Paclitaxel

Paclitaxel is a drug used in the treatment of cancer. It was discovered at Research Triangle Institute (RTI) in 1967 when Monroe E. Wall and Mansukh C. Wani isolated the compound from the bark of the Pacific yew tree, Taxus brevifolia, and noted its antitumour activity in a broad range of rodent tumours. By 1970, the two scientists had determined the structure of paclitaxel, which is extremely complex. Paclitaxel has since become an effective tool of doctors who treat patients with lung, ovarian, breast cancer, and advanced forms of Kaposi's sarcoma (Saville et. al 1995). It is sold under the tradename Taxol. Together with docetaxel, it forms the drug category of the taxanes. Paclitaxel is also used for the prevention of restenosis (recurrent narrowing) of coronary stents; locally delivered to the wall of the coronary artery, a paclitaxel coating limits the growth of neointima (scar tissue) within stents (Heldman et. al 2001).

History

The history of paclitaxel begins with a 1958 National Cancer Institute study that commissioned Department of Agriculture botanists to collect samples of over 30,000 plants to test for anticancer properties. Arthur S. Barclay, one of those botanists, collected 15 lbs of twigs, needles, and bark from Pacific yew trees in a forest near Mount St. Helens. Months later, in 1963, Monroe E. Wall discovered that bark extract from the Pacific yew possessed antitumour qualities, beginning to reveal the tree's hidden treasure. Soon after, Wall and his colleague Mansukh C. Wani were busy isolating and purifying plant compounds for anticancer tests in Research Triangle Park, North Carolina. In 1967 the team had isolated the active ingredient, announcing their findings at an American Chemical Society meeting in Miami Beach. Wall and Wani published their results, including the chemical structure, in a 1971 issue of the Journal of the American Chemical Society.

The paper was noticed immediately by Robert A. Holton who was starting postdoctoral research at Stanford University in natural products synthesis. But, it would be several years before he dedicated his attention to synthesizing pacilitaxel at Florida State University, quelling an emerging environmental controversy; a 40-foot Pacific yew tree, which may have taken 200 years to reach that height, yields only a half gram of paclitaxel, but Holton's group perfected a four-step procedure to convert 10-deacetylbaccatin (a related compound in Pacific yew needles) into paclitaxel.

In the late 1970s, Susan B. Horwitz, a molecular pharmacologist at Albert Einstein College of Medicine in New York City, unravelled the key mystery of how paclitaxel works. Largely in part of an enormous research and development effort, starting in government facilities and later in commercial labs, paclitaxel quickly became an all-time best-selling pharmaceutical. Paclitaxel was brought to the market by Bristol-Myers Squibb in 1993 as Taxol®. Annual sales peaked in 2000, reaching US$1.6 billion.

Production

 Undisturbed Pacific Yew bark contains paclitaxel and related chemicals. The bark must be peeled and processed to provide paclitaxel.Unfortunately, the Pacific yew is one of the slowest growing trees in the world. Furthermore, the treatment of just one patient requires the cutting down and processing of six 100-year old trees. This supply problem combined with the threat to the endangered spotted owl (Strix occidentalis) has prompted researchers to develop actinobacteria from which paclitaxel-like compounds can be obtained by fermentation. Cultures of the fungus Nodulisporium sylviforme can be used to produce paclitaxel itself.

Although other paclitaxel-like compounds may be extracted from various parts of yew trees, these are not as potent as paclitaxel itself. Total synthesis provides one means of accessing paclitaxel from petrochemical-derived starting materials, the results of which are summarised in the Taxol total synthesis.

However, total synthesis is not an economically feasible way of manufacturing paclitaxel, so it is commercially produced by semisynthesis. 10-Deacetylbaccatin can be extracted in relatively large amounts from various yew-related species and is easily converted by several steps of organic synthesis into paclitaxel. Cell-cultures can also be used to provide the starting 10-deactylbaccatin material.

Method of action

Paclitaxel interferes with the normal function of microtubule growth. Whereas drugs like colchicine cause the depolymerization of microtubules in vivo, paclitaxel arrests their function by having the opposite effect; it hyper-stabilizes their structure. This destroys the cell's ability to use its cytoskeleton in a flexible manner. Specifically, paclitaxel binds to the β subunit of tubulin. Tubulin is the "building block" of microtubules, and the binding of paclitaxel locks these building blocks in place. The resulting microtubule/paclitaxel complex does not have the ability to disassemble. This adversely affects cell function because the shortening and lengthening of microtubules (termed dynamic instability) is necessary for their function as a mechanism to transport other cellular components. For example, during mitosis, microtubules position the chromosomes during their replication and subsequent separation into the two daughter-cell nuclei.

Further research has indicated that paclitaxel induces programmed cell death (apoptosis) in cancer cells by binding to an apoptosis stopping protein called Bcl-2 (B-cell leukaemia 2) and thus arresting its function.

One common characteristic of most cancer cells is their rapid rate of cell division. In order to accommodate this, the cytoskeleton of a cell undergoes extensive restructuring. Paclitaxel is an effective treatment for aggressive cancers because it adversely affects the process of cell division by preventing this restructuring. Cancer cells are also destroyed by the aforementioned anti-Bcl-2 mechanism. Other cells are also affected adversely, but since cancer cells divide much faster than non-cancerous cells, they are far more susceptible to paclitaxel treatment.

Marketing

The license to commercialize and market Paclitaxel (as Taxol®) was held by the Bristol-Myers Squibb Co., which was selected for this role by the U.S. National Cancer Institute. Bristol-Myers held an exclusive contract in the harvesting of yew trees from US government lands; it was criticized for having a "cancer monopoly" (Palast p.188).

ABI-007

In January 2005 the Food and Drug Administration (FDA) approved Abraxane® (ABI-007) for the treatment of breast cancer after failure of combination chemotherapy for metastatic disease or relapse within six months of adjuvant chemotherapy. In this preparation, paclitaxel is bonded to albumin as the delivery agent as an alternative to solvent (often toxic) delivery.. The manufacturer heralds the drug as a breakthrough in nanotechnology.

Source: wikipedia GFDL

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Page last modified: May 2007