Neoplastic diseases or cancers, characterized by the proliferation of cells not subject to normal growth regulation, are a major cause of death in humans. An estimated 1,221,800 new cases and 561,000 deaths are expected to occur in 1999. Lung cancer remains the leading cause of cancer-related deaths in the United States; the estimated 158,900 deaths would account for 28% of the total.
Clinical experience in chemotherapy has demonstrated that new and more effective cytotoxic drugs are desirable to treat these diseases. Such experience has also demonstrated that drugs which disrupt the microtubule system of the cytoskeleton can be effective in inhibiting the proliferation of neoplastic cells.
The microtubule system of eucaryotic cells is a major component of the cytoskeleton and is in a dynamic state of assembly and disassembly; that is, heterodimers of tubulin are polymerized to form microtubules, and microtubules are depolymerized to their constituent components. Microtubules play a key role in the regulation of cell architecture, metabolism, and division, and the dynamic state of the microtubules is critical to their normal function. With respect to cell division, tubulin is polymerized into microtubules that form the mitotic spindle. The microtubules are then depolymerized when the mitotic spindle's role has been fulfilled. Accordingly, agents which disrupt the polymerization or depolymerization of microtubules, and thereby inhibit cell growth, comprise some of the most effective chemotherapeutic agents in clinical use.
Such anti-mitotic agents or poisons all kinetically inhibit the normal dynamics of microtubules. There are subtle differences between certain classes of anti-microtubule agents based on their molecular mechanism of action. Colchicine binds to soluble tubulin and then is incorporated into a growing microtubule, vinblastine binds to the microtubule end and thereby suppresses microtubule dynamics. At high concentrations, both colchicine and vinblastine cause the loss of cellular microtubules. Paclitaxel (more commonly known as Taxol™) and related taxanes also inhibit microtubule dynamics, yet at high concentrations these agents cause an increase in polymerized tubulin in the cell and thick microtubule bundles are formed.
Paclitaxel was first isolated in 1971 in the bark of the Pacific yew tree (Taxus brevifolia), and was approved in 1992 by the US Food and Drug Administration for treatment of metastatic ovarian cancer and later for breast cancer. Paclitaxel has attracted unusually strong scientific attention, not only because of its unique anti-proliferative mechanism of action, but also because it is unique anti-tumors. The discovery of the effectiveness of the natural product paclitaxel lead to the production and testing of semisynthetic congeners including docitaxel (Taxotere). These compounds, taxanes, are now recognized as a new class of anti-cancer compounds.
One drawback of paclitaxel is its extreme insolubility: Paclitaxel can be administered effectively only in a solvent including cremophor, which combination can provoke severe hypersensitive immune responses. As a result of these drawbacks, it is considered desirable to explore the use of other naturally-occurring compounds with similar modes of action.
In addition, merely having activity as an antimitotic agent does not guarantee efficacy against a tumor cell, and certainly not a tumor cell which exhibits a drug-resistant phenotype. Vinca alkaloids, such as vinblastine and vincristine, and taxanes are effective against neoplastic cells and tumors, yet they lack or display reduced activity against drug-resistant tumors and cells. One basis for a neoplastic cell displaying drug resistance (DR) or multiple-drug resistance (MDR) is through the over-expression of P-glycoprotein. Compounds which are poor substrates for transport of P-glycoprotein should be useful in circumventing such DR or MDR phenotypes.
Accordingly, the exhibition of the DR or MDR phenotype by many tumor cells and the clinically proven mode of action of anti-microtubule agents against neoplastic cells necessitates the development of anti-microtubule agents cytotoxic to non-drug resistant neoplastic cells as well as cytotoxic to neoplastic cells with a drug resistant phenotype.
Since the discovery of the mechanism of action of paclitaxel, only three other non-taxane chemical classes (epothilones A and B, discodermolide, and eleutherobin and related sarcodictyins A and B) have been identified that possess a similar mode of action. The epothilones were isolated from the myxobacterium Sorangium cellulosum as a result of a large-scale screening effort. The epothilones have generated significant interest, as they retain activity against drug-resistant cell lines. Epothilones have been isolated from a species of bacteria found in soil samples collected from the banks of the Zambesi River in the Republic of South Africa, and have been recently synthesized.
Discodermolide was purified from the marine sponge Discodermia dissoluta as an immunosuppressant and was screened for anti-mitotic activity on the basis of a predictive structure-activity relationship when compared with other tubulin-interacting drugs. Discodermolide promotes tubulin assembly more potently than paclitaxel and it is an effective inhibitor of cell growth in paclitaxel-resistant cells.
Eleutherobin, a potent cytotoxin from the soft coral eleutherobia sp., promotes tubulin polymerization but exhibits cross-resistance to paclitaxel-resistant cell lines. The potential therapeutic usefulness of these new microtubule-stabilizing compounds, and whether they will provide advantages over the taxanes, have yet to be determined.
Accordingly, it remains desirable to identify additional naturally-occurring compounds with modes of action similar to the taxanes, but which display different tissue specificity, solubility, and/or activity against drug-resistant, and particularly multiple-drug resistant, tumors and cells.