Absorption and Distribution
The mean oral bioavailability of gefitinib is 60%, with peak plasma levels occurring 3 to 7 hours after dosing. Food does not alter gefitinib bioavailability to a clinically meaningful extent. Gefitinib tablets can be administered with or without food. Gefitinib is extensively distributed throughout the body with a mean steady state volume of distribution of 1400 L following intravenous administration. In vitro binding of gefitinib to human plasma proteins (serum albumin and α1-acid glycoprotein) is 90%, independent of drug concentrations. Gefitinib is a substrate for the membrane transport P-glycoprotein (P-gp), but it is unlikely to influence gefitinib absorption as P-gp is saturated at higher concentrations.
Metabolism and Elimination
Gefitinib undergoes extensive hepatic metabolism in humans, predominantly by CYP3A4. Three sites of biotransformation have been identified: metabolism of the N-propoxymorpholino-group, demethylation of the methoxysubstituent on the quinazoline, and oxidative defluorination of the halogenated phenyl group. Five metabolites have been fully identified in fecal extracts and the major active component was O-desmethyl gefitinib produced by CYP2D6 metabolism and accounted for 14% of the dose.
Eight metabolites were identified in human plasma. Only O-desmethyl gefitinib has exposure comparable to gefitinib. Although this metabolite has similar EGFR-TK activity to gefitinib in the isolated enzyme assay, it had only 1/14 of the potency of gefitinib in one of the cell-based assays.
Gefitinib is cleared primarily by the liver, with total plasma clearance and elimination half-life of 48 hours after intravenous administration. The inter-subject variability (coefficient of variation) for AUC in healthy subjects was 67%. Daily oral administration of gefitinib to cancer patients resulted in a two-fold accumulation compared to single dose administration. Steady state plasma concentrations are achieved within 10 days after daily dosing. Excretion of gefitinib and its metabolites is predominantly via the feces (86%), with renal elimination accounting for less than 4% of the administered dose.
Specific Populations
Age, gender, body weight, ethnicity or renal function: Population pharmacokinetic analyses suggest that patient age, body weight, ethnicity (populations included) or creatinine clearance (above 20 mL/min) has no clinically meaningful effect on predicted steady state trough concentration of gefitinib. Population pharmacokinetic analyses of Study 1 showed that women had 27% higher exposure than men; however, this difference was not identified in the analyses of other gefitinib clinical studies. No dose adjustment based on patient gender is recommended.
Hepatic Impairment: The systemic exposure of gefitinib was compared between patients with mild, moderate, or severe hepatic impairment due to cirrhosis (according to Child-Pugh classification) and healthy subjects with normal hepatic function (N=10/group). The mean systemic exposure (AUC0-∞) was increased by 40% in patients with mild impairment, 263% in patients with moderate impairment, and 166% in patients with severe hepatic impairment. In a study comparing 13 patients with liver metastases and moderate hepatic impairment to 14 patients with liver metastases and normal hepatic function, the systemic exposure of gefitinib was similar [see Warnings and Precautions (5.2), Use in Specific Populations (8.7)].
CYP2D6 Poor metabolizer: CYP2D6 metabolizes gefitinib to O-desmethyl gefitinib in vitro. In healthy CYP2D6 poor metabolizers, O-desmethyl gefitinib concentration was not measurable and the mean exposure to gefitinib was 2-fold higher as compared to the extensive metabolizers. This increase in exposure in CYP2D6 poor metabolizers may be clinically important because some adverse drug reactions are related to higher exposure of gefitinib. No dose adjustment is recommended in patients with a known CYP2D6 poor metabolizer genotype, but these patients should be closely monitored for adverse reactions. The impact of CYP2D6 inhibiting drugs on gefitinib pharmacokinetics has not been evaluated. However, similar precautions should be used when administering CYP2D6 inhibitors with gefitinib tablets because of the possibility of increased exposure in these patients.
An exploratory exposure response analysis showed an increase in the incidence of interstitial lung disease (ILD) with a greater than 2-fold increase in the gefitinib exposure [see Warnings and Precautions (5.1)].
Drug-Drug Interactions
Strong CYP3A4 Inducer:
Concomitant administration of rifampicin (600 mg QD for 16 days), a strong inducer of CYP3A4, with gefitinib (500 mg single dose on Day 10 of gefitinib administration) reduced mean AUC of gefitinib by 83% [see Dosage and Administration (2.4), Drug Interactions (7)].
CYP3A4 Inhibitor:
Concomitant administration of itraconazole (200 mg QD for 12 days), an inhibitor of CYP3A4, with gefitinib (250 mg single dose on Day 4 of itraconazole administration) to healthy male subjects, increased mean gefitinib AUC by 80% [see Drug Interactions (7)].
Drugs Affecting Gastric pH:
Coadministration of high doses of ranitidine with sodium bicarbonate (to maintain the gastric pH above pH 5.0) to healthy subjects decreased mean gefitinib AUC by 47% [see Drug Interactions (7)].
In human liver microsome studies, gefitinib had no inhibitory effect on CYP1A2, CYP2C9, and CYP3A4 activities at concentrations ranging from 2 to 5000 ng/mL. At the highest concentration studied (5000 ng/mL), gefitinib inhibited CYP2C19 by 24% and CYP2D6 by 43%.
Exposure to metoprolol, a substrate of CYP2D6, was increased by 30% when it was given on Day 15 of gefitinib dosing (500 mg daily for 28 days) in patients with solid tumors.