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How does Docetaxel works ? by liu meimei





How does Docetaxel works ? by
Article Posted: 11/12/2013
Article Views: 139
Articles Written: 168
Word Count: 805
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How does Docetaxel works ?


 
Health
Docetaxel injection is used alone or in combination with other medications to treat certain types of breast, lung, prostate, stomach, and head and neck cancers. Docetaxel injection is in a class of medications called taxanes. It works by stopping the growth and spread of cancer cells.

Absorption and distribution

Intravenous administration of docetaxel results in 100% bioavailability and absorption is immediate. 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 was shown to be greater than 98% plasma protein bound independent of concentration at 37 °C and pH 7.4 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.Polysorbate 80 may be the cause of hypersensitivity reasons in taxanes as recent studies indicated.

The concentration-time profile of docetaxel was consistent with a three-compartment pharmacokinetic model. An initial, relatively rapid decline, with an a half-life of mean 4.5 minutes is representative of distribution to peripheral compartments from the systemic circulation. A ß half-life of mean 38.3 minutes and a relatively slow ? half-life of mean 12.2 hours represent the slow efflux of docetaxel from the peripheral compartment.

Administration a 100 mg/m² dose over a one hour infusion gave a mean total body clearance of 21 L/h/m² and a mean steady state volume of distribution of 73.8 L/m² or 123 L based on the mean BSA (body surface area) of 1.68 m². Area under the plasma concentration-time curve had a mean value of 2.8 mg.h/L. The Cmax of docetaxel was found to be 4.15 ± 1.35 mg/L. 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.

2. Metabolism and excretion

Docetaxel is mainly metabolised in the liver by the cytochrome P450 CYP3A4 and CYP3A5 subfamilies of isoenzymes. 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). M1 and M3 are two diasteromeric hydroxyoxazolidinones and M4 is an oxazolidinedione. Phase II trials of 577 patients showed docetaxel clearance to be related to body surface area and; hepatic enzyme and alpha1 acid glycoprotein, plasma levels.The following model is agreed to represent docetaxel clearance in humans:

CL = BSA · (22.1 - 3.55·AAG - 0.095·AGE + 0.2245·ALB) · (1 - 0.334·HEP12)

where CL is total body clearance (L/h), BSA is total body surface area (m²), AAG and ALB represent alpha1 acid glycoprotein and albumin plasma concentrations (g/L) respectively, and AGE is the patients age (years). HEP12 represents a measure of hepatic dysfunction, affecting clearance of docetaxel. This final model accounted for a modest proportion of patients and identified most of the patients varying from the model (population median of CL = 35.6 L/h) as having hepatic dysfunction, indicating hepatic function as the most unpredictable factor with regards to clearance variability.

Patients with significant hepatic dysfunction had an approximately 30% decrease in clearance of docetaxel and were also at a higher risk of toxicity poisoning from docetaxel treatment. Clearance has been shown from population pharmacokinetic studies to decrease significantly with age, increased alpha1 acid glycoprotein and albumin concentrations and decreased body surface area.

Renal impairment is unlikely to affect metabolism or excretion of docetaxel as renal excretion contributes less than 5% of elimination. Limited data is available for docetaxel use in children with dosage between 55 and 75 mg/m². Two paediatric studies have taken place that show a mean clearance of 33 L/h/m² and concentration-time profiles best fitted by a two-compartmental model of distribution and elimination. Mean distribution half-life was 0.09 hours and mean elimination half-life was 1.4 hours in paediatric studies.

Biodistribution of 14C-labelled docetaxel in three patients showed the bulk of the drug to be metabolised and excreted in bile to the faeces. Of the radioactively labelled docetaxel administered, 80% was eliminated to the faeces with 5% in the urine over seven days, an indication that urinary excretion of docetaxel is minimal. Saliva contributed minimal excretion and no excretion was detected through pulmonary means. The terminal half-life of docetaxel was determined as approximately 86 hours, through prolonged plasma sampling, contrary to the clinically stated terminal half-life of 10–18 hours.

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