Paclitaxel

Paclitaxel - What is Paclitaxel?

Paclitaxel is a mitotic inhibitor used in cancer chemotherapy. It was discovered in a National Cancer Institute program at the Research Triangle Institute in 1967 when Monroe E. Wall and Mansukh C. Wani isolated it from the bark of the Pacific Yew tree, ''Taxus brevifolia'' and named it 'taxol'.

When it was developed commercially by Bristol-Myers Squibb (BMS) the generic name was changed to 'paclitaxel' and the BMS compound is sold under the trademark 'TAXOL'. In this formulation, paclitaxel is dissolved in Cremophor EL and ethanol, as a delivery agent. A newer formulation, in which paclitaxel is bound to albumin, is sold under the trademark Abraxane.

Paclitaxel is now used to treat patients with lung, ovarian, breast cancer, head and neck cancer, and advanced forms of Kaposi's sarcoma. Paclitaxel is also used for the prevention of restenosis.

Paclitaxel stabilizes microtubules and as a result, interferes with the normal breakdown of microtubules during cell division. Together with docetaxel, it forms the drug category of the taxanes. It was the subject of a notable total synthesis by Robert A. Holton.

As well as offering substantial improvement in patient care, paclitaxel has been a relatively controversial drug. There was originally concern because of the environmental impact of its original sourcing, no longer used, from the Pacific yew. In addition, the assignment of rights, and even the name itself, to Bristol-Myers Squibb were the subject of public debate and Congressional hearings.

Paclitaxel is approved in the UK for ovarian cancer, breast cancer, lung cancer. It is also used in the treatment of Kaposi's sarcoma.

It is recommended in NICE guidance of June 2001 that it should be used for non-small cell lung cancer in patients unsuitable for curative treatment, and in first-line and second-line treatment of ovarian cancer. In September 2001 NICE recommended that paclitaxel should be available for the treatment of advanced breast cancer after the failure of anthracyclic chemotherapy, but that its first-line use should be limited to clinical trials. In September 2006 NICE recommended that paclitaxel should ''not'' be used in the adjuvant treatment of early node-positive breast cancer.

The cost to the NHS per patient in early breast cancer, assuming four cycles of treatment, is about $6000.

Similar compounds

The closely related taxane docetaxel has a similar set of clinical uses to paclitaxel. It is marketed under the name of Taxotere.

Much of the clinical toxicity of paclitaxel is associated with the solvent Cremophor in which it is dissolved for delivery.

Abraxis Bioscience developed Abraxane, in which paclitaxel is bonded to albumin as an alternative delivery agent as an alternative to the often toxic solvent delivery method. This was approved by the Food and Drug Administration in January 2005 for the treatment of breast cancer after failure of combination chemotherapy for metastatic disease or relapse within six months of adjuvant chemotherapy.

Cremophor EL is used in paclitaxel possibly as a dose limiting agent because of its toxicities. (A formulation of paclitaxel that uses nanoparticle albumin instead of Cremophor EL is marketed as an alternative under the trade name of Abraxane.

Restenosis

Paclitaxel is used as an anti-proliferative agent 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. Paclitaxel drug eluting coated stents are sold under the trade name Taxus by Boston Scientific in the United States.

Aside from its direct clinical use, paclitaxel is used extensively in biological and biomedical research as a microtubule stabilizer. In vitro assays involving microtubules, such as motility assays, generally rely on paclitaxel to maintain microtubule integrity in the absence of the various nucleating factors and other stabilizing elements found in the cell. For example, it is used for in vitro tests of drugs that aim to alter the behavior of microtubule motor proteins, or for studies of mutant motor proteins.

Paclitaxel is sometimes used for in vivo studies as well; it can be fed to test organisms such as fruit flies or injected into individual cells, to inhibit microtubule disassembly or to increase the number of microtubules in the cell.

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Paclitaxel History

The plant screening program, isolation, and preclinical trials

In 1955 the National Cancer Institute (NCI) set up the Cancer Chemotherapy National Service Center (CCNSC) to act as a public screening center for anti-cancer activity in compounds submitted by external institutions and companies. Although the majority of compounds screened were of synthetic origin, one chemist, Jonathan Hartwell, who was employed there from 1958 onwards, had had experience of natural product derived compounds and began a plant screening operation. After some years of informal arrangements, in July 1960 the NCI commissioned USDA botanists to collect samples from about 1000 plant species per year. On August 21, 1962, one of those botanists, Arthur S. Barclay, collected bark from a single Pacific yew tree, ''Taxus brevifolia'', in a forest north of the town of Packwood as part of a four month trip collecting material from over 200 different species. The material was then processed by a number of specialist CCNSC subcontractors and one of the ''Taxus'' samples was found to be cytotoxic in a cellular assay on May 22, 1964.. They named the pure compound 'taxol' in June 1967.

Wall and his colleague Wani published their results, including the chemical structure, in 1971.

The NCI continued to commission work to collect more ''Taxus'' bark and to isolate increasing quantities of taxol. By 1969 28 kg of crude extract had been isolated from almost 1,200 kg of bark, although this ultimately yielded only 10g of pure material. But for several years no use was made of the compound by the NCI. In 1975 it was shown to be active in another ''in vitro'' system ; two years later a new department head reviewed the data and finally recommended that taxol be moved on to the next stage in the discovery process. This required increasing quantities of purified taxol, up to 600g, and in 1977 a further request for 7,000 lbs of bark was made.

In 1978, two NCI researchers published a report showing that taxol was mildly effective in leukaemic mice.

In November 1978, taxol was shown to be effective in xenograft studies.

Meanwhile taxol began to be well known in the cell biology, as well as the cancer community, with a publication in early 1979 by Susan B. Horwitz, a molecular pharmacologist at Albert Einstein College of Medicine, that showed that taxol had a previously unknown mechanism of action involving the stabilization of microtubules. Together with formulation problems, this increased interest from researchers meant that by 1980 the NCI envisaged needing to collect 20,000 lbs of bark.

Animal toxicology studies were complete by June 1982, and in November NCI applied for the IND necessary to begin clinical trials in humans. These larger trials needed more bark and collection of a further 12,000 pounds was commissioned, which enabled some phase II trials to begin by the end of 1986. But by then it was recognized that the demand for taxol might be substantial and that more than 60,000 pounds of bark might be needed as a minimum. This unprecedentedly large amount brought ecological concerns about the impact on yew populations into focus for the first time as local politicians and foresters expressed unease at the program.

The first public report from a phase II trial in May 1988 showed an effect in melanoma patients and a remarkable response rate of 30% in patients with refractory ovarian cancer.

At this point Gordon Cragg of the NCI's Natural Product Branch calculated that the synthesis of enough taxol to treat all the ovarian cancer and melanoma cases in the US would require the destruction of 360,000 trees annually. For the first time, serious consideration was given to the problem of supply.

Although widely advertised, only four companies responded to the CRADA, including Bristol-Myers Squibb (BMS), who were selected as the partner in December 1989. The choice of BMS later became controversial and was the subject of Congressional hearings in 1991 and 1992. While it seems clear that the NCI had little choice but to seek a commercial partner, there was also controversy about the terms of the deal, eventually leading to a report by the General Accounting Office in 2003 which concluded that the NIH had failed to ensure value for money. In related CRADAs with the USDA and Department of the Interior, Bristol-Myers Squibb were given exclusive first refusal on all Federal supplies of ''Taxus brevifolia''.

This exclusive contract lead to some criticism for giving BMS a "cancer monopoly."

Eighteen months after the CRADA, BMS filed a new drug application (NDA) which was given FDA approval at the very end of 1992.

BMS argued that changing the name would cause confusion among oncologists and possibly endanger the health of patients. BMS has continued to defend its rights to the name in the courts.

BMS have also been criticized for misrepresentation by Goodman and Walsh, who quote from a company report saying that

It was not until 1971 that ... testing ... enabled the isolation of paclitaxel, initially described as 'compound 17'

This quote is strictly speaking accurate: the objection seems to be that this misleadingly neglects to explain that it was the scientist doing the isolation who named the compound taxol and it was not referred to in any other way for more than twenty years.

Annual sales peaked in 2000, reaching US$1.6 billion; paclitaxel is now available in generic form.

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Paclitaxel Production

From 1967 to 1993, almost all paclitaxel produced was derived from bark from the Pacific yew, the harvesting of which kills the tree in the process. The processes used were descendants of the original isolation method of Wall and Wani; by 1987 the NCI had contracted Hauser Chemical Research of Boulder, Colorado to handle bark on the scale needed for Phase II and III trials. While there was considerable uncertainty about how large the wild population of ''Taxus brevifola'' was and what the eventual demand for taxol would be, it had been clear for many years that an alternative, sustainable source of supply would be needed. Initial attempts used needles from the tree, or material from other related ''Taxus'' species, including cultivated ones. But these attempts were bedevilled by the relatively low and often highly variable yields obtained. It was not until the early 1990s, at a time of increased sensitivity to the ecology of the forests of the Pacific Northwest, that taxol was successfully extracted on a clinically useful scale from these sources.

From the late 1970s, chemists in the US and France had been interested in taxol. A number of US groups, including one led by Robert A. Holton, attempted a total synthesis of the molecule, starting from petrochemical-derived starting materials. This work was primarily motivated as a way of generating chemical knowledge, rather than with any expectation of developing a practical production technique. By contrast the French group of Pierre Potier at the CNRS quickly recognized the problem of yield. His laboratory was on a campus populated by the related yew ''Taxus baccata'', so that needles were available locally in large quantity. By 1981 he had shown that it was feasible to isolate relatively large quantities of the compound 10-deacetylbaccatin, a plausible first step for a semi-synthetic production route to taxol. By 1988 he co-published such a semi-synthetic route from needles of ''Taxus baccata''.

The view of the NCI, however, was that even this route was not practical.

By 1988, and particularly with Potier's publication, it was clear to Holton as well that a practical semi-synthetic production route would be important. By late 1989, Holton's group had developed a semi-synthetic route to paclitaxel with twice the yield of the Potier process. Florida State University, where Holton worked, signed a deal with Bristol-Myers Squibb to license this and future patents. In 1992, Holton patented an improved process with an 80% yield. BMS took the process in-house and started to manufacture paclitaxel in Ireland from 10-deacetylbaccatin isolated from the needles of the European yew. This starts from a specific taxus cell line propagated in aqueous medium in large fermentation tanks. Paclitaxel is then extracted directly, purified by chromatography and isolated by crystallization. Compared to the semi-synthesis, PCF eliminates the need for many hazardous chemicals and saves a considerable amount of energy.

In 1993 it was discovered that taxol was coincidentally produced in a newly described fungus living in the yew tree. It has since been found in a number of other endophytic fungi, including ''Nodulisporium sylviforme,'' opening the possibility of taxol production by culturing one of these fungal species.

The initial motivation for synthetic approaches to paclitaxel included the opportunity to create closely related compounds. Indeed this approach led to the development of docetaxel.

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How Paclitaxel Works

Paclitaxel interferes with the normal function of microtubule breakdown. Whereas drugs like colchicine cause the depolarization of microtubules, 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 leukemia 2) and thus arresting its function.

In addition to stabilizing microtubules paclitaxel may act as a molecular mop by sequestering free tubulin effectively depleting the cells supply of tubulin monomers and/or dimers. This activity may trigger the aforementioned apoptosis.

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.

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