After intravenous infusion of irinotecan in humans, irinotecan plasma concentrations decline in a multiexponential manner, with a mean terminal elimination half-life of about 6 to 12 hours. The mean terminal elimination half-life of the active metabolite SN-38 is about 10 to 20 hours. The half-lives of the lactone (active) forms of irinotecan and SN-38 are similar to those of total irinotecan and SN-38, as the lactone and hydroxy acid forms are in equilibrium.
Over the recommended dose range of 50 to 350 mg/m2, the AUC of irinotecan increases linearly with dose; the AUC of SN-38 increases less than proportionally with dose. Maximum concentrations of the active metabolite SN-38 are generally seen within 1 hour following the end of a 90-minute infusion of irinotecan. Pharmacokinetic parameters for irinotecan and SN-38 following a 90-minute infusion of irinotecan at dose levels of 125 and 340 mg/m2 determined in two clinical studies in patients with solid tumors are summarized in Table 9:
Table 9. Summary of Mean (±Standard Deviation) Irinotecan and SN-38 Pharmacokinetic Parameters in Patients with Solid TumorsC max - Maximum plasma concentration AUC 0-24 - Area under the plasma concentration-time curve from time 0 to 24 hours after the end of the 90-minute infusion t 1/2 - Terminal elimination half-life V z - Volume of distribution of terminal elimination phase CL - Total systemic clearance a Plasma specimens collected for 24 hours following the end of the 90-minute infusion. b Plasma specimens collected for 48 hours following the end of the 90-minute infusion. Because of the longer collection period, these values provide a more accurate reflection of the terminal elimination half-lives of irinotecan and SN-38. |
Dose (mg/m2) | Irinotecan | SN-38 |
C max (ng/mL) | AUC 0-24 (ng·h/mL) | t 1/2 (h) | V z (L/m2) | CL (L/h/m2) | C max (ng/mL) | AUC 0-24 (ng·h/mL) | t 1/2 (h) |
125 (N = 64) | 1,660 ±797 | 10,200 ±3,270 | 5.8 a ±0.7 | 110 ±48.5 | 13.3 ±6.01 | 26.3 ±11.9 | 229 ±108 | 10.4 a ±3.1 |
340 (N = 6) | 3,392 ±874 | 20,604 ±6,027 | 11.7 b ±1.0 | 234 ±69.6 | 13.9 ±4.0 | 56.0 ±28.2 | 474 ±245 | 21.0 b ±4.3 |
Distribution
Irinotecan exhibits moderate plasma protein binding (30% to 68% bound). SN-38 is highly bound to human plasma proteins (approximately 95% bound). The plasma protein to which irinotecan and SN-38 predominantly binds is albumin.
Elimination
Metabolism
Irinotecan is subject to extensive metabolic conversion by various enzyme systems, including esterases that form an active metabolite SN-38, and UGT1A1 which mediates the glucuronidation of SN-38 to form an inactive metabolite. SN-38 glucuronide had 1/50 to 1/100 the activity of SN-38. Patients who are homozygous for either the UGT1A1*28 or *6 alleles, or who are compound heterozygous for these alleles, have higher SN-38 AUC than patients with the wild-type UGT1A1 alleles [see Dosage and Administration (2.3), Warnings and Precautions (5.3), and Clinical Pharmacology (12.5)].
Irinotecan can also undergo CYP3A4-mediated oxidative metabolism to several inactive metabolites, one of which can be hydrolyzed by carboxylesterase to release the active metabolite SN-38.
Excretion
The disposition of irinotecan has not been fully elucidated in humans. The urinary excretion of irinotecan is 11% to 20%; SN-38, <1%; and SN-38 glucuronide, 3%. The cumulative biliary and urinary excretion of irinotecan and its metabolites (SN-38 and SN-38 glucuronide) over a period of 48 hours following administration of irinotecan in two patients ranged from approximately 25% (100 mg/m2) to 50% (300 mg/m2).
Specific Populations
Geriatric Patients
The pharmacokinetics of irinotecan administered using the weekly schedule was evaluated in a study of 183 patients that was prospectively designed to investigate the effect of age on irinotecan toxicity. Results from this trial indicate that there are no differences in the pharmacokinetics of irinotecan, SN-38, and SN-38 glucuronide in patients <65 years of age compared with patients ≥65 years of age. In a study of 162 patients that was not prospectively designed to investigate the effect of age, small (less than 18%) but statistically significant differences in dose-normalized irinotecan pharmacokinetic parameters in patients <65 years of age compared to patients ≥65 years of age were observed. Although dose-normalized AUC0-24 for SN-38 in patients ≥65 years of age was 11% higher than in patients <65 years of age, this difference was not statistically significant. No change in the starting dose is recommended for geriatric patients receiving the weekly dosage schedule of irinotecan [see Dosage and Administration (2)].
Male and Female Patients
The pharmacokinetics of irinotecan do not appear to be influenced by gender.
Racial and Ethnic Groups
The influence of race on the pharmacokinetics of irinotecan has not been evaluated.
Patients with Renal Impairment
The influence of renal impairment on the pharmacokinetics of irinotecan has not been evaluated.
Patients with Hepatic Impairment
Irinotecan clearance is diminished in patients with hepatic impairment while exposure to the active metabolite SN-38 is increased relative to that in patients with normal hepatic function. The magnitude of these effects is proportional to the degree of liver impairment as measured by elevations in total bilirubin and transaminase concentrations. However, the tolerability of irinotecan in patients with hepatic dysfunction (bilirubin greater than 2 mg/dl) has not been assessed sufficiently.
Drug Interaction Studies
Clinical Studies and Model-Informed Approaches
Dexamethasone, a moderate CYP3A4 inducer, does not appear to alter the pharmacokinetics of irinotecan.
In Vitro Studies
Irinotecan and the metabolites SN-38 and aminopentane carboxylic acid (APC) do not inhibit cytochrome P-450 isozymes.