Docetaxel (trade name Taxotere) is a clinically well established anti-mitotic chemotherapy medication used mainly for the treatment of breast, ovarian, and non-small cell lung cancer. Docetaxel has an approved claim for treatment of patients who have locally advanced, or metastatic breast or non small-cell lung cancer who have undergone anthracycline-based chemotherapy and failed to stop cancer progression or relapsed. Administered as a one-hour infusion every three weeks generally over a ten cycle course, docetaxel is considered as or more effective than doxorubicin, paclitaxel and fluorouracil as a cytotoxic antimicrotubule agent. Annual sales approx $2bn. Patent expires in 2010.
Docetaxel is of the chemotherapy drug class; taxane, and is a semi-synthetic analogue of paclitaxel (Taxol), an extract from the rare Pacific yew tree Taxus brevifolia. In this T-shaped/butterfly model, a deep hydrophobic cleft exists near the surface of the β-tubulin where three potential hydrogen bonds and multiple hydrophobic contacts bind to docetaxel. The hydrophobic pocket walls contain helices H1, H6, H7 and a loop between H6 and H7 that form hydrophobic interactions with the 3’-benzamido phenyl, 3’-phenyl, and the 2-benzoyl phenyl of docetaxel. 3’-phenyl also has contact with β-sheets B8 and B10. The C-8 methyl of docetaxel has Van der Waals interactions with two residues, Thr-276 and Gln-281 near the C-terminal end of β-tubulin. Docetaxel’s O-21 experiences electrostatic attraction to Thr-276 and the C-12 methyl has proximity with Leu-371 on the loop between B9 and B10. Oral bioavailability has been found to be 8% ±6% on its own and, when co-administered with cyclosporine, bioavailability increased to 90% ± 44%. In practice, docetaxel is administered intravenously only to increase dose precision. Evaluation of docetaxel pharmacokinetics in phase II and III clinical studies were with 100 mg/m² dosages given over one-hour infusions every three weeks. Docetaxel's plasma protein binding includes lipoproteins, alpha1 acid glycoprotein and albumin. Alpha1 acid glycoprotein is the most variable of these proteins inter-individually, especially in cancer patients and is therefore the main determinant of docetaxel's plasma binding variability. Docetaxel interacted little with erythrocytes and was unaffected by the polysorbate 80 in its storage medium. Increased dose resulted in a linear increase of the area under the concentration-time curve and so it is concluded that dose is directly proportional to plasma concentration. Metabolism is principally oxidative and at the tert-butylpropionate side chain, resulting first in an alcohol docetaxel (M2), which is then cyclised to three further metabolites (M1, M3 and M4). This binding stabilises microtubules and prevents depolymerisation from calcium ions, decreased temperature and dilution, preferentially at the plus end of the microtubule. This leads to a significant decrease in free tubulin, needed for microtubule formation and results in inhibition of mitotic cell division between metaphase and anaphase, preventing further cancer cell progeny. Compared with mitoxantrone treatment, docetaxel treated patients showed a 12% overall response rate and mitoxantrone showed a 7% overall response rate. Another large advantage of docetaxel was increased quality of life. Docetaxel showed a 22% response and mitoxantrone had a 13% response. Used in conjunction with prednisone for pain management, docetaxel had a 35% response and Mitoxantrone had a 22% response. This trial leads docetaxel to be a preferred method of treatment to Mitoxantrone where possible.
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