Paclitaxel and Docetaxel

Taxol binds specifically to the [3-tubulin subunit of microtubules and appears to antagonize the disassembly of this key cytoskeletal protein, with the result that bundles of microtubules and 

FIGURE 62.1 Chemical structures of paclitaxel (taxol) and its more potent analog, docetaxel (taxotere). 

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The taxanes (e.g., paclitaxel and docetaxel) are a newer class of agents that rival the anthracyclines in their activity in metastatic breast cancer, becoming (arguably) the most active class of chemotherapy for this disease. 

Other suggested mechanisms for the increased radiosensitization seen with both paclitaxel and docetaxel include an increased alpha component of DNA damage and the fact that docetaxel is toxic for S-phase cells that are maximally radiation resistant (41,42). 

The initial experience with the taxanes and especially with paclitaxel in the realm of combined modality therapy has had a substantial impact on the treatment of cancers both in the United States and worldwide. Paclitaxel delivered in concert with radiation provides a classical model of the development of clinically applicable treatment strategies from laboratory-based studies. The initial in vitro works of Tishler (39) and Choy (40) have translated in a very tangible way into approaches that are clinically applicable and in the next generation of randomized clinical trials their efficacy will be compared to more traditional chemotherapies in the combined modality setting. While the experience to date with both paclitaxel and docetaxel has been largely positive, the mortality rates in many of the solid tumor types remind us that much more needs to be done. 

The taxanes, paclitaxel and docetaxel, act by promoting the assembly of microtubules. The taxanes induce cell cycle arrest in the most radiosensitive phase, G2/M thereby promoting radiosensitivity. In vitro studies have also demonstrated apoptosis in cell culture lines with paclitaxel (32). Paclitaxel dose-dependent synergy is also seen in squamous carcinoma cell lines (33). Much literature has been published reg arding the use of taxanes when combined with radiotherapy. 

Alkaloids are nitrogen-based compounds frequently found in plants. Plant alkaloids that are used in treating cancer in humans include traditional agents, such as vincristine and vinblastine, and newer agents, such as vinorelbine, paclitaxel, and docetaxel (Table 36-4). These agents are also known as antimitotic drugs be-... 

Paclitaxel and docetaxel have been shown to act as spindle poisons, causing cell division cycle arrest, based on a unique mechanism of action.7-10 These drugs bind to the P-subunit of the tubulin heterodimer, the key constituent protein of cellular microtubules (spindles). The binding of these drugs accelerates the tubulin polymerization, but at the same time stabilizes the resultant microtubules, thereby inhibiting their depolymerization. The inhibition of microtubule depolymerization between the prophase and anaphase of mitosis results in the arrest of the cell division cycle, which eventually leads to the apoptosis of cancer cells. 

In this chapter, we describe an account of our research on the chemistry and biology of paclitaxel and taxoid anticancer agents (taxoid = taxol-like compound). The topics covered in this chapter include (i) the development of a practical and efficient method for the semisynthesis of paclitaxel and docetaxel using chiral 3-hydroxy-P-lactams as synthetic intermediates, (ii) structure-activity relationship (SAR) studies of various taxoids that led to the discovery of the extremely potent second-generation taxoids, and (iii) biological and conformational studies with the use of fluorine-containing taxoids as probes. ... 

II. HIGHLY EFFICIENT SEMISYNTHESIS OF PACLITAXEL AND DOCETAXEL BY MEANS OF THE P-LACTAM SYNTHON METHOD... 

DAB (3) through efficient hydrogenation over rhodium on carbon.38 Similar paclitaxel and docetaxel analogues containing cyclohexyl groups were independently reported by Georg and coworkers.56... 

The improved activity of 3 -alkyl and 3 -alkenyl taxoids described above clearly establishes the dispensability of aromatic character at the C-3 position. Next, we investigated the effects of C-2 modification on cytotoxicity by replacing the C-2 benzoate moiety with nonaromatic ester groups. Replacement of the 2-benzoate with simple alkyl and alkenyl esters in conjunction with modification at C-3 provides a series of novel taxoids devoid of all the aromatic groups of paclitaxel and docetaxel.44... 

As Table 8 shows, several of the new taxoids exhibit similar or enhanced activity as compared to paclitaxel and docetaxel. Taxoid 48 (SB-T-104221 nonataxel) with 2-methylprop- 1-enyl groups at both C-3 and C-2 is more active than both paclitaxel and docetaxel, clearly establishing that a benzoate group at C-2 is not necessary for strong cytotoxicity in lieu of the appropriate alkyl, alkenyl, or cyclohexyl group. Taxoids 49-51 bearing a combination of 2-methylprop-1-enyl and 2-methylpropyl groups at C-3 and C-2 were, however, all less potent than 48. The results clearly indicate the considerable sensitivity for the modification at these positions. Based... 

Cytotoxicity of 3 -CF3-taxoids 71 and 72a-h thus obtained were evaluated against human cancer cell lines and the results are summarized in Table ll.45 As Table 11 shows, all these taxoids possess excellent activities, and are substantially more potent than either paclitaxel or docetaxel in virtually every case. The most remarkable results are, however, one order of magnitude better activities of the 10-acylated taxoids 72a-h as compared to paclitaxel and docetaxel against the drug-resistant breast cancer cell line, MCF7-R. The marked difference in cytotoxicity observed between 3 -(2-CF3-ethyl) taxoid, 68 or 69, and 3 -CF3-taxoids 72 reconfirms high sensitivity of the C-3 position to the size of substituent for the biological activity. 

The use of 19F NMR for a variable temperature (VT) NMR study of fluorinated taxoids is obviously advantageous over the use of H NMR because of the wide dispersion of the l9F chemical shifts that allows fast dynamic processes to be frozen out. Accordingly, F2-paclitaxel 65 and F-docetaxel 66 were selected as probes for the study of the solution structures and dynamic behavior of paclitaxel and docetaxel, respectively, in protic and aprotic solvent systems.77 The inactive 2, 10-diacetyldocetaxel (73) was also prepared to investigate the role of the 2 -hydroxyl moiety in the conformational dynamics.89 While molecular modeling and NMR analyses (at room temperature) of 73 indicate that there is no significant conformational changes as compared to paclitaxel, the 19F NMR VT study clearly indicates that this modification exerts marked effects on the dynamic behavior of the molecule.77... 

P-Hydroxy-10-deacetylbaccatin III (75, 14P-OH-DAB) was first isolated from the needles of the Himalayan yew tree (Taxus wallichiana Zucc.) and its structure was determined by X-ray crystallographic analysis in 1992.93 Because of an extra hydroxyl group at the C-14 position, 14P-OH-DAB (75) has much higher water solubility than DAB (3), the key precursor of paclitaxel and docetaxel. We envisaged that new taxoids derived from 75 would improve water solubility and bioavailability, and also reduce hydrophobicity-related drug resistance. These improved... 

It is very important to clarify the minimum structural requirements for paclitaxel and taxoids to exhibit anticancer activity by looking at simplified structure analogues. Along this line, we have investigated the role of the A ring by synthesizing novel nor-seco analogues of paclitaxel and docetaxel.37,96... 

In order to extract paclitaxel and docetaxel (the I.S.) selectively from cell lysates, while simultaneously reducing the quantity of matrix components that were also derived from the samples, the SPE wash and elution conditions were optimized in a manner similar as these described in the paradigm 1. Detailed experimental conditions can be found in a previous publication [5],... 

Fig. 10 Typical LC-MS/MS chromatograms of paclitaxel and docetaxel. Cell lysates were spiked with paclitaxel to a final concentration equaling the LOQ (5 pg/mL) and with the I.S. docetaxel at a final concentration of 300 pg/mL. Both SRM and HRSRM were evaluated, (a) SRM of docetaxel (b) HR-SRM of docetaxel (c) SRM of paclitaxel (d) HR-SRM of paclitaxel (Reproduced with permission from Elsevier)...

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