and other cellular constituents. Under pathological conditions, tau misfolding and aggregation results in axonal transport deficits that appear to have deleterious consequences for the affected neurons, leading to synapse dysfunction and, ultimately, neuronal loss. This review focuses on both progress and unresolved issues surrounding the development of novel therapeutics for the treatment of neurodegenerative tauopathies, which are based on (A) MT-stabilizing agents to compensate for the loss of normal tau function, and (B) small molecule inhibitors of tau aggregation.

Synergistic interactions between peloruside A and other microtubule-stabilizing and destabilizing agents in cultured human ovarian carcinoma cells and murine T cells

PURPOSE

Microtubule-stabilizing agents are an important class of anticancer compounds. Peloruside A and laulimalide bind to a different site on the microtubule to taxoid site drugs such as paclitaxel (Taxol(®)), docetaxel (Taxotere(®)), ixabepilone (Ixempra(®)), the epothilones, and discodermolide. The purpose of this study was to examine the synergistic interactions of these drugs when given in combination in relation to the differences in their binding sites on the microtubule.

METHODS

Human ovarian carcinoma cells (1A9 cells) and murine T cells were treated with different combinations of microtubule-stabilizing or destabilizing agents. The compounds were given individually and in combination, and the antiproliferative activity was assessed to calculate a combination index (CI) from the equation: CI = D(1)/Dx(1) + D(2)/Dx(2) in which D(1) and D(2) are the concentrations of drug 1 and drug 2 that when given together give the same response as drug 1 and 2 alone (Dx(1) and Dx(2)). Thus, a CI value of less than 1.0 indicates a synergistic effect between the two drugs in which the response to the two drugs given together is greater than the additive response of the two drugs if given on their own.

RESULTS

As anticipated from previous in vitro studies, peloruside A and laulimalide did not synergize with each other. They also failed to synergize with the microtubule-destabilizing agents vinblastine and 2-methoxyestradiol. Peloruside A and laulimalide did, however, synergize with the epothilones, as had been previously shown, but not with docetaxel or discodermolide.

CONCLUSIONS

Combining two microtubule-targeting agents with different binding sites does not guarantee a synergistic interaction in cells, and additional factors are likely to be involved. This study highlights the importance of preclinical testing of actual combinations of drugs before proceeding into clinical trials.

Epothilone analogues with benzimidazole and quinoline side chains: chemical synthesis, antiproliferative activity, and interactions with tubulin

A series of epothilone B and D analogues bearing isomeric quinoline or functionalized benzimidazole side chains has been prepared by chemical synthesis in a highly convergent manner. All analogues have been found to interact with the tubulin/microtubule system and to inhibit human cancer cell proliferation in vitro, albeit with different potencies (IC(50) values between 1 and 150 nM). The affinity of quinoline-based epothilone B and D analogues for stabilized microtubules clearly depends on the position of the N-atom in the quinoline system, while the induction of tubulin polymerization in vitro appears to be less sensitive to N-positioning. The potent inhibition of human cancer cell growth by epothilone analogues bearing functionalized benzimidazole side chains suggests that these systems might be conjugated with tumor-targeting moieties to form tumor-targeted prodrugs.

The laulimalide family: total synthesis and biological evaluation of neolaulimalide, isolaulimalide, laulimalide and a nonnatural analogue

We herein describe in full detail the first total synthesis of the antitumor agents neolaulimalide and isolaulimalide as well as a highly efficient route to laulimalide. A Kulinkovich reaction followed by a cyclopropyl-allyl rearrangement is used to install the exo-methylene group. The C(2)-C(16) aldehyde fragment is coupled with the C(17)-C(28) sulfone fragments by a highly (E)-selective Julia-Lythgoe-Kocienski olefination to deliver the key intermediates of all three syntheses. Various conditions for the Yamaguchi macrolactonization are applied to close the individual macrocycles. Finally a carefully elaborated endgame was developed to solve the problem of acyl migration in the case of neolaulimalide. All compounds were tested against several cell lines. The cytotoxicity of neolaulimalide could be confirmed for the first time since its original isolation and it could be shown that it induces tubulin polymerization as efficiently as laulimalide.

Evaluation of biosensor surfaces for the detection of microtubule perturbation

Dual polarization interferometry (DPI) and resonant mirror (RM) methods were used to characterize the growth of microtubules (MTs) on biosensor surfaces. The structure and dynamics of MTs play an important role in cell division and are a target for many anti-cancer drugs. Evidence from DPI demonstrated the growth of MTs on streptavidin-biotinylated-tubulin surfaces from the increase in mass and thickness, with a simultaneous decrease in density. The initial increase in thickness of 0.236 nm/min suggested the elongation of protofilaments before they join laterally to form the MT, where the rate of growth increased to 0.436 nm/min. Continuous mass increases were also observed when tubulin was added to a similar underlying RM surface. Tubulin binding to these surfaces was also temperature dependent, increasing the absolute response with MT stabilizers, while inhibiting binding with destabilizers when temperature was changed from 15 to 37 degrees C. Finally, the initial rates of tubulin assembly (mean+/-SD, n=3) with MT-stabilizer agents were significantly higher at 1.50+/-0.27 and 1.04+/-0.13 arcseconds/s, respectively, compared to 0.37+/-0.11 arcseconds/s for tubulin containing GTP only. In the presence of the MT destabilizers, colchicine and dolastatin 10, the slopes of initial rates were lower than in their absence at 0.05+/-0.01 and 0.27+/-0.08 arcseconds/s, respectively. This provides evidence for the ability of surface-based optical sensors to distinguish between MT stabilizers and destabilizers, while also paving the path to develop other methods to screen for MT-perturbing agents using the same underlying surface engineering.