Nelarabine is a pro-drug of the deoxyguanosine analogue ara-G. Nelarabine is rapidly demethylated by adenosine deaminase (ADA) to ara-G and then phosphorylated intracellularly by deoxyguanosine kinase and deoxycytidine kinase to its 5'-monophosphate metabolite. The monophosphate metabolite is subsequently converted to the active 5'-triphosphate from, ara-GTP. Accumulation of ara-GTP in leukaemic blasts allows for preferential incorporation of ara-GTP into deoxyribonucleic acid (DNA) leading to inhibition of DNA synthesis. This results in cell death. Other mechanisms may contribute to the cytotoxic effects of nelarabine. In vitro, T-cells are more sensitive than B-cells to the cytotoxic effects of nelarabine.
In a cross-study analysis using data from four Phase I studies, the pharmacokinetics of nelarabine and ara-G were characterized in patients aged less than 18 years and adult patients with refractory leukaemia or lymphoma.
Absorption
Adults
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. After infusion of 1,500 mg/m2 nelarabine over two hours in adult patients, mean (%CV) plasma nelarabine Cmax and AUCinf values were 13.9 µM (81 %) and 13.5 µM.h (56 %) respectively. Mean plasma ara-G Cmax and AUCinf values were 115 µM (16 %) and 571 µM.h (30 %), respectively.
Intracellular Cmax for ara-GTP appeared within 3 to 25 hours on day 1. Mean (%CV) intracellular ara-GTP Cmax and AUC values were 95.6 µM (139 %) and 2214 µM.h (263 %) at this dose.
Paediatric patients
After infusion of 400 or 650 mg/m2 nelarabine over one hour in 6 paediatric patients, mean (%CV) plasma nelarabine Cmax and AUCinf values, adjusted to a 650 mg/m2 dose, were 45.0 µM (40 %) and 38.0 µM.h (39 %), respectively. Mean plasma ara-G Cmax and AUCinf values were 60.1 µM (17 %) and 212 µM.h (18 %), respectively.
Distribution
Nelarabine and ara-G are extensively distributed throughout the body based on combined Phase I pharmacokinetic data at nelarabine doses of 104 to 2,900 mg/m2. Specifically, for nelarabine, mean (%CV) VSS values were 115 l/m2 (159 %) and 89.4 l/m2 (278 %) in adult and paediatric patients, respectively. For ara-G, mean VSS/F values were 44.8 l/m2 (32 %) and 32.1 l/m2 (25 %) in adult and paediatric patients, respectively.
Nelarabine and ara-G are not substantially bound to human plasma proteins (less than 25 %) in vitro, and binding is independent of nelarabine or ara-G concentrations up to 600 µM.
No accumulation of nelarabine or ara-G was observed in plasma after nelarabine administration on either a daily or a day 1, 3, 5 schedule.
Intracellular ara-GTP concentrations in leukaemic blasts were quantifiable for a prolonged period after nelarabine administration. Intracellular ara-GTP accumulated with repeated administration of nelarabine. On the day 1, 3, and 5 schedule, Cmax and AUC(0-t) values on day 3 were approximately 50 % and 30 %, respectively, greater than Cmax and AUC(0-t) values on day 1.
Biotransformation
The principal route of metabolism for nelarabine is O-demethylation by adenosine deaminase to form ara-G, which undergoes hydrolysis to form guanine. In addition, some nelarabine is hydrolysed 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.
Elimination
Nelarabine and ara-G are rapidly eliminated from plasma with a half-life of approximately 30 minutes and 3 hours, respectively. These findings were demonstrated in patients with refractory leukaemia or lymphoma given a dose of 1,500 mg/m2 nelarabine (adults) or a 650 mg/m2 (paediatrics).
Combined Phase 1 pharmacokinetic data at nelarabine doses of 104 to 2,900 mg/m2 indicate that mean (%CV) clearance (Cl) values for nelarabine are 138 l/h/m2 (104 %) and 125 l/h/m2 (214 %) in adult and paediatric patients, respectively, on day 1 (n = 65 adults, n = 21 paediatric patients). The apparent clearance of ara-G (Cl/F) is comparable between the two groups [9.5 l/h/m2 (35 %) in adult patients and 10.8 l/h/m2 (36 %) in paediatric patients] on day 1.
Nelarabine and ara-G are partially eliminated by the kidneys. In 28 adult patients, 24 hours after nelarabine infusion on day 1, mean urinary excretion of nelarabine and ara-G was 5.3 % and 23.2 % of the administered dose, respectively. Renal clearance averaged 9.0 l/h/m2 (151 %) for nelarabine and 2.6 l/h/m2 (83 %) for ara-G in 21 adult patients.
Because the timecourse of intracellular ara-GTP was prolonged, its elimination half-life could not be accurately estimated.
Paediatric population
Limited clinical pharmacology data are available for patients below the age of 4 years.
Combined Phase 1 pharmacokinetic data at nelarabine doses of 104 to 2,900 mg/m2 indicate that the clearance (Cl) and Vss values for nelarabine and ara-G are comparable between the two groups. Further data with respect to nelarabine and ara-G pharmacokinetics in the paediatric population are provided in other subsections.
Gender
Gender has no effect on nelarabine or ara-G plasma pharmacokinetics. Intracellular ara-GTP Cmax and AUC(0–t) values at the same dose level were 2– to 3– fold greater on average in adult female than in adult male patients.
Race
The effect of race on nelarabine and ara-G pharmacokinetics has not been specifically studied. In a pharmacokinetic/pharmacodynamic cross study analysis, race had no apparent effect on nelarabine, ara-G, or intracellular ara-GTP pharmacokinetics.
Renal impairment
The pharmacokinetics of nelarabine and ara-G have not been specifically studied in renally impaired or haemodialysed 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). Adults and children in clinical studies were categorized into the three groups according to renal impairment: normal with Clcr greater than 80 ml/min (n = 56), mild with Clcr equalling 50 to 80 ml/min (n = 12), and moderate with Clcr less than 50 ml/min (n = 2). The mean apparent clearance (Cl/F) of ara-G was about 7 % lower in patients with mild renal impairment than in patients with normal renal function (see section 4.2). No data are available to provide a dose advice for patients with Clcr less than 50 ml/min.
Elderly
Age has no effect on the pharmacokinetics of nelarabine or ara-G. Decreased renal function, which is more common in the elderly, may reduce ara-G clearance (see section 4.2).