Absorption: Following intravenous administration of nelarabine to adult patients with refractory leukemia or lymphoma, plasma ara-G Cmax values generally occurred at the end of the nelarabine infusion and were generally higher than nelarabine Cmax values, suggesting rapid and extensive conversion of nelarabine to ara-G. Mean plasma nelarabine and ara-G Cmax values were 5.0 ± 3.0 mcg/mL and 31.4 ± 5.6 mcg/mL, respectively, after a 1500 mg/m2 nelarabine dose infused over 2 hours in adult patients. The area under the concentration-time curve (AUC) of ara-G is 37 times higher than that for nelarabine on Day 1 after nelarabine IV infusion of 1500 mg/m2 dose (162 ± 49 mcg.h/mL versus 4.4 ± 2.2 mcg.h/mL, respectively). Comparable Cmax and AUC values were obtained for nelarabine between Days 1 and 5 at the nelarabine adult dosage of 1500 mg/m2, indicating that nelarabine does not accumulate after multiple-dosing. There are not enough ara-G data to make a comparison between Day 1 and Day 5. After a nelarabine adult dose of 1500 mg/m2, intracellular Cmax for ara-GTP appeared within 3 to 25 hours on Day 1. Exposure (AUC) to intracellular ara-GTP was 532 times higher than that for nelarabine and 14 times higher than that for ara-G (2,339 ± 2,628 mcg.h/mL versus 4.4 ± 2.2 mcg.h/mL and 162 ± 49 mcg.h/mL, respectively). Because the intracellular levels of ara-GTP were so prolonged, its elimination half-life could not be accurately estimated.
Distribution: Nelarabine and ara-G are extensively distributed throughout the body. For nelarabine, VSS values were 197 ± 216 L/m2 in adult patients. For ara-G, VSS/F values were 50 ± 24 L/m2 in adult patients.
Nelarabine and ara-G are not substantially bound to human plasma proteins (< 25%) in vitro, and binding is independent of nelarabine or ara-G concentrations up to 600 μM.
Metabolism: The principal route of metabolism for nelarabine is O-demethylation by ADA to form ara-G, which undergoes hydrolysis to form guanine. In addition, some nelarabine is hydrolyzed to form methylguanine, which is O-demethylated to form guanine. Guanine is N-deaminated to form xanthine, which is further oxidized to yield uric acid.
Excretion: Nelarabine and ara-G are partially eliminated by the kidneys. Mean urinary excretion of nelarabine and ara-G was 6.6 ± 4.7% and 27 ± 15% of the administered dose, respectively, in 28 adult patients over the 24 hours after nelarabine infusion on Day 1. Renal clearance averaged 24 ± 23 L/h for nelarabine and 6.2 ± 5.0 L/h for ara-G in 21 adult patients. Combined Phase I pharmacokinetic data at nelarabine doses of 199 to 2900 mg/m2 (n = 66 adult patients) indicate that the mean clearance (CL) of nelarabine is 197 ± 189 L/h/m2 on Day 1. The apparent clearance of ara-G (CL/F) is 10.5 ± 4.5 L/h/m2 on Day 1. Nelarabine and ara-G are rapidly eliminated from plasma with a mean half-life of 18 minutes and 3.2 hours, respectively, in adult patients.
Pediatrics: No pharmacokinetic data are available in pediatric patients at the once-daily 650 mg/m2 nelarabine dosage. Combined Phase I pharmacokinetic data at nelarabine doses of 104 to 2900 mg/m2 indicate that the mean clearance (CL) of nelarabine is about 30% higher in pediatric patients than in adult patients (259 ± 409 L/h/m2 versus 197 ± 189 L/h/m2, respectively) (n = 66 adults, n = 22 pediatric patients) on Day 1. The apparent clearance of ara-G (CL/F) is comparable between the 2 groups (10.5 ± 4.5 L/h/m2 in adult patients and 11.3 ± 4.2 L/h/m2 in pediatric patients) on Day 1. Nelarabine and ara-G are extensively distributed throughout the body. For nelarabine, VSS values were 213 ± 358 L/m2 in pediatric patients. For ara-G, VSS/F values were 33 ± 9.3 L/m2 in pediatric patients. Nelarabine and ara-G are rapidly eliminated from plasma in pediatric patients, with a half-life of 13 minutes and 2 hours, respectively.
Effect of Age: Age has no effect on the pharmacokinetics of nelarabine or ara-G in adults. Decreased renal function, which is more common in the elderly, may reduce ara-G clearance [see Use in Specific Populations (8.5)].
Effect of Gender: Gender has no effect on nelarabine or ara-G pharmacokinetics.
Effect of Race: In general, nelarabine mean clearance and volume of distribution values tend to be higher in whites (n = 63) than in blacks (by about 10%) (n = 15). The opposite is true for ara-G; mean apparent clearance and volume of distribution values tend to be lower in whites than in blacks (by about 15% to 20%). No differences in safety or effectiveness were observed between these groups.
Effect of Renal Impairment: The pharmacokinetics of nelarabine and ara-G have not been specifically studied in renally impaired or hemodialyzed patients. Nelarabine is excreted by the kidney to a small extent (5% to 10% of the administered dose). Ara-G is excreted by the kidney to a greater extent (20% to 30% of the administered nelarabine dose). In the combined Phase I trials, patients were categorized into 3 groups: normal with CLCr greater than 80 mL/min (n = 67), mild with CLCr = 50 to 80 mL/min (n = 15), and moderate with CLCr less than 50 mL/min (n = 3). The mean apparent clearance (CL/F) of ara-G was about 15% and 40% lower in patients with mild and moderate renal impairment, respectively, than in patients with normal renal function [see Use in Specific Populations (8.6), Dosage and Administration (2.3)]. No differences in safety or effectiveness were observed.
Effect of Hepatic Impairment: The influence of hepatic impairment on the pharmacokinetics of nelarabine has not been evaluated [see Use in Specific Populations (8.7)].
Drug Interactions: Cytochrome P450: Nelarabine and ara-G did not significantly inhibit the activities of the human hepatic cytochrome P450 isoenzymes 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, or 3A4 in vitro at concentrations of nelarabine and ara-G up to 100 μM.
Fludarabine: Administration of fludarabine 30 mg/m2 as a 30-minute infusion 4 hours before a 1200-mg/m2 infusion of nelarabine did not affect the pharmacokinetics of nelarabine, ara-G, or ara-GTP in 12 patients with refractory leukemia.
Pentostatin: There is in vitro evidence that pentostatin is a strong inhibitor of ADA. Inhibition of ADA may result in a reduction in the conversion of the prodrug nelarabine to its active moiety and consequently in a reduction in efficacy of nelarabine and/or change in adverse reaction profile of either drug [see Drug Interactions (7)].