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TARCEVA (ERLOTINIB): CLINICAL PHARMACOLOGY

Mechanism of Action

The mechanism of clinical antitumor action of erlotinib is not fully characterized. Erlotinib HCl (Tarceva) inhibits the intracellular phosphorylation of tyrosine kinase associated with the epidermal growth factor receptor (EGFR). Specificity of inhibition with regard to other tyrosine kinase receptors has not been fully characterized. EGFR is expressed on the cell surface of normal cells and cancer cells.

Pharmacokinetics

Absorption and Distribution:

Erlotinib is about 60% absorbed after oral administration and its bioavailability is substantially increased by food to almost 100%. Peak plasma levels occur 4 hours after dosing. The solubility of erlotinib is pH dependent. Erlotinib solubility decreases as pH increases. Co-administration of Tarceva with omeprazole, a proton pump inhibitor, decreased the erlotinib exposure [AUC] and maximum concentration [Cmax] by 46% and 61% respectively. When Erlotinib HCl (Tarceva) was administered 2 hours following a 300 mg dose of ranitidine, an H2 receptor antagonist, the erlotinib AUC was reduced by 33% and Cmax by 54%. When Tarceva (Erlotinib HCl) tablets was administered with ranitidine 150 mg twice daily (at least 10 h after the previous ranitidine evening dose and 2 h before the ranitidine morning dose), the erlotinib AUC and Cmax decreased by 15% and 17% respectively.

Following absorption, erlotinib is approximately 93% protein bound to plasma albumin and alpha-1 acid glycoprotein (AAG). Erlotinib has an apparent volume of distribution of 232 liters.

Metabolism and Excretion:

A population pharmacokinetic analysis in 591 patients receiving the single-agent Erlotinib HCl (Tarceva) tablets 2nd/3rd line regimen showed a median half-life of 36.2 hours. Time to reach steady state plasma concentration would therefore be 7-8 days. No significant relationships of clearance to covariates of patient age, body weight or gender were observed. Smokers had a 24% higher rate of erlotinib clearance.

An additional population pharmacokinetic analysis was conducted in 291 NSCLC patients administered single-agent erlotinib as maintenance treatment. This analysis demonstrated that covariates affecting erlotinib clearance in this patient population were similar to those seen in the prior single-agent pharmacokinetic analysis. No new covariate effects were identified.

A third population pharmacokinetic analysis was conducted that incorporated erlotinib data from 204 pancreatic cancer patients who received erlotinib plus gemcitabine. Similar results were observed to those seen in the prior single-agent pharmacokinetic analysis. No new covariate effects were identified. Coadministration of gemcitabine had no effect on erlotinib plasma clearance.

In vitro assays of cytochrome P450 metabolism showed that erlotinib is metabolized primarily by CYP3A4 and to a lesser extent by CYP1A2, and the extrahepatic isoform CYP1A1. Following a 100 mg oral dose, 91% of the dose was recovered: 83% in feces (1% of the dose as intact parent) and 8% in urine (0.3% of the dose as intact parent).

Cigarette smoking reduces erlotinib exposure. In the Phase 3 NSCLC trial, current smokers achieved erlotinib steady-state trough plasma concentrations which were approximately 2-fold less than the former smokers or patients who had never smoked. This effect was accompanied by a 24% increase in apparent erlotinib plasma clearance. In a separate study which evaluated the single-dose pharmacokinetics of erlotinib in healthy volunteers, current smokers cleared the drug faster than former smokers or volunteers who had never smoked. The AUC0-infinity in smokers was about 1/3 to 1/2 of that in never/former smokers. In another study which was conducted in NSCLC patients (N=35) who were current smokers, pharmacokinetic analyses at steady-state indicated a dose-proportional increase in erlotinib exposure when the Tarceva tablets dose was increased from 150 mg to 300 mg. However, the exact dose to be recommended for patients who currently smoke is unknown.

Special Populations:

Patients with Hepatic Impairment

Patients with hepatic impairment (total bilirubin > ULN or Child Pugh A, B and C) should be closely monitored during therapy with Tarceva. Treatment with Erlotinib tablets should be used with extra caution in patients with total bilirubin > 3 x ULN.

In vitro and in vivo evidence suggest that erlotinib is cleared primarily by the liver. However, erlotinib exposure was similar in patients with moderately impaired hepatic function (Child-Pugh B) compared with patients with adequate hepatic function including patients with primary liver cancer or hepatic metastases.

Patients with Renal Impairment

Less than 9% of a single dose is excreted in the urine. No clinical studies have been conducted in patients with compromised renal function.

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