Bioprocessing of plant in vitro systems for the mass production of pharmaceutically important metabolites: paclitaxel and its derivatives.

Taxol (paclitaxel) and its derivatives are microtubule-stabilizing drugs widely used in the treatment of several types of cancer, including mammary, prostate, ovarian and non-small-cell lung carcinoma, as well as AIDS-associated Kaposi's sarcoma and other types of tumor. Taxanes stabilize microtubules by enhancing their polymerization and inhibiting depolymerization. Microtubule dynamics are crucial to mitotic spindle formation and function; therefore, cells exposed to taxanes are unable to undergo chromosomal separation during mitosis, become arrested in the G2/M phases of the cell cycle, and are subsequently targeted for apoptosis. Plant cell cultures are used for industrial-scale biotechnological production of important bioactive plant secondary metabolites, including the anticancer agent paclitaxel. In the last two decades, there have been numerous empirical approaches to improve the biotechnological production of taxanes, leading to the conclusion that treatment of Taxus sp. cells with methyl jasmonate or other elicitors is the most effective strategy. However, little insight has been gained into how the elicitors increase taxane biosynthesis or how this process is regulated. In recent years, with the help of "omics" tools, a rational approach has provided new information about taxane metabolism and its control. Once pathway bottlenecks have been identified, it will be possible to engineer Taxus sp. cell lines with overexpression of genes that control the flux-limiting steps, thus boosting taxane productivity. This review describes the chemical and biological characterization of paclitaxel and its derivatives and discusses future prospects for their biotechnological production.

The relationship between TXS, DBAT, BAPT and DBTNBT gene expression and taxane production during the development of Taxus baccata plantlets.

Taxol and related taxane accumulation in plants is regulated by the expression of genes involved in their biosynthesis. Although the metabolic pathway leading to taxol has been almost completely elucidated, comparatively little is known about the rate-limiting steps and their regulation. In this paper we report on a study of taxane production in Taxus baccata plantlets grown in vitro for 1 year. The relationship between taxane patterns and the expression of genes encoding the enzymes taxadiene synthase (TXS), 10-deacetylbaccatin III-10ß-O-acetyltransferase (DBAT), baccatin III 13-O-(3-amino-3-phenylpropanoyl) transferase (BAPT) and 3'-N-debenzoyl-2'-deoxytaxol-N-benzoyltransferase (DBTNBT), involved in early and late steps of the taxane pathway, has been considered. A far higher content was found in the aerial part of the plantlets than in the roots. The most abundant taxane in the aerial parts was 10-deacetylbaccatin III, which increased as the plantlets grew, indicating a low conversion to baccatin III and taxol. In contrast, the levels of 10-deacetylbaccatin III in the roots remained lower than those of taxol. These results correlated with transcript accumulation of the studied genes, since in the aerial parts the expression of DBAT, which codes for the enzyme that converts 10-deacetylbaccatin III into baccatin III, did not increase with the age of plantlets, unlike that of TXS, BAPT and DBTNBT, suggesting that this gene controls a rate-limiting step in the taxane biosynthetic pathway. The lower taxane levels found in the roots also correlated with gene expression, since only the early pathway gene TXS was induced in this organ during the 1-year growth period.

Biotechnological production of taxol and related taxoids: current state and prospects.

Taxol is one of the most effective anti-cancer drugs ever developed. The natural source of taxol is the inner bark of several Taxus species, but it accumulates at a very low concentration and with a prohibitively high cost of extraction. Another problem is that the use of inner bark for taxol production implies the destruction of yew trees. For all these reasons, the growing demand for taxol greatly exceeds the supply that can be sustained by isolation from its natural source and alternative sources of the drug are being sought. Although taxol has been prepared by total synthesis, the process is not commercially viable. Taxol can also be semisynthetically produced via the conversion of baccatin III or 10-deacethylbaccatinIII found in Taxus needles but the cost and difficulty of the extraction process of the semisynthetic precursors are also very high. The most promising approach for the sustainable production of taxol and related taxoids is provided by plant cell cultures at an industrial level. Taxol is currently being clinically used against different tumour processes but due to the difficulty of its extraction and formulation, as well as the growing demand for the compound, new taxol analogues with improved properties are being studied. In this revision we discuss current research in the design of new taxol-related compounds, the chemical structure/anti-cancer activity relationship and new formulations of the drug. We also consider the optimizing strategies to improve taxol and related taxoid production in cell cultures, as well as the current knowledge of taxol metabolism, all of which are illustrated with examples, some of them from our own research.