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 Paclitaxel

Description of Paclitaxel: Paclitaxel is a compound extracted from the Pacific yew tree Taxus brevifolia with antineoplastic activity. Paclitaxel binds to tubulin and inhibits the disassembly of microtubules, thereby resulting in the inhibition of cell division. This agent also induces apoptosis by binding to and blocking the function of the apoptosis inhibitor protein Bcl-2 (B-cell Leukemia 2). Check for active clinical trials or closed clinical trials using this agent. (NCI Thesaurus).

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.

History

According to http://en.wikipedia.org/wiki/Paclitaxel, 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, Washington 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.

Accordingly, in late 1964 or early 1965, the fractionation and isolation laboratory run by Monroe E. Wall in Research Triangle Park, North Carolina, began work on fresh Taxus samples, isolating the active ingredient in September 1966 and announcing their findings at an April 1967 American Chemical Society meeting in Miami Beach. 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.

Production of Paclitaxel

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. In early 1993, BMS were able to announce that they would cease reliance on Pacific yew bark by the end of 1995, effectively terminating the ecological controversy over its use. This announcement also made good their commitment to develop an alternative supply route, made to the NCI in their CRADA application of 1989. Currently, all paclitaxel production for BMS uses plant cell fermentation (PCF) technology developed by the biotechnology company Phyton Biotech, Inc and carried out at their plant in Germany. 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.

DRUG DESCRIPTION

TAXOL (paclitaxel) Injection is a clear, colorless to slightly yellow viscous solution. It is supplied as a nonaqueous solution intended for dilution with a suitable parenteral fluid prior to intravenous infusion. TAXOL is available in 30 mg (5 mL), 100 mg (16.7 mL), and 300 mg (50 mL) multidose vials. Each mL of sterile nonpyrogenic solution contains 6 mg paclitaxel, 527 mg of purified Cremophor® EL* (polyoxyethylated castor oil) and 49.7% (v/v) dehydrated alcohol, USP. Paclitaxel is a natural product with antitumor activity. TAXOL (paclitaxel) is obtained via a semi-synthetic process from Taxus baccata. The chemical name for paclitaxel is 5β,20-Epoxy-1,2α,4,7β,10β,13α-hexahydroxytax-11-en-9-one 4,10-diacetate 2-benzoate 13-ester with (2R,3S)-N-benzoyl-3-phenylisoserine.  Paclitaxel is a white to off-white crystalline powder with the empirical formula C47H51NO14 and a molecular weight of 853.9. It is highly lipophilic, insoluble in water, and melts at around 216-217° C.