The relationship between Taxol and (+)-discodermolide: synthetic analogs and modeling studies

Design, Synthesis, and Evaluation of Carbamate-Substituted Analogues of (+)-Discodermolide

[Structure: see text] The design, syntheses, and biological evaluation of 22 totally synthetic analogues of the potent microtubule-stabilizing agent (+)-discodermolide (1) have been achieved. Structure-activity relationships of the C(19) carbamate were defined, exploiting two synthetically simplified scaffolds, as well as the parent (+)-discodermolide framework.

The clinical development of new mitotic inhibitors that stabilize the microtubule

Microtubule-stabilizing agents are increasingly studied for cancer treatment based largely on the prior success of paclitaxel and docetaxel. In this review, we focus on the clinical development of epothilones and discodermolide, and we discuss salient preclinical and clinical highlights of these two novel natural products. These agents are distinguished by their biochemical features making them poor P-glycoprotein substrates and capable of inducing cytotoxicity in cell lines or in vivo tumor models harboring mutations in tubulin. There is now considerable data regarding the efficacy of the epothilones in human beings and discodermolide holds such promise, as well.

Marine natural products and related compounds in clinical and advanced preclinical trials

The marine environment has proven to be a very rich source of extremely potent compounds that have demonstrated significant activities in antitumor, antiinflammatory, analgesia, immunomodulation, allergy, and anti-viral assays. Although the case can and has been made that the nucleosides such as Ara-A and Ara-C are derived from knowledge gained from investigations of bioactive marine nucleosides, no drug directly from marine sources (whether isolated or by total synthesis) has yet made it to the commercial sector in any disease. However, as shown in this review, there are now significant numbers of very interesting molecules that have come from marine sources, or have been synthesized as a result of knowledge gained from a prototypical compound, that are either in or approaching Phase II/III clinical trials in cancer, analgesia, allergy, and cognitive diseases. A substantial number of other potential agents are following in their wake in preclinical trials in these and in other diseases.

Novel biologically active natural and unnatural products

Natural products have been used as medicinal agents for many years. In addition, these compounds have served as the starting points for semisynthetic analogs with improved properties. This review highlights work on several classes of natural products and their derivatives, including both well established and emerging structural classes that are in, or nearing, clinical use for a variety of important indications.

3D QSAR studies for the β-tubulin binding site of microtubule-stabilizing anticancer agents (MSAAs)

The antimitotic agent paclitaxel continues to play an important role in the cancer chemotherapy. However, its inefficacy on certain resistant cells and toxic side effects have led to the search of new taxanes with improved biological activity. By means of a pseudoreceptor modeling approach, we have developed a binding site model for a series of taxanes. It is the first 3D QSAR model derived from the experimentally determined tubulin structure obtained by electron crystallography studies. The model is able to correlate quantitatively the structural properties of the studied compounds with their biological data.

Simplified discodermolide analogues: synthesis and biological evaluation of 4-epi-7-dehydroxy-14,16-didemethyl-(+)-discodermolides as microtubule-stabilizing agents

Several novel analogues of (+)-discodermolide were synthesized via a convergent strategy that used Wittig reactions to append left and right side chains to a central scaffold and then tested for biological activity. Three of the analogues in the 4-epi-7-dehydroxy-14,16-didemethyl series, 6a-c, had interesting actions. The C3-methoxymethyl ether analogue 6b was more active in antiproliferative cell-based assays as well as in hypernucleation and paclitaxel site competition assays with isolated tubulin than the other analogues, including 6a, which contained a free hydroxyl group at the C3 position. The biological results validated the initial hypothesis that the C7 hydroxy group and the C14 and C16 methyl groups of (+)-discodermolide could be deleted without undermining activity. Although less potent than (+)-discodermolide and paclitaxel, compounds 6b and 6c both showed properties unique to (+)-discodermolide. These properties, particularly the capacity to cause hypernucleation of isolated tubulin at lower temperature than paclitaxel, as well as stabilizing preformed microtubules to cold disassembly, are considered mechanistically superior to those of paclitaxel. Other variations in the right and left sides of the discodermolide scaffold revealed additional structure/activity information.

Tubulin polymerizing activity of dictyostatin-1, a polyketide of marine sponge origin

Dictyostatin-1 had previously been isolated from a marine sponge of the genus Spongia sp. and described as a cytotoxic agent to murine and human cancer cells, but its mechanism of activity was unknown. In a routine screening assay used to detect cytotoxic compounds of marine origin, dictyostatin-1 was identified as a highly active component in an extract from a Lithistida sponge and exploration into its pharmacology was pursued. Initial studies demonstrated that dictyostatin-1 arrested cells in the G(2)/M phase of the cell cycle. Staining of these cells with antitubulin revealed cells having multiple aster formations and microtubule matrix bundling patterns similar to that seen in cells exposed to paclitaxel. Dictyostatin-1 was able to induce the polymerization of purified bovine brain tubulin in vitro and the polymerized tubulin remained stable at cold temperatures. Dictyostatin-1 also proved to be highly potent in two paclitaxel-resistant human cancer cell lines expressing active P-glycoprotein. Together, these results indicate that dictyostatin-1 is a potent inducer of tubulin polymerization and retains activity in cells expressing the P-glycoprotein efflux pump.

Discodermolide/Dictyostatin hybrids: synthesis and biological evaluation

[structure: see text] Two hybrid analogues of discodermolide and dictyostatin (3, 26) have been designed and synthesized. These are the first macrocyclic analogues of discodermolide and biological activities were evaluated and compared with linear discodermolide analogues.

Progress toward the development of agents to modulate the cell cycle

With taxanes continuing to prove useful in the clinical treatment of cancer, the next generation of antimitotic agents has entered clinical trials. Other mechanisms awaiting proof-of-concept for the treatment of antiproliferative diseases include inhibition of cyclin-dependent kinases (Cdks). Flavopiridol and UCN-01 are continuing in clinical trials, and newer more selective Cdk inhibitors are now entering clinical evaluation.

Simultaneous preparation of four truncated analogues of discodermolide by fluorous mixture synthesis

[structure: see text] Four truncated analogues of the natural product discodermolide were synthesized in a single synthetic sequence. Precursors bearing four different groups at C22, each with a unique fluorous p-methoxybenzyl substituent on the C17 hydroxy group, were mixed and taken through an nine-step sequence. Demixing by fluorous chromatography followed by deprotection and purification provided the individual analogues in 3-7% overall yields and with a savings of 24 synthetic steps. Fluorous mixture synthesis is recommended as a new technique to make multiple natural product analogues in a single multistep synthesis.

Novel molecules that interact with microtubules and have functional activity similar to Taxol

Taxol is an antitumor drug approved by the FDA for the treatment of ovarian, breast and non-small-cell lung carcinomas. Originally isolated from the bark of the Pacific yew, Taxus brevifolia, it was the first natural product described that stabilized microtubules. In the past five years, a group of novel natural products, including the epothilones, discodermolide, eleutherobin, sarcodictyins and the laulimalides, all of which have biological activities similar to those of Taxol, has been discovered. In this review, we discuss each of these novel microtubule-stabilizing agents and the search for a common pharmacophore among them, taking into consideration recent advances in our understanding of the taxanes and tubulin.